XR 684+ distortion

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Longshots
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XR 684+ distortion

Post by Longshots » Thu Jan 19, 2023 5:25 pm

My XR 684+ distorts any signal sent to it, instrument or mic. No clipping, at all levels this occurs. It is silent when no signal is present. Any idea what could cause this? I have tried different speakers and tried it at another location and no change.

N0YYM
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Re: XR 684+ distortion

Post by N0YYM » Tue Jan 24, 2023 11:56 pm

I'm not familiar with your equipment, but it could be the reverb/efx bus causing the distortion perhaps?
I hope this helps,
Johnny

Bartman
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Re: XR 684+ distortion

Post by Bartman » Wed Jan 25, 2023 9:28 am

I had an amp that did that one time, and it was a bad biasing diode. I replaced the diode and the distortion went away. The particular sound of the distortion was a slight buzzing that was the same frequency as whatever was going through. The biasing diode prevents the output transistors from completely shutting off at the zero crossing point as the signal goes from positive to negative (think of a sine wave with a +1 to -1 voltage range, and every time it crosses the zero point, there was a distortion sound.)

I believe your amp has an input jack on the front that goes straight to the amplifier(s), bypassing all the input channels? Try sending a signal there (be sure to use a device that you can control the volume sent to it since it will be wide-open to the amplifier). This will determine whether the distortion is happening at the power amp circuit or if originating from the inputs or efx area or other.

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dak
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Re: XR 684+ distortion

Post by dak » Wed Jan 25, 2023 11:32 am

Bartman wrote:
Wed Jan 25, 2023 9:28 am
I had an amp that did that one time, and it was a bad biasing diode. I replaced the diode and the distortion went away. The particular sound of the distortion was a slight buzzing that was the same frequency as whatever was going through. The biasing diode prevents the output transistors from completely shutting off at the zero crossing point as the signal goes from positive to negative (think of a sine wave with a +1 to -1 voltage range, and every time it crosses the zero point, there was a distortion sound.)
Don't know about the XR684+ in particular, but the power amp circuits from Peavey I have seen so far were AB class or very narrowly biased B class. If such a diode has a short-circuit, the amp moves into B class (!). I have a 100W B class amp that has no biasing diodes at all, and it does not really deliver audible crossover distortion (it doesn't use an integrated op-amp for driving the output stage but a discrete differential current balance), so the crossover involves an 1.8V shift (1 Darlington and 1 Sziklai pair) across the driver transistor base capacitance.

If you get crossover distortion, I suspect that the op-amp produces output swing when having to jump the crossover gap (with a capacitive load) due to a shorted biasing diode. Alternatively, the diode drops too much voltage which would be sort of unusual. Of course, if it's entirely open, only half the power stage will work, more or less resulting in a half-rectified output. That would cause tremendous distortion as well as significant DC offset whenever a signal is present.

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