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.

Pierre

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.

Thanks,

Pierre

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 !

Pierre

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 :

• output impedance : intentionally very high (I want to squeeze out the most gain from it, so the impedance has to be highest). Impedance decreases gradually with increasing frequency, as feedback through the Cdom cap comes into effect. This is the standard method of VAS linearization through Cdom.
• drive capabilities : almost none at LF
• hence the buffer transistor !

I have Sennheisers 580... are there any better headphones besides the 600s ? Electrostats do not apply here, thay usually have their own tube amps.