Old and surprisingly good amp - Page 2

WuYit · 22nd March 2011, 01:42 PM

Hi,
well, it can be a bit embarrassing to say, but I would perform a minor circuit clean-up in order to make the sound even more triode-like...

Elvee · 22nd March 2011, 02:13 PM

Quote:

Originally Posted by AndrewT

You are not wrong.
R3 is in series with the emitter, exactly like R8.

R4 does a different job. I don't see it as being wrong located.

What is R4's job then?

Quote:

The dynamic compensation of junction temperature of the power BJTs is via current feedback with R5, R10.
I have simulated that with the temp statement of LTSPice and it works well

This is not "compensation", just degeneration, and anyway, in such a "super-CFP" configuration, the Vbe to compensate is that of the drivers, not the output devices, and that's what the Vbe multiplier does.
The advantage with triples is the low power dissipation/variation in the drivers.

ontoaba · 22nd March 2011, 02:19 PM

Hi, WuYit, what is "minor circuit clean-up"?

I see a half CFA plus bootstrap, I never tried this one.
The best output stages I ever heard and built is EF facing CFA (bumping) in floating driver supply. Its sound is stuning me and my friends.
I never heard any expensive triode amps.

WuYit · 22nd March 2011, 02:28 PM

Howdy,
R4 is doing nothing, causes neither good nor harm.

WuYit · 22nd March 2011, 03:04 PM

ontoaba,
these triples were popular around that time, claimed to provide excellent linearity in some sort of meaning.
I would remove Q9 and Q3 to begin with...

WuYit · 23rd March 2011, 07:06 AM

...furthermore, I would remove the diode, C3, R3 and replace Q10 with a JFET...

tiefbassuebertr · 05:54 PM

Quote:

Originally Posted by hahfran

The design IS Quad303 triple cascade. The dynamic compensation of junction temperature of the power BJTs is via current feedback with R5, R10.
I have simulated that with the temp statement of LTSPice and it works well
up to 100 ° junction temperature of Q1 Q2 Q3 Q8. The qc of Q1 Q2 goes from 50 mA at 30° to 90 mA at 100° that is excellent. There is no time delay as it is in the case of the Vbe multiplier thermally coupled with heatsink .
The performance of this very simple design is amazing when Q1 Q2 are (as tested) 2N2922 which feature good ib/ic linearity up to ic 6 amps and ft 40 mHz.
The compensation C3 can be reduced to 15 pF and C2 to 10pF thus that distortion at 20 kHz is only second harmonic.
Btw making the triple cascade complimentary does not improve the performance, h2 becomes smaller but odd harmonics appear.

Quote:

Originally Posted by Elvee

What is R4's job then?
This is not "compensation", just degeneration, and anyway, in such a "super-CFP" configuration, the Vbe to compensate is that of the drivers, not the output devices, and that's what the Vbe multiplier does.
The advantage with triples is the low power dissipation/variation in the drivers.

In an old article from ELEKTOR (edition December 1972 EQUA project) this is detailled explain, also various differences (disadvantages) to the Quad 303 triplet buffer.
http://www.hifi-forum.de/viewthread-71-8390.html
also this patent could be of interest in this case
http://www.freepatentsonline.com/5726602.pdf

BTW - until this day I haven't find an effective solution to remove the tendency to oscillate in that PP buffer - even not with help of CAD. This is cause the differences between the upper and lower half and I am not able to find a right compensation network.

hahfran · 25th March 2011, 06:42 PM

Another significant advantage of this design is low reactve distortion. The thd is hardly ever measured on complex / reactive loads although every speaker is a complex load.
The good performance is possibly due to current feedback in the triple and to the input BJT. On the other hand CMRR is poor . There are BJTs with even better ib/ic linearity than 2SC2922 and BD139/140

ontoaba · 26th March 2011, 12:05 AM

Quote:

Originally Posted by WuYit

ontoaba,
these triples were popular around that time, claimed to provide excellent linearity in some sort of meaning.
I would remove Q9 and Q3 to begin with...

Then it has half CFA, bootstrap and triple output. If it really good, then is there any clue or issues why it disappeared?

WuYit · 26th March 2011, 11:51 AM

Hi,
there are problems with the triple. When composed of class AB biased bipolar transistors, it´s prone to oscillation and to produce unpleasant crossover distortion. The complementary feedback pair is actually great, but it works best in class A (with feedback through just two devices!)
On the plus side: single-ended input (nice "triode-like" sound, but higher sensitivity to ripple) and current feedback (wide bandwidth, good stability and low reactive distortion).

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22nd March 2011, 01:42 PM	#11
WuYit Banned Join Date: Oct 2010	Hi, well, it can be a bit embarrassing to say, but I would perform a minor circuit clean-up in order to make the sound even more triode-like...

22nd March 2011, 02:19 PM	#13
ontoaba diyAudio Member Join Date: Nov 2009 Location: Kudus, & Malang	Hi, WuYit, what is "minor circuit clean-up"? I see a half CFA plus bootstrap, I never tried this one. The best output stages I ever heard and built is EF facing CFA (bumping) in floating driver supply. Its sound is stuning me and my friends. I never heard any expensive triode amps.

22nd March 2011, 02:28 PM	#14
WuYit Banned Join Date: Oct 2010	Howdy, R4 is doing nothing, causes neither good nor harm.

22nd March 2011, 03:04 PM	#15
WuYit Banned Join Date: Oct 2010	ontoaba, these triples were popular around that time, claimed to provide excellent linearity in some sort of meaning. I would remove Q9 and Q3 to begin with...

23rd March 2011, 07:06 AM	#16
WuYit Banned Join Date: Oct 2010	...furthermore, I would remove the diode, C3, R3 and replace Q10 with a JFET...

25th March 2011, 06:42 PM	#18
hahfran diyAudio Member Join Date: Dec 2007	Another significant advantage of this design is low reactve distortion. The thd is hardly ever measured on complex / reactive loads although every speaker is a complex load. The good performance is possibly due to current feedback in the triple and to the input BJT. On the other hand CMRR is poor . There are BJTs with even better ib/ic linearity than 2SC2922 and BD139/140

26th March 2011, 11:51 AM	#20
WuYit Banned Join Date: Oct 2010	Hi, there are problems with the triple. When composed of class AB biased bipolar transistors, it´s prone to oscillation and to produce unpleasant crossover distortion. The complementary feedback pair is actually great, but it works best in class A (with feedback through just two devices!) On the plus side: single-ended input (nice "triode-like" sound, but higher sensitivity to ripple) and current feedback (wide bandwidth, good stability and low reactive distortion).