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Passive Mid-cut?


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10/4/2000 12:09 AM
Jon Hill
Passive Mid-cut?
Hey -  
 
Just wondering...is it possible to design a PASSIVE circuit that cuts midrange frequencies, while leaving highs and lows intact? I'm thinking of putting such a circuit into a passive bass so that I can get a good slap-bass sound without messing with the amp. Any ideas?  
 
Jon Hill
 
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10/4/2000 12:28 AM
aron

Jon,  
 
Check out this page which describes the Torres Midrange Mod. Alter the values for bass:  
 
http://www.jpbourgeois.claranet.fr/guitar/micos1.htm
 
10/4/2000 1:48 AM
Mark Hammer

BUT....be forewarned that passive mid-cuts eat up signal. If you like a hot output, a passive mid-cut will not give you that, unless you have some sort of compensating circuit like an on-board pre-amp or stomp-box booster. I'm not warning against it - they ARE handy. I'm just saying you don't get something for nothing.  
 
The schematic in the Anderton EPFM book is also a good mid-cut. I built one, and it does some nice re-voicing things, but you lose about 2/3 of your volume.
 
10/4/2000 2:47 AM
aron

Mark,  
 
I've put this in other guitars before but I can't remember paying much attention to the output volume getting clobbered.  
 
Are you saying that even when the tone control is set to "no mid cut effect" that output volume will be cut or do you mean when the inductor/cap is in effect that will cut the volume? If so, that's expected from a passive circuit.  
 
I will try this on one of my passive guitars and see what happens.
 
10/4/2000 3:43 PM
Mark Hammer

There may be some differences in attenuation between the Torres and Anderton circuits, but they are both RCL arrangements, so I'll feel free to generalize a bit. Smack me down if I have pushed the envelope too far.  
 
At minimum cut, the Anderton circuit DOES result in loss. I know because I put mine in a stompbox with a DPDT bypass switch. The Anderton circuit uses a 47k series resistor, though (with a cap, inductor, and 47k pot going to ground), which may be responsible for the attenuation (the pot and series resistor form an attenuator with a cap and inductor stuck in the middle). The Torres, and similar, mid-cuts do not have any series resistor, if memory serves, and use a higher resistance pot (I replaced the 47k in mine with a 100k because of signal loss issues), so it may be less offending in this respect. If the mid-cut pot is a high enough value (say, 500k), then minimum cut (i.e., maximum resistance) should be functionally equivalent to having ANY type of tone control out of circuit.  
 
That being said, mid-cuts are most noticeable as a timbral device when the centre frequency is somewhere within the range of 200hz - 2khz. If you have only one mid-cut frequency, chances are it's in the low part of that range, near the fundamental. What that also means is that bleeding off signal around the fundamental is going to seriously cut the resulting output. It's not just going to sound different, like switching pickups; it will sound quieter. This is different than a treble cut control, which leaves the fundamental intact, and only reduces apparent output minimally, even at maximum cut.  
 
I'm beating this to death, but the user should be aware that there IS this loss. The implication is that if the user intends to stick in a mid-cut in the hopes of shooting for a mid-scoop death-metal sound, the odds of pushing your distortion pedal or amp hard enough are noticeably reduced unless there is some way to compensate for that initial signal loss. If you like a clean tone, play with the volume control at 8 or 9, and use hot pickups to start with, I can't see a passive mid-cut as providing any serious impediment. As always, though, reductions in output from the signal source have implications for overall S/N.  
 
So, to answer your question, Aron, in some cases there will be more overall passive attenuation than others, even with minimim mid-cut, but in all cases maximum mid-cut will produce a noticeable drop.
 
10/4/2000 4:33 PM
Jay Doyle

Boscorelli has the circuit that was used in the old Ampeg amps that produces a notch at 600 Hz. It is supposed to give the guitar a sense of "clarity." But I never thought of what happens to the notes that you play whose fundamentals are around 600 Hz, you would primarily hear the harmonics, wierd. Now I have to try it.  
 
I find that when designing with passive tone controls, it is wise to just go ahead and tack on either a booster to recover the loss or at least a buffer to prevent it from loading down the following "block" or circuit.  
 
FWIW,  
 
Jay Doyle
 
10/5/2000 2:11 PM
Mark Hammer

I'm with you on that one.  
 
As for what happens to notes when the notch is at the fundamental, bear in mind that a notch filter that simple generally doesn't produce a steep n'deep notch, so there is plenty of fundamental left to be heard, but at an audibly reduced level. Same way you can tweak an equalizer and cut a fundamental by 15db, and still hear that fundamental.
 

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