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Old 5th August 2014, 06:41 PM   #14391
acumos is offline acumos  Italy
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Quote:
Originally Posted by 6L6 View Post
acumos - are you sure P2 is turned all the way down? Counter-clockwise does not ensure it being at minimum resistance, all versions of this PCB have one of the bias pots "backwards"

With power off, adjust pot for minimum resistance measured across R3 and R4
yes 1 ohm
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Old 24th August 2014, 08:24 PM   #14392
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by EUVL View Post
> would it be possible to reduce gain of F5 to say 5db or better to unity-gain?

I had this idea around July last year, and discussed with Nelson.
The worry was (in)stability.
We even did some simulations which shows stability should be OK.
did anybody try a unity gain F5 (and compare with F4) ?
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Old 24th August 2014, 08:36 PM   #14393
6L6 is offline 6L6  United States
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If you can live with unity gain, build the F4.

Trust me on this point...
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Old 25th August 2014, 08:56 AM   #14394
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bigun View Post
did anybody try a unity gain F5 (and compare with F4) ?
with +0dB of closed loop gain, there will be an extra 16dB of feedback.
That will probably require major work in determining new/additional stability compensation components.
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Old 25th August 2014, 10:31 AM   #14395
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hello
have built a pair of F5Turbo monoblocks
after months of struggle they´re up and running with choke input psu
bias is 350mA across 1R
32vdc from psu and offset is cruising around +/- 3-4 mv
sink temp is below 50C
yesterday I thought I´d try to replace R2=47K input resistor with a grid choke and the funniest thing happens....the offset goes bananas above 1v and continues so during the warmup period
but the swings rapidly narrows in and finally settles when approaching target bias
will try again tonight, but does anyone have a clue what happens?
the grid choke is 8000H and 1,8mA
not sure about dcr, but probably very low
all input is of interest...
best
Leif
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Old 25th August 2014, 10:35 AM   #14396
AndrewT is offline AndrewT  Scotland
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could be motorboating.

LF oscillation
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Old 25th August 2014, 10:38 AM   #14397
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ok
curable?
anybody else tried grid choke in a similar position?


best
Leif
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Old 20th September 2014, 03:07 PM   #14398
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There was a schematic of f5 amp with 2sk1530 ones with lower voltage.I am searching but could not find it !
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Old 20th September 2014, 03:17 PM   #14399
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And it was without protection transistors.
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Old 20th September 2014, 03:28 PM   #14400
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I found it but want to make an unbalanced one.Euvl does not guarantee to work it
I want to run 4 ohm speakers with higher class a power.This is why i want to have an amp with lower voltage and higher bias.Normal f5 amp does not give high class a power to 4 ohms as much as it gives to 8 ohms
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