Behringer A500 Supply Rail Voltage.

[AndyF585]

Hi,

I'm looking to increase the size of the reservoir capacitors in my A500. The ones currently installed are rated for 100V, but I've measured the voltage across them at just 55. Can anyone see any reason not to replace these with 63V devices? I've read several other threads about changing them, but all seem to replace them with 100V.

Many thanks,

Andy.

 

[Max Headroom]

For a given capacitance value, it is normal to find that higher voltage rating will have lower ESR and higher ripple current rating.
Fitting more but lower voltage rated caps should be quite fine.
That said, use the highest voltage rating that you can fit/afford.

Dan.

 

[AndyF585]

Thanks Dan, I'm getting somewhat more confident that 63V will be fine. This amp has a design I haven't seen before. The ouput transistors appear to pull down the rails thereby discharging the reservoir cap through the load. Other threads suggested the signal 'rides on top of the supply rails', which could mean that one cap might see an appreciable portion of both rails (110V). Anyway, the volt meter doesn't show anything above 55V, so I'll go for it.

I'm wanting to use 63V caps because I'm currently re-capping some very old (25yrs) power amps and I can benefit from a discount buying a box full. The 40 I need for this A500 are almost free

Thanks again,

Andy.

 

[butterwitz]

i own several a500's ive been using to drive my subwoofers. ill be following this thread to see what you do with them

 

[AndyF585]

OK, I'll let you know how it goes

The first mod was to remove the gain control pots. Fairly easy to find the instructions on the web, but basically chop the wire close to the PCB connector, join the red and white wires and connect them to the black via a 22k resistor. You'll need to do both sides. Once done, the amp can no longer run in bridge mode. The gain pots are pretty awful things and removing them makes a very audible improvement.

Next up will be to swap the main reservoir caps with 4 20x470U arrays. I'll also add a couple of poly caps to each, say 2.2 and 1U, to decouple them a bit. This will mean dismounting the pcbs and output devices from the heatsinks, so while they're off I'll swap the input caps for polys. I might also swap the rectifier diodes for some 40 amp schottkys if they don't break the budget.

I need to order the caps, so it might be a couple of weeks before I get around to sorting it. I'll try and post pic's as well.

Andy.

 

[butterwitz]

awesome

 

[Max Headroom]

Next up will be to swap the main reservoir caps with 4 20x470U arrays. I'll also add a couple of poly caps to each, say 2.2 and 1U, to decouple them a bit. This will mean dismounting the pcbs and output devices from the heatsinks, so while they're off I'll swap the input caps for polys. I might also swap the rectifier diodes for some 40 amp schottkys if they don't break the budget.
Andy.

Take a look at the Quasimodo thread....adding critical damping across power transformer secondaries to quench rectifier induced ringing.
Minimal cost and good subjective benefit.
RC networks across supply cap banks is another low cost benefit.

Dan.

 

[AndyF585]

I've had a quick scan through that thread and it looks interesting. I'll bang together the test rig and see what happens.

A cap bank across the bridge is something else I'm looking at. Not sure where I would introduce the R though.

Andy.