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Re: Resonance control, anyone?

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7/5/1999 12:48 PM

Re: Resonance control, anyone?

I've seen a circuit like this (variable damping factor) in one book, I think it's the RDH.

You put a resistor in series with the speaker, and use a variable negative feedback loop around it. This loop is independent from the main feedback loop of the amp. When you increase the amount of feedback applied the effect of the resistor is reduced and you've got more damping.

7/5/1999 4:22 PM

If a the feedback voltage is developed across a series output element, current feedback is developed, as opposed to voltage feedback, which is shunt derived. Negative current feedback increases output impedance, while negative voltage feedback decreases output impedance.

Since the damping factor of an amplifier is the ratio of the load impedance to the actual output impedance of the amplifier, it will change with changes in the output impedance. High levels of negative voltage feedback will result in a high damping factor ("tight" sound - amplifier puts out a relative constant voltage, independent of speaker interaction), and high levels of negative current feedback will result in a low damping factor ("loose" sound - amplifier acts more like a constant current source, and interacts more with the non-linear speaker impedance).

It is possible to combine both negative voltage feedback and negative current feedback in the same amplifier, and make one of the two variable. This allows you to have a continuously adjustable damping factor.

BTW, current feedback is what is used in the Marshall Valvestate, Rocktron, and other solid-state power amps to make them sound more "tube-like", by lowering the normally high damping factor of the solid-state output stage.

Randall Aiken

7/5/1999 3:39 PM

Steve;

I did finially get'er to work. Here's what I did (I don't know if it's right or not! ha, ha) but here goes...

1.) looking at the pot with shaft facing you;

pin#1 = the input (far left)

pin#2 = the output (center)

pin#3 = ground (far right)

2.) Grab either a .001 or .002 cap and solder it to pins #1 & #3. NOTE: .001's seem to work better with Fenders where .002's seem to work better in Marshalls or other high-gain amps but that's just my findings.

3.) Sodler a jumper wire to pin #2 & #3

4.) Remove the feedback wire from the speaker jack and connect it to pin #1.

5.) Solder a new wire to pin #3 and connect it the smae speaker jack that the feedback wire was originally connected to.

That's all there is to it really. Again, Mark Cameron gave this mod to me and I wasn't really sure what the pot value was supposed to be so I messed around and I found a 50K Liner taper to be pretty hip for this. I've used a 1Meg and it works but it mostly depends on the amp it's going on. A 50K seems to work on all amps without a problem.

Also you can vary the cap value and nail down certain frequencies. Anything from a .001 to a .006 will work jujt fine. The .006 gets into the lower mids and doen't really tighten up the bass at all but it that's the desired effect then run with it! (ha, ha)

So anyways that what works for me. I hope this works for someone else!

Trace

7/5/1999 6:10 PM
Niek	Peavey uses a 1MA pot in parallel with a 6.8nF cap on their 5150 amp for the resonance control. Niek

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