Adding mixed mode feedback to any amp..is it simple?

[dschwartz]

Hi all:
I´m wondering if one could put current feedback to any amp by just putting a resistor between the - side of the speaker and ground, and other resistor between the - side of the speaker and the  inverting side of the driver?
For example, if i just add that to a LM3886 or TDA2394 amp, it will work without exploding on fire and blood?

[PRR]

It can be that simple.

Such tricks are treacherous. The '3886 probably won't explode, but your speaker may break out in a 56 Watt howl. Don't test it while the baby is asleep.

Remember that no-load it is completely unstable, no DC feedback.

[dschwartz]

the other option could be putting a high impedance frequency response simulator before a very linear amp. Like some marshall amps. But just adding a couple of resistors seems a lot simpler..

[PRR]

Fender Frontman 15 has current feedback. Shorn of that messy output switching, power and protection, it is this:



Speaker voltage is sensed by R39 100K and the parallel R38 R42 resistors, voltage gain about 238.

Speaker current is sensed by the two 0.68r (0.34r) resistors into the 2:1 divider of the two 820r resistors. If the speaker is 8 ohms, voltage gain is 49.

Therefore voltage sags to 0.2 from infinite load to 8 ohms, and the output impedance is roughly 32 ohms.

C23 C24 shift things around a bit at the bottom of the passband, 40Hz-100Hz.

This is probably meant to approximate the impedance of an old soggy 6V6 and undersized transformer, the classic Fender Tone which was largely lost in 1970's rush to heavy NFB and transistors.

This will allow most of the impedance curve to be "current driven", while still maintaining a little damping at the speaker's bass resonance peak. Unless your speaker is grossly oversized or damped some other way, you probably want to stay a little short of "infinite impedance" to reduce bass-slap.

You've been reading Nelson Pass, right? Is his forum still up and active? The gang there may know some about how to get a practical amplifier.

[R.G.]

http://sound.westhost.com/project56.htm
I have run a TDA2030 in mixed mode. It worked, in that it amplified and produced no flames or sparks, but the sound did not cause an epiphany.

[dschwartz]

i´ve been reading teemuk´s book about SS guitar amps (very good!!) and here and there (ESP and so)..
Thanks for the schem, very interesting!! a TDA2050 for a 15W amplifier? is the current feedback stealing most of the power? or is just conservative power rating?

[dschwartz]

http://sound.westhost.com/project56.htm
I have run a TDA2030 in mixed mode. It worked, in that it amplified and produced no flames or sparks, but the sound did not cause an epiphany.

Thanks RG, i read that, in fact it was that page that got me into writing this thread..