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Old 1st February 2013, 05:51 AM   #1211
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Any dc on the output of the preamp will change bias point of input Jfets, as they will see it as an additional voltage on their gate, and bias is ultimately determined by Vgs.. Add cap on outputof b1 or input of f5t and see what happens.
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Old 1st February 2013, 05:56 AM   #1212
kroyin is offline kroyin  India
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Quote:
Originally Posted by buzzforb View Post
Any dc on the output of the preamp will change bias point of input Jfets, as they will see it as an additional voltage on their gate, and bias is ultimately determined by Vgs.. Add cap on outputof b1 or input of f5t and see what happens.
Thanks Buzz, but as i said with the preamp off (no DC) it still gives me -80mV.

I think that this is related to grounding in F5T (used the grounding scheme as suggested in F5T article-- that is using a 4 prong chassis mount diode and CL60). I must have done something wrong. I'll try the old scheme (no diode just the CL-60 in between the PSU ground and chassis ground) in the other channel.

But then again for the sake of testing, i connected the alpeh-j input (switched off) with F5T input (switched on) get 8mV F5T offset.
Connected DBC1 output (switched off) with F5T input (switched on) get -80mV F5T offset. This points that the F5T grounding is ok

Why cant i have easier issues (like mosfets blowing) :-D

Last edited by kroyin; 1st February 2013 at 06:02 AM.
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Old 1st February 2013, 06:07 AM   #1213
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I don't know. When setting bias, do you short the input? Is the amp oscillating?
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Last edited by buzzforb; 1st February 2013 at 06:13 AM.
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Old 1st February 2013, 07:26 AM   #1214
kroyin is offline kroyin  India
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I don't know. When setting bias, do you short the input? Is the amp oscillating?
Well i clip the AGND & IN wires with a alligator clip while biasing.

Cant tell about oscillation (no scope) but there is no unnecessary heating

Last edited by kroyin; 1st February 2013 at 07:31 AM.
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Old 1st February 2013, 09:07 AM   #1215
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Don't know. Something is changing DC condition slightly. Double check all joints on connectors and inputs. See If you DMMnset on AC picks anything up when it is hooked up to the pre.
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Old 1st February 2013, 09:52 AM   #1216
AndrewT is offline AndrewT  Scotland
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why do you need to be told again and yet still insist it is right?
You have mixed up Vac V rms, Vpk and Vpp
Should I need to repeat what I posted?
BigE could see it
Quote:
...................
1) 29 V P-P should just read 29V peak.
..................
But AudioS is so full of himself, he could not be bothered to actually read what he had typed and take on board the advice re confusion of voltage terms/units.
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Old 1st February 2013, 09:45 PM   #1217
anilva is offline anilva  India
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Originally Posted by kroyin View Post
Well i clip the AGND & IN wires with a alligator clip while biasing.

Cant tell about oscillation (no scope) but there is no unnecessary heating
Is there any audible hum at the speakers. If there is no hum and just 80mv offset, one should not worry. I hope you adjusted the bias and offset after reaching full temperature on the heatsinks.
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Old 2nd February 2013, 05:14 AM   #1218
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that is much normal there is no input caps on the F5 to take care of the remaining ofsett. the alephs have this.
10times ofsett jump is normal. on the other hand. 80mV is not anything to loose any sleep over.

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Originally Posted by kroyin View Post
I completed 1 monoblock of the V2 turbo.

Bias (with output open, input shorted) is 360 mV and 8 mV offset.

I have one observation though

--When i plug in the DCB1 preamp (-1.2 mV offset), even with the preamp off i get a offset of -80mV from F5T. Goes back to 8mV when i unplug the input.

My Aleph J does'nt do that , with or without the preamp connected it gives me 25 mV offset.
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Old 2nd February 2013, 05:22 AM   #1219
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andrew. i hope you dont wright to me. then you are waising your time. you are on my ignore list now.
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Old 2nd February 2013, 05:50 AM   #1220
oiphy is offline oiphy  Norway
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Quote:
Originally Posted by AndrewT View Post
Should I need to repeat what I posted?
BigE could see it
But AudioS is so full of himself, he could not be bothered to actually read what he had typed and take on board the advice re confusion of voltage terms/units.
Hello Andrew.

could you please specify what is wrong. I don't see it
then again... I'm only here to learn so.......
set up the right formula for us, would you?

thanks
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