building a low gain mic pre... but which one?

[frans_13]

Hello, I built the ioaudio MK-U47 mic and it has - lovely as it is and like so many condensers - a healthy output signal. I used it with a host of preamps, most of which got 60 or 80 dB gain, which means I have the input turned way down or even the pad in. As the old Telefunken v72 had fixed 34 dB gain if I remember it right, why not build a low gain micpre? I already built a few preamps, OpAmps, etc.etc. and sniffing the net I am considering building for example the Hairball elements silver, with a 40dB opamp (Melcor, JLM99, etc.etc.) and Lundahl LL1532 1:1 input and some 1:1 output transformer. Building a v72 clone...I don't know... maaaybe as a kit, but I'd rather have something without tubes, already having some Manley, HCL, Neutrik preamps.
Which preamps would you guys think about?

[Jim Williams]

I use cleans stuff, sounds like the source that way. Noise must be extremely low which rules out most older designs.

[frans_13]

Yes, I can agree on that - in cases where such a mic as this is used, the character should be preserved. Which modern designs for low gain? The SSL 9000 series style preamp or the Graeme Cohen-like designs have a lot of gain ...and I think they rule for stuff where you don't shove the mic up close to a source. The whole close-mic-thing gives a host of transients I want to have a lid on anyway in most of what I'm called to record, soo.. (as I'm in no way a master of recording) I'm thinking about which steps along the way help me to stay clear of the transients that are simply too much. Transformers color and bandpass and cushion some transients, but anything in the mix that gets them under control does as well. Of course I could throw in the usual suspects of compression, but I'm rather inclined to "here a bit, there abit" of small steps towards the goal instead of one heavyhanded single step.
I remember a 500-series pre that was specially built for high level sources...uh.. which one was ist again...

[Jim Williams]

Modern transistor mic preamps based on opamps and transistors can be very good or not. Older instrumentation designs have poor linearity at higher gains. Trans-amp versions avoid those problems. Any of them can be fitted with a gain control to operate from about 15 to 60 db. The better designs maintain the same sonic qualities at high or low gains.

[Shameless plug]

The Audio Upgrades PIP module is an excellent trans-amp mic preamp built on a 7/8" square pcb. It drops into a SSM style mic pre chip socket as used by DAV, AMEK, Toft, and many others. It does -129.6 db EIN at 150 ohms, -132 db EIN at 50 ohms noise. THD is 5 ppm, IMD is 1.5 ppm, slew rate is 60 v/us. Cost is fifty bucks.

I also have the venerable "Baby Mic Pre" cards for $125, those are voltage based and have all the blocking caps, etc, built in, a complete design. Rounding up the DIY crowd is the new High Speed Mic Preamp cards for $175 each. Those are a unique current feedback design with all current feedback opamps and a precision SSM matched transistor pair.

1.75" x1.25" square it's a complete SOIC and through hole design. It has -129.6 EIN at 150 ohms, -133 db EIN at 50 ohms. Slew rate is a whopping 2000V/us, bandwidth is 30 mhz, THD is .0005%, IMD is .00015%. Most have never heard the open sonics of a fully current feedback or transconductance mic preamp design, as far as I know, I'm the only one making them.

[The Scum]

So the idea seems OK in by book.

But any design with an opamp at it's heart has a dirty little secret: opamps themselves have absurdly high gain (theoretically infinite, realistically 100,000 (100 dB) or more...) - we use inverting feedback to counteract that gain, and tailor it to an application.

If it's opamp based, the difference between a 30 dB preamp and a 60 dB one is very likely one or two resistor values, used to scale the feedback.

If you really want to make a lower gain preamp, you might look into some really simple transistor designs - it's not hard to get 30 dB from a single BJT or FET. Heres a page with some interesting background on that:
http://myweb.tiscali.co.uk/norcimradioc ... adio15.htm

A ring-of-three plus White follower could be pretty cool, with 1:1 transformers on the in & out to balance/unbalance.

Pass' Bride of Zen might be cool to experiment with, maybe 2 stages in series:
https://www.passdiy.com/pdf/brideofzen.pdf

Or the Hamptone JFET pre could be a candidate as well. It usually has 2 stages in series, one stage all alone would result in less (but probably still plenty) gain.

[frans_13]

I'm not adverse to the ideas of Mr. Williams and the bride of zen schematics don't seem to bad either. I have been looking into http://www.sg-acoustics.ch/analogue_aud ... index.html as well, which brings me... I need at least a schematic and layout to guide me, I'm not fluent enough in electronics alone.
Mr. Williams, I got an AKG 414 you modded - good job! Gets used often.