Discussing audio amplifier design -- BJT, discrete
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From: John Larkin on 6 Feb 2010 13:48 On Fri, 05 Feb 2010 11:32:16 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Fri, 05 Feb 2010 12:36:02 GMT, N0Spam(a)daqarta.com (Bob >Masta) wrote: > >>On Thu, 04 Feb 2010 12:07:16 -0800, Jon Kirwan >><jonk(a)infinitefactors.org> wrote: >> >>>There's another question that comes to mind regarding the >>>output stage. A lot of talk seems to revolve around >>>"crossover distortion." Seems almost very first thing folks >>>talk about when discussing class of operation if not also at >>>other times. >>> >>>Seems to me that in a three-rail power supply situation >>>without an output capacitor involved, the crossover takes >>>place near the midpoint (ground) voltage between the rails, >>>at a time when current into the speaker load is also near >>>zero. (I'm neglecting any thoughts about inductance in the >>>speaker and physical coupling into the air, for now.) In >>>other words, where power at the speaker is near zero. Is it >>>really that important to consider? >> >>>I was looking at that terrible large scale gain plot for the >>>quasicomplementary output stage on the web site recently >>>mentioned in the thread (the lower curve in Figure 4 on this >>>link): >>> >>>http://www.embedded.com/design/206801065?printable=true >>> >>>(It's not that terrible of a plot, as the variation is from >>>.96 to .98 with the "normal" middle at .97.) >>> >>>What's experience say here? Is it really so terrible as to >>>worry too much about something that takes place near zero >>>voltage, anyway? I'm just questioning the concern, for now. >>>I have no understanding about it, at all. Just wondering. >> >>Crossover distortion is a more-or-less fixed >>amplitude, so at low signal levels it becomes a >>large percentage of THD. Our ears are sensitive >>to the relative amplitudes of components, so a >>hypothetical fixed amplitude distortion component >>might be totally inaudible when it is a low >>percentage of a large fundamental, yet be >>obnoxious as a larger percentage of a small >>fundamental. (This is the same essential problem >>as quantization distortion in digital circuits... >>you don't hear it on the peaks, only on the quiet >>parts.) > >Okay. This is consistent with another way I was looking at >this. Since the crossover takes place near where power is >also close to zero, it's effect is fairly constant. If the >amplifier's swing is large scale (nearer the limits it was >designed to provide), it's not likely to be noticed buried >within that. If the amplifier's swing were small (nearer >that zero volt area) then the crossover distortion is quite a >bit more noticeable. > >Makes sense, I think. > >>Unless you have some application that doesn't >>involve soft passages in the signal (like a siren, >>or possibly a PA or musical instrument amp) you >>need to consider the crossover distortion. > >Best I consider it, then. I'm looking to learn, not >specialize in bullhorns. > >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? > >>However, there is still a lower limit to absolute >>detection threshold, regardless of what percentage >>a component might be. If the system levels >>(program, amp, speakers, listening position) are >>set up such some signal component is below 0 dB >>SPL, most people aren't going to hear it even if >>it is the entire signal! > >Hehe. > >>Best regards, >> >>Bob Masta > >Thanks, >Jon A few percent distortion at power levels is essentially inaudible. Speakers do that already. Low-level crossover distortion is obvious and obnoxious. So: make sure the output stage transfer function is fast and continuous, and apply a lot of feedback to straighten it out. John
From: Jon Kirwan on 6 Feb 2010 17:07 On Sat, 06 Feb 2010 10:48:16 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Fri, 05 Feb 2010 11:32:16 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Fri, 05 Feb 2010 12:36:02 GMT, N0Spam(a)daqarta.com (Bob >>Masta) wrote: >> >>><snip> >>> >>>Crossover distortion is a more-or-less fixed >>>amplitude, so at low signal levels it becomes a >>>large percentage of THD. Our ears are sensitive >>>to the relative amplitudes of components, so a >>>hypothetical fixed amplitude distortion component >>>might be totally inaudible when it is a low >>>percentage of a large fundamental, yet be >>>obnoxious as a larger percentage of a small >>>fundamental. (This is the same essential problem >>>as quantization distortion in digital circuits... >>>you don't hear it on the peaks, only on the quiet >>>parts.) >> >>Okay. This is consistent with another way I was looking at >>this. Since the crossover takes place near where power is >>also close to zero, it's effect is fairly constant. If the >>amplifier's swing is large scale (nearer the limits it was >>designed to provide), it's not likely to be noticed buried >>within that. If the amplifier's swing were small (nearer >>that zero volt area) then the crossover distortion is quite a >>bit more noticeable. >> >>Makes sense, I think. >> >>>Unless you have some application that doesn't >>>involve soft passages in the signal (like a siren, >>>or possibly a PA or musical instrument amp) you >>>need to consider the crossover distortion. >> >>Best I consider it, then. I'm looking to learn, not >>specialize in bullhorns. >> >>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? >> >>>However, there is still a lower limit to absolute >>>detection threshold, regardless of what percentage >>>a component might be. If the system levels >>>(program, amp, speakers, listening position) are >>>set up such some signal component is below 0 dB >>>SPL, most people aren't going to hear it even if >>>it is the entire signal! >> >>Hehe. >> >>>Best regards, >>> >>>Bob Masta >> >>Thanks, >>Jon > >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. >So: make sure the output stage transfer function is fast and >continuous, and apply a lot of feedback to straighten it out. Fast, I assume, to deal with high frequencies and still have the needed gain to do it well; continuous in the sense that the feedback isn't itself filled with potholes of some kind or another; a lot because the more, the more linear you get. Of course, that doesn't write equations for me or select specific parts or topologies for me. But it's good so far as it goes. Jon
From: pimpom on 7 Feb 2010 05:40 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.
From: Bob Masta on 7 Feb 2010 08:29 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! Another point to keep in mind about musical instrument amps is that whatever distortion they may have becomes part of the "color" or timbre of the sound. Distortion may be added via dedicated circuits, or obtained by use of tube output stages. (Tubes naturally have a more-pleasant distortion with stronger even harmonics, compared to the harsh odd harmonics of crossover distortion.) Also, since they handle only one instrument at a time, there is less problem with intermodulation. As long as all the distortion components are harmonics, the sound is just more reedy, or richer, or whatever. IM distortion produces sum and difference tones that are usually not harmonically related to any fundamental, so it sounds really bad. 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 7 Feb 2010 08:58
pimpom 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. Correction: I interchanged left and right parts of the image in my description. Sorry. |