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Old 25th June 2012, 04:40 PM   #1
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Default Adding CF to SRPP to lower output impedance?

Hello,

I was thinking about some preamps I have built and they are typically a little hot on the gain. They usually have some hiss to them despite being hum free. I usually use 12AU7s, so it is not a crazy high mu tube.

Anyway, would this work? I moved the volume control to the tail end of the SRPP and added a simple cathode follower to the back end to lower the output impedance further.

Thanks for any input! I left the cap off the cathode of the SRPP because I would not think I have to worry about the SRPP output impedance since the pot and the cathode follower present a high input impedance.

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Blair
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Old 25th June 2012, 06:16 PM   #2
DF96 is offline DF96  England
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You will need a coupling cap between the volume pot and the CF grid.
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Old 25th June 2012, 06:21 PM   #3
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Hi,

Can I just move the coupling cap from in front of to behind the volume pot?

Thanks,

Blair
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Old 25th June 2012, 06:26 PM   #4
DF96 is offline DF96  England
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No because the SRPP needs an output cap too. You could arrange things so the CF gets DC bias from the SRPP, and then the volume pot needs to be floating for DC (put a cap at the ground end of the pot).

Another way to get a lower output impedance is to use a mu-follower instead of SRPP.
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Old 25th June 2012, 06:43 PM   #5
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I see SRPP and Mu Follower getting tossed around quite a bit. What are the circuit differences? They look very similar.

I have already drilled the third tube socket for the cathode follower, so I am interested in pursuing my design for now.

What value cap should go after the pot?

Thank you!

Blair
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Old 25th June 2012, 07:17 PM   #6
DF96 is offline DF96  England
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SRPP and mu-follower are very similar. The only difference is that the mu-follower separates the AC and DC connections to the upper grid so the upper cathode resistor can be bigger. This destroys the balance but reduces the output impedance. It may or may not reduce distortion.

The cap value after the pot depends on where you want to place the LF point.
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Old 25th June 2012, 07:19 PM   #7
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OK,

Thank you! I will just play with values until I get the right corner frequency. I get pretty low distortion with these SRPP values. I just want to lower the output impedance a bit.

Blair
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Old 25th June 2012, 07:20 PM   #8
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Quote:
Originally Posted by deicide67 View Post
Can I just move the coupling cap from in front of to behind the volume pot?
You can put the pot at the input instead of between SRPP and CF. From SRPP just one cap coupled to the CF, minus the pot.
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Old 25th June 2012, 07:27 PM   #9
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Hi,

Yes, that's the way I have always done it, but it leaves full gain at the output regardless of pot position, so the hiss from the gain sits on the output. I thought this might be a good way to drop noise floor.

Thanks! Blair
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Old 25th June 2012, 08:20 PM   #10
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One more question is whether or not I need to add plate resistors to the CF for channel separation since it is a single bottle?

Thanks,

Blair
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