Favourite EL34 circuit

rhone · 23rd May 2010, 01:52 AM

Hi All,
I'm getting the itch to do some major re-working of my current amp, a DIY Hifi Supply Ella. It uses a pretty conventional circuit: SRPP 6922's, 6SN7 LTP , fixed bias EL34 or KT88 with some modifications. Sounds very good so I am wondering if there are better sounding circuits out there.
One that interests me is Allen Wright's PP1 that uses differential, cascoded 6922 into the EL34's.

People with much more experience than me have praised this circuit . I'm thinking I could keep the existing chassis,power supply and fixed bias output stage and slot in the input stage.
Allen doen't show any voltages. Can I calculate what the current is through the CCS in the diff amp part?
Is fixed bias an improvement? Any other circuits I could look at?

ray_moth · 23rd May 2010, 04:02 AM

Allen's all-balanced design is one way of doing it. I'm more familiar with an all-balanced three-stage approach using 6SL7 LTP splitter - 6SN7 diff. driver - EL34s. Coupling between 1st and 2nd stages is done with step networks, to enable reasonable voltages to be used. I like to use cross-coupled balanced NFB loops from final plates to first stage plates. PSRR with these all-balanced designs is really good, giving negligible hum.

Another good topology is the Mullard-style circuit. It's important to use very good smoothing for the 1st stage B+, which can be sensitive to ripple.

23rd May 2010, 01:52 AM	#1
rhone diyAudio Member Join Date: Dec 2005 Location: Melbourne	Favourite EL34 circuit Hi All, I'm getting the itch to do some major re-working of my current amp, a DIY Hifi Supply Ella. It uses a pretty conventional circuit: SRPP 6922's, 6SN7 LTP , fixed bias EL34 or KT88 with some modifications. Sounds very good so I am wondering if there are better sounding circuits out there. One that interests me is Allen Wright's PP1 that uses differential, cascoded 6922 into the EL34's. People with much more experience than me have praised this circuit . I'm thinking I could keep the existing chassis,power supply and fixed bias output stage and slot in the input stage. Allen doen't show any voltages. Can I calculate what the current is through the CCS in the diff amp part? Is fixed bias an improvement? Any other circuits I could look at?

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23rd May 2010, 04:02 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Allen's all-balanced design is one way of doing it. I'm more familiar with an all-balanced three-stage approach using 6SL7 LTP splitter - 6SN7 diff. driver - EL34s. Coupling between 1st and 2nd stages is done with step networks, to enable reasonable voltages to be used. I like to use cross-coupled balanced NFB loops from final plates to first stage plates. PSRR with these all-balanced designs is really good, giving negligible hum. Another good topology is the Mullard-style circuit. It's important to use very good smoothing for the 1st stage B+, which can be sensitive to ripple.

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