Calibration of Quad Bus VCA symmetry on 82e26 RV1

[ULH]

Hi
Couldn't find anything on this subject in the forum. I have a fairly cheap USB 2ch 20Mhz oscilloscope (about 100$) and would like to check that the symmetry is correct on the four 82e26 cards. I'm not certain how to do this.

My understanding is that you lift up the 651 module, send in a 1kHz sine at 4dBU in the patchbay "Pre VCA", measures with the oscilloscope on the "Post VCA" point on the patchbay (unbalanced to the oscilloscope ok?), turn of the QB comp and put the master fader in 0dB. Then adjusts RV1 on 82e26 for lowest THD reading.

The THD calculation from the FFT in the software that came along the oscilloscope is very bad and seems to give a wrong THD value (or because of me not filtering the signal?). But it does give useful mV measurements of the 2nd to 7th harmonic (I can read out the other harmonics on the FFT) which is what you use to calculate THD, rigth? Ie: http://en.wikipedia.org/wiki/Total_harmonic_distortion

Does this seem to be the correct process when not having a dedicated distortion meter?

Thanks guys!

[waltzingbear]

the instructions and directions on the THAT website are your best discussion of this whole issue. Very detailed discussion of all things VCA, and these are the VCA's used in the SSL.

Generally they were talking about nulling the total harmonic distortion, mostly because that was the only real test equipment available at the time. Today we have, as you have, very sophisticated analysis equipment that can give us the individual harmonic makeup of the distortion. Here you can monitor the 2nd harmonic to null for symmetry, or is it the third? I forget, too early and not enough coffee yet, but the THAT documents will clarify.

Cheers

Alan

[ULH]

Hi thanks for the reply. Did a lot of reading on THAT corp website but couldn't find a detailed description of the symmetry adjustment just that it should be tune for lowest THD but not specified to 2nd or 3rd harmonics balance. They mostly describe their newer vca designs that is self-adjusting but I found an interesting plot that show that a manual well-adjusted VCA will have lower THD that their self-adjusting newer versions. Hmmm. This VCA business is for sure big and nerdy science!

I have some issues with my cheap oscilloscope to get a good THD reading but it might be because of me not filtering the signal correctly ie. having too many harmonics in the calculation. Gonna try some software scopes with our Lynx converters for a good thd reading. Do you guys know of any good mac software for this?

[waltzingbear]

not very versed in OSX test software, try this subject, http://www.diyaudio.com/forums/equipment-tools/236566-os-x-audio-testing-software.html

A note, the THAT chips are adjusted, not adjusting for the pretrimmed part.

[ULH]

So we tried using Rational Acoustics Smaart (30days free trial) with our Lynx Aurora converters on our mac pro which has a THD function but we couldn't really get a useful reading (maybe because there were only two decimals in percentage). But zooming in on the high res FFT made it possible to clearly see the 2nd and 3rd harmonics. We adjusted the Quad Bus trim pots so the 2nd harm came down to approx. -106 dBFS (noisefloor around 125dbFS) for a 1 kHz at -16 dBFS and fader in 0dB position. We did this as well for the patchable VCA's.

We think this was a workable solution with clear results of the adjustments. It was difficult to hear any differences though.

You can see one of the measurements on the attached photo.

Cheers,

Uffe

[marcmozart]

Great post! Which VCA-chip is your 26 card using?

[waltzingbear]

unfortunately at those levels its hard to tell what you are seeing. Did you indeed see the levels change and a null area of either the 2nd or 3rd harmonic?

Its possible you saw nothing other than the residual of the test instruments. Do you have baseline data for the signal not going thru the board and is it significantly lower than the results thru the board?

Alan

[ULH]

Thanks Marc and Alan for your replies.

@Marc: DBX202x for both quad bus and the pacthable VCAs.

@Alan: Agreed that the levels seems very low and therefor the measurement setup might not be up to spec.
Yes baseline data straight in/out of converters didn't have the harmonics. When adjusting the symmetry the 3rd harmonics didn't change much but the 2nd could be nulled out. Even small adjustments were very visible. But when trying other fader positions than 0dB it seemed better to adjust the 2nd in balance with the 3rd for the 0db position. This gave more even distortion throughout the fader range. Hope my bad english makes sence ;-)
How do you trim the symmetry? With a special distortion meter?
I think I will send a mail to THAT corp. to ask for direction on this topic and let you all know the answer.

Cheers,
Uffe

[waltzingbear]

sounds like you were seeing things correctly. and your English is way better than my Danish!

Remember that the distortion at that point is a "collection" ie sum, of several different contributors. You are not just seeing the VCA, but the vca, following summing amp in the 202x and  the next gain stage (output buffer/balancer to patchpoint?).

In that context, nulling the 2nd harmonic is probably the best single adjustment. I wasn't quite sure what you were meaning in your description of its actions at different levels.

Alan

[ULH]

I meant that the distortion seemed lower throughout the entire VCA control voltage range if I adjusted the 2nd harmonics closer to the 3rd than nulling it out completely at unity gain VCA control voltage. And yes I measure between the pre VCA insert and post VCA insert at the patchbay for the quad bus which of cause has some opamps for the buffering/balancing. Thanks for pointing that out