Behringer A500 as a DIY project

[smaugAudio]

Originally Posted by bmwman91

I have the exact same problem. So far I have not been able to find anyone else on here that has been able to recommend a fix. When it was still under warranty I had a shop look at it, but they could not reproduce the issue. The warranty expired 5 months ago, so I have been working on it myself. My assumption is that the problem comes from bad input capacitors, and in particular C6.

Annoyingly, any time I transport the amp or open it/touch the PCB the issue disappears for anywhere from 2 days to 2 weeks. When I took it in to the electronics lab at work, it worked perfectly the whole time I had it hooked to the scope. Anyway, my guess about C6 being the culprit is because of the following observations:

When the amp was intermittently cutting out channel A, I was most easily able to remedy the issue by wiggling the TRS input and tapping on the RCA input. Wiggling the XLR input worked too, but it took a lot more effort (had to push on it harder). Looking at the PCB, C6 basically sits against the RCA and TRS input housings. If there is a small internal fault in the cap, I would assume that bumping it with a moving connector body would fix the internal open circuit & make it work again temporarily. Since the XLR connector is far from it, you have to crank on it harder to flex the PCB & move C6.

I de-soldered C6 and hooked it up to a capacitance measurement meter. No amount of abusing it was could get it to give a faulty measurement. Maybe the internal fault "relaxes" over time as the dielectric settles or something.

Then again, perhaps the fault is not in C6! This issue has been driving me nuts! It is unfortunate that one cannot get something properly built for a reasonable sum of money anymore. The A500 price is very reasonably priced, or at least it seemed to be. The component quality seems to be a joke to me! At least only 1 of my 3 A500s (tri-amped setup) is doing this nonsense (the one for the subwoofer).

I also have exactly the same problem with one channel going quiet intermittently. Did anyone try replacing C6?

[ashok]

Originally Posted by sololaptop

I have a Berhinger A500 which is set to 120v input (US model). Can somebody explain to me how to rewire it to be used in 220v input countries? Thanks.

A rather late response.
Check the transformer for additional primary wires. In the unit we have here the blue and black appear to be the primary wires. The power switch is in the blue wire circuit. Black goes straight to the transformer. Then there is an additional red wire that's been dummied.

One would need to check out that red wire. It need not be (!) but could also be a tap for another input voltage ( higher or lower !). I checked some voltages ( in a hurry ) and got some strange readings. I will need to look at it carefully before trying out 230/240V on it ! Don't want to blow up a new amp !

But this winding change was possible with a Behringer CX3400 120 V unit.
Doesn't mean the A500 is similar but could be so and will save an external step down transformer if it is possible. I'll post again if I find it works.
Cheers.

[ashok]

I did check the voltage at the red cable coming from the transformer. With a 120 V input I get about 230 V at that red cable ( currently dummied).
So I'm guessing that it must be the 230 V input cable to the transformer.

Will check the transformer secondary voltage first then cut the blue cable and connect the red and turn on 230 V into it ( using a variac !). I'm guessing it should work OK. Will post results later.

The power amp and input stage mods were not clear. I dislike electrolytic caps in the signal chain if they do not have any dc bias on them. They shouldn't work properly as the electrolyte isn't functioning normally.
Maybe the full stage needs to be changed to take smaller value film caps ! Maybe even remove some caps if possible !
The power amp by itself could be direct wired to the extra socket that was installed in one of the 'modification' posts. The amp will have a gain of about 26.7dB . Even here some mods might be required to use 1uF film caps.

Will try to check this and see if a workable solution is possible. Otherwise I am going to leave things as they are. While the amp sounds nice ,it does have less low end extension than my older amp and midrange isn't as smooth.
But all this is subjective and one needs to do a very careful comparison to ensure that small changes ( like level etc.) aren't responsible for the differences we hear.

I couldn't find any 100V 10,000uF cap in the market that is 1 inch in diameter.
Most are at least 5 mm larger and that will not fit due to the presence of other components around the original cap. 3,300uF / 8 ohms gives a F-3dB of 6Hz and with 4 ohms 12 hz. Number wise it is seems acceptable but will a larger cap actually give an 'audible' difference ?

[ashok]

So I cut the wire that goes from the power switch to the transformer blue wire. Attached the wire from the switch to the red wire from the transformer. The blue tap from the transformer is now not connected to anything. I'll be putting a shrink wrap sleeve on it to avoid shorting with anything else.

The amp works fine at 230 V input !
I did check the dc voltage of the rectified secondary. With the red tap running at 230 V the voltage is similar to the voltage I got with a 120 V supply on the blue wire. So all appears to be well. The amp is running as I write this. It's been powered up for the last 3 hours.

I looked closely at the pcb. The opamp's are below the pcb . SMD's.
Not sure I want to dismantle it to get to it to change it to an OPA2134 and/or make any other changes. Looks like a very fiddly operation ! Lots of things need to be removed.

The amp is nice but mid's are not as good as my older amp.

I guess that's all I will do to it for some time to come !
Cheers.

[audiogeekess]

All I can say is be careful with static electricity and that amp. I blew out one channel one month and then the next channel the following month, both the same way: I was full of static electricity and I touched the knob of my pre-amp attenuator and it sent the charge to the amp blowing it.

[ashok]

Thanks for the warning. At the moment the likely hood of this happening is a bit low as we have relative humidity greater than about 80 % most of the time.
But there is that occasional period when I've seen static charge on the hand when the humidity dips. Get a shock when we touch a grounded metal part !

If the preamp is grounded, it shouldn't have reached the power amp input ?