• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need tube buffer circuit for sub-woofer out

Can anyone recommend a single stage circuit for *buffering* a signal for a sub-woofer output?

Currently building a S.E. 2A3 amp using one gain stage using the 6SL7.

I'm devoting the second half of the 6SL7 as a buffer stage for a sub-woofer output.

Need inversion so I can't use a cathode follower which is non-inverting.

Don't really need any gain. I don't think so anyway.

Thinking about a 'conertina' or split load stage to where I just take the signal off the plate with no gain.

If I set it up as a more typical gain stage, then I guess I'll need a resistive divider of sorts to bring the signal back down to 'line level'.

Probably will need a RC filter of sorts to capacitively couple it to the sub-woofer.

Any suggestions?

The concertina idea seems to have some promise, but high Rp/low gm types, like the 6SL7, are not so good for concertina service. If you're building a stereoblock, you could use a single 6SL7 for voltage gain in both channels and a single 6SN7 for the concertina in both channels.

Instead of a concertina, you could wire a 6SL7 section as a so called plate follower. A "plate follower" is a single stage tubed opamp that uses plate to grid NFB to produce an inverting block with little or no gain. The NFB lowers the O/P impedance, which is good for driving the SW.


2002-01-07 6:02 pm
Gilbert said:
Can anyone recommend a single stage circuit for *buffering* a signal for a sub-woofer output?


Any suggestions?
Use an opamp (with a decent one, it won't make a scat of sonic difference) and parallel the 6SL7 stages to drive the 2A3.
Personally I'd ditch the 6SL7 to drive the 2A3, but that's a 'nother whole discussion........
Single 6SL7 has been used for 2A3 just fine with no loss of highs.

Use shunt feedback to reduce gain. Cathodyne will have non-existant output admittance it'll suck so much.

(Admittance is opposite of impedance, I just thought I'd drag that word out because it's so annoying to have to invert other parts of the sentence when referring to inverse terms!)