Could I get some help building my test rig? Adding an oscillator?

[StompboxAH]

I'm building a Beavis board. I'm wondering if I can add a tone generator into the circuit so that I won't have to use my guitar every time i want to test a pedal. I'm having a hard time finding resources that address a few questions:

1. Where can i find a circuit that does what I'm asking for?

2. Could i activate it using a dpdt on-on switch?

3. If not 2, what switch would i need?

I just need some guidance. I've been on the laptop for the last hour+ and I'm just about depleted of my will for the night.  


Thanks guys.





~~~~~~~~EDIT:

http://www.beavisaudio.com/bboard/images/io_diy.jpg


This is what i want to build, with an integrated oscillator circuit activated by dpdt on on switch.

[GibsonGM]

Hi AH, welcome to the forum!  There are many, many 'test oscillators' out there like you describe.  R.G.'s site GeoFex, link above, has one that is pretty cool!  Yes, you can activate it with a DPDT, no problem.   I believe if you go to his site, there will be a write-up about it.   

If you net search "Valve Wizard", merlinb has built a great test setup with choices of frequency and so on.  A little more complicated, but a real nice piece of test equip.

"DIY signal generator BJT"  or the like would get results, too.  Let us know what one you choose!

[duck_arse]

one word - "phase shift oscillator". easy, one transistor, and it'll get you into practise for all those ea trems and uni-vibes and things.

[therecordingart]

If you want to go nuts:

http://jmkpcbs.com/wp-content/uploads/2014/03/The-Testing-Rig.pdf

Otherwise, I have a PDF for a small oscillator that was made by the same guy above, but he doesn't offer self etch files anymore as far as I can tell. If you PM me I'll send you the file, but I think he went commercial and doesn't offer stuff for free any longer....I don't want him to get angry at me for posting a link to it.

[~arph]

I need one of those testing rigs! Too cold in the shed so I need something with an oscillator and headphones out. Perfect! Thanks for the link

[deadastronaut]

so that I won't have to use my guitar every time i want to test a pedal.

Thanks guys.

buy a loop recording pedal...ive been using mine and its a very useful tool for testing..

[R.G.]

I got annoyed at this same issue some time back and I made up a cyber-guitar.

Well, OK, a fake guitar- note oscillator. 

A Nyquist oscillator runs by the phase shift through it being such that at some frequency, the phase shift makes the signal that arrives from the feedback path be in-phase with the input signal and reinforce it, and by having a forward gain from input to output that is big enough that it makes up for the losses of the feedback network. So it can provide its own input signal.

Strictly speaking, if the gain from input to output and then back through the feedback network is **exactly** one, and there is no disturbance to the circuit, it will not oscillate, but will start oscillating at the slightest noise or external disturbance. If the gain is over one, the signal builds up until distortion limits the signal some way.

But what if the loop gain is just under one? What you have is a resonator, a circuit that rings when it's disturbed, but the ringing dies out. A bell or a taut guitar string is a mechanical resonator, in that its mechanical resonance self-reinforces at only one frequency, but the energy losses in sound make the ringing die out. So if you set up a Nyquist oscillator (phase shift, Wien-Bridge, Twin T, whatever) and then tweak the gain down to just under unity, it will ring when some pulse signal disturbs it, but the signal will die out over time. The closer to 1.00000000 the gain is, the longer it will ring, until at exactly one, it will ring forever.

So I made up a phase shift oscillator with an easily-adjusted loop gain, and a small square wave oscillator that could have its frequency adjusted, nominally about 1-2 pulses per second. Then I coupled in a little of the pulse oscillator output through a resistor and small cap to "pluck" the phase shift oscillator and diddled the loop gain so the signal rang and died out.

Cyber guitar.

[StompboxAH]

so that I won't have to use my guitar every time i want to test a pedal.

Thanks guys.

buy a loop recording pedal...ive been using mine and its a very useful tool for testing..

Thats a great idea. I do have a ditto looper that I could use...

By the way man, I've been checking out the chasm reverb over the last week and I'm super excited to build it once I get my skills in order.

[StompboxAH]

If you want to go nuts:

http://jmkpcbs.com/wp-content/uploads/2014/03/The-Testing-Rig.pdf

Otherwise, I have a PDF for a small oscillator that was made by the same guy above, but he doesn't offer self etch files anymore as far as I can tell. If you PM me I'll send you the file, but I think he went commercial and doesn't offer stuff for free any longer....I don't want him to get angry at me for posting a link to it.

Yeah I actually have that pdf already. I didn't really want to purchase a pre fab pcb. I think i'd learn more with perfboard, but that may be faulty thinking.
I'm looking for a way to integrate an oscillator into the beavis board circuit and activate it with a DPDT on on switch. How exactly would I do that? I'm not too keen on etching just yet, I'd rather use materials i already have if possible.

thanks

[StompboxAH]

one word - "phase shift oscillator". easy, one transistor, and it'll get you into practise for all those ea trems and uni-vibes and things.

Could this be integrated into the beavis board pedal?

[StompboxAH]

Hi AH, welcome to the forum!  There are many, many 'test oscillators' out there like you describe.  R.G.'s site GeoFex, link above, has one that is pretty cool!  Yes, you can activate it with a DPDT, no problem.   I believe if you go to his site, there will be a write-up about it.   

If you net search "Valve Wizard", merlinb has built a great test setup with choices of frequency and so on.  A little more complicated, but a real nice piece of test equip.

"DIY signal generator BJT"  or the like would get results, too.  Let us know what one you choose!

Thanks man. I'll check out Geofex. I haven't had good luck finding the valve wizard. There are a few archived forum posts that I checked out via google but the pictures are gone and the links are dead...

I'll definitely be updating with the finished product once i can finalize my build plan.

[StompboxAH]

I got annoyed at this same issue some time back and I made up a cyber-guitar.

Well, OK, a fake guitar- note oscillator. 

A Nyquist oscillator runs by the phase shift through it being such that at some frequency, the phase shift makes the signal that arrives from the feedback path be in-phase with the input signal and reinforce it, and by having a forward gain from input to output that is big enough that it makes up for the losses of the feedback network. So it can provide its own input signal.

Strictly speaking, if the gain from input to output and then back through the feedback network is **exactly** one, and there is no disturbance to the circuit, it will not oscillate, but will start oscillating at the slightest noise or external disturbance. If the gain is over one, the signal builds up until distortion limits the signal some way.

But what if the loop gain is just under one? What you have is a resonator, a circuit that rings when it's disturbed, but the ringing dies out. A bell or a taut guitar string is a mechanical resonator, in that its mechanical resonance self-reinforces at only one frequency, but the energy losses in sound make the ringing die out. So if you set up a Nyquist oscillator (phase shift, Wien-Bridge, Twin T, whatever) and then tweak the gain down to just under unity, it will ring when some pulse signal disturbs it, but the signal will die out over time. The closer to 1.00000000 the gain is, the longer it will ring, until at exactly one, it will ring forever.

So I made up a phase shift oscillator with an easily-adjusted loop gain, and a small square wave oscillator that could have its frequency adjusted, nominally about 1-2 pulses per second. Then I coupled in a little of the pulse oscillator output through a resistor and small cap to "pluck" the phase shift oscillator and diddled the loop gain so the signal rang and died out.

Cyber guitar.

R.G.,

This stuff is a little over my head. I'll have to look into phase shift oscillators and some of the terminology you used. I'm very much a rookie at this stuff 

[GibsonGM]

You probably just want something like this, StompAH:  http://www.circuitsgallery.com/2012/12/RC-phase-shift-oscillator-transistor.html

About as simple as you can get for an actual sine oscillator.

Your computer SOUND CARD can be used very simply to set up tones for testing, tho!!  Only drawback is you need to be careful or you could short it out...build one like the transistor model and if you kill it - no worries....

[StompboxAH]

You probably just want something like this, StompAH:  http://www.circuitsgallery.com/2012/12/RC-phase-shift-oscillator-transistor.html

About as simple as you can get for an actual sine oscillator.

Your computer SOUND CARD can be used very simply to set up tones for testing, tho!!  Only drawback is you need to be careful or you could short it out...build one like the transistor model and if you kill it - no worries....

This is great. thank you so much.

[GibsonGM]

I meant to put R.G.'s out there too - pretty much as easy a build, and ppl say they like it.  I found it hard to locate on GEOFEX, but probably didn't look under the right terms!  His has more of a description and so would be easier to understand.
http://www.geofex.com/FX_images/q&dosc.pdf

[StompboxAH]

Thanks so much,

I had a really hard time finding it on geofex. the one that i did find had a dead link.

[GibsonGM]

I found the much more advanced one, not used for testing, but for something else.  I got the link to it out of a Small Bear article on biasing JFETS - remembered he also used that tone generator for his tests.  I'm doing similar stuff now, so have an interest and setting up my PC for sine wave output is kind of a pain.

BTW, I just put it together - I could use a "stand alone" tone generator like that too, everything else not from PC I have is square or triangle wave.  It took me about 30 minutes, including finding the parts in my junk box.  I used slightly "off" caps (.018u instead of .01, for instance). 

It's tolerant of weird values....and, the buffer is nice.  I tried loading it a few different ways while observing the pseudo-sinewave on my O'scope, and it stayed pretty stable.   And certainly "siney" enough for what we're doing!   So, if you get this together, it is probably a nice one to box up and use on your bench!   You could make it look pretty, and use banana jacks for outputs, etc.

[StompboxAH]

Do you think having a stand alone box is preferable? I was planning on fitting it all into one box.

[GibsonGM]

Well, I don't use a "beavis board", altho I'm sure that's a great way to troubleshoot/test, too.  So, I prefer to build these things into stand-alone boxes, yup.  Maybe toss a 9V in there (should last a LONG time with an on/off switch on the positive).  Just use banana jacks on the front, label it with a silver marker or something ("Sine 1kHz 100mV" should do).    Plug in jumpers with banana plugs on the end, away you go.

You COULD add this in to a test rig, too.  Just use a DPDT switch that connects to the osc. output on one side, your input jack for guitar on the other, and you can switch between them.  One set of poles can be used for an LED if you want.  All you really need is a SPDT to remove guitar and insert oscillator, as they would share grounds if you built it on board.   If any odd "tone sucking" were to be experienced, you'd lift the ground of the guitar as well, but I'm not seeing that happening...the osc. isn't for any 'quality' measurements, after all, just to trace problems or be sure something is working...

[bluebunny]

I got annoyed at this same issue some time back and I made up a cyber-guitar.

Any mods to make it self-aware?   



Seriously, this is dead neat.  It's the first thing I built.  I should use it more.  It's here, btw.