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Alex Pogossov Alex Pogossov is offline
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Default Quad 405 with current dumping

In another thread Phil Allison commended Quad 405.
Its typical schematic is he
http://schematic.narod.ru/Audio/semi/Amp/Quad405-2.gif

Assuming that Phil knows how it works, and how R38, L2, R6, R20||R21 form a
perfect bridge, blah-blah-blah, here is a question:

What is the purpose of diodes D5 and D6 connected in series?
------------------------
In my opinion, any bridge to achieve effect, needs to be "well balanced".
With typical componebnt tolerances and temperature coefficients in practice
it is difficult to balance the bridge better than to -20dB (unless you do
not select components on test or trim one of the leg of the bridge).

Thus the bridge would typically reduce distortion of the output stage by
20dB (on top of what GNFB does).

I would prefer to reduce distortion in the output stage by design, rather
than making the power stage non-linear and then rely on the bridge.

I have always been impressed by smartness (non-boringness) of Quad design,
but would not be relying on the current dumping for performance.

Regards,
Alex


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Quad 405 with current dumping


"Alex Pogossov"

In another thread Phil Allison commended Quad 405.
Its typical schematic is he
http://schematic.narod.ru/Audio/semi/Amp/Quad405-2.gif



** Err - I commented on the Quad 306.


Assuming that Phil knows how it works, and how R38, L2, R6, R20||R21 form
a perfect bridge, blah-blah-blah, here is a question:

What is the purpose of diodes D5 and D6 connected in series?



** I have seen a few 405s for repair ( the PCB electros fail after about 20
years) but make no claim to be an expert on them.

But seeing as you asked, here is my take: D5 and D6 ensure output
transistor TR9 remains biased off at idle and at low output current while
permitting some current flow through R38 into the load.

Facts to keep in mind about the 405:

1. The output transistors operate in pure class B, ie with zero bias
current.

2. TR7, the "class A" stage transistor drives the upper output transistor
TR9 direct while TR8 is the driver for the lower one, TR10.

3. The bridge network allows the class A stage to drive the load direct when
the output transistors are biased off, so virtually eliminating crossover
distortion.

4. It really worked - Quad were able to use slow, rugged NPN output
transistors in the original 405, have no bias adjustment or thermal
compensation for same and yet get vanishingly low THD and crossover
distortion.

5. Later incarnations of the "current dumping" topology simply added extra
NPN output transistors in parallel with the first pair to increase power and
current output. No special matching of these transistors was required since
there is zero bias.


I would prefer to reduce distortion in the output stage by design, rather
than making the power stage non-linear and then rely on the bridge.


** How boring.


I have always been impressed by smartness (non-boringness) of Quad design,
but would not be relying on the current dumping for performance.


** Good thing you don't have to then.

But Quad sold over 200,000 405s plus large numbers of 306s and various
relatives making them the highest selling hi-fi power amps of all time.


.... Phil




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Alex Pogossov Alex Pogossov is offline
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Default Quad 405 with current dumping

Typo correction. (not very clear on the schematic):

The famous bridge consists of R36, L2, C8, R20||R21.

R36 = 47R;
L2 = 3uH;
C8 = 120pF;
R20||R21 = 500R.

Note that op-amp IC1 is firtually out of the GNFB loop. It is only as a
preamp with gain of 15 and for DC stabilisation.


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Quad 405 with current dumping


"Alex Pogossov"

Note that op-amp IC1 is firtually out of the GNFB loop. It is only as a
preamp with gain of 15 and for DC stabilisation.



** IC1 has four jobs:

1. An inverting stage with gain of 15 times making the total gain 56 times.

2. A DC offset servo with C2, R4 & R5.

3. A sub sonic filter, -3dB at 13Hz and -20dB at 5 Hz.

4. With R11 fitted, it clamps the input signal so the max power becomes
half.

These last three together make to 405 safe to use with an ESL57.


.... Phil











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Alex Pogossov Alex Pogossov is offline
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Default Quad 405 with current dumping


"Phil Allison" wrote in message
...

"Alex Pogossov"
In another thread Phil Allison commended Quad 405.
Its typical schematic is he
http://schematic.narod.ru/Audio/semi/Amp/Quad405-2.gif
What is the purpose of diodes D5 and D6 connected in series?


But seeing as you asked, here is my take: D5 and D6 ensure output
transistor TR9 remains biased off at idle and at low output current while
permitting some current flow through R38 into the load.


=== Alex:
Yes, at idle pre-driver Tr8 will be biased to conducting about 15mA. Power
transistor Tr10 will not even be conducting -- R33 is too small.

But why did they decide to get the output stage run in class A at very low
levels? Probably to reduce distortion at low levels. Running at least Tr8
allows the bootstrap C10R31 to function increasing open-loop gain. Probably
this amp would sound screechy if at low levels the outpud would have been
completely cut off. There is in fact a cross-over kink. Below output stage
operation the feed-through bridge resistor attenuates the signal 6 times
(with 8R load). Plus on top of that the bootstrap not working would made
things even worse.


Facts to keep in mind about the 405:

1. The output transistors operate in pure class B, ie with zero bias
current.


=== Alex:
I think one silicon diode or perhaps even one silicon + one germanium
between the bases of Tr8 and Tr9 would not hurt. The size of the kink would
be reduced while still staying in class B without any risk of thermal
runaway.


2. TR7, the "class A" stage transistor drives the upper output transistor
TR9 direct while TR8 is the driver for the lower one, TR10.


=== Alex:
I noticed that, but I believe another "darlingtonising" transistor in front
of Tr9 would make load on the class A stage more symmetrical, would increase
loop gain on positive waves and overall reduce distortion.


3. The bridge network allows the class A stage to drive the load direct
when the output transistors are biased off, so virtually eliminating
crossover distortion.


=== Alex:
As I said, the crossover exists: at low levels below the output stage
activation the lop gain is at least 6 times smaller. It causes kinks and
distortion which then is marvelously almost (but not completely) compensated
by the bridge. In a conventional class AB output stages we often have kinks
the other way -- at low levels the gain is higher (closer to unity) because
both (idle current taming) emitter resistors work in parallel (class A),
while at high levels -- one at a time.


4. It really worked - Quad were able to use slow, rugged NPN output
transistors in the original 405, have no bias adjustment or thermal
compensation for same and yet get vanishingly low THD and crossover
distortion.


=== Alex:
Indeed, in those days it was quite revolutionary.




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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Quad 405 with current dumping


"Alex Pogossov"

** Dear Alex,

I have answered you bull**** question.

I see now it was a trap.

YOU really wanted ME to debate YOU about YOUR opinions about the Quad 405.

You can keep them.

How about you debate Relativity, Climate Science or the future of the ALP
next ?

You know the forums to go to.



.... Phil




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Alex Pogossov Alex Pogossov is offline
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Default Quad 405 with current dumping


"Phil Allison" wrote in message
...
I see now it was a trap.

YOU really wanted ME to debate YOU about YOUR opinions about the Quad
405.


Rubbish.
Opinions are not debated, but can be exchanged and kept anyway.
I wanted to *discuss* some pecularities of this circuit perhaps to
understand it more.

Strictly speaking this forum is not for transistor amps. I can go to DIY
Audio, for example.


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Quad 405 with current dumping


"Alex Pogossov"

"Phil Allison"

I see now it was a trap.

YOU really wanted ME to debate YOU about YOUR opinions about the Quad
405.



Rubbish.



** It is right there in B&W for all to see - you pathetic ******.


Opinions are not debated,



** Of course they are - you asinine ****head.

**** off.


..... Phil




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