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Different Audio Design
Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/e...s/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 at hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@ |
#2
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On Wed, 17 Nov 2004 00:18:38 -0800, Watson A.Name - "Watt Sun, the Dark Remover" wrote:
Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Hey, Watson. :-) I'm going to level with you, I'm not an expert, I just play one on the internet. But it looks to me like the gain of the output stage - you're talking about the one with an opamp per each output tranny, right? - is strapped such that the opamp's loop gain predominates, and I would not be a bit surprised to see the circuit behave just as you describe (emitter followers are notoriously fast), with two caveats: The slew rate of the opamps, and something about a pole or a zero in the complex impedance at that horrendous huge output cap. But that's just a butt-level[0] feeling, so take it for what it's worth, and let any uselessness go. :-) Cheers! Rich [0] i.e., seat-of-the pants driving by a bench tech. ;-) |
#3
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"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:
Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Start with the "obvious" DC initial conditions: pins 5 and 6 are at 1/2 of the supply voltage. Note average DC current thru R4 must be exactly zero (am assuming zero input bias current in op amps and zero leakage current for C6 and C7). Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of the supply voltage. Now we see the dicey part; theoretically (acting seperately), opamp C will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2 until the inputs "see" its Vos. Housesomever, zee soykut iss a mess. The input conditions that opamp C will try to satisfy is virtually *guaranteed* to be different than the input conditions that opamp D will try to satisfy (the Vos of one will be different than the Vos of the other). Ignoring that feedback, the conclusion seems to be that the opamp with the greater open loop power gain (that includes the transistor) will (mostly) win, meaning the other transistor will either be driven to saturation or zener E-B breakdown. But, remember that no DC current can flow thru R4. It *does* appear that your supposition of oscillation is correct. Now, replace the transistors with E-to-B resistors and one still has the problem. Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. |
#4
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"Rich Grise" wrote in message news On Wed, 17 Nov 2004 00:18:38 -0800, Watson A.Name - "Watt Sun, the Dark Remover" wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Hey, Watson. :-) I'm going to level with you, I'm not an expert, I just play one on the internet. But it looks to me like the gain of the output stage - you're talking about the one with an opamp per each output tranny, right? - is strapped such that the opamp's loop gain predominates, and I would not be a bit surprised to see the circuit behave just as you describe (emitter followers are notoriously fast), with two caveats: The slew rate of the opamps, and something about a pole or a zero in the complex impedance at that horrendous huge output cap. But that's just a butt-level[0] feeling, so take it for what it's worth, and let any uselessness go. :-) Cheers! Rich [0] i.e., seat-of-the pants driving by a bench tech. ;-) One never sees this configuration used in comm'l designs, so I figure there must be a reason, such as problems with stability. I would do a few things, like put fuses in the emitters of the power output transistors. |
#5
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"Robert Baer" wrote in message ... "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Start with the "obvious" DC initial conditions: pins 5 and 6 are at 1/2 of the supply voltage. Note average DC current thru R4 must be exactly zero (am assuming zero input bias current in op amps and zero leakage current for C6 and C7). Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of the supply voltage. Now we see the dicey part; theoretically (acting seperately), opamp C will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2 until the inputs "see" its Vos. Housesomever, zee soykut iss a mess. The input conditions that opamp C will try to satisfy is virtually *guaranteed* to be different than the input conditions that opamp D will try to satisfy (the Vos of one will be different than the Vos of the other). Ignoring that feedback, the conclusion seems to be that the opamp with the greater open loop power gain (that includes the transistor) will (mostly) win, meaning the other transistor will either be driven to saturation or zener E-B breakdown. But, remember that no DC current can flow thru R4. It *does* appear that your supposition of oscillation is correct. Now, replace the transistors with E-to-B resistors and one still has the problem. Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. |
#6
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"Watson A.Name - "Watt Sun, the Dark Remover""
wrote in message Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Depends on the op amp. I stopped taking this circuit page seriously when I saw "741" on the upper two circuit diagrams. Do you know what a 741 is from the standpoint of quality audio? Anathema! Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. It's a variation on what some designers call "current dumping". However let's say the truth - this is an outdated, amateurish design with no known merits over established technology. |
#7
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On Wed, 17 Nov 2004 05:42:37 -0800, Watson A.Name - "Watt Sun, the Dark
"Robert Baer" wrote in message .... Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. ^^^^ Is this an adverb, or an interjection? ;-) Thanks, Rich |
#8
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In article ,
Rich Grise wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Hey, Watson. :-) I'm going to level with you, I'm not an expert, I just play one on the internet. But it looks to me like the gain of the output stage - you're talking about the one with an opamp per each output tranny, right? - is strapped such that the opamp's loop gain predominates, and I would not be a bit surprised to see the circuit behave just as you describe (emitter followers are notoriously fast), with two caveats: The slew rate of the opamps, and something about a pole or a zero in the complex impedance at that horrendous huge output cap. I'm not an expert either, but I have a Bad Feeling about this design. It seems to me that it assumes the existence of theoretically-perfect components with ideal matching (between IC2 and IC2, and between the various Tr1 and Tr2 parallel transistors). I'd be very concerned about the effect of any input offset voltage difference which might exist between IC2 and IC3. It looks to me as if the two op amps could end up "fighting" one another pretty badly. If the input offset voltages are offset from one another in one direction, the bias in the output transistors would probably tend down towards zero, and distortion might result. If the offsets are in the opposite direction, (e.g. if IC2 wanted to see a slightly more positive voltage on its inverting input than IC1 did, for a given noninverting input voltage) then the op amp output voltages would diverge in opposite directions, turning both Tr1 and Tr2 quite hard, and quite possibly driving them out of their safe operating areas. Add to this the fact that the design doesn't include base resistors for the transistors, or emitter ballast resistors for the paralleled Tr1 and Tr2 transistor clusters, and I think you've got a recipe for serious instability (oscillatory and thermal) and for the emission of copious quantities of Magic Blue Smoke. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#9
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"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ...
Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? I just simulated the circuit in LTSpice using LT1113's instead of the TL084. It simulates quite nicely. The feedback causes the base drives to swing through the "crossover" region so that's no problem. Here's a "cut-n-paste" ASCII file for the input to LTSpice: Version 4 SHEET 1 880 680 WIRE -16 384 16 384 WIRE 144 144 48 144 WIRE 16 144 16 384 WIRE 16 384 144 384 WIRE 144 112 96 112 WIRE 96 112 96 240 WIRE 96 352 144 352 WIRE 208 128 288 128 WIRE 208 368 288 368 WIRE 352 176 352 240 WIRE 352 416 352 464 WIRE 352 80 352 32 WIRE 96 240 352 240 WIRE 96 240 96 352 WIRE 352 240 352 320 WIRE 496 240 448 240 WIRE 448 512 448 480 WIRE 448 416 448 368 WIRE 448 288 448 240 WIRE 448 240 352 240 WIRE 560 240 592 240 WIRE 624 240 624 336 WIRE 624 416 624 512 WIRE -16 240 96 240 WIRE -96 240 -128 240 WIRE -128 240 -128 368 WIRE -128 368 -160 368 WIRE -80 368 -128 368 WIRE -320 368 -288 368 WIRE -288 432 -288 368 WIRE -288 368 -240 368 WIRE -288 496 -288 528 WIRE -480 416 -480 368 WIRE -480 368 -384 368 WIRE -480 496 -480 528 WIRE -400 0 -400 48 WIRE -256 48 -256 0 WIRE -80 400 -128 400 WIRE -128 400 -128 448 WIRE 592 240 624 240 WIRE 48 144 16 144 FLAG 176 96 +14 FLAG 176 336 +14 FLAG 352 32 +14 FLAG 176 160 0 FLAG 176 400 0 FLAG 352 464 0 FLAG 448 512 0 FLAG 624 512 0 FLAG -288 528 0 FLAG -480 528 0 FLAG -48 352 +14 FLAG -400 0 +14 FLAG -256 0 +7 FLAG -400 128 0 FLAG -256 128 0 FLAG -128 448 +7 FLAG 592 240 out FLAG 48 144 1st FLAG -48 416 0 SYMBOL Opamps\\LT1113 176 304 R0 SYMATTR InstName U1 SYMBOL Opamps\\LT1113 176 64 R0 SYMATTR InstName U2 SYMBOL Opamps\\LT1113 -48 320 R0 SYMATTR InstName U3 SYMBOL pnp 288 416 M180 SYMATTR InstName Q1 SYMBOL npn 288 80 R0 SYMATTR InstName Q2 SYMBOL res 432 272 R0 SYMATTR InstName R1 SYMATTR Value 10 SYMBOL cap 432 416 R0 SYMATTR InstName C1 SYMATTR Value .1µ SYMBOL cap 560 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value 470µ SYMBOL res 0 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL res -144 352 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL cap -320 352 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value .05µ SYMBOL cap -272 496 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C4 SYMATTR Value .001µ SYMBOL res 608 320 R0 SYMATTR InstName R4 SYMATTR Value 4 SYMBOL voltage -480 400 R0 WINDOW 3 -177 -127 Left 0 WINDOW 123 -172 -91 Left 0 WINDOW 39 -128 98 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 .5 1000) SYMATTR Value2 AC .2 SYMATTR SpiceLine Rser=1k SYMBOL voltage -400 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 14 SYMBOL voltage -256 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 7 TEXT -72 -64 Left 0 !.tran 10m |
#11
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Wed, 17 Nov 2004 05:42:37 -0800, "Watson A.Name - \"Watt Sun, the Dark
Remover\"" wrote: Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. It will not work. Please analyse only output stage: IC C+D and output transistors. Imagine, that you disconnect input pin 10 on C from pin 12 and 7. Now compute bias current thru Q1, Q2 as a function of voltage between pin 10 and 12. You will see, that gain of this stage is near infinity, any positive voltage between 10 and 12 will shortcircuit Q1 and Q2. Now reconnect your circuit and think about bias current as a function of input voltage offset of D and C. How long will it work? -- Pozdrowienia Andrzej Popowski |
#12
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"Watson A.Name - "Watt Sun, the Dark Remover""
schreef in bericht ... "Robert Baer" wrote in message ... "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Start with the "obvious" DC initial conditions: pins 5 and 6 are at 1/2 of the supply voltage. Note average DC current thru R4 must be exactly zero (am assuming zero input bias current in op amps and zero leakage current for C6 and C7). Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of the supply voltage. Now we see the dicey part; theoretically (acting seperately), opamp C will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2 until the inputs "see" its Vos. Housesomever, zee soykut iss a mess. The input conditions that opamp C will try to satisfy is virtually *guaranteed* to be different than the input conditions that opamp D will try to satisfy (the Vos of one will be different than the Vos of the other). Ignoring that feedback, the conclusion seems to be that the opamp with the greater open loop power gain (that includes the transistor) will (mostly) win, meaning the other transistor will either be driven to saturation or zener E-B breakdown. But, remember that no DC current can flow thru R4. It *does* appear that your supposition of oscillation is correct. Now, replace the transistors with E-to-B resistors and one still has the problem. Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. If the offsets of C & D have the 'right' polarity, it will work. But if that is not the case... both transistors will be full on. So you need a couple of TL084's and find a 'good' one. -- Thanks, Frank. (remove 'x' and 'invalid' when replying by email) |
#13
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Robert Baer wrote in message ...
"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Start with the "obvious" DC initial conditions: pins 5 and 6 are at 1/2 of the supply voltage. Note average DC current thru R4 must be exactly zero (am assuming zero input bias current in op amps and zero leakage current for C6 and C7). Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of the supply voltage. Now we see the dicey part; theoretically (acting seperately), opamp C will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2 until the inputs "see" its Vos. Housesomever, zee soykut iss a mess. The input conditions that opamp C will try to satisfy is virtually *guaranteed* to be different than the input conditions that opamp D will try to satisfy (the Vos of one will be different than the Vos of the other). Ignoring that feedback, the conclusion seems to be that the opamp with the greater open loop power gain (that includes the transistor) will (mostly) win, meaning the other transistor will either be driven to saturation or zener E-B breakdown. But, remember that no DC current can flow thru R4. It *does* appear that your supposition of oscillation is correct. Now, replace the transistors with E-to-B resistors and one still has the problem. Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. Yep, I agree with it all... except 'scrap'. The problem is there are 2 feedback loops on the output end opamps, and theyre _connected together_. Thus the 2 feedbacks cant happen independantly. They might cooperate, or they might fight and lead the op trs to an MS-release event (Magic Smoke). But rather than scrap it, this is easily fixed. Add a couple of 0.1 ohm emitter Rs on the op trs, then you can complete the loop separately for each tr. The LS is then being driven thru a 0.1 ohm R, which is quite ok. Think that through... it all works nicely, no fighting. V offsets become a non event, 2mV across 0.2 ohm is trivial. The one issue is opamp speed. Call me cautious but I wouldnt be surprised if they were using something like an LM324. While that does have workable audio performance (reduced max amplitude above 6kHz is fairly ok), it can not shift fast enough to deal with crossover distortion cleanly. To do that would require performance at way above 20kHz. I also have some hesitations about opamp A see the output load capacitance, but again thats easily fixed. NT |
#14
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"N. Thornton" wrote in message om... Robert Baer wrote in message ... "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Start with the "obvious" DC initial conditions: pins 5 and 6 are at 1/2 of the supply voltage. Note average DC current thru R4 must be exactly zero (am assuming zero input bias current in op amps and zero leakage current for C6 and C7). Then pins 7, 9, 10, 12 and 13 are all within an op-amp Vos of 1/2 of the supply voltage. Now we see the dicey part; theoretically (acting seperately), opamp C will drive Q1 until the inputs "see" its Vos, and opamp D would drive Q2 until the inputs "see" its Vos. Housesomever, zee soykut iss a mess. The input conditions that opamp C will try to satisfy is virtually *guaranteed* to be different than the input conditions that opamp D will try to satisfy (the Vos of one will be different than the Vos of the other). Ignoring that feedback, the conclusion seems to be that the opamp with the greater open loop power gain (that includes the transistor) will (mostly) win, meaning the other transistor will either be driven to saturation or zener E-B breakdown. But, remember that no DC current can flow thru R4. It *does* appear that your supposition of oscillation is correct. Now, replace the transistors with E-to-B resistors and one still has the problem. Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. Yep, I agree with it all... except 'scrap'. The problem is there are 2 feedback loops on the output end opamps, and theyre _connected together_. Thus the 2 feedbacks cant happen independantly. They might cooperate, or they might fight and lead the op trs to an MS-release event (Magic Smoke). But rather than scrap it, this is easily fixed. Add a couple of 0.1 ohm emitter Rs on the op trs, then you can complete the loop separately for each tr. The LS is then being driven thru a 0.1 ohm R, which is quite ok. Think that through... it all works nicely, no fighting. V offsets become a non event, 2mV across 0.2 ohm is trivial. The one issue is opamp speed. Call me cautious but I wouldnt be surprised if they were using something like an LM324. While that does have workable audio performance (reduced max amplitude above 6kHz is fairly ok), it can not shift fast enough to deal with crossover distortion cleanly. To do that would require performance at way above 20kHz. I also have some hesitations about opamp A see the output load capacitance, but again thats easily fixed. We're taking a flawed design, and adding band-aids on top of band-aids. You can make a perfectly good amplifier with one opamp and two push-pull emitter followers biased with a couple of diodes, similar to the first design sketched in the OP's link. If that design draws too much idle current, increase the emitter resistor. There's lots of stuff you can do to improve that design without resorting to the demented design being proposed. |
#15
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On Wed, 17 Nov 2004 22:49:08 GMT, "Karl Uppiano"
wrote: We're taking a flawed design, and adding band-aids on top of band-aids. You can make a perfectly good amplifier with one opamp and two push-pull emitter followers biased with a couple of diodes, similar to the first design sketched in the OP's link. If that design draws too much idle current, increase the emitter resistor. There's lots of stuff you can do to improve that design without resorting to the demented design being proposed. It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. John |
#16
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In article ,
John Larkin wrote: We're taking a flawed design, and adding band-aids on top of band-aids. You can make a perfectly good amplifier with one opamp and two push-pull emitter followers biased with a couple of diodes, similar to the first design sketched in the OP's link. If that design draws too much idle current, increase the emitter resistor. There's lots of stuff you can do to improve that design without resorting to the demented design being proposed. It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. Seems like a reasonable idea, *if* you do the feedback loops right. In your case, I'd guess that you have a unity- or small-gain feedback loop wrapped around each individual opamp-and-transistor (tapping off between the transistor source and the ballast resistor?), and then an outer feedback loop from the final output back to an earlier stage (a pre-driver op amp which then feeds the driver op amps?). As long as these loops are speed-compensated properly, this would probably be quite safe... some local feedback and some global feedback. The problemms come up when you have multiple op amps and feedback loops in parallel in a way which allows them to fight, or have "wrapped" feedback loops with inappropriate time constants. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#17
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"John Larkin" It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. ** What fets are P ch, 300 watts and 400 volts ?? Is a bank loan needed to pay for them ?? .................. Phil |
#18
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On Thu, 18 Nov 2004 11:27:45 +1100, "Phil Allison"
wrote: "John Larkin" It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. ** What fets are P ch, 300 watts and 400 volts ?? Is a bank loan needed to pay for them ?? ................. Phil IXTH11P50, 500 volts, 300 watts. My stock report shows $7.55 each, which is likely 100+ pricing. Nice part, very rugged. John |
#19
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"John Larkin" "Phil Allison" ** What fets are P ch, 300 watts and 400 volts ?? Is a bank loan needed to pay for them ?? IXTH11P50, 500 volts, 300 watts. My stock report shows $7.55 each, which is likely 100+ pricing. Nice part, very rugged. John ** Thanks. ............. Phil |
#20
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"John Larkin" wrote in message ... On Wed, 17 Nov 2004 22:49:08 GMT, "Karl Uppiano" wrote: We're taking a flawed design, and adding band-aids on top of band-aids. You can make a perfectly good amplifier with one opamp and two push-pull emitter followers biased with a couple of diodes, similar to the first design sketched in the OP's link. If that design draws too much idle current, increase the emitter resistor. There's lots of stuff you can do to improve that design without resorting to the demented design being proposed. It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. John I'm not saying it can't be done, but for the application at hand (a low power amp running on a single supply) it seems simpler is better. |
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"Arny Krueger" wrote in message ... "Watson A.Name - "Watt Sun, the Dark Remover"" wrote in message Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Depends on the op amp. I stopped taking this circuit page seriously when I saw "741" on the upper two circuit diagrams. The guy's a HAM, cut him a little slack, OK? Do you know what a 741 is from the standpoint of quality audio? Anathema! If you keep the gain down to 10 or less, it's almost Hi-Fi. ;-) Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. It's a variation on what some designers call "current dumping". However let's say the truth - this is an outdated, amateurish design with no known merits over established technology. Well, didn't I say that I didn't think much of it? This guy _is_ an amateur, AKA HAM. Also, if you investigate this guy's web pages, you'll find he's enormously prolific, with gobs of schematics of circuits he's built. In the true spirit of experimentation, you'll notice. And you don't see this particular type of design in copmmercial equipment, as I said. Which leads me to believe there are some inherent disadvantages. I'm asking for insight into what these might be, not destruictive criticism. I have a number of schematics of audio power amps that I think have some disadvantages which I wouldn't use. One is that the design connects one terminal of the speaker to the Vcc, and lets DC thru the speaker bias the output stage. This gives the same effect as bootstrapping. But it also makes the speaker hot relative to ground, which isn't a problem if the amp and speaker are in the same enclosure and isolated. So I would consider this unacceptable, and use the bootstrapping method instead. |
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"Rich Grise" wrote in message news On Wed, 17 Nov 2004 05:42:37 -0800, Watson A.Name - "Watt Sun, the Dark "Robert Baer" wrote in message ... Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. ^^^^ Is this an adverb, or an interjection? ;-) Sorry for the confloosion. It's an adverb. "It works well," he said, with low 'torsion' [distortion]. Hey, give him a break, he has also learned German, and 'Engrish' seems to be his 3rd language(!) Once you learn German, there's no more room for Engrish in your head! :-[ Thanks, Rich |
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"Dave Platt" wrote in message ... In article , Rich Grise wrote: Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Maybe a 1k resistor E to B on the output transistors would 'bypass' this. Or should the amp be biased to work class AB. Hey, Watson. :-) I'm going to level with you, I'm not an expert, I just play one on the internet. But it looks to me like the gain of the output stage - you're talking about the one with an opamp per each output tranny, right? - is strapped such that the opamp's loop gain predominates, and I would not be a bit surprised to see the circuit behave just as you describe (emitter followers are notoriously fast), with two caveats: The slew rate of the opamps, and something about a pole or a zero in the complex impedance at that horrendous huge output cap. I'm not an expert either, but I have a Bad Feeling about this design. It seems to me that it assumes the existence of theoretically-perfect components with ideal matching (between IC2 and IC2, and between the various Tr1 and Tr2 parallel transistors). I'd be very concerned about the effect of any input offset voltage difference which might exist between IC2 and IC3. It looks to me as if the two op amps could end up "fighting" one another pretty badly. If the input offset voltages are offset from one another in one direction, the bias in the output transistors would probably tend down towards zero, and distortion might result. If the offsets are in the opposite direction, (e.g. if IC2 wanted to see a slightly more positive voltage on its inverting input than IC1 did, for a given noninverting input voltage) then the op amp output voltages would diverge in opposite directions, turning both Tr1 and Tr2 quite hard, and quite possibly driving them out of their safe operating areas. I think you need to get a handle on that "safe operating area", I think it's not used in that context. Add to this the fact that the design doesn't include base resistors for the transistors, or emitter ballast resistors for the paralleled Tr1 and Tr2 transistor clusters, and I think you've got a recipe for serious instability (oscillatory and thermal) and for the emission of copious quantities of Magic Blue Smoke. Well, that's what I tend to be concerned about. Thanks. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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"Jim Meyer" wrote in message om... "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ... Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? I just simulated the circuit in LTSpice using LT1113's instead of the TL084. It simulates quite nicely. The feedback causes the base drives to swing through the "crossover" region so that's no problem. Here's a "cut-n-paste" ASCII file for the input to LTSpice: Thanks for the sim. [snip] Swing thru rather quickly I would guess. And doing so with the possibility of overcorrecting? Or hunting about trying to find a place it can never find? Or having two opamps, the phase shift might be too great at high freqs. Or.. |
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"Andrzej Popowski" wrote in message ... Wed, 17 Nov 2004 05:42:37 -0800, "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote: Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. It will not work. Please analyse only output stage: IC C+D and output transistors. Imagine, that you disconnect input pin 10 on C from pin 12 and 7. Now compute bias current thru Q1, Q2 as a function of voltage between pin 10 and 12. You will see, that gain of this stage is near infinity, any positive voltage between 10 and 12 will shortcircuit Q1 and Q2. Now reconnect your circuit and think about bias current as a function of input voltage offset of D and C. How long will it work? Good question. Thanks. -- Pozdrowienia Andrzej Popowski |
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On Wed, 17 Nov 2004 18:59:57 -0800, Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Rich Grise" wrote in message news On Wed, 17 Nov 2004 05:42:37 -0800, Watson A.Name - "Watt Sun, the Dark "Robert Baer" wrote in message ... Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. ^^^^ Is this an adverb, or an interjection? ;-) Sorry for the confloosion. It's an adverb. "It works well," he said, with low 'torsion' [distortion]. Hey, give him a break, he has also learned German, and 'Engrish' seems to be his 3rd language(!) Once you learn German, there's no more room for Engrish in your head! :-[ Oh, the answer to that one is simple. Just go out of your head! %-} Cheers! Rich |
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"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote:
"Rich Grise" wrote in message news On Wed, 17 Nov 2004 05:42:37 -0800, Watson A.Name - "Watt Sun, the Dark "Robert Baer" wrote in message ... Zhoe, vee zee zhat zee soykut iss a mess. sCRAP. But they say it works. Well. ^^^^ Is this an adverb, or an interjection? ;-) Sorry for the confloosion. It's an adverb. "It works well," he said, with low 'torsion' [distortion]. Hey, give him a break, he has also learned German, and 'Engrish' seems to be his 3rd language(!) Once you learn German, there's no more room for Engrish in your head! :-[ Thanks, Rich A long time ago i made an RF amplifier using an Esaki ("tunnel") Diode. It was rather puzzling, in that the gain was very low and could not be improved. Further analysis showed that the dern thing was oscillating like a banshee at a substantially higher frequency. Another thought: the "joke" that if you want an oscillator, build or design an amplifier (and vice-versa). |
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John Larkin wrote:
On Wed, 17 Nov 2004 22:49:08 GMT, "Karl Uppiano" wrote: We're taking a flawed design, and adding band-aids on top of band-aids. You can make a perfectly good amplifier with one opamp and two push-pull emitter followers biased with a couple of diodes, similar to the first design sketched in the OP's link. If that design draws too much idle current, increase the emitter resistor. There's lots of stuff you can do to improve that design without resorting to the demented design being proposed. It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. Bloody hell. That's some amp, probably about 20KW out. This will certainly kick the **** out of you at 50 Hz. How did you get the opamp voltage rating, or was it a discrete one? It forces essentially perfect current sharing, nukes the device tc and part-part variations, and makes lots of gate drive available. So long as the loop is stable its ok. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
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"Robert Baer" A long time ago i made an RF amplifier using an Esaki ("tunnel") Diode. It was rather puzzling, in that the gain was very low and could not be improved. Further analysis showed that the dern thing was oscillating like a banshee at a substantially higher frequency. Another thought: the "joke" that if you want an oscillator, build or design an amplifier (and vice-versa). ** Looks to me like young Robert has just come up with another corollary to the ever growing set of "Laws" popularly attributed to the famous USAF Major Murphy that likely rates recording somewhere for posterity . Or has he perhaps been beaten to it by another ?? ............ Phil |
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"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ...
Swing thru rather quickly I would guess. And doing so with the possibility of overcorrecting? inevitably, since opamps are frequency and slew limited, there will always be a little undershoot then overshoot. But this is not a problem, it just introduces very small ultrasonic artefacts. It happens in every class B with feedback, and is pretty much a non issue. Note that feedback on class B doesnt eliminate distortion, it just moves it up out of the useful frequency band. Or hunting about trying to find a place it can never find? It might possibly do that during idle, but the consequences are none. The output still stays at 0, or extremely close to it, with any artefacts being ultrasonic. again, same as any class B with large nfb. In practice output trs have very low gain when theyre just beginning to conduct, so it would probably be stable in that respect. Or having two opamps, the phase shift might be too great at high freqs. Or.. Thats a problem, especially with a capacitive load. Combined with the amps output R you get an RC that youre taking feedback from. However, I've used simple clas B output tr pair plus corecting opamp before without any problem. The problem in the design offered is the fighting joined feedback loops - but its easily fixed. NT |
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"Arny Krueger" wrote in message ...
"Watson A.Name - "Watt Sun, the Dark Remover"" wrote in message Opinions on this, especially the 2.5W amp schem at the bottom. http://www.intio.or.jp/jf10zl/EF.htm When the amp is idle, and there's no(?) current flowing in either output transistor because there's no forward bias, then the V drop 'resistance' of the E-B junction adds to the 100k feedback resistor, so the loop gain increases to the open loop gain. So it would seem that the amp would attempt to hunt in this region, possibly oscillating? Depends on the op amp. I stopped taking this circuit page seriously when I saw "741" on the upper two circuit diagrams. Lets at least be fair, the earlier circuits on that page make it quite clear its not an attempt at quality audio. He's playing with ideas, and quality is not an inherent part of some designs. But the 741, I would be in no rush to use them - maybe 20 years ago, and even then I found 748s superior. NT |
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On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward"
wrote: It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. Bloody hell. That's some amp, probably about 20KW out. This will certainly kick the **** out of you at 50 Hz. How did you get the opamp voltage rating, or was it a discrete one? It's a transconductance amp, so the fet sources are at the rails (through current-sense resistors) and the drains are the output. So the gate-drive fets work on the +-200 power supply rails (with small floating local supplies) and only need to swing enough to drive the gates. Nice thing here is that all 32 fet drains are common, so they are bolted (actually clamped) to copper heat spreaders on the main heatsink, without insulators. So long as the loop is stable its ok. Well, sure! John |
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John Larkin wrote:
On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward" wrote: It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. Bloody hell. That's some amp, probably about 20KW out. This will certainly kick the **** out of you at 50 Hz. How did you get the opamp voltage rating, or was it a discrete one? It's a transconductance amp, so the fet sources are at the rails (through current-sense resistors) and the drains are the output. So the gate-drive fets work on the +-200 power supply rails (with small floating local supplies) and only need to swing enough to drive the gates. I have recently been playing with both source out and drain out configurations for that super duper amp I mentioned in one post. One issue with the drain out is x-over spikes. I was getting 100A thick x-over spikes in the drain output, verses essentially, none in the source output. The gates tied together prevents the spike when slewing. So, worth noting if you want the utmost in speed without frying the devices in a linear amp. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
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On Thu, 18 Nov 2004 17:37:26 GMT, "Kevin Aylward"
wrote: John Larkin wrote: On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward" wrote: It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. Bloody hell. That's some amp, probably about 20KW out. This will certainly kick the **** out of you at 50 Hz. How did you get the opamp voltage rating, or was it a discrete one? It's a transconductance amp, so the fet sources are at the rails (through current-sense resistors) and the drains are the output. So the gate-drive fets work on the +-200 power supply rails (with small floating local supplies) and only need to swing enough to drive the gates. I have recently been playing with both source out and drain out configurations for that super duper amp I mentioned in one post. One issue with the drain out is x-over spikes. I was getting 100A thick x-over spikes in the drain output, verses essentially, none in the source output. The gates tied together prevents the spike when slewing. So, worth noting if you want the utmost in speed without frying the devices in a linear amp. I'm driving a mostly-inductive load, with overall current feedback, so the current-out stage makes sense for me; it keeps the loop dynamics from depending on the load, which is valuable. An audio or such amp is better of it has an inherently low z-out, sor the same sorts of reasons. As far as crossover goes, that is interesting. My output stage drive signal is grounded, and I have a splitter circuit to convert it into sinking and sourcing currents to get up to the floating +side and -side power stages, which are essentially current mirrors. The splitter must sustain a small idle current on both outputs and also perfectly split the input signal, so that positive inputs increase the sink current and negative sigs increase the source current. I've been tuning this circuit for years, gradually converging on something that works pretty well. It's an interesting, philosophically disturbing, problem. For a voltage-out amp, it would be cool to float a, say, +-12 volt supply on the output node and hang an opamp to close the loop around each fet, making an ideal follower. Then you'd be able to run the fets all the way to the power rails, and you could use very small source sense resistors, with just millivolts of idle voltage. John |
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On Thu, 18 Nov 2004 22:20:57 +1100, Phil Allison wrote:
"Robert Baer" A long time ago i made an RF amplifier using an Esaki ("tunnel") Diode. It was rather puzzling, in that the gain was very low and could not be improved. Further analysis showed that the dern thing was oscillating like a banshee at a substantially higher frequency. Another thought: the "joke" that if you want an oscillator, build or design an amplifier (and vice-versa). ** Looks to me like young Robert has just come up with another corollary to the ever growing set of "Laws" popularly attributed to the famous USAF Major Murphy that likely rates recording somewhere for posterity . Or has he perhaps been beaten to it by another ?? Considering I first heard it in about 1963, I'd say so. ;-) Cheers! Rich |
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John Larkin wrote:
On Thu, 18 Nov 2004 17:37:26 GMT, "Kevin Aylward" wrote: John Larkin wrote: On Thu, 18 Nov 2004 10:09:19 GMT, "Kevin Aylward" wrote: It's actually not a bad idea to have an opamp per output transistor, if you do it right, which this guy clearly hasn't. I make a power amp that uses 32 300-watt fets in the output (16 p-ch, 16 n-ch, +-200 volt rails) and do just that. Bloody hell. That's some amp, probably about 20KW out. This will certainly kick the **** out of you at 50 Hz. How did you get the opamp voltage rating, or was it a discrete one? It's a transconductance amp, so the fet sources are at the rails (through current-sense resistors) and the drains are the output. So the gate-drive fets work on the +-200 power supply rails (with small floating local supplies) and only need to swing enough to drive the gates. I have recently been playing with both source out and drain out configurations for that super duper amp I mentioned in one post. One issue with the drain out is x-over spikes. I was getting 100A thick x-over spikes in the drain output, verses essentially, none in the source output. The gates tied together prevents the spike when slewing. So, worth noting if you want the utmost in speed without frying the devices in a linear amp. I'm driving a mostly-inductive load, with overall current feedback, so the current-out stage makes sense for me; it keeps the loop dynamics from depending on the load, which is valuable. An audio or such amp is better of it has an inherently low z-out, sor the same sorts of reasons. As far as crossover goes, that is interesting. Of course in a real audio amp, one doesn't put a 500Khz square wave in to see this particular "problem". For specmanship it is. One cant do the test with out a fire. My output stage drive signal is grounded, and I have a splitter circuit to convert it into sinking and sourcing currents to get up to the floating +side and -side power stages, which are essentially current mirrors. The splitter must sustain a small idle current on both outputs and also perfectly split the input signal, so that positive inputs increase the sink current and negative sigs increase the source current. I've been tuning this circuit for years, gradually converging on something that works pretty well. It's an interesting, philosophically disturbing, problem. For a voltage-out amp, it would be cool to float a, say, +-12 volt supply on the output node and hang an opamp to close the loop around each fet, making an ideal follower. Again, potential issue of stability. I do have an interesting, somwhat original output circuit that does this with a first order loop enclosing the outputs. It gets me wonderfully low distortion at 20Khz. In one sense topologically its essentially the same as other compound pairs, but this does behave a little different. The feedback diodes are zeners at 10V. The drive circuit on its own 0.0001% THD 20Khz, according to spice that is. Note the feedback transisters can be low voltage ones, hence fast ones. DC loop gain is 135db, open loop more! http://www.anasoft.co.uk/Mospoweramp.jpg Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
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On Thu, 18 Nov 2004 17:37:26 GMT, "Kevin Aylward"
wrote: I'm driving a mostly-inductive load, with overall current feedback, so the current-out stage makes sense for me; it keeps the loop dynamics from depending on the load, which is valuable. An audio or such amp is better of it has an inherently low z-out, sor the same sorts of reasons. Shaker table driver? |
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On Fri, 19 Nov 2004 13:16:44 GMT, "Kevin Aylward"
wrote: Again, potential issue of stability. I do have an interesting, somwhat original output circuit that does this with a first order loop enclosing the outputs. It gets me wonderfully low distortion at 20Khz. In one sense topologically its essentially the same as other compound pairs, but this does behave a little different. The feedback diodes are zeners at 10V. The drive circuit on its own 0.0001% THD 20Khz, according to spice that is. Note the feedback transisters can be low voltage ones, hence fast ones. DC loop gain is 135db, open loop more! http://www.anasoft.co.uk/Mospoweramp.jpg One problem here is current sharing. Linear-mode paralleled mosfets don't do it very well. One of my earlier gradient amps was done with paralleled fets (with source resistors, at least) and we wound up making a fixture so we could match sets of fets for production... a real pain in the sternparts. The advantage of an opamp per fet is that the gate drive becomes very simple and sharing is forced to be perfect. John |
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John Larkin wrote:
On Fri, 19 Nov 2004 13:16:44 GMT, "Kevin Aylward" wrote: Again, potential issue of stability. I do have an interesting, somwhat original output circuit that does this with a first order loop enclosing the outputs. It gets me wonderfully low distortion at 20Khz. In one sense topologically its essentially the same as other compound pairs, but this does behave a little different. The feedback diodes are zeners at 10V. The drive circuit on its own 0.0001% THD 20Khz, according to spice that is. Note the feedback transisters can be low voltage ones, hence fast ones. DC loop gain is 135db, open loop more! http://www.anasoft.co.uk/Mospoweramp.jpg One problem here is current sharing. Linear-mode paralleled mosfets don't do it very well. Yes, yes yes I do know all about this:-) ....but they're not that bad either, when compared to bipolar. One of my earlier gradient amps was done with paralleled fets (with source resistors, at least) and we wound up making a fixture so we could match sets of fets for production... a real pain in the sternparts. On might well be surprised just how well they match in same batches. Its unlikely to be more than a 2/1 mismatch at say 8 volts drive. Dropping a volt or so at max current via source resisters is always an option. It depends on the application. In audio power amps, the reality is, if its running max output and not clipping, then the average power is way way less for music signals. Indeed, in my MOSFET1000 (500W per chan) the transformer was only rated at 800W. I can tell you, transformers never blow under these conditions, despite worst case saying the will. Its knowing when to engineering cheat. My own MOSFET1000 has been going strong for nearly 20 years now, and it has no source resisters in the design. For real applicatons where a amp is actually going to be outputing its ratings contineously, one has to be a bit more carefull. The advantage of an opamp per fet is that the gate drive becomes very simple and sharing is forced to be perfect. Yes, but as I noted, there is the pole of the opamp, and the pole of rout with the fets, plus all the other op-amp parasitics. Its the old frequency verses accuracy trade off. Anyway, its cheating to use an op-amp:-) My none output feedback version is here.http://www.anasoft.co.uk/Mospoweramp2.jpg. This is quite a bit faster as it needs less compensation. It still does 0.005% at 20k, 8ohms, 500 Watts, in spice of course. Why do you think I tied the caps at the gates, instead of he push pull emitters? Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. |
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On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote: Yes, but as I noted, there is the pole of the opamp, and the pole of rout with the fets, plus all the other op-amp parasitics. Its the old frequency verses accuracy trade off. Anyway, its cheating to use an op-amp:-) But Kevin, inside every opamp are lots of little transistors! That should make you happy. John |
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