Topic: Measure distortion and frequency response of preamps

I am just waiting for a "ADI 2 Pro AE". It will soon come from Germany to me in Sweden.
I bought it 90% for the AD converter to record with my preamps.

I would like to be able to measure distortion and frequency response (and also noise) of preamps.

If the preamp gain is 60 dB I must attenuate the incoming signal (from ADI 2 pro out) by 60 dB.
Can I do that with the volume.

Like this (loop back):

DAW(testsignal) -> ADI(out:volume:-60dB) -> PREAMP(+60dB) -> ADI(in) -> DAW(result)

Is this possible or do I have to build some form of low noise attenuator ?
/Göran Nilsson

2

Re: Measure distortion and frequency response of preamps

You need an external attenuator, or the DAC's noise floor will ruin your measurements with such high gain values.

Regards
Matthias Carstens
RME

Re: Measure distortion and frequency response of preamps

I suppose that at least part of the problen is the DAC's DIGITAL VOLUME ?

Would a simple passive voltage divider be enough ?
/Göran Nilsson

4

Re: Measure distortion and frequency response of preamps

Even an analog volume control won't be a full solution, as it is not at the output of the unit, so there is a similar noise floor. A passive voltage divider is not 'enough', but the best solution!

Example for 60 dB Pad

Balanced: 3k3 - 6R6 - 3K3 (6R6 made by 3R3+3R3 if not available)

Unbalanced: 3k3 - 3R3

The ADI output should be set to +19 dBu. For THD measurements set the output level of k1 to -3 dBFS, this gives the best THD and THD+N values.

For best resolution of low level signals the input should be set to +4 dBu. The SNR is 119 dB then, so compared to +19 dBu with 121 dB you can look much deeper into the noise floor. Of course only as long as you don't overload the input this way.

Note that a preamp's noise is usually measured with 150 or 200 Ohm plug at the input. The above Pad brings the preamp to its max performance, equalling a shorted input.

Regards
Matthias Carstens
RME

5 (edited by kegn51 2018-03-21 09:40:46)

Re: Measure distortion and frequency response of preamps

Thank You for your kind answering.

I have to admit that I am not at all good at XLR and balanced things.
But I guess this is what you mean:           

OUT(2)----<3k3>--|---------- PreIn(2)
                            |
                            |
                          6R6
                            |
                            |
OUT(3)----<3k3>--|---------- PreIn(3)

OUT(1)------------------------  PreIn(1)               

/Göran Nilsson

6

Re: Measure distortion and frequency response of preamps

Correct.

Regards
Matthias Carstens
RME

Re: Measure distortion and frequency response of preamps

Can I build this into a plastic box ?
(the ADI 2 PRO AE ( the one I bought and am waiting for) is partly plastic)

Or must I go aluminium ?

/Göran Nilsson

Re: Measure distortion and frequency response of preamps

If it doesn't need to be pro, no box, is also a solution, just build it in the cable. If you get radio interference (probably not) you will hear it. Then you can pack it in aluminium foil and if needed connect that to ground.

Vincent, Amsterdam
https://soundcloud.com/thesecretworld
BFpro fs, 2X HDSP9652 ADI-8AE, 2X HDSP9632

Re: Measure distortion and frequency response of preamps

Thank You All here at RME.

Yes I was in fact wondering if I should connect the box to ground ?


Like this:

OUT(1)-----------|------------  PreIn(1)
                         |
                         |
                     [ BOX or foil]

/Göran Nilsson