Audio gen for guitar amps ?

Do you have a favourite?

In the 'convenience' category, consider an iPod with a few recorded test tracks.

Probably that means you need calibration of the output, too? A ten-turn pot and a trueRMS meter would be helpful.

Battery powered, then; the iPod and an old HP204C (Wien bridge) generator would fit. Or, anything with a good signal-level transformer.

Any generator, with an external attenuator, will do that.

Or, maybe a white-noise output? Mainly, (3) and (3a) will require an arbitrary waveform generator, nowadays; other sine/square combos are mainly inaccurate.

Arb or HP204 will qualify.

Arb should do that.

After warmup, the HP204 will do that. Or, an expensive arb. Or, a good CD player.

I can't think of any single item that does all these (but simple, easily available equipment can do any of 'em).

Combination of a 2Hz to 200kHz low THD oscillator, squarer circuit, 1000:1 stepped attenuator, fine frequency adjust, fixed level transformer isolated output for scope synch, another fixed output for the frequency counter.

Amplitude stabilisation is selectable between a glass bead thermistor or a diode limiter - the latter resulting in zero bounce but 0.2% THD.

Frequency adjustment uses a log wound, double gang 15kohms WW pot - for stability and low noise.

THD of the oscillator is 0.002% and with the aid of a -100dB notch filter I can test nearly anything.

.... Phil

... Phil

You can do all this with some very inexpensive stuff. To cover the whole range from 10Hz to 100kHz [well more like 10Hz to 89kHz, or any specific smaller range you want] with a little C/C++ coding and the ASIO interface to directly work with a 24 bit soundcard, like Creative Labs EMU1212 [old and cheap], you can make a generator to pretty much accomplish everything you want.

You can drive the generator with variable amplitude, with a set of simultaneous tones that will show you passband, phase, and distortion. Use a scope, or the soundcard's inputs to get a lot of information. From memory the drive is low noise, better than 16 bits with amplitude fairly well calibrated. The output is like a pure tone accurately/repeatably set to selectable amplitude.

For passband, I suggest driving with an independent set of tones, not square wave, but tones that just cover the complete range. Monitor the amp's drive with one input, monitor the amp's output with the other, do FFT's on both channels preserving the phases. Then comparing the FFT's will provide a great deal of information regarding not just passband and phase shift, but also the amount of distortion AND noise in the amp. Plus, you can even measure the spectral noise across the band, spot excessive [well, relatively excessive] noise peaks, like AC mains related.

I have my own set up and software for this, but more readily available with a simple to use GUI try Bob Masta's SW and use your own soundcard already in your system. The SW is small and non-obtrusive to a system. I think there's a free trial time for the full blown and if purchased you keep with rights to ALL future downloads. If you opt out after trial you get to keep some features anyway, like the generator capability, a very useful generator. Also this version has LCR Meter capability.

DAQARTA v7.60 Data AcQuisition And Real-Time Analysis Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

Bet a software wanker like YOU has never even seen the insides of one.

... Phil

not a software guru, just had to learn once for a project.

I have two guitar amps. One is a Fender, and yes they are robust construcion [they're in storage and I can't remember model numbers]

My main contribution is pure analog design of a unique mag pickup. Turns a 'cheap' $1100 guitar into sounding like a $10k acoustic guitar. Absolutely the first time I've ever heard great sounding E-A string harmonics. Maybe been around too many cheap guitars, but usually I HATE when those two strings are strummed. Always sounds a bit sour, but NOT with my pickup.

RATOR >suitable for day in day out, knock down drag out bench repair work o n smelly, >dirty, rusty old guitar amps."

Rusty ? you're lucky they got metal in them to rust. In my country it is ha rd to get metal, everything is plastic.

Anyway, what is so knock down drag out about it ? Are you going to do some sort of product testing and rating ? If you are talking guitar amps maybe i nstead of a low distortion generator you want a high distortion generator. they crave distortion in thoise things. In fact the crave the distortion by their favorite amp, at times the will build a seald box and mic the amp fo r a concert, or at least they used to. Really, all those big ass speakers a nd in the back is a little 25 watt Fender or some s*it with THE ORIGINAL SP EAK, in a wood box with foam all over the sides, playing its guts out, but the microphone in the box feeds the big ass 2,000 watt amps outside.

Two basic things I do not see you needing for this applicatiion. you do not need anything over 20 kHz and you do not need low THD. The thing you proba bly could use is a calibrated output level to make sure the amp distorts co rrectly at a certain level input.

When you are lookng for bad outputs and s*it like that, almost any signal w ill do. In fact I tesat regular audio amps sometimes just on a regualr inpu t,like for FM or something. I just null the output and input on the scope a nd see what is left. Divide it up mathematically and you get a total distor tion figure, which includes any phase shift induced error as well as any va riations in the frequency response. It always reads worse than the THD or I M specs but if it is under 1 % you are doing good. At least you can be rela tively sure that there is no malfunction in the amp.

We are not the Institute of High Fidelity, nor are we Julian Hirsch. That's something maybe to get across at Audiokarma. If you can REALLY hear the di fference, there was something wrong. It is hard to tell 1 dB in frequency r esponse and most people cannot hear 1 % distortion, but that varies. Studie s done a long time ago indicated that people were much more tolerant of eve n order harmonic distortion than odd order. And the fact is that most speak ers have so much THD that the spec is never published. It is a secret. A fe w do, but they are not among the cheap.

Anyway, as far as a generator just use resistors to divide it down and use the function genny. Float the DUT with an isolation transformer like the bo ok says. That's the way you do it. However it will not get your money for n othing and your chicks for free.

Are you going to do some sort of product testing and rating ? If you are talking guitar amps maybe instead of a low distortion generator you want a high distortion generator. they crave distortion in thoise things.

So you have never seen a guitar amp - right ?

.... Phil

A/B switch, headphones, attenuator; it's not hard to check uncalibrated tone controls that way. Or, with a math-equipped DSO, it can be quick to measure.

A real square wave has lots of bandwidth, beyond the important, audible, spectrum.

tability.

one

sure.

It is a VERY quick, easy and unambiguous way of seeing if all the controls do what they are supposed to.

spectrum.

Square waves contain only the odd numbered harmonics, predominately 3rd, 5t h and 7th with amplitudes 1/3, 1/5 and 1/7 of the fundamental.

With guitar amps, I use a 250Hz square wave and firstly try to set the cont rols so the output looks square on a scope. Next, I turn each control throu gh is full range and then back to the "flat" position.

Treble boost causes overshoots to appear at the leading edges of the square wave, treble cut rounds them. Mid and bass controls cause the horizontal p arts of the square wave to tilt sharply uphill and down. At the same time, you can see if the controls are working smoothly, free of noise and intermi ttent operation.

.... Phil

It doesn't meet all your specs, but what about a Keithley 2015? They can be had for about $500 on Ebay...

Look on the bay for a Heath IG-58 (several variations). They are very low distortion for the price. (Typically 0.04 to 0.02 % THD, depending on level and frequency). You can get a guaranteed one for $125 or less. Or wait for a bargain and grab one for $50. The Elenco with the big pointer is around 1%. Your smartphone can be used, too. I have not measured the distortion of such apps.

Yeah - that looks pretty good, except for the lack of a decent frequency di al.

My generator has a 3inch aluminium disk that turns with the frequency pot, marked across 290 degrees, turned by a 6:1 reduction drive using ball beari ngs.

I also have a ancient, valve Wien Bridge generator that uses a large, dual tuning gang coupled to a 30:1 reduction worm drive - as you might imagine, frequency adjustment is very silky and resolution is genuinely infinite.

Lotsa bench generators use dual tuning gangs for frequency, cos they give u ltra smooth, noise free adjustment that lasts for - unlike most dual pots.

But watch out if you try to float the internal circuitry and just leave the metal case safety grounded, it does not work. It is essential that that ca se and shielding around the tuning gang all be at the same potential or sup ply frequency leakage from the power transformer secondary injects straight into the oscillator and causes major amplitude modulation.

.... Phil

Yes. I misread my memory. Thanks. I have an early prototype HP200AB with a big knob, which is great for sweeping arrays for resonances but too fuzzy for absolute distortion testing. I am using my Sound Technology generator because it's 10 times quieter than the Heath. I have a crappy little Tek function generator which is great for scifi sound effects. My iPod Touch v3 has a sine wave generator.

I have considered attaching one of those cheap lanel mount vrequency counters. Won't help it adjust more smoothly but the accuracy has to be better than any dial.