Neumann PSVe summing amp - broadcast filter?

[adamasnan]

HI all! Hope someone can tell me if they see any kind of fixed HP filter in the PSVe schematic? I see a steep roll-off from around 30 hz when checking freq response

Schematic attached

Thanks!!

 

[radardoug]

A steep rollof at 30 HZ? Perfectly normal for audio gear. You dont need anything below 30 Hz, in fact its usually a nuisance.

 

[Bo Deadly]

Oops.

 

[abbey road d enfer]

C8 is 0.47uF, so the cut-off there is about 7Hz.
I believe the main factor is the output transformer.
However, I would check the electrolytic caps, that may have drift onsiderably over the years.

 

[ruffrecords]

Isn't this a balanced virtual bus summer? In which case you need to be testing it with the correct series resistors at the input. Not doing so could easily have an adverse effect on the frequency response.

Cheers

Ian

 

[abbey road d enfer]

Isn't this a balanced virtual bus summer? In which case you need to be testing it with the correct series resistors at the input. Not doing so could easily have an adverse effect on the frequency response.

Good catch!

 

[adamasnan]

C8 is 0.47uF, so the cut-off there is about 7Hz.
I believe the main factor is the output transformer.
However, I would check the electrolytic caps, that may have drift onsiderably over the years.

Hi there, thanks, yes all electrolytics have been replaced. I have another unit of the same, but slightly earlier layout, will try that, I assume they have the same output transformers.

Isn't this a balanced virtual bus summer? In which case you need to be testing it with the correct series resistors at the input. Not doing so could easily have an adverse effect on the frequency response.

Cheers

Ian

Yes that's right, I am using 2 x 2550ohm resistors per leg, per channel as per data sheet attached, for +6db gain

I have a lot of different kinds of German broadcast equipment of this nature, and this demonstrates the sharpest roll off in the low freqs

 

[radardoug]

So if you have replaced all caps, and it still rolls off, thats how it was designed. How many dB down is it at 30 Hz? Why do you want to get more out of it below 30 Hz?

 

[ruffrecords]

So if you have replaced all caps, and it still rolls off, thats how it was designed. How many dB down is it at 30 Hz? Why do you want to get more out of it below 30 Hz?

Yes, if you could n post the actual frequency response that would help. The spec says -0.3dB at 40Hz so bound to be a bit lower at 30Hz. What load did you use at the output when you measured this?

Cheers

Ian

 

[abbey road d enfer]

And how did you measure the response?

 

[adamasnan]

Hello! Just a quick and simple sweep, sending though PSVE and direct to recorder from a soundcard. Load should be 10k+

Just checked the other PSVE (with same output transformer) and it does not have this roll off in the lows (and is not recapped), it's about -10db at 20hz, which is audible. The two models have slightly different components inside, I would like to send photos and ask for assistance in identifying what part is causing the filtering.

 

[radardoug]

So maybe you made a mistake when you recapped it.

 

[adamasnan]

So maybe you made a mistake when you recapped it.

nope

 

[adamasnan]

C8 is 0.47uF, so the cut-off there is about 7Hz.
I believe the main factor is the output transformer.
However, I would check the electrolytic caps, that may have drift onsiderably over the years.

The unit I have with the roll off does not have a 0.47 cap at c8/108, but does have instead 2 x 2u2 caps in series it looks like, with some other components, but i can't tell if it's where c8/108 should be exactly. I can take a photo tomorrow

Thanks!

 

[radardoug]

Maybe those components form a high pass filter. You need to work out exactly how those extra bits are wired.

 

[abbey road d enfer]

The unit I have with the roll off does not have a 0.47 cap at c8/108, but does have instead 2 x 2u2 caps in series it looks like, with some other components, but i can't tell if it's where c8/108 should be exactly. I can take a photo tomorrow

You should have started with that. It's quite possible that the output stage has been modded as a Sallen & Key filter.

 

[adamasnan]

You should have started with that. It's quite possible that the output stage has been modded as a Sallen & Key filter.

Thanks! I wasn't able to check the other unit in detail. And I assumed both would be the same before I posted 

Here are some photos attached, it might be well what you describe, it's a bit tricky for me to trace the path, but this board is definitely between output of OA10 #1 and input of OA10 #2

Any suggestions how I could modify what's there??

Thanks for all your help!!

 

[radardoug]

You want help? Take it apart, work out the circuit, and post it. The picture doesn't tell us a lot.

 

[abbey road d enfer]

Any suggestions how I could modify what's there??

Check the actual value of R6 and the resistor on the small add-on board. The caps are 2.2uF, right?

 

[adamasnan]

Check the actual value of R6 and the resistor on the small add-on board. The caps are 2.2uF, right?

Yes, the caps are 2.2uf. If that resistor on the board is R6 then what on the schematic says 47k, this is 2k2.

Does that help?