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Old 25th April 2009, 05:55 PM   #1
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Default Vibrator - what type of output stage do I need?!

I thought the title might get your attention!

Read on....

What type of (preferably monolithic) output stage do I need where, the output power can ramp up quickly to reflect an incoming signal that is tailing off quickly.(to maintain the status quo)

What's this for?

A guitar sustaining device (a string Vibrator - hence the title!).... I've built one, but have found the amount of power needed when a string is far away from the driver vs when it close is quite large (if you don't play guitar the string distance changes between it & the pickup depending where along the fretboard a guitarist places his fingers.)

My signal chain is as follows...

Guitar input (from normal pickup) -> preamp with AGC -> power amp chip ->string driver coil.

The alternating signal fed into the coil at sufficient power starts the string vibrating....but the circuit only has a very limited amount of time to 'control the string' before it fades away past the 'point of no return'.

If the the string happens to be close to the string driver, then getting it to vibrate well is easy, but if the string is a good distance away (ie say 8-10mm or 3/8"), then I'm finding that not the string driver is not getting enough grunt out to stimulate : the string quickly enough....therefore the string fades away as if no sustaining device where even there!

Not enough 'literal stimulation so to speak (wink, wink geddit?!! )

I'm running rings around myself coming up with a new concepts/strategies...I'm sure the answer is simple enough!

Comments welcome! literal stimulation
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Old 25th April 2009, 05:58 PM   #2
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I don't think it's a question of what output stage, more that your vibrator thingy simply isn't meaty enough to do a great deal. Even if it was attached to a big power amp it would probably have the same result.
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Old 25th April 2009, 06:09 PM   #3
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But if the string driver can 'stimulate (hehe) strings when close (& without it getting hot), then why would it be assumed it's not 'meaty enough'... ?

Also, when I squirt a sine wave from a signal generator into the string driver coil (at the same frequency as the string), I can get it to vibrate the string ...albeit I need to really crank the sig gen voltage output level up feeding into my poweramp chip (a TDA7052A). This to my mind, suggests the string driver *is* up to the job of driving the strings at 10mm distance...but there's a missing element/attribute somewhere in the circuit feeding the driver coil (ie when using the guitar pickup as the actual input as opposed to the sig gen)

I guess I'm left pondering whether this inability to drive strings at deistance is...

1. The ouput chip not having enough grunt.
2. The output stage being of the wrong type.
3. The reaction time of the whole circuit not being quick enough
4. The driver needing to be bigger (more meaty)

etc.

How can I nail what the cause is? (ie what test can I do to prove conclusivelyit's because of Xor Y etc). I have access to a scope, sig gen & DVM.
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Old 25th April 2009, 07:13 PM   #4
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Right, so you can get sufficient string movement using a signal generator and your existing power amp? So why then do you think the power amp is at fault? It's clearly got enough guts given the right input. Type of output stage is totally of no concern, there is no wrong type!

I think you just need more gain from your AGC unit. But you run the risk of oscillation, so be careful.
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Old 25th April 2009, 11:43 PM   #5
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thanks...I though I was going insane there - you're completely right of course (it didnt help that I've tapped my AGC 'monitoring feed' off from the wrong part of the signal path. It needs to monitor the input to the circuit (thereby ensuring that what's being sent from the guitar is of a constant level...once I correct this...everyuthing will be alright.

Thanks for your input!
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