Behringer A500 as a DIY project

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SSassen

SSassen

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Got the schematics, thanks very much! Strange topology on the Behringer A500, I was expecting a classic Miller approach, maybe with a folded cascode or some other refinements, but this is dissapointing.

Cheers,

Sander.
 

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AKN

AKN

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Hi,

According to the schematic you don't alter op-amp gain by turning the pot, its just an attenuation.

The feedback loop enters the op-amp non inverting input. This is because the speaker output is already inverted relative op-amp output. Inverting twice would result in positive feedback not negative.

Think very different with this amp.
 
djk

djk

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Joined 2001
The - input of the opamp is not the summing node,
it is not inverting in the sense that you mean. Amplifier gain
is set by R10/R61 at about 32dB.

It would probably be a good idea to have a resistor for R67, if the pot goes open during rotation
the opamp would loose input bias and latch to the rail.
I would suggest about 100K for R67. C11 should have a film bypass too.

I would like to see 'some' resistance on the input to pin 6. If the pot wasn't there that would be R66.
With the pot wide open and a bad ground loop, it would likely ruin IC2B (I've had it happen).

Unless there is more circuitry than what is shown here, the 'clip' LED is more of a distortion alert.
On musical transients the LED will not be visibly bright until it has been on about 40mS or so (40mS
is about what the pesistance of vision is). Audibility of distortion may be on the order of %10 if of less
than 1mS duration, at 40mS the threshold is about 0.3%, so this circuit is more of a distortion
alert than a clip circuit (although on steady-state signals it will be accurate).
 
AKN

AKN

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Ok, that simplified schematic is incomplete. You must include the second part (R10) in the feedback divider (R10-R61). This divider determines the gain; op-amp (-) to output.

Here's how nodes will move from a positive voltage change at input.
 

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megajocke

megajocke

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Joined 2003
Xoc1 said:
On Youtube there is a video that shows increased distortion using the A500 volume control at 60-70%
YouTube - ebaystars's Channel
I thought this might be of interest to A500 users out there!:D
Click to expand...

Not surprising considering the connection of C53. Note that this capacitor does not exist in the QSC schematic and thus it should not have the distortion problem:
http://www.qsc.com/support/library/schems/Discontinued/Series One/1200.pdf

The QSC uses another opamp and a different value of the capacitor in parallell with the feedback resistor however.
 
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