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Solid State Talk all about solid state amplification.

is this a coupling or bypass cap??
is this a coupling or bypass cap??
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Old 18th September 2020, 07:14 PM   #1
ga77a is offline ga77a  United Kingdom
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Default is this a coupling or bypass cap??

hi again everyone, another noob question. is this a coupling or bypass capacitor ? and would changing the value improve the sound ?
this is a rotel model RB-850

regards
gaz
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Old 18th September 2020, 07:18 PM   #2
rayma is online now rayma  United States
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This capacitor is in the negative feedback network.
At high frequencies, the amplifier gain is the ratio of the two resistors, R637/R611 + 1.
At low frequencies, the gain reduces due to the capacitor, until it approaches 0dB (x1) at DC.
This is a common approach in power amplifiers to avoid large output DC voltage offset.
It would be best to leave the capacitance value as it is.

Last edited by rayma; 18th September 2020 at 07:24 PM.
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Old 18th September 2020, 07:26 PM   #3
ga77a is offline ga77a  United Kingdom
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thank you for the reply rayma
so changing the value of this cap will not do anything beneficial ?
and so are these 2 resistors and the cap used like a sort of voltage divider ?


gaz
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Old 18th September 2020, 07:28 PM   #4
MarcelvdG is offline MarcelvdG  Netherlands
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It's a capacitor that is part of the feedback network. It reduces the DC gain to unity to limit the DC offset at the amplifier output. It also gives some low-frequency roll-off below a frequency of 1/(2 pi R611 (C605 + C607)) ~= 3.3862 Hz (assuming C607's value is in uF, which I'm sure it is). You can shift that corner frequency by changing C607 if you like. If C607 is a tantalum capacitor, changing it into an aluminium electrolytic capacitor (preferably a bipolar one) may slightly reduce distortion.

Edit: I see rayma was quicker in replying.
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Old 18th September 2020, 07:29 PM   #5
ga77a is offline ga77a  United Kingdom
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many thanks rayma

regards
gaz
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Old 18th September 2020, 07:32 PM   #6
ga77a is offline ga77a  United Kingdom
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thanks MarcelvdG
i believe it is an aluminium cap, and i think i'll just replace it with the same value.

many thanks
gaz
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Old 18th September 2020, 07:33 PM   #7
MarcelvdG is offline MarcelvdG  Netherlands
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R637, R611 and C605+C607 are indeed a voltage divider. The gain of the amplifier is ideally (if the open-loop gain were infinite) equal to the attenuation of the feedback divider. So when the feedback voltage divider attenuates 22.3 times, the amplifier amplifies almost 22.3 times (almost, because the open-loop gain is never really infinite).
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Old 18th September 2020, 07:44 PM   #8
ga77a is offline ga77a  United Kingdom
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thanks for cluing me up. i thought i had seen that configuration somewhere before.
how did you arrive at 22.3 times ?
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Old 18th September 2020, 07:48 PM   #9
MarcelvdG is offline MarcelvdG  Netherlands
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It's the reciprocal of R611/(R637 + R611) rounded to three digits. I don't like rounding numbers, but it was just an example anyway.

By the way, there is also a voltage divider R601, C601, R603, C603 at the input that reduces the gain a bit, I didn't take that into account.

Last edited by MarcelvdG; 18th September 2020 at 07:51 PM.
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Old 18th September 2020, 07:52 PM   #10
ga77a is offline ga77a  United Kingdom
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i should have studied harder at school !!! cheers for the advise

gaz
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