Behringer A500 power amp problem, please help?

[MurphyUHF10]

Originally Posted by bomellberg

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

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 (Behringer A500 as a DIY project)

I stuffed up the op amp replacement and I' wanting to do the same as you..

Cheers..

[MurphyUHF10]

Originally Posted by MurphyUHF10

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 (Behringer A500 as a DIY project)

I stuffed up the op amp replacement and I'm wanting to do the same as you..

Cheers..

Bump for someone on the opposite side of the world..

[MurphyUHF10]

Originally Posted by bomellberg

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

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.

Input_Bypassed.JPG

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

[bomellberg]

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

[MurphyUHF10]

Originally Posted by bomellberg

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

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)

rca_input.jpg

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