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
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.
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.
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.
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.
Apparently he added this note at some time:
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.
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.
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.
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.
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.
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
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?
I thought that was what a compressor was supposed to deal with ?
btw, problems with large cone movement could be poor speaker design
either the box is wrong
or driver might have low Xmax limit
and if combined with weak magnet system, even worse
pushed hard, and its out of control
personally, I have problems with too weak power amp
so I reckon it would be nice having one with very low output impedance, and thus good control
but I guess there are many different ways to deal with this
btw, problems with large cone movement could be poor speaker design
either the box is wrong
or driver might have low Xmax limit
and if combined with weak magnet system, even worse
pushed hard, and its out of control
personally, I have problems with too weak power amp
so I reckon it would be nice having one with very low output impedance, and thus good control
but I guess there are many different ways to deal with this
Tinitus, there's probably nothing wrong with the amp or speaker, it's likely bass reflex or open back. Years ago (many more than I care to admit), I was in a band where the guitar player was using a Sunn S-100 guitar amp that had a slot-loaded dual 12" speaker bottom. He liked to use it with the bass and treble cranked up, and it would do the same thing if he pushed or thumped the strings, the speaker cone would jump at least 3/4 of an inch.
MuthaFunk. if you're planning to power it with a single 9 volt battery, your opamp options will be pretty limited, it will need to be one with very low quiescent current drain or else you'll be changing batteries often. Frankly for a battery powered opamp based effect, dual 9 volt batteries is the best choice. You should use a low noise chip since it's a high pass filter, something like a NE5532 or an OPA2134 would be good.
Mike
MuthaFunk. if you're planning to power it with a single 9 volt battery, your opamp options will be pretty limited, it will need to be one with very low quiescent current drain or else you'll be changing batteries often. Frankly for a battery powered opamp based effect, dual 9 volt batteries is the best choice. You should use a low noise chip since it's a high pass filter, something like a NE5532 or an OPA2134 would be good.
Mike
Thanks Mike. I've read about the OPA2134's and I think I'll order some of them. I'll actually be running it off of 18V DC supply but would like to design it to run off a battery or a standard 9V pedal adapter in case I ever need to use it off of my pedal board. If it works as I'm hoping I may end up giving it away to one of my fellow bass players and making another down the road. I just want it to be as much like the SFX Tumpinator as possible.
Use a vacuum tube wired as a voltage follower (cathode follower) for your input. There are tubes made for car radios that run with +12 volts on the plate that should work here.
Super high impedance input (1 or 2 megohms depending on the grid resistor) and with the right bias setup you should be able to get a super low impedance out--an exact copy of the input at a very low impedance.
The high impedance input of the tube at the front end preserves that part of it through your chain.
With such a low plate voltage you might even be able to use a design with no capacitors in the signal chain for clean distortion-free response all the way down to DC.
Your +18-volt supply should work nicely for this, feeding both plate and filament if you use the right resistors. It'll be as quiet and noise-free as your power supply is. No solid state thermal noise, for one thing.
This is all speculative. I've never tried this. I could be wrong, but it seems well worth a look.
Google "space charge and other low voltage tubes"
--tubekit
Super high impedance input (1 or 2 megohms depending on the grid resistor) and with the right bias setup you should be able to get a super low impedance out--an exact copy of the input at a very low impedance.
The high impedance input of the tube at the front end preserves that part of it through your chain.
With such a low plate voltage you might even be able to use a design with no capacitors in the signal chain for clean distortion-free response all the way down to DC.
Your +18-volt supply should work nicely for this, feeding both plate and filament if you use the right resistors. It'll be as quiet and noise-free as your power supply is. No solid state thermal noise, for one thing.
This is all speculative. I've never tried this. I could be wrong, but it seems well worth a look.
Google "space charge and other low voltage tubes"
--tubekit
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