An Improved Servo for the THAT1510 and THAT1512

[hazmo]

Thanks for the long and detailed response!

Regarding the choice of topology: is there any major disadvantage with your first circuit, other than the usage of one more opamp? As a matter of fact the pcb layout is actually ready to send to the fab...


Ok, increasing the servo input resistor is what I was gonna do, thanks for the verification. Bigger polypropylene capacitors get too bulky quickly.


Re the bias resistors/input capacitors: with your paper about the phantom menace in mind and following THAT's recommendations, I was going for 47µ and 1.2k, so resulting in a differential input impedance of about 2k. Would you go for something higher? Load-wise it should be ok for practically any microphone I would guess. Is there anything else to consider concerning the noise besides Johnson and bias current? I don't have enough experience to quantify the noise penalty without actually calculating.


Thanks again,
Volker

[mediatechnology]

Regarding the choice of topology: is there any major disadvantage with your first circuit, other than the usage of one more opamp? As a matter of fact the pcb layout is actually ready to send to the fab...

Go for it. I've PM'd bruno to post his sch which is being used in his remote truck. It's the same as page one's schematic. We know it works at the cost of an extra op amp.

Ok, increasing the servo input resistor is what I was gonna do, thanks for the verification. Bigger polypropylene capacitors get too bulky quickly.

I just looked over THAT's original and they used 100nF/510K. (with a Bifet). That puts the -3dB point at ~ 3Hz vs. the 7 Hz I originally showed.
Do be aware that you need a low-bias current (bipolar) op amp to run it up that high. The OP07's bias current is low enough.

Re the bias resistors/input capacitors: with your paper about the phantom menace in mind and following THAT's recommendations, I was going for 47µ and 1.2k, so resulting in a differential input impedance of about 2k. Would you go for something higher? Load-wise it should be ok for practically any microphone I would guess. Is there anything else to consider concerning the noise besides Johnson and bias current? I don't have enough experience to quantify the noise penalty without actually calculating.

I'd stay with what you have.
If you do go with higher value bias resistors the source impedance will usually dominate anyway.
Open-circuit input noise will be higher as Rbias rises and I think that drives THAT's concerns to make them low-ish in value.
At low frequencies the reactance of the caps will add to the source impedance however.
At 20 Hz/47 uF you'll see ~170 Ohms/leg in series with the source.

Is there anything else to consider concerning the noise besides Johnson and bias current?

Capacitor leakage when phantom is applied is not trivial.
Good ones are 1-3 uA (DC) after minutes of "warm-up."
It's not just DC - there can be some significant noise current depending on the state of electrolytic formation.

Send us baby pictures when you get a board built-up.

[mediatechnology]

Here's the link to bruno2000's build of the mic preamp: http://www.ka-electronics.com/images/pdf/WAYNE17A.pdf

[hazmo]

Go for it. I've PM'd bruno to post his sch which is being used in his remote truck. It's the same as page one's schematic. We know it works at the cost of an extra op amp.

Only the engineering mind is the one that is a little unsatisfied . Cost wasn't much of an argument anyway, I get OP07s in single units at 0,29€ from a German distributor. And having four 8-pinners on the board makes for a nicer symmetrical layout...

Capacitor leakage when phantom is applied is not trivial.
Good ones are 1-3 uA (DC) after minutes of "warm-up."
It's not just DC - there can be some significant noise current depending on the state of electrolytic formation.

That sounds funky. And it's one of those things I would never think about myself unless someone told me about it. Where are those ideal parts we were told about in classes?

Send us baby pictures when you get a board built-up.

Sure, will do! And thanks for bruno2000's schematic. Reassurance is always a good thing.

[JR.]

A few uA of leakage sounds high, while the data sheet spec is even more than that, I don't recall it being that much or a problem in practice.

Regarding noise from that leakage, it is terminated by a couple hundred ohms of microphone at audio frequencies so rarely an issue. I recall exactly one production incident when a new model electrolytic cap, got flagged by the factory QA people for noise. My speculation is that, this cap manufacturer made their new improved part too small and cheap to work in that particular application.

So generally not a problem but there are problem parts out there so trust but verify.

JR

[mediatechnology]

I think the best thing you can do is anticipate the need for phantom and allow things to settle for 5-15 minutes before you actually use the preamp for a take.

One of the other issues is that after turn-on, the differential DC offset can be quite large as the C pair charges unequally.
If there is no Cgain capacitor (there was probably a Cgain used in the low-cost realm) the servo has to correct that constantly-changing error for the first minute or two.

I spent hours watching this. Call me a geek - I know it's true.

[mediatechnology]

Take a look at the "improved" improved servo here: viewtopic.php?f=6&t=14&start=114