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September 16, 2014, 07:25:17 AM
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DIYstompboxes.com  |  DIY Stompboxes  |  Building your own stompbox  |  Passive 3 band EQ 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Passive 3 band EQ  (Read 2117 times)
JonasL
Posts: 23



Passive 3 band EQ
« on: August 15, 2013, 11:40:04 AM »

I built a passive 3 band eq from this schematic and put a ehx hogs foot (booster) after it, but it doesn't work. The booster part works though, because the boost knob actually works as a volume knob. But very little sound is getting through, and the eq part doesn't seem to work... Anyone an idea if this EQ schematic works?

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Mike Burgundy
Posts: 1715


Re: Passive 3 band EQ
« Reply #1 on: August 15, 2013, 12:07:43 PM »

Haven't tried to trace a schem out of the vero - but something like this cannot be separated from its surroundings.
EQ circuits like this (Baxandall?) NEED a low-impedance source (which a guitar totally isn't) to be driven correctly, preferable a high input impedance for the next stage, plus they drop a LOT (up to 30dB) which then needs to be pulled back up by something like a booster stage.
Failing to satisfy these demands WILL mean the circuit won't work properly or at all.
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Bill Mountain
Posts: 599


Re: Passive 3 band EQ
« Reply #2 on: August 15, 2013, 02:38:53 PM »

These values are from a Fender Tone Stack which is made to run off of a source impedance of 30-80k ohm.  Your guitar with its 10k or less output impedance will work just fine.  What you will have is significant volume loss because of the way the FMV tone stack is designed.  The original designers didn't care about insertion loss because in an amp they'd have 50V+ signal to work with.  Your guitar is maybe 1V tops.  You'd have better luck putting the booster in front of the tone stack.

What is you ultimate "tone" goal with this design?
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JonasL
Posts: 23



Re: Passive 3 band EQ
« Reply #3 on: August 15, 2013, 03:41:29 PM »

I wanted to make a simple eq pedal to use with my bass guitar. I placed the booster before the eq part and the whole thing works a lot better now! The signal has not too many volume loss. But the only control that clearly seems to affect the sound is the bass control... The other pots do almost nothing. Thanks for the tips so far!
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Mike Burgundy
Posts: 1715


Re: Passive 3 band EQ
« Reply #4 on: August 15, 2013, 04:57:31 PM »

Bill: you're right, I stand corrected.  Grin
mid and treble not working suggests a wiring error - did you make all the cuts necessary, and in the right places? Check for solder bridges?
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Bill Mountain
Posts: 599


Re: Passive 3 band EQ
« Reply #5 on: August 16, 2013, 04:42:59 AM »

Bill: you're right, I stand corrected.  Grin
mid and treble not working suggests a wiring error - did you make all the cuts necessary, and in the right places? Check for solder bridges?

I think the mid and treble do nothing because you put it after a bass booster.  There might not be any mids or treble left to adjust.

The name of that pedal is quite confusing.  It's not necessarily a bass guitar booster but a bass frequency booster.  But...it really just boosts everything and then throws everything above 400 Hz or so away.  It does not retain your full signal.

I don't know what schematic you're using but there should be a 3.3-ish uf cap to ground at the input and a .1-ish uf cap to the power supply in parallel with the collector resistor.  Try removing or lowering the value of either of these to get back the rest of your frequencies.
« Last Edit: August 16, 2013, 04:48:14 AM by Bill Mountain » Logged
deadastronaut
Posts: 9597


Rob H. LONDON


WWW
Re: Passive 3 band EQ
« Reply #6 on: August 16, 2013, 05:25:19 AM »

you may want to build the bass preamp.. Wink

files:
http://www.diystompboxes.com/smfforum/index.php?topic=101898.msg906821#msg906821
demo:
http://www.diystompboxes.com/smfforum/index.php?topic=101898.msg907034#msg907034
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