Impedance Question

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Impedance Question

Postby Stringmaster » Tue Apr 27, 2010 6:09 pm

I'm trying to understand some of the claims for products that do "impedance matching" (a misnomer) such as the Radial Dragster and the Sarno Freeloader and Steel Guitar Black Box, to see if there is anything to this.

Take a typical guitar with a 250K tone pot, and a tube amp with a 1Meg resistor to ground in parallel with triode grid (which is infinite impedance), or 1Meg input impedance. The guitar pickup sees up to 250K in parallel with 1Meg, which equals 200K. This would be the reference situation.

It's easy enough to see that if you plug into something with much less impedance than the guitar amp, the impedance seen by the pickup is reduced and the loading on the pickup is increased. Much as if you roll back the tone pot, mostly what you hear is a loss of highs. A buffer with high input impedance should correct this situation. BTW, this is probably what people refer to as impedance matching, but it would be better to say something like impedance adjustment. Impedance matching has a specific meaning and it is not what you want to do with pickups.

So far, so good, but where I start to scratch my head is when the input impedance is "too high." For example, the Dragster is marketed primarily for use with wireless systems, such as the Shure PG-14, which has an input impedance of 1Meg. There is a review on the Radial site mentioning the use of the Dragster with modelers like the POD XT, with a similar input impedance. I contacted the author of the review, but he was repeating something he had been told and had no further information. The benefit of the device in this case is supposed to be related to feel or dynamics.

Is the concept here that the 1Meg input impedance of a tube amp is somehow different than that of a solid-state device? If you read what Brad Sarno has written on the topic, he claims that it is the direct interaction of the pickup and a preamp tube that creates the magic. (Although the Freeloader is not a tube device, and neither is the Dragster.) Brad claims that the game is over as soon as a digital device is seen by the pickup.

Given the very very high impedance of a tube, normally I would consider that to be nearly perfect isolation. So what interaction are we talking about?

I use a Tone Press at the beginning of my FX chain. I have no idea what input impedance it has, but it sounds great to me.

What am I missing here? Is this another claim like the $300 power cords and oxygen free copper?
56 Fender Stringmaster Triple 8 and Deluxe 8, 61 English Electronics Tonemaster, 2006 Cruztone
Fox 5D6B, Barber Tone Press, Hermida Zendrive, Hardwire RV-7 & Dl-8, Goodrich H10K, Boss PW-10
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Re: Impedance Question

Postby jacobaf105 » Tue May 18, 2010 1:45 pm

I cant explain this in a very technical way, but I am around alot of steel guitar players due to my dad be a pedal steel guitar player, and the sarno box and what they call impedence matching devices in my understanding work like this, their guitar pickups are wound diffrent than our standard guitar coils and when they use a OD box with their guitars I have read/heard that it is hard for the pedal to drive right without the impendence matching box. when they use the "box" first then into the od/ dist pedals it lets them drive more like our guitars do or with a smoother OD if you will. hope this helps a little


Jacob
fender 72 deluxe tele, barber tonepress, digitech gsp 1101, peavey nashville 1000, rocktron velocity 300 and eminence big ben.
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