John Curl's Blowtorch preamplifier part II

[john curl]

Jfets are OK. They are NOT as good as tubes, part for part, in linearity. They can be quieter, and they are complementary, so they can be made to be excellent gain stages. Break-in is, to me, a physics problem, that I must acknowledge, to be difficult to 'prove' but easy enough to hear. Break-in for me, is important, but in the total connection and wire sense. I recommend it.

[wayne]

I like tubes just fine have designed with them and even own some but do like SS consistency and reliability. Very true about break in quite audible doesn't make 100% sense but it is there, our ears are pretty damned good tools still and ear owners buy the product.

[KBK]

Originally Posted by john curl

Well Ed Simon, you have made it very clear why we have to take care as to what feedback resistor we use.
Just to remind everybody, I have been 'harping' about differences in resistors for more than a decade, especially with the feedback resistor, as it is the 'template' that the amp tries to model itself on. Usually to much 'laughter' as well. Well, now it should be clear, VERY clear. '-)

First thing I did when screwing about with amplifiers..is look at the theoretical aspects..then I headed right for the feedback criticals. coupling, PS, and that dang resistor (all concerning the feedback loop).

i started by swapping the resistor out, turning it around, different types, brands, models, solders, each time, a full and proper burn-in in. Skinning the resistors (naked), different mounting, wearing out the leads, via stressing, every thing I could think of. plating the leads, unplating the leads, different field conditions, demagnetizing, etc. It took nearly a year to really get a solid handle on the overall aspects of each and all of those considerations, but since it was the most critical spot in the amp, it ended up explaining much - about all else. This was in the early 90's. I had a couple of basic amp models I used, specifically ones with high feedback, so the effects were multiplied and they (the amps) also had a low number of parallel outputs, so I could actually hear those micro differentials.

[KBK]

Originally Posted by Bob Cordell

See Chapter 13 in my book "Designing Audio Power Amplifiers", specifically pp. 264, 265. There I show a plot of measured distortion of a 1/4-W metal film resistor as a function of frequency when it is dissipating its rated 1/4-W of power due to the AC flowing through it. This measurement largely captures only the thermal-related distortion. I think this resistor had a rated TC of about 100 ppm/C.

I was first introduced to the importance of thermal resistor distortion around 1982 when I designed my MOSFET power amplifier with error correction. I was getting higher distortion than I should have at low frequencies. When I put two 2-Watt metal film resistors in series to form the feedback resistor the LF distortion went way down. This was only a 50-watt amplifier, but the feedback network impedances were fairly low (the feedback resistor was only 4.3k).

Cheers,
Bob

It's a trade off between lowered issues of a transient nature vs inductive interference (flubbery inconstant braking) in the field differentials created by the inability of the resistors to be exactly the same under all complex (simultaneously occurring) conditions. My experience, anyway. (I tried parallel and series, physically close and separate, multiple orientations of field coupling interference)

Is the advantage in thermal (overall and related issues) better than the cumulative effect of the rest? Only the specific result in the specific experiment will tell. Too many variables. My ears said you were right, in most ways or the more 'noticeable' considerations.

[KBK]

Originally Posted by 1audio

Interesting insight on capacitors. Experimentally, I usually see less distortion when there is a dc voltage across a cap. Most obvious with electrolytics but also with film caps. I figured that the dc bias would reduce the effective change across the cap from the AC signal.

Conceptually the same might be true for resistors. Especially for thermal issues. If the resistor has a bias and heat already the effects of the ac would be smaller. I know higher power resistors are more likely to be lower distortion. What is the time constant of the Tc of a resistor? How to measure it?

Maybe this is all naive misunderstanding of the issues?

Do you see it now, JC?

[Joao@AltheaMusica]

Originally Posted by john curl

Jfets are OK. They are NOT as good as tubes, part for part, in linearity. They can be quieter, and they are complementary, so they can be made to be excellent gain stages.

One main aspect for tubes is their their thermal stability (minor shift in the operating points). Disadvantage is their (more) limited lifetime and that you need filament supply.

[Bonsai]

In solid state devices, there are big shifts in the device paramters with the signal voltage. There is a discussion on some of this stuff over on Edmond's thread. A classic example is Cob in a VAS transistor, or the input capacitance on mosfets and JFET's. Designers need to be aware of this stuff, because often it frigs with overall stability.

Now, you overlay the thermal impact on device parameters and its clear nothing is really nailed down. Circuits just flop around, hopefully not banging into any serious limitations. Maybe tubes sound good because they run hot (thermally stabilized?) and the paramters don't shift that much with signal - but I am not a tube expert . . . ;-)

[scott wurcer]

Conceptually the same might be true for resistors. Especially for thermal issues. If the resistor has a bias and heat already the effects of the ac would be smaller. I know higher power resistors are more likely to be lower distortion. What is the time constant of the Tc of a resistor? How to measure it?

For linear TC the second 1/10th Watt heats the resistor just as much as the first (the first order model is a thermal resistance). Pre bias over ambient probably just creates more problems with termination thermo-couples, certainly won't help distortion. Heat is actually a diffusion process and has a distributed time constant. A sudden step excitation in a good bridge setup should allow you to extract the thermal curve.

This stuff has been trade secrets for folks making 18bit + modular A/D's and DAC's for decades.

[simon7000]

Originally Posted by scott wurcer

This stuff has been trade secrets for folks making 18bit + modular A/D's and DAC's for decades.

Good thing because I don't think you can get 16 bits out of a carbon comp. resistor!

[scott wurcer]

Originally Posted by simon7000

Good thing because I don't think you can get 16 bits out of a carbon comp. resistor!

Ed, you keep pointing that stuff out like nobody ever heard of it before. I once scavanged a discrete DAC made for the missile program in the 60's. It used 2W 1% resistors for an 8 bit R2R ladder.