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Old 2nd April 2001, 06:33 AM   #11
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DIY means Do It Yourself. A simple enough statement. It's a wide umbrella which covers everything from someone putting a $4.95 Radio Shack driver into a cigar box to keep the back wave from cancelling the front, to giga systems that would cost more, if bought as manufactured items, than any random three of us could afford if we pooled our resources.
Once upon a time, there was a man named David Hafler. He made his reputation with tube circuits (amongst them the famous Dynaco ST-70, which sold about ninety-nine million units way back when--some people still like 'em), then went on to start the Hafler company. Probably the best known Hafler product was the DH-200, a 100W/ch solid state amp, available as a kit or factory-made unit. A good, reliable, decent sounding unit that went on to sell another ninety-nine million units. I hope David got rich off of them. He deserved it. Good product at a great price.
But for all that it was a separate amp, it was mid-fi. I don't recall a single person who ever felt otherwise. The build quality was excellent (for the price), but the amp tended towards being somewhat harsh on top (a later revision, the DH-220, cured some of the sonic problems) and got positively rough when pushed hard.
The power supply was simple. A standard EI transformer (big toroids were rare in those days) through a bridge rectifier, into a single pair of caps. The value of the caps? 10,000 uF, one for each rail. Now, condsidering that the one power supply took care of both channels, that would work out as being right in line with Slone's rule of thumb for capacitance/watt. Only, the thing was, you could hook a meter to the rails and watch the voltage fluctuate as the music came and went. The first thing most audiophiles did was to tag more capacitance into the amp. Around here they used the Mallory CG series, but in other places they used Sprague or whatever was available locally. It made a huge difference in the ability of the amp to handle peaks...and firmed up the bass. Something even a teenaged kid could hear over a pair of old Advents. He borrowed the caps from his older brother just to see what all this cap stuff was all about. No ego, no preconceived notions, just a couple of borrowed caps and an open mind. The way I heard about it was that the kid brother didn't want to give the caps back when he found out what a big difference it made and the two of them had a bit of an argument about it.
Now, if a curious kid with a ragged-out pair of Advents and a DIY mid-fi kit can hear the difference caps make, it makes you wonder whether Slone ever listens to his stuff, or simply does it as textbook exercises.
Ripple calculations are pretty basic. Within reach of where I'm sitting as I write this I've got ten or fifteen books, all of which will have at least one chapter (or, in the case of Gottlieb, the entire book) on power supplies. In the book shelf on the other side of the room I have another dozen books which will also tell you how to deal with ripple. Kind of them. (Incidentally, for those who might wish for a basic education in electronics [as opposed to Horowitz & Hill, fer instance, which can be pretty opaque to the uninitiated], allow me to recommend the ARRL Handbook. That's the book the Ham radio guys use. No, I'm not a ham operator. Never have been. And yes, there's an awful lot of radio stuff in there which may or may not be of interest to you. But the treatment of basic electronics is second to none in terms of being accessible and practical. Those guys don't have time for a lot of fancy-pants theory, they want a radio that works. There are times when peoples' lives have depended on Ham operators when all other communications have failed due to natural disaster or war or whatever. You can get your theory later from H&H or Diefenderfer or whoever is popular this year. If I'd had the ARRL book when I was a kid, I'd have learned electronics ten times faster.) Anyway, I regard ripple calculations as a good starting point. A bare minimum. To get any performance out of an amp, you're going to need more. Mysterious? No. There's no mystery. When the rails sag, the operating point for the gain devices change, whether bipolar, FET, or tube. When the operating point changes, that causes distortion. (CMRR cannot guard against a sagging rail. That's not what it's for.)
Actually, I guess there is a mystery: Why would anyone defend a circuit that allows dynamic distortions of this nature?
Granted, short of a well-regulated power supply, there will always be some ripple; some collapse of the rails. It's also subject to diminishing returns, as are most things. No problem. But why sit at the bottom of the returns curve with a minimal power supply, when you can slide at least part-way up? (Subject perhaps to having food to eat, spousal-approval-factor, etc.)
Fortunately, there's room under the DIY umbrella for all of us, from speakers in cigar boxes on up. The choice is up to the individual as to how much performance they might want, but to deny that more performance is possible, whether the individual chooses to attain that performance or not, is foolish.
The owner of a Camaro knows that Ferraris are out there, whether he will ever own one or not. His Camaro may even have been hot-rodded to the point where it will accelerate as well as the Ferrari, but will it do everything else as well as the Ferrari? Nope. It's still just a Camaro. But the wise Camaro owner doesn't deny the existence or validity of a Ferrari just because he doesn't own one.
Incidentally, if I had a circuit that required a 60% overvoltage on the rail, I'd start looking back upline to find out where the bottleneck was. Given that it's a car system, there aren't too many places to have to look, although given the limitations of a car's electrical system, some of them may well prove to be intractable.

Grey
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Old 2nd April 2001, 07:27 AM   #12
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It is a waste of time to regulate the current gain stage supplies.It is worthwhile to regulate the voltage gain stage supplies.The Hafler XL280 and XL600 did this.As long as the amp does not clip you cannot hear the difference between 1,000µF and 100,000µF.If you pre-clip the input signal with a soft-clip/limiter circuit your amp will always be under loop control and not make horrible sounds when the feedback loop overloads and the outputs stick on and go into common mode conduction.I sold a hopped up Altec 9440 to a local club.400W/ch with a single pair of 10,000µF caps.A 1.5Kva transformer.It replaced a Crest 6000 professional series.800W/ch with a total of 120,000µF in caps.A 3.6Kva transformer.It cost me $4 for the diodes and caps I put in the Altec.
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Old 2nd April 2001, 03:39 PM   #13
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djk,
I certainly agree that regulating the front end is a good idea. To me, given the relatively low currents in question, it seems to be a no brainer. Regulating the high current to the back...lotsa work. Whether I'm just being lazy or not, I've never gotten around to regulating the back end of my tube amp, although I've regulated the front end to a fare-thee-well.
For club use, which falls more under the category of pro gear (basically mid-fi stuff), I think I'd agree with you. Money and effort invested for little or no return. The speakers are *far* from hi-fi and the intended audience doesn't give a rip. Sell them another beer and crank up the volume another dB or two. Limiting the input is probably not a bad idea as DJs and/or the sound men for live groups are notorious for not caring. (Yes, the better ones do, but you can't count on having a good 'un at the controls.) Clubs are a great application for Crest, QSC, or Hafler gear. Lots o' watts in a compact rack mount chassis.
For home use? As Jan and I were discussing elsewhere--there are better things--although a case can be made for the struggling musician who uses the amp for double duty. But Crest and Altec just don't show up in hi-fi systems inless the wallet is small and the desire for music is large. (Those whose primary use for their home system is for party music are in roughly the same boat as clubs...just a smaller room and the neighbors are more likely to complain when it gets loud.)

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Old 2nd April 2001, 07:50 PM   #14
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djk,

Yes, I think you are right. Working in normal conditions there is no difference between 1000u and 100000u, and I think there is no problem to raise value of the capacitors until the limit of voltage drop. I changed the values some times and I achieved more excursion before clipping. In your case you did you do any test or measurement before the change?

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Old 3rd April 2001, 05:29 AM   #15
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Regulating the input circuits is a very good idea but not allways a must. the advantage of this is that the input can be set at a higher voltage then the output. In this way the output can work at a lower voltage with less dissipation with a lot of drive voltage from the input.
In high power pro equipment a switched mode power supply is a good idea because of space. I have seen a few very good amps with 1U rack height rated at 1000 Watts rms. Not bad.
One thing I have seen in power supplies is that not only a big capacitance is needed (and makes a difference in bass) but low series resistance and inductance is needed too.
I made the same amp twice ones using 4 caps rated 10 mF each/63v of good quality (Philips high current long life type). The second time I used 28 caps rated 2mF2 for a total of 61mF thats only 50% more but the difference was very very noticable and it cost allmost the same.
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Old 5th April 2001, 04:07 AM   #16
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Incidentally, and not considering your bitter words (as I told you before, they are not important here), I was talking about a bench power supply for testing high power car audio amps. There are huge amounts of current running there. It has nothing to do with car battery systems. The SMPS power supply itself has usually near 90% efficient.

In the real life the efficiency of a linear power supply depends heavilly on the output current. Many factors that are not under the designer control in this case or are not possible to solve without very expensive solutions.

Besides, think about .05ohm total circuit wiring resistance over 40 ampéres peak demand plus the VCE of transistor regulator plus the VBE of the rectification diodes and you will have near 60% of overvoltage (considering 12VDC output) needed, right?

In this case, batteries are much better than linear power supplies if properly charged. I'm thinking in change my testing gear on this way.



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Old 5th April 2001, 05:22 AM   #17
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blmn,
Now the curious thing is...some high end folks are running *home* gear off of batteries (super quiet). This is not something I've got a hankering to try at the moment, but there are those who swear by it. (And I'm not sure I want to go to the trouble to series 50 batteries to get enough V+ for tube gear...whew!)
Maybe some day when I've got no other projects on the board (fat chance), I'll get around to it.
Actually, I once owned a tiny Mark Levinson phono head amp that was powered off a 9V battery. I'd forgotten all about that. Cute little booger.

Grey
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Old 6th April 2001, 01:51 AM   #18
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yes, 50 batteries seems to me like a problem waiting to happen. But it must be quiet like a tomb!!!

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Old 6th April 2001, 02:25 AM   #19
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Did I write VBE in the diode junction in my reply? For sure I need a rest...

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Old 10th April 2001, 10:58 AM   #20
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About power supplies, there is an article in April's 2001 Audioexpress magazine about switching power supplies for tubes preamps, it might interest you.

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