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Understanding Amps


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In practical terms as I understand it, Class A is is mostly lower wattage but the amps are running full out, run hotter, and may or may not sound better depending on the music you listen to. It's like a drag racer going full tilt down the track, the only speed is peddle to the metal.

Class A/B is push pull where both sides of the wave get to play, sort of like two people dancing in perfect harmony.

Class Band D I have no idea, but if you listen to The 3 Stooges Alphabet song I'm sure you'll figure it out.

I'm wondering if you are looking to learn about the different toplogies or have a more practical goal in mind. In other words are you trying to arrive at a conclusion as to what type of amp to buy. If it's the latter we need to know about what preamp you have, listening tastes etc.

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That Wikepedia was a good read and I enjoyed reading the the external link to the AES article.

I'm not an amp guy. In my mind, all "perfect" amps should and suspect do sound the same.

However, now we've got a lot of reports that some amps with distortion sound better than perfect amps. It is not just the fringe people.

Again let me suggest the many papers at www.passlabs.com

Wm McD

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10-21-2001 2:08 AM djk

"I hope I dont sound like a complete dolt here but exactly what is the advantage of a low power tube amp vs. solid state stuff. "

Are you sure there is an advantage for tubes?

In the begining there was the phonograph.

And it was good.

And then came the tube.

And it was, well it was louder.

But not necessarily better.

Feedback. Push-pull. FETs. Then transistors.

The first transistor amps were copies of tube amps, just made with transistors. They inherited all the warts from the tube designs and picked up a few of their own. Notice that I haven't said anything bad about the transistor yet.

Lack of odd harmonic disortion in tubes.


All feedback amps have higher odd harmonics. The feedback only cancels even harmonics. This is true whether it is a tube, transistor, or MOSFET.

A single ended amplifier with no feedback does produce predominantly second harmonic distortion driven just into clipping. Beyond 6dB into clipping the spectrum analysis shows the odd harmonics start to rise and look the same as a feedback amplifier.

Electrolytic capacitors don't sound good. The dielectric absorbs part of the signal passing through. When used as a coupling device the electrolytic gives up this charge when the signal goes through the zero crossing point. That means when it is supposed to be dead silent (the zero crossing point) all the hash comes out. Because of the high voltage in tube circuits they don't have electrolytic caps in the signal path. This is one of the main reasons cheap tube gear from the 50's and 60's sounded better than early solid state gear, they didn't have electrolytic coupling caps. When you hear tube freaks looking for 'Black Beauties', 'Orange Drops', and "Vitamin Qs" they are talking about coupling caps. They also didn't need electrolytic caps in the power supply either.

Electrolytic caps in the power supply have poor transient response. In transistor amps this translates into muddy bass. A choke input filter in a tube amp requires only a small high voltage filter cap. A 170W triode amp I used to own only had a single 9µF filter cap, a 'paper-in-oil' type (another tube freak gaga item). A cheap 2A3 based amp found in a console type hi-fi used a choke input filter and was single ended with no feedback. The first transistor amps were electrolytic coupling cap renditions of high feedback push-pull tube amps. They had the extra warts of electrolytic capped power supplies and electrolytic output coupling caps in additon to their electrolytic input coupling caps.

Remarks by 'homeless' tube freaks about the superiority of their glorified 2A3 juke box amps should be taken in this context. The solid state examples they have given as their 'reference' (B&K and Bryston) use un-bypassed electrolytic caps in their signal path and power supply, not to mention high negative feedback. Adding about $4 worth of film bypass caps in the right spots and $1 worth of emitter de-generation resistors to the diff inputs to reduce open loop gain will totally change the sound of these kind of amps. At this point in time the only affordable way to hear a solid state amp without electrolytic caps and feedback is to DIY. The http://www.passlabs.com/ site has good DIY content.

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john curl

"You people worry too much about superficial things."

"Listening is more important than measuring."

"Absolute THD is not very important to me, (because) it is usually below what most references would consider audible."

"I would prefer a transformer to an IC chip. I once designed out (removed from the design) the best IC chip that I could find.... when it failed critical listening tests."

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  • 2 weeks later...

You should check wikipedia and Nelson Pass website.

"Here is something to get you started.

The input signal goes plus and minus in voltage. This is because sound
in air goes plus and minus in pressure. This is because vibrating
things move in both directions. We use a sine wave as an example.
Generally -1, 0, and +1 in an undulating curve.

2) Amplifers have
the effect of taking the input signal and making it bigger. Power
amplifers (the big box) also supply current to drive the speaker.

The amplifer box has a power supply. It just converts the power out of
the wall socket, which is a sine wave itself at 60 Hertz in the USA,
and converts it to d.c. which is a constant voltage. Like a battery
cell. It supplies the power to the rest of the works, but it doesn't
create power............"

Thank you,Wm McD, for taking the time/effort to post this.

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