John Curl's Blowtorch preamplifier part II

[Wavebourn]

...Fourth approach exists that I use, you don't: an asymmetrical class A amp, with a source follower powered by one more bootstrapping source follower, and loaded on a counter-modulated current source. In such case power variations will be minimal due to phase shifts between voltage and current when loaded on a real world speaker.

[1audio]

The concern I expressed above is that the time constant at the sensitive junction for the temperature shift can be near the audio range or at least the range of modulation of audio tones (attack and decay of a note). Even if the heatsink is infinite the resistance to it is not zero and the can be a short term chnage in the devices. Thgis is well understood in IC's and efforts are made to control the modulation of sensitive parts by output devices. However the issue may still be significant in an amp. Is there a way to measure it? Perhaps a change in a low level HF tone while a large change at nearly DC is applied?

[KSTR]

Originally posted by 1audio
Is there a way to measure it? Perhaps a change in a low level HF tone while a large change at nearly DC is applied?

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

[Wavebourn]

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.

This good idea was implemented thirty years ago, about month ago I described it in details on this forum. I used a complex HPF to separate a high tone from a low disturbing one so it won't swing a high pitch tone from an oscilloscope screen. The problem was, first filters I used had non-linear cores so could not be trusted.

[scott wurcer]

Originally posted by stinius

Attached is an example of a slightly better model.

BTW: the thing I showed was not made by SPICE, but by COMSOL

OT: I really hate the limitations in file size that is allowed to attach.

Another issue is that the self heating time constants on the die are orders of magnitude smaller than the heatsink/ambient ones. As an aside Intel is working on thinning wafers to 10 microns to get the heat out (18" wafers BTW ) . Mind-boggling (18" wafers that is).

[KSTR]

Was that really a Zout measurement, Anatoliy?

[Wavebourn]

Originally posted by KSTR
Was that really a Zout measurement, Anatoliy?

No Klaus, it was made for observations of linearity change with voltage swing, I used it to experiment with everything in the amp to see how different changes of topology and regimes were reflected on linearity. I never measured THD, I always used to observe waveforms and to listen to sounds the same time. It was more revealing and useful, like giving thermometers to each and every patient VS averaging of a temperature in the clinic for some kind of a report.

[1audio]

I think the most interesting to me right now would be the transient phase change to the HF signal. I'm not sure how the filter will effectively isolate to tone or if having the filter will cause an effect in the process. Making a good isolation filter that won't distort the tone is the hard part.

[Wavebourn]

Originally posted by 1audio
I think the most interesting to me right now would be the transient phase change to the HF signal. I'm not sure how the filter will effectively isolate to tone or if having the filter will cause an effect in the process. Making a good isolation filter that won't distort the tone is the hard part.

I have a pair of LMI-500 filters by UTC, I can bring them when I come.

Edit: I've found they are LPF actually, 60 dB/Oct, so the task is reversed: like 200 Hz to observe, 20 KHz to disturb. Not so good for phase angle observations, but who knows...

[cliffforrest]

Originally posted by scott wurcer


....

As an aside Intel is working on thinning wafers to 10 microns to get the heat out (18" wafers BTW ) . Mind-boggling (18" wafers that is).

Wow! That will be like a rice-paper frisbee