Re: gEDA-user: Signal Source Setup for Electric Guitar?

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

Michael B Allen wrote:
> On Tue, May 12, 2009 at 6:43 PM, Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
>> Michael B Allen wrote:
>>> Anyway it looks like their AC generator is using 2mV. So the 20mV
>>> value I used to get a good output SIN looks closer to reality which
>>> means my model is probably ok.
>>>
>>> I wish I had a real oscilloscope to find out what my guitar is really
>>> putting out but xoscope doesn't seem to produce voltages (but I'm not
>>> surprised since the sound card is probably oblivious to such things).
>>> Maybe I'll have to get one of those PC oscilloscopes.
>>>
>> I don't know much about E-guitars, other than having enjoyed a live
>> rockband here this weekend. But AFAIK the magnetic pickups are high
>> impedance, above 100kohms. Much higher if piezo. Sound cards in contrast
>> are in the kiloohm range so most likely the signal from your guitar
>> collapsed the instant you plugged it into the sound card. Also, keep in
>> mind that sound card inputs often carry a little DC voltage to feed
>> electret microphones. You can measure it with a voltmeter. That can
>> cause a DC current through the pickup coil and saturate the core.
>>
>> If you want to display the waveforms or an FFT spectrum on the PC
>> cheaply you could use an emitter follower as a buffer so the voltage
>> gets transferred 1:1. Then a high input impedance is provided to the
>> guitar even if you connect a sound card.
> 
> I think I would rather buy a PC oscilloscope only because building the
> buffer circuit requires time and knowledge that I don't really have.
> Again, this is not work - it's just a hobby. I'm still trying to wrap
> my head around the concept of impedance in general and especially it's
> relation to frequency and how capacitors work and so on. So I think it
> would be better to get something that I know is going to just work.
> 
> Has anyone tried the "Parallax USB Oscilloscope"? It's only $139. It
> can certainly handle audio and I assume it will tell me voltages, do
> FFT etc. The only problem I can see is that only has 8 bits of
> resolution.
> 

No idea. All the PC-scopes I've looked at so far only produced yawning. 
The Picoscopes seem pretty good but expensive.


>> Probably the input impedance must be set to a certain value (by means of
>> an extra resistors) so the guitar "likes it" and "sounds alright".
> 
> This is very interesting. I had no idea the difference between the
> impedance of the sound card and a guitar amp was so significant.
> 
> I built the circuit last night (although I haven't tried it yet). So
> now I have to wonder about it's impedance wrt to the sound card.
> 

If you connect it smack to the collector of a transistor (via a 
capacitor, of course) then the impedance will be loading it down. The 
gain of a single transistor stage like yours is roughly equal the 
combined resistance hanging on the collector node divided by the emitter 
resistor. So your gain would drop.


> Another thing I was thinking about doing was using siggen to feed the
> preamp circuit a SIN and then feed the result back into the sound
> card. So what is the impedance of the sound card output jack? Now I
> need a high impedance SIN generator.
> 

That depends on the card you have. Some can drive small speakers 
directly and are in the 10-20ohm range. Others only offer line level 
output and that would be in the kiloohm region.


> I get the feeling the whole sound card solution is not going to be ok.
> I either need a high impedance buffer circuit or an oscilloscope.
> 

Read up on them. I have used the sound card of laptops at clients and 
found noise problem that >$10k audio spectrum analyzers were unable to 
see. Sometimes to the utter amazement of engineers witnessing this. Seen 
some jaws drop ...


> This is a lot harder than I thought. I think I have a lot of reading to do ...
> 

Yep. And the good news is that thanks to the Internet such information 
is essentially free of charge. Or to stay with the tune of this group, 
open source :-)

-- 
Regards, Joerg

http://www.analogconsultants.com/



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