Foreplay III Noise Floor Frequency Spectrum

[bmgrlfl]

Shown below are two plots of the spectrum analysis of my stock Foreplay III.  The first one is the noise floor alone and the second includes a 500 Hz sine wave input around 0.75 rms.  Apparent is some predominate left channel 60 Hz hum and the 120 Hz buzz is only around 70 dB down from the input.  Should the 120 Hz be further down in level or is this measurement in line with what others have measured?  All my voltage and resistance measurements are correct.  Thanks for any input.

 

[Paul Birkeland]

It will be pretty tough to lay hands on a stock Foreplay III to compare, but I would comment that part of the Extended upgrade was aimed at reducing the 120 Hz component. 

It's also important in a measurement like this to check the analyzer with each cable connected with a barrel connector, just to see what might be picked up by the cables (sometimes there isn't much there, sometimes there's some gunk).

 

[Doc B]

Sorry to say it's been so long that I don't recall what we used to measure. I'd try measuring at a signal level of maybe 2V and see what you get for S/N.

 

[bmgrlfl]

I was using the sound card in my computer to generate the signal and it will not output 2V apparently, however my listening level is even lower than the 0.75 VRMS used in the plots so S/N is probably about 50 to 60 dB above the 120 Hz peak.  I was considering replacing the two UF4007's thinking they were adding more noise than they should; but if this looks typical of the noise spectrum for the base Foreplay, I may just leave it alone for now.  Thanks for both of your help.     

 

[mcandmar]

To go a little off topic, what software are you using?

 

[bmgrlfl]

Visual Analyzer 2014.  Its free and works well enough for the spectrum analyzer part of it. 

http://www.sillanumsoft.org/

 

[Paul Birkeland]

What do you get if you connect the output cable to the input cable with a barrel connector?

 

[bmgrlfl]

I didn't connect the output to the inputs, but this was the noise floor of the M-Audio 2496 sound card alone without the Foreplay powered on.

 

[Paul Birkeland]

I didn't connect the output to the inputs, but this was the noise floor of the M-Audio 2496 sound card alone without the Foreplay powered on.

Try it with the .75V of signal going through the cables themselves without the preamp.

That noise floor does show some ambient 60Hz and 120Hz noise/byproducts.

 

[bmgrlfl]

Ok.  When I get a minute, I'll take a reading and post.  Thanks.

 

[Paul Birkeland]

That's always my first step when I bust out the soundcard and FFT software.  Plenty of times I've had to move stuff around to reduce noise pickup.

 

[bmgrlfl]

Here are the plots with the sound card output feeding the inputs with and without the internal 500 Hz signal.  Fairly quiet with no 60 or 120 Hz noise.