John Curl's Blowtorch preamplifier

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#18,501
janneman said:



Basically what this does is to cancel the xformer DC resistance. This concept was patented in around 1982 by Bruce Hofer from AP, and is used in the output transformer circuit of the AP S1 and S2 (don't know about the others).
Presumably, the patent is expired.

Jan Didden
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Hi Jan,

For some reason I thought that Burwen was the first to come up with this general approach of having a virtual zero field in input transformers.

Cheers,
Bob
 
#18,502
syn08 said:


John,

It's really simple 🙂 Step for a second away from that math and look from this perspective.

A MOSFET in subthreshold conduction (weak inversion) has the drain current dominated by a diffusion component. This current depends exponentially to Vgs and the slope is ideally log(10)*Vt=2.3*26mV=60mV/decade of current. In reality, it's everywhere from 70 to 100mV/decade of current, and that's one parameter that affects the noise, see below.

Under these circumstances, the MOSFET is essentially acting a a bipolar transistor, but with three major extra features:

- no rbb to generate voltage noise
- beta can be safely considered infinite
- no current noise

So from the three terms in the BJT equivalent voltage noise we have left only one, the term inversely proportional to the collector/drain current. You can safely assume that the MOSFET subthreshold noise expression is

Vn [nV/rtHz] = SQRT(K/Id)

where K is a constant and Id is the drain current (or, alternatively, you can use the subthreshold transconductance). For a BJT the constant is 2kT*Vt (Vt=26mV), for a MOSFET you can define an equivalent Vt based on the subthreshold conduction slope as mentioned above. Note that this equivalent Vt is precisely 26mV for an ideal MOSFET (with 60mV/decade conduction slope).

The only problem is that Id has to be very low, to keep the MOSFET in weak inversion. Here's why power (large area) MOSFETs are convenient: they are in weak inversion for tenths of microamps, while small signal MOSFETs like 2N7000 needs a under 1uA current... Therefore, from this perspective, high power (large area) MOSFETs have less noise than small signal MOSFETs. High power MOSFETs have unfortunately a higher equivalent Vt (and a few other problems), but overall they are (in terms of noise) still better than small signal devices.

I've published these things a long time ago somewhere in an IEEE journal, I can dig for a reference if you want to further pursue this matter.
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These are all very good points, but I don't think this addresses the 1/f noise contribution. Maybe I'm just out of sync with the discussion.

Cheers,
Bob
 
#18,503
Bob Cordell said:
These are all very good points, but I don't think this addresses the 1/f noise contribution. Maybe I'm just out of sync with the discussion.
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Of course not. In MOSFETs, the 1/f noise is known from the early 70's as being due to the mobility fluctuations in the channel. However, in subthreshold conduction, the diffusion current density does not essentially depend on the scattering coefficient responsible for the mobility fluctuations (this was demonstrated and confirmed by Hung et. al.), therefore the 1/f noise mechanism is, in a first approximation, absent. Or, otherwise said, the corner frequency is low enough so we should not care about.
 
#18,506
syn08 said:


Of course not. In MOSFETs, the 1/f noise is known from the early 70's as being due to the mobility fluctuations in the channel. However, in subthreshold conduction, the diffusion current density does not essentially depend on the scattering coefficient responsible for the mobility fluctuations (this was demonstrated and confirmed by Hung et. al.), therefore the 1/f noise mechanism is, in a first approximation, absent. Or, otherwise said, the corner frequency is low enough so we should not care about.
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Thanks syn08. This is fascinating. I don't consider myself to be too strong in semiconductor physics, so I appreciate your explanation.

Cheers,
Bob
 
#18,509
orjan said:
With some help from MRupp giving the url for searching, I found the german patent 27 10 291 that NTP referenced. It is written in 1977.

However my german is rather bad so I have problem understanding the patent....but I think it is the right one.

örjan
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This is a patent about 'Curing of polybutylene...'

BTW You can get ANY patent from www.pat2pdf.org , for free.

Jan Didden
 
#18,513
Wavebourn said:


Suppose you made such a system "flat" averaging outputs from several microphones. No wonder you will get measured results of standing waves, looking at such a construction as pictured. The same have to be expected on higher frequencies due to so wide placements of tweeters.
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Dick's opinions about massive amounts of power, feedback, and equalization are well known. By all means don't read his opinions on valve amps vs. solid state it will only irritate you. 😉
 
#18,515
I first met Dick Burwen in 1973 while working with Mark Levinson and the Grateful Dead.
Dick had helped Mark Levinson get started with his company by supplying modules that contained a selected RA-911, the very same op amp from Radiation Inc. (Harris Associates, later) that I had chosen for the Grateful Dead console back in 1970.
His sounded better, and I found out why: He selected them for crossover distortion, and perhaps other things, AND he used first class passive parts around them. Large value ceramics were not allowed.
Dick made several other pieces of audio equipment that were very interesting at the time. One was a component that squeezed 3dB into 1dB for recording, and then expanded it on playback. It measured WONDERFULLY! This was one of the first times that I personally heard the difference between what something sounded like, and what something measured like.
In any case, I tried to introduce the Grateful Dead to it.
We made a serious listening test, using a 16 track master tape of the Grateful Dead and we mixed down to 2 tracks, with 2 quality analog magnetic recorders, one of which would use the Burwen's compressor-expander. We could therefore compare the results of the two 2 track tapes and hear what the differences were. We assumed that we would get lower distortion and lower noise with the Burwen system in the path, because the unit itself, measured near perfectly with the test equipment that I had at the time.
However, something amazing happened! The Burwen completely screwed up the dyanamics of the musical performance. It was like putting the music in a washing machine. It was shocking!
We sent the unit back, but it was very well made and interesting in concept.
What is important is that you can't lose the music just to get a 'better' measurement.
The Burwin compressor-expander had one set of problems, but the other problem that I had with Dick was the same problem about negative feedback that I have now with Scott Wurcer.
Dick believes in negative feedback, and simply can't hear the difference between 1 and 100 op amps in series. Working with the Grateful Dead, I learned that op amp concepts were fine for industry, but less feedback and subsequently higher open loop feedback sounded better. It just took more work to get there.
This is what created the Levinson JC-2, and Burwen's modules were phased out, even in earlier products.
 
#18,520
Bob Cordell said:



Hi Jan,

For some reason I thought that Burwen was the first to come up with this general approach of having a virtual zero field in input transformers.

Cheers,
Bob
Click to expand...

I think the zero field input transformer is different from the AP distortion canceling output transformer solution.

The drawing on the Lundahl site is not very clear and doesn't show well what its doing on the input. it seems to use feedback (positive?) to buck the energy from the external signal to keep the magnetic field in the transformer close to zero.

The AP patent (and Lundahl has a similar solution illustrated, patented in Germany around the same time) uses feedback to cancel the distortion mechanism in the output transformer. The field in the output transformer is definitely not zero.
 
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