Microphone preamp idea

Hello all.

I have built a couple mic preamps, and they are doing the job just fine, but I would like to reduce the parts count and cost of input and output transformers. Inspired by the NYD one-bottle preamp, and the cost/quality of the edcor transformers, I came up with this design idea:

- a 6sn7 configured as conventional RC coupled 2 stage amplifier, with volume control between the sections.

- input transformer: Edcor RMX-1
http://www.edcorusa.com/classx/ribbon/rmx/rmx1.htm

- output transformer: Edcor XSM10K/150 configured as traditional series-feed output transformer, like in the RCA bc-2b. Not parallel feed.
http://www.edcorusa.com/classx/matching/xsm/XSM10k-150.htm

- simple unregulated power supply with shunt regulated zener diode phantom power supply.

- extra shielding for transformers via hammond die cast boxes, or folded-up copper boxes or something.

- no feedback.

- gain configurable by choice of output transformer, and use of cathode bypass caps or not (switchable even). And as with most tuube circuits, building a deluxe power supply will probably improve the sound. Maybe a deluxe power supply in a seperate chassis, and a multi-channel mic pre chassis.

This should be a very simple, very inexpensive project. And the components should take up very little chassis real-estate so a multichannel chassis should be possible.

Upgraded transformers are available from Lundahl, etc.

What do you guys think? I'll draw and scan a schematic if anyone is interested.

What do you guys think? Has anyone used the Edcor RMX-1 as a mic input transformer? It looks awesome, but I wonder what the maximum signal level is. And how do you mount these things? Could it maybe be potted in some sort of Hammond box or mu metal can or something?

jsn

Since I haven't worked with the transformers in question I'll forbear commenting on them, and address only the active stages.

I looked at exactly this configuration -- two 6SN7 halves separated by a volume control -- for the design that eventually became "Big Mike", although I was RC coupling the second stage rather than transformer-coupling it. I came to the conclusion that at the levels generated by condenser microphones, the 6SN7 in an open-loop voltage amplifer circuit (I'm talking the first stage here) generated more IM distortion than I was willing to put up with. IM levels were on the order of 7-10% at input levels of something like -20dBu, assuming 20dB of stepup from the transformer. Unless one is looking for an "effects" preamp, that's too much for me. I eventually settled on using a 6SN7 voltage amplifier direct-coupled to a cathode follower, which is much cleaner; something like a feedback pair would also work but presents loading issues.

Still, your suggestion would make a solid "color" preamp.

Peace,
Paul

Hmmmmm, the circuit description sounds lot like what NYDave came up with for Rafafredd, over in the Drawing Board.

I'm no expert, but I'll give this a go: Using the Edor RMX as an input transformer is an interesting idea. It is meant for ribbon mic output, so it might be designed for very low levels only and the intended impedances are much lower than mic input needs. The impedance of the ribbon is tiny and typically gets stepped up to something like 50-100 ohms (normally bridged into something like 100k loading on the sec. of a 1:10 input transformer). For ribbon output, that would mean the expected pri. impedance is 0.2 ohm. :shock:

For your scenario, let's say 100k loading on the sec, 181 ohms reflected to primary. That sounds like matching to a mic's output (not bridging like we usually do nowadays), however the inductance/reactance wasn't intended for these impedances and that can really skew the frequency response. Also note that the higher the ratio, in this case 1:23.5, the more inconsistent the frequency response will be with different loading. Anyway, all that said, it's a cheap transformer so if you can afford to try it you might like the results. Just expect it to be very.... um.... colorful-sounding. :green:

Edit: Please would somebody correct me, so I can go back to chewing on my foot?

JSN,

My tests indicate the the XSM is not suitable for series-fed use, despite what that "AC+DC current" spec on the webpage might lead you to believe. Here are the results of my primary inductance vs. direct current test on a WSM10k/600.

Graph

However, I believe the XSM could very possibly kick ass in shunt-fed applications.

I should have posted this in "Drawing Board". sorry.

Thanks for the feedback and link to that other 6sn7 design. I had hoped to nerge that sort of thing with an RCA bc-2b style design.

Thanks for the note about the Edcor XSM. That is too bad, but parallel feed output is easy enough, and one more cap is undoubtedly worth it.

I am suprized to hear that the 6sn7 resulted in high IMD, as this seems a popular configuration for Hi fi input stages. IMD is not something I understand very well (and by very well, I mean at all), and am not sure how to address it. I have a superstitious fear of cathode followers, so I would prefer to keep everything on the plates. Hoping of course that output transformers decrease the need for a cathode follower.

So perhaps the above design with a parallel-fed output and input and output transformers some combination of Edcors, maybe the mic input, and something from the WSM or XSM series for output. Maybe even a 10k:150 WSM backwards as the input trans...

Jeez, Edcor has some really interesting stuff. What about this as an output autoformer/volume/impedance control:
http://www.edcorusa.com/classx/matching/ex/ex103.htm

Thanks again, this forum is awesome.

jsn

Edit: Oops.

[quote author="jsn"]I am suprized to hear that the 6sn7 resulted in high IMD, as this seems a popular configuration for Hi fi input stages. [/quote]

Sure -- but a typical hi-fi input stage has a nominal input level of -10dBV, or about .3V. Microphone preamps have input transformers, typically 1:10 ratio if you want to get a decent noise figure, and they're fed by hot condenser microphones with people yelling into them. A fixed-gain input stage needs to be able to work on very hot signal levels without massive distortion. That's one reason people use feedback -- to keep gain down, and to keep distortion down in the area below clipping. Hence designs like the Un*versal Audio 610, which is basically a Dynaco PAS output stage, two 12AX7s with feedback around them. Making something with fixed gain and without overall feedback requires something to keep the distortion down, and the cathode follower will do that to some extent.

IMD is not something I understand very well (and by very well, I mean at all), and am not sure how to address it. I have a superstitious fear of cathode followers, so I would prefer to keep everything on the plates. Hoping of course that output transformers decrease the need for a cathode follower.

Click to expand...

Nothing to be terrified about, just a different way of hooking up a tube, with different characteristics.

As for IM, a nonlinear circuit (= any active device) will cause two frequencies present in a signal to generate "sum and difference" frequencies. So, for example, 10kHz and 9kHz will cause the device to generate a frequency of 19kHz (the sum, 10k + 9k) and another one of 1kHz (the difference, 10k - 9k). The device can also, depending on the nature of its nonlinearity, generate higher-order frequencies like 11k ((2 x 10k) - 9k) or 8k ((2 x 9k) - 10k) or 3k ((3 x 10k) - (3 x 9k)). Do this at high enough levels and you've invented the fuzzbox -- notice how much fuzzier the fuzzbox gets when you play two strings at once than when you play only one?

Peace,
Paul

> the Edor RMX as an input transformer is an interesting idea. It is meant for ribbon mic output

If this is mean to couple directly to the ribbon: indeed, the input inductive impedance will be about 0.2 ohms at ~20Hz rising to 20 ohms midband. With a typical dynamic mike rated 200 ohms, the output voltage will be very low in the bass, near-normal at the top of the audio band. (Hmmmm... Edcore says "<0.4 ohms DCR", also "<0.4 ohms impedance", which is screwy.)

You don't have to use a transformer exactly at rated impedance, but going 1000:1 away from nominal is sure to be bad.

> I am suprized to hear that the 6sn7 resulted in high IMD, as this seems a popular configuration for Hi fi input stages.

In hi-fi, levels are standardized. In the studio, levels are all over the place. On large condenser in the audience in front of an orchestra, I can get 0.5V at the mike. Using Paul's 1:10 transformer, that's 5V at the tube grid, over twice what we expect in hi-fi. That's a mere orchestra; many studio source are hotter, say 10V at the grid. If 6SN7 has gain of 14, that's 140V RMS or 200V peak or 400V peak-peak at the plate. If you use a 300V supply, that's impossible.

If you feed more supply voltage or less signal, so it does not grossly clip, you still work the tube so hard that you can not pretend it is linear. Swinging 50V peak on a 300V supply, average gain still 14, gain on positive plate swings may be 12 and for negative plate swings 16. A sine wave is clearly bent: 57V one side, 43V on the other. A complex wave has the high tones modulated by the low tone they ride on (IHF IMD) or by each other (9KHz+10KHZ test signal). While Paul computed IMD, for a simple tube stage you can calculate/measure either THD or IMD: if you have one, you are sure to have the other, and all simple tricks to reduce one will reduce the other.

6SN7 is a reasonable choice for low THD/IMD. There are better choices, but few Twins.

However, few 1:10 transformers are rated for 1V input (because who wants 10V output?). If your signal has any bass at all, it's gonna get very colored or even blatty. A lot of older 1:10 trannies were rated 30mV input, which was ample for dynamics in medium-loud use. Working 300mV in the 6SN7 grid, 5V at the plate, on 200-300V supply, things are quite clean. "PAD" is NOT a dirty word when working with hot sounds and hot mikes. Pads were widely available and used for rock-n-roll in days before transformerless inputs; hot condensors sold on the US market mostly have pads built-in, and the German studios where hot condensers were standard did not have 1:10 input iron.

I think you should just do it, but include a pad and learn to use it.

Shunt-feed works but significantly reduces maximum output level for a given tube. Like 1/6th the output power, 9dB less voltage, or complementary increase of tube heat and size to maintain the same output.

> extra shielding for transformers via hammond die cast boxes, or folded-up copper boxes

Transformers usually need IRON shielding. Often copper/tinfoil too, but that doesn't stop magnetic fields worth a darn, especially at power frequency. I once had spectacular happiness with naked trannies inside a $1 electrical box on a catwalk smothered with dimmered-lamp wires, but this was monitoring not recording and the hum floor may not have been near-zero. Remember that the cheapest shielding is always distance: don't build too small.

So it seems like my first question needs to be: How much gain do I need in a mic preamp?

It sounds like a mic preamp needs to be able to handle signals anywhere from around 2mV to around 1V, and have enough gain to output something like +4dB into 600 ohms.

2mV to +4dB is 56dB of gain.

1V to +4dB is only 2dB of gain!

I might be screwing that calculation up, because the input is into 150 ohms and the output in into 600. I still don't totally have the hang of dB yet.

But I see what you mean about wanting a pad on the input!

So does it make sense to design a mic pre that has around 56dB of gain, but with a pad that is capable of keeping the input transformer and first stage from clipping with 1V of input?

In another thread you (PRR) mention that you prefer "semi-pro" output, which I assume means either less than +4dB or more than 600 ohm impedance. Can someone point me to a thread where you talk more about this?

It would be nice to need less gain since you would get less noise, fewer gain stages, less demanding transformers, etc. If you don't need the whole +4dB, then the design would be much easier. It seems like most of the equipment we feed these mic pres into has some sort of gain control in it, so we might be able to get away with less gain.

jsn

Get your free design work here

[quote author="Gus"]Get your free design work here[/quote]
Are we in totally out of control paranoia mode now?

yeah this paranoia stuff is a little much. I am following this thread as I can because I have no background in tubes and I am learning huge amounts reading honest math, opinions and facts without having to read 2 tons of books to get 10 minutes worth of info.

Gus speaks for the grumpy, hermetic designers, but..... um.... he has a point.

Jsn - A lot of what you asked has been address here a few times. I've found that it's totally worth the time spent searching, since often the answer was most complete the first time it was given. When someone like PRR has answered the same question a few times, they tend to shorten or summarize their previous answers, though this is probably unintentional.

A few quick answers:

> "semi-pro" input impedance

10k for most newer gear

> how much gain needed?

Scan through the "One-Bottle" thread over in the Lab. I think pstamler posted exactly how to calculate the gain staging for your particular design/application. Good stuff.

> input 1V

I've read a few times that this is a rarity, if it ever occurs. I have had instances where a hot condenser mic on the shell of a snare drum seemed to hit this level even with a 10db pad engaged, but that's just a guess and I didn't measure and its the only time I've seen that. Find some PRR posts about recording orchestral percussion. It's all there.

Yeah, I wasn't sure what to make of that comment, but if you think I am trolling for someone to design a product with which I can attain commercial success and a life of luxury built on the backs (and calculator-watches) of these brilliant forum members, well, that is unbelievable. I am new to this forum and if this is how you guys are, this forum is doomed.

And if you want any of *my* schematics, or want to know anything about anything I have ever designed and built, please visit my site (boozhoundlabs.com).

jsn

jsn

Nice web page I should have clicked the link first.

Wow you cut metal with a flycutter in a drill press, scary Al and brass like to "grab" cutters. Flycutters are scary things they seem to work OK with wood I have cut plastic and metal but not any more I was cutting a plastic cutting board(polypro) to make disks for inside some microphone body that I would face in my lathe, even clamped it was not nice cutting. I did see you used clamps to hold the panel to the drill table.

EDIT a few posts before I finsihed this post. I was hoping to post this before stuff started and I did not remove my first post because noone would know what going on.

Jsn,

I don't understand anyone trying to use a single tube and perform some magic with output transformers, including myself . As we stand good transformers are far more expensive than tubes. If you use two dual triodes you can suddenly use many non-exotic irons on the output and make a good sounding piece of gear.
Use two tubes and you can use cheaper transformers. Get some OEP input transformers (1:6.5 ratio) from Newark. They are less than $30 with mu-metal shielding. Use the XSM10K/600, for the output it will be fine.
Take two of the 6SN7 tubes and do some research on different topologies like grounded cathode, buffered grounded cathode, dual triode feedback, mu-amplifier, SRPP, etc. Use an attenuator pot between stages for cheap volume control. You want at least 44dB gain from both stages. (16+44-12=48dB max gain works fine with condensers and dynamics).

Single tube preamps were mostly limited to ~35dB (40dB at most) gain and they worked well in in the context of the studios, 50 years ago. They had separate line amplifiers like today's Hi-Fi folks. Or they had limiters with an additional 20dB to 30dB gain.

Cheers,
Tamas

I hate to continue this off-topic... but I think Gus is more concerned with people basically asking for a design handed to them as a way to avoid learning and calculating for themselves. Just being lazy or opportunistic.

Don't worry about the paranoia. I appreciate your enthusiasm, and I'm really trying to offer helpful advice about making the most of this wonderful forum. The hours I've spent searching, then reading and re-reading posts have taught me so much. Often my questions have been answered several times by the same person, so I see where the veteran members could get agitated about us newbs asking for design help without first UTFSF.

Now back to the tube preamp talk. :green:

Tamas - I think a two-triode design is completely useable, especially in context of a project studio. Jsn has a couple of higher gain, two tube preamps, so he's got that covered.

Besides, he is recording directly into a laptop, so likely a high impedance, unbalanced soundcard input. That's easy enough to drive with a shunt fed design, without a stepdown transformer. Even small triodes, like 12AU7 will do that happily. Also, the limited gain of a two-triode design is not an issue when used with today's high output mics. The only side affect is that we've seen soooooo many questions about pads and attenuators recently.

Point is, I think the twin-triode design is hardly outdated. In fact, I think now is the perfect time to revive the concept and take it even further.

I promise I'll post the 6EM7 design that I'm working on soon. I just have a lot of homework and no time to draw it up right now. :grin:

PS - Tamas, your power transformers shipped this morning, so you'll have them by the weekend.

I have to second Gus on that one, kudos for using a flycutter! I HATE those things but they do have their place in the workshop. I constantly use one to enlarge existing holes..

hey TK, I think you forget about the CJ single bottle pre and the NYD single bottle pre as single tube, cheapo trafos on output. the inputs are another story though..

I built mine for less than 50$ and I will put them up against any other preamp.. :green: they are just cool as hell.