John Curl's Blowtorch preamplifier

[syn08]

Originally posted by john curl
You should cease and desist.

Agreed, it's a waste of time.

[chascode]

Originally posted by john curl
I don't know why an AD825 would be a special problem with I-V, but I am still hopeful to evaluate it for line amp service. The AD815 looks pretty good, also.

Hi John,

These op amps are used by Jeff Rowlands, and also in the Grace Design audio equipment.

[chascode]

Originally posted by john curl
Anyone else with real information on capacitors?

Sorry for this late post, I'm just catching up to this thread.

Way back, we were using large Sprague mica caps in the LC harmonic trap filters of our power converters, removing the 5th, 7th, 11th and 13th harmonics of 400Hz. When Sprague discontinued these caps we tried their polysulfone types, but they just weren't stable enough over the Mil temperature range to hold the notch frequencies. Mylar caps would also drift in frequency, and overheat and exceed their internal temp limit to boot. We also tried banks of Kemet paralene caps, which were a little better. We tried GE polycarbonate caps, which were as good as paralene and available in high voltages, which was important for our application. Finally, Cornell Dubilier made a metal-cased polypropylene cap for us that was small enough to fit in the filter section of the unit, and that did the trick.

Now that Vishay owns just about everything in passive components, there would no longer be any possibility of obtaining this kind of custom packaging.

[m2003br]

Thanks for reply and for advice, Mr. Scott and Mr.John. javascript:smilie('')
I already readed the past discussions about output coil, and is not my intention to clone this amp. I posted this schematic only for a example about a True DC amplifier, without servo, because a recently discussed subject in this thread. (a little delayed, I know...)
I don't like output coils, too. Coincidence or not, I never build an amplifier with coils, since my first one, the low feedback, unconditionally stable, trully unconventional ETI 4000 (470), from Trevor Marshall, ETI may and July 1979.
Best regards,
Marcos

[john curl]

It is not the output coil only, it is the VALUE of the output coil that is the biggest problem. IF the output coil was 1uH or less, I could not really complain.

[john curl]

Actually, DC amps of this type are fairly easy to make. However, we prefer to use a servo, because it is more temperature stable, and generally safer. In 1968 and 1969, I was a DC servo designer. This meant DC response through-out. When I started making amps for the Grateful Dead, I found that DC was just a problem, and I AC coupled it out through the feedback loop. Then I found that the large caps that we were using had distortion contributions, so I went to servos. It is 'ideal' to have a DC coupled amp, but not really practical, and REALLY expensive to do it right.

[scott wurcer]

Originally posted by chengtaw
Dear Scott,

What's your opinion about AD825, AD8065 and AD8067? They are ALL J-FET input Ops with SIMILIAR but DIFFERENT sounds. I did like J-FET input OPs but the appearance of LM4562 did change my mind!

Cheers,
chengtaw

Sorry, but I have no opinion on the matter. Maybe if I had some of the setups at the BA I could tell a difference. Virtually every time someone has claimed a "dramatic" difference there was something pathological going on i.e. their circuit was oscillating on high peaks or some such. I still think a major JFET vs bipolar issue is RFI, again a design issue outside of the IC (mostly).

[JPV]

Originally posted by scott wurcer


Sorry, but I have no opinion on the matter. Maybe if I had some of the setups at the BA I could tell a difference. Virtually every time someone has claimed a "dramatic" difference there was something pathological going on i.e. their circuit was oscillating on high peaks or some such. I still think a major JFET vs bipolar issue is RFI, again a design issue outside of the IC (mostly).

Do you mean higher susceptibility to electrical fields for Jfet because of very high input impedance or is it something else?

JPV

[scott wurcer]

No FET's are better because they are reverse biased and don't present as much a source of rectification at the input. This can be shown in a "touchy-feely" way with a motor like a drill waved over a breadboard. Micro-power bipolars are the worst.

[PMA]

And not only better, their sensitivity to RFI issues is in 2 - 3 orders lower than that of BJTs.