[pastafarello]

I need to reduce the gain of a Creek OBH-11, preferably to unity. It would seem all I need to do is shunt R6/R106. Is that correct? Any advice on this topic? Thanks.

 

[kchapdaily]

That should work, but you will also have to short R5 and R105 as the increased voltage at the inverting terminals of the opamps will draw significantly more current through these resistors. This could cause damage to your output transistors, and could move the operating points enough to cause distortion. Additionally you could damage the opamp gain resistors and output bias resistors.
 
The gain on this amp does seem somewhat high, but this may be in your favor as the decreased current draw from the rails may not be significant if you short the opamp gain resistors. It would take more analysis to determine if the operating points of the output transistors will change due to the decrease in current draw.

 

[pastafarello]

Many thanks for your reply. Now that you mention it, it occurs to me that simply removing R5/R105 would make the op amp function as a unity gain buffer, and this op amp (NE5532) is already internally compensated for unity gain operation. Wouldn't this be the best solution?

 

[kchapdaily]

That will set your gain correctly, but you still need to watch out for the change in current draw affecting your output transistor setpoint.

 

[pastafarello]

Thanks for your help, kchapdaily.
 
I did a bit of testing, and I changed my mind.
 
I compared the OBH-11 against an O2 set to 1x/2.5x. Using a full-scale 100 Hz sine wave (2.26V RMS) for input, I measured the voltage output of the O2, max volume, no load. The results were, respectively, 2.1V and 5.5V RMS. With measurement errors, etc, the results seemed to me to make sense for unity gain and 2.5x. Then I measured in the same way the OBH-11, and the result was 9.7V RMS (with massive clipping). I don't really know if I was interpreting these results correctly, but it seemed to me that the OBH-11's gain could not be anywhere near the 23x suggested by values of R6 and R5. It looked like it was closer to 4x, and this was confirmed by listening (at much lower volumes, of course).
 
So I decided I didn't really understand how this amp works, and I'm not going to spoil a perfectly good, working amp -- at least until I have a better idea of what I'm doing.
 
But thanks again for your help.

 

[kchapdaily]

pasta, it looks like we may have overlooked C3/C103! If you calculate the impedance of C3 and R5 in series at audio frequencies, you will see that C3* is actually quite a dominant part of the impedance. At 100Hz, the impedance of that branch is almost 33.4Mh ohm! This is certainly curious, as it would suggest that the opamp is already at near unity gain, and is only being used as the input buffer. But this cap would be introducing almost 90 degrees of negative phase delay at all audio frequencies, and that would leave the BJT totems to handle signal gain and output stage...
 
*The above is only valid if i am guessing the value correctly. I am assuming it is 47pF, but the schematic i am looking at only says 47/25. 25V is likely the voltage, but if the cap is 47uF the resistor is the dominant component in the impedance.
 
pasta, could you confirm the value of C3/C103 when you get a chance?
 
It is curious though, the clipping would suggest that the gain is indeed higher than 9x. Could you also try testing it again with a different drive signal? If you are using normal audio sources for the test, you could use your phone as the input, they usually run closer to 1Vrms line out. Ideally I would like to know where the amp stops clipping.
 
Also i can put together a spice sim next weekend if this does not pan out.

 

[pastafarello]

  the clipping would suggest that the gain is indeed higher than 9x. Could you also try testing it again with a different drive signal?

Yes, you are right, of course. I did not take into account that the amp was driven into saturation. With a much lower input signal (35mV RMS 1kHz sine wave), the OBH-11's output (without clipping) measured 790mV RMS, which is close enough to 23x. (The unity gain O2 measured 34mV.)