Discussing audio amplifier design -- BJT, discrete
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From: pimpom on 3 Feb 2010 16:40 David Eather wrote: > > If I had a split power supply I would *always* get rid of the > output > capacitor. It is not difficult to get the output DC to withing > 50mv of > gnd. A weird thing I have noticed, and I think you would have > noticed > it sooner, is that no one, even audio "golden ears" pay serious > attention to the output cap. They just stick a plain old > electrolytic > of no particular type (some times it's a bipolar) in the > output, make > it bigger than needed for the LF -3db corner and call it > "good". It > would seem that some attention should be paid to "ripple" > current at > frequencies like 20khz etc, so some low esr caps would seem > mandatory. > That music has relatively less high frequency components is the > only > reason I can think of that this very lax approach might work. > If I may inject a comment here: I strongly support the idea of avoiding an output coupling capacitor. I always use a split-PS, OCL configuration unless some other consideration makes it necessary to use a single-ended PS. The comment about DC offset at the output terminal reminds me of an experience I had more than 20 years ago. I was asked to spruce up the P.A system at our state legislators' main session hall. One of the things I did was to replace the old tube power amp with my own design. I built four 60-watt amps (3 in use, one spare) using 2N3055 BJTs in quasi-complementary configuration (I couldn't easily get true complementary pairs then). Since the existing system distributed audio power to dozens of small speakers, inside and outside the hall, over a standard 100-volt line, I integrated a 4-ohm input, 100V output transformer in my amps. When I first tested the system, one output transistor each in two of the amplifiers warmed up quickly even without any output - not actually hot, but warmer than they should be. After a few moments of puzzlement, I traced the culprit to slight DC offset at the output terminal. It was only a small fraction of a volt and wouldn't have mattered with direct coupling to a speaker. But the DC resistance of the primary winding of the output transformer was so low (a fraction of an ohm) that it forced one of the output transistors to draw a substantil amount of DC current at idle. I further traced the cause of offset to poorly matched transistors at the input differential stage. I didn't include provision for manual balancing of the static DC level, so I tried out a few transistors for the input stage until I got a pair that matched closely enough to reduce the offset to within a millivolt or so (there was no hope of obtaining a factory-matched pair). I know this has no direct relevance to the discussion, but I was partly reminiscing and partly thinking that it may not be a bad idea to give a real-life example of how easy it is to overlook something.
From: Paul E. Schoen on 3 Feb 2010 18:14 "pimpom" <pimpom(a)invalid.invalid> wrote in message news:hkcqg6$vda$1(a)news.albasani.net... > David Eather wrote: >> >> If I had a split power supply I would *always* get rid of the output >> capacitor. It is not difficult to get the output DC to withing 50mv of >> gnd. A weird thing I have noticed, and I think you would have noticed >> it sooner, is that no one, even audio "golden ears" pay serious >> attention to the output cap. They just stick a plain old electrolytic >> of no particular type (some times it's a bipolar) in the output, make >> it bigger than needed for the LF -3db corner and call it "good". It >> would seem that some attention should be paid to "ripple" current at >> frequencies like 20khz etc, so some low esr caps would seem mandatory. >> That music has relatively less high frequency components is the only >> reason I can think of that this very lax approach might work. >> > > If I may inject a comment here: I strongly support the idea of avoiding > an output coupling capacitor. I always use a split-PS, OCL configuration > unless some other consideration makes it necessary to use a single-ended > PS. > > The comment about DC offset at the output terminal reminds me of an > experience I had more than 20 years ago. I was asked to spruce up the P.A > system at our state legislators' main session hall. One of the things I > did was to replace the old tube power amp with my own design. I built > four 60-watt amps (3 in use, one spare) using 2N3055 BJTs in > quasi-complementary configuration (I couldn't easily get true > complementary pairs then). Since the existing system distributed audio > power to dozens of small speakers, inside and outside the hall, over a > standard 100-volt line, I integrated a 4-ohm input, 100V output > transformer in my amps. > > When I first tested the system, one output transistor each in two of the > amplifiers warmed up quickly even without any output - not actually hot, > but warmer than they should be. After a few moments of puzzlement, I > traced the culprit to slight DC offset at the output terminal. It was > only a small fraction of a volt and wouldn't have mattered with direct > coupling to a speaker. But the DC resistance of the primary winding of > the output transformer was so low (a fraction of an ohm) that it forced > one of the output transistors to draw a substantil amount of DC current > at idle. > > I further traced the cause of offset to poorly matched transistors at the > input differential stage. I didn't include provision for manual balancing > of the static DC level, so I tried out a few transistors for the input > stage until I got a pair that matched closely enough to reduce the offset > to within a millivolt or so (there was no hope of obtaining a > factory-matched pair). > > I know this has no direct relevance to the discussion, but I was partly > reminiscing and partly thinking that it may not be a bad idea to give a > real-life example of how easy it is to overlook something. Something else to consider is a bridge output connection. You just need two output stages, invert the phase to one, and make sure the DC voltages on each are balanced. Another bonus is that you can get nearly 24 volts P-P with a 24 VDC single supply. You can get very close to the rails if the driver stage uses a slightly higher power supply voltage, so you can optimize efficiency but at the cost of distortion (clipping). At one time I considered making an amplifier with a dynamically adjustable power supply so that the rails would always be just a couple of volts above the peak output signal. It would probably be workable for a high-power signal generator where some clipping can be tolerated as the output is increased, but for music or other complex signals that vary unpredictably in amplitude, there would need to be a delay in the signal long enough to allow the power supply to adjust to what will be needed. At low frequencies, it might be possible even to have the power supply track the waveshape and even higher efficiency could be obtained. I have used serial analog delay lines which are basically a bucket brigade of switched capacitors, clocked higher than the maximum frequency required. I designed a phase-shifting circuit for power line frequency, using an IC that was sold at Radio Shack at the time, an SAD1024 http://www.geofex.com/sad1024.htm. I think I clocked it at a rate which produced a 90 degree phase shift at 60 Hz, which would be 1024/0.00416 = 246 KHz. But it was prone to distortion, and it was not long before the IC was discontinued and replaced with a dual 512 stage device that was even worse. There are much better ways to accomplish such feats now, but in 1980 or so there were not many alternatives. Now the way to do it might be to digitize the signal and then use a circular buffer to achieve whatever delay is needed. Probably 16 bit audio sampled at 44 kHz so you can get about a 1.5 second delay with a 16 bit x 64k word memory. But with all that, probably a PWM amp would be the way to go. Just draining the brain through my fingers and the keyboard into cyberspace... Paul
From: David Eather on 3 Feb 2010 19:45 Paul E. Schoen wrote: > "pimpom" <pimpom(a)invalid.invalid> wrote in message > news:hkcqg6$vda$1(a)news.albasani.net... >> David Eather wrote: >>> If I had a split power supply I would *always* get rid of the output >>> capacitor. It is not difficult to get the output DC to withing 50mv of >>> gnd. A weird thing I have noticed, and I think you would have noticed >>> it sooner, is that no one, even audio "golden ears" pay serious >>> attention to the output cap. They just stick a plain old electrolytic >>> of no particular type (some times it's a bipolar) in the output, make >>> it bigger than needed for the LF -3db corner and call it "good". It >>> would seem that some attention should be paid to "ripple" current at >>> frequencies like 20khz etc, so some low esr caps would seem mandatory. >>> That music has relatively less high frequency components is the only >>> reason I can think of that this very lax approach might work. >>> >> If I may inject a comment here: I strongly support the idea of avoiding >> an output coupling capacitor. I always use a split-PS, OCL configuration >> unless some other consideration makes it necessary to use a single-ended >> PS. >> >> The comment about DC offset at the output terminal reminds me of an >> experience I had more than 20 years ago. I was asked to spruce up the P.A >> system at our state legislators' main session hall. One of the things I >> did was to replace the old tube power amp with my own design. I built >> four 60-watt amps (3 in use, one spare) using 2N3055 BJTs in >> quasi-complementary configuration (I couldn't easily get true >> complementary pairs then). Since the existing system distributed audio >> power to dozens of small speakers, inside and outside the hall, over a >> standard 100-volt line, I integrated a 4-ohm input, 100V output >> transformer in my amps. >> >> When I first tested the system, one output transistor each in two of the >> amplifiers warmed up quickly even without any output - not actually hot, >> but warmer than they should be. After a few moments of puzzlement, I >> traced the culprit to slight DC offset at the output terminal. It was >> only a small fraction of a volt and wouldn't have mattered with direct >> coupling to a speaker. But the DC resistance of the primary winding of >> the output transformer was so low (a fraction of an ohm) that it forced >> one of the output transistors to draw a substantil amount of DC current >> at idle. >> >> I further traced the cause of offset to poorly matched transistors at the >> input differential stage. I didn't include provision for manual balancing >> of the static DC level, so I tried out a few transistors for the input >> stage until I got a pair that matched closely enough to reduce the offset >> to within a millivolt or so (there was no hope of obtaining a >> factory-matched pair). >> >> I know this has no direct relevance to the discussion, but I was partly >> reminiscing and partly thinking that it may not be a bad idea to give a >> real-life example of how easy it is to overlook something. > > Something else to consider is a bridge output connection. You just need two > output stages, invert the phase to one, and make sure the DC voltages on > each are balanced. Another bonus is that you can get nearly 24 volts P-P > with a 24 VDC single supply. You can get very close to the rails if the > driver stage uses a slightly higher power supply voltage, so you can > optimize efficiency but at the cost of distortion (clipping). > > At one time I considered making an amplifier with a dynamically adjustable > power supply so that the rails would always be just a couple of volts above > the peak output signal. It would probably be workable for a high-power > signal generator where some clipping can be tolerated as the output is > increased, but for music or other complex signals that vary unpredictably > in amplitude, there would need to be a delay in the signal long enough to > allow the power supply to adjust to what will be needed. At low > frequencies, it might be possible even to have the power supply track the > waveshape and even higher efficiency could be obtained. > > I have used serial analog delay lines which are basically a bucket brigade > of switched capacitors, clocked higher than the maximum frequency required. > I designed a phase-shifting circuit for power line frequency, using an IC > that was sold at Radio Shack at the time, an SAD1024 > http://www.geofex.com/sad1024.htm. I think I clocked it at a rate which > produced a 90 degree phase shift at 60 Hz, which would be 1024/0.00416 = > 246 KHz. But it was prone to distortion, and it was not long before the IC > was discontinued and replaced with a dual 512 stage device that was even > worse. > > There are much better ways to accomplish such feats now, but in 1980 or so > there were not many alternatives. Now the way to do it might be to digitize > the signal and then use a circular buffer to achieve whatever delay is > needed. Probably 16 bit audio sampled at 44 kHz so you can get about a 1.5 > second delay with a 16 bit x 64k word memory. But with all that, probably a > PWM amp would be the way to go. > > Just draining the brain through my fingers and the keyboard into > cyberspace... > > Paul > > The adjustable power supply approach has been dubbed type "H", but like type "G", it is almost certainly dying now... (which is sad I think - it does cut down lines of investigation for originality and inspiration) In the search for optimal efficiency: http://www.irf.com/product-info/datasheets/data/irs2092.pdf (irf - the worlds most experimenter unfriendly company)
From: Paul E. Schoen on 3 Feb 2010 23:51 "David Eather" <eather(a)tpg.com.au> wrote in message news:_ZidnV6rPbIxhffWnZ2dnUVZ_rednZ2d(a)supernews.com... > > The adjustable power supply approach has been dubbed type "H", but like > type "G", it is almost certainly dying now... (which is sad I think - it > does cut down lines of investigation for originality and inspiration) > > In the search for optimal efficiency: > > http://www.irf.com/product-info/datasheets/data/irs2092.pdf > > (irf - the worlds most experimenter unfriendly company) Here's what I found for Class H, which used a power supply boost under certain high output conditions: http://www.nxp.com/documents/data_sheet/TDA1562Q_ST_SD.pdf But it has been discontinued, and . I didn't realize that it had already been implemented and had a class letter. I came up with the idea in the early 80s IIRC. And I also tried to design a switching amplifier a few years later. Maybe they were novel ideas then and I should have pursued them. My idea for the switching amplifier planned to use, basically, two programmable switching supplies driven by the input signal and its inverse. But it ignored the necessity of supplying both positive and negative current. The answer, of course, was an H-bridge configuration. And I think sometimes that is known as Type H, as opposed to Class H. Another engineer at the time thought that he could just pass a pulse-width modulated high frequency signal through a ferrite transformer rated at perhaps 200 VA and 40 kHz. It would be modulated by the 60 Hz nominal signal that was to be provided as an output, and would use capacitors and inductors to filter out the carrier. But the only way for this to work at all would be to rectify the output, resulting in half of the waveform. And that would also saturate the core. So it was doomed from the start, but he actually had transformers made and PC boards built before he could be convinced that it had a fatal error. That IRF part sounds like a real beast. There's pretty much no need to design a power amplifier from discrete parts if you can get that IC for about $5 and some $2 MOSFETs to make a reliable, rugged, and efficient amplifier. Their development kit is $200 but that's not bad for a 250W x 2 channel amp. I guess that's what you mean about them being experimenter unfriendly. No reason to design and breadboard and fiddle around with something if it's already been done essentially to perfection.. Paul
From: David Eather on 4 Feb 2010 01:34
Paul E. Schoen wrote: > "David Eather" <eather(a)tpg.com.au> wrote in message > news:_ZidnV6rPbIxhffWnZ2dnUVZ_rednZ2d(a)supernews.com... >> The adjustable power supply approach has been dubbed type "H", but like >> type "G", it is almost certainly dying now... (which is sad I think - it >> does cut down lines of investigation for originality and inspiration) >> >> In the search for optimal efficiency: >> >> http://www.irf.com/product-info/datasheets/data/irs2092.pdf >> >> (irf - the worlds most experimenter unfriendly company) > > Here's what I found for Class H, which used a power supply boost under > certain high output conditions: > http://www.nxp.com/documents/data_sheet/TDA1562Q_ST_SD.pdf > > But it has been discontinued, and . I didn't realize that it had already > been implemented and had a class letter. I came up with the idea in the > early 80s IIRC. And I also tried to design a switching amplifier a few > years later. Maybe they were novel ideas then and I should have pursued > them. > > My idea for the switching amplifier planned to use, basically, two > programmable switching supplies driven by the input signal and its inverse. > But it ignored the necessity of supplying both positive and negative > current. The answer, of course, was an H-bridge configuration. And I think > sometimes that is known as Type H, as opposed to Class H. > > Another engineer at the time thought that he could just pass a pulse-width > modulated high frequency signal through a ferrite transformer rated at > perhaps 200 VA and 40 kHz. It would be modulated by the 60 Hz nominal > signal that was to be provided as an output, and would use capacitors and > inductors to filter out the carrier. But the only way for this to work at > all would be to rectify the output, resulting in half of the waveform. And > that would also saturate the core. So it was doomed from the start, but he > actually had transformers made and PC boards built before he could be > convinced that it had a fatal error. > > That IRF part sounds like a real beast. There's pretty much no need to > design a power amplifier from discrete parts if you can get that IC for > about $5 and some $2 MOSFETs to make a reliable, rugged, and efficient > amplifier. Their development kit is $200 but that's not bad for a 250W x 2 > channel amp. > > I guess that's what you mean about them being experimenter unfriendly. No > reason to design and breadboard and fiddle around with something if it's > already been done essentially to perfection.. > > Paul > > No, I mean they are pricks to try and deal with - unless you have a very large budget. |