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Thread: Can you recommend me a cheap tone generator?

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    Can you recommend me a cheap tone generator?

    Hi all,

    I have been using my android phone as a tone generator for some time, this was fine until today, when the INPUT on the PA system was putting OUT 14vdc, this has blown the headphone amp out on my phone and it's now useless... lesson learnt the hard way.

    I'd like a cheap alternative to exploding android phones, can anyone suggest a cheap battery powered device that can spit out a sine wave?

    Thanks.

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    The is about the cheapest lowish distortion idea I can come up with:
    Click image for larger version. 

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    Taken from pg 12 of this TI app note

    sloa060.pdf

    An output level control would be an essential addition.

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    I usually use my phone as well, or I have an old android tablet. If I had to I would just get a $5 thrift store android phone to dedicate to being a generator, or if you have an old one with crappy battery or something.

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    As high enough DC could also destroy the output stages of other direct coupled signal sources, it seems advisable to isolate the signal source output by a foil coupling cap of 1F or higher.

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    Quote Originally Posted by Helmholtz View Post
    As high enough DC could also destroy the output stages of other direct coupled signal sources, it seems advisable to isolate the signal source output by a foil coupling cap of 1F or higher.
    Or an audio transformer, with an appreciable high voltage isolation rating. I was recently donated a few of these, from an old-school PA/telephone tech.

    If, as our OP states in his opening paragraph, the INPUT to some audio system has 14V of DC riding on it, yes the cap is the best idea.

    FWIW I've popped the output chip on my Loftech TS-1 several times when applying signal, even through a 600V rated capacitor, to some point in a circuit. The sudden application of high voltage (which might be as low as a couple of volts, not necessarily hundreds) can wreck the testing circuit. May be best to bring up the voltage slowly with a variac on the unit under test.

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    Last edited by Leo_Gnardo; 10-03-2019 at 12:09 AM.
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    I have had pretty good luck with the Tenma 72-505. A lot of places sell them for close to $60, but if you look around, you can get them for about $40. I like that it has a synch out I can send to my o-scope.

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    I had a shower thought this morning, I could use one of those cheap bluetooth audio receivers and plug it into whatever needed a signal and send the tone from my phone/tablet/pc while remaining safely air gapped. It won't help me if some tesla coil type event occurs but if I blow up a 5 adapter it's better than a 200 phone!

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    Supporting Member Jazz P Bass's Avatar
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    "until today, when the INPUT on the PA system was putting OUT 14vdc,"
    Hah!
    Had a guy blow up 2 separate looper pedals that way.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Had a guy blow up 2 separate looper pedals that way.
    Yikes! I think I'll be packing a DI box from now on.

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    Quote Originally Posted by eschertron View Post
    Yikes! I think I'll be packing a DI box from now on.
    Speaking of audio xfmrs, Toa makes a couple inexpensive 10k bridging xfmr modules for their 900 series PA mixers that receive these Mic, Line, interface modules, as well as the Peavey modules that also support the Toa mixers. I bought a B-01 10k Bridging Input module for like $8-$9 on ebay some time back, and another one for $13. While they don't have extremely high LF headroom for input level, they'll handle real high signal level from 100Hz on up. No spec on their voltage isolation.

    900_module_guide.pdf

    These TOA modules are usually 'sleepers' on ebay, available cheap.

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    Quote Originally Posted by MichaelNuzum View Post
    I have had pretty good luck with the Tenma 72-505. A lot of places sell them for close to $60, but if you look around, you can get them for about $40. I like that it has a synch out I can send to my o-scope.
    That Tenma looks mighty like a Wavetek unit I first encountered in 1975, being used by PA wiz Don Pearson. Learned an awful lot from that guy! Can't beat it for portability.

    There was another pocket size oscillator offered early 80's by Fostex, intended for tape recorder alignment. Not a big choice of frequencies, I think it was just 40, 100, 1K and 10KHz.

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    I inherited an old AG-8 Heathkit audio generator from my grandfather.
    https://www.worthpoint.com/worthoped...tor-1922287307

    Far from being portable but it is not very heavy. My grandfather worked for Motorola here in Scottsdale, AZ and took part building transponder to communicate with the moon landing of 1969.
    https://www.azcentral.com/story/news...le/1739639001/

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    I know this doesn't help at all, but I use an HP200CD. It's built like a tank and goes up into RF frequencies. All tube and point to point military grade build.
    Click image for larger version. 

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    It's weird, because it WAS working fine.....

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    Before I even open the things, on old PV solid state stuff, I go around the jacks with a volt meter looking to see if any have 15v on the tip. Even if you are not using that jack (one with a shorted diode) it can affect operation. Imagine one in the FX loop jacks. That would mean that the signal path would be squiched up against a 15v rail, causing severe distortion and loss of signal.

    Those old Heathkits are great. I have several, the little IG72 is a useful little unit. The IG5218 is a very popular unit.


    Oh yeah, those old HP200 series are terrific units. I had an ancient one in a rack chassis, but they made 200s for decades.

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    My go-to audio oscillator is one I built over 40 years ago. I think it has about 6 parts built on Vero. Capacitor-coupled output, so protected against DC. It will do 1v p-p max and has pencil marks for typical output levels. Just a 9v battery and single level control for the fixed 1khz signal. I use this for most troubleshooting, though sometimes I need a variable frequency signal to test crossovers or bass amps so have a function generator for those tasks.

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    Thanks for the suggests everyone, I think I'll be asking my other half for a Tenma 72-505 for Christmas, that looks neat. In the mean time I found a bluetooth device for peanuts, less then the price of a coffee, so once that arrives I'll see how it performs. I might build something at a later date, it would make for a good learning project.

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    I've used the ICL8038 on Vero. Here's a complete kit - link

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    I have an old populated board from Maplin that uses the same IC - really versatile.

    There's a cheap XR2206 based generator widely available quite cheaply;

    https://www.ebay.co.uk/itm/XR2206-Si...0769bac9cc3cfb

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    Take a look at this: GW Instek, gag-810.Analog basic one, 200 bucks around, very happy with it.
    https://www.gwinstek.com/en-GB/produ...AG-810_GAG-809

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    Quote Originally Posted by Mick Bailey View Post
    I have an old populated board from Maplin that uses the same IC - really versatile.

    There's a cheap XR2206 based generator widely available quite cheaply;
    That looks good, I only have a 4ftx2ft bench to work on so anything small is preferable. I'll ask the other half to put that on the Christmas list

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