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Quad 303 triple cascade
Quad 303 triple cascade
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Old 3rd October 2012, 11:23 AM   #31
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by tiefbassuebertr View Post
Only truly experienced (and unfortunately very few) developers (like Quad) achieve a design of such triple stages without any oscillation under any load condition.

I recall, that I have for checking a diy amp device, that uses also such - slightly different topology:
The name was "Equa" from German magazine "Elektor" December 1972, page 1216
This amp was an unwanted oscillator (even by p-Spice simulation and in real life only to test by current limited power supply without destroy the output devices).
Only after remove one stage in the output buffers I get satisfy operating.
About this address perhaps there is a complete Equa schematic available:
Audioclub Oost Brabant, clubavonden

I want to have basic articles about such three stage super quasi complementary stages. Who knows more about this?

I know too few English keywords therefore. I recall, that I have read some years ago a JAES article. But I don't know if it was the main topic.
Check out follow replies to post #3403 about
Bob Cordell Interview: Negative Feedback
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Old 4th October 2012, 05:32 AM   #32
Ian Finch is offline Ian Finch  Australia
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I have lost the original magazine, a UK edition but "Elektor" Summer Circuits edition 1980, had a 2N3055 "supertransitor" quasi circuit if I recall correctly. THD was quite low, bias 25-100mA. Perhaps this is of interest.

Recently, I bought a VGC Quad 303 and came across another commercial design in my searches, with the same output stage. If I can find it again, I'll post a link
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Last edited by Ian Finch; 4th October 2012 at 05:38 AM.
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Old 4th October 2012, 11:20 AM   #33
EssB is offline EssB  United Kingdom
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Ian, FYI the Crimson Elektrik amps used this output stage. I think tiefbassuebertr already knows about them
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Old 4th October 2012, 12:46 PM   #34
tiefbassuebertr is offline tiefbassuebertr  Germany
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Ian, FYI the Crimson Elektrik amps used this output stage. I think tiefbassuebertr already knows about them
Yes; the attached file by post #2 about
Crimson (Krimson) Amp Schematics
show a schematic.
Quote:
Originally Posted by Ian Finch View Post
I have lost the original magazine, a UK edition but "Elektor" Summer Circuits edition 1980, had a 2N3055 "supertransitor" quasi circuit if I recall correctly. THD was quite low, bias 25-100mA. Perhaps this is of interest.
In the summer edition from German's magazine Elektor (1983) is in fact a beta enhancer, but with a quadruple (instead a triplett) arround a 2N3055. The actually wrong headline is follow: "2N3055 Darlingtons" (circuit number 73, page 7-92). For the positive half there is at first a NPN (BC546B), then a PNP darlington consist of a BC556B and a BD140 and then the 2N3055.
For the negative half there is at first a PNP (BC556B) and then a NPN triplett consist of a BC564B, BD139 and a 2N3055, but it seems to be a missprint, because the emitter of 2N3055 from the neg. half goes not to the negative rail.

The main issue for me by these methods of beta enhancing is the fact, that there are no clearly design rules.
Please note: by quasi complementary tripletts there are three different arrangements for the topology, and in case of a quadruple yet much more. Also by the idle current for each transistor stage there are very many possibilities.

Last edited by tiefbassuebertr; 4th October 2012 at 12:56 PM.
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Old 7th October 2014, 10:32 AM   #35
tiefbassuebertr is offline tiefbassuebertr  Germany
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are there any news concerning design rules for quasi complementary triplet cascades?
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Old 9th March 2018, 10:06 AM   #36
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by tiefbassuebertr View Post
are there any news concerning design rules for quasi complementary triplet cascades?
by chance I read this statement concerning the output buffer stage of internal circuit of the LM3886 from TI (formerly NS) under
http://www.ti.com/lit/ds/symlink/lm3886.pdf
on page 7:
One common question is whether the output section of the amplifier is biased into Class-A operation. Technically, the answer is “yes,” as the exceptional linearity of the output section design achieves the advantages of pure Class-A operation.
Using the “Intelligent Power Transistor with Gain” system keeps the output section properly biased at all times to eliminate crossover distortion caused by the output transistors during turn-on and turn-off, as opposed to the brute force Class-A biasing techniques used in other inadequate amplifier designs to compensate for poor linearity that result in low efficiency, excessive heat, electrical waste, and the need for power management.


I note, that both driver and output stage at both halfs are create by a NPN darlington configuration - only at the neg. half there is a PNP transistor and two diodes for phase inverting (only small signal area). It seems to be an advantage, that no PNP high- and medium power transistor is in use inside of the output power buffer. Is my estimate right ? If yes, there are no justify of the use for PNP power devices in output stages of new audio power amp projects.
I would like to know, where this approach from LM3886 with only NPN transistors in the output buffer (maybe for the remove of residual class-B distortion) is realized in discrete power amplifiers - thank you very much for an advice.

Last edited by tiefbassuebertr; 9th March 2018 at 10:17 AM.
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Old 9th March 2018, 01:45 PM   #37
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by tiefbassuebertr View Post
[I][B]One common question is whether the output section of the amplifier is biased into Class-A operation. Technically, the answer is “yes,” as the exceptional linearity of the output section design achieves the advantages of pure Class-A operation.
This is of course a pure lie - they say that the stage is as good as class A, not that it is biased in class A, which it is not. A lie.

As to your other questions: the use of NPN-only stems from the time that fast complementary PNP power devices were simply not available and they were 'fudged' with an NPN with a PNP driver.

That they now can turn this ancient, technically inferior method around and present it as something positive is a tribute to a very good marketing department and an advanced technique called lying .

Jan
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Old 9th March 2018, 01:59 PM   #38
john_ellis is offline john_ellis  United Kingdom
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I did not see this description on page 7!.

However, some comments on the Quad 303 triples: in evaluation circuits I have built, only the original RCA-2N3055 (the old slow hometaxial devices) did not oscillate. With the newer faster devices, I had to make some circuit changes to prevent oscillation. I have successfully tested two OP stages using BD139/BD140 pairs in the triples, and ZTX304/ZTX504 input followed by BD139/BD140 with the new 2N3055's, and doubled the frequency response over the original 303.

I have found that, I think along with many others, a straight triple NPN/NPN/NPN and complement are more stable than any combination of inverting triplets.

To set the quiescent current in the Quad, I usually monitor the VAS collector voltage. Since that exhibits greater distortion it is easier to see whether the response has a straight line in the crossover region, or is faster (too low a bias) or slower (too high a bias) - but with the RCA-2N3055(H) the optimal currents were different at 1kHz and 20kHz, so I set it for 20kHz. Not so obvious with the faster devices.

Regarding the 3886, it seems that the PNP's are probably lateral types with low gain. That would allow them to work at high differential input voltages (as they don't have "true" emitter-base junctions) and does not add a particularly high gain which would cause imbalance in the OPS. The kink in the distortion at around 1W looks strange, not something I have seen in the 303 output (have seen a more gradual kink) but the level of distortion may be low enough so as not to be audible.

Eliminating crossover distortion seems best addressed by avoiding Miller capacitors in the VAS stage. Instead, use passive C or CR loads (to ground) e.g. 47-220pF (depending to some extent how much current the VAS can supply) and global compensation or at least output inclusive Miller, but I have not seen the technique used in the 3886 used in a discrete design - probably no need!

Cherry suggested OIM stabilisation (but did not call it that) in his "nested" circuits, which seems to account for the low crossover distortion in his design, rather than the nested feedback, from what I have investigated.

Last edited by john_ellis; 9th March 2018 at 02:05 PM.
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Old 9th March 2018, 02:18 PM   #39
Juergen Knoop is offline Juergen Knoop  Germany
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Quote:
Originally Posted by tiefbassuebertr View Post
by chance I read this statement concerning the output buffer stage of internal circuit of the LM3886 from TI (formerly NS) under
http://www.ti.com/lit/ds/symlink/lm3886.pdf
on page 7:
One common question is whether the output section of the amplifier is biased into Class-A operation. Technically, the answer is “yes,” as the exceptional linearity of the output section design achieves the advantages of pure Class-A operation.
Using the “Intelligent Power Transistor with Gain” system keeps the output section properly biased at all times to eliminate crossover distortion caused by the output transistors during turn-on and turn-off, as opposed to the brute force Class-A biasing techniques used in other inadequate amplifier designs to compensate for poor linearity that result in low efficiency, excessive heat, electrical waste, and the need for power management.
please show us a screenshot! I know NS datasheets have degraded in language and intellectual requirements some years ago. Yet such a statement is not part of the actual datasheet and it was not in the late 90's when I build an amplifier with LM3886.

Last edited by Juergen Knoop; 9th March 2018 at 02:26 PM.
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Old 9th March 2018, 04:54 PM   #40
jan.didden is offline jan.didden  Europe
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The data sheet says 'The LM3886 is a Quasi-AB type amplifier.'

Jan
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