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Please tell me your best SE driving circuit !!! Ask for your design idea.

Posted by Eric Chan on July 11, 2003 at 21:39:08

Since in June last year I've been doing a lot of R&D and different circuit experiment. However up to present I have not come up with a best or optimized drving ciruit for 2x300B PSE.

The amp is supposed to produce 20Watt/channel no matter in Fixed bias or auto bias. Of course low nose level in speaker output terminal --- arround 1~2mV. No interstage is one of the design criteria. In addition, no pentode tube used to drive 300B.

I have tried many driving topology and sticked with all kind of meaty and lush sounding tubes like 6SN7, 12BH7, 12AU7(correct ?). However most of circuit I tried lack the smoothness and meat in mid-high, leaving rather thin sounding sonic signature. I wonder if there is any driving circuit you've tried that is optimized for sonic balance and dynamic achivement.

The circuit I have tried are:

1. 2 stage 6SN7 SRPP. srpp -> cap couple -> srpp -> cap -> 2x300B
Sonic presentation: tight in mid band, overall dynamic OK.

2. common cathode 6sn7 dc couple -> common cathode 6sn7 dc couple -> 6sn7 cathode follower -> cap couple to 2x300B
Sonic presentation: tight and little harsh in mid band, overall dynamic very good.

3. 12au7 common cathode (90V plate volt 4.5mA) -> DC couple to parallel connected common cahtode 12bh7(14mA total) -> cap couple to 2x300B.
Sonic presentation: thin sounding in mid-high, weak dynamic and slow bass response.

The experiment I've come across for the above driving circuits have one charater in common ---- thin and little harsh sound in mid band. Should I look at some other area. The measurement of this amp seem OK as 3.8 % THD for full output and 0.35% at 1 watt. 75mA standing 300B current gaurantee the low 3rd HD. What have I done wrong ?

I sincerely seek for your comment and experience.

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