The Worst Distortion

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There is one inherent advantage in class A amp. Please remember the horrible sound - to me the Worst kind of distortion there is - in many class AB amps - the kind you find in hi-fi shops. When you turn the volume to 4 or even 3.

Every amp I had sounded the same. My old 50 Watt Sansui with feedforward, had a little 25 W Marantz, quadraphonic JVC, my clunky old Radio Shack 50 Watter, the 70 (?) watt Curtes Mathes amp I just repaired my little Sherwood... not much difference.

They all have (had) skimpy power supplies. The power amp reservoir caps are smaller probably than a diyer would use in a Preamp. Because every transformer has internal resistance, this is a part of the REAL schematic not shown. You never see this distortion on a scope running a sine wave at a constant level. You would have to have some kind of burst generator, etc.

Class A amps, running at a constant current drain, don't have this distortion. They can't. The signal never demands any sudden current - it averages out to a net zero change (theoretically for a 50% duty cycle waveform - if I understand rightly). For that reason I don't see a problem in using some resistors in the power supply for filtering. That would be a disaster in a Class AB design. In Class A bias it merely reduces the supply voltage and reduces hum. I don't believe this is an example of clipping, though it causes that.

In my experiments, this bears out. Deliberately driving my experimental class A amp into distortion, I noticed the sound only got distorted - sounding ugly, of course, but that effect I Loathe was absent - that dynamic range compression which makes the music sound like it's getting buried under an avalanche is not present. I never want to build an amp capable of that ugly sound.

What is the name of this distortion ? Does it have a name ? I haven't read anything I can remember about this anywhere, yet it obviously Must have been addressed.

If this type of distortion were to be listed along with the other specs, a whole lot of amps would be looking pretty sorry. 0.05 % distortion ? The truth is, it may be more like 20 % I think, under REAL conditions. This is not make-believe, it could be measured. I do wonder about how pernicious this effect may be.

Time and time again I have turned up the volume (even only driving headphones !!) and had the music suddenly turn sour. Even at a setting as low as 2 (which makes one suspect it's something else in that case; whatever it is, it's bad).

This is one of the reasons I like class A bias better. I know that crossover distortion is supposed to be hideous - but in my experience it isn't as bad as the other - and I wonder how much is Really audible, especially in some of these designs you see where the designer can't even measure it. There was one amp I recollect - I Know it was AB - and it was maybe the best I ever heard - it also weighed and cost a lot. I believe it had a real power supply inside.

It was natural enough that I found Nelson Pass's site and looked at some designs. I built a kind of Zen amp, the performance of which I want to describe next.

I am beginning to explore hi fi.

What I have said here is, I think, somewhat obvious, seeing all the diyer amps with gigantic cans in the power supply. As long as you put those in your class AB amps, and hopefully a stout tranny, none of this will even be on your radar screen. It's what you might experience when you buy some hi-fi type amp, very real and ugly sounding. It's one big reason a diyer can create a superior amp.
 
Some people like some R in AB powersupplies, but then they are using diy amounts of capacitance- 35k uF+ per rail.

One approach in AB tube amps is the use of a swinging choke LC filter.

Alot of guitar amps have deliberately droopy rails, but this is instrument use and not hifi.
 
same

Thanks for all responses :)

Yes, Tweeker, I have noticed that, in some guitar amps. An example : A little 12 watt Marshall. The schematic showed a bipolar supply, two 1000 uF caps, and the preamp op amp fed from a 1 k resistor and decoupling cap. The whole amp was very simple.

It sounded MUCH better than the Peavey Decade, which used a lot more capacitance in the supply, and more complex.

Maybe a good reason for resistance in the supply of an AB or any amp is to try to reduce something that preys on my mind, which is the thought of harmonics being sprayed into the supply rails b/c of the bridge diodes (?) On both my pwr amp & pre amps I inserted a power resistor between the bridge and caps, in the hope of reducing it. I couldn't tell a difference, tho, I suspect it does nothing. I can imagine it may reduce clipping harshness on AB amps.
 
Re: same

JCM said:
Maybe a good reason for resistance in the supply of an AB or any amp is to try to reduce something that preys on my mind, which is the thought of harmonics being sprayed into the supply rails b/c of the bridge diodes (?) On both my pwr amp & pre amps I inserted a power resistor between the bridge and caps, in the hope of reducing it. I couldn't tell a difference, tho, I suspect it does nothing. I can imagine it may reduce clipping harshness on AB amps.

That resistor isn't really in the right place, because you're still charging/discharging the cap with pulses from the rectifier. Try an RC network after the first capacitor, and bypass the second cap with a good-sized polypropylene.
 
>Talk about hideous big box POS. I have a Sony HT receiver that >says 100W x 5 on the front, but says energy consumption 370 >watts on the back. Only sound OK when it's switched off.

>There used to be FTC laws against this kind of ****.

That reminds me - bought a Radio Shack 50 Watt / ch receiver, the Optimus STA 5500. (I'm not sure what I was thinking then.) The power consumption : 95 W. How that amp manages to break the laws of physics, I'm not sure.

I've noticed, along with bell bottoms and sideburns and all, another fad of the 70s : a lot of "100 Watt" boom boxes all over the place. They pulled that in the 70s, and yep, they're at it again, even the "hi fi" products.

>That resistor isn't really in the right place, because you're still >charging/discharging the cap with pulses from the rectifier. Try >an RC network after the first capacitor, and bypass the second >cap with a good-sized polypropylene.

I tried different small caps, but couldn't see any difference on the scope. Mainly it's broadband noise now (there's an 8 Henry inductor in the preamp supply). My thought was that while the pulses remain, the amperage would be reduced. For both supplies, I used so many capacitors, it's almost the whole thing, & 4 resistors in the pwr amp supply, so there's 4 RC networks.
 
The one and only
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JCM said:
>Talk about hideous big box POS. I have a Sony HT receiver that >says 100W x 5 on the front, but says energy consumption 370 >watts on the back. Only sound OK when it's switched off.

Actually I recommend Sony products if you just want to buy
something inexpensive and have it meet expectations.

I think the rating relects more of what the unit will actually draw
in real life. Figuring that you have peaks of 200 watts per
channel, real music will result a draw considerably less than
the 370 watts quoted.
 
Here's my complaint. They claim 100W x 5 rms. For all of the older (good for its day) equipment I've ever seen, 100W rms means 100W rms continuous, from 20 to 20khz, both channels driven and the nameplates on the back claim power consuption 2x the advertised 8 ohm power output. My Pioneer SX950 is rated 80WPC and consumes 370W, 450VA. My Kenwood KRC750 is rated 60WPC, and I think it pulled 320W if memory serves. And with todays music compression levels, it's constant maximum power draw with no dynamics. We need rms continuous ratings more than ever.

Another trick they're pulling is rating recievers at 6 ohms. This inflates the ratings by another 33% over the 70's FTC regulated ratings. It's still dishonest.
 
Getting a little off topic, I would like to add my vote and say I too have had trouble with Sony products. Most recently the display on my DVD player died. But in all fairness it still works without the display. In addition a cdr drive they built last to three years and then died.

It’s good to vent.

Some people swear putting a 0.1ohm resistor between the rectifier diodes and the caps they are charging makes things sound softer. Theory is it reduces spikes. I have never tried it, but hope to some day.

Leve
 
Sony

I deleted my post yesterday about my take on Sony, I figured maybe it was too off-topic, but can't resist now to say something, seeing Nelson post.

I kept seeing good quality in their products. Something you'd think would have no meaning, like a tiny transistor radio that pulled in more stations, and wierdly, even sounded better. That was the first thing. Almost some kind of bizarre theme, I kept running across some speaker or tape or whatever, that stuck out as being noticeably better, and would always turn out to be Sony.

I had this real marginal tape deck, and wanted to make lots of recordings, tried every single brand of tape I could get, you guessed it, only Sony tapes held the sound, the rest deteriorated quickly after recording. They still do 19 yrs later.

Only one more example - there were so many. My nephew had a cheap compact stereo. The turntable on it, I imagined, would be so bad, I wanted to hear it. I knew my "hi fi" cartridge on my new Sherwood turntable would blow it away. It didn't. I played an old record I didn't care about. This seemed like the Twilight Zone to me. And this little stereo had separate bass and treble tone controls, 8 " woofers, 2" tweeters also 2 shortwave bands.

It seemed to me the engineers squeezed every dime on it, it was practically threatening to equal my system quality at a fraction of the price - almost. Of course it was a Sony.

By that time I'd seen enough, seemed to me there was someone in that company that actually cared about quality a lot. That's when I bought a new deck, a Sony, instead of another brand, which, turned out to be the one model they made that had a design flaw. The salesman confirmed it when I took it back, no refund of course.

Aside from that, every one of their products I had or ran across seemed to stick out like a sore thumb in terms of better quality. Or maybe I imagined it ? I really don't know, but it kept happening.
 
jcm-
By that time I'd seen enough, seemed to me there was someone in that company that actually cared about quality a lot. That's when I bought a new deck, a Sony, instead of another brand, which, turned out to be the one model they made that had a design flaw. The salesman confirmed it when I took it back, no refund of course.
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when you look at the price Sony is a little pricy compared with
what you compare it with, compare it with something like
musical fidelity and come back to tell me about it

Guess what is the best japanese brand, Sharp! dont tell i told you
so....simply the best....:D
 
>when you look at the price, Sony is a little pricy compared with
>what you compare it with, compare it with something like
>musical fidelity and come back to tell me about it

>Guess what is the best japanese brand, Sharp! dont tell me i told you so....simply the best....

Maybe so. I can't say. Every time I turned around, there was some speaker or tape or amp or whatever that impressed me, and turned out to be a Sony.

It got to the point it was almost like, "What are the hi-fi gods trying to tell me ?" And the bit of irony of the defective tape deck.

I don't know how it is today, or if it was just a bunch of flukes or whatever. Shrug.
 
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