Discussing audio amplifier design -- BJT, discrete
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From: John Larkin on 26 Jan 2010 19:15 On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >I'd like to take a crack at thinking through a design of an >audio amplifier made up of discrete BJTs and other discrete >parts as an educational process. > >I imagine this will be broken up into three sections; input >transconductance, transimpedance VAS, and output driver. But >other arrangements (such as combining the VAS and output >driver using a signal splitting BJT) would work for me, in >learning. > >I said "BJT" and "discrete" but I'm also open to the idea of >using BJT pairs, such as the BCV61 and BCV62. In the case of >current mirrors, that may make sense. But not high-priced, >elite and/or hard to get, or obsolete. And no FETs. This is >to be about learning to design with BJTs. > >SMT vs through-hole isn't an issue for learning about a >design, I suppose. If I need to build up some section and >test it with a signal, I'll probably want to do it quickly >and without having to buy services every step of the way. So >I may 'dead bug' SMT parts to get there. (The basic idea >here is to learn, not to make something tiny.) > >Although I have some other applications, right now I'd like >the target use to be as a computer speaker system (not unlike >those dirt cheap, sadly almost all of them 10% THD, systems >sold today into this market. Except that I'd like to work >through the design on my own, from start to end. > >Given what I understand right now from a very short search on >the topic, the input should be taken as a maximum of 1.0Vrms >and the input's load should appear to be something like 10k >ohms. If someone knows different from that, I'll accept the >criticism and change that spec. > >I'd like to consider a tone control and a volume control to >be included. > >Output is to be into a small 8 ohm speaker. With that >maximum 1.0Vrms at the input and the volume control set to >maximum the wattage into 8 ohms should be around 10 watts. >Since human hearing won't tell much difference between 8 >watts and 12 watts, this is a bit of a sloppy spec and I'm >open to anything in the area of 5-20 watts... though I'm >really wanting to keep the rail voltages down to something >modest and the BJTs not having to tolerate hugish Vce. > >Now that I say this, an odd idea comes to mind because the >CFL light bulbs include two TO-220 BJTs that can handle quite >a high Vce on them. I could cannibalize those. But to be >honest, I'm still not needing high watt outputs. So there's >no reason to think about scavenging such parts. > >I would like to design it to work into 4 ohms as a margin >bound and not as a design goal, but even 5.6 ohms would be >acceptable. > >I'm not looking for this to be done quickly, either. If it >takes months of only occasional back-and-forth, I'm fine with >that. Also, I expect to do my work and don't expect someone >else to hand-hold me from complete ignorante to complete >enlightenment. :) I just need someone to slap my face when >I say something terribly stupid and/or point in a truely >useful direction when I need it. Or else someone who is >wanting to explore this with me and willing to work for it. > >Is anyone here willing to consider a sincere discussion? > >Jon Back when transistors were young, and transistor manuals (GE, RCA) were published, there were tons of such circuits around. They all pretty much converged to a few forms, and haven't changed much since. I could post some circuits from the old manuals, it that wouldn't spoil what you want to do. 10 watts into computer speakers sounds like a lot. Most AM radios didn't make one watt. You might experiment first to see how much power you really need. John
From: Jon Kirwan on 26 Jan 2010 21:46 On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>I'd like to take a crack at thinking through a design of an >>audio amplifier made up of discrete BJTs and other discrete >>parts as an educational process. >> >>I imagine this will be broken up into three sections; input >>transconductance, transimpedance VAS, and output driver. But >>other arrangements (such as combining the VAS and output >>driver using a signal splitting BJT) would work for me, in >>learning. >> >>I said "BJT" and "discrete" but I'm also open to the idea of >>using BJT pairs, such as the BCV61 and BCV62. In the case of >>current mirrors, that may make sense. But not high-priced, >>elite and/or hard to get, or obsolete. And no FETs. This is >>to be about learning to design with BJTs. >> >>SMT vs through-hole isn't an issue for learning about a >>design, I suppose. If I need to build up some section and >>test it with a signal, I'll probably want to do it quickly >>and without having to buy services every step of the way. So >>I may 'dead bug' SMT parts to get there. (The basic idea >>here is to learn, not to make something tiny.) >> >>Although I have some other applications, right now I'd like >>the target use to be as a computer speaker system (not unlike >>those dirt cheap, sadly almost all of them 10% THD, systems >>sold today into this market. Except that I'd like to work >>through the design on my own, from start to end. >> >>Given what I understand right now from a very short search on >>the topic, the input should be taken as a maximum of 1.0Vrms >>and the input's load should appear to be something like 10k >>ohms. If someone knows different from that, I'll accept the >>criticism and change that spec. >> >>I'd like to consider a tone control and a volume control to >>be included. >> >>Output is to be into a small 8 ohm speaker. With that >>maximum 1.0Vrms at the input and the volume control set to >>maximum the wattage into 8 ohms should be around 10 watts. >>Since human hearing won't tell much difference between 8 >>watts and 12 watts, this is a bit of a sloppy spec and I'm >>open to anything in the area of 5-20 watts... though I'm >>really wanting to keep the rail voltages down to something >>modest and the BJTs not having to tolerate hugish Vce. >> >>Now that I say this, an odd idea comes to mind because the >>CFL light bulbs include two TO-220 BJTs that can handle quite >>a high Vce on them. I could cannibalize those. But to be >>honest, I'm still not needing high watt outputs. So there's >>no reason to think about scavenging such parts. >> >>I would like to design it to work into 4 ohms as a margin >>bound and not as a design goal, but even 5.6 ohms would be >>acceptable. >> >>I'm not looking for this to be done quickly, either. If it >>takes months of only occasional back-and-forth, I'm fine with >>that. Also, I expect to do my work and don't expect someone >>else to hand-hold me from complete ignorante to complete >>enlightenment. :) I just need someone to slap my face when >>I say something terribly stupid and/or point in a truely >>useful direction when I need it. Or else someone who is >>wanting to explore this with me and willing to work for it. >> >>Is anyone here willing to consider a sincere discussion? >> >>Jon > >Back when transistors were young, and transistor manuals (GE, RCA) >were published, there were tons of such circuits around. They all >pretty much converged to a few forms, and haven't changed much since. I remember reading in popular electronics about some audio amps that I couldn't even come close to following at the time. The series of them with the name 'tiger' in them. I'm not so much interested in _circuits_, per se, as I am in learning about topologies, various ideas built upon them, and then the specific details of designing towards a specific implementation. For example, I enjoyed learning about bootstrapping as a general idea _and_ as it applies to a couple of specific areas. Having both theory _and_ specific practice helps firm up the ideas better. >I could post some circuits from the old manuals, it that wouldn't >spoil what you want to do. It may serve as a point of discussion. Would you be willing to discuss their details and the broader theories as applied? >10 watts into computer speakers sounds like a lot. Most AM radios >didn't make one watt. You might experiment first to see how much power >you really need. Oh, I figure one watt is enough, too. As a practical matter and as a consumer using a device and not as a designer trying to learn something. That's what... 3Vrms? Into 8 ohms? A voltage gain of 3, given 1Vrms input? I'm wanting to learn some things, not place one BJT (okay, not really, but it almost seems like that) down as an emitter follower and then calling it good. ;) Up front, I thought I'd like to deal with perhaps something on the order of about 10Vrms into 8 ohms. I figured that is enough 'bad' that I'd have to cope with some interesting corners along the way; but not enough 'bad' that I'd have to deal with too much all at once. For example, at around 10 watts or so, it's enough that I may need to seriously consider avoiding class-A operation of the output stage and move to class-B, instead. But it is low enough that there is some room to discuss each, as well as class-AB biasing, too. More power and I'm almost certain I'm pushed into class-B. Less power and.. well, who cares that much? At one watt or so, just class-A and be done with it? I won't learn the reasoning behind trade-offs that way. There's more. I just figured at about 10 watts I'm likely to learn some things but not be forced to learn so much that I'm overwhelmed. I'm open to specific advice about all this, of course. Jon
From: Jon Kirwan on 26 Jan 2010 21:48 On Wed, 27 Jan 2010 10:11:40 +1000, David Eather <eather(a)tpg.com.au> wrote: >Jon Kirwan wrote: >>><snip of offer to discuss amplifier design> >> >> Oh, my gosh! This is great to hear!! > >Don't get too excited, my brain has been badly scrambled. I often miss >things on a first pass - don't be afraid to check - I won't take offense. ><snip of trailer unresponded to> hehe. Okay. It's just a topic I'd like to play with and I'm glad there might be someone else out there to talk it over with. What can I say? Except _thanks_! Jon
From: David Eather on 27 Jan 2010 02:31 Jon Kirwan wrote: > On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan >> <jonk(a)infinitefactors.org> wrote: >> >>> I'd like to take a crack at thinking through a design of an >>> audio amplifier made up of discrete BJTs and other discrete >>> parts as an educational process. >>> >>> I imagine this will be broken up into three sections; input >>> transconductance, transimpedance VAS, and output driver. But >>> other arrangements (such as combining the VAS and output >>> driver using a signal splitting BJT) would work for me, in >>> learning. >>> >>> I said "BJT" and "discrete" but I'm also open to the idea of >>> using BJT pairs, such as the BCV61 and BCV62. In the case of >>> current mirrors, that may make sense. But not high-priced, >>> elite and/or hard to get, or obsolete. And no FETs. This is >>> to be about learning to design with BJTs. >>> >>> SMT vs through-hole isn't an issue for learning about a >>> design, I suppose. If I need to build up some section and >>> test it with a signal, I'll probably want to do it quickly >>> and without having to buy services every step of the way. So >>> I may 'dead bug' SMT parts to get there. (The basic idea >>> here is to learn, not to make something tiny.) >>> >>> Although I have some other applications, right now I'd like >>> the target use to be as a computer speaker system (not unlike >>> those dirt cheap, sadly almost all of them 10% THD, systems >>> sold today into this market. Except that I'd like to work >>> through the design on my own, from start to end. >>> >>> Given what I understand right now from a very short search on >>> the topic, the input should be taken as a maximum of 1.0Vrms >>> and the input's load should appear to be something like 10k >>> ohms. If someone knows different from that, I'll accept the >>> criticism and change that spec. >>> >>> I'd like to consider a tone control and a volume control to >>> be included. >>> >>> Output is to be into a small 8 ohm speaker. With that >>> maximum 1.0Vrms at the input and the volume control set to >>> maximum the wattage into 8 ohms should be around 10 watts. >>> Since human hearing won't tell much difference between 8 >>> watts and 12 watts, this is a bit of a sloppy spec and I'm >>> open to anything in the area of 5-20 watts... though I'm >>> really wanting to keep the rail voltages down to something >>> modest and the BJTs not having to tolerate hugish Vce. >>> >>> Now that I say this, an odd idea comes to mind because the >>> CFL light bulbs include two TO-220 BJTs that can handle quite >>> a high Vce on them. I could cannibalize those. But to be >>> honest, I'm still not needing high watt outputs. So there's >>> no reason to think about scavenging such parts. >>> >>> I would like to design it to work into 4 ohms as a margin >>> bound and not as a design goal, but even 5.6 ohms would be >>> acceptable. >>> >>> I'm not looking for this to be done quickly, either. If it >>> takes months of only occasional back-and-forth, I'm fine with >>> that. Also, I expect to do my work and don't expect someone >>> else to hand-hold me from complete ignorante to complete >>> enlightenment. :) I just need someone to slap my face when >>> I say something terribly stupid and/or point in a truely >>> useful direction when I need it. Or else someone who is >>> wanting to explore this with me and willing to work for it. >>> >>> Is anyone here willing to consider a sincere discussion? >>> >>> Jon >> Back when transistors were young, and transistor manuals (GE, RCA) >> were published, there were tons of such circuits around. They all >> pretty much converged to a few forms, and haven't changed much since. > > I remember reading in popular electronics about some audio > amps that I couldn't even come close to following at the > time. The series of them with the name 'tiger' in them. > > I'm not so much interested in _circuits_, per se, as I am in > learning about topologies, various ideas built upon them, and > then the specific details of designing towards a specific > implementation. For example, I enjoyed learning about > bootstrapping as a general idea _and_ as it applies to a > couple of specific areas. Having both theory _and_ specific > practice helps firm up the ideas better. > >> I could post some circuits from the old manuals, it that wouldn't >> spoil what you want to do. > > It may serve as a point of discussion. Would you be willing > to discuss their details and the broader theories as applied? > >> 10 watts into computer speakers sounds like a lot. Most AM radios >> didn't make one watt. You might experiment first to see how much power >> you really need. > > Oh, I figure one watt is enough, too. As a practical matter > and as a consumer using a device and not as a designer trying > to learn something. That's what... 3Vrms? Into 8 ohms? A > voltage gain of 3, given 1Vrms input? I'm wanting to learn > some things, not place one BJT (okay, not really, but it > almost seems like that) down as an emitter follower and then > calling it good. ;) > > Up front, I thought I'd like to deal with perhaps something > on the order of about 10Vrms into 8 ohms. I figured that is > enough 'bad' that I'd have to cope with some interesting > corners along the way; but not enough 'bad' that I'd have to > deal with too much all at once. > > For example, at around 10 watts or so, it's enough that I may > need to seriously consider avoiding class-A operation of the > output stage and move to class-B, instead. But it is low > enough that there is some room to discuss each, as well as > class-AB biasing, too. More power and I'm almost certain I'm > pushed into class-B. Less power and.. well, who cares that > much? At one watt or so, just class-A and be done with it? I > won't learn the reasoning behind trade-offs that way. > > There's more. I just figured at about 10 watts I'm likely to > learn some things but not be forced to learn so much that I'm > overwhelmed. > > I'm open to specific advice about all this, of course. > > Jon I like 10 watts as a starting size - at this size you have to start doing things the way the big amps do, but it is not so big as to be outrageously expensive, for example you still use a relatively small power supply, heatsinks, and inexpensive transistors, and at the end you can use it with your PC and really blow those 320 watt PFPO (peak fantasy power output)speakers away. I like your hesitation on class A. You want an amp with some power output and class A is very inefficient, never more than 25% and often way less. This would add greatly to the cost - a 40 watt power supply, heatsinks capable of getting rid of the same as heat while keeping the transistor junction temperature low, and beefier transistors. You also get to put up with a shorter service life from all that heat. The "big thing" with class A is there is no crossover distortion, which can lead to better overall distortion figures, but the cost is huge - a kit for 2 x 20 watt class A sells for $600. My particular bias for an amp this size is to go class AB with a split power supply. The majority of quality audio amps follow this topology and this is, I think, I great reason to go down this design path (what you learn is applicable in the most number of situations). I should hunt down a schematics of what I'm seeing in the distance (which can/will change as decisions are made) - some of the justifications will have to wait The first step is to think about the output. The basic equations are (1).....Vout = sqrt(2*P*R) With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts - actually +/- 12.7 volts with a split power supply. (2).....Imax = sqrt(2*P/R) This comes out to 1.6 amps. You should probably also consider the case when R speaker = 4 ohms when initially selecting a transistor for the output 2.2 amps - remember this is max output current. The power supply voltage will have to be somewhat higher than Vout to take into account circuit drive requirements, ripple on the power supply and transformer regulation etc. Are you OK with connecting mains to a transformer? or would you rather use an AC plug pack (10 watts is about the biggest amp a plugpack can be used for)? The "cost" for using an AC plug pack is you will need larger filter capacitors. I should also ask if you have a multi meter, oscilloscope (not necessary but useful)and how is your soldering? But it would be wise to keep this whole thing as a paper exercise before you commit to anything.
From: Jon Kirwan on 27 Jan 2010 04:26
On Wed, 27 Jan 2010 17:31:00 +1000, David Eather <eather(a)tpg.com.au> wrote: >Jon Kirwan wrote: >> On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan >>> <jonk(a)infinitefactors.org> wrote: >>> >>>> I'd like to take a crack at thinking through a design of an >>>> audio amplifier made up of discrete BJTs and other discrete >>>> parts as an educational process. >>>> >>>> I imagine this will be broken up into three sections; input >>>> transconductance, transimpedance VAS, and output driver. But >>>> other arrangements (such as combining the VAS and output >>>> driver using a signal splitting BJT) would work for me, in >>>> learning. >>>> >>>> I said "BJT" and "discrete" but I'm also open to the idea of >>>> using BJT pairs, such as the BCV61 and BCV62. In the case of >>>> current mirrors, that may make sense. But not high-priced, >>>> elite and/or hard to get, or obsolete. And no FETs. This is >>>> to be about learning to design with BJTs. >>>> >>>> SMT vs through-hole isn't an issue for learning about a >>>> design, I suppose. If I need to build up some section and >>>> test it with a signal, I'll probably want to do it quickly >>>> and without having to buy services every step of the way. So >>>> I may 'dead bug' SMT parts to get there. (The basic idea >>>> here is to learn, not to make something tiny.) >>>> >>>> Although I have some other applications, right now I'd like >>>> the target use to be as a computer speaker system (not unlike >>>> those dirt cheap, sadly almost all of them 10% THD, systems >>>> sold today into this market. Except that I'd like to work >>>> through the design on my own, from start to end. >>>> >>>> Given what I understand right now from a very short search on >>>> the topic, the input should be taken as a maximum of 1.0Vrms >>>> and the input's load should appear to be something like 10k >>>> ohms. If someone knows different from that, I'll accept the >>>> criticism and change that spec. >>>> >>>> I'd like to consider a tone control and a volume control to >>>> be included. >>>> >>>> Output is to be into a small 8 ohm speaker. With that >>>> maximum 1.0Vrms at the input and the volume control set to >>>> maximum the wattage into 8 ohms should be around 10 watts. >>>> Since human hearing won't tell much difference between 8 >>>> watts and 12 watts, this is a bit of a sloppy spec and I'm >>>> open to anything in the area of 5-20 watts... though I'm >>>> really wanting to keep the rail voltages down to something >>>> modest and the BJTs not having to tolerate hugish Vce. >>>> >>>> Now that I say this, an odd idea comes to mind because the >>>> CFL light bulbs include two TO-220 BJTs that can handle quite >>>> a high Vce on them. I could cannibalize those. But to be >>>> honest, I'm still not needing high watt outputs. So there's >>>> no reason to think about scavenging such parts. >>>> >>>> I would like to design it to work into 4 ohms as a margin >>>> bound and not as a design goal, but even 5.6 ohms would be >>>> acceptable. >>>> >>>> I'm not looking for this to be done quickly, either. If it >>>> takes months of only occasional back-and-forth, I'm fine with >>>> that. Also, I expect to do my work and don't expect someone >>>> else to hand-hold me from complete ignorante to complete >>>> enlightenment. :) I just need someone to slap my face when >>>> I say something terribly stupid and/or point in a truely >>>> useful direction when I need it. Or else someone who is >>>> wanting to explore this with me and willing to work for it. >>>> >>>> Is anyone here willing to consider a sincere discussion? >>>> >>>> Jon >>> Back when transistors were young, and transistor manuals (GE, RCA) >>> were published, there were tons of such circuits around. They all >>> pretty much converged to a few forms, and haven't changed much since. >> >> I remember reading in popular electronics about some audio >> amps that I couldn't even come close to following at the >> time. The series of them with the name 'tiger' in them. >> >> I'm not so much interested in _circuits_, per se, as I am in >> learning about topologies, various ideas built upon them, and >> then the specific details of designing towards a specific >> implementation. For example, I enjoyed learning about >> bootstrapping as a general idea _and_ as it applies to a >> couple of specific areas. Having both theory _and_ specific >> practice helps firm up the ideas better. >> >>> I could post some circuits from the old manuals, it that wouldn't >>> spoil what you want to do. >> >> It may serve as a point of discussion. Would you be willing >> to discuss their details and the broader theories as applied? >> >>> 10 watts into computer speakers sounds like a lot. Most AM radios >>> didn't make one watt. You might experiment first to see how much power >>> you really need. >> >> Oh, I figure one watt is enough, too. As a practical matter >> and as a consumer using a device and not as a designer trying >> to learn something. That's what... 3Vrms? Into 8 ohms? A >> voltage gain of 3, given 1Vrms input? I'm wanting to learn >> some things, not place one BJT (okay, not really, but it >> almost seems like that) down as an emitter follower and then >> calling it good. ;) >> >> Up front, I thought I'd like to deal with perhaps something >> on the order of about 10Vrms into 8 ohms. I figured that is >> enough 'bad' that I'd have to cope with some interesting >> corners along the way; but not enough 'bad' that I'd have to >> deal with too much all at once. >> >> For example, at around 10 watts or so, it's enough that I may >> need to seriously consider avoiding class-A operation of the >> output stage and move to class-B, instead. But it is low >> enough that there is some room to discuss each, as well as >> class-AB biasing, too. More power and I'm almost certain I'm >> pushed into class-B. Less power and.. well, who cares that >> much? At one watt or so, just class-A and be done with it? I >> won't learn the reasoning behind trade-offs that way. >> >> There's more. I just figured at about 10 watts I'm likely to >> learn some things but not be forced to learn so much that I'm >> overwhelmed. >> >> I'm open to specific advice about all this, of course. >> >> Jon > >I like 10 watts as a starting size - at this size you have to start >doing things the way the big amps do, but it is not so big as to be >outrageously expensive, for example you still use a relatively small >power supply, heatsinks, and inexpensive transistors, You appear to confirm my instincts. >and at the end you >can use it with your PC and really blow those 320 watt PFPO (peak >fantasy power output)speakers away. Well, mostly I'm just trying to learn... not impress others about the results. :) >I like your hesitation on class A. You want an amp with some power >output and class A is very inefficient, never more than 25% and often >way less. This would add greatly to the cost - a 40 watt power supply, >heatsinks capable of getting rid of the same as heat while keeping the >transistor junction temperature low, and beefier transistors. You also >get to put up with a shorter service life from all that heat. The "big >thing" with class A is there is no crossover distortion, which can lead >to better overall distortion figures, but the cost is huge - a kit for 2 >x 20 watt class A sells for $600. Egads. My instincts said class-A would add a lot to weight and cost, but no idea a mere 20W kit could sell for $300! >My particular bias for an amp this size is to go class AB with a split >power supply. The majority of quality audio amps follow this topology >and this is, I think, I great reason to go down this design path (what >you learn is applicable in the most number of situations). I should hunt >down a schematics of what I'm seeing in the distance (which can/will >change as decisions are made) - some of the justifications will have to >wait I'm fine with taking things as they come. As far as the class, I guessed that at 10 watts class-A would be too power-hungry and probably not worth its weight but that class-AB might be okay. I have to warn you, though, that I'm not focused upon some 20ppm THD. I'd like to learn, not design something whose distortion (or noise, for that matter) is around a bit on a 16-bit DAC or less. I figure winding up close to class-B operation in the end. But I'd like to take the walk along the way, so to speak. >The first step is to think about the output. The basic equations are > >(1).....Vout = sqrt(2*P*R) > >With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts - >actually +/- 12.7 volts with a split power supply. If you don't mind, I'd like to discuss this more closely. Not just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC. Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R) equation. Which suggests the +/-12.7V swing. Which further suggests, taking Vce drops and any small amounts emitter resistor drops into account, something along the lines of +/- 14-15V rails? Or should the rails be cut a lot closer to the edge here to improve efficiency. What bothers me is saturation as Vce on the final output BJTs goes well below 1V each and beta goes away, as well, rapidly soaking up remaining drive compliance. >(2).....Imax = sqrt(2*P/R) > >This comes out to 1.6 amps. You should probably also consider the case >when R speaker = 4 ohms when initially selecting a transistor for the >output 2.2 amps - remember this is max output current. The power supply >voltage will have to be somewhat higher than Vout to take into account >circuit drive requirements, ripple on the power supply and transformer >regulation etc. Okay. I missed reading this when writing the above. Rather than correct myself, I'll leave my thinking in place. So yes, the rails will need to be a bit higher. Agreed. On this subject, I'm curious about the need to _isolate_, just a little, the rails used by the input stage vs the output stage rails. I'm thinking an RC (or LC for another pole?) for isolation. But I honestly don't know if that's helpful, or not. >Are you OK with connecting mains to a transformer? or would you rather >use an AC plug pack (10 watts is about the biggest amp a plugpack can be >used for)? The "cost" for using an AC plug pack is you will need larger >filter capacitors. I'd much prefer to __avoid__ using someone else's "pack" for the supply. All discrete parts should be on the table, so to speak, in plain view. And I don't imagine _any_ conceptual difficulties for this portion of the design. I'm reasonably familiar with transformers, rectifiers, ripple calculations, and how to consider peak charging currents vs averge load currents as they relate to the phase angles available for charging the caps. So on this part, I may need less help than elsewhere. In other words, I'm somewhat comfortable here. >I should also ask if you have a multi meter, oscilloscope (not necessary >but useful)and how is your soldering? But it would be wise to keep this >whole thing as a paper exercise before you commit to anything. I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS handheld, two oscilloscopes (TEK 2245 with voltmeter option and an HP 54645D), three triple-output power supplies with two of them GPIB drivable, the usual not-too-expensive signal generator, and a fair bunch of other stuff on the shelves. Lots of probes, clips, and so on. For soldering, I'm limited to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and somewhat wider spade tips in the 1.5mm area. I have tubs and jars of various types of fluxes, as well, and wire wrap tools and wire wrap wire, as well. I also have a room set aside for this kind of stuff, when I get time to play. Jon |