API 325 vs Auditronics a30 output card - methods of gain control?

[plumsolly]

I am using Auditronics a30 cards for buffered, transformer-balanced direct outs on my console. Stock, they are made to provide 20-30db of gain. I wanted something closer unity gain, because it provided more sensible gain-staging in my console, so I jumpered r7? (the one in the feedback loop in series with the trimpot) and then turned the trimpots down to get 100% feedback/unity gain. This seemed to work fine - I haven't had any problems with oscillations that I have noticed. But I was looking at the 325 schematic, and saw that they accomplish gain control in a different way. I was wondering what the benefits or drawbacks might be to doing it either way. Thanks, Ben

 

[plumsolly]

maybe this should have gone in the drawing board...

 

[emrr]

This thread addresses your question better than I can rephrase:

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=330#p3722

 

[plumsolly]

Thanks Doug - I didn't think about the gain pot going to ground on the 325 - but input lo and ground are the same in this instance? So I am thinking now about the feedback cap: with feedback r on the order of an ohm or so, the corner frequency of the low-pass created by the feedback cap and feedback r is off of the charts. Maybe that doesn't matter if the opamp isn't oscillating, but, to be honest, I don't know it isn't for sure - I don't have scope to check if its oscillating out of the audio spectrum. I calculated the corner frequency in the 325 to be around 75k, so maybe what I'll do is make rfeedback 15k so I don't have to change cfeedback for a 75k corner freq. Then I'll make rg 15k and have a gain of 6db which should be tolerable. What do you think?

Thanks, Ben

 

[electroslut]

I have been dealing with kinda the same thing with adm 2668's and 2780's and noticed they do sound a little darker and "dead" at lower
gain settings, so much that clients have asked for output pads because they liked the sound much better at higher gain settings and
wondered if the way api does it would solve this problem, I think so. post your findings!

 

[plumsolly]

Does the value of Rg affect the pole frequency of the filter made by Cf?

Thanks, Ben

 

[EvLoutonian]

Post your findings!  :~)

Just interested to hear where you chaps all got to with this question here in the end..

I'm debating whether to complete an old Auditronics 110-8 (24 ch/8 buss "Grandson II") console that I have here, using the Auditronics style output cards (with the Jensen JE-123-S transformers etc), or change / upgrade the output stages - particularly seeing as that I don't have the actual original A30 output cards as such, to slide straight in this desk, so some mods would be required here anyway..  (I have a dozen of the similar output stages from a later "532" series Auditronics console, where the whole output stage is integrated into each of the "532 OA" (Output Assign) modules' boards.

I think the circuit on these is probably essentially (or exactly) the same as the A30 modules, though.
ie.  Buffer amp with 5534's, and the Jensen output transformer.
..mmm, actually, looking at my 532 boards a bit closer, there is a bunch more circuitry..  but I'm sure I could use those boards in a different context one way or another.