distortion with Class B (with NFB) vs Class AB

I had a feeling that was part of my "homework" to make it "less horrible."

Michael

e that.)

l-wart-powered class B, or AB, or boosted-opamp thingie, and would like to know *why* it works, too (so that kind of defeats the purpose of chipamps).

after taking the electronics class at Davis. Took me a long time to like electronics again after that.

I'm sure there are some good ones, but the ENG 17 class was all theory and nothing fun. Even worse, the professor sold us at the bookstore a photocop y of ALL her lecture slides, and I found myself yawning during class becaus e there was nothing to write.

One notable thing: she told us the day before an exam that a student oncee drew a picture of a Star Trek phasor when she asked for a "phasor diagram," saying "this one is set to stun," "this one is set to kill" and threatene d to fail anyone who did something like that again.

It wasn't until my brother-in-law repaired the car stereo (that *his* broth er had installed with the polarity reversed) that I recovered my interest i n electronics. He de-soldered the chip amp and soldered in a new one.

Michael

You might want to have a look at the print for the Phase Linear 400 Series Two.

They specify the bias as a voltage across R128, which is a 10 ohm resistor across the bases and the commons of the emitter resistors of the outputs. I t specifies 0.35 to 0.40 volts.

It seems that the intention is to actually have the drivers drive the load at very small power levels. The outputs would not kick in until you have ab out another tenth of a volt or so, across that ten ohm resistor.

Yet the thing seems to meet distortion specs, or did. I don't have the equi pmnt for that but of course it was tested when they were new and for sale. Right ?

The thing must have a quite high open loop gain, because I don't see how th is could be linear. Obviously the feedback is taking care of it. With a rat ed damping factor of 1,000, it's more than just having ten ohm shunt resist ors on the outputs.

If you play with the bias of an amp whie watching the waveform coming off t he voltage amp, you will see the crossover distorion, well not really, you are seeing the correction applied by the feedback to attempt to eliminate t he crossover distortion.

Another thing that didn't take off but could be a viable option is high fre quency AC bias. It may actually be more efficient. Keep it well away from t he audible range and that little output choke will eliminate it, yet it sti ll varies the quiescent operating point enough to get rid of any crossover distortion. It probably doesn't really need to thermally tracxk the outputs ' temperature either, as long as no significant load is presented to it at that frequency.

s Two.

r across the bases and the commons of the emitter resistors of the outputs. It specifies 0.35 to 0.40 volts.

d at very small power levels. The outputs would not kick in until you have about another tenth of a volt or so, across that ten ohm resistor.

uipmnt for that but of course it was tested when they were new and for sale . Right ?

this could be linear. Obviously the feedback is taking care of it. With a r ated damping factor of 1,000, it's more than just having ten ohm shunt resi stors on the outputs.

the voltage amp, you will see the crossover distorion, well not really, yo u are seeing the correction applied by the feedback to attempt to eliminate the crossover distortion.

requency AC bias. It may actually be more efficient. Keep it well away from the audible range and that little output choke will eliminate it, yet it s till varies the quiescent operating point enough to get rid of any crossove r distortion. It probably doesn't really need to thermally tracxk the outpu ts' temperature either, as long as no significant load is presented to it a t that frequency.

Oh, wow, a complete schematic included with the manual? :D

Impressive THD and IMD specs!

As John Larkin said, build some small projects first... and only move up when none of those "flame out" ;-) ...Jim Thompson

I think that manual is in error. The THD was stated at 0.09 %, not 0.009 %.

I doubt that particular circuit is capable of getting it that low.

Of course rare is the ear as well as the source material that enables one to hear 0.09 % THD.

Scans of th original manual are at BAMA.

ries Two.

stor across the bases and the commons of the emitter resistors of the outpu ts. It specifies 0.35 to 0.40 volts.

load at very small power levels. The outputs would not kick in until you ha ve about another tenth of a volt or so, across that ten ohm resistor.

equipmnt for that but of course it was tested when they were new and for s ale. Right ?

ow this could be linear. Obviously the feedback is taking care of it. With a rated damping factor of 1,000, it's more than just having ten ohm shunt r esistors on the outputs.

off the voltage amp, you will see the crossover distorion, well not really, you are seeing the correction applied by the feedback to attempt to elimin ate the crossover distortion.

h frequency AC bias. It may actually be more efficient. Keep it well away f rom the audible range and that little output choke will eliminate it, yet i t still varies the quiescent operating point enough to get rid of any cross over distortion. It probably doesn't really need to thermally tracxk the ou tputs' temperature either, as long as no significant load is presented to i t at that frequency.

struction-manual.pdf

Yes, I plan to! I'm encouraged that reasonable volume can be made from jus t the 5V coming out of USB...

And then bias the transistors on some.

out of USB... "

Some people would be surprised at how loud just one watt is. Take whatever amp you got and put a meter on it, on an eight ohm speaker run it up to whe re there is 2.83 volts read on the meter going into the speaker.

Pretty loud actually, louder than alot of people listen to music or the TV, whatever.

Then take it to double the voltage which will be four times the wattage, an d see how much louder it ISN'T.

This is why crossover distortion needs to be addressed. Things are a bit di fferent down in the milliwatts. Really, alot of people listen to things at like a quarter watt or so. Down there is where you can really hear crossove r distortion. Crank it up to ten watts and it'll be clear as a bell, kinda. The distortion is still there but it is only a small part of the output.

Let's not even go into what an acoustic watt is. Levels like that'll blow y ou right out of there. I forgot what it is, but an acoustic watt is LOUD.

No kidding. Guitarists seem especially bad about this. I'm constantly hearing guys insist they need these high powered tube amps (pushing two, four or eight 12" speakers) or similar, and yet they'll probably rattle their teeth loose if they turn it up past 3. A lot of other things factor into SPL (speaker, cabinet, what freqs are emphasized, etc) but my main amplifier (Laney ProTube AOR) is rated 22W RMS into the speaker. It is PLENTY. I had a 50W version of the same amp and it was ungodly loud- too loud (and heavy) for me to use on a regular basis. I can't imagine how deafening it was in the 1970s when touring musicians played in front of a wall of 100W Marshall JTMs going full tilt. Amazing sound but those volume levels were absolute madness I'm sure. Madness I say!

Meanwhile, I have a pile of LM386 chipamps that I practice with regularly. I think a couple are 250mW and one is 500mW. They're quite adequate. I've made a few for some guitar buddies and they're always amazed at how loud they are for being so low-powered.

-J

ming out of USB..."

ver amp you got and put a meter on it, on an eight ohm speaker run it up to where there is 2.83 volts read on the meter going into the speaker.

TV, whatever.

, and see how much louder it ISN'T.

t different down in the milliwatts. Really, alot of people listen to things at like a quarter watt or so. Down there is where you can really hear cros sover distortion. Crank it up to ten watts and it'll be clear as a bell, ki nda. The distortion is still there but it is only a small part of the outpu t.

ow you right out of there. I forgot what it is, but an acoustic watt is LOU D.

Oh! How many watts are typically required for a rock concert?

What ever it takes to get those rocks a flying!

Jamie

On 5/27/2014 1:01 AM, snipped-for-privacy@gmail.com wrote: >> >> -J > > > Oh! How many watts are typically required for a rock concert? > > M >

All of them!

As I'm sure you know the modern-ish (and better) solution is miking the amp at practice volume and sending it through the PA to fill the room. Even clubs and small shows do this for a better, more even sound. The last couple of times I saw Dick Dale there weren't any amps on stage at all, he was connected to some hidden backline via wireless.

I don't know why they didn't figure that out long ago, when you had to hit the back of the stadium with stage volume.

-J

Actually none.

Like this?

:D

Nothing like cute girls who listen to heavy rock. I'm a bit concerned about the THD of that rug it's sitting on, though.

-J

She looks doable...

As for the THD, to eliminate that, glue it to the floor.

If she protests your advances, glue her to the floor as well.

Problem solved. And don't start bitching about that, some girls really like it.

Just hope for the best, if not, see you in twenty years or whatever.

Better yet, cute girls who play heavy metal.

Yum!