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You don't have to do this stuff. This is just a sample circuit. You can put coils in there for a CLC filter if you want or nothing at all. With a little decoupling I'm getting noise figures on the order of a hundred micro volts or so. I consider that to be quite good. I have seen better but I rarely achieve it in amplifiers with more than 25 watts or so. People often ask about how much capacitance should be on either side of the filter. The answer is there is no hard rule. I usually do half and half (the even-Steven rule) but you don't want to be stingy on the first half. The ripple currents are much bigger than you think – like 10 times bigger. I see examples of amplifiers which still work fine but the heat-shrink plastic sleeve on the capacitor bodies has continued to shrink from high temperatures until it's a belt. That tells me it's been running too hard because it's taken all the ripple current of short-duration high-current pulses. The energy dissipated is proportional to the square of the pulse size divided by the pulse length so you want to make a serious allowance for it. By the way, you can use the voltage drop across the resistors to measure the bias current of the amplifier.
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It's the same thing except that you can split the RC networks out to each channel separately. These sorts of filters are very effective at lowering noise, particularly the higher-order harmonics of the ripple noise. You're free to put large values of capacitance here. It doesn't bother me at all. Here is the actual amplifier interior before I got it to work properly. Unless I'm mistaken, this is end of the presentation. Any questions?
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Q: "The degeneration resistors - the most common kind would be the sand-filled block things. I think they use some nichrome wire. I think I've measured distortion in those before. Do you just recommend those or any other material?"
A: I use metal-oxide film types. You can get very snazzy resistors and at some point spend a lot of money but get diminishing returns. The whole thing of eye-candy parts is great. I have no objections to people using gold-plated anything and there's some really nice parts out there. I'm the last person to argue they're not better. What they are for sure is more expensive. And I'll be honest. One thing I can tell you for sure about DIYers is that they're really cheap guys... (audience laughs)...
"But Mr. Pass, that transformer costs $30! I can't afford that sort of thing!" So you understand why I don't automatically point people to expensive parts. As a default I pick cheap/available parts—read: crummy—for these projects. When I'm asked what calculation led me to a 220uF value for a capacitor, I explain that I have thousands of them on the shelf. When you look at the wire on my display today, you will see nice-quality clean copper wire from Fry's. It works fine. I should also say that I have some very nice wires that I think work a little better. The way I look at it? If you build one of my amps from the cheap parts and it sounds good, the design gets all the credit -:)
If you want to take it up a notch that's great and I have no argument with that. I don't even consider it a waste of money. I personally spend a lot on my own toys.
Q: "About the power supply, it turns out that Duncan Amplifiers makes something called PSUV originally intended for tube amplifiers. It turns out that it does a wonderful simulation of these kind of voltages and it shows everything, it shows the current pulses, whatever you want, and it's a wonderful simulation tool and highly recommended."
A. The other thing you can do is download a copy of LT Spice which is a little bit of work (I'm really bad at Spice but I can get what I want by beating on it). You can simulate anything and it will tell you a lot about power supplies. Perhaps the easiest of them is the free MicroCap student edition. It's nice and dumb and easy to use. And the last time I looked, John Curl was still using it so I'm with him... (more audience laughter)...
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Q: "The specs on British and American power amplifiers are almost identical, very similar, but the tonal character are differentiated because the Brits have the windings start on the opposite end of the coil that flips the polarity. You say that if we do a little bit more second-order harmonic on the top or bottom side it changes the character. Have you tried flipping the polarity on the speaker and see how that..."
A: Absolutely. One of the most fascinating things is this whole subject of absolute phase. How much can you hear absolute phase? That's not about whether your speakers are in or out of phase with each other but absolute phase. It has become quite apparent that it matters. It matters in the context of how you look at the second harmonic structure of the amplifier as it relates to the speakers and to what comes out of the recording process. I mentioned how positive-going phase for the second harmonic has a particular sound. If you flip that characteristic you get a different sound. I'm not here to tell you what to like. I've noticed that when you get reasonably experienced listening to that effect, you can go through your record collection often deciding which recordings are in phase and which ones are out of phase. I find that totally fascinating.
It relates to something I can talk about briefly as one of my favorite soap-box subjects. If you go into the literature of psychoacoustic perception, there is a very good book by Diana Deutsch at UC San Diego called The Psychology of Music. In it there are several chapters talking about how the low-level neural networks of the brain take the data from the ear. What they do with it is like the bureaucracy at the DMV. You have an army of these things and for each of them the job is to make a decision – what goes with what. These are called grouping mechanisms. Each bit of the network takes disparate bits of audio information and decides whether they go together or not. The system is sensitive to such things as loudness, timing, pitch, harmonic structure and phase. Decisions are made at very low levels and then get passed upward for increasingly more abstract decisions. The final result, the 'executive summary', gets handed to the guy who sits behind your eyes at the control panel and imagines he's in charge.
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So what are we doing when we play with the distortions of an amplifier? Well, we're just fooling ourselves, fooling the ear and the brain. And sometimes that's a good thing. It's plausible to me that if you tag the sound with a particular characteristic—I'm not claiming that expertise—it seems to slip more easily through these neural systems like poop through a goose. The decision-making process gets easier. There is a lot of work going on in the brain when we are talking about listening. A vast army of neurons work this thing. If you make their life easier, they aren't working as hard. We are talking about listener fatigue, about people who get tired after a half hour and shut the music off versus guys who go through their entire record collection all night long. We are literally talking about fatigue. The brain gets tired.
So why do we try to fool the ear? It makes people happy. It helps them to relax while they listen to music and try to forget all the terrible problems in the world. I'm not here to deliberately create distortion but if my simple little circuits are going to have some distortion anyway, I can at least try to organize it the way I want. Perhaps you say that it's not accurate? I say it's entertainment!... (thunderous applause)...
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© 2012 Nelson Pass
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