Discussing audio amplifier design -- BJT, discrete
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From: Jon Kirwan on 29 Jan 2010 20:10 On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >George Herold wrote: >> On Jan 27, 9:51 pm, Jon Kirwan <j...(a)infinitefactors.org> >> wrote: >> >> "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.)" >> >> First Jon I know less about amplifier design than you do... >> That said, >> I would be careful about replacing the diodes in the push-pull >> stage. >> Way back in college I had a Sony stero amp that I had to fix. >> It came >> with a nice circuit diagram. I seem to recall that the bias >> diodes >> in the push pull stage were thermally attached to the same heat >> sink >> that held the output transistors. As the output transistors >> warm up >> their Vbe drop decreases. You want the bias diodes to track >> this >> change. Or else the whole thing could 'run-away' on you. ... >> degenerative emmiter resistors (as you suggest) will help some. > >I like the biasing scheme mentioned by Jon and use it for all my >designs except the early ones using germanium transistors, though >I don't know the name either. The biasing transistor can be >mounted on the output transistors' heatsink for temperature >tracking. > >I like it because it's versatile and a single transistor can be >used to bias several transistors with their b-e junctions in >series as long as they are mounted on a common heatsink. >http://img691.imageshack.us/img691/2075/bias.png Yeah. That's what I was thinking about. Just some thought here to add. As you carefully point out, tying it to the heat sink of the output BJTs to hold them near each others' temperatures seems important. Vbe varies a great deal over temperature. The voltage across this structure is: Vbe * (1 + R1/(R2+R3)) However, it's also true that the output BJTs are also experiencing similar (but _not_ the exact same as they aren't necessarily even from the same manufacturer or family) changes in Vbe. So it's actually a kind of "good thing" to have the voltage held between the output BJT bases vary as the output BJTs temperatures vary. Question is, is a random selection of a BJT for this purpose okay? Or does it need to be carefully considered, taken together with the output BJT characteristics? It seems to me that some care is needed here, even assuming good temperature coupling occurs. Also, I think I've seen some examples where there is a collector resistor added to this structure, with Q2's base kept tied directly to Q1's collector lead. What is the reasoning here? (I believe in the cases I saw, there was a current source [not a resistor] feeding at the top. I started to work the equations to show the relationship, but then realized that there is also base current drive to the upper side of the output transistors involved and then decided to just ask, instead of wandering all over the place right now.) >My personal preference is to place the bias adjustment pot R3 in >this position rather than with R1. It ensures that any accidental >loss of contact by the pot's wiper arm will reduce the total bias >whereas placing it with R1 will have the opposite effect and >could cause excessive quiescent current in the output >transistors, possibly getting them to overheat. Makes sense. Thanks, Jon
From: Paul E. Schoen on 29 Jan 2010 21:05 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:l6n6m5pd53835g9q745t2t4ifbavkghaua(a)4ax.com... > > A concern I care not the least about. My _real_ preference, > were I to impose it on the design, would be to use ONLY > PN2222A BJTs for all the active devices. One part. That's > it. Why? Because I've got thousands of them. ;) > > Literally. Something like 22,000 of the bastards. I give > them away like popcorn to students at schools. Got them > _very cheaply_. So if I were pushing something, I'd be > pushing a 10W PN2222A design, use signal splitting approach > probably (because it's the only way I think think of, right > now), and distribute the dissipation across lots and lots of > the things. I design a lot of things that way as well. I have (hundreds of) thousands of parts that I got about 20 years ago, and I really like to use them wherever possible. You can check my website where I have some of these parts listed as surplus sales and I'd really like to get rid of them where they might be used rather than hauling them to the dump. Take a look and if you can use anything I'll see if it's worthwhile to send them to you for little more than the cost of shipping (probably USPS flat rate). My website is www.pstech-inc.com, and just look for the link to surplus parts. If that doesn't work, try http://www.smart.net/~pstech/surplus.htm and http://www.smart.net/~pstech/PARTS.txt and http://www.smart.net/~pstech/PARTS.xls. I have a lot of MPSA06 NPN transistors, so I use them wherever possible. I also have a few thousand MJE170 PNP Power transistors (40V, 3A, 12W). And lots of 2N6312 in TO-66 metal cans (PNP 40V 5A 75W) and about 600 Thermalloy 6060 heat sinks that can be used for them, as well as other case types. If you like SCRs I have about 600 of 2N6504 which is 35A at 50 V. If you need a transformer for a power supply I have a couple hundred Signal 241-6-16 which can be used to make a raw +/- 8-10 VDC supply and with a few more capacitors and diodes makes a nice +/- 20 VDC supply at about 1 amp. And I have an armload of capacitors such as 500 uF 50V, 1500 uF 50V, and even some 4500 uF 50V in big blue metal cans. And a few handsful of 1N4003 and 1N4004 rectifiers. If you could come to my place near Baltimore, MD I could give you a "shopping spree" where you could fill a few bags and boxes with all sorts of goodies. Lately I am realizing that almost any new design I do will be with SMT components and newer parts, and there are only a few one-off projects that I might make using these older components. Some of them have been stores so long in a damp, unheated building that the leads are difficult to solder, and some resistors have actually soaked up enough moisture to change value. (That is what a friend told me, and he also said they were restored to normal by baking them for a while). I've sent "care packages" to others in the past. I don't expect to make any money selling/giving away these parts but I just want to be compensated for shipping cost. I don't know if I have some of these parts and if I do I might not even be able to find them, but I think I can supply enough parts for you to build a good amplifier and other projects. Paul
From: Paul E. Schoen on 29 Jan 2010 21:14 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:iou6m5haf0vm46v085k50slhsi6mtgsgs6(a)4ax.com... > On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom" > <pimpom(a)invalid.invalid> wrote: > > >>My personal preference is to place the bias adjustment pot R3 in >>this position rather than with R1. It ensures that any accidental >>loss of contact by the pot's wiper arm will reduce the total bias >>whereas placing it with R1 will have the opposite effect and >>could cause excessive quiescent current in the output >>transistors, possibly getting them to overheat. > > Makes sense. As a quick precaution against thermal runaway, a thermistor from base to emitter, and thermally tied to the case, should shut off base drive if things get too hot. Or just put a thermal switch on the heat sinks and use it to shut off the supply to the whole shebang. Paul
From: Jon Kirwan on 29 Jan 2010 21:40 On Fri, 29 Jan 2010 21:05:02 -0500, "Paul E. Schoen" <paul(a)peschoen.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:l6n6m5pd53835g9q745t2t4ifbavkghaua(a)4ax.com... >> >> A concern I care not the least about. My _real_ preference, >> were I to impose it on the design, would be to use ONLY >> PN2222A BJTs for all the active devices. One part. That's >> it. Why? Because I've got thousands of them. ;) >> >> Literally. Something like 22,000 of the bastards. I give >> them away like popcorn to students at schools. Got them >> _very cheaply_. So if I were pushing something, I'd be >> pushing a 10W PN2222A design, use signal splitting approach >> probably (because it's the only way I think think of, right >> now), and distribute the dissipation across lots and lots of >> the things. > ><snip> > >If you could come to my place near Baltimore, MD I could give you a >"shopping spree" where you could fill a few bags and boxes with all sorts >of goodies. Lately I am realizing that almost any new design I do will be >with SMT components and newer parts, and there are only a few one-off >projects that I might make using these older components. Some of them have >been stores so long in a damp, unheated building that the leads are >difficult to solder, and some resistors have actually soaked up enough >moisture to change value. (That is what a friend told me, and he also said >they were restored to normal by baking them for a while). > >I've sent "care packages" to others in the past. I don't expect to make any >money selling/giving away these parts but I just want to be compensated for >shipping cost. I don't know if I have some of these parts and if I do I >might not even be able to find them, but I think I can supply enough parts >for you to build a good amplifier and other projects. > >Paul Paul, I'll write under separate cover, directly. There are some thoughts I'd like to explore more, if that's okay. I can also provide a 501(c)3 for tax purposes, as well as cash compensation. That may also help a little. But we can talk about that off-line. Jon
From: John Larkin on 29 Jan 2010 23:01
On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: > >"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.)" "Vbe multiplier." The classic output stage biasing scheme uses small emitter resistors and biases the output transistors to idle current using a couple of junction drops between the bases, or a Vbe multiplier with a pot. Both are good ways to have a poorly defined idle current and maybe fry transistors. Two alternates are: 1. Use zero bias. Connect the complementary output transistors base-to-base, emitter-to-emitter. Add a resistor from their bases to their emitters, namely the output. At low levels, the driver stage drives the load through this resistor. At high levels, the output transistors turn on and take over. 2. Do the clasic diode or Vbe multiplier bias, but use big emitter resistors. Parallel the emitter resistors with diodes. In both cses, the thing will be absolutely free frfom thermal runaway issues and won't need adjustments. Bothe need negative feeback to kill crossover distortion. Or... 3. Use mosfets John |