Close but no cigar
We got close. Really close. The technician finished stuffing the new input stage, our chief engineer Bob Stadtherr brought it up and tested it, he and I spent a fun afternoon tweaking the square wave response and then 5pm rolled around. 5pm Friday December 21st. The last day of work for a week for many folks at PS Audio. Now I have to wait another week till the holidays are over before we connect the input stage of the amp to the amp module and listen to it. I too am heading out this morning to spend a week with my family. I am bummed about having to wait a week and impatient with the holidays getting in the way of listening to my new amp. Where's my lump of coal? In the meantime let me tell you about tweaking the square wave response and a little more about this stage. Round about 4pm Terri walked into the lab where Bob and I were huddled around the scope stroking ourselves over the beauty of the square wave of the new amplifier input. We beamed with pride. "Check that out!" Said I to Terri. You know that look people give you when they think you've slipped a cog? "Seriously? Really? You two nerds are admiring that thing on the scope and want to know what I think?" No, we probably didn't want to know what Terri thought. Bless her heart. There's nothing like a good square wave response on an input stage. I know this sounds funny because we don't listen to square waves, but in fact square waves tell us a great deal about how a circuit is going to sound. Not everything, mind you, but a lot. A square wave is a collection of sine waves. There's a fundamental sine wave, the main one at the frequency you set, and then declining higher frequency sine waves that fill in the edges. Over the years, as a designer, you look at a lot of these to see how your amplifier is doing. You tell a lot by how it looks at the beginning and the end of the square wave. Here's a picture I borrowed from a Stereophile review. It isn't important what this product is that the image is taken from. Could be anything. Let's focus instead on how the example looks at the beginning of the square wave. See the little "tit" or rise at the beginning? The little sharp edge? This will tend to sound a little bright when you have this rise. You see this type of imperfection when you use feedback. You can see it in other situations as well, but when it's a result of feedback, there are sonic consequences. I am sure the technocrats will howl over this, but I am simply sharing with you some years of experience. If this tack-sharp blip is present in an amplification stage with GNFB (global negative feedback), almost invariably the amp will have a hard or slightly bright sound indicative of that type of topology. Yes, a designer can add a capacitor to smooth out the little tit, but that in itself will not help the sound, only the way this looks on the scope. If the tit is present because of feedback, lowering or eliminating the feedback is one surefire way to get better sound. It's also possible to fix the open loop circuit (no GNFB) such that when you close the loop (add back GNFB), the tits aren't present. But that's fodder for another discussion. This is the kind of stuff you find out over years of looking at scopes and listening to the sound of circuits. Now take a look at this square wave. See how nice, rounded and gentle this looks? Compare it to the other one. If you see this type of response on your scope and it's the output of a simple circuit with or without GNFB the audible results will be far nicer sounding (e.g. musical). This is the stuff we nerds get excited over at work. Is it any wonder I am antsy about listening to the new circuit?
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