True. The key is it should be a non-polarized one and probably isn't. 2x the power supply rail of the OP amp. 50 V is a good number. Usually the higher the voltage, the better the reliability. The lower the voltage the better the ESR (Effective Series Resistance)
Neither matter much in that area. If it does turn out to be that cap, you might replace the other one.
Neither matter much in that area. If it does turn out to be that cap, you might replace the other one.
Use a polarized one to test. Check the other channel for a band or a marking for polarity.
I've seen a lot of amps that used polarized caps there and they fail eventually. Typically I saw 1 uf electrolytic caps when they should have been metalized polyester which is NP.
What really should go in there is a 10 uf 63 V metalized polyester cap. Digikey has them for about $4.50 each. I did't think they made them that high.
I've seen a lot of amps that used polarized caps there and they fail eventually. Typically I saw 1 uf electrolytic caps when they should have been metalized polyester which is NP.
What really should go in there is a 10 uf 63 V metalized polyester cap. Digikey has them for about $4.50 each. I did't think they made them that high.
OK. I'll start with the polarized test and work from there.
I can get metalized polyester from elfa.se:
10u/63V is about $3. But it's nearly an inch long, so it probably won't fit.
Best regards,
/Bo
I can get metalized polyester from elfa.se:
An externally hosted image should be here but it was not working when we last tested it.
10u/63V is about $3. But it's nearly an inch long, so it probably won't fit.
Best regards,
/Bo
I dismounted Channel A completely from the cooling fins. I will now be able to reach all solder points on this channel.
Where do you suggest I start? I have never done any troubleshooting before (apart from my LYNX amp, which blew up a couple of times). I have scope, signal generator, multimeter and schematics.
The problem again: As I turn volume up, noise is immediately present at the output. When volume knob reaches a certain point, output increases by itself, clipping occurs (visible at the led on the PCB as well as the indicator on the front), the circuit resets, then starts again. It's a sort of slow oscillation.
Any hint is obviously extremely welcome!
Best regards,
/Bo
Where do you suggest I start? I have never done any troubleshooting before (apart from my LYNX amp, which blew up a couple of times). I have scope, signal generator, multimeter and schematics.
The problem again: As I turn volume up, noise is immediately present at the output. When volume knob reaches a certain point, output increases by itself, clipping occurs (visible at the led on the PCB as well as the indicator on the front), the circuit resets, then starts again. It's a sort of slow oscillation.
Any hint is obviously extremely welcome!
Best regards,
/Bo
Update:
Before Christmas I got some help from a Swedish forum, faktiskt.se. Together with two other guys I found out that the OPAMP on the input stage was faulty. The easy fix would have been to replace the faulty one and be happy.
Instead I found that you can easily bypass the entire input stage and volume knobs, by directing the input signal to the middle pin of the volume knob connector head. I drilled a hole for a new rca-plug on the back panel and routed this to the middle pin and ground.
This mod turns the A500 into a no-nonsense power amplifier, without "input level matching" and volume controls. I only have to solve a ground loop humming problem, but I think this is the way to go.
Some pictures:
I'll be back with more pictures of the complete solution once it's finished.
Best regards,
/Bo
Before Christmas I got some help from a Swedish forum, faktiskt.se. Together with two other guys I found out that the OPAMP on the input stage was faulty. The easy fix would have been to replace the faulty one and be happy.
Instead I found that you can easily bypass the entire input stage and volume knobs, by directing the input signal to the middle pin of the volume knob connector head. I drilled a hole for a new rca-plug on the back panel and routed this to the middle pin and ground.
This mod turns the A500 into a no-nonsense power amplifier, without "input level matching" and volume controls. I only have to solve a ground loop humming problem, but I think this is the way to go.
Some pictures:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I'll be back with more pictures of the complete solution once it's finished.
Best regards,
/Bo
Hi All,
My guitar teacher brought his A500 over for me to look at, as his A500 has one normal channel (2 or Right) and one channel (1 or Left) that is faint and doesn't appear (by listening) to be distorting but with no bass. Both Gain Pots are at max.
Although I have been visiting here over the years, I might need a little hand holding. So can anyone make suggestions as where to look. My only Test Equipment is a nice DMM (Agilent U1242A).
I checked all the mechanical things like the jacks, etc. and that all seems intact.
Also any cheap and easy mods that improve it would be appreciated too.
Thanks!
Regards//KP
My guitar teacher brought his A500 over for me to look at, as his A500 has one normal channel (2 or Right) and one channel (1 or Left) that is faint and doesn't appear (by listening) to be distorting but with no bass. Both Gain Pots are at max.
Although I have been visiting here over the years, I might need a little hand holding. So can anyone make suggestions as where to look. My only Test Equipment is a nice DMM (Agilent U1242A).
I checked all the mechanical things like the jacks, etc. and that all seems intact.
Also any cheap and easy mods that improve it would be appreciated too.
Thanks!
Regards//KP
How did you go with your mod??
Judging by the circuit diagram, if you direct the signal to the center pin on the volume control, you are still going through one of the Op amps (IC2)..
If you want to bypass the entire input side, you need to inject signal at junction of C19 & R65.. I'm just waiting for confirmation if this will work form another thread here (http://www.diyaudio.com/forums/solid-state/77959-behringer-a500-diy-project.html)
I stuffed up the op amp replacement and I' wanting to do the same as you..
Cheers..
Hi there,
I also have just completed this mod..
Instead, I actually removed all components from the input stage and inject signal directly to pin 6 of IC2 & IC3 (see my diagram) using the volume socket (X6 & X14).. Because I removed all input I was able to use the vacted hole on the connector panel to screw in a new RCA socket.
I kept a 20K resistor across pin 6 and ground, I'm not sure if I actually needed to? but I thought I would I just keep it as the 4K7 resistors shown in the circuit diagram aren't actually present..
First impression is that the amp now seems 'alive' and more bright. More detail between instruments..
But the amp is not as quiet as it once was!! There is quite a noticable hissing noise from the speakers when no sound is playing.. With sound on low you can still here hissing, but as you turn up the volume on the pre amp you cant here it anymore..
Any suggestions on how hissing has been introduced?
Any solutions to test and eliminate this hissing??
Cheers,
Murphy
That's great, and it is exactly what I did myself, although I didn't bother to desolder all of the surrounding components.
For clarity: I have added an extra RCA-input per channel. The signal goes to the middle pin of the volume socket, X6-2, shield goes to ground, X6-3. No 20K resistor is used in my case (volume knobs completely removed). I had a humming problem when I used both channels, but since I just use it in bridged mode for now, I'm not bothered by it.
I added a 10K-resistor between the new rca-shield and signal. I also added a 1K-resistor in series with the input signal:
Another mod is the addition of a proper soft start:
Regards,
/Bo
For clarity: I have added an extra RCA-input per channel. The signal goes to the middle pin of the volume socket, X6-2, shield goes to ground, X6-3. No 20K resistor is used in my case (volume knobs completely removed). I had a humming problem when I used both channels, but since I just use it in bridged mode for now, I'm not bothered by it.
I added a 10K-resistor between the new rca-shield and signal. I also added a 1K-resistor in series with the input signal:
An externally hosted image should be here but it was not working when we last tested it.
Another mod is the addition of a proper soft start:
An externally hosted image should be here but it was not working when we last tested it.
Regards,
/Bo
Hi Bomellberg,
Thanks for getting back to this thread..
Yes, I have the exact same problem!! In stereo mode I get a loud buzzing from the speakers, but in bridged mono it's dead quiet!! I too use them in mono.. I injected the signal the same way as you, and I actually removed all unused components and connectors too.. I have noticed improved sound by replacing stock op-amp with opa2134's.. Also, I put a 20K resistor across pin2 and pin3 of volume input (just to keep the same value as stock unit when the volume is at max)
On one of my A500's I had an interesting issue.. After my initial testing when I mounted heatsink back into case and screwed eveything together, i get a bad distortion, but if i separate the heat sinks from the amp chasis and leave the amp spread out (but still wired up) it sounds great!! So one of my amps I'm using it with case open and heat sinks separated from chasis,, weird??
btw, your attachments are not working??
Cheers..
Murphy
Hi
I made the modification like you ( removing input, inject X6-2, 10K-resistor between rca-shield and signal), but I have to add a 20k resistor in series with the input signal, else I have a powerfull ground loop (I thinks it is).
Shield is direct connect to case for ground, nothing connect in X6-3 or 1.
This resistor can alter hight frequencies ?
Even with this resistor, still have a little noise when no input sound.
I'm in bridged mod, I keep the volume B because if I remove it's look like I loose power of channel B, when you are in bridged mono, what do you do for Volume B ?
Thanks, and sorry for my English
I made the modification like you ( removing input, inject X6-2, 10K-resistor between rca-shield and signal), but I have to add a 20k resistor in series with the input signal, else I have a powerfull ground loop (I thinks it is).
Shield is direct connect to case for ground, nothing connect in X6-3 or 1.
This resistor can alter hight frequencies ?
Even with this resistor, still have a little noise when no input sound.
I'm in bridged mod, I keep the volume B because if I remove it's look like I loose power of channel B, when you are in bridged mono, what do you do for Volume B ?
Thanks, and sorry for my English
Behringer A500
Hi
Just bought a A500 and wanted to use it as an amp for my Disub Woofer, so i need 2 channels, not bridged, i did a few mods, got rid of the pots, better in and output connectors, when i use one channel at the time, so i unplug the powersupply to one of the 2 boards, all is well, no hum no noise, good quality sound, as soon as i connect both boards, there is serious distortion at relatively low level on both channels, does anyone know a solution to this problem?
thankes for help,
Hi
Just bought a A500 and wanted to use it as an amp for my Disub Woofer, so i need 2 channels, not bridged, i did a few mods, got rid of the pots, better in and output connectors, when i use one channel at the time, so i unplug the powersupply to one of the 2 boards, all is well, no hum no noise, good quality sound, as soon as i connect both boards, there is serious distortion at relatively low level on both channels, does anyone know a solution to this problem?
thankes for help,
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