Discussing audio amplifier design -- BJT, discrete
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From: Bob Masta on 8 Feb 2010 08:20 On Mon, 8 Feb 2010 02:53:51 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >In my late teens almost 40 years ago, during the only time I ever >worked under someone else, my boss in a research lab once asked >me why I kept doing calculations in my head when I could use a >calculator. I said that it helps keep my brain sharp and also >lets me visualise the outcome even before I arrive at the final >figures. I do also use pen and paper, and a calculator for >complex sequences. But it's only very recently that I've started >using a calculator (mostly Windows' scientific calculator) for >routine work. > >That said, I think it's also a good idea not to rely too much on >mental imaging. We do sometimes make mistakes, and what we >visualise may not at all be what actually happens. > >In a way, I kind of miss the old days when the average EE and >tech didn't have access to computer simulation. Back then, when I >designed a tube amp for instance, I drew a load line on the tube >characteristics curves and marked the outputs for as many grid >voltages as possible. Then I plotted the input-output curve on a >separate graph paper, chose an operating point and marked the >appropriate points for a Fourier analysis of harmonics expected >without feedback. One thing we had "back in the day" was slide rules. For any whippersnappers out there who've never used one, you only got answers to 2 or 3 decimal places, and you had to keep track of the exponent yourself... you'd read 1.23 and have to know whether that was 123000 or 0.000123 or whatever. That "shortcoming" of slide rules meant you had to have a feel for the answer you were expecting, with the slide rule just giving you more precision on your guesstimate. I think that forced you to stop and think a bit instead of blindly plugging in numbers, and it meant that you never put blind faith in your result, so you looked for other validation, considered the problem from multiple angles, etc. Not a bad thing. 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: Jon Kirwan on 8 Feb 2010 14:54 On Tue, 2 Feb 2010 22:30:31 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Sat, 30 Jan 2010 01:11:28 +0530, "pimpom" wrote: >> >>> <snip> >> I've seen this as a modification. In ASCII form: >> >>>> A >>>> | >>>> ,---+---, >>>> | | >>>> | \ >>>> | / R3 >>>> \ \ >>>> / R2 / >>>> \ | >>>> / +--- C >>>> | | >>>> | | >>>> | |/c Q1 >>>> +-----| >>>> | |>e >>>> \ | >>>> / R1 | >>>> \ | >>>> / | >>>> | | >>>> '---+---' >>>> | >>>> B >> >> We've already decided that R1 might be both a simple resistor >> plus a variable pot to allow adjustment. The usual case I >> see on the web does NOT include R3, though. However, I've >> seen a few examples where R3 (small-valued) exists and one of >> the two output BJTs' base is connected at C and not at A. >> >> The above circuit is a somewhat different version of the Vbe >> multiplier/rubber diode thing. The difference being R3, >> which I'm still grappling with. > >I've seen R3 used in that position too, but never gave it much >thought until you brought it up. Offhand I still can't see a >reason for it either. Maybe for stability against a local >oscillation? Perhaps taking some time to think about it will >bring some revelation. Or someone else can save us the trouble >and enlighten us. ><snip> I had earlier said I thought you might be right about this R3 value. Now, I don't. I think it deals with something else -- unregulated rail voltage variations. In this thread, you've posted circuits with a resistor on one side and the VAS on the other of this structure. The VAS yanks one end around while the other side mostly follows it around. However, the current through the resistor varies, of course. Even if one of those BJTs+2 diodes thingies is used for a current source instead of the resistor, which does improve things, it still isn't very constant. Using the 2 BJT structure doesn't change that fact, though it does impact variations. No matter how you arrange it, resistor or current source, the fact is that the current into the Vbe multiplier device changes around as the VAS yanks around one side of it. This variation means that Q1's Ic varies. To accomodate that variation, Vbe varies. Since Vbe varies, so does the multiplied value. And for no _other_ reason than variations in signal. That changes the bias. Changing the bias changes the quiescent current. Etc. (Also, I suppose, the Early effect will add yet another slight modification, since the Vce is slightly changed so is the Ic for the same Vbe. The higher required Ic (assuming the current source or resistor is supplying more current, instead of less) requires a higher Vbe, as stated. So the multiplied voltage at Vce is higher. But that multipled voltage also slides over on the Vce axis for whatever Vbe that has become and that suggests still more Ic due to Early effect, so it is a positive feedback contributing to the already existing problem, I think. I haven't tried to work out just what percent it contributes, though.) So a cludge fix for this is to insert a resistor in the collector, which will act in the opposite direction to some degreee. I'd imagine this would create a second degree poly curve, with a maximum somewhere but gentle 'arms' outward, which means less variation of the Vbe-multiplied value with a tweakable peak point based upon a nominal Ic. I imagine this is NOT nearly as important for class-A operation, though, since it is already "biased up" and variation at that point of operation probably isn't so important. It _would_ matter, I think, in other classes of operation. Thinking as I am that I don't want to go with class-A, I am trying to think of still better ways of replacing R3 with (or adding) an active device to further improve it. Anyone have a suggestion there? Jon
From: Jon Kirwan on 8 Feb 2010 14:55 On Mon, 08 Feb 2010 11:54:42 -0800, I wrote: >I think it deals with something else >-- unregulated rail voltage variations. __and__ variations due to the VAS driving the output stage. Jon
From: John Larkin on 8 Feb 2010 17:04 On Sun, 7 Feb 2010 16:10:14 +0530, "pimpom" <pimpom(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Sat, 06 Feb 2010 10:48:16 -0800, John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>> A few percent distortion at power levels is essentially >>> inaudible. >>> Speakers do that already. Low-level crossover distortion is >>> obvious >>> and obnoxious. >> >> Yes, I think that's now much clearer to me now than it was >> say two weeks ago -- without needing my ears to say so. Just >> on understanding better _what_ crossover distortion is and >> does. >> >Jon, to see a graphical illustration of JL's point, see this >screenshot of some simple simulations I just ran: >http://img694.imageshack.us/img694/8967/crossoverdistortion.png > >On the right, the complementary output stage is driven without >any bias, . The upper trace shows the output when the input >amplitude is +/-1V peak. The transistors are operating in Class C >and manage to conduct for less than half of each half cycle. Now >that's going to sound awful by any standard. I _know_ it sounds >awful because, when I was doing a lot of repairing work on >consumer products in the 80s, I came across some amps whose >biasing circuit had developed a fault. > >Do a Fourier analysis and you'll get lots of harmonics. Reduce >the input amplitude even further and there won't be any output at >all below a certain amplitude. > >The lower trace shows the output with +/-9V input. Crossover >distortion is much reduced, though still evident. This may or may >not be acceptable depending on the application. For anything that >needs good audio quality, any waveform distortion that can be >clearly seen in graphical form is still too high, especially in a >low-resolution bitmap trace like this. > >On the left, we have the same amp with diode biasing added. >Visible distortion of the waveshape has disappeared. The slight >irregularities in the sinusoidal shape are due to limitations of >the low-res, non-antialiased bitmap image. > In your "unbiased" circuit, try adding a 1K resistor from the bases to the output. Now the flat spots in the output waveform become slopes. But the transfer function is now continuous, so negative feedback can reduce distortion without ugly slewing problems. And with zero bias, there's no idle power dissipation and no possibility of thermal runaway. The second circuit, with the bias diodes, is a likely firebomb. John
From: John Larkin on 8 Feb 2010 17:08
On Sun, 07 Feb 2010 13:29:07 GMT, N0Spam(a)daqarta.com (Bob Masta) wrote: >On Fri, 05 Feb 2010 11:32:16 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > ><snip> > >>Interesting to me that you say that crossover distortion >>might not be such an issue for a musical instrument >>amplifier, though. I take it you must mean for stage work >>where the power is going to be set pretty high, generally? >> > >Yes. There might be a *very* quite venue >somewhere, where (say) the final note of a song >might trail away into audible crossover >distortion, but I'm skeptical. > >However, note that the previously-mentioned >quantization distortion was first discovered as a >problem on early CDs where a final piano note >decayed into silence. When it got very soft, it >also got very "chunky" since it only used a few >active bits, and it sounded gritty just before it >became inaudible. The fix was to add dither >(noise) when recording, which effectively gave >PWM that miraculously eliminated the distortion at >the cost of a small increase in noise. > >(You can get a fairly dramatic demo of this using >Daqarta. See <www.daqarta.com/dw_0gbb.htm>. This >is done using just the Generator, so it is totally >free.) > >Alas, there is nothing like dither to "fix" >crossover distortion! Mag tapes had inherently gross crossover distortion. The fix was to add a pretty high-level "bias" oscillator to the record path to smear it out. The bias voltage might be 20 volts at 60 KHz, way bigger than the record signal going into the head. John |