I have an Ashdown bass amp and when played at low volumes you cna hear what sounds like crossover distortion where the power transistors are not biased correctly. There is no trimmer pot on the power amp board. How can I sort this out?
Schematic attached.
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Schematic attached.
An externally hosted image should be here but it was not working when we last tested it.
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That description also fits a rubbing voice coil on a speaker. Does the amp still sound this way through a different speaker cab?
If it is the amp, you want to verify what the problem is before changing the bias circuit. You need to scope it to see if it is crossover and not some other distortion. You need to verify the power supplies are both up to the same voltage and free of ripple. You also would want to check the signal leaving the preamp to see that it is clean.
Bias would be adjusted my making the R25/R17 variable. I see the note "back of board" by R25, which leads me to wonder if it was a factory add-on to tweak bias already.
If it is the amp, you want to verify what the problem is before changing the bias circuit. You need to scope it to see if it is crossover and not some other distortion. You need to verify the power supplies are both up to the same voltage and free of ripple. You also would want to check the signal leaving the preamp to see that it is clean.
Bias would be adjusted my making the R25/R17 variable. I see the note "back of board" by R25, which leads me to wonder if it was a factory add-on to tweak bias already.
Thanks for your reply Enzo. I already tried a different cab, that was my initial thought that the speaker was no good. I will check the power rails and put a scope on various points tonight and report back.
I checked the underside of the PCb for bad solder joints yesterday and I don't remember seeing any resistors on the underside, but it was late so I will check again.
One other thing TR15 that says "On Heatsink" was actually sitting about 3mm away from the heatsink there was some heat transfer paste between the transistor and heatsink but it would not have an ideal transfer path.
I checked the underside of the PCb for bad solder joints yesterday and I don't remember seeing any resistors on the underside, but it was late so I will check again.
One other thing TR15 that says "On Heatsink" was actually sitting about 3mm away from the heatsink there was some heat transfer paste between the transistor and heatsink but it would not have an ideal transfer path.
Checked psu rails and it all appears to be fine +- 61V. and +-15 on the preamp board is fine. I have an old valve oscilloscope with incredibly bad pots so scope measurements are difficult but the signal coming out of the preamp section looks like a nice clean sinewave.
I connected the DI out and the Tuner/Line out to a mxing desk and the signal is clean. You can see some anomalies on the attached scope traces.
Sorry about the poor quality, I will try to get access to a decent scope this weekend
[img=http://s11.postimage.org/osnsacxsj/20130307_214907.jpg]
image ru
[img=http://s9.postimage.org/iqnlgsjv3/20130307_214832.jpg]
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I connected the DI out and the Tuner/Line out to a mxing desk and the signal is clean. You can see some anomalies on the attached scope traces.
Sorry about the poor quality, I will try to get access to a decent scope this weekend
[img=http://s11.postimage.org/osnsacxsj/20130307_214907.jpg]
image ru
[img=http://s9.postimage.org/iqnlgsjv3/20130307_214832.jpg]
upload pic
Hi, did you get to the bottom of this? Mine is doing exactly the same. The distortion isn't particularly noticeable once you turn the volume up, but I only use it for home practice and it's really bugging me.
If the solution is replacing components, I'm ok with "replace R7/C4/TR12" type instructions but don't know what I'm looking at otherwise and I'm certainly not going anywhere near it to measure components whilst it's live!
Any help would be appreciated.
If the solution is replacing components, I'm ok with "replace R7/C4/TR12" type instructions but don't know what I'm looking at otherwise and I'm certainly not going anywhere near it to measure components whilst it's live!
Any help would be appreciated.
Agree and add: you might place a 10Kpot in its place, set it to 10K value (max), **carefully** lower it while monitoring voltage across , say, R20 until you get some 10mV across it.
If you used a regular pot hanging from wires, turn amp off, desolder pot carefully without turning it, measure its resistance off-circuit, and get closest standard value resistor , then you solder it in the place marked R25.
If you used a preset pot and mounted it well , leave it there if you wish.
That's what the Factory *should* have used to begin with.
If you used a regular pot hanging from wires, turn amp off, desolder pot carefully without turning it, measure its resistance off-circuit, and get closest standard value resistor , then you solder it in the place marked R25.
If you used a preset pot and mounted it well , leave it there if you wish.
That's what the Factory *should* have used to begin with.
I presume they don't for two reasons:
1) It's cheaper to fit a resistor than a preset.
2) It stops customer 'twiddling' it and blowing the amp.
Personally I've never designed an amp without a preset for bias, and always as part of a Vbe multiplier.
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