GGG EQ build hiss problem

[Dave Simpson]

I just finished building two GGG EQ's and I'm having a problem with a high end hiss with both of the pedals. I was hoping someone could give me some tips in where to start looking for the problem.

One thing I will mention is that with both pedals I had to wire the gain pot with lugs 2 and 3 together which is reverse from the diagram for the gain knob to work clockwise. Any advice would be gratefully appreciated.

http://www.generalguitargadgets.com/pdf/ggg_eq6b_lo3_wiring.pdf

http://www.generalguitargadgets.com/pdf/ggg_eq6b_sc.pdf

[Dean Hazelwanter]

Hi Dave,

One thing I will mention is that with both pedals I had to wire the gain pot with lugs 2 and 3 together which is reverse from the diagram for the gain knob to work clockwise. Huh Any advice would be gratefully appreciated. icon_mrgreen

Yes, it looks like that got transposed on JD's wiring diagram.

I'm having a problem with a high end hiss

Ok, silly question time. Does the amount of hiss change when any of the EQ band pots are turned? When you turn each EQ band pot, does the sound change as you would expect?

[Dave Simpson]

Thanks for the reply Dean!

The hiss level definitely gets more noticeable when the 125Hz is turned down, and gets more noticeable when the 2k Hz and 1k Hz are boosted. The hiss is there even if I have all the frequencies cut. But, yes the pots all cut and boost as they should.

I haven't tried running it with a battery yet, so the power supply filtering might still be a possible cause of the hiss. I'll have to install a battery clip in one of the pedals and give that a shot as well.

[Dean Hazelwanter]

Hi Dave,

Try installing the 220K resistor immediately to the left of IC1, between pins 3 and 5, as in http://www.generalguitargadgets.com/pdf/ggg_eq6b_lo2.pdf. That will tie the input of this opamp stage (that is unused in JD's 5 band version) to ground, instead of letting it float.

Let me (us) know how you make out.

[Dave Simpson]

Thanks Dean! I'll give it a try and see if it helps.

[kvb]

mine hisses too. I've changed some caps - to no avail.  I'm going to try some tlo84 ops - aren't these higher current lower noise? We'll see this weekend.

I do like the EQ, but I can't use it for cleans. It sounds good with high gain stuff that is already noisy.

I have it set up +/- 9v and it can handle a mini-booster at its input no problem. (it can make the mini-booster even louder!)

[Dave Simpson]

I really like the way the EQ sounds too. But until I get the hiss under control it won't be usable for me. Hopefully the 220k resistor is the answer.

Let me know how the TL084's work. I might try those in one of my EQ's if that makes any difference for you.

[Dave Simpson]

The resistor didn't help. I even tried adding the .033 capacitor as well. Tried powering the pedal with a battery to rule out power supply filtering and the hiss just got worse. Any more ideas?

[oskar]

There is a 10uF cap in the feedback path of IC1B (gain stage).   
Bypass this one... the OP-needs a DC-path... right?
To me it seems like OP-1B is operating from whatever leaks through the electrolyte.

oskar

[oskar]

You just substitute the crossed-over cap with a wire.

[Dave Simpson]

Oskar, thanks for the idea. I tried it and the hiss is still there. I guess we can rule out the gain stage as being the cause now.

[oskar]

Oskar, thanks for the idea. I tried it and the hiss is still there. I guess we can rule out the gain stage as being the cause now.

Darn, I thought I had a pretty solid case there...   
Test it with a battery and report back, with some voltage readings too.

[Dave Simpson]

Here's the voltage readings.

IC-1                  Battery                 
Pin 1 2.2mV      8.09V
Pin 2 2.2mV      8.06V
Pin 3 0             1.462V
Pin 4 3.06V       8.84V
Pin 5 0             1.8mV
Pin 6 -4.6mV    155.6mV
Pin 7 124.5mV  1.071V
Pin 8 -1.6mV    .979V
Pin 9 -1.7mV    .979V
Pin 10 0           .416V
Pin 11 -6.49V   1.023V
Pin 12 0           1.772V
Pin 13 1.5mV   3.44V
Pin 14 1.5mV   3.44V

IC-2                  Battery
Pin 1 2.1mV      3.63V
Pin 2.1mV         3.64V
Pin 3 0             1.838V
Pin 4 3.06V       8.59V
Pin 5 0             1.84V
Pin 6 -2.1mV    3.43V
Pin 7 -2.1mV    3.43V
Pin 8 75.5mV    2.9V
Pin 9 70.7mV    2.1V
Pin 10 68.8mV  1.88V
Pin 11 -6.5V     1.02V
Pin 12 0           1.82V
Pin 13 -1.8mV  3.45V
Pin 14 -1.8mV  3.46V

Diode
-6.5V  3.05V

Gain 10uF cap
.265V  -4.6mV

Input 10uF
-146.6mV  -4.6mV

Output 10uF
0      .253V

+Power 10uF
3.05V  0

-Power 10uF
-6.49V  0
   

[PerroGrande]

I think you're probably going to have to isolate various stages of the circuit to identify the source of the hiss.

One quick thing to try -- short the input to ground and listen for a change in the hiss level.  If there is no change, then the initial stage is probably not the culprit.  If the hiss drops out, then you've probably got an issue in the first stage (around IC-1B). 

I need to look at it a little more to make sure I'm not speaking out of line, but I'm not too comfortable with the design of that last stage (IC-2C).  This *may* be the cause of what you're experiencing.  It is wired up like a differential amplifier, but lacks a few things that I'd expect to see in such a design.  However, before I say something really stupid, I need to spend a few more moments with the schematic. 

[oskar]

Here's the voltage readings.

IC-1                  Battery                 
Pin 1 2.2mV      8.09V
Pin 2 2.2mV      8.06V
Pin 3 0             1.462V
Pin 4 3.06V       8.84V
Pin 5 0             1.8mV...

...Just hold it there Amigo!   
The battery will have one voltage when it's unconnected and a lower voltage (depending on the current) when it is driving a circuit!
You put the probe connected to COM (on the DVM) to one spot, usually at the ground, and then keep it there while moving around the other probe
New readings please...

[Dave Simpson]

Oskar the voltage reading on the right is with the AC Adapter and the one under the Battery heading is the voltage with the battery powering the pedal. Man, don't make me do that all over again!

I just noticed that on the 5 band version the 1k resistor that is on IC1 Pin 1 is not on the diagram. Maybe that's a possible cause for the hiss? Probably not, but I'll give it a shot and see.

[oskar]

Oskar the voltage reading on the right is with the AC Adapter and the one under the Battery heading is the voltage with the battery powering the pedal.

But they shouldn't differ much at all...

Man, don't make me do that all over again!

I just can't shake a scene from 2001 out of my head ( Hal2000 -Dave, I can see that your upset! )
I'll refer to the left column from now on.

I just noticed that on the 5 band version the 1k resistor that is on IC1 Pin 1 is not on the diagram. Maybe that's a possible cause for the hiss? Probably not, but I'll give it a shot and see.

If you only use 5 bands what goes is CA, CB and the 1k resistor of that stage. You keep the 220k to ground (or just short it with a wire) to
stabilize that OP-amp at zero.

[Dave Simpson]

I'm starting to feel like the charcater in 2001!

There's nothing in CA or CB. I tried putting the 220k resistor and the .033uF capacitor in but the hiss was still there. I have not tried it with the 1k resistor yet. If I keep going I might as well add the 5k potentiometer and drill another hole in the enclosure. That is, if there was room for another potentiometer!

[oskar]

There's nothing in CA or CB. I tried putting the 220k resistor and the .033uF capacitor in but the hiss was still there. I have not tried it with the 1k resistor yet. If I keep going I might as well add the 5k potentiometer and drill another hole in the enclosure. That is, if there was room for another potentiometer!

1. The 33n cap won't do anything and isn't needed, just connect the 220k resistor or substitute with a wire.
2. Pin 6 -4.6mV    155.6mV       <--- pretty high offset! Because...
The 10u cap at the gainpot still has to go... replace with a lead (even if it doesn't change the hiss!).
check out fig. 32 in this pdf... and the accompanying text...
http://zebu.uoregon.edu/~rayfrey/431/notes9.pdf
Could someone else comment on this? Dean? It has your name on it...   

Say after me... OP-amps need a DC-path!
3.

Thanks for the reply Dean!
The hiss level definitely gets more noticeable when the 125Hz is turned down, and gets more noticeable when the 2k Hz and 1k Hz are boosted. The hiss is there even if I have all the frequencies cut. Tongue But, yes the pots all cut and boost as they should.

Ok. The hiss increase when you boost the high frequencies. Sounds logical.
The hiss increase when you cut the 125Hz (IC1D) out. Doesn't immediately strike me as correct...
Try swaping the IC's with eachother... If it is one of the op-stages that's failing the error would possibly change. Like hiss on other
frequencies/level. While you have the OP-amps out of the sockets and the power unplugged, you also meassure the resistance from
A. Ground to V+
B. Ground to V-
4. It think you "might" need bigger capacitors for decoupling... increase to 100uF and decrease the 10k resistors to 4k7... for more current.
These are the ones in the lower left of the drawing that decouples the battery and generates virtual ground...

other than that... gee,  make sure no powercables are nearby... also shut off your nokkia! (might as well pull down your curtains!)

oskar

[PerroGrande]

Well, I certainly agree with Oskar about the DC Path in the feedback loop...   The circuit works because both the input and the feedback are AC-coupled.  However, in my book, this is a risky practice.  Eliminating the cap will make the circuit far more robust,  but won't cure the noise.

Thinking out loud for a moment...

Each of the "EQ" stages is a second-order op-amp high-pass filter that should roll off at around 12db/octave, if I've got the circuit correctly sorted out.  The original signal is split -- some going to the final stage inverting input and some going into a series of attenuators.   The "sum of attenuation" is then returned, still inverted, to the non-inverting input of the final stage.  In other words, the original goes to the inverting side, and the "reduced" original goes to the non-inverting side.

This configuration is known as a differential amplifier stage.  The final stage amplifies the difference between the two inputs.  When the inputs are the same, the output goes to zero (no difference).  It is one of the golden rules in action. 

So -- consider this last stage  -- and this is why this circuit worries me...

What are the two inputs getting?   Well, one is getting the original signal.  The other is getting the original signal with selected frequencies attenuated at varying degrees, depending on how the pots are set.  So what gets amplified?  Well, the differences do...  including, and especially any and all noise introduced by the attenuator stages. In effect, this thing is summing up and then amplifying 5 op-amps + resistors + caps + RFI + EMI worth of noise...      Instead of this noise being canceled/controlled via feedback, it is what is getting amplified -- along with the portions of the original that are different.  The actual EQ circuit attenuates, but because of the differencing of the last stage, it effectively gives the attenuated stuff a boost... (it is even marked "boost" on the circuit).  Unfortunately, any and all noise that wasn't in the original signal also gets a boost...

If I've got the analysis correct (and please forgive me if I've got it wrong -- it has been a l o n g day), it may be quite difficult to de-noise this circuit without some design changes.