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Peter Walker and his current dumping principle
Peter Walker and his current dumping principle
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Old 17th September 2018, 04:39 PM   #111
traderbam is offline traderbam  Europe
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It's a tricky design challenge. The bridge can only stop Vbe feedback through the 120pF & 500 ohm parts. The bridge cannot stop parasitic feedback in the transconductance amplifier: for example the Ccb of the driver transistor Tr7 (RCA 40872). Quad has added C9 and C11 feedback paths too, presumably for stability reasons. I did not notice this mentioned in those articles but I would note parasitic feedback paths as another reason the Vbe feedback distortion can not be completely eliminated in practice.

Stability may also relate to Chris Daly's citation of Keith Snook's "proper location of C8". In the theoretical model, the 120pF and 500 ohm resistor should both connect to the same point of input to the transconductance amp. In the 405 that would mean 4 BJTs sequentially in the signal path of the NFB loop. Asking for trouble. I didn't read it carefully but I think Snook had to bypass one of them in order to relocate C8; maybe for this reason.

Another question in my mind concerns the output stage. The transconductance amp bias is about 45mA and this drives the BDY77s. Sort of short on current compared to conventional designs. The current limiter (about 8A?) is only on the positive rail but I doubt the negative could reach that with only 45mA drive current when the BDY77 has max gain of 140. Ok so you don't absolutely need 6A at normal listening into efficient speakers but it will run out of puff with 4 ohm loads and below.

The circuit is arguably more complex than the 303 and not as powerful. Does it sound better than the 303?

Some user comments: Quad 405-2 vs 303
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Last edited by traderbam; 17th September 2018 at 05:06 PM.
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Old 17th September 2018, 05:46 PM   #112
steveu is online now steveu  United States
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In those days, BJT amps blew up a lot. Going class-C helped a bit, but did nothing to reduce high frequency float and shoot through, and lack of high power operation heat sink. I fixed a couple 405s. The slow power transistors available were the primary culprit, and the lack of power PNPs, and poor SOA. I think Harmon-Kardon were ahead of their time and used ross-coupled complimentary power transistors. Most amps were just barely stable because pushing slow transistors to 20KHz was close to the feedback limit. Most amps had very poor base pull-downs so when you loaded the amp with a long cable, it would become unstable, oscillate, float on, shoot through and SOA failure.
But today we have 40MHz BJT power transistors including PNPs, but most of all FETs and class-D which provide as good or better fidelity than class-B and much reduced heat.
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Old 17th September 2018, 07:54 PM   #113
traderbam is offline traderbam  Europe
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Default Original Quad 405 schematic from PW's article

FYI the bridge arrangement that was ingeniously used in the output stage is one of these: Wheatstone bridge - Wikipedia
Attached Images
File Type: png Quad405 PW schematic.png (351.9 KB, 89 views)
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Last edited by traderbam; 17th September 2018 at 07:56 PM.
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Old 17th September 2018, 09:49 PM   #114
VivaVee is online now VivaVee  New Zealand
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Quote:
Originally Posted by traderbam View Post

The current limiter (about 8A?) is only on the positive rail but I doubt the negative could reach that with only 45mA drive current when the BDY77 has max gain of 140.
Current limiting is on both positive and negative rails - the current sense resistors are the two 0R08 resistors on the schematic you posted. Mislabeled as R35 in both positions on your schematic
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Old 17th September 2018, 10:03 PM   #115
traderbam is offline traderbam  Europe
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Quote:
Originally Posted by VivaVee View Post
Current limiting is on both positive and negative rails - the current sense resistors are the two 0R08 resistors on the schematic you posted. Mislabeled as R35 in both positions on your schematic
Good catch, thx. And TR8 allows more than 45mA to TR10 so the neg protection can kick in. Should have looked more closely at that bottom section.
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Last edited by traderbam; 17th September 2018 at 10:07 PM.
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Old 17th September 2018, 11:30 PM   #116
VivaVee is online now VivaVee  New Zealand
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Originally Posted by traderbam View Post
Good catch, thx. And TR8 allows more than 45mA to TR10 so the neg protection can kick in. Should have looked more closely at that bottom section.
I remembered because I used a two short lengths of Constantan (acquired by devious means) to make the required 80 milliOhm resistors.

These days you can just search on Digikey and voila
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