Sound Quality Vs. Measurements

[john curl]

Nigel, this has been thought out thoroughly for the last 50 years. IF you increase the current in the differential pair too much, you get increased current noise contribution, and it is not quiet unless you drive with 50 ohms or less. That is one good reason to not go above 2ma with bipolar transistors. Also, bias current goes up, almost proportionally with increased current. However slew rate will remain the SAME if the gain-bandwidth of the amp remains the same, because the increase in differential stage current increases the Gm proportionally, and then your compensation cap increases proportionally. Only degeneration using emitter resistors or increasing the gain bandwidth of the amp will get you increased slew rate. That is the 'KEY' in Solomon's paper. Unfortunately, emitter resistors WILL add input noise. So, very low noise is out.
The rational alternative is the low noise jfet pair, with inherently lower Gm, so you don't need any source degeneration resistors, and immunity from current noise. NOW you can run any source current you want, mostly, because the Gm does NOT track the current, so you are not 'chasing your tail' so to speak. I hope this helps.
In any case, please look at successful working examples of amps and preamps and note that they are often pretty much the same, as far as operating current is concerned.

[a.wayne]

Originally Posted by Wavebourn

If you have hands-on experience and well trained imagination no simulation is needed. But in order to obtain this skills you need to work with breadboards and measurement tools. Like pocket calculators deprived people from ability to calculate numbers, simulators deprived people from ability to see, feel, and hear circuit diagrams.

+10 ,

Basic skills are lost due to such , best to start with basics before moving on to technology..

[nigel pearson]

Thank is for that . Good explanation . I had supposed that gain of 1 was a priority and determined the whole reason d'etre of the design , maybe so ? NE5534 gives us the choice with external compensation option . I sometimes wonder if 5534 is disliked because people are so use to other op amps being unity gain stable that when asked to tweak they don't want to to ?

[jcx]

bjt diff pair noise with higher bias isn't a major issue in power amp front ends where we are expecting line level signal V - today we expect low Z drive at up to 2 Vrms from digital sources/SS preamps - can use lower Z volume pot - or can skip the volume pot for digital

where input current noise isn't limiting then we can use higher bias, degeneration R to make bjt front ends more linear than jfets: "What's your reasoning?" and not "What's your belief?".

[bcarso]

Originally Posted by a.wayne

+10 ,

Basic skills are lost due to such , best to start with basics before moving on to technology..

I didn't succumb to simulators until I was nearly 50. I think I waited too long. I did use calculators fairly early on, although the first I acquired was an HP-25, a machine I still miss although it didn't run all that long on the battery pack.

When Brad Plunkett found out I was using simulators he told me he was very disappointed, mostly joking. When he finally began using them a bit, only a few years ago, he presented a circuit to me that he'd come up with, having "unbelievable" performance. It was a good idea nevertheless, and was built, incorporated into a design that very much needed it, and went into mass production.

I pointed out that the shortfall between its performance, which was wholly adequate to the task at hand, and what the simulator predicted was mostly due to his use of a default transistor model, and that he could pull down a list of real parts including a 2N3904 and greatly improve the accuracy of his results.

I then THOUGHT about the mechanisms that still limited the performance and conjectured ways to correct for them. This led to a series of additional designs, verfied by simulation AND construction.

Simulators must not become a substitute for thought, although there are factions within the communities that advocate things like "genetic programming", where actual topologies are created "at random" and then "prove" their robustness in a sort of Darwinian fashion. It's somewhat reminiscent of quality assurance people who believe various methodologies can replace people with real insight, experience, and expertise with a lot of duller or at least uninformed folks doing experiments and using statistics. Pease of course loved to skewer the more egregiously silly examples, like the voltage regulator that worked better after enhancement via Taguchi approaches because it eliminated line regulation, something that hadn't been included among the measurables.

But one does need to be open to about everything. It's easy for an old guard to get complacent, nay even defensive. When Planck reluctantly presented his empirical formula for solving the "ultraviolet catastrophe" a number of then-traditional physicists said they could fix things without such a preposterous notion by using various "devices" with which they felt comfortable (Nernst in particular had some ideas which seem absurdly hidebound to us today, but must have seemed quite plausible at the time).

[bcarso]

Originally Posted by nigel pearson

Thank is for that . Good explanation . I had supposed that gain of 1 was a priority and determined the whole reason d'etre of the design , maybe so ? NE5534 gives us the choice with external compensation option . I sometimes wonder if 5534 is disliked because people are so use to other op amps being unity gain stable that when asked to tweak they don't want to to ?

Despite its antiquity the 5534 remains an amazingly good part. Besides the external compensation access, those same terminals allow the use of alternative input stages, including JFETs. Pretty soon the added parts may rather dwarf the chip that was to be so convenient and space-saving, and one can be pushed closer to an all-discrete design, but such is life.

[john curl]

There is better, today, and not much difference in price, like the lme4562

[jan.didden]

Originally Posted by Wavebourn

Like pocket calculators deprived people from ability to calculate numbers, simulators deprived people from ability to see, feel, and hear circuit diagrams.

"In Plato's Phaedrus there is the story of king Thamus who reigned over a city in upper Egypt. He had a friend Thot who was an inventer, amonst others inventing script. After Thot has exalted all the virtues of writibg ("This is an invention that will make Egyptians wiser and stronger - my creation is beneficial for memory wisdom").
Thamus: This will bring forgetfullness in the soul of those who learned because they no longer exercise memory, and by relying on written texts, they will not find wisdom from within but from strange symbols from outside. Your pupils will only find the appearance of wisdom, not wisdom itself.They will read much without learning, but will be awkward to deal with because their so called wisdom is only imagined."

Apologies for the slight OT, but I was just reading Neil Postman's "Technopolie". L'histoire se repète, as the French say.

jan

[Pano]

The guys who do the memory championships say much the same as King Thamus.

[jan.didden]

Originally Posted by Pano

The guys who do the memory championships say much the same as King Thamus.

Yet I think in the end he was wrong. One central theme in Postman's writing(s) is that technological developments not just improve the way we do things. Cell phones not just make it easier to stay in touch, they actually change our culture, the way we as a society communicate and live together. Cars not just make it easier to get around, they have fundamentally changed our culture.

In Naples I've seen streets where the young gather in their cars at night, and they literally past up the windows with newspapers and that is their world where they talk, fight, court each other and make the occasional baby. The car, in their society, radically changed the way they court and select mates and prepare for family life.
And take TV, another game changer if there ever was one, way beyond just being a medium to bring programs in the living room.

Simulators are not just an easier way to 'test' circuits, they changed the game. People with no real understanding of circuits can now design a circuit that on the face of it works quite well and might even do so in real life. Question is, where is the harm in that - pursuit of happiness and all that.

BTW Postman hates TV, you should read his thought-provoking "Amusing ourselves to death".

jan