Calrec PQ1549 Preamp Square Wave Testing

[jasonallenh]

Hi everyone, I built myself a test jig to get some modules ready for racking. My first module seems to be functioning fairly well, however I notice the preamp is not giving me a clean square wave. In isolation, the eq, filters, and output (borrowed from the MC1520 small fader module) have all given me clean squares compared to the generator. I'm not sure whether what I'm looking at is considered within a normal expectation or a sign of a problem. This preamp has a hi and lo gain amp and they both show this same sloping. Output of the line input section too. The slope would suggest I've lost some low frequency information, so I'm thinking a recap may be necessary. However, I've not really done a ton of testing like this before, so perhaps I'm making too big a deal of this response. Yellow is the preamp output, blue is the test signal.

 

[ruffrecords]

The obvious thing to try next would be a frequency response sweep to see if there is any loss at the low end.

Cheers

Ian

 

[gyraf]

yes, there's a low cut there, like we suspect from a transformer - question is, as Ian says, just where it's at

 

[thomasdf]

I’ve had similar results with output transformer being loaded incorrectly (or even unloaded)
Maybe that’s a thing you can (double) check ?

 

[jasonallenh]

I’ve had similar results with output transformer being loaded incorrectly (or even unloaded)
Maybe that’s a thing you can (double) check ?

Hi Thomas, I believe my output transformer is fine- I've bypassed the preamp to check the eq and filter sections and they look pristine. It's the input transformer that seems to be giving me problems.

 

[jasonallenh]

like we suspect from a transformer

The thing that gives me pause about this is the preamp seems to only rely on the transformer in lo-gain mode, and the line input doesn't have have a xformer at all. I'll certainly do a freq sweep and report back though