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Old 23rd May 2010, 01:52 AM   #1
rhone is offline rhone  Australia
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Default 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.
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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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Old 23rd May 2010, 04:02 AM   #2
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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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