Why is this a buffer?

[aslee]

Hello,

Found a schematic on a tube buffer with 12AU7. It seems that the design is different from other buffers, as the output is not from cathode.

Could anybody explain. Thanks!

Regards,
Francis

 

[Mooly]

Being non cathode bypassed it could be a form of anode follower. There is polarity inversion of the output signal. Quite what the overall gain might be would I think depend on the mu of the valve.

 

[aslee]

Thanks for your reply.
Just wonder how to due with the polarity inversion. Should I reverse the polarity of speaker output?

 

[Mooly]

Yes, that would correct any phase inversion but you should try doing that anyway to prove (or disprove) that absolute phase even matters to you.

 

[Davey]

That doesn't look like a buffer to me. I think you'll get a decent amount of gain from that circuit. At least times 4 or so.

Dave.

 

[kodabmx]

This is a grounded cathode amplifier. By the looks of it it will give a signal gain of 10-14.

A buffer is made from a cathode follower 99 percent of the time.

By adding a cap to the input (22nf), rb = 1k, rk = 47k taking the output from out2 through a 1uf cap you will turn the circuit into a less than unity buffer. I would also lower the grid stopper from 22k to about 300R.

 

[piano3]

It's rather hard to see what the circuit was actually designed for with only moderate gain and fairly high output resistance.

 

[kodabmx]

It's rather hard to see what the circuit was actually designed for with only moderate gain and fairly high output resistance.

It looks like the front end of a power amplifier, the output would feed a phase splitter?

 

[piano3]

But it's already push-pull

 

[kodabmx]

But it's already push-pull

It's two single ended grounded cathode amps in the same drawing, nothing push pull about it.

 

[DF96]

Buffers should have gain=1 (or thereabouts), high input impedance and low output impedance. This circuit has just one of those properties, therefore it is not a buffer.

It looks more like one of those circuits you find on a general circuit website, with a description something like "here is a useful/universal amplifier". It is useful in the sense that it has some gain; it is universal in the sense that it would be equally unsuitable for most amplification purposes.

 

[piano3]

Kodabmx, yes, it's just my habit of never duplicating channels in my own circuit diagrams that made me see it that way.

 

[aslee]

I looked up the website that publish the circuit again. It was mentioned as "one tube gain stage". So it was my mistake. Apologise for the confusion.
Anyway, thanks for the input!