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<p>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?</p>
<p>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.</p>
<p>You know that look people give you when they think you’ve slipped a cog?</p>
<p>“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.</p>
<p>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.</p>
<p>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.</p>
<p><a href="http://www.pstracks.com/wp-content/uploads/2013/12/Overshoot.jpg"><img class="alignleft size-full wp-image-12776" alt="Overshoot Close but no cigar" src="http://www.pstracks.com/wp-content/uploads/2013/12/Overshoot.jpg" width="450" height="302" title="Close but no cigar"/></a></p>
<p>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.</p>
<p>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.</p>
<p>This is the kind of stuff you find out over years of looking at scopes and listening to the sound of circuits.</p>
<p>Now take a look at this square wave.</p>
<p><a href="http://www.pstracks.com/wp-content/uploads/2013/12/good-one.jpg"><img class="alignleft size-full wp-image-12777" alt="good one Close but no cigar" src="http://www.pstracks.com/wp-content/uploads/2013/12/good-one.jpg" width="450" height="299" title="Close but no cigar"/></a></p>
<p>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).</p>
<p>This is the stuff we nerds get excited over at work. *Is it any wonder I am antsy about listening to the new circuit?</p>
<br/><span class="c4"><img src="http://www.pstracks.com/wp-content/plugins/wp-spamfree/img/wpsf-img.php" width="0" height="0" alt="" class="c3"/></span>
[Source: http://www.pstracks.com/pauls-posts/close-cigar/12775/]
<p>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.</p>
<p>You know that look people give you when they think you’ve slipped a cog?</p>
<p>“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.</p>
<p>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.</p>
<p>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.</p>
<p><a href="http://www.pstracks.com/wp-content/uploads/2013/12/Overshoot.jpg"><img class="alignleft size-full wp-image-12776" alt="Overshoot Close but no cigar" src="http://www.pstracks.com/wp-content/uploads/2013/12/Overshoot.jpg" width="450" height="302" title="Close but no cigar"/></a></p>
<p>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.</p>
<p>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.</p>
<p>This is the kind of stuff you find out over years of looking at scopes and listening to the sound of circuits.</p>
<p>Now take a look at this square wave.</p>
<p><a href="http://www.pstracks.com/wp-content/uploads/2013/12/good-one.jpg"><img class="alignleft size-full wp-image-12777" alt="good one Close but no cigar" src="http://www.pstracks.com/wp-content/uploads/2013/12/good-one.jpg" width="450" height="299" title="Close but no cigar"/></a></p>
<p>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).</p>
<p>This is the stuff we nerds get excited over at work. *Is it any wonder I am antsy about listening to the new circuit?</p>
<br/><span class="c4"><img src="http://www.pstracks.com/wp-content/plugins/wp-spamfree/img/wpsf-img.php" width="0" height="0" alt="" class="c3"/></span>
[Source: http://www.pstracks.com/pauls-posts/close-cigar/12775/]