Please help -- PAIA Gator key threshold

[aloupos]

I've build the gator, and everything seems to work fine except the input trigger seems to require a much higher gain than a straight guitar signal in order to have a working range. 

Here's the project: 

http://gaussmarkov.net/wordpress/circuits/gator-clone/


With guitar as input and the threshold all the way open, I need to hit the strings hard to open the gate.  When using a tone source with a louder output I get a usable range on the threshold.  In bypass mode, where running through 2 of the op amps I get a hot signal and can get some boost with the gain control. 

Only sub is that I've used a CA308 instead of LM308.  I think the CA308 is RCA's version of the LM308. 

Voltages on IC5 as follows;

pin1: 6.12
pin2  6.05
pin3  6.05
pin4  0
pin5  5.88
pin6  5.26
pin7  11
pin8  12

Note the V+ is around 12 volts.  any clues hugely appreciated. 

[Mark Hammer]

Try a 9v battery and see what happens.

[aloupos]

Just tried 9v, exactly the same.  With a guitar (LP) the threshold needs to be all the way open to get any sound through, although sending a boosted signal through works fine and I get a usable range of threshold.  Is it possible that the CA308 has a different output than the LM308?  I don't really understand how the actual gate works in this circuit, but is there someway I can change a resistor or two to increase the key gain? 

[Paul Perry (Frostwave)]

I don't like the look of those voltages around IC5B.
With 5.88 on one input & 5.26 on the other.. something is wrong.. (unless it's a problem from loading down by the multimeter impedance).
Think of this: IC5B has a gain of around 1,000, so if VRef is say 6V, the op amp is going to be whacked right over to one rail.
I could be wrong, but my guess is having IC5A and IC5B in series without a decoupling cap is a potential source of unhappiness.
Put a 10uF cap in series with R15 (either end) and see if it helps. That's what I would try, anyway.

Troubleshooting hint: if an op amp is acting as a straight amplifier with negative feedback, the two input pins have the same voltage... the way your IC5A has 6.05 on each input.

[aloupos]

Thanks for taking a stab paul ... 10U in parallel with R15, same performance.   I should note for clarity that with the exception of the threshold gain being too low, the circuit functions perfectly, and if I boost the input signal I get a usable range of threshold. 

Any more ideas?

[Paul Perry (Frostwave)]

Thanks for taking a stab paul ... 10U in parallel with R15, same performance. 

Ah, I did say series, not parallel...

[Marcvv]

Hi,

First: Did you build it from the Paia kit, if so, did you contact Scott Lee at Paia because he always is very helpfull?

Second: I encountered the same problem when I was using it with the guitar going straight into the Gator. It is actually build to be driven by line level signal, e.g. at the end of your chain. To solve this you could  add some resistance from the R12/R13 junction to  V+.  For me 6.1M did the trick (so you'll have to select some parallel or series combinations of resistors to get this, or, use a trim or pot that is nearly this value with a fixed resistor in series for an adjustment)
For me that worked.

Marc

[aloupos]

Paul -- I meant series, not sure why I typed parallel.  Long day I guess

Gitaar0, I didn't buy the kit from Scott, and as far as I know it's no longer available.  I built it from a project at gaussmarkov's site. 

If it is designed to be driven line level than what I'm seeing makes 100% sense in that only a boosted signal produces a usable threshold, but, when using it as a noise gate, you'd probably want to run a clean signal to the trigger, and the effected signal to the input, no?  Anyway, your fix will likely give me what I'm after.  Maybe I'll put a toggle in to select from line level or instrument level, switching the 6.1M resistor in for guitar.  I'll test and report back to officially close the thread! 

Thanks for saving me days of time!!   

[aloupos]

Hmm, just checking the schem, and the J at R12/R13 goes to the inverting input on IC5A, not to V+.  Maybe my schem is different from the original.  Can you take a look please?  :

http://gaussmarkov.net/layouts/gator/gator-schem.png

[Marcvv]

Hi,

Sorry for not responding sooner.

I think you misunderstood the description: the suggestion is to put a resistor of 6.1M between the junction of r12/r13 and V+. You might have to change that value to what works best for you.
Is this more clear??

The schem is exactly as the original with the difference of the original using a quad opamp 4136 and the one you are using had two dual opamps TL072.



best, Marc