Preamp freq response. Huge boost past 20K. Does it matter?

[JW]

Hey folks,

I've been running some frequency response tests on some of my gear. Here is one from a Yamaha PM1000 preamp strip. Does it matter that there is around a 40dB boost right past 20K? (I've tried terminating the transformer with a 620 ohm resistor to see of that had any effect, but it did not.

Does it matter? It's ruler flat otherwise.

 

[dfuruta]

Are you sure this isn't an artifact of your measuring equipment?

 

[abbey road d enfer]

Are you sure this isn't an artifact of your measuring equipment?

+1

 

[CJ]

could be hi freq oscillation caused by scope ground, try iso xfmr on scope,

or cap on the ground probe, us AC input on scope,

unless you are using a laptop which would take us out of the conversation completely,

 

[emrr]

Looks like an artifact to me

 

[JW]

Yeah,

I admit it was the fuzzmeasure program running a sweep out of my converters (less that 50ohm output impedance) into the Pm1000 (with input switch up high at mic level) I had to run the sweep out of the converters at a very low level to be able to use the inputs of the preamp up high like that. The intent was to try to see what my frequency response was like when running these preamps at a general approximate level used for mics.

Output of the PM1000 is a 600 ohm output loaded with around 15K input impedance of the converters.

Does that perhaps make sense with what I'm measuring?

I'm curious that boost would happen RIGHT at 20K.

 

[abbey road d enfer]

Yeah,

I admit it was the fuzzmeasure program running a sweep out of my converters (less that 50ohm output impedance) into the Pm1000 (with input switch up high at mic level) I had to run the sweep out of the converters at a very low level to be able to use the inputs of the preamp up high like that. The intent was to try to see what my frequency response was like when running these preamps at a general approximate level used for mics.

Output of the PM1000 is a 600 ohm output loaded with around 15K input impedance of the converters.

Does that perhaps make sense with what I'm measuring?

I'm curious that boost would happen RIGHT at 20K.

Looks like a glitch from the program; can you extend the measurement frequency range, to see if it happens at let's say 40 kHz.

 

[warpie]

Are you measuring at 44.1kHz? If so, try 88.2kHz.

 

[ruffrecords]

Yeah,

I admit it was the fuzzmeasure program running a sweep out of my converters (less that 50ohm output impedance) into the Pm1000 (with input switch up high at mic level) I had to run the sweep out of the converters at a very low level to be able to use the inputs of the preamp up high like that.

That's your problem. Build a 60dB attenuator then you can use the converter outs at a more normal level.

Cheers

Ian

 

[JW]

Thanks guys. H- pad then? I know where to find the info to build those. Rane or Uneeda or something like that. . . .

 

[dfuruta]

Nyquist?

 

[abbey road d enfer]

Thanks guys. H- pad then? I know where to find the info to build those. Rane or Uneeda or something like that. . . .

U-pad is enough, even L-pad should be adequate. H-pad is for impedance-matching; who cares? 

 

[ruffrecords]

22K and 22 ohms will be close enough or use 2 x 11K to make it balanced.

Cheers

IAn

 

[lassoharp]

I'm curious that boost would happen RIGHT at 20K.

What is the bandwidth of your test software?  Many of the scope and sweep programs only have 20Hz-20K.


If you have a true 40 db rise at 20K - you will hear it.  Plug in a mic or some program material.

 

[Gene Pink]

there is around a 40dB boost right past 20K?

Just a hunch, do you have any CF squiggle bulbs, or other fluorescent lights with newfangled electronic ballasts near your workbench? Try shutting them off.

They have switching supplies that run just above audio frequencies, and spew out enough RF crap to get into any sensitive measurement. I have to shut off mine when measuring, even with proper grounding techniques.

Any attachments to this post, DO NOT CLICK, I'm fighting a root virus.

Gene