Simple Single Ended Mic Pre

[Samuel Groner]

Hi

I need a simple and cheap mic pre to record my singing lessons; mic is a dynamic one, PSU a +5 V from an old HD.

I took a design by Hood and tried to adapt it to 5 V ([removed]); however, I have trouble setting nice operating points. Base of Q1 and Q3 are OK, collector of Q2 and Q4 is not (< 1 V). Is there a way to set these collectors to ~2.5 V? Playing around with the resistor values did not help me.

How about biasing Q1 with more current for lower noise? Lowering R3 seems not to help that much?

R9 will be a pot, BTW.

Thanks!
Samuel

[bcarso]

Samuel, note that the d.c. gain of each stage is small.  Were it not for the input transistors' emitter currents dropping across R5 and R14 the output V would be Vbe lower than the input V set by the voltage dividers (R1 R2 for example).

So the input d.c. bias must be higher to raise the output V.  Q1 doesn't need much from C to E to work so this shouldn't be a problem.

As far as putting more current through Q1, if R3 is smaller then R5 has to be as well to preserve the new bias.  R5 smaller may mean R6 smaller at that point as well, and to preserve the gain then R4 proportionally to R5.

After a while in this program of reduction the decoupling R's may need to come down a notch as well.

Linearity would be enhanced a bit of R6 and R15 were current sinks, but this would be more significant in the second gain stage, and would have the disadvantage of reducing output swing a bit.



PS: Good luck with your singing lessons ;-)

[bcarso]

PS: I see no need for the bigger output device in the second gain block.  A 4403 shold be fine here as well.

[PRR]

> mic pre to record my singing lessons; mic is a dynamic one, PSU a +5 V from an old HD.

Hard to believe you want a fixed gain of 1,000 for that. I would expect levels over 1mV from any adult singer through a dynamic mike. Your output level is at best 5V/2.8= 1.78V, and probably more like 1V; this implies input overload at 1mV.

Yes, R9 is an obvious place to put a gain control, giving input overload ~10mV. I'd want more.

> adapt it to 5V

Working at low voltage is tough. All the "negligible approximations" trip you up. At a glance, R5 has "small" voltage drop, about 0.7V*10K/6K8. "Only a volt". But one volt is big in 5V system.

Assuming infinite Beta: Q1 Base is 2.7V, Q1 Emitter is 2V, R5 drops 1V, Q2 Collector wants to sit at 2V-1V= 1V to satisfy the bias. For realistic Beta, it sits lower.

Also the maximum voltage gain of a resistor loaded BJT stage is Vs/30mV, and half that if you want some voltage swing. For 5V, max gain is 160 to 80, less in real life.

Also note that the current gain around the feedback loop is just Q2's Beta. If that is 100, and R5 taps 10% of R6 current, and Q1 signal current is 10% of R5 R4 signal current, you have no feedback.

If you already built it: reduce R5 to 3K. That lessens its DC drop, and also gets the AC gain more in line, R13 and R14 should be like 100 and 3K for the same reason. Yes, the feedback is a heavy load in the stage: but this is a tight design.

Then reduce R1 (and R10) to bring the collector voltages up. Considering the heavy load, 2/3rd of Vp or about 3V may be optimum.

And put a gain pot for R9.

[PRR]

Here's another way to get large adjustable gain with a 5V supply.


The commercial drawback is that R4 has to be adjusted for each Q1. In DIY, you can try 500K, 680K, 1M, 1,5M until the output sits around 2.5VDC.

Bias will drift with temperature; I assume your lesson room does not have huge temperature swings.

R10 is regular Audio taper.

[bcarso]

Agree with PRR that the gain is a mite high.

I toyed around with this design a bit in sim and realized that Linsley-Hood did some thoughtful distribution of the gains w.r.t. distortion.  With that in mind my earlier remark about the I source loading of the second stage now applies to the first stage as well.

What is additionally interesting is that if you flip the second amp upside down you can adjust the gains of first and second so as to get cancellation of even-order distortion.  With I source loads for both and a bit of tricking around the THD for about 600mV rms out is less than 20 ppm, a pretty impressive result for a gain of a thousand and 6 transistors.

[Samuel Groner]

Thanks for the suggestions - fast and competent as always!

I really need 60 dB - that Sony F-99LT has frightening low output. The pot will give usuable gain control between 40 dB and 60 dB, just in case I start to sing louder...

I agree that there are better ways to change gain, but I had bad experience regarding stability with point-to-point stuff that uses abjustable feedback networks, so I leave that as it is.

I don't think it is worth working on linearity here, but the idea with "turning" the second stage sounds very interesting indeed (I have a single ended "pro" design in my mind as well); how would we need to distribute gain?

Now revision 1 is out:
[remove]

Are the operating points fine? I skipped the higher bias for Q1 as it caused to much voltage drop in some resistors.

Step response doesn't look good to me; in addition to this, the max. output level is very low (< 0.5 V without the peak) and clipping causes polarity inversion. Anything to do about this?

Samuel

[bcarso]

The first stage bias now looks too high at first glance, as you do want a little more C-E voltage across Q1, and the attempted closed loop gain of > 200 looks excessive.  Maybe I will email you schematics of suggestions, now that you have confirmed that you need 60dB.  Feel free to post them.

I presume that for your pro design you can use a few more parts and a more respectable rail voltage?  As PRR points out, a lot of issues arise with the 5V supply.

The alternating stage polarities ides to cancel even-order is fun, and maybe has appeal among minimalists, but with a few more parts becomes pretty much academic as the individual section distortion gets so low.  As I did this last night I was amused that I got within one transistor of Tamas's initial design, more or less.

[Samuel Groner]

The first stage bias now looks too high at first glance

Where do we want the collector of Q2 to sit? Is between supply and GND no good?

Maybe I will email you schematics of suggestions

That would be very helpful indeed!

I presume that for your pro design you can use a few more parts and a more respectable rail voltage?

Of course, 40 V sounds nice to me...

The alternating stage polarities ides to cancel even-order is fun

Thinking over it, it seems that this must rely heavily on matching of some parts and currents and thus may not proof well conditioned in real life - look how hard it is to make a diff. input stage cancel even order stuff...

Samuel

[bcarso]

It's fine to have Q2's collector at about half of the supply, although if you are really working at these high gains overall, all it really has to do is swing enough not to grossly overload the next section.

When I said first stage bias I meant the base voltage of Q1 btw.

Yes, I will hope to get some examples off to you soon.  I have some deadly-boring work to do for a client today which despite needing the money I'm trying to get out of, but it may not be possible so it may be a while.

BTW what does the noise of your 5V power supply look like, i.e., how critical are those R-C filters?  We are losing precious voltage in them.  If the PS noise is mostly high frequency they could be replaced with little inductors.

PS: "Thinking over it, it seems that this must rely heavily on matching of some parts and currents and thus may not proof well conditioned in real life - look how hard it is to make a diff. input stage cancel even order stuff... "

Well as I say a couple more transistors here and there make life easier.  However, since both amplifiers have some feedback the matching problem is less by guess and by gosh than it might sound.  As it is you will be doing around 0.1-0.2%, which as we musicians used to say (wryly) is good enough for jazz...

Also, you're making (only) one of these, right?

[Samuel Groner]

When I said first stage bias I meant the base voltage of Q1 btw.

Yes, I got that. But raising the base voltage of Q1 seems to be the only way of getting the collector of Q2 above 1 V...

Yes, I will hope to get some examples off to you soon.

No hurry!

BTW what does the noise of your 5V power supply look like

I have to check that. It's just a cheap wall wart, so no big expectations. It's rated for 1.5 amp and IIRC it has an unloaded output of above 6 V, so this is for sure handy here.

If it's really impossible with 5/6 V, I'll get a DC-DC, but the cheapest I can easely source here is above 15$, and my students budget is stressed anyway, so...

Samuel

[bcarso]

Ahhh---so it's just a d.c. wall wart.  Yes, the regulation will be poor which is a good thing as it does give us more volts to work with.

Yes, please measure that with about a 30mA load, both for d.c. voltage and ripple.

Also, there will be the question of how stable your mains power is.  We may need to give up that extra volt to a regulator to keep serious bumps in the line from propagating to your recorder, or sacrifice some low frequency extension, or both. These circuits don't have good power supply rejection. Do you have any TL431's, or at least some zener diodes?

[bcarso]

PS: Is the mic Z really 500 ohms, or more like 150?  I've been assuming the latter...

[CJ]

Just curious, are you looking for better sound  than that of a cheap Radio Shack portable casette with built in condenser mic?

Never mind. I just answered my own question.  :oops:

[bcarso]

Samuel, have you measured you power supply yet?  I need that data.  Also, I need to know the mic impedance.  You are going to make two channels for the stereo, correct?

[Samuel Groner]

Sorry, not yet. I quickly measured it open loop, and I got 5.1x V, so no freebies here. However, noise was below 2 mV (did not check the BW of my DVM yet) - not sure how this behaves with load. Should get to it this evening.

DC resistance of the mic is around 450 ohm. I guess we get to even higher impedances at audio freqs.

I'll do a stereo version, yes.

Samuel

[bcarso]

Hmmm---5.1V means it almost certainly has a regulator in it---good! A bit of a load will confirm.  It should be fine.

I just emailed you a schematic.  Despite being based on 150 ohms it will still work fine with 450---it will have a little bit different PS rejection, since that lower impedance helps a bit in this case.  But there is a tweak for PS rejection anyway.

I'm quite happy about the predicted performance, although it is sobering to realize that if the mic has thermal noise like an equivalent resistor (probably pessimistic a bit since there is a reactive component at some point) the best signal-to-noise is severely limited by source noise and output swing at 60dB gain.

[Samuel Groner]

Here's Brads schemo: simple 5V mic pre.pdf

Thanks for your design effort (though I suspect that this was a simple job for you :wink: )!

What law should R3 be? Is linear way off?

I measured 5.16 V DC and 1.x mV AC on my wall wart with 30 mA load, so we got a regulator there.

CJ, I got a mic and a MD recorder without mic pre, so I thought replacing my friends mic pre which I have to give back would be the cheapest solution plus some fun and learning as well.

Samuel

[bcarso]

R3 probably reverse log, but you're not going to be adjusting it that much so linear will be touchy but o.k. for the occasional adjustment.

This circuit should be way better than you need, but I liked the design challenge of 5V and four transistors.  Noise and distortion are very low and bandwidth goes to about 400kHz (with a 150 ohm source) at a gain of 60dB (that's a 400MHz gain-bandwidth product!).  Transient response is exemplary and it's not very sensitive to C loading either, within reason.

[jmccanna]

Hi,

Old post but new question from a newbie.  Could the last schematic just above this from Samuel Groner titled "simple 5V mic pre.pdf" be powered by the usb port with the line out going to the line in on a netbook?

And, would it work with an electret capsule like the Primo EM-172?  The Spec sheet for the Primo I think says it operates at 2.4k ohms +/- 30% at 1kHz.  (I unfortunately do not know what that means).

James