John Curl's Blowtorch preamplifier

[john curl]

My JC-1 power amp could be added as a buffer. That might drive the feedback loop properly.

 

[CG]

The feedback network values are in kOhm, not Ohms,
otherwise you wouldn't get 64dB of gain

It is so surprising ... it can be done with a calculator, easily enough, or even pencil and paper.

Perhaps the 0.94 uF caps aren't really 0.94 uF then. They're each about 17 Ohms at 10 KHz.

 

[PMA]

Is it UG stable? /JC-1/

 

[PH104]

Take 797 signal from pin 5 into a JC-1, think how many stages of amplification you can get rid off.

 

[syn08]

The feedback network values are in kOhm, not Ohms,
otherwise you wouldn't get 64dB of gain

It is so surprising ... it can be done with a calculator, easily enough, or even pencil and paper.

Talking to me? See the entire schematic in the GPP thread for which those measurements apply. We are talking here only about the input stage that has a gain of about 71.

 

[john curl]

Better get an AD811 in there, SYN08. But then Scott has allegedly made SUPER IC OP AMP that slews current at any demand, with perfect linearity.

 

[CG]

I've used the AD797 effectively in a phono stage. Using an output buffer (like an LT-1010) inside the feedback loop works pretty well. The distortion cancellation mechanism works in that case when the cap is connected to the LT1010 output.

That preamp may not be up to Vendetta or P-5xe standards, but with the right regulation, decent parts, and a phono cartridge output > 1 mV, it's pretty good. No parts sorting required.

 

[PMA]

(like an LT-1010)

Yeah. There is a very good reason to use just this one.

 

[john curl]

Properly used I think that the AD797 is one of the best IC op amps in the business. I use them for the input stage of my ST analyzer (5K load) and in my new Parasound phono stage (800 ohm load, worse case). But 3 ohms?

 

[dimitri]

 

[CG]

Then please put the right value 35.7 Ohm, I'm not sure about other values now

I'm really confused right now, and can't really offer any comment. It looks like my earlier question was as labeled: silly.

 

[PMA]

BTW John, how about Halo P 7 ?

 

[CG]

Yeah. There is a very good reason to use just this one.

With the bias resistor set to give symmetrical slew rate, it's really pretty good.

 

[john curl]

PMA, I had nothing to do with the P7 and I don't even have a schematic. How about that?

 

[CG]

Wow, CG, you hit the JACKPOT!

What do I win?

 

[john curl]

Our applause!

 

[PMA]

I thought you designed it (P7); it seems to be very well accepted.

 

[john curl]

No.

 

[CG]

Our applause!

Crap! The computer audio was muted.

 

[syn08]

Silly question of the day:

What happens to the AD797 when the phone cartridge encounters a "tick" (no, not the kind of insect that causes Lyme disease) on a record? The load impedance on the output is only a few Ohms at 10 KHz, if I read the schematic right.

Here's a silly answer

Again, I have to refer you to the entire schematic in the GPP thread http://www.diyaudio.com/forums/showthread.php?postid=1720612#post1720612

The max output swing of the AD797 is 1.4Vpeak, under worst conditions (20KHz). This is because the output stage clips at +/-22V peak. 1.4Vpeak at 20KHz is the equivalent of an input POP of 70mV. You would agree that it is unlikely for a 0.5mV nominal cartridge (at 5cm/s) to generate a pop of 70mV. This is called "headroom".

Within these limits, AD797 can easily deliver the required output current in the feedback network. Not to mention that I don't see any requirement to linearly reproduce the vinyl pops up to +/-22V output. Can you imagine what would be the required headroom of the rest of the chain?

The rest is John's fabulations about PIM, TIM, bla, bla. The one and only reason for this blabla is John's more or less subliminal need to trash opamps in general. Otherwise said, just another episode of "three legs good, eight legs bad". Nothing good has happened in audio in the last forty years, everything was already done forty years ago.