Simple tube Mic preamp help

Hi everyone, this is my first post on this forum. I've already learned a lot reading the archives, but I still have a lot to learn.

I'm looking to build multiple channels of a simple single tube preamp. I'd post a schematic but I dont have anywhere to host one. It would basically consist of a 1:10 input tranny to the grid of a 12ax7, around 300v B+, 150k grid to ground (as jenses suggests for the jt-115k-e) 220k Rp, around 1k for Rk, 2uf cath bypass cap. This direct couples into a cathode follower with 100k Rk then a cap coupled 2:1 output transformer to drive a 600 ohm load. If my calculations are correct I should net about 50dB of gain, which should be plenty for what I'll be using them for.

My main question is this: where should I put an attenuation control? I'd like it to be adjustable, either switch or pot, but I don't know the best place for it. Any tips? Before the input transformer? between the gain stage and CF, after the CF?

Also, anyone see any drawbacks to this design?

-Kevin

alright, schematic ...

Use a 12AT7 instead it has a little less gain that the 12AX7 but better current drive capability for the OPT and use a 4:1 ratio instead.
Welcome on board

Analag

It's "too simple" to have a good place for a gain control.

The 2:1 output iron limits output level. The cathode follower is working at say 1.5mA. Assuming 600Ω at the secondary, primary impedance is 2.4K. Maximum signal there is 1.5mA, times 2.4K is 3.6V peak, times 2:1 is 1.8V peak output. That's lower than we generally want. If you don't really have 600Ω load, the midband output will be higher, but if the transformer is rated 600Ω then it will BE 600Ω near is bass cut-off frequency. Taking your estimated 50dB gain, input overload point is 1.8Vpk/300= 6mV peak, 4mV RMS. About 100dB SPL in a dynamic mike.

Heck, you might just make the 150K resistor a pot and control gain there. That's not ideal for noise, but output noise will only be 30uV-60uV which is low enough by line level standards.

Generally, a 2-triode amp is barely enough gain for mike uses. Using a cathode follower denies you some of the power gain possible in a tube. Be sure this minimalist plan does what you need before you build a bunch.

Wow, thanks for the reply. I don't quite understand all the stuff you're saying, so I guess I have quite a bit of learning to do still.

I was kicking around the idea of a two stage 12at7 design with a negative feedback gain control with low/high cut/boost switching in the loop. more on that later. But I thought a very simple single stage design would be good for close miking some drums where tons of gain isn't needed.

I'll draw up rev 2 and repost.

Thanks!
Kevin

Possibly some relevant info here:

http://www.normankoren.com/Audio/FeedbackFidelity.html

[quote author="Baltimore"]I was kicking around the idea of a two stage 12at7 design with a negative feedback gain control with low/high cut/boost switching in the loop. more on that later. But I thought a very simple single stage design would be good for close miking some drums where tons of gain isn't needed.[/quote]

For a two-stage preamp with a negative-feedback gain control, check out the metas for New York Dave's "One-Bottle" preamp thread. It doesn't have the EQ you're talking about, but that's probably going to be a bit hard to do in a variable-gain design of that sort.

A simple single-stage design can indeed yield nice results -- well, almost single-stage, since the cathode follower is really another stage, albeit a gainless one. A 12AX7 direct coupled to a 12AU7, or a 6SL7 direct coupled to a 6SN7, will do a very nice job, although it may overload at a lower point than you'd like for drums. A low-gain option is the preamp I published in audioXpress a few years ago, Big Mike, which was a 6SN7 voltage amplifier direct-coupled to another 6SN7 cathode follower. Total gain, with input transformer and a single pair of tube sections, was about 39dB, adequate for condenser mics into a -10dBu unbalanced input, not adequate for dynamic mics (unless they're miking drums?) or for use with a stepdown transformer to drive 600 ohms. The big version used two of those stages, with a level control in between.

Note that if you use a voltage amplifier and a cathode follower, they really should have separate filament supplies, with the follower's supply hoisted up to 100V or so, to prevent cathode-filament problems.

Peace,
Paul

Note that if you use a voltage amplifier and a cathode follower, they really should have separate filament supplies, with the follower's supply hoisted up to 100V or so, to prevent cathode-filament problems

Click to expand...

Not necessarily so.

[quote author="pstamler"]

For a two-stage preamp with a negative-feedback gain control, check out the metas for New York Dave's "One-Bottle" preamp thread. It doesn't have the EQ you're talking about, but that's probably going to be a bit hard to do in a variable-gain design of that sort.

[/quote]

I had been looking at that design, but I wanted something even simpler. As far as the feedback EQ, it would just be simple switching in and out certain caps to boost certain frequencies, like in the $ebatron VMP series preamps.

as Far as this preamps overloading at too low a level, how do I calculate this? Can someone point me to some resources?

-Kevin

[quote author="Baltimore"]as Far as this preamps overloading at too low a level, how do I calculate this? Can someone point me to some resources?
-Kevin[/quote]

First, figure out what gain you're using on your current preamps, and figure out how much signal they're putting out. Also figure out what signal level causes your recorder to go into overload. If, for example, your recorder overloads at +20dBu in, then that's the maximum you can tolerate from your preamp (even if the preamp itself will put out more signal).

Let's say your typical signal maxes out on your recorder at -6dBFS (I assume you're using a digital recorder, but the principle's the same) using a preamp witn a gain of 30 dB. If you use a preamp with a fixed gain of 40dB, you'll be pushing the recorder to +4dBFS, at which point the TILT light goes on.

That's recorder overload. What about preamp overload? To figure that out, look again at the situation with the recorder. We said that 0dBFS happens at +20dBu, and your signal peaks out at -6dBFS. That means it's coming out of the preamp at +14dBu. If your current preamp is set to 30dB gain, that means the input signal is 14-30, or -16dBu.

Apply that to a new preamp design. If the new preamp has a fixed gain of, say, 50dB (typical of a 12AX7 direct-coupled to a 12AU7, with an unbypassed cathode resistor on the 12AX7 and a 1:10 transformer at the input), and it's known to overload at +24dBu, you can do the arithmetic for your typical signal. We decided in the last paragraph that was -16dBu, and the gain of 50dB will boost that to +34dBu, except that the preamp clips at +24dBu, so we're way into clipping.

You can do similar calculations with any preamp and mic combination.

Peace,
Paul

Thanks for the info. It will probably take me a bit to digest it.

New question: How can I calculate the output impedance of a Cathode follower circuit?

[quote author="Baltimore"]Thanks for the info. It will probably take me a bit to digest it.

New question: How can I calculate the output impedance of a Cathode follower circuit?[/quote]

It's about the reciprocal of the transconductance, which is Mu divided by the plate resistance. So, it's the plate resistance divided by Mu. The cathode resistor is in parallel with this value but shouldn't be a significant factor if it is sized properly---i.e., don't make it too small.

[quote author="Baltimore"]Thanks for the info. It will probably take me a bit to digest it.

New question: How can I calculate the output impedance of a Cathode follower circuit?[/quote]

There's a bunch of design info about cathode followers and variations like the White CF at www.tubecad.com too. I'm building NewYorkDave's one bottle pre right now, the two bottle version. It uses some nice tubes which are inexpensive too.

Kiira

hey Kiira, what kind of gain are you expecting from that two bottle version?

I've been wanting to build newyorkdave's one bottle design for some time but finding a decent 1:16 input transformer (or even 1:10) is looking next to impossible. I've got a lead on one but I've almost exhausted my sources. Might be I have to settle with some other design with more gain, which would be a shame.

What advantages in general are you expecting from that two bottle version?

Cheers,
Mike

[quote author="Kingston"]hey Kiira, what kind of gain are you expecting from that two bottle version?

I've been wanting to build newyorkdave's one bottle design for some time but finding a decent 1:16 input transformer (or even 1:10) is looking next to impossible. I've got a lead on one but I've almost exhausted my sources. Might be I have to settle with some other design with more gain, which would be a shame.

What advantages in general are you expecting from that two bottle version?

Cheers,
Mike[/quote]

Well NYDave quoted 60+ into a 10:1 and 78dB into unbalanced out and that would be enough juice to do a better job on my Beyer ribbon I think. I also just got a lil carried away wanting to put as much as I could into this cute little csu/dsu box I scavanged so it has mic/line/instr. in and balanced and unbalanced out so it can be a nice DI too. It's too crowded even for a on/off switch! I'd post a pic but I'm held up now waiting for the line xformer. I can't do the layout without all the parts I'll mess it up for sure.

Here in the USA 1:10 are easy to find dunno about Helsinki tho. :sad: I talked on the phone with the guy at edcor... they'll wind a wsm custom for you for only $5 which makes it like $14... plus shipping :roll:

Kiira

Doh! I just re-read newyorkdave's plan and it quite clearly quotes +23dB for the original plan when adding that new stage. Which is nice!

Thanks for the info kiira.

I just found out it's reasonable cheap doing custom transformers here as well. I don't know about the quality, but I expect them to be ok. My little box is going to be crowded as hell, too. Trying to do a kind of two tube swiss knife brick here as well...

With the 5879 input stage added (aka, the "two-bottle" version), the approximate gains are as follows:

Input stage: 23dB
Output stage: 32dB

With a 4:1 output transformer (12dB loss), the net gain--not counting step-up from the input transformer--is 43dB.

Net gain with a 1:4 input xfmr: 55dB
with a 1:5 input xfmr: 57dB
with a 1:10 input xfmr: 63dB
...and so on, and so on.

If you're don't need the output xfmr--if you're SURE you'll always be running it into a high-Z load--add 12dB to the gain figures above.

60+ into a 10:1

Click to expand...

No, 60-plus dB with a 4:1 output transformer. You must have mixed that up with the 1:10 input transformer.

You'd better call Edcor back now before they start winding your 10:1!

thanks newyorkdave.

then I might settle with 1:8 and 5:1 jensens which I was offered already. And if it turns out alright (I know it will) I'll be doing something along these lines with slightly differing tubes and transformers. might end up doing some swap-in testing with the first one.

[quote author="Kingston"]hey Kiira, what kind of gain are you expecting from that two bottle version?

I've been wanting to build newyorkdave's one bottle design for some time but finding a decent 1:16 input transformer (or even 1:10) is looking next to impossible. I've got a lead on one but I've almost exhausted my sources. Might be I have to settle with some other design with more gain, which would be a shame.

What advantages in general are you expecting from that two bottle version?

Cheers,
Mike[/quote]

well NYDave quoted 78 into unbalanced and 60+ (?) into a 10:1 output I think. I mainly want the extra gain for my Beyer ribbon. I also just got carried away wanting to make this lil box versatile and neato w/ line/mic/instr. in and balanced and unbalanced out so it can be a DI too. I don't have room for an on/off switch! I'm held up waiting for a xformer from edco tho... if I try and do the layout before I have the parts I will surely mess it up.

Here in USA 10:1 are easy to find. Dunno about Helsinki :sad:

good luck!

Kiira