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Laurie
Double Platinum Member
Canada
4854 Posts |
 Posted - 11/17/2008 : 17:01:59
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"Impedance matching" of pedals is not as straightforward as it might appear at first glance.
For the record, "matched impedance" is a wonderful thing because it gives maximum power transfer. This is important when you are transfering substantial quantities of power around, like in a power-amp to speaker connection.
In pedals, however, the thing we are most interested in is signal transfer. Here are a couple of examples that look at impedance vs signal transfer, and show why "matched" impedance is not what we seek for pedal connections.
Assume a guitar is plugged into a high impedance pedal input (pedal #1) - the pickups won't be loaded down and the guitar will perform as required (there is a lot of literature on pickup loading and the effect on tone - I won't go into it here).
Now imagine a second pedal connected to the output of the first pedal (pedal #2).
Situation 1: Assume that pedal #1 has a low impedance output and that pedal #2 has a high impedance input. In any circuit like this the maximum voltage is developed across the highest impedance in the circuit. If pedal #2 has an input impedance 100 times bigger than the output impedance of Pedal #1, then 100/101 of the signal will be applied to the input of pedal #2 (and 1/101 of the signal will drop across the output of pedal #1).
Situation 2: Assume that pedal #1 has a high impedance output and pedal #2 has a high impedance input (the output of pedal #1 is "matched" to input of pedal #2). Again, the maximum voltage is developed across the highest impedance in the circuit, but if both pedals are the same impedance, then half of the signal will be applied to the input of pedal #2 (and half of the signal will drop across the output of pedal #1)...
So what does this mean?
In situation 1 the input voltage to pedal #2 is 100/101 (or close to 100%) of the output of pedal #1, and 100% = no signal loss with a low-to-high impedance connection.
In situation 2 the input voltage to pedal #2 is 50% of the output of pedal #1, and 50% = 3dB signal loss with a "matched" impedance connection.
Conclusions
A "perfect" pedal would have infinitely high input impedance (in reality Meg-Ohms) and zero output impedance (in reality a few hundreds of Ohms).
The Boss FA-1 is a pretty good example of this...
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Edited by - Laurie on 11/17/2008 20:47:02 |
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pawnshop_trash
Gold Member
USA
603 Posts |
Posted - 11/17/2008 : 18:29:57
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thanks Laurie! 
can you (or anyone else) recommend the "best" (or a "quick and dirty") method for the average musician to measure the output or input impedance of a pedal, using a digital multimeter? would measuring resistance instead of impedance be close enough? would the pedal have to be switched on to accurately measure either value? would you treat a buffered bypass pedal differently from a true-bypass pedal? |
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DeFrag
Moderator
USA
3409 Posts |
Posted - 11/17/2008 : 18:53:15
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Measuring Input Impedance:
1. Create an AC source and connect that in series with a 1M variable resistor and the pedal.
2. Use a DMM to measure the frequency and amplitude of the source at the junction between the source and the variable resistor. Suppose you read 500Hz and 100mV RMS.
3. Start measuring ACV rms at the junction between the variable resistor and the pedal�s input.
4. Adjust the variable resistor until the ACV reading is 50mV at the junction.
5. Now disconnect everything and use your DMM to measure the resistance in ohms of the variable resistor.
That resistance is a practical version of impedance at 500Hz. For comparison, repeat the measurement for lower and higher frequencies.
Measuring Output Impedance:
1. Create an AC source with a known frequency, say 500Hz, and connect that to the input of your pedal.
Connect the the variable resistor to the pedal�s output and to ground.
2. Use a DMM to measure the frequency and amplitude of the output of the pedal, say 500Hz and 100mV RMS.
3. Start measuring ACV rms at the junction between the variable resistor and the pedal�s output.
4. Adjust the variable resistor until the ACV reading is 50mV at the junction.
5. Now disconnect everything and use your DMM to measure the resistance in ohms of the variable resistor.
An alternative, but similar approach is to replace the variable resistor with a fixed resistor (R) and measure the AC voltage across the resistor (VR) and across the source (VS). Then calculate the impedance as (VS-VR)�R/VR. |
Edited by - DeFrag on 11/17/2008 18:58:48 |
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pawnshop_trash
Gold Member
USA
603 Posts |
Posted - 11/17/2008 : 20:05:30
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| thanks, DeFrag... but what if all you have is a basic DMM? can input or output impedance be accurately estimated by measuring resistance with a multimeter? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 11/17/2008 : 20:45:38
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quote:
Originally posted by pawnshop_trash
thanks, DeFrag... but what if all you have is a basic DMM? can input or output impedance be accurately estimated by measuring resistance with a multimeter?
No... sorry dude :-)
PM me and i can walk you through how to do it with the gear you have. |
Edited by - Laurie on 11/17/2008 20:59:29 |
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DeFrag
Moderator
USA
3409 Posts |
Posted - 11/17/2008 : 21:26:36
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Impedance describes a measure of opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases.
As long as your DMM can measure RMS voltage you should be able to get some idea. But if your DMM doesn't measure frequency like mine does (Fluke 189), then you simply won't be able to tell how the impedance changes at different input frequencies. |
Edited by - DeFrag on 11/17/2008 21:27:11 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 11/19/2008 : 10:38:34
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Boss gives you the approximate numbers for their pedals in the specifications.
For example:
CS-3: Zi ~ 1 MOhm, Zo ~ 1 kOhm
SD-1: Zi ~ 470 kOhm, Zo ~ 10 kOhm
So if you were to connect CS-3 followed by SD-1 the signal loss would be 1/471, which is probably negligible.
The have some mistakes/typos too. They give for DS-1 Zi ~ 470 Ohm which is nonsense. |
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 11/26/2008 : 22:53:50
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This might be a stupid question, but which resistors set the input and output impedance in these pedals?
The quoted values seem to match the transistor bias resistors, not the resistors in line with the input/output signals. Clearly I'm misunderstanding something... |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 11/26/2008 : 22:59:30
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quote:
Originally posted by nathanscribe
This might be a stupid question, but which resistors set the input and output impedance in these pedals?
The quoted values seem to match the transistor bias resistors, not the resistors in line with the input/output signals. Clearly I'm misunderstanding something...
Ah, grasshopper... you ask very good questions!
The resistors do set the impedance - but it's not necessarily simple. This site has some introductory stuff:
http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
This is simplified - the "real" calculation of impedance that incorporates all the physical attributes of the circuit is very complex (and the subject of advanced Electrical Engineering courses), but the calculations above are really good approximations.
Opamp impedances are a bit trickier to calculate, and there is a bunch of reference material on the WWW.
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Edited by - Laurie on 11/26/2008 23:09:27 |
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 11/26/2008 : 23:05:51
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Thanks, I think! A bit more complex than op-amp circuits, by the look of it. With op-amp designs, I kind of know what I'm doing... well, enough to be dangerous.
(I'm self-taught wiith this, and until someone who knows explains it clearly, it can be difficult to grasp some of it. As I have found, you cannot ask a badly-written book even a simple question.)
EDIT: just seen your mention of opamps. Doh!
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Edited by - nathanscribe on 11/26/2008 23:07:52 |
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DeFrag
Moderator
USA
3409 Posts |
Posted - 10/01/2009 : 15:50:41
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| *bump* for an awesome topic! |
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Pedal impedance and signal loss
