Miller capacitance

[ding]

I was reading an article yesterday by lynn olsen (spelling??) about why tube amplifiers are often preferred (by listeners) to solid state amplifiers even though their specs aren't as good. Lynn hypothesised that this difference which measurements such as THD (etc. etc.) could not pick up was in fact due to miller capacitance of solid state devices. He said something along the lines of transisters being very poor capacitors (worse than electrolytics) and when the distortion of the capacitance within the device is amplified by the gain of the circuit... well you can see where he was going with this.

So does anyone have any thoughts on this.? Possibly circuit suggestions which could combat this miller capacitance. I was thinking that some devices have lower capacitance than others, are these the ones we should be using?

Dan

P.S. At present I don't have an opinion on this it's just what I read and I would like to hear what others have to say.

[GRollins]

Dan,
Lest someone get the wrong idea, tubes have Miller capacitance, too. That said, vacuum is a *very* high quality dialectric. None better.

Grey

[ding]

Sorry,

The last thing I want to do is misrepresent Lynn. Grey, he did mention this, in fact I think he said it very similarly to what you just did. He basically said that the dielectric of the transistor was very poor in comparison to tubes. I will post the link to the article tonight when I get home so I don't cause anyone any confusion.

Dan

[Hoffmeyer]

Hi Ding.
A circuit wich can, if not circumvent the Miller-capacitance, then at least minimize this, is tha cascode design, of wich mr. Pass has an article:
http://www.passlabs.com/articles/cascode.htm
It'll not only increase bandwidth, the slewrate will benifit from it, as well as the risk of self-oscilation will be less.

[hifiZen]

I really value and respect Lynn's opinions (and his designs). He's authored some fine articles, and I highly recommend anything you come across with his name on it.

That said, there are of course numerous precautions which can be taken to mitigate the aforementioned effects... including cascode design. I wouldn't worry too much about the miller capacitance though. In the primary gain stage of most solid state amplifiers, you'll have a compensation capacitor (usually of high quality) whose value should swamp the miller capacitance of the gain transistor, possibly in combination with a cascode. On top of that, you've usually got plenty of feedback to help linearize the system as a whole.

There are several more subtle effects I've come across which i think deserve somewhat more attention. One of these is transient thermal distortion. There's an interesting site on that here: http://peufeu.free.fr/audio/

If you look at many of the Pass designs from the perspective of thermal memory distortion, you'll see that they're probably rather immune to these effects... high-biased class-A MOSFETs aren't going to feel much parametric change from very small instantaneous power dissipation changes. A close look also reveals that MOSFETs have much higher gate capacitance (miller capacitance) than BJTs. You may draw your own conclusions from this...

[ding]

Here is the URL for the article

http://whoville.ucsd.edu/~stark/audio/PF1.html

You'll have to scroll down quite a way to find the Q&A on tubes Vs. SS.

Thanks for the links Hoffmeyer and HifiZEN I will have a look at these sites.

Dan

[hifiZen]

oh yes, something else which may be working against solid-state designs (mainly those with class-B outputs) is non-monotonic distortion. Class-A designs should be relatively immune to this problem, so long as the driver stages don't contain anything too funky...

I have another theory which I've been investigating... focussing on open loop output impedence. This is very different from damping factor. If the open loop output impedence is non-zero, the load impedence affects the open-loop transfer function of the amplifier, prior to the application of feedback. You can see how open-loop output impedence could play an important role in how an amplifier sounds, and why OL output impedence also plays a critical role in amplifier stability (especially in high-feedback designs.. eg solid-state). With low feedback or OL designs, this problem is much less of a factor, perhaps contributing to better sound. It's interesting to note the parallel between the subjective sound of higher-feedback tube designs and SS amps.

If we don't want to reduce the feedback, the only option is to reduce the open-loop output impedence as much as possible, something which seems to have escaped the attention of many SS amp designers. Notably, the many paralleled output devices used in Pass amplifiers have the effect of reducing open-loop output impedence, in addition to providing more output capacity...

My most recent test designs all feature very low open-loop output impedence, and i'm very pleased with the results so far... In addition, some of the circuit's i've tried happen to minimize memory distortion too, and my limited experimentation with this concept seems to indicate some subjective improvement. I'm eager to try more experiments with various high-power circuit configurations to help reduce open-loop output impedence further, with the goal of eliminating the output inductor ordinarily necessary for power amp stability.

[Alex M]

"If we don't want to reduce the feedback, the only option is
to reduce the open-loop output impedence as much
as possible, something which seems to have escaped the
attention of many SS amp designers. Notably, the
many paralleled output devices used in Pass amplifiers have
the effect of reducing open-loop output
impedence, in addition to providing more output capacity..."

Some designers avoid multiple output devices, as they feel
these compromise sound quality. For instance, Naim have
always used single output transistors (hugely over-specified
in their new NAP500!). This may be one reason why many
prefer the sound of relatively low-powered amplifiers.

If this were true, it would seem to counter the output
impedance argument (although I can certainly see sense
in the latter).


Any comments?

Alex

[hifiZen]

Actually, i think it works in favour of the output impedence argument... In general, multiple output device stages require emitter (or source) resistors to keep quiescent currents balanced between transistor pairs. So, if one were to use single, very large output devices, it is entirely possible to reduce the open-loop output impedence by eliminating the need for these current-sharing resistors...

One salient aspect of low-power amps is that the magnitude of the most difficult problems is drastically reduced (usually by the square of output amplitude or more). So, I think it is a much easier proposition to design a good low-power amp than a high-powered amp. This, perhaps even more than output impedence, works very much in favour of the low-powered amp. To list a few of the advantages:
- lower output currents (for greater linearity)
- lower rail voltages (more robust outputs)
- lower slew rates
- more feedback available (similar open-loop gain)
- less thermal stress
- power supplies can be proportionally bigger
etc. these are just a few that come to mind, but all are potentially very important to sound quality.

Anyway, my theory basically lies on the premise that it seems desireable to make the operation of the amplifier as independent as possible from the load, and this includes not only the measured output of the amp, but also what's happening inside the amp circuit... eg, what kind of error signals are floating around in there, and how is the load affecting the currents, voltages and thermal stresses being seen by components inside the amp circuit?

[hifiZen]

I think many of these discoveries (that single transistor / many parallel transistor / high bias current / single-ended -output stages sound better) may simply be serendipitous, because no-one seems to have come out and pointed the finger squarely at open-loop output impedence. Yet, in all of these cases, it can be seen that investigation will likely reveal that there is a lower open-loop output impedence than the alternative. I find it very appealing that all of these cases can be explained with one easy-to-understand theory.