Studer A101 opamp

[spacewig]

Evening, people.

Can the A101 run on 15 volt rails?

Is there any compensation neccessary for unity gain application?

TIA

 

[spacewig]

Also, how does one count the pin outs? There are 9 pins, 1 which is placed further to the left from the other 8.

 

[spacewig]

Well, apparently it works in my initial test.

I found info on the pin-outs and I'll post them now for those whom might be interested:

PIN#
2 + Input
5 - Input
4&9 +V
6 -V
7 Output 1
8 Output 2

There is a 330 Ohm resistor between output 1 & 2

 

[thermionic]

Hi,

Here's the schematic for the latest version: http://recordist.com/studer/A101%20latest%20version-4.pdf (Thanks Jakob!)

If you just build a circuit with the op-amp, merely putting a feedback resistor + divider resistor off the inverting input to pin 8 and Gnd respectively (e.g. as a non-inverting topology), you're likely to get an oscillator, not an op-amp - right? In order to stop it oscillating, you need to put a small-value cap between pins 1+7 (aka a 'cdom' cap) - correct?

Secondly, what does pin 3 do?

Thirdly, I read an 'opinion' at another audio forum (the name of which I won't mention...) stating that even the later op-amps that use the trusty BC550/560 combo go noisy over time in the A80... Why might this be? Surely this can only be due to an electrolytic drying up, or an issue with a passive?


Comments?

Many thanks in advance.

Justin

 

[abbey road d enfer]

If you just build a circuit with the op-amp, merely putting a feedback resistor + divider resistor off the inverting input to pin 8 and Gnd respectively (e.g. as a non-inverting topology), you're likely to get an oscillator, not an op-amp - right? In order to stop it oscillating, you need to put a small-value cap between pins 1+7 (aka a 'cdom' cap) - correct?

Yes, you need a C (sometimes series RC circuit there for compensation. [/quote] The value may be optimised in regard to the application (high closed loop gain requires less compensation, positive gain requires more than negative gain)

Secondly, what does pin 3 do?

Pin 3 can be used for a number of things: increasing the LTP current (for more speed, low source impedance noise optimization...), also for so-called "current feedback", eliminating Q2 and directing NFB to Q1's emitter, and probably other things I can't figure out...Seems like the Studer guys have left open a lot of possibilities for optimisation. Same thing with pin 8, which allows increasing the output idle current

Thirdly, I read an 'opinion' at another audio forum (the name of which I won't mention...) stating that even the later op-amps that use the trusty BC550/560 combo go noisy over time in the A80... Why might this be? Surely this can only be due to an electrolytic drying up, or an issue with a passive?

You're right, these are the most likely causes of noise build-up. Other possible, but less likely cause is contamination of transistors due to porous package (it happens some time). Don't neglect the possibility of dirty connectors or dirty PCB. Anyway, all of these problems are quite easy to trace and solve once and for all.

 

[thermionic]

Pin 3 can be used for a number of things: increasing the LTP current (for more speed, low source impedance noise optimization...), also for so-called "current feedback", eliminating Q2 and directing NFB to Q1's emitter, and probably other things I can't figure out...Seems like the Studer guys have left open a lot of possibilities for optimisation. Same thing with pin 8, which allows increasing the output idle current

Thanks, Abbey.

I feel slightly embarrassed for not seeing that pin-3 can be used to increase tail current...

If the 10u polarised cap indicated in the diagram is an electrolytic, potted inside the Studer package, that could explain the noise issue, I suppose.

Cheers,
Justin

 

[sponzorb]

Hi.. reviving a really old thread with a question about the 10uF cap in the A101. I'm not seeing why it's needed after simulating the op amp. It seems to cause distortion on the 240kHz bias signal and I'm generally not seeing any benefit to it being in the op amp. Does anyone know why it's needed?

 

[verbos]

I have used this opamp in several products. In one case, I came to the same conclusion as you and eliminated that cap. Turns out the voltage range is drastically reduced by taking that cap out. I don't know why, but in that case putting the cap back allows the output to swing the full range.

 

[sponzorb]

I have used this opamp in several products. In one case, I came to the same conclusion as you and eliminated that cap. Turns out the voltage range is drastically reduced by taking that cap out. I don't know why, but in that case putting the cap back allows the output to swing the full range.

Oh interesting. It might be acting as a feedback bypass capacitor like you’d see in the standard common emitter amplifier.