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Old 19th September 2011, 03:13 AM   #1
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Default Need to modify this low impedance input to allow guitar pickups in?

I have figured out a few things in adapting the following circuit for a bass guitar application but am stuck on the input impedance requirement. As stated in the literature

Quote:
It is imperative that this circuit is driven from a low impedance. The actual input impedance is greater than 47k at all frequencies, but the source impedance should ideally be no more than 100 ohms or so (although as noted above, even as high as 10k will cause few problems).
Subsonic / Rumble Filter for Phono preamps and Sub-Woofers

A bass guitars pickups would be of high impedance and I'm not sure how to modify this circuit to allow higher impedance in. Any thoughts or recommendations are very very welcome. Thanks for your time to look at this thread.
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Old 19th September 2011, 03:39 AM   #2
cbdb is offline cbdb  Canada
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Im not sure why you need a hi pass filter on yor bass, but the easiest way to increase the input impedance and not effect the filter is to put a buffer (another opamp) before the filter.
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Old 19th September 2011, 12:44 PM   #3
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Yeah, I thought I might have to do something like that.

This board just got a little bigger with a second chip to mount.

The reason for the pedal is to remove inaudible low transients from slapping and aggressive playing that appear below 30hz to relieve the amp and speakers from the undue stress. The goal is to achieve the same sound but without the large cone movement allowing for a smaller amp to achieve higher volumes. See the SFX Thumpinator video on you tube. It's pretty convincing but the cost seems a little much for just a filter and buffer.
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Old 20th September 2011, 01:39 PM   #4
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Apparently he added this note at some time:

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Although it is stated below that the input impedance of this filter should be less than 100 ohms, it may be directly connected to the Project 06 phono preamp. Testing shows that the overall frequency response is changed by less than 0.1dB at any frequency above 30Hz. Naturally, low frequency response is affected by the filter as it should be. Even with an input impedance as high as 10k, there is no significant deviation from the expected response curve, and only a tiny (about 0.2dB) loss of overall level.
His concerns for flat response are relevant to hifi, but for making music odds are your speaker isn't totally flat to begin with (nor would you want it to be). I say build the filter and try it, buffer if needed. Who knows, you might like the change in response, which is an option the hifi guys don't have.
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Old 20th September 2011, 11:04 PM   #5
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Discrete FET Guitar Preamp
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Old 21st September 2011, 12:33 AM   #6
Enzo is offline Enzo  United States
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I have this question: are you actually experiencing some low frequency PROBLEM, or is this just a theoretical concern? As a career veteran in pro audio, I work with all brands of commercial bass amplifiers, and I have yet to see one with an input high pass filter of this sort. Low end rolloff is not a feature expected on a bass amp, and in fact percussives and other artifacts of slapping and popping strings is PART of the sound. If I were really concerned about the speakers, I would put a low filter at the powr amp rather than the input of the preamp.

There would be zero stress from any of this on the preamp. And any subsonic output from the power amp would be limited to the same amplitudes of the audible output, which is generally the power supply level, at least in solid state amps. SO in general, if the speaker can handle the amp, it will handle whatever the amp throws at it.

Of course that assumes the speaker is not under-specified, or the amp too, for that matter.
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Old 21st September 2011, 12:41 AM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by MuthaFunk View Post
...
A bass guitars pickups would be of high impedance and I'm not sure how to modify this circuit to allow higher impedance in. Any thoughts or recommendations are very very welcome. Thanks for your time to look at this thread.
Add a unity gain buffer between the input the the bass. This is justt an simple opamp with gain set to one. You will need this more for for the bass than for the filter. A bass or guitar amp needs to have about 1M input impedance so as not to load the pickup coils.
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Old 21st September 2011, 01:10 AM   #8
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Enzo View Post
I have this question: are you actually experiencing some low frequency PROBLEM, or is this just a theoretical concern? As a career veteran in pro audio, I work with all brands of commercial bass amplifiers, and I have yet to see one with an input high pass filter of this sort. Low end rolloff is not a feature expected on a bass amp
Yes, you don't typically see a filter on the preamp section but it's there. The frequency response of a bass amp is not flat to 0Hz. Don't you typically see the 3dB points at about 30Hz, give or take?

Even a high-end HiFi amp is flat to only about 20Hz then rolls off fast.

All that said, I doubt to OP needs that because as said above, his amp will already roll off subsonics already
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Old 21st September 2011, 03:26 AM   #9
jjman is offline jjman  United States
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A bass often has it's own preamp built in that I believe can tolerate a lower impedance than a standard type bass/guitar pickup would. Not sure how low however.
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Old 22nd September 2011, 12:21 AM   #10
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Quote:
Originally Posted by Enzo View Post
I have this question: are you actually experiencing some low frequency PROBLEM, or is this just a theoretical concern? As a career veteran in pro audio, I work with all brands of commercial bass amplifiers, and I have yet to see one with an input high pass filter of this sort. Low end rolloff is not a feature expected on a bass amp, and in fact percussives and other artifacts of slapping and popping strings is PART of the sound. If I were really concerned about the speakers, I would put a low filter at the powr amp rather than the input of the preamp.

There would be zero stress from any of this on the preamp. And any subsonic output from the power amp would be limited to the same amplitudes of the audible output, which is generally the power supply level, at least in solid state amps. SO in general, if the speaker can handle the amp, it will handle whatever the amp throws at it.

Of course that assumes the speaker is not under-specified, or the amp too, for that matter.
I have a small single 12" Speaker Combo amp with very high powered amp. I have noticed quite a bit of cone movement while slapping the high G string and realized it was my sloppy technique allowing my palm to softly bounce or bend the thickest B string. Even though the notes I was plucking were not in the sub sonic range my speaker was bouncing quite a bit. I wish to combat this phenomena with a DIY filter. Here is a video from the boutique manufacturer SFX who made one called The Thumpinator:

[sfx] micro-Thumpinator - YouTube

He wants big $$ to make one and I thought it couldn't be all that hard to accomplish a steep 4th order sub 30Hz filter and a flat frequency response up to 20 Khz to preserve my actual bass tone.

Right now I'm thinking I'll add a simple op-amp at unity gain before the phono sub filter and use 100nF caps to get me in the approx right frequency range on the phono filter. Any recommendations on an appropriate op-amp? The TL072's spec'd in the original project have a reputation as being cheap sounding.... Any thoughts on something better?
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