Discussing audio amplifier design -- BJT, discrete
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From: David Eather on 27 Jan 2010 20:17 Jon Kirwan wrote: > 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! Well it would cost something less at 10 watts. > >> 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. > 10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and a few detours to see what would help or harm that. >> 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. Mostly not needed, if you use a long tailed pair for the input / error amplifier, but you might prefer some other arrangement so keep it in mind if your circuit "motorboats" > >> 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. Ah, then there are questions of what voltage and VA for a transformer. So there are questions of usage (music, PA, PA with an emergency alert siren tied in etc) and rectifier arrangement and capacitor size / voltage to get your required voltage output at full load. > >> 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. OK. Next serious project, I'm coming around to your place! Your gear is better than mine. I had to ask, rather than just assume just in case my assumptions got you building something you didn't want to, and got you splattered all over the place from the mains, or suggesting you choose the miller cap by watching the phase shift of the feedback circuit - I don't read a lot of the posts so I didn't know what you could do. > > Jon Have a look at http://en.wikipedia.org/wiki/Electronic_amplifier The bits on class A might be interesting as it says 25% efficiency and 50% obtainable with inductive output coupling (i.e. with a transformer) which is what I said, not what blow hard Phil said.
From: Jon Kirwan on 27 Jan 2010 21:51 On Thu, 28 Jan 2010 11:17:02 +1000, David Eather <eather(a)tpg.com.au> wrote: >Jon Kirwan wrote: >> On Wed, 27 Jan 2010 17:31:00 +1000, David Eather >> <eather(a)tpg.com.au> wrote: >>> <snip> >> >>> 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. > >10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and >a few detours to see what would help or harm that. Hehe. I'm thinking of some numbers I saw in the area of ..002% THD. I hate percentages and immediately convert them. In this case, it is 20e-6 or 20 ppm. Which is darned close to a bit on a 16-bit dac. That's why I wrote that way. I just don't like using % figures. They annoy me just a tiny bit. Regarding .1% to .05%, I'm _very_ good with that. Of course, I'm going to have to learn about how to estimate it from theory as well as measure it both via simulation before construction and from actual testing afterwards. More stuff I might _think_ I have a feel for, but I'm sure I will discover I don't as I get more into it. But speaking from ignorance, I'm good shooting for the range you mentioned. It was about what I had in mind, in fact, figuring I could always learn as I go. >>> 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. > >Mostly not needed, if you use a long tailed pair for the input / error >amplifier, but you might prefer some other arrangement so keep it in >mind if your circuit "motorboats" Okay. I've _zero_ experience for audio. It just crossed my mind from other cases. I isolate the analog supply from the digital -- sometimes with as many as four caps and three inductor beads. There, it _does_ help. >>> 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. > >Ah, then there are questions of what voltage and VA for a transformer. >So there are questions of usage (music, PA, PA with an emergency alert >siren tied in etc) and rectifier arrangement and capacitor size / >voltage to get your required voltage output at full load. I figure on working out the design of the amplifier and then going back, once that is determined and hashed out, with the actual required figures for the power supply and design that part as the near-end of the process. Earlier on, I'd expect to have some rough idea of how "bad" it needs to be -- if the initial guesses don't raise alarms, then I wouldn't dig into the power supply design until later on. The amplifier, it seems to me, dictates the parameters. So that comes later, doesn't it? >>> 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. > >OK. Next serious project, I'm coming around to your place! You come to the west coast of the US and I'll have a room for you! >Your gear is >better than mine. I had to ask, rather than just assume just in case my >assumptions got you building something you didn't want to, and got you >splattered all over the place from the mains, or suggesting you choose >the miller cap by watching the phase shift of the feedback circuit - I >don't read a lot of the posts so I didn't know what you could do. To be honest, I can do a few things but I'm really not very practiced. My oscilloscope knowledge is lacking in some areas -- which becomes all too painfully obvious to me when I watch a pro using my equipment. And I'm still learning to solder better. It's one of a few hobbies. >> Jon > >Have a look at >http://en.wikipedia.org/wiki/Electronic_amplifier Done. >The bits on class A might be interesting as it says 25% efficiency and >50% obtainable with inductive output coupling (i.e. with a transformer) >which is what I said, not what blow hard Phil said. What I first see there is the amplifier sketch at the top of the page (I don't really care too much about arguing about efficiencies right now -- I'm more concerned about learning.) The input stage shown is a voltage-in, current-out bog standard diff-pair. First thing I remember about is that R4 shouldn't be there and better still both R3 and R4 should be replaced with a current mirror. R5 should be a replaced with a BJT, as well. I assume the input impedance of that example is basically the parallel resistance of R1 and R2, but if we use split supplies I'd imagine replacing the two of them with a single resistor to the center-ground point. There's no miller cap on Q3, I'd probably replace the two diodes with one of those BJT and a few resistor constructions I can't remember the name of (which allows me to adjust the drop.) The feedback ... well, I need to think about that a little more. There's no degen resistors in the emitters of Q4 and Q5. Um.. okay, I need to sit down and think. Mind is spinning, but I've not set a finger to paper yet and there is lots to think about in that one. I could be way, way off base. Jon
From: Jon Kirwan on 27 Jan 2010 21:55 On Wed, 27 Jan 2010 18:51:00 -0800, I wrote: >The amplifier, it >seems to me, dictates the parameters. So that comes later, >doesn't it? By "that" I mean "the power supply." In case it isn't clear. Jon
From: pimpom on 28 Jan 2010 03:30 Jon Kirwan wrote: > On Thu, 28 Jan 2010 11:17:02 +1000, David Eather > <eather(a)tpg.com.au> wrote: > >> >> Have a look at >> http://en.wikipedia.org/wiki/Electronic_amplifier > > Done. > >> The bits on class A might be interesting as it says 25% >> efficiency >> and 50% obtainable with inductive output coupling (i.e. with a >> transformer) which is what I said, not what blow hard Phil >> said. > > What I first see there is the amplifier sketch at the top of > the page (I don't really care too much about arguing about > efficiencies right now -- I'm more concerned about learning.) > The input stage shown is a voltage-in, current-out bog > standard diff-pair. First thing I remember about is that R4 > shouldn't be there and better still both R3 and R4 should be > replaced with a current mirror. R5 should be a replaced with > a BJT, as well. I assume the input impedance of that example > is basically the parallel resistance of R1 and R2, but if we > use split supplies I'd imagine replacing the two of them with > a single resistor to the center-ground point. There's no > miller cap on Q3, I'd probably replace the two diodes with > one of those BJT and a few resistor constructions I can't > remember the name of (which allows me to adjust the drop.) > The feedback ... well, I need to think about that a little > more. There's no degen resistors in the emitters of Q4 and > Q5. > Either the circuit was designed by someone with a limited experience or it was deliberately presented this way for clarity as an illustration of the basic topology. In a practical design using an unregulated power supply, R1 should be split into two with a capacitor from the split point to ground. This is to decouple the input stage for stability as well as for hum filtering. R6 should also be split and the split point bootstrapped with a capacitor to the mid point of the output stage. Talking about RCA's 70W amp got me nostalgic about those days. Here's a 1W amp using _germanium_ transistors: http://img716.imageshack.us/img716/2583/1wamp.png This is one of my early solid-state designs based, of course, on topologies I'd learned by studying others' designs. It's no hi-fi by any stretch of imagination, but I actually constructed a few of these in the early 70s for myself and for friends. One of them fed the input from an early Sony Walkman to drive an 8-inch Philips dual-cone "Hi-Q" speaker and gushed over how good it sounded!
From: Jon Kirwan on 28 Jan 2010 06:04
On Thu, 28 Jan 2010 14:00:18 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Thu, 28 Jan 2010 11:17:02 +1000, David Eather >> <eather(a)tpg.com.au> wrote: >> >>> >>> Have a look at >>> http://en.wikipedia.org/wiki/Electronic_amplifier >> >> Done. >> >>> The bits on class A might be interesting as it says 25% >>> efficiency >>> and 50% obtainable with inductive output coupling (i.e. with a >>> transformer) which is what I said, not what blow hard Phil >>> said. >> >> What I first see there is the amplifier sketch at the top of >> the page (I don't really care too much about arguing about >> efficiencies right now -- I'm more concerned about learning.) >> The input stage shown is a voltage-in, current-out bog >> standard diff-pair. First thing I remember about is that R4 >> shouldn't be there and better still both R3 and R4 should be >> replaced with a current mirror. R5 should be a replaced with >> a BJT, as well. I assume the input impedance of that example >> is basically the parallel resistance of R1 and R2, but if we >> use split supplies I'd imagine replacing the two of them with >> a single resistor to the center-ground point. There's no >> miller cap on Q3, I'd probably replace the two diodes with >> one of those BJT and a few resistor constructions I can't >> remember the name of (which allows me to adjust the drop.) >> The feedback ... well, I need to think about that a little >> more. There's no degen resistors in the emitters of Q4 and >> Q5. > >Either the circuit was designed by someone with a limited >experience or it was deliberately presented this way for clarity >as an illustration of the basic topology. Well, having looked a little more at the web site, I see them talking about everything from opamps to servo amps so maybe they just did the basics. But they missed the signal split technique aka the old tube days, then. >In a practical design using an unregulated power supply, R1 >should be split into two with a capacitor from the split point to >ground. This is to decouple the input stage for stability as well >as for hum filtering. Now _that_ makes a lot of sense. I missed it. >R6 should also be split and the split point >bootstrapped with a capacitor to the mid point of the output >stage. That one I really need to think about. This is what I wanted to happen here. Throwing out things (I'm assuming correct things, of course) that force me to consider and think. Thanks. >Talking about RCA's 70W amp got me nostalgic about those days. >Here's a 1W amp using _germanium_ transistors: >http://img716.imageshack.us/img716/2583/1wamp.png >This is one of my early solid-state designs based, of course, on >topologies I'd learned by studying others' designs. It's no hi-fi >by any stretch of imagination, but I actually constructed a few >of these in the early 70s for myself and for friends. One of them >fed the input from an early Sony Walkman to drive an 8-inch >Philips dual-cone "Hi-Q" speaker and gushed over how good it >sounded! Okay. So lets talk about some aspects. It'll expose my terrible ignorance, but what the heck. Input loading. I think I can ignore the R2 feedback as it is 10k. At least, for now. C1 will present about Z=800 at 20Hz, Z=160 at 100Hz, and Z goes down from there. R1 is 1k, obviously in series with C1. Then there is R3=1k in parallel with Q1's impedance, which maybe I can approximate as R4 times beta, or call it 50*33 or about 1500 ohms? So about 600 ohms counting that and R3 in parallel, that itself in series with 1k and whatever C1 presents? So call it around 2k ohms loading, or so? (Which adds to the idea that the R2 feedback can be mostly ignored as a load.) Would that be an okay, off-the-hip guess? Or how would you go about it? D1 is, I guess, silicon and given that you said _germanium_, I'll take that to suggest that the Vbe on those are about half that of a silicon BJT. Which is why only one DR25 was needed there. DC bias point of Q1... hmm. Well, assuming no signal, SPK1 is roughly a dead short, so R5 is tied one side to a rail. The other side moves Q2's base and Q2's emitter follows. As Q2's emitter rises with it, R2 and R3 act to split that as 1/11th to the Q1 base. Q1's emitter follows up for a ways, allowing DC current via R4 which must go through R5, dropping Q2's base and thus Q2's emitter, lowering Q1's base voltage in opposition. So there will be a middle point found. Assuming Q1's Vbe should be something on the order of 300mV (random guess), and I(R4) roughly equals I(R5), let's establish where Q1's base will wind up. Call it Vb. The value at Q2's emitter (which is also the other side of R2 from the Q1 base) will be 11 times higher because R2 and R3 split things that way. And Q2's base will be 300mV (same random guess, again) higher than that. The difference between there and the 9V battery voltage sets the current in R5 and, by implication, in R4 as well. Of course, Q1's emitter is 300mV away from that Vb value we are fussing over. The equation looks like: I(R5) = (9V - Vb*11 - 300mV) / 560 I(R4) = (Vb - 300mV) / 33 I(R4) = I(R5) So, (9V - Vb*11 - 300mV) / 560 = (Vb - 300mV) / 33 33/560 * (9V - Vb*11 - 300mV) = (Vb - 300mV) Vb = 33/560 * (9V - Vb*11 - 300mV) + 300mV Vb = 33/560*9V - 33/560*Vb*11 - 33/560*300mV + 300mV Vb + 33/560*Vb*11 = 33/560*9V - 33/560*300mV + 300mV Vb * (1 + 33/560*11) = (33/560*9V - 33/560*300mV + 300mV) Vb = (33/560*9V - 33/560*300mV + 300mV) / (1 + 33/560*11) or, Vb = 493mV and thus the current routing through R5, D1, Q1, and R4 is about 193mV/33 or 5.85mA. That's not the total quiescent current because D1 uses that 5.85mA to develop a voltage across it that is probably on the order of 700mV. With that between the Q2 and Q3 bases, both Q2 and Q3 are passing collector currents, rail to rail. Hard to know how much without data sheets, I suppose. But something. Their shared emitter node would be on the order of 11*490mV or about 5.4V. That neglected the base current for Q1 flowing via R2. As I'm now guessing almost 6mA as Ic, and since we are talking germanium here, I will pick a beta of about 60 and figure about 100uA base current, then. That's about another 1V across R2, less than that a little because that lowers Vb a bit which lowers the 5.85mA figure a bit, which probably then gets things very darned close to the midpoint of 4.5V one might wish there. Not too bad given I have no idea about the BJTs and am using a lot of random guesses as I go. R2 is not only a DC divider but also NFB, I think. Can you talk a little about how you figure on calculating both the NFB you want _and_ the DC biasing of this thing, both of which affect R2's value, I think? And although I've _seen_ miller feedbacks in the small nF range, could you talk a little about how that was set at 2.2nF? Also, I think I _almost_ get the idea of hooking one side of R5 to SPK1 instead of to the (-) side of 9V... but not quite sure. Can you talk about that choice, as well? Have at me. I probably got a lot wrong in the above, but that's my thinking exposed like a soft worm to be crushed. If I learn in the process, crush away! Jon |