Does a resonance control really work?

[leadfootdriver]

I seen it as a pot with a .0047 uf cap in line with the presence wire to the OT secondary. Does this really add a big bass to the sound?

[loudthud]

In a tube amp it acts like a bass boost but at the same time it is increasing the output impedance at bass frequencies. The increased output impedance will accentuate bass frequencies where the speaker has an impedance peak.

[leadfootdriver]

With your thread of the 3rd stage of the TW Express, you've obviously analyzed that amp. Do you think that amp could handle a Resonance control?

It's the amp I'd like to add low end to.

[loudthud]

Since the TW Express is known for it's critical layout problems, I would first try adding the pot/cap network on the component board near the feedback resistor or even on the back panel near the speaker jacks. If I thought it brought a wider range of useful tones that I wanted from the amp, then I would drill a hole in the front panel and try it there. One guy I talked to who had experience with a Peavey 5150 amp thought it was a useful feature.

My experience is in bass amps. I thought it gave a little low end texture that you can't get from a bass control alone. It alters the looseness of the low end and you have to adjust your playing technique to make it usefull.

[Chuck H]

Since the Express overdrives the tubes: power tubes, then PI, then preamp, any EQ change from altering the feedback loop gets lost. That is, the presence control doesn't actually add presence anymore. It just increases clipping in the already clipped top end. I expect the same would be true of a resonance control on that amp. It wouldn't add low end. It would just increase clipping in the already clipped low end. Probably not a great circuit for that amp since that would only serve to reduce definition in the low end. On an amp like the 5150 the distortion is generated by the preamp. So the power amp is free to add or subtract via the feedback loop. Very different animals. Unless you run your Express clean. Which sort of defeats the point of that amp. Or worse yet, with a master volume. Which also defeats the point of that amp.

The best way to add bottom end to an Express in by changing the speakers. A 4x12 is the standard cabinet for use with that amp. And a good place to start if you don't have one. A deep 2x12 will do IME provided the speakers have strong bottom end.

[Gaz]

Chuck H is absolutely right. My personal experience has shown this, that a resonance control works wonderfully on a an amp with an emphasis on pre-amp distortion and a pre-PI master volume, but don't do squat on a non-master amp.

[jpfamps]

Additionally I'm always slightly concerned about adding another time constant into the feedback loop of an established design, as you may encounter stability issues.

[Chuck H]

Additionally I'm always slightly concerned about adding another time constant into the feedback loop of an established design, as you may encounter stability issues.

Which the Express design already has. Even KF built units often suffer instability when too much top end EQ, presence and gain is employed.

[leadfootdriver]

My amp is solid.

I tried a 1M/.0047 and the amp responded well. I even hardwired the resistor into the feedback line for a bit! But now that the honeymoon is over, I'll go back to the pot config for more subtlety and versatility.

'That said, I've been studying any schematic I could find with a Resonance control, and have found that the majority of them had a higher value cap in the line. I'd like to try a .01uf which should pass only the very low bass, accenting the thump of the cab, while keeping the rest of the lows tight.

[Enzo]

It's not the parts, it's the circuit. You can't just ask if a .01uf is too much or too little, nor can we say a 10k feednack resistor is too much or too little. It all depends upon the circuit they find themselves in. These NFB circuits take the output signal and feed a portion of it back to the earlier stages of the amp. To make up examples, whatever the output signal voltage is is sampled through a couple resistors forming a voltage divider. SO am 820 ohm feedback resistor feeding into a 47 ohm resistor to ground is roughly a 20/1 voltage divider. But so is 8.2k into 470 ohm. So when looking at these circuits, don;t focus in on just a cap, look at it in the context of the whole feedback circuit.

[leadfootdriver]

It's not the parts, it's the circuit. You can't just ask if a .01uf is too much or too little, nor can we say a 10k feednack resistor is too much or too little. It all depends upon the circuit they find themselves in. These NFB circuits take the output signal and feed a portion of it back to the earlier stages of the amp. To make up examples, whatever the output signal voltage is is sampled through a couple resistors forming a voltage divider. SO am 820 ohm feedback resistor feeding into a 47 ohm resistor to ground is roughly a 20/1 voltage divider. But so is 8.2k into 470 ohm. So when looking at these circuits, don;t focus in on just a cap, look at it in the context of the whole feedback circuit.

I realize the OT tap and NFB resistor is part of the total level of feedback. But, isn't the cap just a passive filter that bypasses freqs? Wouldn't said cap bypass the same freqs no matter what level of NFB was designed into the amp?

[Chuck H]

I realize the OT tap and NFB resistor is part of the total level of feedback. But, isn't the cap just a passive filter that bypasses freqs? Wouldn't said cap bypass the same freqs no matter what level of NFB was designed into the amp?

Not even close. The knee frequency of a cap changes with circuit impedance. Example: A typical Marshall uses a 100k/4.7k feedback divider (21.27 to 1 ratio) and a .1uf cap for the presence control. I have an amp design that uses a 22k/1k divider (22 to 1 ratio) and I need to use a .47uf cap to affect the same frequencies as the .1uf cap in the Marshall circuit. Sorry I can't tell you the math on this but as you can see it's almost a straight inversion. My divider is about 1/5 the Marshall value and my cap is about 5X the Marshall value.

[leadfootdriver]

Why can't u tell me the math?

So, the Express with a 100k/5k fd, has a ratio of 22:1. So what set's the impedance? The resistor (100k)?

Someone must have a calculator program that shows what the cap will affect.

[imaradiostar]

The feedback ratio isn't based on the values of the resistors, it's based on the open loop gain of the output section (PI and power tubes) with NO feedback then the resistor math is done based on the amount fed back from the output. I don't know the math off the top of my head but I'm sure someone does! Say the amp makes 10 volts at the output with one volt into the phase inverter. By feeding a portion back via resistive divider and converting to a logarithm you know how much feedback you have in dB. Yes, that's an oversimplification!

The interaction of a cap and resistor is generally determined by F=1/(2piRC) where R and C are the resistance in Ohms and capacitance in Farads and F is -3db frequency (or corner frequency) in Hertz.

jamie

[Chuck H]

Why can't u tell me the math?

'Cause I ain't learned so goodly in electronical stuff.

I don't know the nomenclature or formula structure for the equations so I can't read them. Something I should work on because though my math skills are undeveloped my math conception is very high (according to tests). Never went to college. Hated math in school.

So what set's the impedance?

The shunt resistor in the feedback voltage divider (in the 100k/4.7k example it would be the 4.7k)

Someone must have a calculator program that shows what the cap will affect.

I use several on line simulators and pre configured calculators. Duncan TSC is great and can be reconfigured to show all kinds of circuits other than tone stacks. Get it here:TSC

Another simple but versitile calculator:Combinations of Resistors, Inductors and Capacitors