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General Discussions => Drawing Board => Topic started by: BluegrassDan on March 29, 2020, 02:43:26 PM

Title: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 02:43:26 PM
Howdy,

I am elated with this design. It sounds great and has great specs on paper. Just one more question.

Do I need any sort of decoupling caps on the B+ anywhere in the circuit, and if so, where?

This is powered with an external PSU and umbilical cable. My thought is what might happen when adding multiple channels in a single chassis. Will there be potential for crosstalk without decoupling caps.

Thanks in advance.

Title: Re: One last check. B+ decoupling caps necessary?
Post by: gridcurrent on March 29, 2020, 03:09:07 PM
from RDH3, chapter 4
Title: Re: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 03:27:34 PM
I have read that, but I was curious about when a third stage is a White follower. Plus the other two cathode followers.
Title: Re: One last check. B+ decoupling caps necessary?
Post by: ruffrecords on March 29, 2020, 04:43:53 PM
I would certainly decouple the first stage HT because its PSRR is poor and there is a lot of gain following it. You definitely want the quietest supply you can get for the first stage. A 10K series resistor and a 10uF decoupling cap will give you over 35dB attenuation of 100Hz. Up it to 100uF and you get over 55dB.

Cheers

Ian
Title: Re: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 05:43:08 PM
How's this look?

Title: Re: One last check. B+ decoupling caps necessary?
Post by: ruffrecords on March 29, 2020, 06:11:17 PM
How's this look?

Looks good to me. Not sure about the 7K5 cathode resistors in the two triode gain stages. What current does that make them run at?

Cheers

ian
Title: Re: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 06:15:36 PM
The RCA BC3C runs 8.2k 1W cathode resistors.
Title: Re: One last check. B+ decoupling caps necessary?
Post by: gridcurrent on March 29, 2020, 06:49:33 PM
The RCA BC3C runs 8.2k 1W cathode resistors.
yes, and un-bypassed ,  as the  preamp is wrapped in feedback.
the EIN of a BC-3c preamp  hardly stellar.
Title: Re: One last check. B+ decoupling caps necessary?
Post by: ruffrecords on March 29, 2020, 07:13:56 PM
yes, and un-bypassed ,  as the  preamp is wrapped in feedback.
the EIN of a BC-3c preamp  hardly stellar.

 But in the OP's amp the first stage cathode R is bypassed so the resistor makes no contribution to the noise performance. Looking at the data sheet it looks like quiescent current will be around 0.5mA which is probably OK.

Cheers

Ian
Title: Re: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 07:24:38 PM
EIN is not bad at all.
Title: Re: One last check. B+ decoupling caps necessary?
Post by: ruffrecords on March 29, 2020, 07:45:32 PM
EIN is not bad at all.
I am not sure how that pic shows EIN.

Cheers

Ian
Title: Re: One last check. B+ decoupling caps necessary?
Post by: BluegrassDan on March 29, 2020, 07:50:14 PM
It doesn’t, per se. But it’s one example of noise floor.