Analogue signal generator (and maybe more related questions)

[charangohabsburg]

I am tempted by Trevor's point of view (from another forum thread):

Feed the output from your computer to your amp/speaker and you should be right, but adjusting the frequency is not as convenient as twiddling an old analogue knob.

I think I'll go for the knob twiddling.
Without worrying too much I scored this one on eBay:
(click on image for detailed view)
It should arrive in a few days and I wonder now how I can get the generated frequencies to a loudspeaker!
Yes, exactly, I have no great idea of electronics, last time I got deeply involved with it I was a teenager, buying used TVs for $1, hitting them here and there until they worked again, an selling them then for $50.

Of course, I'll have to send the signal to an amplifier.

• But what kind of amplifier?
• Can it be directly wired or will I have to put some electronics between the two devices?

Technical data says:

• Sine output amplitude: 0...>=6V
  Internal resistance: ca. 600 Ohm

Peter Coombe's setup also looks very nice. Peter, does your tuner display frequencies also in Hertz or does it indicate the deviation from standard notes just in cents like any other electronic tuner? I really like the idea to not always depend on a computer.

It might be the most expensive knob I ever bought!

[matthew]

send the output directly to the line/aux/cd input of any audio amp. And any suitable speakers will be fine. it's just an audio signal.

the only problem with the analogue generator is knowing precisely what freq you are actually outputting. The analogue dial isn't so precise. But if you have a digital tuner or an iphone app that will do the trick.

[MattW]

Nice score

Any Hi-Fi Amp or PA amp or any Audio frequency amp will drive the loudspeaker for this. From a quick read of Trevor's post you linked, and from Peter Coomes' setup, it might require a sizable speaker (10-12") and a decent amount of power (100+ watts) from the amp.

You wont need any electronics between the output of the Signal generator and the input of the amp. The 6 Volt output may be a bit large for a hi fi amp though if its turned up full steam and cause distortion. This distortion will add harmonics to the sine wave, so you wont have 100hz, but 100,200,300,400 etc in decreasing amounts.

The 600 Ohm spec refers to the Sig Gens ability to drive a load. So the lower the better. A Hi Fi amps input load is around 2-5K ohms or so, so when the signal generator is driving this load the actual voltage out drops significantly, reducing the drive on the input.

As matthew said the dial indicator isnt going to be very accurate.

[charangohabsburg]

Thank you very much Matthew. Great to know I'll just can plug in.

Yes, I also thought that I must measure the output frequency, the dial indicates only 5 hz steps around 100 hz, so 2.5 hz or maybe 1.66 hz might be the most accurate I could go with the dial only (the sound generator got recently calibrated).

Of course I could (and probably will) use a software like Spectrum Lab which I find very nice to read off precise data including the actual max. peak frequency, while Visual Analyzer displays "nicer" curves (thanks Trevor, for mentioning these programs in your book).

A frequency counter would be nice to have but would add another $100-150 to the bill.
Not to speak about my lack of knowledge how to feed the microphone signal to the frequency counter. Well, maybe a frequency counter would be sort of technical overkill.

[charangohabsburg]

Nice score

Any Hi-Fi Amp or PA amp or any Audio frequency amp will drive the loudspeaker for this. From a quick read of Trevor's post you linked, and from Peter Coomes' setup, it might require a sizable speaker (10-12") and a decent amount of power (100+ watts) from the amp.

Thanks a lot Matt.
Maybe Peter's setup is a bit different from Trevor's one. Trevor talks about 10 Watt RMS (speaker? amp?), and he uses much smaller speakers, held over the instrument and driving the antinodes +/- directly.

Your mentioning of overdriving the speaker reminds me of those beeping "tuners" like the one I find built in my electronic metronome (Korg Ma-20): when I looked at it's "440" hz signal, not one peak was at 440 hz , but the peaks were were on 2200, 1760, 1320, 2640 etc, 6160 hz being the highest peak, and 16280 hz being only 4 db weaker than 1320 hz. Trevor addresses this phenomena (and it's uses) in his book.

The 600 Ohm spec refers to the Sig Gens ability to drive a load. So the lower the better. A Hi Fi amps input load is around 2-5K ohms or so, [...]

So 600 Ohm doesn't seem to be bad at all.

[...] a digital tuner or an iphone app that will do the trick.

Maybe I'll get an iphone + app, just to have a decent tuner, even if I won't use it for phone calls!
Actually I am still waiting for the iphone that will be usable as a hotplate for preparing hot hide glue.

[kiwigeo]

Actually I am still waiting for the iphone that will be usable as a hotplate for preparing hot hide glue.

iGlue is due for release next week.

[matthew]

Any 4"-6" speaker will be fine. You are not needing much sound energy if you can get the speaker close to the plates. A bigger speaker wouldn't hurt. You are not going to be using very high frequencies.

I use a big old Plessey 12" woofer, but there again, I build much bigger instruments with stiffer plates.

You could use your computer to check the frequency. If your computer has a mic input, I'm sure something like this http://www.aguitartuner.com/ would even do.

[peter.coombe]

Mine is a fairly powerful setup with 140W hi fi power amp and 120W speaker. I usually set the speaker in the frame and mount the mandolin plate above it, but you can do it the other way around, and in many cases it is better to do it the other way around, especially with guitars. If you are able to put the speaker right up close to the guitar plate then you don't need so much power because you can move it around and drive the antinodes. A mandolin plate is carved so I can't get it really close. My signal generator is a digital sine wave generator, so I get an accurate digital readout of the frequency. Basically it consists of a 511 timer chip that generates a square wave, a filter to convert the square wave into a sine wave, a few switches and pots to adjust the frequency, a frequency counter, and a LED display driver. The frequency does drift a little bit because the 511 is actually an analolgue timer, but it is fairly minor so not really much of a problem. You do need fine control of the frequency, especially if what you are measuring has a high Q, otherwise you can miss it entirely. Mine has course and fine adjustment of the frequency and has a range of fractions of a Hz up to around 1Mhz (is overkill). So, the more power the better, and the more fine control and accurate the frequency reading the better. It is much easier to twiddle a knob than to stuff around with a computer. It is also a good idea to have some sort of speaker protection in the amp becasue it is easy to blow up a speaker with a sine wave, especially if you overdrive the amp.

[charangohabsburg]

Back from my TV-kick-repair years I had a box full of speakers of every size. About 6 years ago I entrusted the whole treasure to the dumpster.
Now I have a problem to find a halfway decent and affordable speaker.
Probably I'll just get one out of my car. Three speakers are enough for a car.

Actually I am still waiting for the iphone that will be usable as a hotplate for preparing hot hide glue.

iGlue is due for release next week.

Martin, tomorrow at 8pm GMT I'll offer some perfectly quartered, straight grained, nicely figured iWood at Buy, Swap and Sell. The best of all: it will not occupy your precious shed space, it will only cost your hard earned $$$

[charangohabsburg]

Thank you Peter for your detailed description. Now I understand why you need so much power. Also, you and Matthew handle carved, relatively thick plates which certainly require more power to produce the Chladni patterns compared to the ca. 2 mm guitar tops.

I am not interested in the free plate modes, so the speaker-above method is the only way to go. Hence, a huge speaker is not very tempting. My hand already starts to ache when i imagine holding a 2.5 kg heavy vibrating monster over the instrument. A frame holding the speaker over the instrument would be a bit awkward to work with.

It is also a good idea to have some sort of speaker protection in the amp becasue it is easy to blow up a speaker with a sine wave, especially if you overdrive the amp.

Am I right if I suppose that every HIFI amp has some kind of speaker protection? Or are they just underpowered for the recommended speakers?

[Dominic]

I was looking at old SG on ebay but given Peter has successfully used a digital one I started looking at them on ebay. In the end I decided to try a small unboxed circuit board with a display stuck on it. Cost about $40 and I imagine it would be similar to Peter's one built from a kit. I also got a 40w component amp from Jaycar and wired that up. As I mentioned in a previous thread, the SG on the computer was not really working, output too low and resolution low. I also got a decent speaker which was discontinued and so was quite cheap. All up it will have cost just over $100 including a power block for the amp and a bit or wiring. I'll let people know how the SG board works. They had others for a lot more money that seemed to have the same specs but had a box. Seemed a lot of money to pay for a platic box and a few dials.
I imagine there will be a few people buying this gear so we should keep in touch and compare how we go with the various options available.
Good luck with your set up.
Cheers
Dom

[peter.coombe]

Am I right if I suppose that every HIFI amp has some kind of speaker protection? Or are they just underpowered for the recommended speakers?

Most do have some sort of protection, but many are more for the protection of the amp than the speaker so don't rely on them entirely. The thing is, with a continuous sine wave you are putting a lot more continuous power through the amp than with music. Continuous power produces heat in the voice coil of the speaker and the insulation and voice coil melts. Speakers have a much lower power rating for continuous RMS sine waves than for music type signals. If the amp is driven into clipping, then your speaker is even more likely to blow up so it pays to have an amp bigger than what you need, and a speaker protector that triggors before the amp starts to clip. Most power amps will clip at +-2V input because that is the maximum output of a CD player according the red book specifications. So if your sine wave generator outputs more than +-2V then you are in danger territory. It only takes a few seconds and the speaker is toast. The easiest and simplest protection is a fast blow fuse or polyswitch. I have a polyswitch in the amp that triggors below 120W. The polyswitch is connected in series with the speaker and warms up as the power goes up and after a certain current level (and heat) is reached it's resistance shoots up, thus greatly reducing the current and hence power to the speaker. Turn down the volume and wait, the thermister cools down and everything goes back to normal. You need to match the triggor point of the polyswitch with the individual speaker power rating.

[peter.coombe]

I have done a quick search and looks like my device is no longer available. There is a new kit that is a true digital signal generator and it is available from Altronics for around $150.

More information - http://www.siliconchip.com.au/cms/A_111885/article.html
Altronics page - http://www.altronics.com.au/index.asp?a ... m&id=K2553

[kiwigeo]

Do you reckon this speaker would be big enough for the job?

[Dominic]

I have done a quick search and looks like my device is no longer available. There is a new kit that is a true digital signal generator and it is available from Altronics for around $150.

More information - http://www.siliconchip.com.au/cms/A_111885/article.html
Altronics page - http://www.altronics.com.au/index.asp?a ... m&id=K2553

Peter, I don't know much about this stuff but these kits look like what I ordered from ebay http://www.ebay.com.au/itm/280715678459 ... 1439.l2649
Or there is a more sophisticated looking device from a local supplier for $115. http://www.ebay.com.au/itm/110732990085 ... 1438.l2649

I can’t tell much difference in specs between these and the more expensive kits but I don’t know what I am looking for. But they are used to test audio equipment etc and have a resolution of 0.01Hz which seems to imply they are reasonably accurate. Do you have any thoughts on these devices Peter?

Cheers
Dom

[Trevor Gore]

If you can bear with me for a short while, I'll put up some specs of a sig gen designed and built specifically for Chladni testing. I'm just collecting some more info and pics together. It's a small unit with a built-in power amp, so you won't need separate units. It is not the cheapest, however, at about US$500. Depends how you value your time, I guess. Those on low budgets and who know their way around electronics will undoubtedly find less expensive ways.

Trevor.

[peter.coombe]

Dom

Looks like both would do the job, but they are not the same thing as the Altronics kit. The Altronics device is from the Aussie Silicon Chip magazine, designed here in Australia, not in China. It has more functionality so is useful for other things that you would probably never need.

[kiwigeo]

There seem to be quite a few software based tone generators available in Mac and Windows flavour. I don't see why you cant feed output from a PC or Mac into an audio amp and speaker.

I'm currently playing with this share ware program on my Macbook Pro:

http://www.katsurashareware.com/pgs/audiotest.html

[Kim]

I don't know how effect it would be, but the Peterson V2 tuner or its successor the V-Sam both have a tone generator. If I recall correctly, my V2 has set steps from 32Hz through 6000Hz but can be adjusted manually between the steps 1Hz at a time so you can select what you like by just turning a control pot until the number your after displays on the LCD screen...its been a while since i played with it though so would need to double check and confirm that if anyone is interested. I do know that it out puts to an amp via a normal 1/4" jack because I ran it through a practices amp one time just fooling a round when I first got it.

My point is that if you look around you can pick up the later model V-Sam of the same thing for just a few of hundred bucks these days and if the tone generator is the same quality as everything else Peterson does, then you wind up with an excellent virtual strobe tuner throw in with the deal. Peterson say their generator is accurate to 0.5Hz BTW.

http://www.petersontuners.com/index.cfm?category=92

Cheers

Kim

[matthew]

you can use computer based tone gens perfectly well into an amp. the downside is the contr mechanism of mouse or buttons is not use friendly as a knob.

a tone gen with increments instead of continuous sweep will not work well.

[Kim]

I did mention that I think my model Peterson has pre-sets but also can be adjusted between those pre-sets @ 1Hz increments. Not sure what the advantage of a sweep would be over that?

EDIT: I will also add that the V-Sam replaced my V2 in the Peterson range and I do not know if they kept that function or done away with it when they included an internal speaker.

Cheers

Kim

[Dominic]

Martin, what resolution does it have. The one I found that worked easier enough on windows only went up in 2 Hz increments. And given it comes from the headphone jack it is a very weak signal in the lower frequencies. Other software SG I found you had to set the frequency by going into the options rather than use a slider to sweep. If anyone finds a decent software SG let me know. But the setup I will end up with is all on one small mdf board with one power plug so is pretty basic.

But I think the more important set up is the mike and spectral analyser. Haven't tried yet but I have been using a mike into Strobosoft and using the tap tuning function to record freeplate frequencies on my last 8 guitars. The mike cost about $80 and seems to do fine but I'll run so tests into Visual Analyser and see how weel it picks up the profiles.

Cheers
Dom

[Dominic]

I did mention that I think my model Peterson has pre-sets but also can be adjusted between those pre-sets @ 1Hz increments. Not sure what the advantage of a sweep would be over that?

Given the level of accuracy we are trying to tune our various frequencies to and the closeness of notes in the lower range it might be worth knowing that your plate actually resonates at 169.6 rather than 170. From memory some of the gaps are only 4Hz. So a sweep setup along with spectral analysis using a mic seems like the best option.
Dom

[charangohabsburg]

I'll put up some specs of a sig gen designed and built specifically for Chladni testing.

That will be helpful for many of us!

Do you reckon this speaker would be big enough for the job?

Should be just fine...

...for this one:

youtu.be/

Martin, of course a good software signal generator is much cheaper and does the job, but I find it easier to turn a real knob to adjust the frequency (while holding the speaker over the guitar!) than to fiddle around with a software knob or typing in a number each time I want to change the frequency. It's all about the knob (and for me about beeing a noob regarding Chladni patterns).

I got two steps further now.

• =>
  
  I ordered a decent 6" bass speaker 50..3000hz (vs. dismantling my car) which cost me about $20.- (including shipping from Germany to Switzerland).
• =>
  
  And I also ordered a low cost digital frequency counter (another $20.-, including shipping from Hong Kong to Switzerland) which only displays in 1hz steps, but this will be good enough as I mainly want it to assign the modes T(1,1)2, T(1,1)3, T(1,2), T(2,1) and T(3,1) to the tapping response curve without doubts left.

Next week I'll have to rescue my old, kit-built "guitar" amp to see if it fits the requirements, and if it doesn't I'll go for a new kit. All in all I'll land close to $200 including power supply, which means that I must not spend more than $300 worth of my precious time (comparing with what Trevor mentioned before).

[kiwigeo]

Martin, what resolution does it have. The one I found that worked easier enough on windows only went up in 2 Hz increments. And given it comes from the headphone jack it is a very weak signal in the lower frequencies. Other software SG I found you had to set the frequency by going into the options rather than use a slider to sweep. If anyone finds a decent software SG let me know. But the setup I will end up with is all on one small mdf board with one power plug so is pretty basic.

Audiotest has a slider for setting frequency or you can type desired frequency into a window. Sweep doesn't have an increment setting...it's only continuous between two frequencies.....these are also selectable by slider or typing into a window.