Discussing audio amplifier design -- BJT, discrete
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From: David Eather on 11 Feb 2010 03:15 Paul E. Schoen wrote: > "pimpom" <pimpom(a)invalid.invalid> wrote in message > news:hkr77v$lvl$1(a)news.albasani.net... >> John Larkin wrote: >>> 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. >>> >> DC bias of the recording head was also used in many cheap portables. The >> erase head was a permanent magnet that swiveled out of the way on >> playback. On recording, the erase head magnetised the tape to saturation >> in one direction. The dc bias is polarised in the opposite direction, >> with enough strength to place the operating point in the linear region. >> The noise level is higher than with AC erase and bias, but it works. > > I had one of those cheap cassette recorders, and it worked OK. But I had an > 8-track tape player in my car and I wanted to be able to record, so I built > an AC bias circuit (I think mine was 40 kHz), using a circuit that I found > in an old databook. It incorporated the RIAA non-linear amplitude curve as > well. I also made a device which used a cheap turntable and crystal pickup, > with two J-FET (2N3819) linear amplifiers and VU meters. > Didn't all that stuff make the car hard to drive? > http://en.wikipedia.org/wiki/RIAA_equalization > http://freecircuitdiagram.com/2008/11/27/gramophone-pre-amp-a-pre-amplifier-with-riaa-response-curve/ > > This was in 1970, and the only decent piece of test equipment I had was a > refurbished HP140A scope, with 100 kHz bandwidth and a fast blue phosphor > CRT made for photography. I still have everything except the 8-track tape > player for the car. It was stolen, along with most of my tapes, when I > foolishly neglected to put them in the trunk when I parked in a marginally > bad neighborhood in Washington, DC in 1972. > > Paul > >
From: David Eather on 11 Feb 2010 06:59 Jon Kirwan wrote: > On Fri, 05 Feb 2010 17:06:54 -0800, I wrote: > >> On Fri, 05 Feb 2010 16:46:22 -0800, I wrote: >> >>> <snip> >>> I tracked down a very nice transformer in my box which may be >>> okay. It has two secondaries and was intended for 60Hz use. >>> It weighs in at 2.8 lbs (1.25 kg.) >>> >>> Primary: >>> 115VAC, 5.0 Ohms, 16 gauge >>> Secondaries: (Tested using 120.5VAC RMS on primary) >>> 16VAC RMS CT, 0.05 Ohms, 14 gauge >>> 30.4VAC RMS CT, 2.6 ohms, 22 gauge >>> >>> The 16VAC RMS outer winding across a 56 ohm resistor yields >>> 15.88VAC RMS. (I don't have a large wattage resistor with >>> lower values of resistance, so that needs to suffice.) Half >>> of the 30.4VAC RMS winding (CT to one side) yields 14.75VAC >>> RMS loaded with the same 56 ohm resistor. Across the entire >>> 30.4VAC windings it is 28.9VAC RMS. (The poor thing is just >>> a 5W, so I didn't measure for longer than a few seconds.) >>> >>> The 30.4VAC secondary looks reasonable, I think, for the two >>> amplifier rails and ground. The 16VAC might make another >>> supply for some other reason or, perhaps, provide another >>> pair of rails to use for a 2 ohm speaker? >>> >>> I hadn't thought about that aspect, but as you earlier >>> pointed out the 25.2VAC CT standard transformer might be a >>> little light for a 10W amplifier... unless I spec'd a 4 ohm >>> speaker, I suppose. Then it might be fine. >>> >>> Anyway, it looks like it may be a reasonable choice as >>> something I have available and ready from the junk box. >> Second thoughts. The 30.4VAC RMS CT secondary shows 2.6 ohms >> and is 22 gauge. That's 1.3 ohms per half. I believe from >> calculation that the peak diode current _might_ be 8-10 times >> the load current in the ideal case (0 ohms.) Taking into >> account the winding resistance, I may need to think more >> closely about using this transformer in this application. The >> winding resistance will limit the current and thus the energy >> per unit time that can be transferred to the caps and that >> will very likely lower the achievable rail voltage on the >> other side of the bridge since the bridge itself simply won't >> ever see the idealized peak voltage even right up to the >> moment of peak where the dv/dt goes to zero. By the time >> that happens, the cycle will already be on a decline again >> while the resistance continues to limit inflow of charge. >> Cripes. >> >> Darn it. Back to monster caps to get a slight decent rail >> voltage there. >> >> Jon > > An addition or two. The transformer mentioned above uses 18 > gauge, not 16 gauge, for the primary. And since I'm still > wrestling with why there are 5 ohms, measured, I think it's > likely that is merely the wiring to the outside and that > perhaps even smaller diameter wire was used to wrap around > the core. So externally labeled gauge probably is NOT a > precise indication of what was used in the core. > > I took a look at the weight of some similarly shaped 60Hz > power transformers, available from Stancor. It seems that > similar weights are on the order of 80 VA. (I've seen a few > rated 100 VA, but I'm betting less.) > > For now, I'll assume that transformer is too small, not > because of my guessed-at VA rating but because of the > measured resistance in the 30.4VAC secondary. > > I want to get back to the power supply design and finish > that. > > I understant that at 10W, the 8 ohm speaker will experience > sqrt(2*10W*8 ohms) or 12.65Vrms and 1.58Arms. I could always > modify this to fit what I have available but rather than do > that, I think it's better to "stay on target" and see where > that goes. > > So that's what is expected "at the speaker." The peak figure > required will be rounded up to 18V. To reach that peak, the > output BJTs will need some headroom of their own. Staying > out of saturation and assuming the output stage might use two > BJTs on either side, requiring perhaps two diode drops if > either of these quadrants uses both NPN or both PNP, I would > best figure another 4V of headroom on each side. So 22V > minimum there, __under load.__ > > And I begin to see why 25V isn't a bad target. > > Which brings in the question about a linear regulator. It's > my vague feeling that there is NO need for one. I should be > able to arrange the circuity (current sources, etc.) so that > they are sufficiently immune to modest ripple that the 60Hz > (and other components due to loading causing cap voltage > changes, as well) can be rejected well enough. Besides, a > linear regulator would mean just that much more headroom and > wasted power/heat. So unless something very difficult is > shown to me, I'd like to take the position that a linear > regulator is a lot extra trouble without worthwhile payback. > (And dealing with the added poles/zeros would seem to make my > worries compounded, as if the rest weren't enough.) > > The filter capacitors will probably have to be spec'd at 50V > given what I've read here. It seems 35V wouldn't be entirely > safe, given the comments about regulation at 15% and another > 7% margin, as well. And something else that is bothering me. > Charging only takes place for short bursts and happen > _before_ the windings reach peak voltage. So there is a > small duty cycle during which usable energy is transferred. > Does this suggest that one might _under specify_ the VA > rating for the transformer to save cost and weight and get > away with it? > > Jon I didn't give a full answer. The small duty cycle (and large I**2R losses) mean you have to over specify the VA of the transformer - but because of the low duty cycle (for a domestic application) the net result can be a transformer smaller than the amplifier output wattage. There are common figures used for various configurations. If I ever knew how they were derived I don't remember now. Can someone step in here?
From: Paul E. Schoen on 11 Feb 2010 21:18 "David Eather" <eather(a)tpg.com.au> wrote in message news:VMOdnQ-MerwJIe7WnZ2dnUVZ_tP_fwAA(a)supernews.com... > Paul E. Schoen wrote: >> >> I had one of those cheap cassette recorders, and it worked OK. But I had >> an 8-track tape player in my car and I wanted to be able to record, so I >> built an AC bias circuit (I think mine was 40 kHz), using a circuit that >> I found in an old databook. It incorporated the RIAA non-linear >> amplitude curve as well. I also made a device which used a cheap >> turntable and crystal pickup, with two J-FET (2N3819) linear amplifiers >> and VU meters. >> > > Didn't all that stuff make the car hard to drive? Not at all. My car was a 1965 Chevy Malibu that was good on the curves. Most importantly, it had a Class A driver :) Paul
From: pimpom on 12 Feb 2010 02:52 Paul E. Schoen wrote: > "David Eather" <eather(a)tpg.com.au> wrote in message > news:VMOdnQ-MerwJIe7WnZ2dnUVZ_tP_fwAA(a)supernews.com... >> Paul E. Schoen wrote: >>> >>> I had one of those cheap cassette recorders, and it worked >>> OK. But >>> I had an 8-track tape player in my car and I wanted to be >>> able to >>> record, so I built an AC bias circuit (I think mine was 40 >>> kHz), >>> using a circuit that I found in an old databook. It >>> incorporated >>> the RIAA non-linear amplitude curve as well. I also made a >>> device >>> which used a cheap turntable and crystal pickup, with two >>> J-FET >>> (2N3819) linear amplifiers and VU meters. >>> >> >> Didn't all that stuff make the car hard to drive? > > Not at all. My car was a 1965 Chevy Malibu that was good on the > curves. Most importantly, it had a Class A driver :) > Was the driver good on curves _inside_ the car?
From: Jon Kirwan on 16 Feb 2010 23:28
On Thu, 11 Feb 2010 17:51:27 +1000, David Eather <eather(a)tpg.com.au> wrote: ><snip> >It is not a big stress. You can always use the junk-box transformer and >if it really isn't suitable replace it latter. For your consideration - >the RMS power of even compressed samples of music is only about 20% of >the peak. > >There are a few variations on that figure. RCA did a lot of research in >the area and found that Radio broadcasts of compressed FM signals of >"Rock Music" - an undefined term, was the most demanding at 15%. Some >companies are trying to redefine that. IRF who call the same figure 1/8 >of max power (12.5%) - which just happens to make their newest audio >mosfets look really good. It might be the other way around. They may >really believe it, and designed the mosfets to match. I forget where but >some group stated the 20% figure with respect to new modern music >styles. IIRC they were regarded as technically competent in the area and >had no axe to grind or wheelbarrow to push - so I filed the info away. >In any case an overestimate leads to a more conservative design and 5% >is not much. I'd be wary of definition of "modern music" too - badly >played organ music can be a stream of full amplitude waveforms that only >change in frequency at random intervals. > >I'd use the junk box transformer and forget about allowing for the >electricity company slackness and just choose good sized caps that are a >reasonable price. I think a learning experience allows for a little >compromise. Okay. I'm back to the power supply, again. (I'm convinced that my junkbox unit will work fine -- I think it can hold maybe 18V minimum under load on each rail. Which seems more than enough headroom for 12.7V, plus output stage overhead. I take a little issue with your use of terms in this phrase, "RMS power of even compressed samples of music is only about 20% of the peak." Power is average and I don't think RMS applies to power. Volts-to-power is a squared-phenomenon. So are amps-to-power. RMS makes sense for those two. But power is an average (integrated Joules divided by time.) So I believe I have to interpret your meaning as suggesting that the short-term power required (also an average of some ill-defined kind, I suppose) when playing music can be a factor of 5 times more than its long-term average power. You also mentioned a figure as low as 12.5%, which would suggest a factor of 8 used as a margin instead of 5. But a requirement to support short-term power levels is really just a compliance requirement on the power supply rails, isn't it? So put another way, if I wanted a long-term average of 10W output and I wanted the extra margins required to support the worst case estimate of a factor of 8 for short-term power bursts, then I'd need to design rails that support a voltage compliance level substantially higher. The parts would need to withstand it, too. And because of the much higher rail voltages that need to be dropped most of the time, the output BJTs would need to have just that much more capacity to dissipate. Or put still another way, assuming that my output swing at the output stage emitters cannot exceed a magnitude of 15V and that everything is sized for dissipating 10W, does this mean the amplifier is a 10W amplifier that can support a peak of 14W=(15^2/(2*8))? (Which isn't so good, considering your comments above regarding "music?") What is meant when one says, '10 watts?' This gets worse when I consider the class of operation, doesn't it? I mean, class-B might be specified as 10W into 8 ohms, but wouldn't that be 20W into 4 ohms? But if class-A, it's pretty much 10W no matter what? I'm beginning to imagine amplifiers should be specified as to their peak output voltage compliance into 8, 6, and 4 ohms; instantaneous and sustained without damage to the unit. For example, 35V into 8 ohms instantaneous, 15V sustained. Or 80W instantaneous, 15W sustained. That way, someone might have some knowledge about how well it might handle _their_ music at, say, 15W average power. And could compare that against another unit specified as 20V into 8 ohms, 15V sustained. How does one know what they are buying? What a headache. Jon |