Adding a volume control to my filter

[Halion]

Hi guys,

Few days ago my Electro Hamonix Mini Q-tron came in, and I love it! Fatness galore, however, it gives me a slight volume boost as well, which I don't want. There's no volume control, and I can't roll back the volume on my bassguitar (I use it mostly for bass) because that would make a filter effect less pronounced as well. Is there an easy way for me to add a volume control to the unit? I only need to be able to lower the volume, and there's plenty of space in the unit.

Thankx in advance,

Tim

[gez]

I'm assuming it's true bypass and uses a stomp switch?  If so, trace the output jack wires back to the board.  The one connected to a cap will probably connect to a resistor to ground too.  You can disconnect this resistor, insert another in between to form a divider and take the output from the junction of the two resistors.  

A better option would be to replace the (pulldown) resistor with a pot (one outer lug to output cap, one outer lug to ground) and take the output from its wiper.

[Halion]

Yes it is true bypass (with a 3PDT switch). Would a 100k log potmeter do? And I just leave the middle lug unconnected to anything? (I'm not that good with electronics, but I can solder a bit).

[gez]

Yes it is true bypass (with a 3PDT switch). Would a 100k log potmeter do? And I just leave the middle lug unconnected to anything? (I'm not that good with electronics, but I can solder a bit).

Assuming they've used an op-amp/buffer at the output then a 100k would be fine.  The output cap attaches to one outer lug, the other outer lug to ground and the middle lug (the wiper) is the output (goes to jack).  If there's a resistor from output cap to ground (pulldown resistor) then get rid of it.

[Halion]

Ok, it's not working. I probably did something wrong but here's what I tried:

Outer lug 1 to ground: done
Outer lug 2 to Cap: done, however, both the cap and the lug are now connected to ground.
Inner lug to output: done

When the effect is on, there is signal at all. When it's bypassed, I can control the volume with the potmeter.

I couldn't find the pulldown resistor (there is a resistor running from one leg of the output jack to the ground leg of the output jack, I snipped it and my bypassed signal was gone, so I reconnected it). There seems to be a resister connected to ground, I snipped that one out to, which made the volume control a bit better (nicer slopen), however, still no effect and the volume control controles the bypassed signal, which I just want to leave alone. Any thoughts on what I'm doing wrong?

[gez]

Outer lug 1 to ground: done
Outer lug 2 to Cap: done, however, both the cap and the lug are now connected to ground.
Inner lug to output: done

When the effect is on, there is signal at all. When it's bypassed, I can control the volume with the potmeter

:?:   How have you managed to ground the output cap?  Are you sure you've got the right cap? (should connect to the output jack) Have you connected the wiper to one of the outer lugs by mistake?  The lead which was coming off the output cap should now connect to the wiper (middle lug).  The wiper shouldn't connect to anything else and your output cap shouldn't connect to ground! (you're just shunting the signal to ground if you do this).  

I couldn't find the pulldown resistor (there is a resistor running from one leg of the output jack to the ground leg of the output jack, I snipped it and my bypassed signal was gone, so I reconnected it). There seems to be a resister connected to ground, I snipped that one out to, which made the volume control a bit better (nicer slopen), however, still no effect and the volume control controles the bypassed signal, which I just want to leave alone. Any thoughts on what I'm doing wrong?

Seeing as we're working blind here...er, so to speak, it's a bit difficult to tell what's what.  Follow the leads/traces from the output cap till you find a resistor that connects across the output, i.e. from the output cap to ground.  This is the resistor that you want to get rid of.  If the output impedance is nice and low (buffered output) then it won't be a big deal to leave it in.  Lets just concentrate on getting the vol pot working!  

[gez]

Er, you are taking the wire coming off the pot's wiper to the switch first aren't you, and not just connecting it to the output jack?

When I said follow the output jack back to the board, I meant via the switch.  It's the wire that connected from the switch to the output cap that should now connect to the wiper.

[Halion]

doh! heh, well like I said, I can do it, I just won't get it ;-)

I'll try that another day, I figured that won't be a problem anymore once I make may bassguitar active (I have heard that for some people, the output is actually lower than the bypassed signal, so I guess providing a higher signal would solve the problem).

Thanks for that help.

[puretube]

the pot should be hooked up between the mother-contact of the HP/BP/LP switch, and R13:
desolder R13 from the PCB on the side where it goes to the switch;
solder one outer lug of the pot to the mother-contact of the switch;
wiper to the loosened end of R13;
other outer lug to ground;

leave everything else like it was from factory;
rather use a 250k linear pot;

If the rotation-sense is wrong (inversed) swap the 2 outer lug connexions.

> if you use an active bass, you probably overdrive the thing and get lots of distortion <

[Halion]

Now that i understand thanks a bunch, will try that soon.

I don't think an active bass will overdrive it, do you have know for a fact that it will?

[puretube]

just think so because of the fact that I heard,
that some active basses can put out
about 10 times higher volumes than guitar pickups
(~ 7.5V),
while guitar FX are designed for (very roughly said:) 100 to 750mV.

If the "activity" of the bass is just an impedance-converter
without extra voltage-boost, it might be OK...


btw.: in the mini-Q-Tron there is not a single output-cap
comparable to several other FX-pedals,
so the descriptions given before, can be applied to lots of other FX,
but not in this special case.

[Halion]

ah! I didn't think I found the right one already, thanks alot for the help mate.

And an active bass does not just give you a fatter sound, also more output. Generally, most bass amps have 2 imputs, one of them attenuating bij 20 dB. I'll bring the pedal to my teacher on friday, he has 3 active basses. I've heard from more bassist that it works fine however, I really doubt it will start to clip (I know for a fact that even though my bass is passive, it has a decently high output, and I can strike like a mad dog on the lowest string, and still not a sign of distortion).

[gez]

Whoops, sorry about that Halion!  That's what happens when there's no shematic available/linked to. :oops:

Thanks to PT for correcting my blunder.

[Halion]

ah no problem mate, you couldn't have known.

[puretube]

I would have proposed exactly the same as gez if I`d have to do it "blind"...

   :wink:

[Halion]

It works! and like a charm too! I used a 100k log pot anyway, because I didn't have a 250k, but no problem at all. I taped it inside the box because for me it's a set-it-and-leave-it thing anyway (I only have 1 bass, and I can always make it an outside pot). Makes it so much more usefull (won't have to run to my amp to fix the volume now at all )

Once more, thanks loads guys.