QSC RMX matched filter caps required?

[tedmich]

Hi all.

I got an RMX850 for about $150 and, as is my habit, I took it apart and cleaned it out and did their suggested mod for "church quiet" to slow down the fan until it heats up (R17 120ohm/2w-> 300ohm/2w), also putting in a very quiet fan from DigiKey ( 259-1506-ND , $4.50!) with a similar flow rating.

I noticed there are 2 empty snap-in filter caps perhaps that get populated in the larger 1450 and wondered if anything is to be gained by popping in 2 more identical filter caps (Panasonic ECO-S1KP472CA I believe)?

In theory it would have more current available for bass notes BUT on closer reading I hear that these amps have unregulated PS! This means the filter caps MUST be closely matched or the rails can become asymmetric (?!). Does this mean when replacing the filter caps in older amps, you have to
a) match the caps very carefully
b) rebias the whole amp
c) both

?

Do most other pro amps require matched filter caps and/or rebias when adding/replacing filter caps? Will the amps likely clip if caps are added to "stiffen" bass response (a popular mod)?

[jrfrond]

I can't say that we've ever had an issue with filter caps need to be matched on those, nor does QSC specify this in any service docs or tech bulletins, so I think you are just fine. The only issue with adding extra capacitance might be popping a fuse due to the higher inrush current when the caps charge.

BTW- you DO know that this is a flying rail amp, don't you? The power transistor outputs are grounded and the filter caps are grounded through the speaker load.

[tedmich]

yes I am aware of their design, check this QSC Engineer discussion
QSC Audio Forum • View topic - RMX 850

Q:would +/-73VDC rail be a bad thing versus +/-67VDC?? More output?

the posting also seems to suggest capacitor (ESR) matching is pretty important and that re-biasing does not help rail symmetry without matched caps

also see Thanh's 4th post here:
QSC Audio Forum • View topic - RMX 2450 POWER SUPPLY IMBALANCE

ESR has to match!

maybe getting same lot good Mfg. caps takes care of this, as the 2450 poster subbed 1/2 the PS caps with Illinois, and the originals are (I think) Panasonics

[Jazz P Bass]

Never heard of matched caps.
Please do not confuse the QSC posts.
The first one had a slight imbalance.
Next step: check for assymetrical clipping.
These amps do have a "slight" clip on the positive rail, before the negative.
It is no biggy.
The second post, the guy had blown caps.
The amp was not checked out. (ie: what blew the caps)
Mr; Thahn Nguyen is doing his best to walk the guy through.
There are obviously issues with the amp.
As to the RMX 850, channel A draws a slightly higher current on the one rail.
(sadly, I do not recall which one)
If it is low, that is why.
Again, no biggy.

[Enzo]

Little to be gained by slamming more caps in there. And as to keeping them matched, we have enough to worry about already. To be concerned that the rails will become assymetric with different caps, is to worry that at some low freq the amp is not quite up to the task now, and that the extra capacitance would allow some increase in performance. Not likely. Push the low end peak down 4 more Hz? In other words, any increased range would be outside the published range. Like having my Fender Twin all of a sudden good down to 40Hz.

[jrfrond]

The RMX series aren't rocket-science. Class AB flying-rail design. If it will make you feel better to match caps, then go right ahead. Nothing to lose, but really nothing to be gained either.