Behringer A500 as a DIY project

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Here you go with 2 others schematics


TYVM sir, your schematic post will help me greatly. I have an A500 on my bench now, with Ch. 2 dropping into protection. I tested a bit over 100 mv DC on that output, kinda thinking that's why it only happens intermittently.

Can someone tell me what VR1/VR3 and VR4/VR6 do in the output stage? VR2 and VR5 appear to be the bias pots, and I think they have to do with DC output balance, but 2 for each side is a bit strange to me.

The install has 2 identical amps. The other sounds fine, but the meter bridge is dead in Ch. 2. I really liked these amps when I put them in, but both having issues after 4 years of light-medium duty use (funeral home PA) makes me sad.
 
Hi all,

I just performed the pot modification bypassing the pots on the front with 22 k resistors as described in post #65.

I got a lot of distorsion ( > 50 % ) from both channels!

The strangest thing was that unplugging one input cord ( anyone of them ) made the distorsion go away!!

As I can remember the same happened the first day I used the amp when I tried to set both pots to max. Backing down 2 clicks on any of the pots cured the problem! How is this for amp design? I have since then used max level minus 2 clicks on both channels but tonight doing the pot mod the problem was back!

I managed to solve the problem by soldering in a simple voltage divider instead of the 22k resistor as per post #65. The voltage divider consisting of 1,8k + 18k between red ( in ) and black ( ground ) connecting white ( wiper/out ) between the resistors.

147268d1258592394t-behringer-a500-diy-project-behringer_a500_single_channel.png


Please consider the values preliminary as this was result of "first try". I´m just so pleased to get this "rats *** amp" working properly again...
 
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Has anyone experienced problems with A500 channels randomly going quiet?

I've got 8 (yes, eight!) A500s running my cinema room and ChA on one of them goes quiet every so often. Its deffo the amp as I've swapped amps around and it follows that particular amp. The only way to cure it is to wiggle the XLR or jack. I've tried both thinking it might be a duff XLR connector but the symptoms persist. I've had it in bits, gone over all the solder joints with a magnifying glass and an iron, cleaned and reseated all the connections, no joy. :confused:

Also has anyone done distortion measurements with the OPA2134 opamp mod? I think I've got a tube of SOIC package OPAs somewhere :)

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).
 
Hello All,

I also modded my A500 with OPA2134 and got good results, i did add a bypass capacitor of 470nF to the +/- supply of each op-amp. I also added some extra low ESR capacitors (10.000uF + 6800uF + 330uF + 220nF per side) after the bridge rectifier and replaced the rectifier diodes with Schottkey 20TT100 devices.

I also had the same problem with the XLR input (CH1), but the RCA input has no problems, I couldn't pin point the faulty component either. So I replaced the XLR connector, the opamp used for balanced to unbalanced conversion and re-soldered all other components used in this conversion with some solder flux and a (soldering) heat gun set to 400C. It now works fine again..

Hope it helps!
 
I bought one of these and received it today in the mail. I like it so far and have made one mod so far: removed the entire lcd panel inside--by disconnecting all 4 sets of wires to it; I hope this is okay--if anything, shouldn't this help the sound quality? Is it possible what I did could make it sound worse?

The next mod I want to do is to add a stepped ladder stereo attenuator, replacing the two potentiometers. Something interesting about this amp is that the left pot resistance measures 19.6 and the right one measures 18.7--this explains why I have to bump up the left volume a click or two to get it in balance. I am kind of surprised there is so much difference between the two--makes me believe they are pretty low quality.

Isn't the input impedance of the amp 22k? Why are the pots 19.6 and 18.7? (Unless my multimeter is off--have no clue how good of a meter I have.)

I can get a 20k stepped ladder attenuator for $12 on ebay plus like $5 shipping--from China. I have no clue how good they are but they claim they are very good. Or I can get a big--HUGE--one where I solder on resisters myself.

I am thinking about, for the time being, just making a voltage divider with two resisters and have the amp at a fixed level. What should be the ideal sum of the resisters? 20k? 22k? (I think I read somewhere the amp is 22k input impedance). Assuming it is 22k, say I got 2 x 11k resisters, would that work?--assuming I'd want 50% gain. Do I simply SUM the two resister values?

EDIT: Also, would the ideal source have an output impedance of 22k? I need a usb dac since the source will always be my computer. Perhaps there is a USB DAC that has an output impedance of 22k.
 
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For each channel, I am going to unplug the board connector to each potentiometer; then I'll remove the potentiometer wires and potentiometers completely. This will eliminate both the connection point and wiper blade contact. I will then solder, directly onto each amp board, a voltage divider giving the amplifier max current.

The input impedance of the amp is 22k, but the potentiometers used in the amp, I assume are supposed to be 20k (but they measure 18.7k and 19.6k).

So I wanted a voltage divider of : (0, 20k). I also want a good brand and decided to go with Holco H8. But I couldn't find a 20k from the supplier, so I got 19.5k. So the voltage divider is going to be (0, 19.5k)--I hope this is okay.

I really don't much about electronics, but learning more every day.

Someone was mentioning that they were having distortion problems with a voltage divider of (0, 22k) and they ended up going with (1.8k, 18k). But I am thinking the problem is that the impedance of the source gets added in to the 22k, which is a slight bit too much impedance for the amp. So hopefully (0, 19.5k) will work well--it just about matches one of the potentiometers in the amp now.

I'll let ya know how it sounds after I'm done :) Suggestions very welcome as I don't really know what I am doing lol.

EDIT: Other mods I am doing to this amp are: to completely remove the LCD and knobs. I'll throw away the plastic face plate and buy a chunk of 1/2" thick curly maple and drill 4 holes through it--at the corners--and screw it on the amp, (using the existing 4 screw holes that plastic face plate used). I'll sand, stain and do a nice varnish job on it. It will have one power button with an indicator light. I haven't decided on the power switch or light -- recommendations very welcome; thanks.
 
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Ok, I just now finished replacing the potentiometers with fixed voltage divider (0, 20k)--i.e. max gain.

The 20k resister is 1/4 watt metal film 1% tolerance, that I bought from my local electronics supplier. It's not HOLCO but it still sounds great--it's black perfectly cylindrical.

I feel like crying out of happiness because it has VERY MUCH opened up. Virtually every instrument is distinguishable now and separated with air between them. This is the best audio I've personally ever heard, so far; (It's playing into a pair of Infinity RS-2001 monitors with my onboard motherboard dac as source: Gigabyte MB with ALC 888).

I can't wait to try out some nice outboard dacs. I have an spdif dac on order and will be getting it tomorrow: fiio d3. I am going to start by feeding it spdif from the toslink spdif connector on my motherboard. I'll get a USB to SPDIF converter if I need to.
 
so is this what you did?

Ok, I just now finished replacing the potentiometers with fixed voltage divider (0, 20k)--i.e. max gain.

The 20k resister is 1/4 watt metal film 1% tolerance, that I bought from my local electronics supplier. It's not HOLCO but it still sounds great--it's black perfectly cylindrical.

My amp arrives today and I'd like to do the same... is this pretty much all you did? Was the 20k in the R2 position?
 

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My amp arrives today and I'd like to do the same... is this pretty much all you did? Was the 20k in the R2 position?

Yes R1 = 0k and R2 = 20k. Btw, I use the RCA unbalanced connection bypassing the balanced converter circuitry. I think someone that used balanced connection had a problem with R1 being 0--so he went with 1.8k for R1 and 18k for R2.
 
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Hi,

I wanted to add, that for bridge mode the volume pot for channel 2 - according to the manufacturer - must be set to gain off.

In that case - if I understood it correctly - R1 = 20k, R2 = 0K was the desired setting.

If that is correct, I would recommend to replace the current mode switch with a multi way switch, that will additionally switch the resistor on channel 2. Would be most elegant.

Or disable the mode switch. Otherwise, if the modded amp will be sold, the new owner might wonder why bridge mode does not work.

Please correct me if I am wrong,

regards Tom
 
TYVM sir, your schematic post will help me greatly. I have an A500 on my bench now, with Ch. 2 dropping into protection. I tested a bit over 100 mv DC on that output, kinda thinking that's why it only happens intermittently.

Can someone tell me what VR1/VR3 and VR4/VR6 do in the output stage? VR2 and VR5 appear to be the bias pots, and I think they have to do with DC output balance, but 2 for each side is a bit strange to me.

The install has 2 identical amps. The other sounds fine, but the meter bridge is dead in Ch. 2. I really liked these amps when I put them in, but both having issues after 4 years of light-medium duty use (funeral home PA) makes me sad.

Any answer for this?
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.