DBX 786 Schematic Request

[glorymorris]

I've checked the groupdiy account and searched all over, but I can't find DBX 786 schematics. DBX tightly controls their intellectual property, so it's no surprise. If anyone here has schematics, please let me know.

 

[JsLR]

Hello,

I´m also trying to find the dbx786 schematics or service manual. I´ve just bought one in very good shape, but I'd like to adjust it properly. I´ve also checked the documents page but no luck.

On the board there are two trimmers, one labeled "p4 meter ref.", wich I suppose is for Vu adjustment, and "p2 20HZ THD", maybe to adjust the harmonic distortion (i've tried that feeding a 1khz signal, but seems to have no impact on the measured out THD).

Any info will be highly appreciated.

Thanks in advance and best regards.

 

[mad.ax]

Hello and welcome aboard!

1KHz THD is supposed to be about 0.002%... Your measuring equipment is reliable that low?
At any rate, p2 is meant to adjust distortion at 20Hz, not 1K. this white paper seems to confirm it https://dbxpro.com/en/product_documents/786wppdf

"Most equipment that incorporates a transformer in the output stage will have a fair amount of distortion at high levels and low frequencies. The 786 uses a patented circuit in conjunction with a proprietary oversized output transformer, designed by Jensen, which overcomes these distortion problems."

So I guess you should inject 20Hz, scope the output, and see what effect p2 have on the waveform.

 

[JsLR]

Hello mad.ax and thanks for your quick response! And also greetings to everyone at the forum.

Yes indeed, I'm going to try it with a 20Hz signal. I don't know why I assumed that the adjustment was to be made with a 1khz signal, as in many of my other audio equipment, because the trimmer is labeled clearly!

Thanks a lot again, and if anyone has the schematics, just let me know!