How do I make a dual supply op amp circuit run on 9V battery? - diyAudio
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Old 17th September 2011, 11:42 PM   #1
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Default How do I make a dual supply op amp circuit run on 9V battery?

I found a company (SFX) that makes a product called the Thumpinator for bass players. It's basically a steep high pass filter at 30Hz, allowing a bass amps speakers and amp a break from producing those low inaudible transients from 5 string basses. I was doing some research to DIY one myself and came up with this page:

Subsonic / Rumble Filter for Phono preamps and Sub-Woofers

It's designed for phono applications but I thought I could modify the circuit to suit my needs as far as impedance's go. My question is how could I modify this circuit to run off of a typical 9V supply like most other guitar pedals?

Thanks in advance for any guidance or expertise you fellas can provide.
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Old 18th September 2011, 12:04 AM   #2
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Join Date: Sep 2004
Location: New Zealand
Purple haze, smoke on the water and close to the edge,

You don't need anything so complicated or expensive.
The simplest thing would be to solder a capacitor restricting very low frequencies into the guitar lead. A capacitor wired in series will change the sound slightly as it always has a 90 degree phase. The formula for frequency is 1 / 2Pi x R X C I can solder a phone jack with such a capacitor for you to try, let me know how long a lead you need. Values between 100nf and 680nf should work OK to restrict low frequencies but depends on other components such as resistors also in the circuit. A surface mount cap will easily fit in to the guitar lead.

Cheers / Chris

Last edited by Chris Daly; 18th September 2011 at 12:17 AM. Reason: Practical solution
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Old 18th September 2011, 01:00 AM   #3
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Location: Minneapolis, Minnesota, USA
No, a 1st order high pass filter will have a 45 degree phase shift only at Fc, more below Fc and less above. Three octaves above Fc phase shift will be less than 10 degrees. 90 degree shift is about 10 octaves below Fc. As for powering that circuit from a single 9 volt battery, you would have to create a mid-point voltage source to act as a virtual ground using something like a voltage divider, and AC couple the input and output. It would also be wise to use a opamp that will work OK at reduced voltage levels and doesn't draw much current to keep battery drain reasonably low. Good info on running opamps on batteries can be found here: How to Build the CMoy Pocket Amplifier

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Old 18th September 2011, 03:01 AM   #4
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Location: New Zealand
Thanks Michael for reminding about phase and 6db filters, but why amplify or buffer with a op amp, when a simple 1 st order 45 degree phase shift at Fc capacitor will achieve a high pass filter, and be fairly kind to audio preservation. The the need as I understand is to stop the possibility of sub bass frequencies damaging equipment.

Cheers / Chris
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Old 18th September 2011, 02:47 PM   #5
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Thanks for the responses gentlemen! I really appreciate it.

I never considered going with a passive system for two reasons. One, I was under the impression that a first order filter wouldn't give me enough slope to be useful unless I were to raise the cutoff frequency up high that brings me to two, my true bass sound would be compromised. I'm not too concerned with component count and complexity as I've DIY'd things before and am comfortable with larger projects.

The goal is to have the steepest slope that's reasonable with no phase shift and centered around 30hz. I didn't think it would be too complicated to make a fourth order filter with a couple of dual op-amps. For me it's all about keeping the bass guitar sound in tact and NOT inducing any audible frequency changes. Do you guys think the phono sub filter would suit this purpose and be modified to run on a single 9v battery. I guess op-amp selection would be key to high fidelity and low power consumption. Any suggestions?

@ Chris, it's so very kind of you to offer to soldier a phone jack for me to try! I have a great thermostatically controlled soldiering iron and some quad eutectic high silver soldier. I really appreciate your offer but I would be fine making my own. I might just give it a try just to see as it wouldn't take all that long...

Last edited by MuthaFunk; 18th September 2011 at 03:11 PM.
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Old 28th November 2011, 04:28 AM   #6
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Location: Adelaide South Oz
Resurrecting and old thread.
What you need to run a dual rail op amp circuit off a single 9V battery is something like this chip.
It generates the "false 0V" at half battery voltage for you.
I would be willing to best a small amount that other manufacturers (e.g. Maxim) probably have something similar.

Last edited by gingertube; 28th November 2011 at 04:34 AM.
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Old 3rd December 2011, 10:53 PM   #7
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Thanks for the info!
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