Damping Factor >1000

[Steven]

Originally posted by PRR
[B>... The other "minimum" is HIGH current. It actually degrades a hair when you get into Class A: the two sides fight each other. But in the range far above the usual 28mV but below Class A, there is a broad stable minimum.
[/B]

PRR,

I agree that 20-30mV across Re is a narrow optimum that is difficult to keep stable in practice. Turning up the idle current at least moves the point where Gm is halved away. So for small signals Zout is quite stable. But, I'm a little surprised that your simulations shows that it degrades when you get into class A. I would assume that as long as you are in class A without one of the transistors starving for current the output impedance would be mainly determined by Re//Re, which is very stable.
Actually, I don't understand what you mean with 'below Class A'; as long as the amplifier is idling, we are in the class A region.

Steven

PS I'm currently busy with simulations for the 'Spreading the heat in Class A' case. I changed the circuit such that I don't have jumps on the collector of Q1 anymore that invalidates the Class A benefits. I will post the answer this weekend in that thread.

[PRR]

> as long as the amplifier is idling, we are in the class A region.

Point taken about idling (though like the tree in the forest, can we talk of Class when there is no signal?)

I mean: idling at a very high current that allows full-power Class A operation.

Let me check some more things. Unless my pants catch fire, I probably won't post over the weekend (56K modem, new definition of "poverty").

[DrG]

I've been away a bit... I don't know if I agree with the following, PRR:

Velocity is proportional to voltage, and for cone speakers the acoustic output is mostly a function of velocity, not force.

The relationship between applied voltage and flowing current is not linear in a speaker. This being so, it is *only* the current in the coil which generates flux and thence force. The voltage which caused the current is irrelevant.

Acoustic output is not a function of cone velocity per se. It is a function of cone displacement which is proportional to net force, which is determined by coil current, which results in a net acceleration a=F/m. At no time does the oscillating diaphragm possess velocity in the constant sense. Unless you mean angular velocity...

And impedance characteristics varying as they do wrt frequency, I repeat my perception that current drive of a speaker appears more natural/logical...

[millwood]

Originally posted by DrG
current drive of a speaker appears more natural/logical...

agreed. But why is that there is few current drive amps out on the market?

I remember that doing similar things (using a current sample resistor in serial to the speaker coil as feedback) in tube days but it seems to have died out (I tried it and other than tighter bass it did nothing audiable).

I guess is that the differernce between a current vs. voltage drive amp is minimal, at best.

[DrG]

agreed. But why is that there is few current drive amps out on the market?

I don't know...

I guess is that the differernce between a current vs. voltage drive amp is minimal, at best.

Perhaps. But I'm not sure and I'd like to hear from the brains trust out there what the concensus is. Anybody tried it?

I saw a little amp some years back incorporating the speaker into the feedback loop, with a small 0R22 shunt resistor. What are the drawbacks of such an arrangement?

[Tube_Dude]

Hi

See some exemples of current amplifiers (transconductance):

http://www.ultrasound-hifi.com/Us_wh..._frame_en.html

Cheers

[millwood]

Originally posted by DrG
I saw a little amp some years back incorporating the speaker into the feedback loop, with a small 0R22 shunt resistor. What are the drawbacks of such an arrangement?

it is exactly what I was talking about from my tube days. I don't know of any drawbacks but I do know that it doesn't do a whole lot of good either.

So the question is "why bother?".

[DrG]

So the question is "why bother?".

The answer is "why not!" Seriously though, my personal approach to everything audio is to try to be as unconventional as possible. I figure the chances of creating something really special are greater when choosing the road less travelled by. And admittedly there is also an ego thing... I did it MY way etc. Each to his own.

Thanks for the link Tube_Dude

[jcarr]

>I saw an amp incorporating the speaker into the feedback loop.<

The remote feedback system was invented by Shin Nakagawa, and was subsequently used in consumer audio amplifiers from the likes of Kenwood and Toshiba.

>What are the drawbacks of such an arrangement?<

The feedback loop will now include substantially more reactive elements than if it had been kept within the amplifier only, which can lead to a variety of stability and recovery issues. The phase compensation for a remote feedback amplifier will need to be fairly heavy-handed to keep the amplifier under control.

Good approach for a self-powered subwoofer, but if you want a good-sounding full-range power amplifier, I think that there are better ways than the remote feedback approach.

>my personal approach to everything audio is to try to be as unconventional as possible.<

Unless it makes you fee better about yourself, I don't think that it means all that much to be unconventional or conventional. Rather, I suggest that you do whatever is appropriate for the task at hand. If there is a specific engineering or performance goal that you are aiming for, and if you cook up an original and unusual method to achieve that goal, fine.

But if not, I don't see the point in being different just to be different.

regards, jonathan carr

[DrG]

I don't see the point in being different just to be different.

You needn't. Each to his own, like I said. And you're right, I do feel better about myself when I achieve something via an alternate route. "It works well and I did it my way" does more for my ego that "It works well because I copied someone else's established method." But that's just me.

Jonathan, I admit a problem-oriented approach using the best circuit element for each situation is probably best from an engineering approach. But I'm not a commercial audio engineer.