Same problem: A500 channel going quiet
I also have exactly the same problem with one channel going quiet intermittently. Did anyone try replacing C6?
I also have exactly the same problem with one channel going quiet intermittently. Did anyone try replacing C6?
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.
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 ?
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 ?
230 V test
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.
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.
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.
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 ?
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 ?
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Dear Brit,
Wrong about using an electronic crossover, because one can calculate the value of the required capacitor to put at the input of an amplifier to have it work as a high-pass filter for the tweeter amplifier. And by the way the RCA single ended input of the Behringer A500 amplifier is inverting and its XLR balanced input in non-inverting. It also sounds better with a two wire power cord with it polarity reversed from its factory default that's wrong.
Respectfully submitted,
George S. Louis
Wrong about using an electronic crossover, because one can calculate the value of the required capacitor to put at the input of an amplifier to have it work as a high-pass filter for the tweeter amplifier. And by the way the RCA single ended input of the Behringer A500 amplifier is inverting and its XLR balanced input in non-inverting. It also sounds better with a two wire power cord with it polarity reversed from its factory default that's wrong.
Respectfully submitted,
George S. Louis
Behringer A500 Fuses
The direction of the two fuses in the Behringer A500 is absolutely critical for the best sound. I had to reverse them in the A500 that I purchased from Guitar Center Online. I'll be trying other fuses as well. I put mortite on the fuses to damp them. The amplifier sounds better to me with its cover removed. I have the amplifier sitting on a pillow that's filled with buckwheat hulls. I tried using a single-ended RCA to XLR adaptor on its XLR balanced input but that cut the gain by approximately 6db and didn't sound very good when I raised the volume 6db.
George S. Louis
The direction of the two fuses in the Behringer A500 is absolutely critical for the best sound. I had to reverse them in the A500 that I purchased from Guitar Center Online. I'll be trying other fuses as well. I put mortite on the fuses to damp them. The amplifier sounds better to me with its cover removed. I have the amplifier sitting on a pillow that's filled with buckwheat hulls. I tried using a single-ended RCA to XLR adaptor on its XLR balanced input but that cut the gain by approximately 6db and didn't sound very good when I raised the volume 6db.
George S. Louis
Hello. I just stumbled on this thread thread and thought I could point you at some of my thoughts and work with this A500 amp.
Amp/Preamp Asylum
Objective Device Testing - Anything Electronic or Electro-machanical - Page 4
Hopefully some of you can find this usefull.
Amp/Preamp Asylum
Objective Device Testing - Anything Electronic or Electro-machanical - Page 4
Hopefully some of you can find this usefull.
Without having read it all - have you tried addressing the problem of high distortion depending on input level setting? There's a demo on Youtube where you can clearly hear it, it's at about 0.5% or so 3rd in places - nothing minor for sure. I presume it's an input impedance distortion problem, as it would result from feeding a bipolar input pair running at highish tail current directly from a highish-value pot. A better-behaved buffer stage (balanced, I presume) may be required to eliminate it.
Hi i have seen the same video.
He says that with the front volume pots at maximum there is no distortion
But i do not understand how it is possible.
If the amp stage gives distortion for a certain Vin the same will happen if the same Vin will be provided by a preamp before it.
I mean ... if the amp distorts with a 0.3Vin if with pots at maximum a preamp will send 0.3Vin the amp will distort at the same way.
Very very strange. Anybody can please explain ?
Another question for who knows weel this amp ... better unbalanced or balanced connections ?
I guess that after the balanced connection there must be a circuit that makes the signal unbalanced and then it goes to the pot
Am i right ?
Thanks a lot, gino
P.S. I have ordered one ...
He says that with the front volume pots at maximum there is no distortion
But i do not understand how it is possible.
If the amp stage gives distortion for a certain Vin the same will happen if the same Vin will be provided by a preamp before it.
I mean ... if the amp distorts with a 0.3Vin if with pots at maximum a preamp will send 0.3Vin the amp will distort at the same way.
Very very strange. Anybody can please explain ?
Another question for who knows weel this amp ... better unbalanced or balanced connections ?
I guess that after the balanced connection there must be a circuit that makes the signal unbalanced and then it goes to the pot
Am i right ?
Thanks a lot, gino
P.S. I have ordered one ...
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The source impedance presented by a volume pot is highest at -6 dB, at which point it comes out to Rpot/4.Hi i have seen the same video.
He says that with the front volume pots at maximum there is no distortion
But i do not understand how it is possible.
At maximum volume, source impedance is pretty much dominated by what's in front of the pot, and usually much lower. Likewise, a very low volume setting also presents a low source impedance.
Input impedance nonlinearity would have to be quite significant and pot value quite high for the effect to be as pronounced as this, but with an input LTP run at a mA or more and a pot in the 10s of kOhms I would not consider it impossible.
I do not remember whether he adjusted source volume to keep the same output volume. That would obviously tax whatever circuitry there is in front of the volume pot more at lower settings. I don't think he did though...
The best way of solving this mystery would probably be tracing out the input circuitry. Maybe it's something else entirely.
Thanks a lot for the very interesting explanation.
Main problem is that Behringer is not willing to provide the schematic
I have already asked. This is a real pity.
Maybe i am wrong but this is a fundamentally nice power amp.
With the right mods can sound quite fine.
As the pots are single i think that they are placed after the opamp connected to the balanced inputs.
The easiest solution should be to bypass completely the pots
Someone told me that it is a little noisy and with too much gain.
But that the sound is nice. I should receive it soon.
Thanks a lot again, gino
There are mods on the net that do away completely with the front panel (pots are replaced by fixed resistors, no VU meter).
I modded this amp (replacing op-amps, adding capacitors @ op-amp power supply, ...) beware that Behriner glues and solders IC's!! Using a desoldering hot-air gun works best to remove components as this also melts the glue.
Hi and thanks for the reply
Actually i was thinking to put a new pair of RCAs and connect them directly to the amp inputs bypassing the volume pots.
I will use it only unbalanced of course.
Morevoer ... do you think that reducing the amp gain a bit could cause instability ?
kind regards, gino
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