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Topic: For Mike Brown: Pre-EQ Patch |
Paul Graupp
From:
Macon Ga USA
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Posted 11 Jul 2002 8:54 am |
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Mike; could you expand a bit on your article in OSGA's Newsletter about this patch capability. I'm a little stuck on your statement of a foot volume control introducing a varying resistance to the pickup. The pickup is parallel to the volume pot being tied to it HI and LO. The arm going to the amp might show the amp input as varying but R1 and R2 in your drawing are DC blocked as you noted.
I don't see the pot as changing to the Pickup but tied in with the amp input it may show a varying impedance to the PU and since the PU is essentially inductance, then maybe it is Z that causes the degradation of the PU signal.
Regards, Paul  |
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Paul Graupp
From:
Macon Ga USA
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Posted 17 Jul 2002 10:54 am
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| Bump for Mike..... |
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Mike Brown
From:
Meridian, Mississippi USA
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Posted 18 Jul 2002 7:01 am
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My statement from the article is as follows;
"A typical volume pedal patch is made between the guitar and the amplifier input. However, doing so could cause the tone of the guitar to change as the pedal is moved up and down with changing volume requirements, because this patch introduces a varying resistance as seen by the guitar pickups."
When using a "pot" type volume pedal before the input of an amp, the "position" of the pot will one of the determining factors as to the extent of the signal loss. The pickup value would be constant, but the varying resistance of the pot is what effects the tone change. The point is that by inserting the volume pedal in the circuit "after" the first gain stage of the amp eliminates this problem.
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 19 Jul 2002 8:26 am
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Mike is correct. Paul, a potentiometer, or pot, is nothing more than a "voltage divider".
That is what a pot does, divides voltage. When you change voltage, you change impedance. When you change impedance tone "MUST" change. With a pot pedal, you change tone as you change volume. If a pot pedal is the first thing your signal sees after leaving the coils of your guitar pickup, then the guitar pickup sees from zero to 500K ohms of resistance. If a pickup sees really low reistance, and capacitance, then high end frequency is lost. The signal also sees the capacitance in the pot, which can be considerable capacitance. By the way, capacitors are used in tone controls to cut highs. Capacitance is found in guitar cords also. Long guitar cords have more capacitance. If the signal is made stronger before it sees the pot,or long guitar cords, then many of the properties that eat at good signal quality are overcome. |
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Paul Graupp
From:
Macon Ga USA
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 20 Jul 2002 8:10 am
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| Paul, what is changing is the amount of signal sent to ground. This creates an ever changing impedance on a signal source that is very, very weak. A steel guitar pickup is not powered. Anything weak can be easily altered. That is why the capacitance in the Pot, and the capacitance in the length of the guitar cables so easily degrades the highs a pickup puts out. From reading the electronic work others have published on pickup outputs,a pickup needs to see a minimum of 150K resistance, and hopefully more. I have tested this and found they are correct. From my research a pickup needs to see very high impedance. If you run the guitar cord from the pickup to a pre amp, "FIRST",for example- in a Peavey amp,hence the 3 cord hook up method of Mike Brown, or if you run to one of my pre amps, the pickup will see the following; The pickup will see an integrated circuit or chip/OP-amp. The input on these OP-amps are around a million Ohms. Very high impedance and that is what a guitar pickup needs to see to avoid being "milked". If you look at a Pot, the pickup could see anything from 0 to 500K to ground. The other important aspect is the output of the pre amp. This needs to be low impedance, and that is what a OP-amp has, plus it creates a powered strong signal that maintains the integrity of the original pickup quality. The signal that leaves a pre amp is stronger than the signal that entered. On the other hand, a Pot is not powered and the signal leaving has less strength than the signal that entered. Just some simply observations that I hope helps you. |
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Paul Graupp
From:
Macon Ga USA
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Posted 20 Jul 2002 9:43 am
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Keith: You know everything you're saying makes very good sense and I agree with what you and Mike have said because I tried the approach in the 60's with a tube kit preamp. I ran the PU in the mag input of the preamp and used a cathode follower to get my lo-Z volume control effect. It worked fine but I felt I was out of the main stream because JD, BE and BG were doing it the "RIGHT WAY !"
I'm sure every steel player has at one time or another, hooked his volume pedal in backwards. Now the arm is going to the PU and it sees the 0-500K swing of the control.
The sound degradation is enough to make you think your amp has blown out. The point I'm making here is that if the PU does see the 0-500K change, the tone degradation is intolerable. The opposite must be true if the pedal is inserted properly but there is still the tone degradation (though not as severe..) that your pedal and Peaveys volume control approach eliminates.
Some thing is not clicking for me here and it's like a skeeter bite; I wish it would stop itchin' !!
BTW: I surely appreciate you taking the time to join this conversation. I recognize your expertise in this area and if I don't get too old and don't loose my playing job (probably my last...) my hope is to one day have one of your pedals. From what I've read and what Herbie Wallace has posted here many times; they are the ultimate and I know that !!  
Regards, Paul |
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Lee Baucum
From:
McAllen, Texas (Extreme South) The Final Frontier
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Posted 20 Jul 2002 3:08 pm
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If only someone would produce a device to plug in between the guitar and the volume pedal to correct this problem. That would be great!
Click Here
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Lee, from South Texas |
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 20 Jul 2002 6:34 pm
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| Paul, thanks for the kind words. Don't know what else to say other than I would like to help you. Lee, if it wasn't for Thomas Edison we would all be watching television in the dark. |
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