Help in design: adding a Baxandall diode across emitter in Quasi Complimentary Stage - diyAudio
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Old 18th November 2014, 07:07 AM   #1
ygg-it is offline ygg-it  Italy
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Default Help in design: adding a Baxandall diode across emitter in Quasi Complimentary Stage

Hello can you please help me to add a Baxandall transdiode or diode across R139 or (may be better) across R316 without doing a disaster?

The amplifier is National Panasonic SA-73 (1971)


1) Is it better a power diode across R319 (1 Ohm) or a transdiode across R366 (39 ohm)

2) Can I add directly in parallel or I have to change any components, like rise R316 equal to R317 (180 ohm)? Which resistor value in parallel?

3) Should I have to add a few nF capacitor in parallel ?

4) Which diode/transistor type do you suggest (see below cross ref)? I would like to maintain a vintage configuration so i need to add components exsisting in the seventies

5) What voltage/bias level I should read to be sure that everything works, after adding the Baxandall?


Legenda:

R318, R319: 1 ohm
R314,R317: 180 ohm
R316: 39 ohm


R306 (wrong): 3K9 ohm

TR308,309: 2SD154
TR306: 2SC853
TR307: 2SA545

TR301,302: A2SA564
TR303: CS1383
TR304: 2SC828
all diodes: DS410


PS: I'm trying to simulate with MicroCAP: the problem are the missing models. May be:

2SC853 == BC639 ??
2SA545 == BC640 ??
2SD154 == BD241B, 2N3054 ??

2SA564 == BC213, BC308, BC558 ??
S1383 == 2N4919, 2N4236 ??
DS410 == 1N4148 ??
2SC828 == BC183, BC238, BC548 ??

PS: According to:

http://ochtedete.free.fr/!-amplis/qu.../QuasiComp.PDF
I should add a 10nF~22nF parallel capacitor but I cannot see the pic here...
Attached Images
File Type: jpg SA-73.jpg (624.8 KB, 263 views)

Last edited by ygg-it; 18th November 2014 at 07:10 AM.
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Old 18th November 2014, 07:31 AM   #2
Mooly is online now Mooly  United Kingdom
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A diode across R316 would be the correct location for a "Baxandall Diode" and using a general purpose device such as 1n4001. I wouldn't like to say whether the resistor should be tweaked or not. Typically you might see nearer 100 ohm here... one to simulate

(models in simulation probably have only a minor effect of what you are looking at, just use generic common devices)
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Old 18th November 2014, 10:08 AM   #3
Orit is offline Orit  Italy
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While on this subject, I'd like to point out that neither Shaw nor Baxandall actually invented that circuit modification, as their articles are dated 1968/69, if I'm not mistaken. The Scott Stereomaster 342 / 382 receiver, released in 1965, already had that circuit.
Probably Baxandall and Shaw were the first to actually analyse and divulgate that modification to the technical public.
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Old 18th November 2014, 11:25 AM   #4
AndrewT is offline AndrewT  Scotland
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The Quasi output stage only occurs at the two NPN devices.
Prior to that the amp is complementary.

What the diode/capacitor/resistor do is ensure the NPN and PNP driver transistors see equal loading on their emitters.

The sch shows the NPN driver emitter loaded with R314||baseTR308
That base looks like a diode and a capacitor in parallel. The effective load on the NPN driver is R314||diode||capacitor.
Now look at the PNP driver.
It is loaded with R316, nothing else.
The parallel effect of the output transistor is missing.
That's where the Shaw and Baxandall modifications come in.

One ADDS the parallel loads to TR307's emitter to mimic what TR306 sees.

Once you see that, you know where to put the added C & D, in parallel to R316.

However, we are not finished.
The two NPN transistors are not loaded equally.
TR308 has an emitter resistor, TR309 has none. Three little piggies went to market.
Add an emitter resistor to TR309.

Now compare the upper combination R314 TR308 R318 arrangement to the lower side, R317 TR309, Re.
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Last edited by AndrewT; 18th November 2014 at 11:31 AM.
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Old 18th November 2014, 12:52 PM   #5
ygg-it is offline ygg-it  Italy
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Thanks, but can you please advice what value should be R316 (317) and R319 if I just put a diode across them?

I don't want to make to much changes overall adding new resistors....
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Old 18th November 2014, 01:25 PM   #6
Mooly is online now Mooly  United Kingdom
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One of the problems I forsee with this amp (modifying it) is that the bias stability may suffer.

I would tend to disagree with Andrew about adding an emitter resistor too. The "Shaw" scheme would add the diode in series with R319 and with the emitter of the driver trasistor being returned to the collector of TR309 and not the output line as it is now. You would still bypass the diode and collector resistor with the 39 ohm, perhaps making that a bit higher.

The Baxandall scheme adds a series diode to the driver transistor emitter. This would probably also be returned to the collector of TR309 rather than the output line. The diode could also be bypassed with a resistor and small cap.

What you do to the lower driver as regards its emitter connetion you do to the top, so that means transferring R314 from its present location to being simply across base-emitter of TR309.

(it is an ideal candidate to simulate actually... you could have a lot o fun trying the various configurations out)
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Old 18th November 2014, 07:59 PM   #7
Mooly is online now Mooly  United Kingdom
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The three schemes, Quasi, Baxandall and Shaw. Values carried over from your circuit and not optimised. Iq set to 10 ma for all three. I've written the T.H.D. on each image. Output was at 10.5 volts RMS 1kHz. This was the max swing available before asymmetric clipping on the negative side.

Enjoy
Attached Images
File Type: png Quasi.PNG (197.6 KB, 183 views)
File Type: png Baxandall.PNG (199.3 KB, 173 views)
File Type: png Shaw.PNG (199.4 KB, 173 views)
Attached Files
File Type: asc Quasi Amp.asc (5.6 KB, 6 views)
File Type: asc Quasi Amp Baxandall.asc (5.8 KB, 9 views)
File Type: asc Quasi Amp Shaw.asc (5.9 KB, 8 views)
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Old 18th November 2014, 10:08 PM   #8
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I came across the Baxandall diode when I ventured into quasi complimentary output amplifiers.
In my design I also found I needed a capacitor 100pf across CB on Q4.
Without it I got a bit of oscillation on the output.
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Old 18th November 2014, 10:11 PM   #9
sreten is offline sreten  United Kingdom
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Hi,

I like the Baxandall scheme more than the Shaw, its just
a lot more practical in terms of the diode used, and allows
you to adjust R3 more usefully I'd suggest than Shaw.

TBH I'm not too sure the examples shown by Mooly actually apply to
the original circuit, and without a lot of further investigation of the
Panasonic circuit AFAICT a diode won't help. It already has a
symmetric loop that probably addresses the problem, or not,
I can't be sure, but cannot see the simple cases Mooly shows.

FWIW I really don't like the schematic, its very confusing.

rgds, sreten.
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Old 19th November 2014, 06:07 AM   #10
ygg-it is offline ygg-it  Italy
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Default Two Baxandall Schottky ??

Hi, many many thanks for the LTspice models.

Improvements are really visible!

There are few components uncorrect, so I redesigned it according to original schematic.

After many simulations I found:

1) original = 0.017% THD
2) with Baxandall diode (added 100 pf Cap, as suggested) = 0,007% THD
3) with two serie Schottky = 0.005% THD
4) optimised R316 resistor = 47 ohm (no big change THD, so I will stay with original 39 value)
5) a transdiode is performing worse than the diode


What do you think about two serie Schottky ?
Attached Images
File Type: jpg Simil SA-73.jpg (72.5 KB, 153 views)
File Type: jpg Baxandall.jpg (79.9 KB, 76 views)
Attached Files
File Type: asc Quasi Amp Baxandall mod.asc (6.1 KB, 13 views)
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