Excellent, excellent article. I only read it lightly, but found nothing to argue with. I too believe one of the "differences" between SS and VT amps to be just how they are affected by instantaneous temperature changes in the active devices.
These are ideas that I have intuitively considered to be at play, but never bothered to attempt to measure.
I Look forward to more.
University of Utah
Thanks for the comments Marvin ! This is what I hope for : wake up people who have been thinking about this kind of things before, and think together about it. Read the new articles, we'll talk about it.
I find some of your comments especially interesting- particularly that the brain compensates for or ignores the constant distortion of the SET. The function of the brain in what we think we hear is to me the big black hole wherein probably lies a lot of the apparent difference in peoples perception of sound.
I am somewhat surprised that you write off TIM as I would think that what you describe is a very valid TIM mechanism. ie change of operating points with the possibility of increasing distortion. Particularly if the amplifier is direct coupled to the output with resulting change in output bias. The distortion of the output stage is very dependent on bias settings. regards pat
Some research reports that the ear has lots of distortion by itself. I might be speculating, but this might explain why the high distortion figures of SET amps don't seem to matter. Just like if the brain could filter out large amounts of low-order distortion, on the condition that the spectrum be stable, which is not the case with transistors.
You are also positively right about TIM ! I think that conventionnally TIM is associated with slew rate limitation, but this memory effect could also be labeled transient intermodulation as it is caused by the signal itself... It is a matter of vocabulary actually. Personnally I'd rather call it memory to differentiate from slew rate effects.
I have now read your ideas on the SS amplifier on the page http://peufeu.free.fr/audio/ and found that your results correspond very closely to my efforts.
The use of a cascode stage to drive the output is very important as well as the use of good CCS for both the input stage and VAS.
I will try the magic resistor tonight and see what I can hear. You comment that during listening tests you could still hear the hiss and suggested it may come from the input signal, I would hazard a guess and say it is coming from the zener in your CCS.
Thanks ! Working alone, and not having other listeners at hand to confirm what I thought made me a little nervous. Was I hearing things ? Now, I receive emails that confirm from experience, that structures with lower memory have better sound....
I would suggest that you look at using the Philips BF469/470 as they are much better sounding and do not inhibit the slew rate as do the BD139/140. There is also the MJE 340/350 series but they also inhibit slew rate but have better driving abilities than the BD139/140
For small signal I use BC556 or BF421/423 which seem to be similar apart from the VCEO.
Power transistors : I have tried the Toshiba 2SA1302 /2SC3281 but found them to be very unstable over about 35 Volt rails. I am now using the MotorolaMJL21193/21194's with good results.
MOSFETs do work well but are not as good sounding as the Bipolars and I have not worked out why yet.
Maybe this is because MOSFETS are much less linear than Bipolars... their crossover region looks very bad unless biased into class A. Even then, they are less linear than bipolar.
I would also stay away from the use of CFP stages as they tend to get unstable and the local feedback errors generated cannot be controlled by the global feedback.
I tried CFPs too, they seem to like oscillating very much. Stabilizing them (with caps) does not seem to do good to the sound. The good old EF stage is IMHO the best compromise.
Another worry is what effect the normal VBE multiplier has in the bias arrangement for the output stage, as I think this also has a detrimental effect on the sound due to it trying to present a stable voltage during transient conditions as well as providing bias stability during thermal changes to the output stage. The use of a capacitor helps but then you get into capacitor problems again. I would prefer to use a simple resistor with the thermal characteristics being taken care of in another way. Russell.
To be honest I don't know if the memory occuring in the output stage is important. I have got to make other simulations on this. It is actually an other form of memory than the one I already studied, as it is not the DC drifts that are annoying (these are verry little), but rather the crossover distortion being modulated by the quiescent variations caused by transistor heating/cooling. The cure might simply be to use large emitter resistors.
I think that the Vbe multiplier is slow enough... The transient is finished for long once its thermal effects start to arrive at the sensor transistor. The memory effects I simulated are much faster (ie. at the second scale, similar to the beat of the music, or even faster, ie. the bass notes below 100 Hz). Output stage memory and interaction with the Vbe multiplier is at much slower speeds, like the alternance of loud and soft moments in music. This might be very important, too, and the easy solution appears to be Class-A, with all assorted heat problems.
Unlike memory in the rest of the amplifier, the output stage has been quite studied before. I will try to simulate it nonetheless...
Thanks for your feedback !
These are messages I got on the Audio Asylum, and their replies.
Thanks for sharing your facinating work. I do have some questions -
1.Your Vas circut ,cascode or cascoded cascode, would, I believe, have a very high output impedence. With the addition of an emmitter/source follower you have a defacto, functional op amp. A wonderful building block. Have you investigated the succeptability of followers to memory distortion, and its cure? If so could you update us on your findings?
2. Have you expolered the application of your findings to balanced/differential circutry? Hope your original work bears fruit and look forward to the day I can buy a 'minimal memory' op amp from your multinational conglomerate.
I failed to take note of the cap providing local feedback around the (cascaded-) cascode. Could you comment on the output impedence and load driving capabilities (modeled or actual) with the circut as it now stands?, as well as the previous question on followers?
The VAS :
The memory in the buffer transistor is quite negligible compared to the VAS transistor. Imagine the VAS is confronted to a 50k load (base resistance of the buffer), with a 10mA current sink. Heating the VAS transistor by one degree C will vary its Beta by 3%, which means a drift of 10mA * 3% * 50k = 15 volts that will have to be corrected by feedback. Heating the buffer by 1°C will only make it drift 2mV... The essence of the thing is that memory is multiplied by the gain of the stages downstream ; the VAS has a lot of gain, the follower has none.
If you use an emitter follower for amplification (with for instance 10 Ohm from emitter to ground and 1k from collector to V+) as is done in an input diff pair, you're into trouble, because the Vbe variations are amplified by the gain of the stage. They have to be compared to the input signal, not the output signal.
With a buffer and a small output stage, it is actually an opamp, even if a complicated one... It could be used for anything actually.
Early Effect :
I don't think memory distortion is related to the early effect. This one does not depend on temperature (or so I think). This is more about Vbe variations...
I have Sennheisers 580... are there any better headphones besides the 600s ? Electrostats do not apply here, thay usually have their own tube amps.