trafoless microphone preamp wish list

[pstamler]

[quote author="chrissugar"]If the Gordon is done with only two stages I can only think that probably that was the best compromise and that is the way to go. I supose two stages is the best balance between noise, distortion and change in sound.[/quote]

It's worked for me, anyway. Well, sort of. In the solid state pre designs, which were really intended to feed unbalanced -10dBV outputs, first stage had 24dB gain, second had 26 or 28dB gain depending on which one you looked at. The balanced output stage was separate, and of course added 12dB.

The tubed design had only two stages: ~39dB for the first stage, ~20dB for the second stage (if present). Add transformer for balanced out. Because the input stage had relatively high gain (no global feedback), I kept a bunch of inline pads in the suitcase.

Which brings up a point: I've been allergic to any kind of switching in a mic input ever since I almost got fired when an A/B switch on our (old) console at channel 9 went on the fritz during the fund-raising auction. The director cued me, I brought up the fader on the channel with the KM-84, the floor guy cued the string quartet, and -- dead silence. After several seconds of panic, we cut to a videotape. While that was on, just as a hunch, I worked the A/B pushbutton switches back and forth a couple of times. The signal came back. Explaining to the operations director was an experience I wouldn't want to repeat.

So I use inline pads now rather than switchable pads. If our design includes any switching, particularly in vulnerable low-level spots, it should be done by switches of irreproachable quality.

Peace,
Paul

 

[bcarso]

[quote author="pstamler"]
Which brings up a point: I've been allergic to any kind of switching in a mic input ever since I almost got fired when an A/B switch on our (old) console at channel 9 went on the fritz during the fund-raising auction. The director cued me, I brought up the fader on the channel with the KM-84, the floor guy cued the string quartet, and -- dead silence. After several seconds of panic, we cut to a videotape. While that was on, just as a hunch, I worked the A/B pushbutton switches back and forth a couple of times. The signal came back. Explaining to the operations director was an experience I wouldn't want to repeat.

So I use inline pads now rather than switchable pads. If our design includes any switching, particularly in vulnerable low-level spots, it should be done by switches of irreproachable quality.
[/quote]

Very good point. I was actually in the switching system business for a while and it was not a trivial affair.

Later, for the evil empire I was designing an attenuator system that used mercury-wetted reed relays and Vishay bulk-metal-foil resistors.

One question: if we propose to do with discrete gain steps, what gain resolution would be acceptable?

 

[Svart]

I'm getting more into tracking straight from preamp to tape/HDD with no console in between so I'd like to see a few more smaller steps of maybe 15db each instead of leaps of 25db+ so that I don't have to go too hot or too soft to the track.

:thumb:

 

[chrissugar]

[quote author="Svart"]I'm getting more into tracking straight from preamp to tape/HDD with no console in between so I'd like to see a few more smaller steps of maybe 15db each instead of leaps of 25db+ so that I don't have to go too hot or too soft to the track.

:thumb:[/quote]

What is the point to use an external high quality preamp if you compromise the signal by routing it into the mixer and then to the recorder.
I always conect the preamp out directly to the recorder and I use the mixer only for monitoring.

[quote author="bcarso"]One question: if we propose to do with discrete gain steps, what gain resolution would be acceptable?[/quote]

I think 3 or 6dB steps would be ok. With 24 step switch you can have 3dB steps to cover 72dB gain. Or with 12 step switch you can have 6dB/step.

chrissugar

 

[bcarso]

Thanks Chris.

Svart and I are thinking of some options, and the approach would facilitate the smaller steps. Gets a trifle complex though.

 

[Svart]

"Gets a trifle complex though."

:green: That's supposed to be the fun isn't it? :green:

Chrissugar, yes you are completely correct. only recently have I been able to build up enough DIY preamps to get around using the mixer pres. The sound is indeed better, but I still mix back through the console which doesn't sound all *that* bad. I have been doing mods which bypass the internal pre, and ones that bypass the whole channel only using some DIY differential transmitter/receiver over the snake which took me FOREVER to make.. :green:

 

[fum]

While we're waiting for this one to get hammered out, I went ahead with a
board layout of the pre that PRR slapped together in the this thread.

http://www.groupdiy.com/index.php?topic=2043

Some areas that need pondering:

I did the layout using standard electro caps. What are the likely best candidates for higher quality (film) caps? If someone will shout out the cap numbers, I'll change the layout to accomodate these parts.

I need to reread the gain adjustment section, as there seemed to be some questions of how to best do the gain staging. Any opinions on this appreciated.

I've only done a few circuit boards in the past, so any comments there are appreciated also.


Regards

ju

 

[pstamler]

[quote author="bcarso"]One question: if we propose to do with discrete gain steps, what gain resolution would be acceptable?[/quote]

If we can do it without compromising quality, I'd really rather have at least some way of adjusting level +/- a couple of dB on a continuous basis. My reason: I have a couple of pairs of mics that are supposedly matched but really aren't. The frequency responses match well enough, but the sensitivities are up to 1.75dB different. For stereo recordings, it'd be nice to balance them out. Yes, I can do it later in the DAW, but then I don't hear it right when I'm monitoring, so I'd rathe get it right to begin with.

There are good pots. They aren't exactly cheap, but they're out there.

Peace,
Paul

 

[Samuel Groner]

Obviously, please don't feel you need to wait for me.

Your word: [removed]
A pair of them should make a nice frontend, be it switched or fixed gain.

Basically an extended GainBloke. Open loop gain is very high (> 90 dB @ 10 kHz). Step response looks fine except some few-hundred-MHz overshoot which disappears at gains above 6 dB.

There is some negative slewing (well above 40 V/us), which I was not able to get rid off.

As with every single-ended approach there are DC issues which probably need a more elaborate solution than what I showed here.

Q1 could be replaced with a lower-noise type. As shown, NF should be pretty good already. Dissipation for Q1 is rather high. Overall, this thing eats about 40 mA...

Samuel

 

[tk@halmi]

Sam,

Would C2 have to be charged through R5 for each cycle and limit slew rate? (Actually, I am uncertain if C2 gets charged by Q1 or the output)

Instead of a darlington output I get good results with using a darlington VAS to increase open loop gain. I just use something like an MPSA77 transistor for that purpose. Stabilizing through the VAS using local feedback is straightforward as well.

If you want even lower input voltage noise than the 2N4401 "flip" the circuit upside down (complemetary circuit) and make the input a PNP transistor. The OnSemi 2N4403 is pretty durn good, low noise, cheap and available. Another adventage is that now your VAS uses NPN transistors which may come in very high gain versions such as the MPSA18.

Two of these opamps you have drawn make a good differential input amplifier using one of PRR's circuits. I am trying to develop it into the first stage of a mic pre just on a breadboard for now. The second and unknown stage will have to provide more gain and rebalance the signal.

What I found, through measurements, when increasing the gain within the amp is that the second and fourth order distortion would disappear and only the third order became detectable above the noise floor. The third order got reduced a little too, but not enough to fall into noise level.

Cheers,
Tamas

 

[Samuel Groner]

Thanks for your thoughts! Need some more time to think them through. First two hours of correpetition to do!

Would C2 have to be charged through R5 for each cycle and limit slew rate? (Actually, I am uncertain if C2 gets charged by Q1 or the output)

I'm uncertain about the source of the slewing as well. However, R5 does not have an influence on it. Making C2 even smaller does not eliminate slewing as well. Replacing Q1 with a CFP (see my new thread on this) seems to remove this - my uneducated guess is that the lower impedance at the feedback connection point (collector of Q2) converts this design from a current/voltage-feedback mixture more towards a ideal current feedback, which has inherently no slew limit.

Samuel

 

[bcarso]

I don't think the series R is necessary.

 

[Samuel Groner]

I don't think the series R is necessary.

It seems to dampen out some resonances. Making the value much smaller points out some overshoot in the step response. At least that's what SPICE is lying to me... :grin:

I used this series resistor in my balanced CF-design as well, for the same reason.

Samuel

 

[bcarso]

I guess I see where that could work. Possibly it's a term involving Ccb of the input transistor coupling through to Q4, even though at most frequencies the impedance at the R2 Q1 junction is mostly resistive (= ~R2). I had also not noted that C2 is a feedback C rather than a shunt C to common. In a way R5 limits the input current to what amounts to being a big inner-loop integrator so as to give it more time to respond to a charge delta function.

Might be worth looking at it open-loop, and with and without C2, to see what's going on. Just fine-tune a d.c. offset at the input to stabilize the output d.c., something that simulators let us do that would never be practical in a bench setting.

 

[Samuel Groner]

Instead of a darlington output I get good results with using a darlington VAS to increase open loop gain.

Can you help me a bit? What is a/the darlington output in my schemo?

If you want even lower input voltage noise than the 2N4401 "flip" the circuit upside down (complemetary circuit) and make the input a PNP transistor.

I just ordered a good bag full of 2SC3329 (from www.lobtron.de) which were much cheaper than the complementary, that's why I want to stay with the NPN input.

What I found, through measurements, when increasing the gain within the amp is that the second and fourth order distortion would disappear and only the third order became detectable above the noise floor.

One would have thought that this circuit should generate mainly 2nd order, except with heavy loads where the output stage starts to distort.

I had also not noted that C2 is a feedback C rather than a shunt C to common.

Any advantage by changing this? Increasing local feedback sounds like a better idea than to wast open loop gain to ground?

Samuel

 

[tk@halmi]

[quote author="Samuel Groner"]

Instead of a darlington output I get good results with using a darlington VAS to increase open loop gain.

Can you help me a bit? What is a/the darlington output in my schemo?
Samuel[/quote]

My bad. It is a follower enhanced VAS. I just use a darlington transistor for Q5.

 

[bcarso]

[quote author="Samuel Groner"]

Instead of a darlington output I get good results with using a darlington VAS to increase open loop gain.

Can you help me a bit? What is a/the darlington output in my schemo?

If you want even lower input voltage noise than the 2N4401 "flip" the circuit upside down (complemetary circuit) and make the input a PNP transistor.

I just ordered a good bag full of 2SC3329 (from www.lobtron.de) which were much cheaper than the complementary, that's why I want to stay with the NPN input.

What I found, through measurements, when increasing the gain within the amp is that the second and fourth order distortion would disappear and only the third order became detectable above the noise floor.

One would have thought that this circuit should generate mainly 2nd order, except with heavy loads where the output stage starts to distort.

I had also not noted that C2 is a feedback C rather than a shunt C to common.

Any advantage by changing this? Increasing local feedback sounds like a better idea than to wast open loop gain to ground?

Samuel[/quote]

The 3329's are one of the rare cases where the NPN should be no noisier than the PNP complement, so I think you will be fine with them.

On the distortion issue, be very careful to look at a variety of levels. For the typical nonlinearities second will rise proportional to signal and third proportional to the square of the signal*---so what looks good at one level may be quite a different story at a higher level. Since we can always in principle get rid of second with a balanced structure, minimizing third and higher is probably the best strategy if one is striving for clean.

As far as the feedback C vs. shunt C to common, that's intricate. The sims are probably pretty reasonable though. I recommend doing the cascaded notch filter, possibly followed by bandpass filters at the harmonics, if you have trouble getting the fourier analysis to function (which is often the case for my sims---the dreaded "line too long" bomb). I'm going to try synchronous rectification next :green: and lowpass filtering.

Another thing the program gags on is paralleled devices. I am trying to devise equivalent transistor models by playing with Is and the capacitances, so that a single device can be deployed in sim.


*good discussion in Feucht, I believe crediting Bruce Hofer for the analysis.

 

[bcarso]

AH. A bunch of currents, resistances and capacitances to change, but the equivalent multiple transistor strategy appears to work!

It will make the schematics look simpler too ;-), however illusory that is.

 

[PRR]

> Just fine-tune a d.c. offset at the input to stabilize the output d.c., something that simulators let us do that would never be practical in a bench setting.

Also remember that a 100,000 Henry choke is free in SPICE, though unavailable or unaffordable on the bench. You can get unity DC gain feedback yet high open-loop AC gain.

Watch for unexpected resonances. 100,000H and a few stray pFd winds up in the audio band.

 

[Bauman]

Tim R. is cooking something transformeless here, looks interesting:

http://www.seventhcirclestudios.com/SCA/C84/docs/c84_sch.pdf

:thumb:
Fabio