Will this work as audio amplifier?

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While trying to understand how a differential pair works, I came up with this idea. The differential pair Q3 and Q4 can be dragged up close to the rail, because the CCS1 can be so low in voltage requirement. If the differential is close to the rails, it can perform as VAS. So Q3 and Q4 is VAS differential. Q4 will drive output stage, but it will need signal depended current change in Q3.

The input is fed to Q1. Q1 is only biased about 156uA (very low) to get sensitive operation. I dont know about noise figure, maybe bad. Q1 drives 25k resistor that fed the signal to Q3.

The problem is, how to get current fluctuation on Q1 that is dependent on input signal. If the emitors of Q1 is connected to CCS, it wont work, because Q1 will only form a common base transistor. So, the emitors of Q1 will need fixed value for reference. This reference is formed by Q2 (from its base). Q2's base is driven by a servo, I assumed about -1V2, so the output is steady at ground voltage.

Q2 is the escape path for the current in Q1+Q3 and works also for "reference" for Q1.
I can use Q1 directly to Q4's emitor, without 25k and Q3 so it will perform folded cascode. But I wanted to asked about this particular design.

I know the dissipation in the transistors are big, but it can be tackled by small heatsinks. The output also will drop some significant voltages from the rail.

1. Whats the effect that the input stage is running in very low bias (156uA at Q1)? Will it have bad noise figure (audible noise)?

2. What will be the harmonic properties in this configuration. Will it have bigger 2nd harmonic than ordinary 3 stage design, because input is only fed to 1 transistor (Q1)? Differential at input stage tends to cancel 2nd harmonic, dont they?
 

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lumanauw said:
What is the mechanism of 2nd order canceling in differential pair?


If i understood it correctly, odd harmonics are symetrical distortions,
even harmonics asymetrical distortions.
As a diffpair works symetrical, (1st transistor closes, 2nd opens
exactly the same amount) it can only produce odd harmonics.
It doesn't cancel K2, it simply can't create them.

If you add a currentmirror, this looks completely different...

Mike
 
The reason I put Q1 and Q2 that way is to advoid 2nd harmonic cancelation (because wanted to try how "nice" it is to have 2nd harmonic not canceled, some sort of experiment, if it works).

If I put Q1 and Q2 same type Nch transistor, it will perform differential.
From how I look at differential, it always have 3 current path, 1 is downward (usually to CCS) and 2 upward to each diff transistor.
In the configuration I make, I use 1 Nch and 1Pch, with both emitors attached to each other, like differential. The difference is that in this configuration, it will only 2 current path (from C of Q1 to C of Q2), not 3 like ordinary differential.

But the consequence is that we have to put a reference voltage in base of Q2, so it will have signal depended current change flowing from Q1 and Q2----> needed to activate the VAS differential of Q3 and Q4, to perform audio amplifier.

The drawings I attach if for DC coupled power amp. If we put cap in the input (before 1k resistor attached to base of Q1) then base of Q1 will need another resistor to ground, for putting ground reference in base of Q1 (so it will be 3 of them, 1=1k=input resistor, 2=100k=feedback resistor, and 3=for ground stand point).

If we have #3 resistor attached, there will be current flowing from collector of Q1 to collector of Q2, when the base of Q2 is about <-1V2 (>of 2 VBE) from ground (because Q1's resistor is about ground voltage by R#3, and Q2's base is modulated about -1V2 by the servo, or by a trimpot. I use servo, because the change in Vbe because temperature will add, not cancel each other like differential, because we use 1Nch and 1Pch. In differential this thermal Vbe change will cancel each other--->DC offset can be maintained without servo).
Q1 and Q2 will perform some kind of signal depended current switch.
 
I've spent some hours yesterday with spice, in theory it should
be possible to build a diff amp (or something else) that cancels
out 3rd harmonics, but its very difficult. Had no real luck yet...
The problem is, that even harmonics are by nature shifted 90deg
in phase, so adding (subtracting) complementary signals automatically
cancel them (-90deg <-> +90deg -> 180deg = negative).

The odd harmonics are not phaseshifted, so the idea might be to
somehow "reverse" the transfercharacteristic of a bjt.

I believe there is a reason, why most famous ClassA-Amps don't
use diffamps. (like zenquito,hiraga,JLH etc.)

The simplest solution yet is to use CFP in the diffamp, it does a
great job in reducing harmonics, but does not cancel them.
(CFP "only" linearizes, reducing thd of the diffamp to ~0.02%,
without GM-degenerating, and boosting the gain to 1:1500)

At this moment i'm trying a circuit with CFP in the diffamp, i think
it should be functional this evening. I can't wait hearing it...
But maybe there is a reason, why nearly noone uses CFP in the
diffamp ? (Except steven and some cyrus-amp)

Mike
 
I also wonders about this CFP differential. MikeB, since you have spice, could you trace how low bias current can to in the first transistor in CFP differential? The Cyrus is about 100-150uA, but is that can be made lower?
Doug Self stated that the optimal value for this CFP front resistor is 2k2 (that gives about 270uA bias). He said if the resistor is bigger than this, the noise figure will increase.
What happens if this resistor is 10k or 22k? What noise will be generated? Bias so low=bad noise?
 

fab

Member
Joined 2004
Paid Member
MikeB said:
...
But maybe there is a reason, why nearly noone uses CFP in the
diffamp ? (Except steven and some cyrus-amp)

Mike

hi Mike

At least Halcro, Lavardin and me use CFP in the input diff amp. I have also seen other amp circuits from what I remember. If it is not widely used it does not mean that it is not good. If it was so obvious to know what is the best topology in an audio amp then everybody would use the same basis.

Fab
 
Hi lumanauw, the reason is very simple: the two emitters of
the diffamp have a voltageswing of the middle of both inputs,
that means nearly exact the inputvoltage (plus some harmonics)
for a voltageswing of 0.7v at voltagerail 15v, this means that
the current fed into the diffamp differs about 4.6%.
The higher the voltagerail, the less this effect. At some voltage
the plain resistor can give a better constant current then a ccs.

I finished my amp this night, but this late in the night i was
not able to make extensive listeningtests. But i can say, that
i'm very impressed by the quality. The sound is crystalclear and
clean, and the instrumentseparation is a dream. Also it's like
you can see/feel the singer. (I tested with "brother in arms")
(hmm, i hate monolistening)
And no sign of any hard sound.

I used 2K for the CFP-Resistors, the CFP-BJTs are bc556b, the
input transistor are sk170. I skipped the currentmirror, to get
maximum bandwidth (-3db at ~1mhz :bigeyes: )
Outputdevices are MJL1302A & co.
I already hear some noise, but only when pressing the ear directly
to the tweeter. It seems that i already have some thermal
problems in the diffampstage, the DC drifts ~60mv with a
frequency of ~30secs.
I still have to investigate this phenomen, maybe the diffamp is
not balanced enough. (2.2mA to 2.8 mA)
In spice i tested with upto 10k for the CFP-resistors, but the
difference in THD was minimal to the 2k.
Maybe i will downgrade to 1k, if the thermal drift keeps...
Or i reduce the current to the diffamp.
I have to recheck the datasheet for the sk170...

Mike
 
Is this good or bad?

the current fed into the diffamp differs about 4.6%.
The current fed to differential will varied if the rail voltage is low (like only +/-15V and using only plain resistor, not CCS). But is this good or bad (audibly and measurement)

This is kind of scary but we must know. Finding good sounding is not the same as finding good measurement.

True story -- There's a guy named Andrew Hefley that is a very good solid state designer. Do a web search and you'll see he has a long and distinguished resume. You can even see some of his circuits in an app note he wrote for Motorola (now available at On Semiconductor).

About six or eight years ago, he heard his first single-ended triode. And do you know what he did? He quit the business. Now he makes instrumentation for light aircraft.

He told me that after 30 years of designing solid state audio amps that he figured he knew it pretty well. But after hearing the SET, it showed him that pretty much everything he knew and believed was wrong. Instead of going back to the drawing board, he literally quit the field.

There are other guys here, like Nelson, who didn't quit. Instead, they started exploring a completely different approach than "perfect steady-state performance on a test bench into a resistive load".

Thinking is good. Listening is better. Listening and thinking about what you are hearing is best.
 

fab

Member
Joined 2004
Paid Member
Re: CCS and plain resistor

lumanauw said:
FAB,
How big is the CFP resistor can be (how low the current minimal in the first transistor)? Smaller than 50uA is OK?

hi lumanauw,

Unfortunately, I have not preformed a study on the optimal value for that resistor. I use 2k2 value. I think the resistor value depends also on the ratio of first input transistor collector current versus the second transistor collector current. Gerrard Perrot (Lavardin Technology) in his patent 5,635,874 suggests to use 1/4 of second bjt collector current in the first bjt collector current as an example and I believe that he may have performed a thourough analysis and testing to determine this ratio of collector current. I also think that if you use a too small current you may have an increase of noise depending on the input transistor noise characteristic.

Fab
 
Hi lumanauw !

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.

I will check with the ccs to the diffamp this evening, but from
my imagination, using a resistor with low voltagerails should
create 2nd harmonics. (but maybe only with asymetrical topology)

BTW, i did some extensive listeningtests with my CFP-testamp,
and i must say: Wow, CFP rulez ! is it really patented ?
Of course it's still preferable to find a topology for ClassAB that
cancels out odd harmonics.

Mike
 
thanh said:

Ok! I re-say some word from a member of diyaudio
"Some one like music and some one like distortion"
I think "some one like HIFI and some one like distortion"
I hate odd harmonics too!
:)

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 !
(Okay, played only through pcspeakers) But this means that
simple THD-measurements are useless to describe the soundquality
of an amp. (Not a new discovery at all...)
I still don't know what i prefer, HIFI (ultraprecise reproduction),
or some nice harmonics... I am still on the quest to a ultraprecise
ClassAB. (Maybe found it last night) I think i "simply" need to
construct a single ended ClassA, not showing any odd harmonics.
(And then compare them)
Mike
 
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