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#41
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MOSFET output stage
"Gareth Magennis" No, this is good fun, and you might eventually tell me why my supposition is untrue. ** Cos nothing makes it true. YOU ****ING Z- GRADE IMBECILE !!! ..... Phil |
#42
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MOSFET output stage
"Gareth Magennis" wrote in
message Not if you clip them like you might a valve amp for example. Straw man. It is entirely practical to build power amps that are never clipped in actual use. If you don't like how your power amp clips? Get one with enough output so that it never clips. Would a Mosfet amp clip more "nicely" than a BJT amp? Power amps aren't just devices, they are circuits. Circuit design can easily trump device characteristics. In fact the sharpness of the clipping of a power amp relates to things like how much negative feedback it has, all things considered. If you have a circuit with lots of negative feedback, it is very likely to clip very sharply and ideally. If you have a circuit with less negative feedback, the clipping will be softer, but will occupy a larger proportion of the transfer characteristic. I'm thinking driving bass speakers. If they need lots of power, get a powerful power amp. Forget about what's inside the box, worry about how the box works. Is this what some of these "audiophools" or perhaps PA guys are getting at by saying they sound better? 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. |
#43
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MOSFET output stage
On a sunny day (Wed, 17 Sep 2008 16:12:25 -0700 (PDT)) it happened RichD
wrote in : Who do MOSFET sound better than bipolar, as an audio amp output driver? MoSfEtS zound batter becuaze thOze work wiz EleKtRONS, LikE TubeS. Traanzisters WOrk wiz HOLES, AND Thoze HoLES YoU wILL HeAr in Ze Muzick. |
#44
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MOSFET output stage
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. 2) If the global warming is not an issue, then the class A stage made of drain follower loaded by the current source can be very linear indeed. 3) FETs are free from BJT high injection effects and the charge accumulation in the base. 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |
#45
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MOSFET output stage
Gareth Magennis wrote: "Eeyore" wrote Gareth Magennis wrote: Er, perhaps. Mosfets don't have the same thermal characteristics as BJT's, which are more prone to thermal runaway. So perhaps when you are runing Mosfets hard, there is some kind of compression thing going on which sounds nicer than a BJT amp exploding after clipping a lot. Competently designed BJT amps don't 'explode'. It's not difficult, but the Chinese haven't quite mastered it yet. The point is that when people prefer one amp over another it may not be easy to tell technically what it is they prefer. Flatter frequency reponse perhaps ? If a Mosfet amp compresses the bottom end slightly over a BJT, for example, this might in the long term be a nicer sounding amp. Maybe absolutely nothing to do with crossover distorion, linearity, feedback blah blah blah. How would this 'compression' occur ? Graham |
#46
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MOSFET output stage
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." Not quite as inane as some comments so far. Graham |
#47
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MOSFET output stage
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 ? Graham |
#48
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MOSFET output stage
Phil Allison wrote: "Eeysore" I know of no 'PA' amps currently using mosfets. ** Huh ???? All those hundreds of thousands of MOSFET audio power amps made since the mid 1980s have not justs disappeared you know - power amps made by H-H & Harrison Electronics plus C-Audio in the UK , Perreaux and ZPE of NZ, Australian Monitor and ARX (still in full production) and Jands here in Aussie - plus many other less well known brands. Not to mention Studiomaster's Mosfet series of old. Initiated by none other than myself. The majority are STILL in use, maybe looking just a bit the worse for wear. Even in the UK - Chevin Research ( based in Yorkshire) A-series amps are all lateral mosfet designs. http://www.chevin-research.com/products_a_series.php Goodness are they still going ? By current, I did mean in current manufacture. So there are a few. But all the big boys are bipolar only now. Graham |
#49
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MOSFET output stage
Arny Krueger wrote: "Gareth Magennis" wrote Not if you clip them like you might a valve amp for example. Straw man. It is entirely practical to build power amps that are never clipped in actual use. If you don't like how your power amp clips? Get one with enough output so that it never clips. Would a Mosfet amp clip more "nicely" than a BJT amp? Power amps aren't just devices, they are circuits. Circuit design can easily trump device characteristics. In fact the sharpness of the clipping of a power amp relates to things like how much negative feedback it has, all things considered. If you have a circuit with lots of negative feedback, it is very likely to clip very sharply and ideally. If you have a circuit with less negative feedback, the clipping will be softer, but will occupy a larger proportion of the transfer characteristic. Precisely so. Graham |
#50
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MOSFET output stage
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. We discovered that EV's SX500 was particularly bad in this respect in PA for example. Now that is certainly audible. Graham |
#51
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MOSFET output stage
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. Another one off with the faeries. Graham |
#52
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MOSFET output stage
Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Graham |
#53
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Graham Where did you put the dominant pole cap? - still around the voltage amplifier I'm guessing. It is vital that the dominant pole cap is the only one that shows up in the phase response up to unity O/L gain, so buffering of the fet gate caps is pretty much a given. The big problem you need to overcome with mosfets is that they have an essentially log response, resulting in (or from, depending how you look at it) huge - maybe 10/1 Gm changes over the operating range of currents. This is all easily taken care of by global nfb, of course, but it does make the sewing together of the crossover point a bit trickier than a bipolar design. The Gms are dropping at a much sharper angle and the doubling spike consequently more obvious. All unimportant, of course, given enough nfb. d |
#54
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Where did you put the dominant pole cap? - still around the voltage amplifier I'm guessing. It is vital that the dominant pole cap is the only one that shows up in the phase response up to unity O/L gain, so buffering of the fet gate caps is pretty much a given. It wasn't dominant pole compensated. But you got the location right after a fashion. The big problem you need to overcome with mosfets is that they have an essentially log response, resulting in (or from, depending how you look at it) huge - maybe 10/1 Gm changes over the operating range of currents. Not on laterals. Graham |
#55
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Where did you put the dominant pole cap? - still around the voltage amplifier I'm guessing. It is vital that the dominant pole cap is the only one that shows up in the phase response up to unity O/L gain, so buffering of the fet gate caps is pretty much a given. It wasn't dominant pole compensated. But you got the location right after a fashion. How did you keep it stable? The big problem you need to overcome with mosfets is that they have an essentially log response, resulting in (or from, depending how you look at it) huge - maybe 10/1 Gm changes over the operating range of currents. Not on laterals. Graham Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. d |
#56
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Where did you put the dominant pole cap? - still around the voltage amplifier I'm guessing. It is vital that the dominant pole cap is the only one that shows up in the phase response up to unity O/L gain, so buffering of the fet gate caps is pretty much a given. It wasn't dominant pole compensated. But you got the location right after a fashion. How did you keep it stable? There's an extra pole and zero. Hence not dominant pole. Works a charm. I've used that on every amp design since ~ 1990. Sometimes in more than one place in the loop. The big problem you need to overcome with mosfets is that they have an essentially log response, resulting in (or from, depending how you look at it) huge - maybe 10/1 Gm changes over the operating range of currents. Not on laterals. Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Graham |
#57
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MOSFET output stage
Don Pearce wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. Had better luck locating the later plastic package devices and they actually show the relevant transfer characteristic. http://www.datasheetcatalog.com/data.../2SK1058.shtml http://www.datasheetcatalog.com/data...1/2SJ162.shtml Graham |
#58
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Vladimir Vassilevsky wrote: 4) FETs do not require high base currents of BJTs; that simplifies the driver stage. So what charges and discharges the damn gate capacitance you dozy bugger ? The classic Class A voltage gain driver stage probably needs as much current as if you were driving darlington output devices with a simple design.. In my ultra-low THD design I had a bipolar complementary Class A emitter follower driving the mosfet gates ! It also removes the capacitive loading from the voltage gain stage, increasing phase margin and loop HF response. Where did you put the dominant pole cap? - still around the voltage amplifier I'm guessing. It is vital that the dominant pole cap is the only one that shows up in the phase response up to unity O/L gain, so buffering of the fet gate caps is pretty much a given. It wasn't dominant pole compensated. But you got the location right after a fashion. How did you keep it stable? There's an extra pole and zero. Hence not dominant pole. Works a charm. I've used that on every amp design since ~ 1990. Sometimes in more than one place in the loop. Ok. The big problem you need to overcome with mosfets is that they have an essentially log response, resulting in (or from, depending how you look at it) huge - maybe 10/1 Gm changes over the operating range of currents. Not on laterals. Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Graham Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. You do it with local feedback by emitter degeneration - a fraction of an ohm does it. By the time you have added enough to bring the Gm down close to that of a mosfet the bipolar is almost perfectly linear, while the mosfet necessarily shows its square law. And the next problem is how tidily the devices run out of Gm at low current. The bipolar drops away gently while the mosfet simply runs straight into the ground - there is no place you can make a clean crossover without a lot of effort in controlling the crossover point. d |
#59
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Graham |
#60
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MOSFET output stage
Don Pearce wrote: Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. You do it with local feedback by emitter degeneration - a fraction of an ohm does it. Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least which would be intolerable in practical designs for obvious loss reasons. There is a cleverer way to do it which blows away all the classic ideas of biasing bipolar output stages but it's my secret. That basic design was good for 0.008% THD and I wasn't even trying hard. It borrows on your idea though, just not the same way. Graham |
#61
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Graham Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! You can cross over a bipolar output stage long before you hit that kind of number. d |
#62
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MOSFET output stage
On Wed, 17 Sep 2008 16:12:25 -0700, RichD wrote:
Who do MOSFET sound better than bipolar, as an audio amp output driver? Because you believe they will. Cheers! Rich |
#63
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! It averages out including higher currents at about 1S. You can cross over a bipolar output stage long before you hit that kind of number. And you'll still get crossover distortion. Graham |
#64
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. You do it with local feedback by emitter degeneration - a fraction of an ohm does it. Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least which would be intolerable in practical designs for obvious loss reasons. 0.1 ohm is plenty in a high current stage - it makes all the difference and brings the Gm down to a pretty flat 9ish - there isn't much to be gained going lower. The variable resistance it is fighting is 25/(Ic * 1000). By the time you hit 200mA or so that is pretty much inconsequential - everything above controlled by the external resistor. There is a cleverer way to do it which blows away all the classic ideas of biasing bipolar output stages but it's my secret. That basic design was good for 0.008% THD and I wasn't even trying hard. It borrows on your idea though, just not the same way. Obviously I can't comment. You can reveal it here safely of course - just cite the posts as proof of prior art if anyone tries to nick it. d |
#65
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MOSFET output stage
Rich Grise wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? Because you believe they will. Because they CAN. By a country mile. But all people want today is cheap. And if they want esoteric they want the high distortion of tubes instead of ultra-low THD of well-engineered mosfet amps. Graham |
#66
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! It averages out including higher currents at about 1S. You can cross over a bipolar output stage long before you hit that kind of number. And you'll still get crossover distortion. Graham You get crossover distortion whatever the topology. d |
#67
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MOSFET output stage
In article , Eeyore wrote:
Rich Grise wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? Because you believe they will. Because they CAN. By a country mile. But all people want today is cheap. And if they want esoteric they want the high distortion of tubes instead of ultra-low THD of well-engineered mosfet amps. Graham Seems like most people like the Hafler amp of old. Is the design still around ? greg |
#68
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MOSFET output stage
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. 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. 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. Fact - The fidelity of the sound depends on much more than just device properties. |
#69
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. You do it with local feedback by emitter degeneration - a fraction of an ohm does it. Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least which would be intolerable in practical designs for obvious loss reasons. 0.1 ohm is plenty in a high current stage - it makes all the difference and brings the Gm down to a pretty flat 9ish - there isn't much to be gained going lower. The variable resistance it is fighting is 25/(Ic * 1000). By the time you hit 200mA or so that is pretty much inconsequential - everything above controlled by the external resistor. Sorry 0.1 is not enough. I don't even go that low for current sharing purposes ! More like 0.15, 0.22 or even 0.33. You're still missing the point about crossover distortion though. OK for 0.1 % THD maybe but who's going to buy that ? Graham |
#70
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MOSFET output stage
Don Pearce wrote: You get crossover distortion whatever the topology. The mosfet curves match into each other far far better, plus you're already using more feedback too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's output. Graham |
#71
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MOSFET output stage
GregS wrote: Eeyore wrote: Rich Grise wrote: RichD wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? Because you believe they will. Because they CAN. By a country mile. But all people want today is cheap. And if they want esoteric they want the high distortion of tubes instead of ultra-low THD of well-engineered mosfet amps. Seems like most people like the Hafler amp of old. Is the design still around ? I hope not. Some people like antiques too. Graham |
#72
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MOSFET output stage
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. It TOTALLY proves wrong your assertion "The device properties of BJTs are superior to those of MOSFETs in all respects" 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. 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... What the **** is this 'offset' you're talking about. Do you mean biasing ? Fact - MOSFETs have lower offset than BJTs. Fact - The fidelity of the sound depends on much more than just device properties. You're a COMPLETE IDIOT. You need a boundary condition up the backside. It's clear that you know zilch about high-performance audio, whilst I've been doing it for 37 years. Graham |
#73
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! It averages out including higher currents at about 1S. You can cross over a bipolar output stage long before you hit that kind of number. And you'll still get crossover distortion. Graham Take a look at these two graphs I copied from Doug Self's power amplifier book. They show the voltage gain of the output pair against operating point (input volts)for a variety of bias conditions. This can be used to select a best bias. For the bipolars at the top, the fourth curve up is clearly the best with gain varying from 0.97 down to 0.963 across the range. This is easily tamed, and even a small error doesn't do much damage. Now contrast this with the fets (2SK135/2Sj50) below. First there is no stable flat line - the gain goes on rising all the way out to the 15V which is the maximum he measured. Secondly there is no decent bias current that will control the crossover. I guess that again the fourth from the bottom is as good as it gets but that gives a gain variation from 0.83 down to 0.77, with a much sharper turnaround into the Gm doubling region (spiky crossover products result). http://89.174.169.10/odds/crossover.gif That is why it is so much easier to control crossover distortion in bipolars. d |
#74
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: You get crossover distortion whatever the topology. The mosfet curves match into each other far far better, plus you're already using more feedback too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's output. Graham I'm sure it did, but that is the amp, not the output stage. See my next post. d |
#75
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
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. It TOTALLY proves wrong your assertion "The device properties of BJTs are superior to those of MOSFETs in all respects" No, it doesn't. 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. 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... What the **** is this 'offset' you're talking about. Do you mean biasing ? No, of course not. If I'd meant biasing I would've said biasing. BTW, I don't see how one component's biasing can be "better" than another one's - it's simply a design step that's necessary to make it work. I mean, nobody would say "this amplifier's frequency compensation is so much nicer than that one's"... 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. 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. |
#76
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! It averages out including higher currents at about 1S. You can cross over a bipolar output stage long before you hit that kind of number. And you'll still get crossover distortion. Take a look at these two graphs I copied from Doug Self's power amplifier book. They show the voltage gain of the output pair against operating point (input volts)for a variety of bias conditions. This can be used to select a best bias. For the bipolars at the top, the fourth curve up is clearly the best with gain varying from 0.97 down to 0.963 across the range. This is easily tamed, and even a small error doesn't do much damage. Now contrast this with the fets (2SK135/2Sj50) below. First there is no stable flat line - the gain goes on rising all the way out to the 15V which is the maximum he measured. Secondly there is no decent bias current that will control the crossover. I guess that again the fourth from the bottom is as good as it gets but that gives a gain variation from 0.83 down to 0.77, with a much sharper turnaround into the Gm doubling region (spiky crossover products result). http://89.174.169.10/odds/crossover.gif That is why it is so much easier to control crossover distortion in bipolars. Nonsense. I've seen Doug Self's 'blameless amplifier' diagrams. The crossover 'pip' is clearly visible in all of them. It's quite good but not that good. I can beat that standing on my head. When I said the crossover of my big Mosfet amp was 'invisible' that IS what I meant. INVISIBLE and a THD 14% above the AP analyser residual. For some reason I can't reach your gif btw. Graham |
#77
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: You get crossover distortion whatever the topology. The mosfet curves match into each other far far better, plus you're already using more feedback too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's output. I'm sure it did, but that is the amp, not the output stage. See my next post. What good is an amp without an output stage ? Or vice-versa. I'm not interested in how many fairies can dance on the head of a pin. I'm intereted in real products you can build reliably in quantity production.. Graham |
#78
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don Pearce wrote: Eeyore wrote: Don't know laterals - haven't kept up. Gimme some numbers and I'll go have a look. You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer manufactured. Equivalent types now sourced by Semelab and Exicon. http://www.profusionplc.com/pro/gex/...teral%20mosfet Oh, I thought there was something new going on. They have exactly the Gm characteristic I was talking about. Here's the thing. Bipolar Gm is enormously bigger than mosfet, and you can use that in an output stage. Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The Exicon data sheet doesn't have the equivalent plot for some reason. The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's crap and that's where crossover distortion comes from. You just can't get rid of it. Yup, looking now. The transfer characteristic is the alomst-square-law curve I was expecting; I don't think you can get anything else from a mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of 0.6! It averages out including higher currents at about 1S. You can cross over a bipolar output stage long before you hit that kind of number. And you'll still get crossover distortion. Take a look at these two graphs I copied from Doug Self's power amplifier book. They show the voltage gain of the output pair against operating point (input volts)for a variety of bias conditions. This can be used to select a best bias. For the bipolars at the top, the fourth curve up is clearly the best with gain varying from 0.97 down to 0.963 across the range. This is easily tamed, and even a small error doesn't do much damage. Now contrast this with the fets (2SK135/2Sj50) below. First there is no stable flat line - the gain goes on rising all the way out to the 15V which is the maximum he measured. Secondly there is no decent bias current that will control the crossover. I guess that again the fourth from the bottom is as good as it gets but that gives a gain variation from 0.83 down to 0.77, with a much sharper turnaround into the Gm doubling region (spiky crossover products result). http://89.174.169.10/odds/crossover.gif That is why it is so much easier to control crossover distortion in bipolars. Nonsense. I've seen Doug Self's 'blameless amplifier' diagrams. The crossover 'pip' is clearly visible in all of them. It's quite good but not that good. I can beat that standing on my head. When I said the crossover of my big Mosfet amp was 'invisible' that IS what I meant. INVISIBLE and a THD 14% above the AP analyser residual. For some reason I can't reach your gif btw. Graham Sorry, wrong address http://81.174.169.10/odds/crossover.gif d |
#79
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Don Pearce wrote: Eeyore wrote: Don Pearce wrote: You get crossover distortion whatever the topology. The mosfet curves match into each other far far better, plus you're already using more feedback too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's output. I'm sure it did, but that is the amp, not the output stage. See my next post. What good is an amp without an output stage ? Or vice-versa. I'm not interested in how many fairies can dance on the head of a pin. I'm intereted in real products you can build reliably in quantity production.. Graham We're talking about the difference between fet and bipolar output stages. It is simple to reduce distortions in amplifiers with either to negligible proportions, which is why, for the purposes of the chat, it is necessary to restrict the chat to output stages per se. d |
#80
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Jorden Verwer wrote: Eeyore wrote: What the **** is this 'offset' you're talking about. Do you mean biasing ? No, of course not. If I'd meant biasing I would've said biasing. BTW, I don't see how one component's biasing can be "better" than another one's - it's simply a design step that's necessary to make it work. I mean, nobody would say "this amplifier's frequency compensation is so much nicer than that one's"... 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. Oh for Christ's sake grow up. An amp is a closed loop (often DC) servo almost. Any offset depends on the INPUT transistors you brainless jerk. You have the tiniest idea what you're talking about. You must be a lecturer to be this stupid. Graham |
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