Presence control - set up

[überfuzz]

I'm going to try and set up a presence control, in my tinker amplifier. I think I'll try some Fenderish set up with a presence pot to ground with a capacitor to ground from the wiper.



I only have a vague idea of what values to start with, in hope of stumbling upon something wonderful. So first of all, are there other presence control circuits I should know about, possibly opt for instead of this one. Secondly, how do I chose starting values in my search for a nice presence control. The values in the attached picture are just nicked from a Fender schematic.

[überfuzz]

I tinkered a bit with presence and NFB now. I can see myself adjusting the presence control every now and then while using the amp. The NFB control as it is now. Meh... I have to keep on messing with it to see whether I can get a useful set up.

[Dave H]

So first of all, are there other presence control circuits I should know about, possibly opt for instead of this one. Secondly, how do I chose starting values in my search for a nice presence control. The values in the attached picture are just nicked from a Fender schematic.

There's the circuit which has the cap and a 25k pot in series with the combination in parallel with the 4k7 resistor. It has nearly the same effect as your circuit but without the scratchyness.

A 0.2u cap and the Presence on full (1) will start to have an effect at 170Hz. That's possibly a little low I'd try a 0.1u cap.

[Dave H]

The NFB control as it is now. Meh... I have to keep on messing with it to see whether I can get a useful set up.

You have the maximum value of the feedback resistor set at 1000k. The amplifier doesn't have huge open loop gain like an op-amp does so it will be almost open loop for values much over 100k. I'd set the max value to be something like 100k to 200k to see if that gives the control a more useful range.

[Chuck H]

If you liked the amp already without a presence control, a presence control should be done on the existing feedback loop or something reworked to be essentially the same as the existing loop. If your amp doesn't have a feedback loop now you will need to change the basic tone of the amp to add one. So... Did your amp have a feedback loop? And if yes, what were the circuit values?

[überfuzz]

In the final, well at least work in progress, version I opted for component values that gave me the same cut frequencies as when the negative feed back was fixed. I don't have any schematic to post as I just calculated values on the fly. Without the amp here I seem to remember opting for parts giving me a high pass filter with a cut frequency of 735 Hz, or was it 725 Hz. Well in that ballpark... I've played the amp a bit now and I like the presence control as it is now, so I'll leave it for now.

The NFB dropping potentiometer however needs more attention. Thanks for the advises. I thought the 1M pot would give me a BIG range for which I could find a, lets call it, sweet range. I don't know about that. I have to recheck the circuit to see whether I did something funky, I guess.

[Dave H]

I thought the 1M pot would give me a BIG range for which I could find a, lets call it, sweet range. I don't know about that. I have to recheck the circuit to see whether I did something funky, I guess.

A 1M feedback resistor is attempting to set a gain of about 1000 [(1000+4.7)/4.7] which would be fine for an op-amp with an open loop gain (gain without feedback) of 1000000 but this circuit only has a gain of about 30 without feedback so even if you try to set it at gain 1000 it will only have a gain of 30 because that's all the gain it has. That's why I suggested a lower max value for the feedback resistor. A 1M pot will have all the useful variation cramped at one end of its rotation.

[überfuzz]

I've been messing about with the NFB-control now, a bit. I can't get any evident, audible, difference. I suspect I have to push the phase inverter harder to get some sort of dampening happening as a result of the NFB-control.

[Chuck H]

Can you post a current diagram of your circuit?

[überfuzz]

Yes , here it is.

[Chuck H]

Should I assume that the split resistors in the circuit are either switches or pots of odd values? The schem shows (for example) "pres 1" and "pres 2" but there is no switch present.

The first thing I notice is that your NFB loop is at 10:1 and inserted at the 50% point on the PI tail. That's a huge amount of NFB. And the presence capacitor, at .2(uf?) across an 11k or 22k load is only going to audibly affect frequencies above 10k!

[Enzo]

The way I see it, look at the tone controls and volume, they are drawn the same. I suspect that just represents pots set to mid point - this circuit is for simulations.

[Chuck H]

The way I see it, look at the tone controls and volume, they are drawn the same. I suspect that just represents pots set to mid point - this circuit is for simulations.

I guessed the same for the same reason. But 22k and 47k aren't typical DIY pot values. So I thought I'd ask. Part two still applies.

[Enzo]

No, but they are standard resistor values. A 50k pot becomes a 47k resistor in the sim.

[überfuzz]

Well I bought a bunch of pots from a local vendor, among them there are values like 22k and 47k pots. There is third party potentiometer model suitable for spice-ware, but I just vent with resistors, e.g. Pres1 and Pres 2.

I guess you nailed the problem with the NFB + phase inverter. However, I didn't quite understand your input Chuck. Is it possible to break it down further?