Measuring output problem

[simon111]

I want to compare to pedals output gain.
I connect each one up to a signal generator set at 100mV ac (sine wave), then measure the output voltage in AC on my meter.
How come both are reading within 20mV of each other - i.e. 850mV @ 1Khz, but when I plug the output into my amp one is noticeably louder than the other.

WHY?

Is there something that I am not measuring correctly?

I am wanting to make both my fuzz faces (one for backup) have the same output and this I thought would be the best way was to measure the outputs to get an exact level matching, then on the louder one I could put in a volume pot to reduce it to the same level as the quieter one. It seems that my ears can tell better...surely thats wrong? I am obviously not measuring something right.

Any thoughts would be welcome.

Simon 

[brett]

Hi
one factor that differs between your meter and the amp is the input impedance.
The meter's input impedance will be much higher than the amp's input impedance, and so the readings will be biased (upwards) for any circuit with high output impedance (e.g. a fuzzface). 

To get the output volts equal, you'll need a true RMS measuring voltmeter (rather than an averaging one) and a load that is similar to your amp (usually 200k to 500k ohms).

But even then, one will almost certainly sound louder than the other.  As well as volume (volts) sounding loud, gain also sounds loud.  As you would know, some amps amps can get amazingly loud at relatively low wattage because they can run at high gain without the distortion sounding bad. Tube amps do this, and the Ruby does it quite well, too.

I'm not sure why high gain seems loud, but I suspect that because many components of the ear (such as the ear drum) are accelerated more by higher gain (delta pressure/delta time), and because these components have momentum (however small), there might be some "overshoot" that gives the impression of higher volume. 
cheers 

[simon111]

I might be able to get my hands on a Fluke Meter, which claims to be TRUE RMS. Would this help? And how do you mean adding in a load of 250K-500K? Do you mean put a similar sized resistor between the meters leads when testing (parallel)? or in series with one of the leads?


Is there a better way to test tonal differences then?


Simon.