Adding Presence Control?

[sevenisthenumber]

I am wanting t get a grid for adding a presence control to a couple pedals like an 808 and a Suprex from ROG. Any advice on this?

[Mark Hammer]

Depends what you consider to be "presence".

Traditional presence controls on tube amps vary how much negative feedback is applied from the output (post-transformer) back to the driver stage.  The original intent was to keep the amplifier more linear and clean, which is why some of the first instances had them wired "backwards" (fully clockwise was a warmer and cleaner sound).

Can you do that with a solid-state emulation?  Not in the same way, since there is no output transformer or power-tube stage to distort or behave non-linearly.  Can you adjust the amount of top end so that it starts to sound sort of like what a presence control would do?  Yeah, sort of.

Consider the 808/TS9.  Here I'll use the now-classic illustration from GEOFEX.

The tone control provides two reciprocal paths to ground through the .22uf cap and 220R resistor, such that as one of those paths goes lower-resistance, the other goes higher resistance.  In one direction, the resistance of that leg of the tone pot goes lower and lower until eventually the .22uf/220R network is placed in parallel with the other .22uf resistor to lower the corner frequency of the lowpass filter formed by the 1k/.22uf network. 

Rotating in the other direction, that resistance gets higher (reducing the treble cut effect of the .22uf/220R network), but the resistance from the wiper to pin 6 of the op-amp gets lower.  This not only increases the gain of that stage, but changes where the bass is rolled off.  When the control is at full bass, the rolloff of A2 will be at 35hz, and the gain of the tone stage will be just over unity.  When the tone control is at full treble, however, the gain of that stage will be around 5.5 and the bass rolloff will occur around 3.3khz.  So, while you can get a warm tone from the unit, you can't really get top end without having to forfeit bottom.

Could we fix that?  Yes.  Let's say we had a 10k pot, from pin 6 (or its functional equivalent in your particular build), in series with a 330R fixed resistor and a .1uf cap, with the whole thing going to ground.  We are essentially providing another parallel path to ground from the inverting pin of that op-amp.  Note that there will be some interaction between this and the existing tone control, and in some settings the one will over-ride the other.

But let's say the tone control is at its midpoint, with 10k on either side of the wiper.  Not much treble boost, but not that much treble cut either.  The added control, at max resistance (which gives us 10k+330R) won't really provide much that is audible, since it provides a higher resistance path than the tone control does.  As we reduce the resistance of that path, however, we start to add gain to the op-amp stage, and as gain is increased, that gain gets applied more selectively to higher frequencies, until finally at minimum resistance, we have a 330R resistor and .1uf cap to ground, giving a gain of 4x for content above 4.8khz.  Note that this boost will be added to the frequency content set by the current location of the tone control.  I've deliberately set the max gain at 4x, simply because you want a little more sheen or bite, not hiss.

Try it out and let us know if it works as intended.  Again, I caution that the add-on and the stock tone will interact, so don't expect to hear bite if you've set the tone control for full treble cut: you can't boost what you never received.

[Mark Hammer]

And now that I've seen your other thread on the Supreaux, it would appear you want a presence control not to boost but to cut.

To do that, stick a 1k-2k pot, i series with a .068-.1uf cap, in the feedback loop of the tone-stage op-amp.  At full resistance, it will behave as normal.  As the pot resistance descreases, it will cut more and more of the treble off.  When the pot resistance reaches zero ohms, you'll have a rolloff starting around 1.6khz using .1uf and around 2.3khz using .068uf.  It will function independently of the tone control.

[sevenisthenumber]

And now that I've seen your other thread on the Supreaux, it would appear you want a presence control not to boost but to cut.

To do that, stick a 1k-2k pot, i series with a .068-.1uf cap, in the feedback loop of the tone-stage op-amp.  At full resistance, it will behave as normal.  As the pot resistance descreases, it will cut more and more of the treble off.  When the pot resistance reaches zero ohms, you'll have a rolloff starting around 1.6khz using .1uf and around 2.3khz using .068uf.  It will function independently of the tone control.

Nice.. ONly prob is that the Supeaux doesnt have an op amp??

[Mark Hammer]

Take a look at the King of Tone clone over at the Madbean site.  Like the Supreaux's output, it also has two cascaded lowpass section.  The "presence" control on the KoT s simply a pot in series with the second cap to ground to provide adjustable trimming of top end.

[sevenisthenumber]

Take a look at the King of Tone clone over at the Madbean site.  Like the Supreaux's output, it also has two cascaded lowpass section.  The "presence" control on the KoT s simply a pot in series with the second cap to ground to provide adjustable trimming of top end.

Mark, your response is gone?? I was about to try it. Weird...

[sevenisthenumber]

bump? I think our conversation got deleted when the site was acting weird...?

[Mark Hammer]

The Supreaux has two 12k/3n3 pairs to form a 2-pole lowpass filter at around 4khz.  That's reasonably low, but when it comes to lowpass filters, the amount of treble content is a function not only of the corner frequency, but the steepness of the rolloff as well.  With only one lowpass section (a single 12k+3n3), a reasonable amount of treble pass through.  If you stick a variable resistance between the second 3n3 cap and ground, you can adjust the steepness of the filter by reducing the action of the second section.  This will allow finer adjustment of the upper treble.  Try a 25k pot.

[sevenisthenumber]

The Supreaux has two 12k/3n3 pairs to form a 2-pole lowpass filter at around 4khz.  That's reasonably low, but when it comes to lowpass filters, the amount of treble content is a function not only of the corner frequency, but the steepness of the rolloff as well.  With only one lowpass section (a single 12k+3n3), a reasonable amount of treble pass through.  If you stick a variable resistance between the second 3n3 cap and ground, you can adjust the steepness of the filter by reducing the action of the second section.  This will allow finer adjustment of the upper treble.  Try a 25k pot.

So I tried this and there is no audible difference?

[Mark Hammer]

How was the "Tone" control set?  The premise of what I suggested is that there would still be some top end to trim back.  It won't add back anything you've eliminated earlier in the circuit.

[sevenisthenumber]

How was the "Tone" control set?  The premise of what I suggested is that there would still be some top end to trim back.  It won't add back anything you've eliminated earlier in the circuit.

I tried it with all positions of the tone but the presence control is not audible.. Hmmm? I even tried 4 or 5 different pot values...