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Old 19th January 2009, 06:51 PM   #1
Puffin is offline Puffin  United Kingdom
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Default DC Blocking on Pre-Amp

I have copied this from the SOAP thread as it might grab a wider audience.

Can any of you geniuses out there (a few regular posters spring to mind) give me a definitive answer as to whether this board (SOAP) has any DC blocking on it.

I had a problem with mine at the weekend and when testing/playing with it, it blew an old speaker! A strange noise and then....phut!





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Old 20th January 2009, 01:13 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
provide a link to a schematic
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Old 20th January 2009, 01:33 PM   #3
Puffin is offline Puffin  United Kingdom
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Andrew. I did try to get one from the SOAP thread, but it does not seem to want to work. Zei who made the boards says there is DC blocking on the output. If you look at the last few posts on that thread you will see that the problem may have been the way I had it wired to the pot.
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Old 20th January 2009, 07:37 PM   #4
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Quote:
Originally posted by AndrewT
Hi,
provide a link to a schematic

Puffin, what would you do without me?
Attached Files
File Type: zip soap_mkiii_psu_sch.zip (52.3 KB, 49 views)
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Old 20th January 2009, 07:43 PM   #5
Puffin is offline Puffin  United Kingdom
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Why thank you. I have tried wiring the SOAP the other way as suggested by Zei and it is still playing up!
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Old 20th January 2009, 09:22 PM   #6
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Default Re: DC Blocking on Pre-Amp

Quote:
Originally posted by Puffin
give me a definitive answer as to whether this board (SOAP) has any DC blocking on it.
Yes, it has DC blocking at the non-inverting input and at the output. That is, if you built it according to the schematic Tripmaster attached.

DC is not the only reason, why a speaker would blow. Did you actually measure DC at the SOAP's output?

Your power amp obviously has no DC protection of its own. Did you measure, if the DC is coming from the power amp?

Could it be that the voltage regulators offset the ground, because they are slightly disadjusted? Then you could indeed get DC at the output through the 100k resistors.

Was the attenuator full open, and the signal too strong for the speaker, because the SOAP adds a gain of two?
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Old 21st January 2009, 08:06 AM   #7
Puffin is offline Puffin  United Kingdom
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pacificblue. I will do as you suggest and report back.
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Old 22nd January 2009, 05:42 PM   #8
mjf is offline mjf  Austria
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hello.
perhaps oscillations? (wiring,missing decoupling cap,......).
greetings.............
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Old 22nd January 2009, 08:08 PM   #9
Puffin is offline Puffin  United Kingdom
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mjf. I haven't had time to go over the board yet, but you may be right.
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