Repeat question "volume drop with 6 band EQ of GGG"

[fozzy]

Hi All  (didn't think I should renew older post with same question),

However, I am having the same issue that was posted (using GGG schematic 5 band), signal is fine through the pedal, then turn on and volume drops.  I need to crank volume of amp to hear the pedal work.  I've checked all values and even changed cap C7 to 1uf but still no change, and I am using a single supply 9v. I have scoured everywhere for a resolve, but nothing, either no one else has issues or no one is building this pedal. 

Thank you,
Fozzy

[GibsonGM]

Hi Fozzy, welcome.  Is there any chance you did not build the pedal correctly?   I would assume GGG, such a great effect maker, would compensate for that if this is the case!   

Please post a link right to the schematic and we'll have a look. 

[fozzy]

Hi Mike, 

I wouldn't think GGG would have a schematic available if there was an error (http://www.generalguitargadgets.com/pdf/ggg_eq6b_sc.pdf).  I used a 250k pot instead of the trimmer. If you look at the schematic and notice all is good, I will pull it all apart again and rebuild, I was hoping someone has a solution to try this or watch out for that. However, I do enjoy trying to resolve these issues, but sometimes just have to wave that HELP ME FLAG!  Again, thank you.   

Happy New Year

Fozzy

[mth5044]

Does the 250k pot do anything? Did you use audio or linear? Voltages for the IC's and/or audio probe? Do the boost/cut controls do anything?

Rebuilding is a pain in the buns when you might be able to pinpoint the problem with some troubleshooting.

[fozzy]

Hi Matt,

The 250k is linear and does work, more so increase volume but does add gain.  I will check on the volts for the IC's and audio probe.  The boost/cut controls do effect tone, but again only noticeable when turn up the amp volume.  Thank you.

[Slowpoke101]

I built two of these units a few years ago. Set up to run on 9 Volts and used a 250K pot for the gain trimmer.
Both units worked perfectly with no problems or "funnies". I used the GGG PCB design. TL074 ICs were also used.

I didn't bother using slider pots as milling the required slots in the enclosure was just too much bother, so rotary pots were used.
With the EQ pots set to their centre range I could easily get unity gain with the gain pot set to about a quarter travel CW from minimum. There was still plenty of gain to go.

I would suggest that you check your circuit for any errors that may have crept in. The circuit that GGG presented does work and seems pretty well bullet-proof.

[vigilante397]

I've also built this and can confirm the schematic and layout are legit. Are you aware that the entire point of the 250k trimmer is to set the level? The idea is that you set all of the EQ levels to 12 o'clock then adjust the trimmer (or external pot if you prefer) to get unity gain. So if you have a volume drop you just need to adjust the trimmer.

You said that the trimmer "does work ... but does add gain." Well you're increasing the resistance in a feedback loop, that's what happens That first op-amp stage is basically a buffer before you go into the EQ stages, and the idea is to get the gain right before you worry about filtering. Obviously it's hard to tell without hearing it, but it sounds to me like it might be working after all...

[fozzy]

Thanks for the responses and time.

I did audio probe and only had strong signal up to the trimmer from C8. On the other side of the trimmer volume drop and I did try another pot and trimmer same result. When I adjust the gain it does not balance, just brings down the signal from C8 and nothing changes on the other side of the trimmer. Also nothing coming from Pin 7 on IC1 (I did replace IC1 with a another one). Does it sound like I am missing some power some place?  Below are my voltages (battery was only 8.4)

IC1
.8 .8 1.4 6.0 2.3 2.9 5.4 .8 .8 .8 2.3 1.4 .8 .8
IC2
.9 .9 1.4 6.0 1.4 .9 .9 5.4 5.4 5.4 2.3 0 0 .8

Not giving up on this.  GGG's schematics have been good for me, and as someone has mentioned, probably not the schematic but some connection or value I have incorrectly done.

[R O Tiree]

Assuming you have provided the voltages in "pin order", starting with pin 1 and ending at pin 14 for the 2 ICs, then there is a definite problem... Pin 4 on both should be 9V (you have posted only 6V on both) and pin 11 on both should be 0V (you have posted 2.3V on both).  So, your supplies to your ICs are clearly not right.  Even if your battery is only pushing out 8.4V, then that reading should be 8.4V on pin 4 on both ICs and pin 11, regardless of how strong or weak your battery is, should definitely be 0V.

First thing is to get the power supply lines right, then re-measure.

[Slowpoke101]

You may have a battery connection issue. With ground and virtual ground confusion thrown in.
When you use a single 9 volt battery the positive goes to point A and the negative goes to point B (as shown on the GGG schematic). R26 and R27 set a virtual ground point for the circuit. Measure the voltages on the ICs using point B as the multimeter's negative (black test lead) connection. Your measured voltages will then make more sense.

At the moment do not bother with a DC power jack or switching the battery using J1's ring connection. Power the EQ directly from the battery only and see what happens.

Just remembered; The circuit worked perfectly but I did have an issue with powering it and switching it with the sleeve and ring connections on J1. Due to the virtual ground you have to power the EQ with its own power pack. You can't daisy chain it with other pedal's power cables. The other problem with the virtual ground is that you can't switch the battery negative using J1's ring and sleeve connections (usual way of isolating the battery when not being used). In order to achieve the switching function I used a fully switched stereo socket that has two isolated internal SPDT switches in the socket. I used one of the SPDT switches to connect/disconnect the batttery's negative.