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Preface... total amateur. I picked up 8 Pasco WA-9857 String Vibrators off of eBay for an art project. I need some way of driving them individually.... cheaply. So I bought a $12 XR2206 Function Generator DIY to try it out. Because that XR2206 kit outputs DC there is a 10MF capacitor on the Sin output to mimic AC current (at least that's my understanding).

I'm using a 12V 2.5amp PSU, Sin, and 3K - 65KHz settings on the Function Generator. The result is a high pitched squeal from the Pasco unit if I play with the Amplitude, Fine and Coarse potentiometers but no movement from the vibrator.

Is my problem here the 12V power supply, the settings on the function generator, the not-so-awesome function generator itself, or something I'm not familiar with/considering? If it is the function generator is there something I can use that is cheep and in a small form factor like what I bought.

Function Gen and String Vibrator

Mark David Gaal
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3 Answers3

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You need up to 10 watts of power to drive the vibrator. There's no way the Exar chip can provide that. Try feeding it through an audio amplifier. The 10Ω nominal impedance should be compatible with pretty much anything capable of driving 8Ω speakers.

Dave Tweed
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@Dave Tweed is right,

The cheapest solution, without putting your home stereo amp at risk is also found on E-Bay. For under $12. https://www.ebay.com/itm/1-X-10-Watt-4-Ohm-Class-AB-Audio-Amplifier-DIY-Kit-TDA2030-10W-Mono-Amp/350586610872?epid=1431792372&hash=item51a09724b8:g:K1sAAMXQjUNR1AsC

You would connect the output of the function generator you have, (KEEP THE CAPACITOR IN CIRCUIT!!), to the amp input, then the amp out put to the string vibrator. The power supply you mentioned will work fine for this project. Reply with any questions.

Sara Heart
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  • Thanks Sara, just a quick note I mentioned that I want to drive 8 of these. Let say I drove all 8 with the same signal (freq) and then just adjusted string tensions to get my desired effect. Is there a DIY amp kit that could satisfy power requirements of amplifying the signal to 8 of these Pasco Units daisy chained? – Mark David Gaal May 19 '18 at 13:17
  • i'm having trouble finding any low frequency, mono channel, DIY amplifiers that run on AC current that would be suitable to run 8 of the Pasco units wired together. Any suggestions? – Mark David Gaal May 20 '18 at 17:09
  • @markdavid, the answer would in large part be in what your budget is. Because you need about 80 watts of amplified signal, that's were the cost comes in. The rest is finding some DIY or used electronics to satisfy your requirements. – Sara Heart May 20 '18 at 23:19
  • <$100 would be nice (I only paid $140 for the 8 Pascos). I think i found a few options... but they all seem to be DC...? http://www.droking.com/100w-high-power-converter-dc-10-32v-to-15-35v-6a-boost-power-supply-module-adjustable-voltage-regulator-dc-12v-24v-adapter?search=amplifier&description=true If there isn't a better option i'd need some guidance on how to convert back to AC before going to the Pasco unit – Mark David Gaal May 21 '18 at 01:55
  • What I saw on that droking link is definitely not what you need. – Sara Heart May 21 '18 at 04:12
  • Because of your price limitations, and the specificity of what you are needing, you would need to make a 80W 10Vdc power supply, and 4 of these DIY amplifiers (as a for instance) https://www.ebay.com/p/20w-Lm1875t-Mono-Channel-Stereo-Audio-HIFI-Amplifier-Board-Module-DIY-Kit/21009783134?iid=122813305766 This would require a significant amount of building time. The most expensive components would be the transformer and the heatsinks for both the amps, and Power Supply. – Sara Heart May 21 '18 at 07:32
  • thanks!, what would you recommend if price where not a consideration? I also thought this might work... https://www.ebay.com/itm/1pcs-TDA7293-Digital-Audio-Amplifier-Board-Mono-Single-Channel-AC-12v-50V-100W-/152097380081?_trksid=p2349526.m4383.l4275.c10#viTabs_0 – Mark David Gaal May 21 '18 at 12:49
  • I'll skip the higher budget question for now, and say that for the cost and much reduced hassle, If you used a Transformer with 100V output, center-tapped so you could make a +/- 50Vdc power supply, the module you showed could in theory drive all 8 vibrators in series. A higher impedance (80 Ohms) wouldn't hurt the amplifier. Nice find! Plus I don't think you would need to run the vibrators at their full 10W rating anyway. But you will need one big heatsink for the amp module. and probably a fan as well, but that could be determined by test. – Sara Heart May 21 '18 at 21:03
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You have two things to keep in mind:

  1. Required power

  2. Required frequency

The answer from Dave Tweed covers power. I'll address the frequency.

Start by reading the manual for the string vibrator.

Note that the generator settings (where mentioned) are 100Hz or below. Also note that highest sampling rate mentioned for the measurements is 1000Hz.

From this, we can assume that the operating frequency of the vibrator will be less than 500Hz. We can assume this because of the nyquist rate. I'll let you read that rather than explain it myself.

The next thing is that the vibration frequency is related to the length of the string and the tension it is under.

The string used in the experiments is approximately the length of the strings on a guitar or a bass guitar. But, it is under no where near as much tension.

Because of this, you can expect the string to vibrate at lower frequencies than the strings of a guitar, and probably lower than those of a bass guitar.

So, frequencies in the kilohertz range are right out.

Conclusion:

  1. You need an amplifier to use your signal generator to drive the string vibrator.

  2. You need to use frequencies below 100Hz to drive the string vibrator.

  3. Your current signal generator will do, if you pair it with an amplifier.

Check to make sure the amplifier can operate at low frequencies. Many cannot. An amplifier made to drive a woofer or sub woofer will probably do better than a typical stereo amplifier.

A small note concerning power:

10VAC at 1A is the maximum power rating. I think you will find that the vibrator will operate at lower power.

More power means the amplitude of the vibration can be larger.

So, it might operate at lower power but you might need more power to make the amplitude high enough for what ever you are demonstrating.

Also, you will probably need more power at higher frequencies - though that might not be a large factor over the frequency range you can use.

JRE
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