variable damping factor?

[Tube_Dude]

Originally posted by sreten


Hi,

To be more correct they had variable gain via varying the amount
of feedback. The lower the gain the lower the output impedance.

/sreten.

Hi Sreten

Not so. They use a mix of voltage feedback and current feedback .

The current feedback come from a small resistor in series with the load.

Using a pot , they change the feedback to the input stage , from the normal low output impedance obtained by voltage feedback, to the high impedance of current feedback .

A classic example.

http://www.audiofanatic.it/Schemi/Ti...tkitW6M_PP.jpg

[forr]

Hi,
In tube amps, there was the old trick of positive current feedback with which you can get more than infinite damping factor, a negative output impedance. This is mentionned in Thiele's JAES paper about bass-reflex loading but there were number of papers about this fascinating technique before, for example by Werner Clements.
You just need three resistors to implement it in a solid state amplifier, by bootstraping the input. It can be useful if you need to obtain a lower Qt for a speaker.

[Johan Potgieter]

Ye-e-e-s,

But that method is fraught with traps. The moment you use that sort of positive feedback it becomes dependant on the load current (magnitude and phase), which is different from a simple 'independant" low df topology.

Combined positive and negative feedback in tube amplifiers were used from an early stage (early 50s, e.g. the Connoisseur amplifier), but sometimes caused havoc because of the above fact. The advantage was that it could completely cancel the loudpseaker dc resistance (that was in reality the only way to achieve high "real" df.), but the sensing network ideally had to be the converse of the loudspeaker impedance. In that sense it was best done as a committed circuit for a particular loudspeaker. A very practical application could however be to apply over a limited bandwidth, say from low up to 200 Hz. This did audible improvement for low frequency response. (It was quite exciting to put a finger to the cone of a loudspeaker driven thus - one could feel the "effort" of the cone increasing as one tried to limit its amplitude.)

Regards.

[unclejed613]

actually, with negative feedback, the voltage across that 5.5r resistance gets "opposed" actively by the amplifier. with my second model, it did not matter what the open loop output impedance of the amp was. once there was enough feedback, the amp maintained a constant .01r or lower output impedance, effectively cancelling any (except for a very small, about 0.008v) applied voltage at the output terminals. even with 1r source resistors...... it's basically applied feedback theory, the amp will do WHATEVER it takes to maintain the inverting and noninverting inputs at the same voltage, even if the error is supplied to the output by source external to the amplifier, it is still treated as a feedback error, just like distortion and other errors, and cancelled out by the amp maintaining the input balance.

[Johan Potgieter]

Unclejed613,

If I understand you correctly, yes I agree with the operation of nfb, but it cannot cancel what it cannot see. To put it better, let us think in terms of current. Let us accept that the amplifier has zero output impedance by whatever method. The loudspeaker is "braked" (damped) by the current in the circuit as a result of back emf. This current is still limited by the total resistance in the circuit. If the loudspeaker voice coil resistance is 5 ohm, that will be the current limiting element. If the voice coil resistance could be made 2,5 ohms, twice the current would flow and the damping would be better. This has nothing to do with the amplifier.

For the purpose of this argument the output device equivalent circuit is a voltage generator in series with a resistor. The amplifier cannot correct for a voltage across its output terminals that does not exist, because the output impedance is zero. It is equivalent to testing a battery for the short-circuit current, by shorting the terminals with, let as assume, a perfect short. Smaller batteries with higher internal resistance will give a lower current than low resistance batteries. As said, if I understood you correctly.

Regards.

[forr]

Johan Potgieter
---The advantage was that it could completely cancel the loudpseaker dc resistance (that was in reality the only way to achieve high "real" df.), but the sensing network ideally had to be the converse of the loudspeaker impedance.---

Completely removing the DC resistance would make the loudspeaker having a Q of 0, behaving like a differentiator, with an ascending slope of 6 dB/o towards the high frequencies.

A problem with a high value of negative resonance is that the inductance of the voice coil, Le, makes a resonant LC circuit with the Ces of the motionnal impedance. The inductance of the voice-coil having not a stable value, it is quite difficult to find an optimal value of negative impedance which will nullify it.

Short simulations have shown me that for a negative resistance equal to half of the DC resistance of the voice coil, Re, there is nothing needing to be compensated for, and that Le should be compensated for values of negative resistances higher than 2/3 Re.

As these values are easy to implement, I find that negative resistance is a useful tool to divide the Qe of a driver by 2 or 3 to suit particular needs.

[Johan Potgieter]

Indeed, forr

The "cancel the dc resistance" was over-simplification, more to convey a concept. That is why I included that reality would have to include loudspeaker impedance. Thanks for the other information. My own experience with this was years ago, and I would not have been able to supply the information you just did.

Regards

[unclejed613]

i am not looking for a NIC. too much opportunity to "let the smoke out". just a way of controlling damping factor without a lot of difficulty. i think the best way to go here will probably be adding resistance between amp and speakers, since controlling it electronically will introduce distortion. i thought of using a MOSFET controlled by DC bias, but that could be a source of distortion as well.

[sakevich]

Such amplifiers already are - http://sakevich.ru/eng_german/eng1200/1200eng.htm

"Reference power amplifier with the variable output resistance SK1200 Studio3"