olafmatt
Well-known member
[..]
high-gain tube mic pres
- Thread starter olafmatt
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NewYorkDave
Well-known member
Why not write to the keeper of that website and ask if he can supply further information? It wouldn't hurt.
Otherwise, there are many options if you want a tube mic pre with high gain. A couple of my own designs, posted here, might fit the bill. And they're inexpensive to build, too. There's the "One Bottle" preamp, which can be coupled with my line amp design to give about 70dB of gain. There's also the "Two Bottle" (which is the One-Bottle with an additional input stage added) which can do about 70dB into a high-Z load, or 60dB into a low-Z load. Or you can cascade two "One-Bottle" circuits for a whopping 84dB (into a high-Z load) or 72dB (low-Z load). And there's lots of other high-gain options out there besides those. For instance, there's the Altec 1566A, which may not be the quietest or the cleanest, but it sure has loads of gain. Look around...
Otherwise, there are many options if you want a tube mic pre with high gain. A couple of my own designs, posted here, might fit the bill. And they're inexpensive to build, too. There's the "One Bottle" preamp, which can be coupled with my line amp design to give about 70dB of gain. There's also the "Two Bottle" (which is the One-Bottle with an additional input stage added) which can do about 70dB into a high-Z load, or 60dB into a low-Z load. Or you can cascade two "One-Bottle" circuits for a whopping 84dB (into a high-Z load) or 72dB (low-Z load). And there's lots of other high-gain options out there besides those. For instance, there's the Altec 1566A, which may not be the quietest or the cleanest, but it sure has loads of gain. Look around...
Remember that at those gains, you only come out with a weak 50-57dB signal/noise ratio - at best...
Jakob E.
Jakob E.
You can use a Jensen 1:10 input for the V76. I think they are about 80 bucks.
Same sound, just not designed to take a 30 db hit from a bass drum into a hot condenser mic.
Same sound, just not designed to take a 30 db hit from a bass drum into a hot condenser mic.
clintrubber
Well-known member
[quote author="olafmatt"]For use with ribbon mics as stereo room mics I'm searching for a tube mic pre that has around or even more than 70dB of gain. I know about the V76, which seems to be expensive to DIY (because of the input transformer). On my search I came across this one:
http://www.aesproaudio.com/newimages/emimicpre1.jpg
It's a 'EMI 806' pre with 40 - 80 dB gain on 10dB steps, exactly what I am looking for. Does anybody know anything about these pres? They were made by EMI for use in tube consoles. You can see one here: http://www.aesproaudio.com/emi.htm
Olaf[/quote]
I may have overlooked it, but which tubes are used in that '806 ?
For such gains another option is to use 'normal' mic-pre's (be it tubed or solid state) and when ribbon-room-use is in order insert the circuit from this thread:
http://www.groupdiy.com/index.php?topic=5743&highlight=active+ribbon
Depending on preferences it may be 'less glamorous' perhaps or just not what you're looking for, but imho it looks a sane approach to the question at hand.
Bye,
Peter
http://www.aesproaudio.com/newimages/emimicpre1.jpg
It's a 'EMI 806' pre with 40 - 80 dB gain on 10dB steps, exactly what I am looking for. Does anybody know anything about these pres? They were made by EMI for use in tube consoles. You can see one here: http://www.aesproaudio.com/emi.htm
Olaf[/quote]
I may have overlooked it, but which tubes are used in that '806 ?
For such gains another option is to use 'normal' mic-pre's (be it tubed or solid state) and when ribbon-room-use is in order insert the circuit from this thread:
http://www.groupdiy.com/index.php?topic=5743&highlight=active+ribbon
Depending on preferences it may be 'less glamorous' perhaps or just not what you're looking for, but imho it looks a sane approach to the question at hand.
Bye,
Peter
clintrubber
Well-known member
BTW, how's that level-meter done ? This is pre-LED-stuff, right ? Bulbs ? Some plasma-thingy ? Doesn't look a magic eye/band thing...
Hey Olaf, I think you are more on the spot with a V76 design. I´tried to
mod one of 76´s to a ~ 72 , to break out to the output Tx after the sec.
tube (804) but in high gainsettings from 64 and above it was not stable
anymore. I did not try any compensation or other things.I just went back
to the 4 tube design and widrawed the rolloff-filter0,85H induktor and the 100pF cap.
You could try with a Lundahl 1577 (1:14) setting,it´s just 7 dB lower than
the org. 1:30. If you as NY-Dave suggest make an output stage with a WCF you dont have to loose those 19 dB at the output with a > 2kohm
load.
Actually the V76 has a gain structure from in to out Tx(~+30dB),V1(+22dB),V2(+33dB), -6 dB in the filter section, V3+V4(~+16dB)
and outTx(-19dB),wellthat suppose to be 76 dB gain in totally.
If we remake this into our "new" design it could turn out this way.
A small calculation could be; 1577 (+23dB) the first half of V76
(+55dB) and the WCF direct out (0) db. If I count right it beats
the V76 with 2 dB to be a "V78",but that name is occupied, anyway.
Cheers Bo
mod one of 76´s to a ~ 72 , to break out to the output Tx after the sec.
tube (804) but in high gainsettings from 64 and above it was not stable
anymore. I did not try any compensation or other things.I just went back
to the 4 tube design and widrawed the rolloff-filter0,85H induktor and the 100pF cap.
You could try with a Lundahl 1577 (1:14) setting,it´s just 7 dB lower than
the org. 1:30. If you as NY-Dave suggest make an output stage with a WCF you dont have to loose those 19 dB at the output with a > 2kohm
load.
Actually the V76 has a gain structure from in to out Tx(~+30dB),V1(+22dB),V2(+33dB), -6 dB in the filter section, V3+V4(~+16dB)
and outTx(-19dB),wellthat suppose to be 76 dB gain in totally.
If we remake this into our "new" design it could turn out this way.
A small calculation could be; 1577 (+23dB) the first half of V76
(+55dB) and the WCF direct out (0) db. If I count right it beats
the V76 with 2 dB to be a "V78",but that name is occupied, anyway.
Cheers Bo
> a tube mic pre that has around or even more than 70dB of gain.
Just put in more tubes.
In general, a lowest-noise tube input for a low-Z dynamic mike needs a 1:7 to 1:15 input transformer just to overcome grid noise. That's 20dB right there: not "free", not even cheap, just necessary. Each triode gives gain of 10dB to 30dB. A key detail is your output stage: if you need low-Z transformer output your last tube will be big and nearly no gain. So transformer, 12AX7, 12AX7 is 20dB+25dB+25dB= 70dB of gain, a 6F6 triode and output iron has gain of unity, add another 12AX7 to get 10dB more gain with 10dB feedback around the output stage. Just 2.5 bottles, 2 bottles if you use 6EM7 for the output stages. Octal 6SL7 would be a suitable input tube. There are also 12SL7 and 13EM7 which may be cheaper and can be fed with a 12VDC heater supply
-124dBu input noise, times gain of 70dB, is -54dBu output noise. That's only 58dB S/N re: +4dBu, 74dB re: +20dBu. Since we now have 94dB dynamic range devices, it should not be necessary to jam that much noise onto the recording. Don't use that much gain. Fix it in the mix.
Anyway, as bovox says: "in high gainsettings from 64 and above it was not stable". It is really difficult to get 70dB of gain on one chassis. Why do you think Langevin stuck with 40dB and 50dB modules? The total path from mike to recorder would have at least one of each for 90dB total gain, though usually 20dB loss in mix-network. Fancy configurations would have 40dB+40dB+50dB in gain, and -20dB in EQ/echo and -20dB mix and another -20dB in normal fader settings, 70dB gain nominal with a lot in reserve for weak sources. But even at the high noise level and high line levels of AM broadcasting, you'd never go over 82dB working gain unless it was the dying words of a great man being broadcast live. If that's your normal recording, go for it.
The stability problem multiplies when you have several channels next to each other. I'm stunned that EMI was able to do it in that size; the dedicated power supplies may be critical.
Just put in more tubes.
In general, a lowest-noise tube input for a low-Z dynamic mike needs a 1:7 to 1:15 input transformer just to overcome grid noise. That's 20dB right there: not "free", not even cheap, just necessary. Each triode gives gain of 10dB to 30dB. A key detail is your output stage: if you need low-Z transformer output your last tube will be big and nearly no gain. So transformer, 12AX7, 12AX7 is 20dB+25dB+25dB= 70dB of gain, a 6F6 triode and output iron has gain of unity, add another 12AX7 to get 10dB more gain with 10dB feedback around the output stage. Just 2.5 bottles, 2 bottles if you use 6EM7 for the output stages. Octal 6SL7 would be a suitable input tube. There are also 12SL7 and 13EM7 which may be cheaper and can be fed with a 12VDC heater supply
-124dBu input noise, times gain of 70dB, is -54dBu output noise. That's only 58dB S/N re: +4dBu, 74dB re: +20dBu. Since we now have 94dB dynamic range devices, it should not be necessary to jam that much noise onto the recording. Don't use that much gain. Fix it in the mix.
Anyway, as bovox says: "in high gainsettings from 64 and above it was not stable". It is really difficult to get 70dB of gain on one chassis. Why do you think Langevin stuck with 40dB and 50dB modules? The total path from mike to recorder would have at least one of each for 90dB total gain, though usually 20dB loss in mix-network. Fancy configurations would have 40dB+40dB+50dB in gain, and -20dB in EQ/echo and -20dB mix and another -20dB in normal fader settings, 70dB gain nominal with a lot in reserve for weak sources. But even at the high noise level and high line levels of AM broadcasting, you'd never go over 82dB working gain unless it was the dying words of a great man being broadcast live. If that's your normal recording, go for it.
The stability problem multiplies when you have several channels next to each other. I'm stunned that EMI was able to do it in that size; the dedicated power supplies may be critical.
There is a ton of NFB in the V76. Even at 76 db, there is still plenty of NFB going on. I believe this keeps things stable?
There was some genius around here that was toying with the idea of a tube stage in the ribbon mic itself, so if you are after less noise and enough gain to drive a 40 db box..... but it would be quite an undertaking I suspect.
There was some genius around here that was toying with the idea of a tube stage in the ribbon mic itself, so if you are after less noise and enough gain to drive a 40 db box..... but it would be quite an undertaking I suspect.
Wow, do you really need that much gain with a ribbon mike? 70dB seems like an awful lot.
I would even suggest going as far as 1:20 transformer. These are common for moving coil phono step-ups. I get over 72dB SNR with my all tube phono stage using 1:10. But that's unfair, as the RIAA equalization is cheating.
jh
I would even suggest going as far as 1:20 transformer. These are common for moving coil phono step-ups. I get over 72dB SNR with my all tube phono stage using 1:10. But that's unfair, as the RIAA equalization is cheating.
jh
> There is a ton of NFB in the V76. ... I believe this keeps things stable?
Not by magic. If you bring the output of a 76dB amp anywhere near its input, it will find a way to oscillate. That could be putting the input and output leads too close (especially the unbalanced grid and plate leads); it could be through the power supply, even the heater supply (hi-gain radio IF amps have to have RF decoupling in the heater leads).
> Wow, do you really need that much gain with a ribbon mike? 70dB seems like an awful lot.
I would even suggest going as far as 1:20 transformer. These are common for moving coil phono step-ups.
Different unstated assumptions for different fields of audio.
Moving-coil pickups are ~~10Ω impedance with very low reactance and motional impedance.
Microphones are commonly transformed to "line" impedance, 100-200Ω for most types of cables we use. We have to do that because, unlike a phono, we often have the mikes a long-long way from the mike-amp. A phono needle is rarely more than arm's-length from its amp; from the studio to the booth may be the next room (typical small studio) to 4 floors up (BBC used to have ALL its tubes in one room, everything went there) to the far end of a football stadium.
While 150-250Ω is the current fad, in the past some operations transformed mikes to 37, 60, and other lower impedances. This suggests less-long runs of more-fat cable, which probably was the case when studios were not so huge in size and relatively huge in cost. Some of this "need for high gain" with older ribbons may just be using 37Ω mikes with 200Ω inputs: a 37Ω:200Ω input tranny would bump the level another 7dB.
> 1:20 transformer. These are common for moving coil phono
A 20Ω pickup, transformed 1:20, is a 8,000Ω secondary. 500pFd of combined winding and grid capacitance is a 40KHz roll-off, fine. A 200Ω mike, transformed 1:20, is a 80KΩ secondary and 4KHz roll-off, not OK. We can partition the winding, but that increases transformer leakage inductance which would already be a problem. Good 60K windings have been made in the past; but lower-Z windings are easier to make good, and at good price. We can only reduce grid capacitance so far: we need a fairly big (12AX7 size) tube to get noise voltage down, we can't use a Pentode because of partition noise. We could of course try Cascode, but that's hard on heater insulation, and if we just want Gain we could use a plain cascade of the same two tubes and get so much extra gain that tranny-gain is not a big deal.
Transformers look like "free gain" but rarely are. The higher you wind the secondary, the lower the bandwidth, the higher the cost of not losing so much bandwidth.
So in a tube amp, where we do not have a BIG range of device noise (running a bigger tube at higher current lowers input noise voltage, but not by much before power demands become obscene), we just transform the source resistance thermal noise voltage up above the tube's noise voltage. Tubes can run 0.5uV to 2uV (and up!) across the audio band. 200Ω mike is ~0.2uV noise. 1:7 works well with selected fat tubes like 6DJ8. 1:10 is less fussy about the tube: most clean general-purpose triodes will work OK. And they can be run at 0.5mA-1mA, instead of the 2mA-6mA of a rich 6DJ8, which makes a big difference in power decoupling size/cost.
And then, being one-off people, we have to take the iron we can get. No tranny-winder is going to take us out to lunch to get our business, or slave over a hot winder to get the perfect balance of inductance and capacitance. This is not a killer problem because good 1:7 1:10 1:15 transformers can be found at Jensen, Sowter, and by eBay archeology.
Gain-structure may be as important as gain. We know we will NOT need 80dB EVERY gig. Plate to cathode feedback is limited: if we need to reduce gain with feedback, the second tube has to be big to supply the feedback current into the first tube.
That foils my 6SL7/6EM7 plan: we should probably expect 100mV at the input, 1V at the first grid. At reasonable B+, the second plate can't swing more than 40V RMS. That means the two tubes have to be held back to a gain of 40. If the first cathode resistor is 1K, the feedback resistor is 40K, which is a heavy load for 6SL7. And suggests significant supply current, which has to be VERY free of buzz and of output current flowing back along the B+ chain. If we increase the first cathode resistor, we linearize the first tube but not the second tube. And we'll want to reduce first tube current, which will increase its noise voltage.
Many ways to skin the cat. Langevin used a transformer in the second plate, which allows much more output power, and also allows transformer-tap feedback instead of lossy resistor feedback. Langevin had people chained to core-winders; we don't. At today's prices, a fat cathode follower as the third stage would be cheaper than a custom transformer, and it could be any of many unloved old TV tubes still glutting the market.
This is the point where a motivated designer starts walking around, mumbling, holding alternatives in each hand and weighing them, and stares at clouds. Until inspiration strikes, the project is abandoned, or "we have to do something" so some scraps of ideas are tossed together until sound comes out.
Not by magic. If you bring the output of a 76dB amp anywhere near its input, it will find a way to oscillate. That could be putting the input and output leads too close (especially the unbalanced grid and plate leads); it could be through the power supply, even the heater supply (hi-gain radio IF amps have to have RF decoupling in the heater leads).
> Wow, do you really need that much gain with a ribbon mike? 70dB seems like an awful lot.
I would even suggest going as far as 1:20 transformer. These are common for moving coil phono step-ups.
Different unstated assumptions for different fields of audio.
Moving-coil pickups are ~~10Ω impedance with very low reactance and motional impedance.
Microphones are commonly transformed to "line" impedance, 100-200Ω for most types of cables we use. We have to do that because, unlike a phono, we often have the mikes a long-long way from the mike-amp. A phono needle is rarely more than arm's-length from its amp; from the studio to the booth may be the next room (typical small studio) to 4 floors up (BBC used to have ALL its tubes in one room, everything went there) to the far end of a football stadium.
While 150-250Ω is the current fad, in the past some operations transformed mikes to 37, 60, and other lower impedances. This suggests less-long runs of more-fat cable, which probably was the case when studios were not so huge in size and relatively huge in cost. Some of this "need for high gain" with older ribbons may just be using 37Ω mikes with 200Ω inputs: a 37Ω:200Ω input tranny would bump the level another 7dB.
> 1:20 transformer. These are common for moving coil phono
A 20Ω pickup, transformed 1:20, is a 8,000Ω secondary. 500pFd of combined winding and grid capacitance is a 40KHz roll-off, fine. A 200Ω mike, transformed 1:20, is a 80KΩ secondary and 4KHz roll-off, not OK. We can partition the winding, but that increases transformer leakage inductance which would already be a problem. Good 60K windings have been made in the past; but lower-Z windings are easier to make good, and at good price. We can only reduce grid capacitance so far: we need a fairly big (12AX7 size) tube to get noise voltage down, we can't use a Pentode because of partition noise. We could of course try Cascode, but that's hard on heater insulation, and if we just want Gain we could use a plain cascade of the same two tubes and get so much extra gain that tranny-gain is not a big deal.
Transformers look like "free gain" but rarely are. The higher you wind the secondary, the lower the bandwidth, the higher the cost of not losing so much bandwidth.
So in a tube amp, where we do not have a BIG range of device noise (running a bigger tube at higher current lowers input noise voltage, but not by much before power demands become obscene), we just transform the source resistance thermal noise voltage up above the tube's noise voltage. Tubes can run 0.5uV to 2uV (and up!) across the audio band. 200Ω mike is ~0.2uV noise. 1:7 works well with selected fat tubes like 6DJ8. 1:10 is less fussy about the tube: most clean general-purpose triodes will work OK. And they can be run at 0.5mA-1mA, instead of the 2mA-6mA of a rich 6DJ8, which makes a big difference in power decoupling size/cost.
And then, being one-off people, we have to take the iron we can get. No tranny-winder is going to take us out to lunch to get our business, or slave over a hot winder to get the perfect balance of inductance and capacitance. This is not a killer problem because good 1:7 1:10 1:15 transformers can be found at Jensen, Sowter, and by eBay archeology.
Gain-structure may be as important as gain. We know we will NOT need 80dB EVERY gig. Plate to cathode feedback is limited: if we need to reduce gain with feedback, the second tube has to be big to supply the feedback current into the first tube.
That foils my 6SL7/6EM7 plan: we should probably expect 100mV at the input, 1V at the first grid. At reasonable B+, the second plate can't swing more than 40V RMS. That means the two tubes have to be held back to a gain of 40. If the first cathode resistor is 1K, the feedback resistor is 40K, which is a heavy load for 6SL7. And suggests significant supply current, which has to be VERY free of buzz and of output current flowing back along the B+ chain. If we increase the first cathode resistor, we linearize the first tube but not the second tube. And we'll want to reduce first tube current, which will increase its noise voltage.
Many ways to skin the cat. Langevin used a transformer in the second plate, which allows much more output power, and also allows transformer-tap feedback instead of lossy resistor feedback. Langevin had people chained to core-winders; we don't. At today's prices, a fat cathode follower as the third stage would be cheaper than a custom transformer, and it could be any of many unloved old TV tubes still glutting the market.
This is the point where a motivated designer starts walking around, mumbling, holding alternatives in each hand and weighing them, and stares at clouds. Until inspiration strikes, the project is abandoned, or "we have to do something" so some scraps of ideas are tossed together until sound comes out.
Yeah. that's where I am now. Looking at a range of mics, condensers are pretty hot at 10mV to 20mV. And it seems they can put out a LOT more on peaks. The headroom specifications of these mics is astounding. Dynamics have lower output in the 2mV to 5mV range.
Seems to me 40dB gain covers most situations. Or am I in left field here?
jh
pstamler
Well-known member
[quote author="hagtech"]
Yeah. that's where I am now. Looking at a range of mics, condensers are pretty hot at 10mV to 20mV. And it seems they can put out a LOT more on peaks. The headroom specifications of these mics is astounding. Dynamics have lower output in the 2mV to 5mV range.
Seems to me 40dB gain covers most situations. Or am I in left field here?[/quote]
No, you're landing where a lot of us have landed. 40dB for the first stage, with a pad in front of the transformer to deal with the really hot condensers, and more gain in a second amplifier after a level control.
The trick is to find something that runs quietly enough to not add significant noise to a ribbon mic's inherent (thermal) noise. A 150 ohm mic, loaded by 1.5k, has an inherent noise in a 20kHz bandwidth of about 0.21uV, or -131.2dBu. Designing a preamp that will add no more than 1dB of noise to that is a challenge; with transformer-input circuits, your biggest obstacle becomes the winding resistances of the tranny.
Peace,
Paul
Yeah. that's where I am now. Looking at a range of mics, condensers are pretty hot at 10mV to 20mV. And it seems they can put out a LOT more on peaks. The headroom specifications of these mics is astounding. Dynamics have lower output in the 2mV to 5mV range.
Seems to me 40dB gain covers most situations. Or am I in left field here?[/quote]
No, you're landing where a lot of us have landed. 40dB for the first stage, with a pad in front of the transformer to deal with the really hot condensers, and more gain in a second amplifier after a level control.
The trick is to find something that runs quietly enough to not add significant noise to a ribbon mic's inherent (thermal) noise. A 150 ohm mic, loaded by 1.5k, has an inherent noise in a 20kHz bandwidth of about 0.21uV, or -131.2dBu. Designing a preamp that will add no more than 1dB of noise to that is a challenge; with transformer-input circuits, your biggest obstacle becomes the winding resistances of the tranny.
Peace,
Paul
> Seems to me 40dB gain covers most situations. Or am I in left field here?
You're in the ballpark; sometimes the drummer isn't.
I once recorded a classical percussion solo and needed LOSS between mike and recorder.
Mike sensitivities:
Output at 74dB SPL:
1.0 mV to 2.5 mV - German-tradition large condensers
0.2 mV to 0.6 mV - small condensers
0.1 mV to 0.2 mV - US dynamics
vintage ribbons: 5dB to 10dB lower than dynamics, assuming ~150Ω nominal impedance
Few mikes want to be impedance-matched. Some don't mind. Others specify >1K or >2K loading. Some mikes expect some load: some of the BBC 300Ω ribbons wanted 600Ω loading to tame the bass. Many people prefer SM-58 with a heavy load.
Dynamics can drive a dead short without harm. The response will deviate from the spec, but it won't go to hell. Some condensers with class AB output will drive a short, most are rated 100Ω minimum. Condensers with single-ended output device may have trouble driving less than 1KΩ (or long cables). General custom is to give a 2KΩ input; you can always add a loading resistor if you like the sound better. A current fad is "variable impedance" implemented several ways: plain loading, or tapping a transformer. One designer advocates zero impedance inputs, though he has to put a few ohms in so the condenser mikes don't freak. Very high impedance inputs are not favored because when the mike cable is unplugged, they buzz (and maybe hiss) badly, though Valley People sold a lot of 200K mike inputs.
Performance acoustic levels:
Harpsichord in audience: 74dB SPL
String quartet in audience: 94dB SPL
Full orchestra, in audience, rare peaks: 126dB SPL
Trumpet at 4 feet: 118dB SPL; at 1 foot, ~130dB SPL
High power speaker, near-field: 130dB SPL
Using AKG414 (1.25mV at 74dB SPL) around orchestra, I assume that once or twice a season I will have 500mV coming off the mike.
I also do harpsichord in that hall: about 1.25mV off the mike which would need 400 times more gain than the orchestra to peak at zero dBfs. In fact I use only 10 times more gain because the room rumble is not a lot lower than the harpsichord (though a lower frequency range). The high-end noise of mike and room is 15dB SPL, so I can only get 60dB dynamic range from the performance. I don't have to jam this into the top of the 95dB range of 16-bit recording. (Also the applause may be louder than the harpsichord, and I'm too lazy to reach for the knob when the music stops.)
When dynamics ruled, it was customary to spec input iron for 30mV for small acts, 100mV for orchestral work. 1.25mV/0.2mV is about 5:1 which is about the difference between my LD numbers and the old dynamic-mike mixer specs. This is a good benchmark for "normal" performance. Of course since Dick Dale got Leo and Jim to build him a GITAR AMP, and the drum makers caught up, "normal" is abnormal.
Large classic percussion, vigorously played, miked at 8 feet, with a 2.5mV/74dBSPL condenser, gave peaks over 2V. Indeed that seems to be the upper un-padded spec for many inputs of today. If you don't give 2V input overload, someone is going to clip.
I have heard that dynamics in front of some singers or inside a bass drum can make a Volt of signal. That would be 1V/0.2mV= 74dB above 74dBSPL or 148dB SPL. I have a hard time accepting this, but am not prepared to reject it either.
As Paul says, noise of the passive mikes runs 0.2uV, and they give up to 0.2mV at 74dB SPL. Assuming noise floor near 14dB SPL, these mikes need near-zero noise figure in the preamp. (Studios have acoustic noise from below 10dBSPL to over 30dBSPL, so total noise is not always dominated by the mike and amplifier.)
The large condensers give 1mV-2.5mV output because they have internal amps, which make noise, 1uV for my AKG414. These do not need low noise figure amps: a 10dB NF input would be just noticeable. The logic is impeccable: if you adopt the condenser capsule, you are stuck with an amp in the mike. But now that you have an amp, you may as well bring the level up to overcome line noise and allow a low-price console input. The board-amps for the hot condenser mikes can be seen as high-gain line-inputs, rather than low-noise inputs. OTOH, there are condensers with outputs (and output noise) scaled to dynamic levels, so you can't just go by "type".
You can build a low-noise amp, you can build a low-gain amp, you can even build a low-noise low-gain amp. But it gets very hard to build a low-noise amp with gain that varies from high to near-unity. And since hot mikes can easily exceed a volt, and console internal level may be less than a volt, you may need gain of unity or less.
It would seem logical to build an input with <0.2uV noise and 100mV overload, and supply a 5:1 pad for use with hot condensers. It is hard to build a 5:1 14dB pad with appropriate in and out impedances; if hot-condenser is the only goal, you can let the input impedance be less than 1K. For general use, you wish the input to be over 2K and output under 200Ω, which suggests at least 20dB pad, but that puts a hot-condenser's self-noise below the thermal noise of a 200Ω input. We live with imperfection.
> Seems to me 40dB gain covers most situations. Or am I in left field here?
Take my 500mV orchestral peaks and amplify 40dB. We have 50V signal! I could engage the 10dB pad on the mike, but the lines run past dimmered lamps so I'd rather not. I could pad 10dB-15db at the amp, but then why did I pay for 40dB gain and throw-away a lot of it?
What I use is 20dB gain, pot, 20dB gain, 2Vfs recorder. For the largest orchestra works, I run 28dB loss in the pot. I can bring 5mV or 86dBSPL signals up to 0dBfs. I don't try to bring 74dBSPL harpsichord up to 0dBFS; let it peak at -12dB, it aint supposed to be loud, and the digital dither garbage is hidden under room and mike noise.
Note that the 20dB gain in the first stage is nearly all I can run with +/-15V supplies. 500mV times 20dB is 5V, just 6dB-8dB below clipping. Actually I would be OK with 26dB gain: 126dB SPL peaks are so rare and so strained that touching clipping would not ruin the recording. It just happened that the commercial boxes I used were easily set to 20dB, fussy to set to 26dB.
Note that, with passive pot, I need gain after the pot. I need 34db-40dB of gain-range, for the same mikes in the same room for different acts. If I took all the gain before the pot, then to get 2V at the recorder with 34dB attenuation for the big orchestra, I'd need 100V level out of the preamp! Yeah, we could do it with tubes or high-dope transistors.
If I used dynamics, I would need 14 to 20dB more gain. Say 40dB, pot, 20dB.
Ribbons could need 5dB to 10dB more, even if they are nominal ~200Ω impedance. Say 46dB, pot, 20dB.
If I worked at +4dBu nominal levels, I'd need 12dB more gain. For hot condenser: 20dB, pot, 32dB. For ribbon: 46dB, pot, 32dB. This IS 78dB maximum gain, and only pulls 86dB SPL up to recorder max input.
So there are a lot of issues. Mike sensitivity varies from 0.05mV to 2.5mV, 24dB. Acts range from 74dB SPL to >130dB SPL. Nominal electronic level can be -10dBV or +4dBu (or +8dBm), 2.8V peak or 17V peak, or some other level (or no level discipline at all).
Mikes: 24dB range
Acts: 56dB range
Level: 12dB range
So we could need 92dB range of gain.
In fact 74dB SPL acts don't need to be peaked; with modern recorders a 90dB SPL level is reasonable, so only 76dB range of gain.
The 12dB difference between Tascam and Pro levels is usually covered by a switch or added jack (or no-support): 64dB range. Many sound-folk know to use a pad on bass-drum: 44dB range.
The situation is very different from phono-amps. The noise and max signal on a disk is fairly well fixed: ultra-virgin vinyl may have 10dB lower noise, some 12" singles may have 10dB bigger wiggles, but nothing like the 50dB range of acts found on stage. Phono needles have converged on two basic types: 5mV and 0.25mV, and you don't change pickups often. Mikes ran 0.05mV to 2.5mV and we do swap them frequently to find "the sound".
> I get over 72dB SNR with my all tube phono stage
You can only claim the last 12dB of that. The LP standard aimed at 60dB from surface noise to nominal recording level. The 5mV 47K pickup standard evolved because the surface noise would be slightly higher than the thermal noise of 10K-30K impedance at resonance and the grid noise of a semi-selected tube. (An MC pickup with a transformer works about the same as an MM pickup, though a little less trouble from parasitics.) Improved vinyl and cutters have got noise down a bit, your skilled tube-work has improved on early 6SC7 phono inputs to keep up. And I suspect that you reach over-kill: if you listen with the needle up, then drop it in a blank groove, doesn't the hiss increase? You "don't have to be that good" (though I agree that pure hiss is different from surface noise, less interesting, should be kept far back).
You're in the ballpark; sometimes the drummer isn't.
I once recorded a classical percussion solo and needed LOSS between mike and recorder.
Mike sensitivities:
Output at 74dB SPL:
1.0 mV to 2.5 mV - German-tradition large condensers
0.2 mV to 0.6 mV - small condensers
0.1 mV to 0.2 mV - US dynamics
vintage ribbons: 5dB to 10dB lower than dynamics, assuming ~150Ω nominal impedance
Few mikes want to be impedance-matched. Some don't mind. Others specify >1K or >2K loading. Some mikes expect some load: some of the BBC 300Ω ribbons wanted 600Ω loading to tame the bass. Many people prefer SM-58 with a heavy load.
Dynamics can drive a dead short without harm. The response will deviate from the spec, but it won't go to hell. Some condensers with class AB output will drive a short, most are rated 100Ω minimum. Condensers with single-ended output device may have trouble driving less than 1KΩ (or long cables). General custom is to give a 2KΩ input; you can always add a loading resistor if you like the sound better. A current fad is "variable impedance" implemented several ways: plain loading, or tapping a transformer. One designer advocates zero impedance inputs, though he has to put a few ohms in so the condenser mikes don't freak. Very high impedance inputs are not favored because when the mike cable is unplugged, they buzz (and maybe hiss) badly, though Valley People sold a lot of 200K mike inputs.
Performance acoustic levels:
Harpsichord in audience: 74dB SPL
String quartet in audience: 94dB SPL
Full orchestra, in audience, rare peaks: 126dB SPL
Trumpet at 4 feet: 118dB SPL; at 1 foot, ~130dB SPL
High power speaker, near-field: 130dB SPL
Using AKG414 (1.25mV at 74dB SPL) around orchestra, I assume that once or twice a season I will have 500mV coming off the mike.
I also do harpsichord in that hall: about 1.25mV off the mike which would need 400 times more gain than the orchestra to peak at zero dBfs. In fact I use only 10 times more gain because the room rumble is not a lot lower than the harpsichord (though a lower frequency range). The high-end noise of mike and room is 15dB SPL, so I can only get 60dB dynamic range from the performance. I don't have to jam this into the top of the 95dB range of 16-bit recording. (Also the applause may be louder than the harpsichord, and I'm too lazy to reach for the knob when the music stops.)
When dynamics ruled, it was customary to spec input iron for 30mV for small acts, 100mV for orchestral work. 1.25mV/0.2mV is about 5:1 which is about the difference between my LD numbers and the old dynamic-mike mixer specs. This is a good benchmark for "normal" performance. Of course since Dick Dale got Leo and Jim to build him a GITAR AMP, and the drum makers caught up, "normal" is abnormal.
Large classic percussion, vigorously played, miked at 8 feet, with a 2.5mV/74dBSPL condenser, gave peaks over 2V. Indeed that seems to be the upper un-padded spec for many inputs of today. If you don't give 2V input overload, someone is going to clip.
I have heard that dynamics in front of some singers or inside a bass drum can make a Volt of signal. That would be 1V/0.2mV= 74dB above 74dBSPL or 148dB SPL. I have a hard time accepting this, but am not prepared to reject it either.
As Paul says, noise of the passive mikes runs 0.2uV, and they give up to 0.2mV at 74dB SPL. Assuming noise floor near 14dB SPL, these mikes need near-zero noise figure in the preamp. (Studios have acoustic noise from below 10dBSPL to over 30dBSPL, so total noise is not always dominated by the mike and amplifier.)
The large condensers give 1mV-2.5mV output because they have internal amps, which make noise, 1uV for my AKG414. These do not need low noise figure amps: a 10dB NF input would be just noticeable. The logic is impeccable: if you adopt the condenser capsule, you are stuck with an amp in the mike. But now that you have an amp, you may as well bring the level up to overcome line noise and allow a low-price console input. The board-amps for the hot condenser mikes can be seen as high-gain line-inputs, rather than low-noise inputs. OTOH, there are condensers with outputs (and output noise) scaled to dynamic levels, so you can't just go by "type".
You can build a low-noise amp, you can build a low-gain amp, you can even build a low-noise low-gain amp. But it gets very hard to build a low-noise amp with gain that varies from high to near-unity. And since hot mikes can easily exceed a volt, and console internal level may be less than a volt, you may need gain of unity or less.
It would seem logical to build an input with <0.2uV noise and 100mV overload, and supply a 5:1 pad for use with hot condensers. It is hard to build a 5:1 14dB pad with appropriate in and out impedances; if hot-condenser is the only goal, you can let the input impedance be less than 1K. For general use, you wish the input to be over 2K and output under 200Ω, which suggests at least 20dB pad, but that puts a hot-condenser's self-noise below the thermal noise of a 200Ω input. We live with imperfection.
> Seems to me 40dB gain covers most situations. Or am I in left field here?
Take my 500mV orchestral peaks and amplify 40dB. We have 50V signal! I could engage the 10dB pad on the mike, but the lines run past dimmered lamps so I'd rather not. I could pad 10dB-15db at the amp, but then why did I pay for 40dB gain and throw-away a lot of it?
What I use is 20dB gain, pot, 20dB gain, 2Vfs recorder. For the largest orchestra works, I run 28dB loss in the pot. I can bring 5mV or 86dBSPL signals up to 0dBfs. I don't try to bring 74dBSPL harpsichord up to 0dBFS; let it peak at -12dB, it aint supposed to be loud, and the digital dither garbage is hidden under room and mike noise.
Note that the 20dB gain in the first stage is nearly all I can run with +/-15V supplies. 500mV times 20dB is 5V, just 6dB-8dB below clipping. Actually I would be OK with 26dB gain: 126dB SPL peaks are so rare and so strained that touching clipping would not ruin the recording. It just happened that the commercial boxes I used were easily set to 20dB, fussy to set to 26dB.
Note that, with passive pot, I need gain after the pot. I need 34db-40dB of gain-range, for the same mikes in the same room for different acts. If I took all the gain before the pot, then to get 2V at the recorder with 34dB attenuation for the big orchestra, I'd need 100V level out of the preamp! Yeah, we could do it with tubes or high-dope transistors.
If I used dynamics, I would need 14 to 20dB more gain. Say 40dB, pot, 20dB.
Ribbons could need 5dB to 10dB more, even if they are nominal ~200Ω impedance. Say 46dB, pot, 20dB.
If I worked at +4dBu nominal levels, I'd need 12dB more gain. For hot condenser: 20dB, pot, 32dB. For ribbon: 46dB, pot, 32dB. This IS 78dB maximum gain, and only pulls 86dB SPL up to recorder max input.
So there are a lot of issues. Mike sensitivity varies from 0.05mV to 2.5mV, 24dB. Acts range from 74dB SPL to >130dB SPL. Nominal electronic level can be -10dBV or +4dBu (or +8dBm), 2.8V peak or 17V peak, or some other level (or no level discipline at all).
Mikes: 24dB range
Acts: 56dB range
Level: 12dB range
So we could need 92dB range of gain.
In fact 74dB SPL acts don't need to be peaked; with modern recorders a 90dB SPL level is reasonable, so only 76dB range of gain.
The 12dB difference between Tascam and Pro levels is usually covered by a switch or added jack (or no-support): 64dB range. Many sound-folk know to use a pad on bass-drum: 44dB range.
The situation is very different from phono-amps. The noise and max signal on a disk is fairly well fixed: ultra-virgin vinyl may have 10dB lower noise, some 12" singles may have 10dB bigger wiggles, but nothing like the 50dB range of acts found on stage. Phono needles have converged on two basic types: 5mV and 0.25mV, and you don't change pickups often. Mikes ran 0.05mV to 2.5mV and we do swap them frequently to find "the sound".
> I get over 72dB SNR with my all tube phono stage
You can only claim the last 12dB of that. The LP standard aimed at 60dB from surface noise to nominal recording level. The 5mV 47K pickup standard evolved because the surface noise would be slightly higher than the thermal noise of 10K-30K impedance at resonance and the grid noise of a semi-selected tube. (An MC pickup with a transformer works about the same as an MM pickup, though a little less trouble from parasitics.) Improved vinyl and cutters have got noise down a bit, your skilled tube-work has improved on early 6SC7 phono inputs to keep up. And I suspect that you reach over-kill: if you listen with the needle up, then drop it in a blank groove, doesn't the hiss increase? You "don't have to be that good" (though I agree that pure hiss is different from surface noise, less interesting, should be kept far back).
Interesting.
Those 5n caps across the input might need to be tweaked for more high end, since the represent a short to the upper signal.
Those 5n caps across the input might need to be tweaked for more high end, since the represent a short to the upper signal.
pstamler
Well-known member
[quote author="CJ"]Those 5n caps across the input might need to be tweaked for more high end, since the represent a short to the upper signal.[/quote]
With a 200 ohm source impedance, they'd create a rolloff that was -3dB at 318kHz. Probably not too big a deal.
Peace,
Paul
With a 200 ohm source impedance, they'd create a rolloff that was -3dB at 318kHz. Probably not too big a deal.
Peace,
Paul
hey- i recognise that micpre- i had a couple of really really screwed ones in about a year or so ago- theyre from the REDD era of gear, i think... the meter is a really small magic eye, i had the same one one on a Westminster valve reel-to-reel that i had as a kid....
ill see if my buddy still has them tomorrow and try and borrow one to reverse engineer. but they were *REALLY* screwed- missing input transformers, broken turret boards, etc.. Noel from Oasis has a few of them in his studio- he really rates em, apparently.
will keep ya posted,
rrrrrich
ill see if my buddy still has them tomorrow and try and borrow one to reverse engineer. but they were *REALLY* screwed- missing input transformers, broken turret boards, etc.. Noel from Oasis has a few of them in his studio- he really rates em, apparently.
will keep ya posted,
rrrrrich
underthebigtree
Well-known member
IMHO, the choice of preamp designs here really depends upon the type of material you are planning on recording with the ribbon. I have an AEA R84 that I use all the time. For loads of gain on quiet sources, I use a Millenia HV-3D, which delivers loads of crystalline signal. For vibe, and on percussion sources such as toms or timpani, I often use a V76, which gives a richer, more thunderous and velvety sound. It has been my experience, though, that the V76 in combination with a ribbon creates a pretty dark, chocolatey signal. If you are looking for lots of detailed high harmonics, you might think about a condenser through a tube preamp, or the ribbon through a solid state (John Hardy opamp? Maybe the JLM Audio stuff?)
The ever-popular Gates SA-134 / MO-3638 remote amp strip is a fairly useful three stage circuit using triode connected 6J7, gain control, 6SJ7, and paralled single-ended 6SN7 driving a 12K output primary to achieve 80-82db gain. There's a million of them out on the used market, and it should be pretty easy to build with new parts too. The input trans is a typical 50/250 ohm to 60K ohm.
BTW, great post on signal levels and mic impedances PRR!!
BTW, great post on signal levels and mic impedances PRR!!
