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Impedance & Amplifiers

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codan

New Member
Hi Everybody,

This is going to be a bit long winded-sorry!

I have been reading up on Impedance, for Amplifiers in particular, it
appears there is Low Impedance or High Impedance inputs & outputs.

My question is, at what Impedance range in Ohms is a circuits input or output classified as either Low or High Impedance?.

Next Question.

I found an old Peavey Chorus Classic 130 Amp at our local recycling centre that i purchased for $5 --condition unknown.
It had the mains plug cut off it so i thought it may not work & be good for studying & to scavenge some components out of it:).

I found no burnt components or anything that appeared to be shorting out with a DMM so i connected another mains plug & it works nicely.

On the rear panel it has:

240v-60Hz-400Watts

65W RMS
23v RMS
8 Ohm minimum load each channel.

It has two large speakers in it.

I want to make a preamp circuit for my electric guitar so i can amplify the guitar pickup signal & plug this into the main amplifier & study soft clipping.

My question with this is:

What is the maximum output (Volts, Amps or Watts) i can have from the preamp circuit to plug into the main amplifier?
The guitar is a very cheap model & the pickup output signal can be amplified a lot but i am unsure how far i can take this to safely plug it into the amplifier, i would like to take it as far as reasonably possible, for studying & to make more noise --of course & then try some sound effect circuits?

Next question:

The normal connection setup for a foot pedal is to plug a lead into the guitar & the other end into the foot pedal & then another lead from the foot pedal to the Amplfier.
I have noticed that some people plug guitar into the amp & then out of the amp to the foot pedal & then back to the amp, using the main amplifier as a preamp into the footpedal--Feedback Loop??. Apparently this gives a feedback sound effect as well or something?

I would like to try this & i want to make sure this is ok & how to wire this with the leads properly into & out of my amp.

The Amplifier has two inputs on the front panel:

Low Gain & Gain

On the rear panel it has four mono sockets marked:

PWR AMP IN ------ PRE AMP OUT
---1 V(0dbV)
-----1 ---------- 1



PWR AMP IN ----- PRE AMP OUT
---1 V(0dbV)
-----2 ---------- 2

Any help , hints or advice would be appreciated.

Thank You
 
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audioguru

Well-Known Member
Most Helpful Member
A guitar pickup works best when feeding a very high impedance preamp. There is a FET preamp on the internet with an input impedance of 3M ohms. the output of the preamp can feed your pedal.

Your Peavy amp is stereo (two channels). Since a guitar is mono then simply connect the two inputs of the amplifier together.
 

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codan

New Member
Hi audioguru,

Thanks for that, i see they use a J201 FET, i only have MPF102 & 2N3819 FETs at the moment, looking at the data sheets i don't think there suitable?
I will have to find some.

If you have the time, ok if you haven't, could i bother you for an example or an LTspice simulation of how the newer Amps Clip compared to how the older Tube Amps Clipped.

I find this interesting & would like to study the effects but not knowing what i am looking at or for at this stage makes it difficult to get a good start.


I was searching some older posts & found a Schematic you posted, i have attached it.
I have attached the ASC as well as i can't seem to get the circuit to simulate as you did?

I added an 8 0hm load resistor, not sure if that's correct, obviously i am doing something wrong that i can't work out?

Thank You
 

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audioguru

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i see they use a J201 FET, i only have MPF102 & 2N3819 FETs at the moment, looking at the data sheets i don't think there suitable?
The guitar preamp circuit can use just about any FET.

If you have the time, ok if you haven't, could i bother you for an example or an LTspice simulation of how the newer Amps Clip compared to how the older Tube Amps Clipped.
I have never seen a simulation of an old vacuum tube amplifier. Just measurements. they have poor low frequency and poor high frequency performance. They soft-clip.

(Re: The Fuzz-face circuit) I added an 8 0hm load resistor, not sure if that's correct, obviously i am doing something wrong that i can't work out?
The circuit cannot drive an 8 ohm speaker because it is not a power amplifier. It drives a power amplifier that has a high input impedance then the power amplifier drives a speaker.

Test the simulation without the 8 ohm speaker. The 500k resistor is actually the input impedance of a power amplifier.
 
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codan

New Member
Hi audioguru,

----"The guitar preamp circuit can use just about any FET."-----


Oh, ok i shall test some FET's out.


-----"I have never seen a simulation of an old vacuum tube amplifier. Just measurements. they have poor low frequency and poor high frequency performance. They soft-clip." -----


Hmm interesting, yes, it is the soft clipping i would like to have a close look at, something to study.



----"The circuit cannot drive an 8 ohm speaker because it is not a power amplifier. It drives a power amplifier that has a high input impedance then the power amplifier drives a speaker.

Test the simulation without the 8 ohm speaker. The 500k resistor is actually the input impedance of a power amplifier."----


I get it now, the strange thing is that i tried it with & without various load resistors & LTspice just showed a very erratic output waveform?.

I tried it without the 8 Ohm resistor this morning after restarting the computer & now i get very close to a squarewave form--Attached. Certainly very different than your result?


---"The "Fuzz Face" circuit drives a power amplifier of high input Impedance"--

Hypothetically speaking, just say i had a Power Amp that was of low input Impedance, is it possible to get things working correctly.
Can you simply just add more Impedance--Resistance to the power amp input to get it to work?


When testing a Power Amps output, obviously it should be loaded, what is the correct proceedure for applying a Dummy Load without the noise.

Can you just disconnect the speakers & use a suitable load resistor across the wires, i am not sure about this as i thought the speakers were a resistive & inductive load?


Thanks again for the help!
 

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audioguru

Well-Known Member
Most Helpful Member
Your input level is much higher than mine (100 times more?) so of course your output is a saturated square-wave.
The value of the output capacitor is calculated for a 500k ohms load. With only 8 ohms for a load then the output capacitor passes only the high frequencies which messes up the waveform at lower frequencies.
 

codan

New Member
Hi audioguru,

That's where i was getting confused, i was using a 100mV signal, that being the average ouput from my pickups.
So, the output from the pickups has to be regulated at 1mV for the circuit to function as designed, do i have that correct?

The LTspice sim works now i have reduced the input signal.

I breadboarded the FET preamp circuit you suggested & tried some different FET's, it works nicely & has nice sound, an improvement over the original sound output from the guitar, it's nice.

Thank You
 
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audioguru

Well-Known Member
Most Helpful Member
There is an article about the Fuzz Face circuit on the web.
At low levels it has low distortion.
At higher levels it clips but not symmetrically (so the fuzz is even and odd harmonics).
Then at higher levels it changes the sound.
 

codan

New Member
Hi audioguru

I see how it works now, i did a search & found some articles on it as you suggested. I also had a listen to some Jimmie Hendrix guitar work & i can tell you i play more like Jimmie Buffet:eek:

I see i have a lot to learn about Harmonics as well & will study up on this, i have to say i am not in favour of distortion with guitar & would like to create clean sustain like sound effects.

Are all guitar sound effects, maybe not sustain, just distortion of one kind or another?



One last question, well two of course, is there a suitable replacement for the CA3080 Opamp & also can i replace the LM1458 with the TL072.

Thanks for you help once again, i need to study up on some things.
 

audioguru

Well-Known Member
Most Helpful Member
The CA3080 is not an opamp. It is a different kind of amplifier IC. It is 40 years old and nobody uses it anymore so maybe nobody makes it anymore.
A National LM13600 and LM13700 are similar but are not exactly the same same since they are duals and are a little different.
Digikey has hundreds of the LM13700 in stock today.

A TL072 dual low noise opamp is much better than a lousy old LM1458 dual opamp.

Most old guitar effects had high distortion and had the high audio frequencies removed.
 
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