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PostPosted: Sat Dec 03, 2016 11:13 pm 
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Joined: Sat Dec 03, 2016 10:39 pm
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I'm having an issue with the first (bass) EQ stage on the Para-EQ. I'm only getting half the boost on that stage. The cut works fine, as does the boost and cut on all the subsequent stages. All other parameters also work as expected. Also, when I pull the 1st TL074, the problem moves to the second (now first) EQ stage. In other words, I pull the Bass IC, and the previously working midrange section inherits the problem. I pull the second IC, and the Treble section then inherits the problem. The first stage always has the issue, regardless of where and what it is.

I tested the EQ response with a white noise loop and Voxengo Span spectrum analyzer. I also used an external sine wave generator set to the EQ's center frequency and poked around with my Tektronix scope. This did reveal something interesting. The input signal to the first stage (post R4) drops down when the EQ is boosted, as if negative feedback is getting into it. This doesn't happen on the subsequent stages (post R6, R19). The input signals are rock solid regardless of boost.


Has anyone else noticed this type of behavior? I've been doing DIY for a very long time and went over my build multiple times, checking and rechecking. So I'm pretty confident that I've eliminated the obvious, like bad joints, bridges, or misplaced components. Plus the problem can move to a completely different set of components. Any insights would be appreciated, especially if anyone with a working build can test their bass boost level compared to the cuts and other boosts.


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PostPosted: Sun Dec 04, 2016 1:17 pm 
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Location: Albany, NY
I am remarkably ignorant of how this particular BYOC circuit works, but that won't stop me from tossing out a few "basics" that could possibly help:

1) Humor us and post a set of photos--large, well-focused & well-lit shots of both sides of the PCB and all of the switch and jack wiring, please--so that we can have a look-see at your build. I can't tell you how many times we've seen even experienced DIY'ers look past a problem that jumps right out at another set of eyes. It's the old "can't see the forest through the trees" cliché....but there's a good reason why such things become clichés.

2) Have you checked the DC voltages coming out of the 1044 to see if they're correct?

3) Try swapping another 4558 (or any other general purpose dual op amp) into IC1 and see if it makes any difference. Prolly not, but since that IC is common to all the stages, it seems like something reasonable (and easy!) to try.

Hopefully, someone with more thorough/relevant knowledge of this circuit will happen by soon....

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My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2


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PostPosted: Mon Dec 05, 2016 8:59 am 
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Joined: Sat Dec 03, 2016 10:39 pm
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Those are good suggestions. I'll give them a shot and also post some pictures.


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PostPosted: Sun Dec 11, 2016 1:33 pm 
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Joined: Sat Dec 03, 2016 10:39 pm
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I did a little more testing on this and have some results to post:

1. I didn't take any photos because my board is starting to look like a bombed out battle field, with all the lifting and substitutions I've been doing. The 5K1 resistors that came with the kit were too large to sit nicely on the board which made it look like a mess before I even started hacking. After hacking, I have some 1/4W resistors jammed in there to replace ones that I broke when lifting them. It looks like a mess, but I still think it is OK. The behavior hasn't changed over the course of my hacking.

2. The charge pump is working well enough, putting out ~ +8.5, -7.5V. I also bypassed it with regulated +9V/-9V without any change.

3. I swapped out the buffer opamp IC1 without any effect. I also tried some off-board buffers without any improvement as well.

4. I fixed the problem by jumping R4. I was playing around with a resistance substitution box in place of R4 and found that the higher the series resistance I had, the less relative boost I got from the first stage. Notice that there are no series resistors on subsequent stages after the signal is rebuffered. I didn't try adding other series resistors in between stages 1/2 or 2/3, but my gut tells me that they would introduce the same problems there.

At this point the problem is academic for me (other than the mess my board has become), but I'm still curious if anyone else can observe this effect in their build. If so, any other builders might want to jump R4, especially if the pedal is being used for bass.


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