Discussing audio amplifier design -- BJT, discrete
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From: Bob Masta on 27 Jan 2010 08:23 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. > <snip> When I was first getting interested in power amp design (back in the '70s) I started collecting schematics for all the power amps I could get my hands on, to compare them. I noticed that the schematics for simple bipolar op-amp ICs were remarkably similar to those for big discrete power amps. If you have an old National Linear Databook (or don't mind a lot of rooting around on the Web for individual datasheets), you might take a look. You can build a pretty decent amp with only a handful of transistors. The same basic circuit can be used for a wide range of output powers, just by changing the power supply voltages and the output device ratings. Best regards, Bob Masta DAQARTA v5.00 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!
From: pimpom on 27 Jan 2010 13:24 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. >>> .......<snip>......... >> >> 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. > That was probably an adaptation of RCA's 70-watt power amp, published in their transistor manual of the mid-60s. A true 70-watt continuous output was pretty hefty then. Still is, in fact, for many applications. The design was state-of-the-art, using their 4000 series transistors which were specifically made for audio. THD was <0.25% at 70W (pretty good for the time). IIRC, Popular Electronics published it virtually unchanged (I don't think they credited RCA with the design). I vaguely recall their calling it 'tiger' something and the article title included the word "indestructible". The latter term was because it included current clamping for the output transistors as short-circuit protection, a thermal fuse and a normal fuse. PE claimed that they abused it with short-circuits and reactive loads, and the worst they got was a blown fuse. I remember that design fondly, if not perfectly, because I spent a lot of time analysing it and others in the RCA manual.
From: Jon Kirwan on 27 Jan 2010 14:19 On Wed, 27 Jan 2010 13:23:28 GMT, Bob Masta 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. >> ><snip> > >When I was first getting interested in power amp >design (back in the '70s) I started collecting >schematics for all the power amps I could get my >hands on, to compare them. I noticed that the >schematics for simple bipolar op-amp ICs were >remarkably similar to those for big discrete power >amps. If you have an old National Linear Databook >(or don't mind a lot of rooting around on the Web >for individual datasheets), you might take a look. > >You can build a pretty decent amp with only a >handful of transistors. The same basic circuit >can be used for a wide range of output powers, >just by changing the power supply voltages and the >output device ratings. > >Best regards, Thanks, Bob. Audio amplifiers, especially ones delivering _some_ power, seem to offer such an excellent way to learn. The basic idea, at a behavioral level, is fairly simple. An implementation requires some knowledge and thought in the end. So the destination is arrived at by taking a great path to walk, with such wonderful vistas to see, I think. Much of interest is along the way of getting there. I may have an old National databook on linear parts somewhere. I keep a lot, but I also have several thousand books in my library which covers all of the walls in one of the rooms. I'm at a point now where to get room for more books, others must be boxed and stored or simply destroyed and pulped. So it's a _maybe_. One of the nice things (to me) about this kind of a path, too, is that what I learn can be used for lots of things. An audio amplifier is, in effect, not that much different from an op amp. There is the usual basic idea of open loop gain and closed loop gain with negative feedback, phase margins, problems to solve over a frequency range spanning many decades, and so on. A completely separate project I'd like to play with, which this learning will help prepare me for, is designing a pin driver. I'd like to sink or source a programmable current spanning decades from perhaps 100nA to perhaps 100uA while reading the voltage at the node, as well as being able to program a low impedance voltages spanning from -15V to +15V there and read the current, or read a voltage at the same node while presenting a fairly high impedence to it. I imagine what I learn here will aid me there. And I'd like to do this at some speed, as well. I may then start with a BJT tester, for example, making up only three of these to start and tying them into a micro for playing. Expanding that for other purposes, later. It would be fun. Jon
From: Michael A. Terrell on 27 Jan 2010 14:25 pimpom 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. > >>> > ......<snip>......... > > >> > >> 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. > > > > That was probably an adaptation of RCA's 70-watt power amp, > published in their transistor manual of the mid-60s. A true > 70-watt continuous output was pretty hefty then. Still is, in > fact, for many applications. The design was state-of-the-art, > using their 4000 series transistors which were specifically made > for audio. THD was <0.25% at 70W (pretty good for the time). That was the RCA 404xx series of their house numbered transistors. I think they used: 1 40406 1 40407 1 40408 1 40409 1 40410 2 40411 and a couple 1N series metal cased diodes for temperature sensing. It's been 40 years since I built that PE Tiger amp and the preamp that went with it. > IIRC, Popular Electronics published it virtually unchanged (I > don't think they credited RCA with the design). I vaguely recall > their calling it 'tiger' something and the article title included > the word "indestructible". The latter term was because it > included current clamping for the output transistors as > short-circuit protection, a thermal fuse and a normal fuse. PE > claimed that they abused it with short-circuits and reactive > loads, and the worst they got was a blown fuse. > > I remember that design fondly, if not perfectly, because I spent > a lot of time analysing it and others in the RCA manual. -- Greed is the root of all eBay.
From: Jon Kirwan on 27 Jan 2010 14:28
On Wed, 27 Jan 2010 23:54:15 +0530, "pimpom" <pimpom(a)invalid.invalid> 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. >>>> >......<snip>......... > >>> >>> 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. > >That was probably an adaptation of RCA's 70-watt power amp, >published in their transistor manual of the mid-60s. A true >70-watt continuous output was pretty hefty then. Still is, in >fact, for many applications. The design was state-of-the-art, >using their 4000 series transistors which were specifically made >for audio. THD was <0.25% at 70W (pretty good for the time). > >IIRC, Popular Electronics published it virtually unchanged (I >don't think they credited RCA with the design). I vaguely recall >their calling it 'tiger' something and the article title included >the word "indestructible". The latter term was because it >included current clamping for the output transistors as >short-circuit protection, a thermal fuse and a normal fuse. PE >claimed that they abused it with short-circuits and reactive >loads, and the worst they got was a blown fuse. > >I remember that design fondly, if not perfectly, because I spent >a lot of time analysing it and others in the RCA manual. Thanks for that bit. I remember looking at the first article and wondering about trying my hand at building it. There was no way I was prepared to understand it, though. The largest problem I faced at the time, besides my own limitations in education, was funds. I couldn't afford to even buy the boards they offered, let alone the parts. So it was a non- starter for me. I got my parts by scavenging TV sets and radios others threw away. One of my larger hauls was when a tornado knocked down a bowling alley and I called up the owner and received permission to walk through the mess and extract parts. I _never_ paid for anything. (Dad had died when I was 7 and I literally had to work the fields picking vegetables to earn enough to survive.) Now, I might go back. But to be honest, I'd much prefer being able to ask questions as they arise and work on refining as I go. I learn more from a "movie in progress" than studying a "snapshot," I guess. Jon |