Home |
Search |
Today's Posts |
#1
Posted to rec.audio.tubes
|
|||
|
|||
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 |
#2
Posted to rec.audio.tubes
|
|||
|
|||
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 |
#3
Posted to rec.audio.tubes
|
|||
|
|||
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. |
#4
Posted to rec.audio.tubes
|
|||
|
|||
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 |
#5
Posted to rec.audio.tubes
|
|||
|
|||
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. |
#6
Posted to rec.audio.tubes
|
|||
|
|||
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 |
#7
Posted to rec.audio.tubes
|
|||
|
|||
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. |
#8
Posted to rec.audio.tubes
|
|||
|
|||
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 |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Dumping old Cakewalk audio data | Pro Audio | |||
Dumping Pro-Tools for Linux. Is it feasable? | Pro Audio | |||
Dumping Pro-Tools for Linux. Is it feasable? | Pro Audio | |||
Dumping 5000 plus Audio/Video Service Manuals | Marketplace | |||
Dumping analog to digital question | Pro Audio |