New mosfet amp from ESP

[rode]

The description used above referrs to a bootstrapped input stage, which is a completely different usage of the technique. There are 2 forms of bootstrapping -

1) input stages, to obtain a high input impedance using low value resistors, and
2) VA Stages, where the bootstrap acts as a constant current source.

Obviously, it was not my intention in the MOSFET amp to boost the input impedance, and the technique is used to provide a constant current to the VAS.

A regulated supply is not necessary at all, and a great many of these amps have been built (and used) with no problems encountered (other than construction errors).

Two of the amps (supplied as fully built units) are installed in recording studios in Australia, with another about to follow suit. This has been after extensive listening and comparison with other 'high end' amps. The comments so far indicate that the amp has excellent detail and accuracy, two properties that are essential for studio monitoring applications. Obviously, the overall sound quality has been considered superior to other amps that have been auditioned.

As for 'low efficiency', this is not really true either. Admittedly, under heavy loading, the amp will not provide as much peak-to-peak swing as a bipolar amp (or a MOSFET amp with auxiliary supplies), but in terms of real losses, the difference only amounts to about 1dB. At (or near) speaker resonance (where the highest instantaneous swings occur) the difference is far less.

Naturally, all of the criticism that has been levelled at this amp is easily resolved by building and listening, but it is far easier to make negative comments based on what one has heard from someone else (who didn't build one either). Hardly a scientific approach

Cheers,

[mikeks]

Have you tried connecting the bootstrap capacitor to the intersection of complementary cross-coupled predriver followers to isolate it from load impedance variation??

cheers

[mikeks]

...note that this will not work with CFP outputs...

[sam9]

"The description used above referrs to a bootstrapped input stage, which is a completely different usage of "

<< That's suppossed to indicate embarrassment. That's what happens when I read posts on the fly (and try to comment) with a lot of local excitment and a lot of noise vs. signal around and behind me. Looking at it on a calm quiet morning, I find the correct D.Self reference which says the technique may look "old fashioned" but works as well more modern solutions. He makes the claim that very low speaker impedances can cause a problem, however, it ocurrs to me that this may be offset by the zobel.

In any case I think it is interesting to speculate on how the amp works, but rather silly to bad mouth it if you haven't built it. I don't want to be distracted from a couple of other things I'm struggling with currently or I would have my check in the mail to ESP for a rair of boards. I enjoyed the P3A and expect this one as well will perform better than seems reasonable from such a semi-minimalist design.

[rabbitz]

I built one and I posted some info here.
Elektor crescendo ME vs ESP project101

I couldn't care less what technology is in the amp........ as long as it sounds great, I don't care and it does sound great.

I don't know how it compares to the P3A but there has been a bit of discussion on the ESP forum.

http://www.instantboard.com/users/rode/

[lumanauw]

What I learn from AKSA (thanks Hugh), is that bootstrapped VAS produces nice to hear amp. This is due to harmonic produced, and behavior for low frequencies.

The designers using this, like AKSA or RodE, are very clever guy. They are not putting bootstrapped VAS because they dont know there is CCS VAS existed. They put it there on purpose.

But is this implementable to small-medium size power amp only? Why I never see bootstrapped VAS on high output power amp, like pro power amp? Is it when we are into the area of huge rating watt, CCS VAS is a must?

I also dont understand about TRANSCONDUCTANCE in mosfets. I can read the definition, but cannot imagine what is it. HFE is easy to understand, it is amplifying factor. What is TRANSCONDUCTANCE? What is the difference between one mosfet has low transconductance, and other mosfet have high transconductance? The transconductance figure is somehow inverted. One with 0.3 is lower transconductance than one with 0.03 transconductance? Is the gain factor of mosfet depends on load resistance (from transconductance formula)?

Other question (maybe someone can help). IF I wanted to put 5 pairs of K1058 for final output stage, with rail about +/-80V, what is the bias current needed in the VAS enough for driving all output stage?

[traderbam]

I also dont understand about TRANSCONDUCTANCE in mosfets. I can read the definition, but cannot imagine what is it. HFE is easy to understand, it is amplifying factor. What is TRANSCONDUCTANCE?

beta or hfe is the ratio of output current to input current.
transconductance is the ratio of output current to input voltage
transresistance is the ratio of output voltage to input current
voltage gain is the ratio of output voltage to input voltage

In the case of a mosfet the input voltage is the voltage between gate and source. The output current is the drain current. Because of biasing requirements the transconductance usually refers to the slope of the Id/Vgs curve at a particular Id.

[mikeks]

Originally posted by rode

Overload (clipping) recovery is something that I'm almost paraniod about, and if I see "rail sticking" (where the signal sticks to the maximum +ve or -ve output voltage for many microseconds, then suddenly returns to normal) then IMO there is something wrong with the design.

Cheers, Rod

So...Rod .....What are your specific remedies for 'rail sticking' ?

[Nelson Pass]

Most cases of rail sticking I've seen are due to slow saturation
recovery of bipolar devices. This often appears in amplifiers
where the front end Vas has a separate (higher) supply.

Apart from avoiding saturation altogether, it's best to make the
fastest devices in the chain saturate first, avoiding saturation
in the slower devices. Alternatively, you look at the use of a
Baker clamp (look it up).

[mikeks]

Originally posted by Nelson Pass
This often appears in amplifiers
where the front end Vas has a separate (higher) supply.:

One would've thought the opposite is the case...!?

Originally posted by Nelson Pass

Apart from avoiding saturation altogether, it's best to make the
fastest devices in the chain saturate first, avoiding saturation
in the slower devices. Alternatively, you look at the use of a
Baker clamp (look it up).

Tried baker clamp....distortion unacceptable....