Discrete Opamp Open Design

[bear]

Just in case anyone did not get the acronym/abbreviation:

gnfb = global negative feed back

...this is near to the limit of my ability to constructively participate.

_-_-bear

[benb]

Originally Posted by RNMarsh

You missed a few theads leading up to this --> Actually, it is also about discrete low gnfb vs. IC or high gnfb.

So a prerequisite for this thread is BOTH Blowtorch threads?

[wayne]

For transistors the Toshiba 2SK2145 dual is still available 50 Volt, 300 mW 15 mS and noise figure under .5 dB. Bipolars like the KSC945 and KSA773 are readily available, complimentary, 300 mHZ and low noise. Dual P Fets are history though. The 2SK2145 has the disadvantage of sources tied though.
Maybe a nice tube for voltage gain. I will defer this to Wavebourn.

[PMA]

Originally Posted by Wavebourn

I am going to try today in the field Behringer 2x300W per channel amp, class D with SMPS. I paid $185 for it including shipment. No heatsinks (almost), just a fan.

We tried something similar 2 years ago. It was not Behringer, but similar brand, same output power, class D.
It was an open-air garden listening session, with JBL 250TI LE speakers.

http://www.uwe-neidhardt.de/assets/i...i_Jubilee2.png

The class D was unable to compete with complementary-differential power amplifier, also 2x300W, 2SK170/2SJ74 input, cascoded, MOSFET VAS and 4 pairs MJL1302/3281 output. The class D sound was more harsh, less fluent, less full-bodied.

[fitzfish]

Any information on 2SK2145 device matching between the two? Did not see it in the datasheet.

Dave

[bcarso]

Originally Posted by scott wurcer

The one I was thinking of is well known (Bernie Oliver?), just the right drop on the emitter degeneration resistors (output devices) will do some first order distortion cancellation. I don't remember the exact numbers.

The paper was in the Hewlett-Packard Journal and is reproduced in The Selected Papers of Bernard Oliver, pp. 311-317. I have that book handy for a change

An excerpt: "However, the drop across this resistance produced by the operating current is only Io/2go = 1/2 kT/q to Io/go = kT/q or from 13 to 26 millivolts. Over the temperature ranges from 0 C to 100 C, the junction drop of a silicon transistor will change typically by 250 mV, Thus, unless the biasing diodes ... track this change within a few percent, Io will be very unstable ... At present the most practical solution to the temperature stability problem appears to be to make Rc [the emitter resistors] many times larger than 1/go, and to rely on negative feedback to reduce the resulting distortion."

Also in the collection is his great paper Thermal and Quantum Noise, from Proc. IEEE, May 1965.

What a wig he was.

[bcarso]

Originally Posted by fitzfish

Any information on 2SK2145 device matching between the two? Did not see it in the datasheet.

Dave

I don't think they ever said.

[wayne]

The matching of the 2SK2145 seems pretty good on spot testing. We buy them on reels of 3,000 and they are pretty consistent on the reel. Nice dual part for all but the lowest noise apps. I mostly current source them in circuits using the Y type.

[Wavebourn]

If you close local loop before buffer, and add resistor from buffer's input to output, you will get a version of "current dumping" amp. Try and compare. However you can't get high gain from such an amp, but low distortions on a wide band for sure.

Speaking of transistors, why you don't consider Zetex devices? I would call them "Toshiba of modern days", in terms of innovations.

[RNMarsh]

Originally Posted by bcarso

The paper was in the Hewlett-Packard Journal and is reproduced in The Selected Papers of Bernard Oliver, pp. 311-317. ... At present the most practical solution to the temperature stability problem appears to be to make Rc [the emitter resistors] many times larger than 1/go, and to rely on negative feedback to reduce the resulting distortion."

Bingo. See my output stage for headphone amp. The whole package doesnt drift at all. very temp stable circuit. Surprised no one commented before as to why circuit was so temp stable.