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Old 13th April 2011, 09:18 PM   #1
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Default El34 PP advice needed

Hi to All
As my new amplifier is at the stage of quotes and pricing , I decided to give it another go and "mix" a little with the old one.
Amp is based on Jolida schematic as below:
302b.pdf
PSU has been slightly improved :CLC filter ,bridge diodes paralled with caps, higher values of electrolytics , electrolytics paralled with PP caps , I used stepped lader attenuator and also added switchable PSE configuration.
I wonder if there is room for an improvement in pre- stage , phase splitter ( use different tubes , circuit , etc.) or output stage .
Any advice would be very welcome
Cheers
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Old 13th April 2011, 11:22 PM   #2
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Location: Monroe Township, NJ
The topology used by Jolida is a variation on the "classic" Mullard 5-20 theme. That's very well proven. The selection of a 12AT7 as the LTP phase splitter is excellent. I have reservations about a "totem pole" 12AX7 serving as the voltage amplifier. I'd like to see something with higher gm there. Also, put a 10M45S constant current sink (CCS) in the LTP's tail, instead of a resistor. Forced symmetry is SWEET.

BTW, watch out for excessive gain. "Gobs" of gain are not needed, when the 2 VRMS of a CD player are the driving signal.
Attached Images
File Type: jpg Mullard 5-20 Schematic.jpg (380.3 KB, 451 views)
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Eli D.
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Old 13th April 2011, 11:31 PM   #3
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Location: Adelaide South Oz
Fairly standard circuit arrangement.

EDIT: Ah!! I see Eli reponded while I was still typing - seems we both agree about a current source for the driver.

If I were building it I would make the 12AT7 driver anode loads R7 and R8 the same value = 22K and replace the R6 resistor with a current source. A simple "Ring of 2" current source would be fine. Set current source for approx. 3mA.
C1, C3, C4 need to be good quality caps, I suggest Wima Polypropylene Film/Foil.

Cheers,
Ian
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Old 14th April 2011, 04:01 PM   #4
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Would it be wrong to use just one CCS set at 6mA and -90V ?
Where can I find schematic for one?
Best Regards
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Old 14th April 2011, 06:51 PM   #5
DF96 is offline DF96  England
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There is an interesting cascade of knock-on effects in that amp. Adopting fixed bias means lower value grid leaks for the output. This means a lower impedance phase splitter is needed, hence 12AT7 instead of 12AX7. This reduces gain a little, so choosing a triode input (presumably for reduced noise) means a totem pole/SRPP can restore some of the lost gain. You can just about get away without a CCS tail for a 12AX7 PS, but 12AT7 needs one because of the lower mu.

One thing to watch when making changes to the 5-20 circuit, or derivations of it: the original had only marginal LF stability, with a subsonic peak. Changes can push you further down this road. The forward path through the amp has three LF roll-offs: R4/C1, R9/C3 (+ R21/C4), and the OPT. The original circuit had an LF shelf in the output stage cathodes, which might have helped or hindered a little, but fixed bias gets rid of this.
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Old 14th April 2011, 07:27 PM   #6
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Hi
Thanks for reply , excuse me ignorant question but I am not very familiar with CCS. As I understand one of the advanteges would be close to perfecr DC balance , what else . What does wait for me on the dark side?
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Old 14th April 2011, 07:33 PM   #7
DF96 is offline DF96  England
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A CCS tail gives perfect AC balance, so you should get cancellation of even-order distortion. It does not affect DC balance, which is set by the grid voltages and the triode matching. You may get a little DC imbalance due to the grid current for the second PS triode causing a voltage drop across R10 (in the original 5-20 circuit).
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Old 14th April 2011, 07:38 PM   #8
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Any specific CCS circuit worth to look at?
Do I need to make couple 3mA CCS's per triode or one 6mA?
Cheers
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Old 14th April 2011, 07:48 PM   #9
DF96 is offline DF96  England
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One CCS, replacing R6 the tail resistor.
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Old 14th April 2011, 09:09 PM   #10
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How to built(calculate) CCS , where should I start ?
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