Valve mic preamp design incoherent rambling

[Cqwet Dbdfte]

Last night I finished up a bench top power supply that I will use to start building up and testing circuits. It's been a while since I've had much time to put into this project as I've been doing a lot of recording, lots of repair work, building out a new patch bay and spending lots of time with my family. Thanks so very much to Mr. Thompson-Bell (@ruffrecords) for supplying the PCBs and fielding some of my amateur questions and thanks to everyone here for your continued inspiration. Also - we should collectively come up with a good solution for cutting IEC openings. I used a Dremel tool and a bastard file. It worked out okay but there must be an easier way.

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Board looks like it would fit sideways closer to frontpanel, away from toroid transformer.
Use a filtered input jack for the AC.

 

[Alva Goldbook]

Hi friends -

I'm going to be making a whole pile of notes here, kind of mostly for me to talk things out...out loud.

It's kind of long winded so I get it if you'd rather move on. Again. If you have some insight you'd like to share, I would seriously appreciate it. I'm in no rush - I figure I might live another 40 years or so I have time.

Designing valve based microphone amps

I am interested in designing some simple microphone amplifiers using tubes. I have a fairly decent knowledge of guitar and bass amplifier circuits and have been regularly studying, repairing and modifying guitar amps for about 8 years or so. I’m no genius but I can more or less understand how all the components work and generally how stages interact on a very basic level. I feel confident that I could copy a design and layout and wire someone else's design now, but I'm just looking to learn, progress and have fun.

I have been studying designing tube based guitar and bass amps by Merlin Blencowe for quite some time and am fairly familiar with his writings. But between that and Small Signal Audio Design by Douglas Self, I’m at a bit of a loss for where to turn. I have studied NYDave’s one bottle and Mila and checked out some of the Gyraf stuff including the dual micamp as well as looked at some classic circuits including the Pultec MB-1 and have been following some of the other threads here as well like Professor Dan’s. Of course I’m pulling knowledge from forum members PRR, abbey road d enfer, ruff records and Winston OBoogie who tend to dig in on the valve design stuff. Among others of course.

So some of the things I don’t understand (I have zero formal training) I think are pretty basic but I’m not entirely sure of even how to ask them.

For example, I more or less understand that we want reasonably high input impedances and also wish to have output impedances as low as we can really. At least I think those things to be true. There are somethings about creating oscillations that give some real limitations and the fact that we can’t have infinite input impedance and zero output impedance. I don’t think so anyway. I don’t really understand what kind of signal we are going to see at the output of the microphone and how much signal we should be trying to aim for at the output? Are we just looking for +4dBu (3.472 Vpp)? There is only some much head room in whatever is following the preamp be it an external EQ, compressor, desk, tape machine, converters, etc. so we don't need to amplify more than that but we do need enough gain for low output mics and quiet signals. Would all of these devices look for the same (nominal?) level at their inputs? Would most of them have approximately the same input impedance? I am using fairly high end gear at my studio limited now perhaps by my 003 interface which will likely be replaced in the coming 2 years (Probably long before I finish this project)

So I guess starting from the beginning, we are going to have an XLR input, positive voltage on pin 2 with respect to pin 3. That’s a good standard. I think I can figure out getting phantom power there, there are enough reasonable examples of how to do that, I can also figure out getting a pad there, no problem. I could easily figure out how to get a polarity switch here too, but I’m not sure I would do that here or at the output? I’m not sure why you would choose one over the other…? Anyway most of this stuff is covered by the Jensen papers. No big deal. So what’s next - we need to get to the first gain stage.

Here’s where I think I’m really running off the rails.

Okay so let’s say I want to have a transformer input…if I look at one version of NYDave’s One Bottle there is a 150:15K which I think would step up my voltage by a factor of 100 (Is that right? That seems like a massive step up? - Surely I'm missing something there, should I be taking the square root of this?) and reduce the current from by the same factor? On another schematic of that circuit I see a different transformer spec’d which is 200:50K upping my voltage by a factor of 250 (again - this seems like I'm wrong - same as above?) and reducing my current by the same amount? The Gyraf Gyratec IX shows an input transformer with a 1:6.45 ratio which only would step up the voltage by a factor of 6.45 right? These are good right? (Probably, I suspect that these things are well designed ) The microphone output would be relatively small and the reduction of current for gain in voltage is good. I want to send a fluctuating voltage signal to the grid. But why such a difference between the Gyratec and the one bottles? Because the one bottle wants to have a similar amount of gain as the Gyratec but with half the gain stages, so we get a boost up front by the transformer? It also gives us a little bonus with the signal to noise ration as well?

NOISE: So one reason we might add an input transformer is because the tube itself will have some noise - that noise we can’t change I don’t think. We can step up the voltage of the signal, but we are also stepping up the noise from the microphone too - but because we’re stepping both up by the same amount, the S/N ratio of the mic itself stays the same and the noise from the tube (when added) will account for a smaller percentage of that noise? Am I getting that right?

I’m going to pull a few numbers here from space…bear with me.

Transformerless:
1 mV of signal from the mic
1 µV of noise from the mic
5 µV of noise from the tube

So if the tube has a gain of 100. We would have 100 mV of signal and 105µV of noise at the output?

So the tube noise is 4.67% of the total noise.

w/ transformer (1:6.45)

1 mV of signal from the mic via transformer gives us: 6.45 mV
1 µV of noise from the mic via transformer gives us: 6.45 µV
5 µV of noise from the tube

So again with a gain of 100 we'd get 645 mV of signal and 650 µV of noise (yes - I do know these numbers are whack - but is this at least correct in theory?)

And here the tube noise is 0.7 %?

I still don’t understand how what the signal from the mic looks like. I can look at the sensitivity of a microphone, for instance my SM7B which seems to be a somewhat popular microphone, relatively inexpensive and somewhat of a low output based strictly on a comparison to my other microphones, but it doesn’t really mean much to me yet. I don’t really understand how to read or make sense of it. So - at 1 kHz, open circuit voltage (whatever that means) I see -59 dBV/Pa (1.12 mV) - 1 Pascal = 94 db SPL.

So I guess if the microphone capsule sees 1 pascal of pressure (at a freq. of 1 kHz) the the mic will output 1.12 mV or -59 dBv? Am I mixing up RMS and PP?

The reading material for the mic says that typical speech three inches from the grill will need a microphone preamp with +60 dB of gain and that many modern microphone amps only provide 40-50 dB of gain.

You see, I kind of get how the circuit's DC function works. I understand the types of bias, how to read the DC load lines, more or less how to spec parts for each section kind of/not fully in terms of bandwidth (plate resistor, cathode resistor/bypass cap, interstage coupling capacitors, grid leak, grid stopper) I have a rudimentary understanding of negative feedback and how it works for the most part, but I feel like I’m missing some key knowledge when it comes to the small signal side of things. What do those signals look like - where are we wanting to take them? How much should we be amplifying those signals. I feel like one of D. Self’s Ten Commandments is thou shall not amplify and then attenuate but maybe another is also thou shall not attenuate then amplify…I would like to be able to design to fit those criteria. I know that he’s talking semiconductors but I would assume that it’s probably best practices for both? Ultimately the less attenuating we’re doing the less resistors we have in series with the signal and the less Johnson noise? Am I getting that right? Should I even be worrying about these noise issues when working with tubes or does the tube noise itself swamp any worry of other forms of noise? I don't need a pristine noiseless preamp. I have a nice GML 8403 and a few other nice mic amps for that.

I’m not looking to make a new product to take to market. I just want to have some fun and make some tools to use in my home studio. Thanks to this community and some other helpful folks, I feel like I’m getting closer but I am really having a hard time understanding what it is that I don’t know.

I will make another post here shortly with some more concrete planning that I've been circling around on.

To me, it seems like the Putnam 610 and Redd47 circuits tells you most of what you need to know. You need an input gain stage of some sort. And and 2nd gain stage. Maybe it's parallel, or SRPP or something. I wouldn't worry about output impedance too much, since you can just let an output transformer handle that for you. The other feature to think more carefully about is negative feedback. It seems to be a pretty common feature on mic pres to prevent gnarly distortion. You can even put your volume and EQ (if any) right in the NFB circuit. And that's about it. I'm currently working on a design that combines both the 610 and Red47 just to see what happens. I have a bunch of little 5725 pentodes laying around that I may try on the input. That or maybe 6AU6s. We'll see.