THD Usefulness Thread = Son of Opti-MOS ? - Page 2 - diyAudio
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Old 13th March 2001, 01:43 AM   #11
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Join Date: Oct 2000
Location: Austin, Texas, USA
The Zen is designed to roll off at about 15kHz. Look at passlabs.com. He gives the frequency graph. It's short life... I have no idea. Everything was fine except the MOSFETs. Maybe I had a bad one. Maybe it just couldn't handle the heat. I rechecked the schematic a dozen times. I got it right according to the web site. But, I can't speak for the accuracy of that. I'm assuming it's good since a few people here built one.

I currently have 40,000uF. 20,000 per rail. Do you think I need more? When I switch off the amp it continues to operate for about 10 seconds! It even has pretty good bass for 5 or 6 seconds. And that's playing it very loud.

I thought about opening my McIntosh again and looking to see how much reserve it has. I may be surprised.

pixie

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Old 13th March 2001, 02:55 AM   #12
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Join Date: Feb 2001
Location: Columbia, SC
Pixie,
As a fellow I once knew was fond of saying: If some is good, then more is better, and too much is just enough!
The proper way to talk about storage on rails is not in capacitance, but in Joules. If I recall correctly, the formula is J=1/2CV**2 (somebody check me on this, I'm at work, quoting from memory), where J=Joules of storage, C=capacitance in Farads (NOTE: *Not* microfarads!), and V= volts on the rail.
N.B.: Tube amps get a head start here, since the voltage is squared, and the rail in tube equipment typically is much higher than that of solid state equipment.
So what voltage are you running on your rails? I seem to recall that you said that the Opti-MOS circuit has variable output, presumably by applying higher rails.
Me? I'd look into the possibility of at least, say, 50,000 uF/rail. There's no magic number...it's just a question of money, and caps get expensive, fast. Search the surplus websites. Hit scrap yards, looking for junked electronic equipment that might still contain caps. Old computer equipment, particularly tape drives, can provide caps with suprisingly high voltage ratings. (Not to mention humongous transformers...)
Sound quality...don't expect a lot of difference in the treble by putting in more electrolytics, but the bass will firm up quite nicely. To do anything for the high frequencies, start bypassing with film caps. In a pinch, I've even thrown in a handful of old Sprague Orange Drop (Mylar, aka polyester) caps out of my junk box. A mere 5-10 uF will make a surprising difference with cymbals, etc. I know Slone/Self don't think much of this, but a little simple math will demonstrate what I'm talking about. Look up the ESR for a typical electrolytic, then apply that number to the capacitance...electrolytics cease to be useful at pretty low frequencies.
There is a law of diminishing returns, here. Infinite amounts of capacitance are *not* infinitely better than, say, 100,000 uF. When is enough, enough? That's between you, your speakers, your ears, and your wallet. Not to mention such practical considerations as spouses, and where to put the damned things so that they're not under foot.
I'll mess with your head another way: You can view power supply caps from another perspective. Think of them, not as a reservoir from which to draw power, but as a filter. You can take this in two ways. First, it's a filter to take out residual ripple from the incoming AC. Okay, everybody knows that. But here's the head-messer-upper: Think of the power supply caps as a shunt to ground for the audio frequencies that backlash through the rails. Pixie, this will particularly apply to you, since Slone designed that amp to be biased class B, and B throws ferocious amounts of hash back up into the rails. (Incidentally, this is a point in favor of class A & AB that Self/Slone kinda never get around to discussing...ahem.) Slone speaks of using decoupling caps to help isolate the front end. Good. But let's just get rid of the stuff while we're at it. A low impedence to ground is your best avenue. How do you accomplish this? With lotsa caps, of course! It's also helpful if the impedance doesn't vary too much from 20-20kHz (and beyond), so that's where the film caps come in. They reduce the impedance seen by the high frequency audio AC on the rails when it's looking for somewhere to go, since the electrolytics dropped out of the race a long time ago, frequency-wise.
Incidentally, the difference between 15kHz and 20kHz is somewhat less than 1/2 octave; each doubling (or halving) of frequency represents one octave.
Re: Running for 10 seconds after the power switch is hit. I once had an amp that would run for nearly a minute. That was, of course, with an ever-so-slightly augmented power supply...the depth and power of the bass was quite extraordinary.

Grey
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