Matching Transistors for Valley People MicPre

[student]

ok. two (make that three) more newbie questions just to freshen up my brains:

...we apply 1mV. we get 10mV/100Ω= 0.1mA emitter current...

Is this a typo? Should it say ...we apply 10 mV...?

if the transistors are biased at 3mA, then 1/Gm is about 10Ω each

Gm is Ic/(kT/q), right? Or, 3/25 = 0.12 (at room temperature)

or am I completely lost?

That's just one kind of differential amplifier, sometimes called "long tail". It is an especially handy form, so we see it more than all others put together, at least when we are not doing something fancy.

...

Generally the presence of (one or two) tail-paths does not affect the signal gain. The between-Emitters resistor is much smaller than the tail-resistors so it dominates the gain equation.

It's clear to me now how this thing works with diffmode signals, but how about common mode signals. Here, the tail resistors play a role, right?

Thank you for your help once again!

/stu

 

[student]

[quote author="student"]ok. two (make that three) more newbie questions just to freshen up my brains:

...we apply 1mV. we get 10mV/100Ω= 0.1mA emitter current...

Is this a typo? Should it say ...we apply 10 mV...?

if the transistors are biased at 3mA, then 1/Gm is about 10Ω each

Gm is Ic/(kT/q), right? Or, 3/25 = 0.12 (at room temperature)

or am I completely lost?

That's just one kind of differential amplifier, sometimes called "long tail". It is an especially handy form, so we see it more than all others put together, at least when we are not doing something fancy.

...

Generally the presence of (one or two) tail-paths does not affect the signal gain. The between-Emitters resistor is much smaller than the tail-resistors so it dominates the gain equation.

It's clear to me now how this thing works with diffmode signals, but how about common mode signals. Here, the tail resistors play a role, right?

My guess is that the single tail resistor configuration could be seen as two tails in paralell and that the common mode gain could be calculated as:

G_CM= RC/(re + RE + Rtail + Rtail)

Thank you for your help once again!

/stu

 

[bcarso]

quote: "It's clear to me now how this thing works with diffmode signals, but how about common mode signals. Here, the tail resistors play a role, right? "

Depends on what the common-mode rejection of the next stage is. Often we use current sources in lieu of resistors so as to alleviate the requirements of CMR of the following stage. From a noise perspective though the resistors are always quieter, given the same available rail voltages.



Glad you are enjoying the forum. I think you will find a lot of very helpful and knowledgeable people here, and a general absence of rancor, compared to other forums, at least when I remember to take my medication. Joke. No, really....

 

[PRR]

> Gm is Ic/(kT/q), right? Or, 3/25 = 0.12 (at room temperature)

Equation looks right. But your numeric solution appears to forget k and q. Whatever they are. (Yeah, I know what they say they are, but all this stuff is black magic to me.)

And "3mA" is really 0.003A.

I just remember 30Ω at 1mA. Gm is one-over that, but working with resistance is more comfortable for me.

If you look up the values of k and q you actually get numbers like 26Ω or 28Ω, but "30" or "33.333" is easier and pesimistic.

 

[CJ]

Thanks,Bill, Paul and especially Student for asking the questions that I was going to ask!

One last question, how does adding extra transistors in parallel change the gain?

cj

 

[bcarso]

"One last question, how does adding extra transistors in parallel change the gain? "

It doesn't, to first order.

 

[CJ]

Danke!

 

[CJ]

OK, one last last question: If I were to stick an x-former on the output of the last op amp, what would be a suitable ratio?
(I should know this)

 

[tk@halmi]

CJ,

If you use the TL072 I would not hang anything less than 2k off that thing. (Assuming 600 ohm secondary for all these transformers.) Even that is a questionable load for the TL072 so a 3:1 may be better, but your total gain will start drooping significantly. The OPA2134 will swing it all into a 2:1 easily. Put a GainBloke on the output in a 6dB config and you will be able to drive anything in your collection.

On a different note, how difficult would it be to make decent 2k:2k output transformers? Most chip opamps can drive that load with low distortion. You would not be able to drive vintage stuff with 600 ohm inputs, but most modern gear has 10k inputs these days. It would make adding an output transformer to any gear simple.

The Radio Shack ground loop isolator works, but the lows are missing badly.


T-$

 

[SonsOfThunder]

[quote author="tk@halmi"]If you use the TL072 I would not hang anything less than 2k off that thing.[/quote]
Total agreement!

The OPA2134 will swing it all into a 2:1 easily.

or TLE072...!

But I thought part of the charm of this baby was that it is trafo-less?

Go CJ! I'm excited you are making progress!
Charlie

 

[Carl_Huff]

So CJ ....

What does your current circuit look like?

How far have you drifted away from the original TransAmp design?

 

[CJ]

I will re-draw everything and post after I tweak this thing.

Weird co-incidence: We were just talking about photo-electric effect at the current source thread in relation to using LED's . Well, my eyes are not the greatest, so last night I was using my hi-powered underwater diving light to illuminate the VP circuit board so I could check a resistor value. I happened to be listening to the amp thru my headphones while I was shining the light on the circuit. As soon as I hit the circuit with the light, snap, crackle, pop, rice krispies! What the....
There are four zener diodes used as a clamp on the front end of the Valley pre to keep from nuking the E-B junctions of the input array. I happen to use four glass diodes. The light was caising a pretty good amount of hash coming thru the amp. The worst place to have noise generation is right at the input. Anyway, I clipped out the diodes and the noise dis-appeared. I was wondering why the VP was a bit noisy at high gains. That was co-incidence number one.

Co-incidence number two: I mentioned pots not going all the way to zero in a Pultec thread. Well, guess what. A 2.5 k audio pot that I tried in the gain position did not go all the way to zero! 47 ohms was the lowest. Well, the VP needs 10 ohms across the neg feedback lines for full gain. So I was misssing quite a bit. Also, the pot was noisy at the high gain setting with that 47 ohms. So I replaced the pot and now have the full gain and less crackle.

Also, I clipped out a pair of matched transistors in my array. I was using 8 instead of the stock 4. Now I have 6. Helped noise even further.

So between the zener diodes, the potentiometer and the removal of the transistor pair, I have managed to lower the noise floor quite a bit. Almost as good as my V76 mic pre.

One thing to note: the resistor from the middle current source trans-diode to ground. I have two schematics. One uses a 10 k bias resistor, the other an 86 K. The 86 k gives slightly less noise.

Lesson to be learned: If you are using glass diodes in an audio circuit, make sure you cover them up good, especially if they are on the input to your mic pre!!!


cj

 

[bcarso]

CJ, remember to have your inputs terminated in a representative load. Noise current will greatly increase your noise with an open input, which is not going to be how you use the preamp. It may well be that you are at an optimal point with fewer transistors, but you won't know without the proper source impedance attached.

Heh light sensitive zeners...it does make sense that they would be particularly noisy.

 

[CJ]

Thanks, Brad, yes, I strapped a 100 ohm resistor when listening. Also note that any noise from glass diodes in this circuit will be common mode and thus will get cancelled. I noticed that I got the most noise when shining the light on only one side of the circuit. And also, most people run their stuff with the lid on, but I was doing some circuit tweaking, so.....lid off.

Never-the-less, something to be aware of.

Thanks for the hint!

 

[SonsOfThunder]

Sounds like a good reason not to use glass-cased diodes!!

:thumb:

 

[CJ]

If there potted they will be fine, also, a diving light is a pretty concentrated hi intensity beam, so a little ambient daylight....

 

[SonsOfThunder]

I know I'm resurrecting from the dead here. Sorry!

CJ!! What's the word on this? Run out of steam??

Peace!

 

[CJ]

Well, I scored four VP modules from a friend on the forum, so it looks like my VP matching days are over. I did build a pre out of the transistors I matched and it sounds great.
Still messing with the output circuit, which is transformerless. Stuck a Triad HS 56 on the output and it sounds good.

cj

 

[SonsOfThunder]

CJ,

Thanks for the update. Is the schematic basically the same as your last one here in the thread or do you have an update?

Did you get my email re The Cove? Maybe I have an old funky email address!

Peace!
Charlie