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#121
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MOSFET output stage
On Sep 18, 7:40*am, Eeyore
wrote: Arny Krueger wrote: wrote RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower odd harmonic distortin. on and on. Not in any relevant way for audio power amps. The first two are highly relevevant in ANY circuit using NFB. Basic stability criteria. For Christ's sake has everyone except Phil's and my brain turned to jelly overnight ? Checking brain ..... Mashed potatoes detected Mine hasn't. Graham |
#122
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MOSFET output stage
"Kevin McMurtrie" wrote in message
In article , "Arny Krueger" wrote: "Eeyore" wrote in message Kevin McMurtrie wrote: In article , Damon Hill wrote: RichD wrote in news:b78fc9c2-fe9c-444c-8ac5- : Who do MOSFET sound better than bipolar, as an audio amp output driver? They do? Seems like it's possible to design good amplifiers either way. --Damon Exactly - zero difference in quality capabilities. It's usually a matter of impedance matching. Silicon transistors have a fixed loss around 0.5 volts. MOSFETs have a resistive loss inversely proportional to their voltage rating. That usually makes MOSFETs less expensive for low impedances and transistors less expensive for high impedances. TOTAL AND COMPLETE GARBAGE. Agreed. Actually, what Kevin said is the exact reverse of generally accepted practical knowledge. Bipolar is generally more efficient when the impedances get really low. MOSFETs were trendy for a while, but most new power amp designs seem to have bipolar outputs. Time to check the specs for this decade. Been there done that. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. The low voltage rating takes this device out of consideration for serious audio amps for home or professional use. It may or may not be up to snuff for automotive use. Try coming up with a relevant example - something that can take at least 100 volts. Now I remember why I left this newsgroup. I lack the time or bad judgment required to feel any remorse at all over that. If you can't stand the heat - get out of the kitchen! |
#123
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MOSFET output stage
On Sep 18, 2:25*am, "Jorden Verwer" wrote:
RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? The device properties of BJTs are superior to those of MOSFETs in all respects, except for offset - there MOSFETs have the advantage. Whether you will actually hear this depends on many more factors. Radiation hardness Safe operating area Bandwidth |
#124
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MOSFET output stage
"Phil Allison" wrote in message
"Kevin McMurtrie" Time to check the specs for this decade. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. ** So you can use it to replace the engine starter relay in a car. Yawnnnnnnn .... Actually, it is marginal even for that. Most automotive guys like devices that are safe at higher voltages. However, the solar panel inverter guys might love it to death. |
#125
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MOSFET output stage
"MooseFET" Rumors aside MOSFETs are not really easier to operate in parallel. You still need emitter / source resistors. ** Which MOSFETS ???? Amazing how so many MOOSE like idiots have no idea there are TWO kinds !!!!!!!!!!!!!! The kind known as " lateral " share current just fine when in parallel in LINEAR applications with no source ballast resistors. Septic Tank Imbecile. ...... Phil |
#126
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
"Eeyore" wrote in
message Arny Krueger wrote: wrote RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower odd harmonic distortin. on and on. Not in any relevant way for audio power amps. The first two are highly relevevant in ANY circuit using NFB. Basic stability criteria. Yes, but we're talking about modern devices, not legacy devices. A ton of very acceptable power amps were built with power devices with FT on the order of a MHz. Every modern bipolar device designed for power amps that I see has FT above 10 MHz. I'm not saying that 100 KHz devices are optimal for audio, I'm saying that in 2008, higher bandwidth than commonly available devices for audio power amps is irrelevant. |
#127
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sep 18, 9:55*am, "Jorden Verwer" wrote:
Eeyore wrote: Jorden Verwer wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? The device properties of BJTs are superior to those of MOSFETs in all respects, How about SOA for one you UTTER MORON ? Fine, fine, but that doesn't have any direct influence on what you'll hear, because it's a boundary condition. When I hear BLAM-FIZZZT-POP-KAPOW from an SOA problem, I gain and understanding of how the SOA of a BJT is not better than that of a MOSFET. Do you even know what SOA is ? Like I said, it's a boundary condition. It can influence the performance of the circuit, but only indirectly, through other design decisions. You can design around a problem with a device but that doesn't make the device not have that problem. except for offset - there MOSFETs have the advantage. Whether you will actually hear this depends on many more factors. YET MORE INSANE ******** Now you're being the moron (not that I admit to being a moron before). It seems that either you don't know what you're talking about, or personal attacks are a hobby of yours. Because, frankly, everything I said was true... Fact - MOSFETs have lower offset than BJTs. How are you defining offset? Fact - The fidelity of the sound depends on much more than just device properties. Yes a Class-C audio output sounds quite bad regardless of the type of device used. It sure gets rid of that annoying hiss. |
#128
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Spehro Pefhany wrote: the renowned John Larkin wrote: Kevin McMurtrie wrote: "Arny Krueger" wrote: "Eeyore" wrote in Kevin McMurtrie wrote: Damon Hill wrote: RichD wrote in Who do MOSFET sound better than bipolar, as an audio amp output driver? They do? Seems like it's possible to design good amplifiers either way. --Damon Exactly - zero difference in quality capabilities. It's usually a matter of impedance matching. Silicon transistors have a fixed loss around 0.5 volts. MOSFETs have a resistive loss inversely proportional to their voltage rating. That usually makes MOSFETs less expensive for low impedances and transistors less expensive for high impedances. TOTAL AND COMPLETE GARBAGE. Agreed. Actually, what Kevin said is the exact reverse of generally accepted practical knowledge. Bipolar is generally more efficient when the impedances get really low. MOSFETs were trendy for a while, but most new power amp designs seem to have bipolar outputs. Time to check the specs for this decade. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. Now I remember why I left this newsgroup. IR is famous for creating incredible specs, with tiny footnotes retracting the wilder numbers. No TO-220 is going to last long at 300 watts; not many milliseconds. John Anything approaching 1600A would blow the leadwires off a TO-220. The fusing current of a long wire of that cross-section area is only in the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter. Hence those ceramic power puks and the like. Graham |
#129
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
"Eeyore" wrote in
message Arny Krueger wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? Who do? That's voodoo! Only to be expected from Arny "Any amp with less than 0.1% THD at full power sounds the same as all other amps." As usual, you're misrepresenting me. Nonlinearity corresponding to 0.1% THD can be audible given choice of musical program material. So, that number is way too high. Measurements made at just one power level, like full power, are obviously bogus. Obviously, the amp has to be able to perform within spec while driving a real-world tough (but not badly designed) speaker load. That all said, the world is full of good power amps. There are some bad ones, too. |
#130
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sep 18, 10:39*am, "Jorden Verwer" wrote:
[... BJTs are better ....] As for offset, here's one explanation (in the context of opamps):http://en.wikipedia.org/wiki/Operati..._imperfections Note that I never claimed that this is relevant in audio applications - but it's there. You may not have claimed it mattered but you sure implied it. That aside since we are talking about the output section of an amplifier, this sort of offset has no effect at all on the quality of the op-amp. If you look for an op-amp with an extremely low offset voltage, you will find that it uses MOSFETs to obtain that extreme low offset. As for your personal attacks towards me, I should mention that I'm under the impression that your experience with electronics outside audio applications is fairly limited, given that you've apparently never heard of the term offset. |
#131
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Arny Krueger wrote: "Eeyore" wrote Arny Krueger wrote: This whole "amp sounds better" stuff was fully debunked 30 years ago. Good power amps sound the same and they sound like a piece of wire with gain. There are tons of power amps that can't be distinguished from a piece of wire with gain, while driving well-designed speakers. There are quite a few amps that meet the same criteria while driving even the weirdest speaker load. A really good power amp will destroy a poorly-designed speaker before it starts sounding bad, and really good power amps aren't all that unique. The speaker is a large part of it. Highly reactive ( i.e. lots of inductance or capacitance ) speakers can prematurely trigger device protection. Agreed. Speakers that require heroic amplifiers (heroic amps definitely exist and are easy enough to obtain) are IMO not well designed. We discovered that EV's SX500 was particularly bad in this respect in PA for example. Now that is certainly audible. Agreed. However we have only part of the equation - the name of the speaker. What was the amp? With our own D Series amps. It was the only speaker we had trouble with. The VI limiting was quite generous for a 4 ohm rated amp but not enough for an SX500. I slackened off the VI protection and in the later AX series took a totally different approach to protection. The SX 500i EDS suggests that it might be a handful for weak-kneed amps. Its impedance curve does go down to 4 ohms at several points below 1 KHz. It's the phase angle that's insane more than the modulus of impedance. A D series amp would drive 3 typical 8 ohm speakers in parallel quite happily. http://www.electrovoice.com/documents/ev/sx500-eds.pdf Using pairs paralleled can be a short road to amp problems. In contrast, the sequel ZX-5 stays above 8 ohms over the same range: http://www.electrovoice.com/document...X560PI_EDS.pdf Needless to say, I have 4 ZX-5s. ;-) A better choice I imagine. I didn't even like the sound of the SX500s, very harsh and boxy. Graham |
#132
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
MooseFET wrote: Rumors aside MOSFETs are not really easier to operate in parallel. You still need emitter / source resistors. Not needed for lateral mosfets ! It's a whole different ball game. Graham |
#133
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Arny Krueger wrote: "Kevin McMurtrie" wrote in message Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. The low voltage rating takes this device out of consideration for serious audio amps for home or professional use. It may or may not be up to snuff for automotive use. Try coming up with a relevant example - something that can take at least 100 volts. More like 230-250V ! Graham |
#134
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
"Eeyore" wrote in
message Arny Krueger wrote: "Kevin McMurtrie" wrote in message Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. The low voltage rating takes this device out of consideration for serious audio amps for home or professional use. It may or may not be up to snuff for automotive use. Try coming up with a relevant example - something that can take at least 100 volts. More like 230-250V ! Well, yes, now that they are so easy to obtain. I was trying to make things easy for him. In fact we built a lot of pretty good power amps with 60-90 volt parts, back in the day. |
#135
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
MooseFET wrote: "Jorden Verwer"wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? The device properties of BJTs are superior to those of MOSFETs in all respects, except for offset - there MOSFETs have the advantage. Whether you will actually hear this depends on many more factors. Radiation hardness Safe operating area Bandwidth Not too worried about the first of those but gimme loads of the next two ! Hmmm - turn-off time 60ns @ 4A. That's why it pays to drive them from a complementary emitter follower stage. Just like SMPSs. Graham |
#136
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MOSFET output stage
Phil Allison wrote: "MooseFET" Rumors aside MOSFETs are not really easier to operate in parallel. You still need emitter / source resistors. ** Which MOSFETS ???? Amazing how so many MOOSE like idiots have no idea there are TWO kinds !!!!!!!!!!!!!! The kind known as " lateral " share current just fine when in parallel in LINEAR applications with no source ballast resistors. Septic Tank Imbecile. It's quite clear that there are just TWO people in this thread who really understand the use of lateral audio mosfets. Graham |
#137
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Fri, 19 Sep 2008 11:41:15 +0100, Eeyore
wrote: Kevin McMurtrie wrote: "Arny Krueger" wrote: "Eeyore" wrote Kevin McMurtrie wrote: Damon Hill wrote: RichD wrote in Who do MOSFET sound better than bipolar, as an audio amp output driver? They do? Seems like it's possible to design good amplifiers either way. Exactly - zero difference in quality capabilities. It's usually a matter of impedance matching. Silicon transistors have a fixed loss around 0.5 volts. MOSFETs have a resistive loss inversely proportional to their voltage rating. That usually makes MOSFETs less expensive for low impedances and transistors less expensive for high impedances. TOTAL AND COMPLETE GARBAGE. Agreed. Actually, what Kevin said is the exact reverse of generally accepted practical knowledge. Bipolar is generally more efficient when the impedances get really low. MOSFETs were trendy for a while, but most new power amp designs seem to have bipolar outputs. Time to check the specs for this decade. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. These are NOT used in audio amps. Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. Graham That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. |
#138
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Arny Krueger wrote: "Eeyore" wrote Arny Krueger wrote: wrote RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower odd harmonic distortin. on and on. Not in any relevant way for audio power amps. The first two are highly relevevant in ANY circuit using NFB. Basic stability criteria. Yes, but we're talking about modern devices, not legacy devices. It's relevant regardless ALWAYS. "Basic stability criteria" as I said above. Those Hitachi style lateral mosfets are STILL faster than any audio specific bipolar. Plus no SOA issues. Show me an audio bipolar that'll switch off in 60ns. Graham |
#139
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MOSFET output stage
Arny Krueger wrote: I'm not saying that 100 KHz devices are optimal for audio Even early 2N3055s were 8x better than that. I'm saying that in 2008, higher bandwidth than commonly available devices for audio power amps is irrelevant. Plenty still have an fT of only 4MHz (published). Graham |
#140
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Arny Krueger wrote: I'm not saying that 100 KHz devices are optimal for audio Even early 2N3055s were 8x better than that. I'm saying that in 2008, higher bandwidth than commonly available devices for audio power amps is irrelevant. Plenty still have an fT of only 4MHz (published). Graham High gain bandwidth products are important for a good reason. If you can keep device phase shifts well up in frequency, the dominant pole (I know you didn't implement one specifically, but I'm sure it was still there) can also move up in frequency. That means that global feedback will carry on working instead of disappearing towards the top of the audio band, at which point it has usually changed to local feedback in the voltage amp, leaving output stage non-linearities uncorrected. d |
#141
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MOSFET output stage
On Sep 18, 7:06*am, Vladimir Vassilevsky
wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? In most cases, this is fallacy, and the result is just the opposite. Reason: FETs have lower transconductance compared to BJTs. It is impossible to build a half bridge stage with an ideal transfer curve. However there are few special cases when a FET output stage has an advantage: 1) With FETs, it is simpler to control bias current, because of the negative dependency from the temperature. That simplifies the life. Bewa Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. This means that biasing them to a low idle current isn't so easy. |
#142
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Arny Krueger wrote: "Eeyore" wrote Arny Krueger wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? Who do? That's voodoo! Only to be expected from Arny "Any amp with less than 0.1% THD at full power sounds the same as all other amps." As usual, you're misrepresenting me. It's not 'as usual' at all. I can clearly recall you making that claim. Nonlinearity corresponding to 0.1% THD can be audible given choice of musical program material. I'm sure I've mentioned before I did a test when I thought the 0.1% spec was pretty good against another amp with 0.01% (both bipolars). It was chalk and cheese. The differences were NOT subtle. Nor were they 'level matching' issues or whatever. It stood out like a sore thumb. One sounded 'gratey' the other sounded 'clean'. Since then a 0.01% spec is my minimum target. I like to get into the 0.00X% area though. So, that number is way too high. Measurements made at just one power level, like full power, are obviously bogus. I agree. As Chris Hornbeck has noted, low-level linearity is crucial. Full-power specs are almost verging on the irrelevant but make good advertising copy. Obviously, the amp has to be able to perform within spec while driving a real-world tough (but not badly designed) speaker load. That all said, the world is full of good power amps. There are some bad ones, too. I'm not quite sure at the moment whether the overall trend is positive or negative. It's certainly debatable. Graham |
#143
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Arny Krueger wrote: "Eeyore" wrote Arny Krueger wrote: "Kevin McMurtrie" wrote in message Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. The low voltage rating takes this device out of consideration for serious audio amps for home or professional use. It may or may not be up to snuff for automotive use. Try coming up with a relevant example - something that can take at least 100 volts. More like 230-250V ! Well, yes, now that they are so easy to obtain. I was trying to make things easy for him. In fact we built a lot of pretty good power amps with 60-90 volt parts, back in the day. That was a while back ! Even 2N3773s were good for 140V IIRC. I still have half a tray of them somewhere from when I used to replace crap transistors with something that would handle the job and not go phut. Graham |
#144
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: Kevin McMurtrie wrote: "Arny Krueger" wrote: "Eeyore" wrote Kevin McMurtrie wrote: Damon Hill wrote: RichD wrote in Who do MOSFET sound better than bipolar, as an audio amp output driver? They do? Seems like it's possible to design good amplifiers either way. Exactly - zero difference in quality capabilities. It's usually a matter of impedance matching. Silicon transistors have a fixed loss around 0.5 volts. MOSFETs have a resistive loss inversely proportional to their voltage rating. That usually makes MOSFETs less expensive for low impedances and transistors less expensive for high impedances. TOTAL AND COMPLETE GARBAGE. Agreed. Actually, what Kevin said is the exact reverse of generally accepted practical knowledge. Bipolar is generally more efficient when the impedances get really low. MOSFETs were trendy for a while, but most new power amp designs seem to have bipolar outputs. Time to check the specs for this decade. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. These are NOT used in audio amps. Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). Graham |
#145
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Arny Krueger wrote: I'm not saying that 100 KHz devices are optimal for audio Even early 2N3055s were 8x better than that. I'm saying that in 2008, higher bandwidth than commonly available devices for audio power amps is irrelevant. Plenty still have an fT of only 4MHz (published). High gain bandwidth products are important for a good reason. If you can keep device phase shifts well up in frequency, the dominant pole (I know you didn't implement one specifically, but I'm sure it was still there) can also move up in frequency. Yes, there was ultimately one after the initial pole-zero which as you know helps keep phase response (and therefore margin) under control. That means that global feedback will carry on working instead of disappearing towards the top of the audio band, at which point it has usually changed to local feedback in the voltage amp, leaving output stage non-linearities uncorrected. Interesting. In the distant past, I've toyed with seperate feedback paths from both those points. Graham |
#146
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MOSFET output stage
MooseFET wrote: Bewa Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. This means that biasing them to a low idle current isn't so easy. Indeed. You'll need lossy ballast resistors. Laterals are different that way. Graham |
#147
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MOSFET output stage
Eeyore a écrit :
"Mr.T" wrote: "Fred Bartoli" " " wrote Please explain how you obtain even distortion products from a symmetrical (suppose paired mosfets) push-pull. Any symmetrical function produces odd (and no even) harmonics. But isn't that part of the problem, producing Mosfets with identically symmetrical characteristics in N-ch and P-ch ? Of course good circuit design can compensate for many such factors, so the final result is what is important, not some evangelical belief, IMO. Fred is thinking of push-pull transformer coupled arrangements. He needs to advance about 4-5 decades. Uhhh? What I wrote is just basic maths. Try a pencil and paper... That's one reason I never call modern amps push-pull but complementary symmetry instead. Graham -- Thanks, Fred. |
#148
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MOSFET output stage
Eeyore a écrit :
Fred Bartoli wrote: Eeyore a écrit : Trevor Wilson wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? **Non-sequitur. If you're saying that MOSFET outputs sound better, they don't. As a device, operating at typical bias currents (say) 10-50mA, MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents are elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost as good as BJTs. MOSFETs are very tough, have an exceptional ability to deliver high power, high frequency audio (and RF), but distortion is very high. They require lots of Global NFB in order to operate linearly. MOSFETs do not sound better than BJTs. At best, they can sound as good. All things being equal. With the required amount of feedback (which isn't rocket science - just good design) they have no trouble meeting VERY low THDs such as one I designed with 0.0008% THD @ 1 kHz. They sound better because of the predominance of 2nd harmonic distortion and almost complete absence of 3rd and ditto up the range AIUI with even and odd harmonics. Uhhh. Please explain how you obtain even distortion products from a symmetrical (suppose paired mosfets) push-pull. If you're such an expert, built one and see for yourself. And use LATERAL mosfets designed for audio. TWIT. You don't need to build one. It's just basic maths that you don't seem to grasp... Oh, and vertical or lateral mosfets has nothing to do with that. -- Thanks, Fred. |
#149
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MOSFET output stage
Fred Bartoli wrote: Eeyore a écrit : "Mr.T" wrote: "Fred Bartoli" " " wrote Please explain how you obtain even distortion products from a symmetrical (suppose paired mosfets) push-pull. Any symmetrical function produces odd (and no even) harmonics. But isn't that part of the problem, producing Mosfets with identically symmetrical characteristics in N-ch and P-ch ? Of course good circuit design can compensate for many such factors, so the final result is what is important, not some evangelical belief, IMO. Fred is thinking of push-pull transformer coupled arrangements. He needs to advance about 4-5 decades. Uhhh? What I wrote is just basic maths. Try a pencil and paper... Try measuring one ! Graham |
#150
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MOSFET output stage
it.
Not at all in any remote way shape or form. Don't you at least agree there are many similarities between 1/f noise and offset? Actually, I do. By and large, they amount to the same thing. Its all low frequency variations. For example, if one designs a chopper amp to get low offset, it also kills/corrects for 1/f noise as well. If one has 1/f problems in an system, one immediately thinks about using a chopper..well I do any way... Kevin Aylward www.anasoft.co.uk SuperSpice |
#151
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MOSFET output stage
"MooseFET" Vladimir Vassilevsky 1) With FETs, it is simpler to control bias current, because of the negative dependency from the temperature. That simplifies the life. Bewa Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. ** Every switching MOSFET made has a negative gate threshold voltage tempco. Equates to a VERY positive drain current tempco. Many times more severe than a BJT. ...... Phil |
#152
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MOSFET output stage
RichD wrote:
On Sep 18, "Kevin Aylward" wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? **Non-sequitur. If you're saying that MOSFET outputs sound better, they don't. As a device, operating at typical bias currents (say) 10-50mA, MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents are elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost as good as BJTs. MOSFETs are very tough, have an exceptional ability to deliver high power, high frequency audio (and RF), but distortion is very high. They require lots of Global NFB in order to operate linearly. MOSFETs do not sound better than BJTs. At best, they can sound as good. All things being equal. Well, I like mosfet outputs because they are easier to design with, imo. Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a tad harder to stabilse the feedback loop because of each stage pole. Basically, you only need about 6 low current transistors, or so, to achieve silly distortion and bandwidth figures, with mosfets. As far as "sounds better", that's all moot. Any competently designed amp should have thd, imd below audibility. Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Do you have any experience designing audio amps? er... yes...e.g. http://www.studiomaster.com/1984%20-%201986.htm "This was the amplifier pro sound companies were waiting for; many buy up to 100 units. " Which did you use? Right now I use a few, one of which is a studiomaster AX2500 (750+750), which I did not design. I also have a Behringer 1280S mixer/amp, and a Carlsbro (600+600) PA..and a.etc...etc... Kevin Aylward www.anasoft.co.uk SuperSpice |
#153
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MOSFET output stage
Chronic Philharmonic wrote:
"Eeyore" wrote in message ... Arny Krueger wrote: wrote RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower odd harmonic distortin. on and on. Not in any relevant way for audio power amps. The first two are highly relevevant in ANY circuit using NFB. Basic stability criteria. Only if the bandwidth and the phase margins are small relative to the target audio bandwidth, which is unlikely. Ahmmmm.... Lets say we are after a 50khz closed loop BW, i.e. so that there is negligible loss at 20khz. Now suppose we are after silly distortion levels, say , 0.005% at 20Khz. Typically this means large amounts of feedback, say a minimum of 40db at 20khz, maybe up to 60db even. This means that we need say, a loop unity gain bandwidth of say 5Mhz min. Now, if the Ft of a big, high voltage high current, bipolar was say 50Mhz, which is a tad on the fast side, its current gain would have dropped to 10, which aint so great. Furthermore, it would be really pushing a 50Mhz ft transistor to get an overall stable loop at 5Mhz., not forgetting that there will already be, by design, a dominant pole rolloff, prior to the output stage. Lets do some sums: Cin of a bipolar ~= gm/2.pi.ft X re/RL, in emiter follower mode, i.e. Cin = 1/(2.pi.ft.RL) For a 50Mhz bipolor this would be 800pf. at 4 ohms. A mosfet, would be Cin ~= Cgs/(gm.RL), which at typically 1A/V and 600p, would be Cin=125pf. So, despite the much larger gm of a bipolar to back off its inherent large Cbe in source follower mode, they still typically need much more high frequency drive than mosfets. Furthermore, without additional buffering, this larger capacitance kills the h.f gain of the class a main gain stage, as already mentioned by Graham. Indeed, in the early 80s, such 50Mhz devices were made from unobtainium. There are a lot of other details, but I really don't have the time to go into any more detailed technical design at the moment. So... try putting full on voltage on a mosfet without a heatsink for a while, then try that with a bipolar!. And hopefully I haven't made any errors in my calcs;-) Kevin Aylward http://www.kevinaylward.co.uk/ee/index.html |
#154
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MOSFET output stage
Eeyore wrote: It's quite clear that there are just TWO people in this thread who really understand the use of lateral audio mosfets. Oops THREE. I forgot Kevin Aylward. Graham |
#155
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MOSFET output stage
Eeyore wrote:
Kevin Aylward wrote: Trevor Wilson wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? **Non-sequitur. If you're saying that MOSFET outputs sound better, they don't. As a device, operating at typical bias currents (say) 10-50mA, MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents are elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost as good as BJTs. MOSFETs are very tough, have an exceptional ability to deliver high power, high frequency audio (and RF), but distortion is very high. They require lots of Global NFB in order to operate linearly. MOSFETs do not sound better than BJTs. At best, they can sound as good. All things being equal. Ahmm.... welll....here we go... Well, I like mosfet outputs because they are easier to design with, imo. Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a tad harder to stabilse the feedback loop because of each stage pole. Basically, you only need about 6 low current transistors, or so, to achieve silly distortion and bandwidth figures, with mosfets. As far as "sounds better", that's all moot. Any competently designed amp should have thd, imd below audibility. Seems to be a few that miss that though. Esp those Chinese copies of copies. Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm Kevin, you'd be just the guy to do a harmonic analysis of the distortion spectrum of a properly biased bipolar vs lateral mosfet amp wouldn't you ? Any chance ? It has just occurred to me, that you probably live quite close to me. I live in Stevenage now, just a tad away from the old Luton hunting grounds. I would say a joint visit to the pub for some Guinness might not go amiss. Oh.. check out my current venture. Available for bookings...www.blonddee.co.uk Kevin Aylward www.anasoft.co.uk SuperSpice |
#156
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MOSFET output stage
On Fri, 19 Sep 2008 15:28:33 +0100, Eeyore
wrote: John Larkin wrote: Eeyore wrote: Kevin McMurtrie wrote: "Arny Krueger" wrote: "Eeyore" wrote Kevin McMurtrie wrote: Damon Hill wrote: RichD wrote in Who do MOSFET sound better than bipolar, as an audio amp output driver? They do? Seems like it's possible to design good amplifiers either way. Exactly - zero difference in quality capabilities. It's usually a matter of impedance matching. Silicon transistors have a fixed loss around 0.5 volts. MOSFETs have a resistive loss inversely proportional to their voltage rating. That usually makes MOSFETs less expensive for low impedances and transistors less expensive for high impedances. TOTAL AND COMPLETE GARBAGE. Agreed. Actually, what Kevin said is the exact reverse of generally accepted practical knowledge. Bipolar is generally more efficient when the impedances get really low. MOSFETs were trendy for a while, but most new power amp designs seem to have bipolar outputs. Time to check the specs for this decade. Visit IRF. You can get a surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms on resistance, 300W dissipation, 24V rating, and a 1600A surge rating. These are NOT used in audio amps. Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). Graham An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. That will optimize the hell out of the power supply, fets, and heat sink, giving a lot more safe power for the buck, especially in complex-signal non-sinusoidal apps like audio and NMR gradient drivers. Use a bunch of smaller fets rather than a few big ones; that speeds things up and spreads the heat out across the heat sinks better. For the audio version, use two "hot" heat sinks, with no insulators under the fets. Include full BIST. It's worth it for the savings in production test alone. The output stage will be so quiet and linear that no overall feedback is needed or helpful. The audiophools will hate this. John |
#157
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MOSFET output stage
Eeyore wrote:
Kevin Aylward wrote: Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm This link seems to be bad. I did find this though http://divinecables.co.uk/mains-powe...i-power-cable# All I can say, is that I am still stunned..and shocked...shocked and stunned...all I said was that I was taller than Jesus, not that I was bigger than Jesus... Kevin Aylward |
#158
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MOSFET output stage
Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Trevor Wilson wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? **Non-sequitur. If you're saying that MOSFET outputs sound better, they don't. As a device, operating at typical bias currents (say) 10-50mA, MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents are elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost as good as BJTs. MOSFETs are very tough, have an exceptional ability to deliver high power, high frequency audio (and RF), but distortion is very high. They require lots of Global NFB in order to operate linearly. MOSFETs do not sound better than BJTs. At best, they can sound as good. All things being equal. Ahmm.... welll....here we go... Well, I like mosfet outputs because they are easier to design with, imo. Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a tad harder to stabilse the feedback loop because of each stage pole. Basically, you only need about 6 low current transistors, or so, to achieve silly distortion and bandwidth figures, with mosfets. As far as "sounds better", that's all moot. Any competently designed amp should have thd, imd below audibility. Seems to be a few that miss that though. Esp those Chinese copies of copies. Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm Kevin, you'd be just the guy to do a harmonic analysis of the distortion spectrum of a properly biased bipolar vs lateral mosfet amp wouldn't you ? Any chance ? It has just occurred to me, that you probably live quite close to me. I live in Stevenage now, just a tad away from the old Luton hunting grounds. I would say a joint visit to the pub for some Guinness might not go amiss. Sounds ok to me. I'm in St Albans. Will contact you off group. Oh.. check out my current venture. Available for bookings...www.blonddee.co.uk OK will do. I know someone who might even book you. Graham |
#159
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MOSFET output stage
Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm This link seems to be bad. I did find this though Works here. http://divinecables.co.uk/mains-powe...i-power-cable# All I can say, is that I am still stunned..and shocked...shocked and stunned...all I said was that I was taller than Jesus, not that I was bigger than Jesus... I'd like to start a movement to have all these liars and fraudsters shut down. Graham |
#160
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MOSFET output stage
Eeyore wrote:
Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Trevor Wilson wrote: "RichD" wrote Who do MOSFET sound better than bipolar, as an audio amp output driver? **Non-sequitur. If you're saying that MOSFET outputs sound better, they don't. As a device, operating at typical bias currents (say) 10-50mA, MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents are elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost as good as BJTs. MOSFETs are very tough, have an exceptional ability to deliver high power, high frequency audio (and RF), but distortion is very high. They require lots of Global NFB in order to operate linearly. MOSFETs do not sound better than BJTs. At best, they can sound as good. All things being equal. Ahmm.... welll....here we go... Well, I like mosfet outputs because they are easier to design with, imo. Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a tad harder to stabilse the feedback loop because of each stage pole. Basically, you only need about 6 low current transistors, or so, to achieve silly distortion and bandwidth figures, with mosfets. As far as "sounds better", that's all moot. Any competently designed amp should have thd, imd below audibility. Seems to be a few that miss that though. Esp those Chinese copies of copies. Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm Kevin, you'd be just the guy to do a harmonic analysis of the distortion spectrum of a properly biased bipolar vs lateral mosfet amp wouldn't you ? Any chance ? It has just occurred to me, that you probably live quite close to me. I live in Stevenage now, just a tad away from the old Luton hunting grounds. I would say a joint visit to the pub for some Guinness might not go amiss. Sounds ok to me. I'm in St Albans. Yeah, just down the road...16.7M according to the AA. Although with a suggested time of 20mins, obviously, the AA don't know how to drive... Will contact you off group. Look foward to it. My phone # is on my websites. Oh.. check out my current venture. Available for bookings...www.blonddee.co.uk OK will do. I know someone who might even book you. Oh.. nice... Kevin Aylward |
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