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Old 15th May 2012, 05:26 PM   #1
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Default Thomann t amp S-150

Hi folks,

Just wanted to share this, in case anyone needs a schematic for the S-150 t-amp. Thomann don't seem to release this info.

After spiking the input of one channel accidentally, I found that gain had dropped considerably and anything at voice frequencies or lower had dropped completely (i.e. no bass at all).
This makes your speaker sound as though you've just killed a woofer (which I thought I had done in this case! Luckily, not).

Usually, power amps go or don't go (the latter resulting in some dead component/s). This was a weird one and isn't a problem I'd had to fix before, but I knew it had to be a bypass/coupling capacitor.

After reassuring myself that my speaker was not dead, I started to draw out the schematic for the amp, studied it for a while and eyed up the suspect capacitor - yep, a bypass cap on the driver bias circuit - C110. Don't let them fool you - this tested ok on a meter but still failed in-circuit!

I've neglected to label a couple of diodes as I overlooked them and the cover is back on the case now.

I have hand-drawn the rest of the op-amp inputs and some circuitry for bridged/parallel operation, but I'll post the output stages here. If anyone wants a scan of the rest, send me a PM.

Hope that is helpful, and I've seen quite a few posts of people complaining of the same amp symptoms. Hopefully google will lead them here.

Since small caps are inexpensive, I'd advise doing a blanket replacement of all signal level electrolytics, if, as in this case, the amp is old enough to warrant it.

btw, I'm using two of these amps, bridged for my home system and they perform really well.
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Old 16th May 2012, 09:48 PM   #2
lohk is offline lohk  Europe
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Thank you for the circuit drawing - we have several amps in the storage, already replaced by others. They tend to change the quality by time (obviously after heavy use) - this should be possible to cure.
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