My amp is motorboating, improve it with b+ regulator ? - Page 3 - diyAudio
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Old 13th May 2012, 04:26 PM   #21
DF96 is offline DF96  England
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I'm puzzled. How would adding a few low value caps to the PSU affect subsonic oscillation? Where is R18 - I can't find it. Disconnecting the feedback will often work, but in this case the OP has already tried that.
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Old 13th May 2012, 05:20 PM   #22
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because the inherent noise that voltage regulators form on their ground. r18 is B+ ground and chassis ground which can cause common mode noise to develop at the input of the unbalanced I/O circuit.

with tubes removed and power applied, look at the heater 12V and 0V with the oscilloscope connected to signal ground. what you should see is a strait line. If the heaters were AC, it should be a clean sine wave. with the tubes plugged in and high voltage B+ and bias diodes removed, there should be a strait line when measuring the cathode, with the heater power only applied to the tubes.
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Old 13th May 2012, 05:26 PM   #23
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also, electrically isolate the regulators from the chassis ( using mica insulator heat sink goop and nylon hardware )
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Old 13th May 2012, 07:23 PM   #24
DF96 is offline DF96  England
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The regulators may or may not be noisy, but the problem is motorboating (low frequency oscillation) not noise. Can we stick to the problem?
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Old 13th May 2012, 07:35 PM   #25
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Quote:
Originally Posted by Chris Hornbeck View Post
Also, what's the value, and purpose, of R13? Seems to degrade supply impedance.
I can't remember the exact value, maybe 1k. i basically set it to reduce the brightness on the Led.
maybe i could use the zener string to regulate the B+, and get the voltage for the led from somewhere else.


@Davesnothere : Sorry R18 doesnt exist. thats leftover from the original circuit. i have only one connection from signal earth to chassis earth, and thats R1 on the preampcircuit.
the regs are not connected to the chassis at all, the only semconductors connected to the chassis are the regs (7812) and the ccs for the driver stage.
They have this silicon thingy inbetween and i used nylon as well.

seeing that my amp has nylons already maybe it just needs a bit of latex and it will be fixed......

ahm if anything sounds weird, feel free to ask , i am not a native speaker, well, not english that is.

Last edited by yurgs; 13th May 2012 at 07:50 PM.
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Old 13th May 2012, 07:43 PM   #26
DF96 is offline DF96  England
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Remove R13. If the LED is too bright then put a resistor in parallel with it. R13 may be the cause of your problem as it provides a feedback path between the first two stages.
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Old 13th May 2012, 07:51 PM   #27
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Quote:
Originally Posted by DF96 View Post
Remove R13. If the LED is too bright then put a resistor in parallel with it. R13 may be the cause of your problem as it provides a feedback path between the first two stages.
Thanks, i will try that next, or maybe just take it out of the loop for now
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Old 13th May 2012, 11:07 PM   #28
MelB is offline MelB  Canada
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Did you decrease C23 yet?
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Old 14th May 2012, 05:31 PM   #29
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Originally Posted by MelB View Post
Did you decrease C23 yet?
Hello Mel, thanks for the reminder.
i haven't, mostly because they are soldered in and a bit of a pain to remove where i put them, and i remember calculating them with f= 1Hz,
rk' =rcathode//rk = 682Ohm
Ck=1/(6.28*f*rk')= 273uF
i chose next size i had, 470uF (seeing that there are lots of designs with 1000uF),
and that gives me f=0.5Hz... so ithought that whould be ok.
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Old 14th May 2012, 07:00 PM   #30
DF96 is offline DF96  England
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Within a feedback loop it all gets more complicated than that.
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