measuring input impedance

[dualaud]

hiya,

if i would want to measure the input impedance of a pedal, could i just plug in a cable and measure tip to sleeve with a multimeter? 

thanks for your help,
dual

[R.G.]

No.

Input impedance is an AC quantity. Using a multimeter like that  you can only measure whatever DC resistance happens to be there. Almost all pedals use an input capacitor with perhaps a DC pulldown, so you'd only be meausuring that, not what the effect does.

The simplest good way to measure input impedance is to hook up a large value pot, perhaps 1M to 5M, in front of the pedal as a variable resistor, from 0 ohms to full. Then put a certain amount of signal in; 100mV should be fine. Leaving the signal source alone, turn up the pot until you only read half the original input voltage (i.e. 50mv if you put in 100mv). Now measure the pot resistance. It's equal to the input impedance of the effect at that test frequency.

Input impedance really ought to be measured at a number of frequencies, as it varies, but most people measure it at some middle frequency, like 500Hz or 1kHz and let it go at that.

The method works because you are making the resistor be part of a divider with the input impedance as the other part. When the voltage divides equally, the pot and input impedance are "eating" an equal amount of signal, and you CAN measure the pot value.

[dualaud]

so to measure the input impedance i'd have to look at the schematic and calculate, or is there an analysis setup to determine the input impedance?

thanks,
dual

[R.G.]

I'm confused. What I described to you was the setup to determine input impedance: test signal oscillator -> variable resistor -> circuit, AC voltmeter measures voltage at circuit input.

As to schematic and calculate, it's generally easy to do that by inspection to an accuracy which is useful for stompboxes. We're generally not interested in whether the input impedance is 103,452 ohms or 110,376 ohms. We usually want to know is it about 100K or greater than 1M. Knowing exactly what the input impedance is is usually useful only for publishing specifications.

If you can tell me more about what you're trying to do, I can help more.

[Paul Marossy]

This is how to find the approximate input impedance of a guitar effect circuit:
http://www.diyguitarist.com/PDF_Files/ImpedanceTester.pdf

[dualaud]

ok, where to start ... (while keeping it simple).

first of all, thanks for your help.
@R.G., look at the posting date and your edit date. can it be that you edited the instructions later on? so i only had the first part of your answer?
anyway, that was what i saw looking at the post, and therefore my question. i couldn't see the instructions, sorry to confuse you.
@Paul, thanks for the link. it really helped me to visualise it.


i am located in germany, and i dug out an old german VU meter signal generator and analyzer; it was used in telecommunication tech.

the generator can be switched to have 0Ω or 600Ω output, which one to choose?
i have four 'outputs' on the generators front. plus and minus (i guess) and one which looks like a capital 'T' and one, which is a little bit towards the bottom of the device which has a 'Ground' symbol on it (a little like an upside down christmastree).

i wonder how to connect the setup. do i have to connect minus, or 'T' to the effects ground?
i get different readings when i connect either minus, or 'T' or the 'Ground' symbol to my setup.

as a side note, i can only switch the db value on the device, and switching it to 0db (plus the Ω switch to 0) gives me about 0.777V when i measure (ac) plus to minus and 0.205V to either, 'T' or 'Ground'. will this be ok to measure the input impedance?

the pots i have are 4M7 and 1M btw. i guess the 1M would be a better choice to be able to scale with finer adjustment, wouldn't it?

heck, i'll shoot a pic or two of the device with my mobile and link it.



thanks for your patience,
dual

[dualaud]

btt.

i am trying to solve my fuzz and/or buffer woes, which also brought me to measuring all my pedals for input impedance, to optimize my setup.
would i use the procedure described above for measuring output impedance as well?

thanks,
dual

btw,
the signal flow is:

guitar (Fender Jaguar)->
Boss-LS2 output 1->Strobostomp
Boss-LS2 output 2
->Foxx Fuzz/volume/wa (modified: external power supply; true bypass; volume pedal function disabled): in this position in the chain the fuzz sounds bad. 
->George Dennis Blues Wizard
->Boss CS-3 (modified: swapped components for better tone)
->EH-MicroSynth (modified: half guitar/half bass version; seperated circuit ground from internal tranny ground/chassis to prevent hum; modified circuit for pop free switch; On/Off Status LED)
->EH-MemoryMan Deluxe (modifed: On/Off Status LED; quieter opamps)
->Sherman Filterbank (modified: input overdrive kicks in later for cleaner filter sweep)
(->Oberheim Echoplex Digital Pro parallel, i.e. not in the signal flow when loop isn't running)
->Fender Deluxe Reverb circuit in Super Reverb chassis and cabinet

power on the pedal board is one regulated Boss tranny split to the Foxx, LS-2, Strobostomp and Blues Wizard.
the MicroSynth and MemoryMan have their own tranny.
the CS-3 has its own Boss tranny.

[slacker]

Most fuzzes sound best with your guitar plugged straight into them. That way the relatively low impedance of the fuzz interacts in a nice way with your guitar. If you put a fuzz after a pedal with a low output impedance like the LS2 then you ruin the interaction of the guitar and the fuzz and the fuzz probably won't sound as good.
I think the only real solution is to put your fuzz at the start of you pedal chain. 

[R.G.]

@R.G., look at the posting date and your edit date. can it be that you edited the instructions later on? so i only had the first part of your answer?
anyway, that was what i saw looking at the post, and therefore my question. i couldn't see the instructions, sorry to confuse you.

That's because I got interrupted after only the first paragraph. I had to come back and add the instructions.

the generator can be switched to have 0Ω or 600Ω output, which one to choose?

0 ohms.
The "T" symbol is ground, the "christmas tree" is an arrow. On the right hand side it's pointing out, indicating an output. On the receiver side, it points in, indicating an input.

You have available outputs of less than 0dbm. 0dbm is 0.775V, but you can pick -20dbm, about 0.0775Vrms, which is very close to guitar pickup level. I would use that one.

It has been my experience that only a few vintage fuzzes care about being plugged right into the guitar. The Fuzz Face is one, there are others; but modern fuzzes usually have a buffer inside them.

i am trying to solve my fuzz and/or buffer woes, which also brought me to measuring all my pedals for input impedance, to optimize my setup.

What are those woes? You may not be looking in the right place.

[dualaud]

@slacker, thanks for your hint. i have extensively researched this topic, and i am planning on building something like Skrydstrups "Fuzz Magic Box" into the Foxx, for i think i'd be good to have a buffer as a first device.
it is basically a simulation of a guitar pickup ('reactive' resistance and capacitance) and the following network of tone and volume pot as a low pass filter.
i put the Skrydstrup between my buffer (LS-2) and the Foxx and it sounded very good, but when i switch off the Foxx the Skrydstrup device stays in my signal chain, and i just want the lowpass to active when the Foxx is active, so it doesn't make sense for me; but build in the Foxx it will be switched off with the Fuzz.
a similar circuit is explained on the AMZ site, but doesn't feature a 'real' pickup coil. but i'd recommend it to anyone with the same problems; it sounds ok and would be appropriate for most applications.

@r.g., i'll get back to the topic when i'm @home in front of the generator. thanks.

[dualaud]

hey,

i took my measurements and i am going to write down my measurements following my signal flow contrasting input and output impedance. (i measured the output impedance the same way as i measured the input impedance; i figured i should be able to apply the same procedure)
all pedals on:

LS2
905K Input Ω
1K Output Ω

Foxx
51K Input Ω
11K Output Ω

Blues Wizard
418K Input Ω
1K Output Ω

CS3
869K Input Ω
1.1k Output Ω

MicroSynth
61k Input Ω
1K Ouput Ω

MemoryMan
90K Input Ω
2k Output Ω


all pedals (except LS2) off:


LS2
905K Input Ω
1K Output Ω

Foxx
True Bypass

Blues Wizard
394K Input Ω
1K Output Ω

CS3
879K Input Ω
1.1k Output Ω

MicroSynth
True Bypass

MemoryMan
92K Input Ω
7.6k Output Ω


for the record output and input Ω of the LS2 are the same, no matter if the device is on or off.

what is striking to me is that:
- the Boss devices, and the George Dennis Blues Wizard all feature appropriate input/output Ω in either on or off state.
- i have 'mismatches' between, e.g. the CS3 and the Memory Man, or is 92K considered high impedance? the pdf posted by Paul Marossy reads that an ideal input Ω of an effect device to take guitar signal would be around 200k-500k .
- the MicroSynth has only 61K input impedance, when turned on.
- the sound of the Foxx is not desirable when switched on behind the buffer, and i don't think this is only an impedance problem.

whats your opinion about:
- the Boss buffers. are they ok to use? 
- the 'mismatch' of boss output Ω against EH input Ω? does the connection need to be buffered, or can i adapt the EH devices to have a higher Ω input easily?
- why is the Memory Man not featuring a high Ω input? i read somewhere there is a buffer circuit in there. would this compensate the 'wrong' input Ω it has?
- not only did i choose the Memory Man as last device because of its effect type, but also because i read it features a buffer, and could drive the cable to my amp properly. is that a good, or bad idea?
- the main problem: the gritty sounding Foxx after the Boss buffer. with its 51K input Ω the Foxx is not a wise choice as first stompbox to be fed via a long cable, is it? so would you advise to build a small 'debuffer' similar to the Skrydstrup Fuzz Magic Box or like explained on the amz website (guitar pickup simulator), but with a custom made coil into the Foxx?

whew, thats a lot
thanks for your help,
dual

[slacker]

I'm not as expert on this as some people but basically the general rule of thumb is that you want the input impedance of a pedal to be at least 10 times greater than the output impedance of the thing driving it. This means that you don't get any unwanted losses in your signal. This normally means for non true bypass pedals you want a high input impedance and a low output impedance. So your Boss pedals and the Blues Wizard are fine.
As the Microsynth is true bypassed the low input impedance doesn't really matter as long as you're happy with the sound of the pedal. You can increase the input impedance by changing one resistor, check out Craig Andertons mods for more details. http://hammer.ampage.org/files/Device1-6.PDF
As the Memory is always driven by about a 1k output impedance in your setup the input impedance is probably high enough. It has a low output impedance so it will be able to drive the cable to your amp fine.

If the only problem is that you don't like the sound of the Fox when it's after the LS2 I'd just put it in front of the LS2 so your guitar is going straight into it. As it's true bypass it wont affect the sound when it's off and when it's on if it sounds how you want it to that's fine.
I haven't tried the various pickup simulators but they are supposed to work. I don't think the guitar's controls will interact with the pedal in the same way as if it was plugged straight in though so depending on how much you rely on turning down your guitar to get different sounds you might lose some something.

[dualaud]

hey,

do you think the Foxx would work ok with a 10 meter cable between it and the guitar?
putting it in front of the LS2 would be the best solution, i figure.
and if you advise that 2k to 8k as output from the Memory Man to my amp would be good through a 10 meter cable, then i'd do it that way.

thanks for reminding me of Device; i totally forgot about that. 
*opening the MSynth right now.*

cheers,
dual