John Curl's Blowtorch preamplifier part II

[john curl]

Power amps have thicker substrates.

[kamskoma]

Nelson Pass,

Once you recommended an "how to" book about working with aluminium, do you remember the author and title of this book!

This book can be very useful for preamp DIY builders.



Kamskoma

[Bob Cordell]

Originally posted by 1audio
How does the transient thermal change affect the signal passing through the amp? This is clearly not easy to measure with steady state tests and it may be significant. I have been reading about delay vs. temperature in very precision systems and the numbers are in the 10 pS/degree C in a carefully compensated system. I seriously doubt anyone would hear something that small as a steady state change but an abrupt change could modulate a signal enough to be audible (if we knew what was audible).

I worry the most about transient under-bias of a class AB output stage. If I have to pick my poison, I'll always go with overbias and some possible gm-doubling consequences.

Cheers,
Bob

[Bob Cordell]

Originally posted by KSTR
Sounds like a good idea. Something like measuring small signal HF Zout during/after a power burst, by backdriving the amp with a LF current. Maybe in a high (noise) gain config to make it more readily apparent.

- Klaus

This is a good idea. A variant would be to operate the amplifier at 20 kHz at the power level that nominally yields the highest amount of crossover distortion. Then back-drive the amplifier with a low frequency at moderate power at perhaps 10 Hz. Then look at the 10 Hz modulation of the 20 kHz distortion residual on a scope. Note that the high damping factor of the amplifier under test keeps most of the low-frequency back-drive signal out of the distortion analyzer.

Cheers,
Bob

[PMA]

Originally posted by Bob Cordell



I worry the most about transient under-bias of a class AB output stage. If I have to pick my poison, I'll always go with overbias and some possible gm-doubling consequences.

Cheers,
Bob

Good point again. No we can discuss a measure of overbias, with respect to mentioned thermal transients as well. And soft transition.

[Nelson Pass]

Originally posted by kamskoma
Nelson Pass,

Once you recommended an "how to" book about working with aluminium, do you remember the author and title of this book!

This book can be very useful for preamp DIY builders.

Did I? Hmmmm...

[SY]

Originally posted by kamskoma
Nelson Pass,

Once you recommended an "how to" book about working with aluminium, do you remember the author and title of this book!

This book can be very useful for preamp DIY builders.



Kamskoma

Though tube-centric, "Building Valve Amplifiers" is a very good guide to general metal-bashing for audio.

[h_a]

Well, maybe I'm missing something here, but thermal effects on THD should be quite easily investigated.

Just run a 2 tone test into the DUT, say 1kHz plus a 40kHz sine. Apply bandpass to get rid of the 40kHz. Estimate from standard 1kHz THD the additional intermodulation products and subtract. Certainly a little matlab or similar is usefull for better precision.

Have fun, Hannes

EDIT: or mount transistors on high power Peltier-elements.

[kamskoma]

Yes, I will remember it was a Harley Davidson related book, or at least motorcycle metal works.



Kamskoma

[h_a]

The art of motorcycle maintenance?