John Curl's Blowtorch preamplifier

[Bob Cordell]

Originally posted by PMA
comp diff HAS lower THD, I showed it here a year ago and it can be seen on my web page.

I don't believe this to be true, if each is designed with equal care and expertise. I beleive this applies whether or not negative feedback is used.

I guess we're back to the subject of the Unipolar vs Complementary thread.

Could you please elaborate briefly on the particular reason why you assert that the complemntary has lower distortion?

BTW, it is most certainly the case that the complementary pair approach can incur GREATER distortion if the usual pair of Miller capacitors is returned back to the input of each of the upper and lower VAS stages. This is due to VAS fighting. The individual Miller feedback capacitors provide shunt feedback, which lowers output impedance. We thus end up with the upper and lower VAS stages as a pair of low-outpt-impedance stages connected in parallel - bad design practice - and they will fight, creating distortion.

They will fight to the extent that the output voltage they wanted to produce is not equal. This inequality will occur if the miller feedback capacitors are mismatched by as little as 5%. It will also occur if the transconductance of the P input pair and the N input pair do not match. This is a special problem for N/P channel JFET complementary pairs.

It is notable that John's JC-1 DOES NOT use the usual pair of Miller capacitors back to the VAS inputs, and so appears not to be subject to this problem.

BTW, I'll be up at FSI in Montreal tomorrow through Sunday. Anybody else going to be there?

Cheers,
Bob

[Edmond Stuart]

And the remedy is ?..... a CMCL !

[john curl]

I was accused of 'verbal diarrhea' 50 years ago by a high school teacher. Close enough?

[stinius]

Originally posted by john curl
I was accused of 'verbal diarrhea' 50 years ago by a high school teacher. Close enough?

verbal diarrhea:

A serious disease which, once it has control of a person, causes them to spew forth incoherent babble from the bowels of the voicebox. Often extremely frustrating for the victim and extremely hilarious for the observer

And about when were you cured from this disease?

[john curl]

Especially when people do not have the prerequisites to understand what is being discussed.

[ostripper]

By bob cordell - it is most certainly the case that the complementary pair approach can incur GREATER distortion if the usual pair of Miller capacitors is returned back to the input of each of the upper and lower VAS stages. This is due to VAS fighting.

Mr. Cordell , would that "vas fighting effect" also hold true
with unipolar/comp. "hybrids" like the APT or symasym ??

OS

[Edmond Stuart]

Originally posted by john curl
Especially when people do not have the prerequisites to understand what is being discussed.

How much pot will be needed to understand that?

[john curl]

Don't smoke the stuff, so I wouldn't know.

[Jon Lord]

By bob cordell - it is most certainly the case that the complementary pair approach can incur GREATER distortion if the usual pair of Miller capacitors is returned back to the input of each of the upper and lower VAS stages. This is due to VAS fighting.

Bingo !

Thanks god, that someone with more "prerequisites" and as famous as Mr. Curl attends again this discussion !

[anatech]

Hi John,
Here! You're skipping there!

On your complimentary differential input preference...

In a perfect world, yes, it may have lower distortion, and it will have lower DC offset for a given beta and tail current.

Now for the depressing part .....

In every production amplifier I have seen designed with complimentary differential input, the best I have seen was each polarity with a reasonable match. Of course we now have some DC offset on the order of a single differential. This because complimentary differential input pairs tend to be run at higher tail currents because the offsets will mostly cancel. Measured distortion is not remarkable, being about the same (both sets close) or worse than a single matched pair (normally).

The average case ...

Normally I find none of the input transistors match. This doesn't sound very good for one, and it sure measures worse. Completely expected, no surprises here. After matching these parts by hand (a very time consuming process), both the distortion and DC offset are reduced considerably. DC offsets normally run less than 5 mV after this. The audible improvement is noticeable. Of course, no way are production lines going to halt and wait for weeks before enough matched sets are gathered to begin the run. No, they will stick in what they have received from the distributors. The 2SA798 and 2SC1583 dual transistors were an attempt to make this work reasonably in production environments. I find that over time, these even drift apart in beta.

In closing ....

As far as I'm concerned, complimentary differential inputs only work in hand built products. In a normal mass market product, they are a no-flier and everyone is better off with single matched pairs. Servo or no. They are more expensive to service if done correctly due to the extra time required to match 2 pairs rather than 1 pair.

I'm not disagreeing with you John. Just illustrating the real world after some years have passed, or mass production examples.

-Chris