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Old 15th May 2010, 03:04 PM   #1421
AndrewT is online now AndrewT  Scotland
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Unhappy oops

Today I reached forward to adjust the volume of the stereo and spark.
A static discharge from my body to the metal knob of the switched attenuator made a sharp "crack" coming from the speakers.
I adjusted the volume down and on one channel the woofer cone jumps almost end stop to end stop.

If I turn down the source signal, I can hear that the same channel has gone noisy, about 1mVac to 2mVac of substantially white noise (from both woofer and tweeter.

Checked the DC at the amp. OK. The DC blocker at the input is doing it's job.
Checked the DC at the B1 output. Oh dear, 9mVdc wavering to 10mVdc.

Turned the source back on and music plays pretty normally from both channels and hides the white noise.

It appears that the B1 jFETs did not like that static discharge and have suffered some damage.

I don't know yet whether it's the Source Follower or the CCS that has gone noisy. Maybe both.

Any ideas on how to prevent this happening again on the DCB1?
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Old 15th May 2010, 03:23 PM   #1422
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Ground your attenuator knob to a chassis I think is the answer...otherwise discharge yourself somewhere else
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Old 15th May 2010, 03:30 PM   #1423
Salas is offline Salas  Greece
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The LF pumping makes me suspicious of firstly, the Vref Jfet, secondly the error amp's active collector load Jfet. Oh that devilish spark...
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Old 15th May 2010, 03:37 PM   #1424
AndrewT is online now AndrewT  Scotland
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the attenuator switch is soldered straight into the attenuator PCB. There is only a signal ground on the PCB.

I suppose I could try to find a plastic knob but that is not ideal and will it provide sufficient isolation?
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Old 15th May 2010, 03:42 PM   #1425
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by Salas View Post
The LF pumping makes me suspicious of firstly, the Vref Jfet, secondly the error amp's active collector load Jfet. Oh that devilish spark...
the good channel has no DC offset. I doubt the pumpimg can be regulator connected, since both B1s run from the same reg.
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Old 15th May 2010, 03:45 PM   #1426
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Is the knob connected to the signal ground you are telling you have on the PCB? If you, you could try to tie it through a ground breaker with just the diode bridge:
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
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Old 15th May 2010, 03:46 PM   #1427
Salas is offline Salas  Greece
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You could jam a wire between the threaded neck and body of the attenuator, and connect it to chassis? Is the chassis grounded to main's earth?
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Old 15th May 2010, 03:48 PM   #1428
Salas is offline Salas  Greece
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Quote:
Originally Posted by AndrewT View Post
the good channel has no DC offset. I doubt the pumpimg can be regulator connected, since both B1s run from the same reg.
Since its only in one channel, then the suspicion firstly turns to the audio jfets naturally.
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Old 15th May 2010, 03:57 PM   #1429
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by Salas View Post
You could jam a wire between the threaded neck and body of the attenuator, and connect it to chassis? Is the chassis grounded to main's earth?
chassis is not connected to Safety Earth.

Using the nut & thread might allow a separate wire feed to ground but it would have to be to signal ground.
The same arrangement is going to be used in the Mezmerize, as a dedicated pre-amp with remote control. Ah! don't fit any knobs on the outside. Totally remote control. Nah.
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Old 15th May 2010, 04:25 PM   #1430
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What I have is a pot for my future lightspeed which has the shaft made of plastic. My knob is made of aluminium, but it is isolated from the case and the pot.
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