Will this work as audio amplifier?

[lumanauw]

Hi, Mike,

I did some checkings on VAS with Spice, it seems that the VAS mainly creates 2nd harmonics, but i don't know if this
is still so if the transistors are matched. Cascoding the VAS
had a very big increase in qualitity

Are you simulating ordinary VAS or my differential VAS in post #1?
How about folded cascode VAS? Is it creating the same thing as VAS in post #1?

Yes, odd harmonics are agony... and the human ear is extremely sensitive to them. I experimented a little, add 3% 3rd harmonic to music (done with software), and you instantly puke, add 3% 2nd harmonic, and you wonder if there really is a difference !

Interesting. So 2nd harmonic is not adding "Niceness". It's just not detectable.

But this means that simple THD-measurements are useless to describe the soundquality of an amp. (Not a new discovery at all...)

Yeah, thats what they say. Measurement figures doesn't give a clue about how a power amp sound.

If 2nd harmonic is just "not detectable" instead of giving "nice sound", what is really happening in SET tube? There is measurement comparison with SET tube amp (CARY 300SEI) with "very good figures" amp. SET amp gives very bad measurement result.
But in listening test, this SET excells the other amp. Every test subject says the same. If this is not caused by 2nd harmonic, what cause this SET sound "NICE"?

[lumanauw]

Just finished prototype of my own idea in post #1. IT WORKS as audio power amp.
BUT, the servo type doesn't work. It seems the voltages in the 1M resistors drop so much, that the servo cannot see what happens in the output stage. Maybe other kind of servo will work for this, but not the servo in #1.
The one that works is the conventional VR type. I put VR in base of Q2, to set the reference voltage for Q1. (100k, left to -15, right to+15, CT to base of Q2)---> it needs manual adjustment. I'm afraid of DC offset drifting with heat, but from 1/2 hour, it looks steady.

I'm glad that this Q1-Q2 arrangement can made a signal depended current path. So there is another option for this front end current switch besides differential pair.
Q1 and Q2 works as 1 way current path. Differential works as intersection of 3 current path.

If differential pair cancels 2nd harmonic, hopefully this 1 way current path doesn't. Maybe can lead to "good sounding amp" not "good measurement amp"?

[MikeB]

Hi lumanauw,

I did these checks on ordinary VAS, i checked symetrical and
asymetrical design. The symetrical design has 2 times the gain,
which is logical. I will check your design this evening...

"So 2nd harmonic is not adding "Niceness". It's just not detectable."

On a PCspeaker, yes. I have to make this test again, with the PC
connected to the Stereo... Maybe my ears are not good enough
for a representive test. (This i Psychoacoustic, varies between
different people) Maybe it's not only 2nd harmonic, but a
specific combination of 2nd & 4th ?

Because i want to know this, i will build a SE-ClassA, not tube,
i am not familiar with these devices.


Does anybody know if the jFet-model in Spice is accurate ?
Because if it is, it would mean that jFets have a far better
transfercurve then BJTs (Audiophile !) In Spice they produce
nearly no 3rd order, only 2nd order, and no highorder harmonics at all. If it is, i would make a SE-Amp nearly FET-only.

Mike

[Justcallmedad]

Referring to Post 1:

Your servo circuit can't work this way !
You don't have a reference for Q1 (resistor to ground), or your reference via Q2 is not 0V but 2*0.7V (check V at the base of Q1) Your DC-servo is in the inverting mode ?
I think that Q1 must "run" with more current, about 1 or 2 mA.

Your design is a CFP input, folded cascode VAS.

Maybe you can post your new design, or at least your new Q1 reference to see what's wrong ?

For CFP:

Have a look here ...

Post 42

And graphs here ...

Post 46

My actual design is a little different but it's sounds music !

Output stage not yet choose ...

[lumanauw]

In Spice they produce nearly no 3rd order, only 2nd order, and no highorder harmonics at all.

What about MOSFETs for front end. Like Passlab uses IRF610 or IRF9610. Are they producing the same harmonics as Jfets?

Your servo circuit can't work this way !

Well, it is not working, but why?

You don't have a reference for Q1 (resistor to ground), or your reference via Q2 is not 0V but 2*0.7V (check V at the base of Q1)

I'm aware that Q1 must have ground reference. Look at post #7. This design is for DC coupled. For AC/Capacitor coupled, we must add R#3 to set ground in Q1.

Your DC-servo is in the inverting mode ?

It has tobe inverting to work. You can trace the current, like what happens if the output is too positive.

I think that Q1 must "run" with more current, about 1 or 2 mA. Your design is a CFP input, folded cascode VAS.

Yes, you are right. Q1+Q3+25k resistor can be seen as 1 component, CFP transistor. They can be replaced with just one transistor, if you like. I'm still confused in this CFP configuration, what is the good value to be flowing in each transistor. FAB suggested 1/4 of total.

[MikeB]

Originally posted by lumanauw

What about MOSFETs for front end. Like Passlab uses IRF610 or IRF9610. Are they producing the same harmonics as Jfets?

In Spice they show the same behaviour, seems FET = FET
The MOSFETs have the advantage of having a much higher
max voltage (>100v), while jFets are up to ~40v.
But jFets like sk170 have a much higher gain, nearly as high
as a BJT.

I checked the behaviour of a diffamp fed with a plain resistor
from a low railvoltage. The 3rd harmonics nearly stay the same,
but the 2nd harmonic explode. (in my testcase from 10nV to 1mV,
tested with jFETs)

Mike

[Justcallmedad]

Hi, lumanauw.

Differential mode cancels 2nd harmonic only if you sum the ouput signal from the collectors of the differential pair. In your case, it's just a Common Emitter in CFP config.

For your CFP, the current in Q1, depends on what you plan for this stage, it depends of your choice for Q1/Q2 and their Hfe. You can start with 2mA (1/10 of Ic Q2).

Sorry about DC-Servo ! When i wrote this, on last post, i forgot that the output is "out-phase".
If you want to go at the end of your idea, maybe you can fix the problem with 2 diodes in series between the base of Q2 and the 1k from the IC, you need two resistors (10k) between the diodes and +V / -V to set the current (5mA). In this way with a trim between these diodes and the center to the base of Q2 you can adjust the offset. The op-amp in this circuit will "always go" for 0V at his output (0V reference). If you prefer, you can set the + input of the op amp at -2*Vbe.

[lumanauw]

Sorry, my English is bad.

Differential mode cancels 2nd harmonic only if you sum the ouput signal from the collectors of the differential pair. In your case, it's just a Common Emitter in CFP config.

Is this mean that if we take only 1 output from differential collector, it wont cancel 2nd harmonic? Aleph take only 1 output from the differential. Differential in Aleph doesn't cancel 2nd harmonics?

Again, sorry my English is bad.

If you want to go at the end of your idea, maybe you can fix the problem with 2 diodes in series between the base of Q2 and the 1k from the IC, you need two resistors (10k) between the diodes and +V / -V to set the current (5mA). In this way with a trim between these diodes and the center to the base of Q2 you can adjust the offset. The op-amp in this circuit will "always go" for 0V at his output (0V reference). If you prefer, you can set the + input of the op amp at -2*Vbe.

I dont get it. Could you draw a picture of your idea?

I found something interesting. I played CD of classic guitar instrument. The sound of the guitar seems have more "vibration" and "depth" than usually. Is this mean the "nice 2nd harmonic" is added to the amp (or at least not canceled as in ordinary 3 stages amp)?

Trying to get closer to the destination---->Nice sounding SS amp. I've heard SET amp, Manley 300B. It sounds nice. But for me too small power.

[thanh]

In Spice they produce
nearly no 3rd order, only 2nd order, and no highorder harmonics at all. If it is, i would make a SE-Amp nearly FET-only.

OOOH! Are you sure? Which circuit did you simulated? I'm using Orcad 9.2.My sim says that single ended Jfet produces 2nd ,3rd very much and a little high order harmonics .
I think Jfet with no source resistor only produces 2nd but in the real world Jfet alway is wired with a resistor
http://www.its.caltech.edu/~musiclab...per-crobat.pdf

[Justcallmedad]

Hi, lumanauw.

I got time to check your circuit, and tune it.
For the moment, forget my last post, it was just some design advises.

I think this general diagram should be ok.

R8 in my schematic, is just for simulation, remove it and insert your output stage bias between I3 and Q4.
You can replace I1 by a resistor between base and emitter of Q3 about 330 to 680 Ohms, for 1-2 mA of current for Q3.

I use OPA 627 or AD 743 for DC-servo but you can do it with an TL0701.
A small amount of local feedback is provided by R3,R4,R5, for stability and linearisation.

THD 2 about 1%, is really not important for the quality of sound, most significant are the higher order harmonics 5,6,7 etc... even at very low levels.


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