5 Band EQ Power Puzzle

[rhys]

I've built the 5-band EQ from GGG.  http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=46&Itemid=26

I can't figure out what's with the power though. 

The pedal works when powered directly by a standard Boss 9v adaptor, and it works when powered by a cheap-o Radioshack 9v adaptor.  So the pedal will work properly. 

But when I power it through my skb-25 pedalboard, it sounds like it's being starved of power.  The light comes on; if I play hard I can barely here a weak, farty signal.  It sounds like when you starve a fuzz pedal of voltage. 

I measured the voltages on the power supplies and I can't figure out why the EQ isn't working through the pedalboard supply.  Straight from the wall the Boss supply reads 9.18; the reading from one of the daisy chains is 9.17.  And with the Radioshack supply I get 11.5 from the wall and about the same through the pedal board.  But even when the Radioshack supply is feeding 11+ volts through the daisy chain, it doesn't power the EQ. 

Any ideas (other than buying another dedicated power supply for this pedal)?

[Sock Puppet]

Hi,

Looks like your daisy chained pedal board could effectively be connecting the -Ve to ground (point B to C on the schematic) through the jack grounds.

i.e. something in the pedal board already connects the -Ve to ground and this is shorting points B and C when the audio lead is connected to that and the eq.

S.

[DDD]

By the by there's a mistake on the shematic diagram: the first OA (1b) has no DC negative feedback and its inverting input is "floating". So the OA won't function properly. But maybe PCB is correct.

[Dean Hazelwanter]

So the OA won't function properly.

This project is based on a 10 band EQ found in ETI magazine - from '77 I believe. While I didn't build that version, I have built 4 or 5 of these 5 and 6 band versions, and they all worked.

Rhys, this was always meant to be powered with a split supply (+/-9v). If you use it with a single supply (+9), and that supply is also powering other pedals, this will be shorting -V to ground. Sorry, but I don't know any workaround, unless... For the audio cable between the EQ and the effect before and after it, try using shielded cables with the shield disconnected on one side.

Hope that helps!

[slacker]

If you want to run this on 9 volts sharing a power supply with other pedals I think you just need to do a couple of mods.

1. Disconnect point C from ground, and use point B as ground instead. That makes your negative battery terminal or negative supply voltage ground, so the pedal will work with others on the same supply. Point C is now your standard 4.5volt vref like you get in most FX pedals that use opamps.

2. Connect the input and output jack socket sleeves and the bottom of the final 10k resistor to point B, so that they are connected to the new ground. If you're using the GGG layout you can do this just by moving the jumper that normally connects those to point C to point B instead.

3.  Flip the input cap so the positive end faces the 100k resistor and remove the 10uF cap in the feedback loop of the IC1b and replace it with a jumper. That will correctly bias the opamp now that it's referenced to 4.5volts instead of ground.

That should be it. I can't see any reason why it shouldn't work, but if it doesn't the mods are easy enough to undo

[DDD]

"...I have built 4 or 5 of these 5 and 6 band versions, and they all worked. Huh..." - so we have to assume:
1. THERE IS negative DC feedback via leaky 10 uF cap, since all of that caps have leakage current much more than OA's input current. That's not reliable way to bias OA.
2. There is a "hidden" resistor on the PCB, omitted on the schematic diagram.
In any case the el. caps around the first OA have no DC bias, and their capacitance is too big, at least 50-80 times bigger than it necessary for the lowest operating frequency.
To be frank, there's too many mistakes :-(

[slacker]

Unless I've misunderstood something, there doesn't need to be any DC feedback the way it's drawn. It's running on a bipolar supply so the opamps are referenced to ground and there should be no DC on their inputs and outputs.

[Dave Simpson]

Let me make sure I'm going to do this right.

I remove the 10uF capacitor that's circled and replace it with a jumper.Also I  reverse the 10uF with the + drawn next to it. As for moving the jumper from point C to B. Do I just tap into the B trace at some point?

[slacker]

Yeah that's correct, you can probably just jumper to the pad marked B and then hook up your ground power supply/battery negative to one of the jack sleeves.

[Dave Simpson]

Thanks Ian! I'll give it a try.

[Dave Simpson]

Just wanted to give an update if anyone was interested in the results of the mod Ian suggested. It works! Thanks Ian! My EQ now works with my other pedals using a shared power supply.

[slacker]

cool, glad it worked out

[rhys]

That worked great--thanks Slacker. 

This pedal is so much fun.  I built it probably 6 months ago and haven't played it much because of the power issue.  Now, I think this one will be a regular on the board.  One drive pedal with this EQ can get you tons of very different tones. 

[DimebuGG]

Is there any difference in the sound using single power supply?

[Dave Simpson]

I think the only advantage to the bi-polar power supply might be a little more clean headroom.

[DimebuGG]

I think the only advantage to the bi-polar power supply might be a little more clean headroom.

Yeah, you're right dave. I think i'm going to clone the DOD FX40-B and compare it to the GGG EQ. I just finished drawing the PCB.

[Dave Simpson]

Cool, good luck with your project.