Vactrol switching

[Matt T.]

I built a 2-channel (footswitchable) amp years ago that's very loosely based on the SLO. I just based it on the switching I saw somewhere else and fudged it from there...not really knowing what I was doing. I used a total of 4 Vactrols (P/N: VTL5C1) and used the 5v rectifier filament winding for my switching power supply, bridge rectified. All 4 Vactrols are ON/OFF at the same time...all 4 ON (dirty) or all 4 OFF (clean). I've only got about 5vdc at the rectifier...and after I've filtered it/ground referenced it/etc. it's only about 2vdc with all four Vactrols running (this 2vdc is what is connected to all four anodes of the LEDs). The cathodes of all 4 LEDs go to ground through a 22ohm resistor and I'm only getting about 3.5mA current per LED...so I've still got some resistance in the photocell but it looks to be <1K ohms.

Questions:

The amp sounds great...switching seems to work pretty well (there is an extremely quick 'surge' in volume when switching from dirty to clean but it's not really that bad)...so should I even worry that I'm not really running these Vactrols 'hot' enough to get the photcell resistance way down?

Any guesses on why it 'surges' when switching from dirty to clean? FYI...two Vactrols 'add' the dirty channel to the clean channel (one at the input and one at the 'summed' output), one Vactrol shunts the clean channel output to ground, and the fourth one connects the ground of the pre-PI MV (it's 'lifted' in clean mode)...I think it's that last one that is responsible for the surge. Is there an easy way to speed up (or slow down) one Vactrol relative to the others?

Do I even need a resistor between the LED cathodes and ground? It seems that maybe it was for reducing current but I don't think I could burn them up with only 2vdc...the spec sheet for these Vactrols says the minimum photocell resistance (~200ohm) occurs at about 40mA per Vactrol...but even at only 10mA it's still only ~600ohm.

[Ray Ivers]

Matt,

I'm sort of tempted to say "if it works, it works", but as you know, your supply should effortlessly produce >1A at 5VDC or more.

after I've filtered it/ground referenced it/etc. it's only about 2vdc with all four Vactrols running

Can you post a schematic, or a Web link to an identical (or nearly so) circuit?

IMO there's no need to burn those LED's at 40mA, but I would suggest 10 to 15mA or so - also, you really don't want any ripple on the LED DC at all, especially in a high-gain circuit operating at lower LED currents.

A pre-PI MV like in a Marshall 2204 is at a very high-impedance circuit node; you may want to add a dedicated dirty-channel MV, or at least shunt your LDR with a very large-value resistor (you may have to live with having your MV somewhat 'channel-interactive', to what extent I don't know).


Ray

[Matt T.]

Thanks Ray. I know I have the schemo around somewhere but I just can't find it right now...I'll find it when I'm not looking for it.

Circuit is simple enough that I think I can describe it:
Bridge rectifier - series 15 ohm R - filter cap - series 22 ohm R - divider (68 ohm to ground and 68 ohm to LED anodes). The switch just shunts the anodes/voltage to ground...when the switch is closed the voltage at the filter cap node drops just a little...say from 4.9v to 4.6vdc or so. I assumed I needed all these really low resistor values (low impedance circuit?) because the circuit is essentially being grounded...but I'm guessing.

The filter cap has a band around it (to secure it) so I can't read it and I can't remember the value but it's very big...like 1Ku. The amp is extremely quiet even at max gain...spooky quiet actually.

The pre-PI MV is labeled 'Lead Master' because that's how it operates (to the user)...it's functional when in Dirty mode (pot grounded) and pretty much non-functional when in clean mode (pot ground 'lifted'). I'd never seen anyone do it that way and was all proud of myself for coming up with it all by myself...just because it's so simple (I love simple!)*. The series FX loop (which is always in circuit) Return Level pot is labeled 'Master Volume' because that's how it operates (to the user).

* - the series resistance from the 'top' of the pre-PI 'Lead Master' pot when in clean mode doesn't seem to be a problem, I thought about shunting around it with another Vactrol but it didn't seem necessary plus...that was in the direction opposite of 'simple'.

FWIW...I just recently opened this amp up again and I modded it to get to the above condition (both 22ohms were 33 ohms; 68 ohm to ground was 47 ohm)...I gigged with it for years that way at about 1.75mA per LED.

[Don Symes]

I think part of the problem is in the phrase 'ground reference'.

You can safely 'float' the entire switching supply and circuit - not ground ANY of it. If you do that, though, you need a switch jack that doesn't connect to ground either. A TRS (stereo) jack is a good choice - connect your control switch between TIP and RING - leave the SLEEVE terminal open.

Of course your voltage measurements then need to be between that circuits + and - rails. GND doesn't enter into it.

Hope this helps!

[Ray Ivers]

Matt,

It sounds to me like you're happy with it just the way it is - and there's nothing simpler than doing nothing at all.

Ray

[Enzo]

Do I even need a resistor between the LED cathodes and ground? It seems that maybe it was for reducing current but I don't think I could burn them up with only 2vdc...

Remember, LEDs are still just diodes. Imagine youself asking this question: DO I need a resistor to connect this 1N4007 across 2 volts?

SOmething has to soak up the rest of the 2v after the LED takes its 1.2 or whatever it takes. LEDs are damaged by excess current, the voltage is irrelevant with a proper limiting resistor. WIthout the resistor, how much current can the 2v supply provide?