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SHiFTY 13th March 2004 09:08 PM

Cathode Follower? Yay or nay?
 
2 Attachment(s)
I am building the circuit below (Angela easy 2A3), and it sounds quite good, certainly better than SRPP or simply 1/2 a 6SL7. Sounds very neutral and not too euphonic. Nice to listen to however. Bass is tight and power is up.

However there seems to be a prejudice against cathode followers, people really don't like them. Anyone tell me why?

In the below circuit, I would also like to try a 6SN7 as the driver tube (i.e. half 6SN7 per channel), how would I do this? Is it as easy as changing the 100K cathode R to a 47K?

Sch3mat1c 13th March 2004 09:43 PM

Sure, or even 22k. I'd remove the cathode bias from the 2A3, run the CF ground return to a negative (-100V or better) supply, move the coupling cap to before the CF and apply fixed bias to its grid. That way you can run a smidge of grid current, making for more graceful clipping when at full power, important with only 5W to spend.

As for people hating CF's, well, people are stupid. :devilr:

Tim

Steve Eddy 13th March 2004 10:19 PM

Re: Cathode Follower? Yay or nay?
 
Quote:

Originally posted by ShiFtY
However there seems to be a prejudice against cathode followers, people really don't like them. Anyone tell me why?
Aside from personal tastes and preferences, the argument is that cathode followers (or emitter followers or source followers for that matter) employ 100% negative feedback and of course as everyone knows, negative feedback is bad. :)

It's a bit ironic really. Those who don't care for the CF because of negative feedback tend to prefer triodes over pentodes, even though pentodes have less inherent negative feedback than triodes.

Go figure.

se

fdegrove 13th March 2004 10:56 PM

Hi,

Quote:

Aside from personal tastes and preferences, the argument is that cathode followers (or emitter followers or source followers for that matter) employ 100% negative feedback and of course as everyone knows, negative feedback is bad.
Is it?

The more advanced argument should be:

Some CFs are bad sounding, the one in the schem in post #1 being a typical example, not because of the local NFB but because it's just a lousy circuit.
See Tubecad for a more indepth explanation.

Overuse of global NFB loops is also a no-no in good audio design, local FB loops as used in well designed CFs and triodes is generally preferred.

Penthodes: well, isn't that a thermionic device with a local feedback inhibiting grid?
And what is used to linearise that entire circuit? You guessed it, overall NFB, the more the merrier.

That's just scratching the surface though, suffice it to say that believing that any amount of feedback is a cure all is a sad case of selfdelusion...

First of all try to understand how a circuit works, than see what FB can do and can't do to help it along if it can at all...

Cheers,;)

Steve Eddy 13th March 2004 11:14 PM

Quote:

Originally posted by fdegrove
Is it?
Depends on the individual it would seem. And my comment was intended sarcastically. However some do in fact wholly reject negative feedback in all its forms. Or at least those forms that they're aware of. :)

Quote:

The more advanced argument should be:

Some CFs are bad sounding, the one in the schem in post #1 being a typical example, not because of the local NFB but because it's just a lousy circuit.
See Tubecad for a more indepth explanation.

Overuse of global NFB loops is also a no-no in good audio design, local FB loops as used in well designed CFs and triodes is generally preferred.

Penthodes: well, isn't that a thermionic device with a local feedback inhibiting grid?
And what is used to linearise that entire circuit? You guessed it, overall NFB, the more the merrier.

That's just scratching the surface though, suffice it to say that believing that any amount of feedback is a cure all is a sad case of selfdelusion...

First of all try to understand how a circuit works, than see what FB can do and can't do to help it along if it can at all...

I couldn't agree more. Very well put, Frank.

se

fdegrove 13th March 2004 11:30 PM

Hi,

Quote:

And my comment was intended sarcastically.
Sure, I know you well enough for that...:D

Fortunately there other ways than FB to reduce distortion if that's the holy grail.

Unfortunately, to achieve low Zo from a tube circuit the means are rather limited; other than a good old buffer I see...Ah, xformers.
The good news is that at least those are getting better as years go by....
Not something I can wholeheartedly say about tubes even though the situation isn't all doom either.

Just another thought, some of the best tube amps I've ever heard, with the exception of OTLs, often used CF windings on the OPT and very low amounts of global FB.
This could well be a coincidence although somehow I doubt it.

Cheers,;)

P.S. Steve, I finally got a hold of Dr. WDC. I'll try to summarise what he told me in an e-mail to you asap.

Steve Eddy 14th March 2004 12:35 AM

Quote:

Originally posted by fdegrove
Sure, I know you well enough for that...:D
Jes' makin' sure. :)

Quote:

Fortunately there other ways than FB to reduce distortion if that's the holy grail.

Unfortunately, to achieve low Zo from a tube circuit the means are rather limited; other than a good old buffer I see...Ah, xformers.

True. And with transformers, rather like feedback, your lower Zo comes at the expense of signal gain. So you need a certain amount of excess gain in either case.

Quote:

The good news is that at least those are getting better as years go by....
Yes. And one nice thing about them, they tend to have a much longer in-use lifespan than tubes or solid state so we'll have them around for quite some time yet.

Quote:

Not something I can wholeheartedly say about tubes even though the situation isn't all doom either.
Tubes are your department so I'll just have to take your word on that. :)

Quote:

Just another thought, some of the best tube amps I've ever heard, with the exception of OTLs, often used CF windings on the OPT and very low amounts of global FB.
This could well be a coincidence although somehow I doubt it.

No idea.

Though it would be interesting to discover how much of the anti-feedback sentiment out there is driven purely by prejudice which has a nasty habit of coloring our subjective experiences.

Quote:

P.S. Steve, I finally got a hold of Dr. WDC. I'll try to summarise what he told me in an e-mail to you asap.
Thanks, Frank. I'll keep an eye out for it.

se

DrDeville 14th March 2004 01:06 AM

Frank said:
Quote:

Some of the best tube amps I've ever heard, with the exception of OTLs, often used CF windings on the OPT and very low amounts of global FB.
Hmmmm... :yummy:

Care to share any specific examples, especially schematics? :innocent:

Something with 50+WPC? :drool:

Best,

George Ferguson

fdegrove 14th March 2004 01:34 AM

Hi,

Quote:

Care to share any specific examples, especially schematics?
Sure, but I'm bound to forget some:

McIntosh (maybe not everyones' cupper but alot of tweek potential), same goes for the Quad II, more recently: Jadis, Audiomat....

None of the diagrams however reveal the exact amount / percentage of FB to the cathodes,~ 20% is my guess.

As for schematics, the Quad II is on the net, as is most of Frank McIntosh's work.
Audiomats' older amps I know by heart but I shouldn't divulge the schematics, same goes for the Jadis.Sorry.

Let's not forget either that the ferryman needs to be paid; what you gain in linearity has to be paid in drive voltage and gain in the preceding stage(s).

So, unless you have a competent winder as your best friend, you're still stuck...

Come to think of it, I can't think of any DIYer having such an amp on the net...

On the bright side, I vaguely recall at least one source for replacement Mac iron...
So, if you don't mind using the 6L6 family, KT88 and relatives, you should at least have one source of suitable iron for a project.

Cheers, ;)

GaryB 14th March 2004 01:49 AM

Quote:

Originally posted by fdegrove
McIntosh (maybe not everyones' cupper but alot of tweek potential)
Frank,
I recently picked up a pair of MC30 amps and have been thinking about digging into them and tweaking a bit. Care to share any of your thoughts on tweaking MACs?
Thanks,
---Gary


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