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Device Characterization

Device Characterization

Device characterization means measuring a device's performance with instruments and creating a report showing the measured performance, thus "characterizing" that individual unit. Device characterization is useful not only to know that a given unit is performing well, but it can be useful when selling a unit, as you can prove that it is working as it should, or that its performance has been improved by some sort of modification or repair. 

Imagine that you are considering the purchase of an expensive vintage audio amplifier, and all the seller can tell you about its performance is that "it works great," and maybe that it "sounds good." Now imagine that the seller can provide you with a document from an experienced audio technician, bearing the model and serial number of the unit in question, and showing performance graphs and figures as measured on a certain date. Would that not make you feel better about your potential purchase? And if you were the seller, wouldn't that make it possible (and reasonable) to ask for a little more money for your unit?

I have recently invested in the equipment and software to allow me to measure frequency response, total harmonic distortion, intermodulation distortion, wow and flutter, noise, and more. And of course, I have always been able to measure maximum output power of an amplifier into a given load. I can verify RIAA equalization accuracy in phono preamplifiers, speed accuracy of turntables and tape decks, and more, and provide you with computer-generated graphs and a written report for your unit. Here are some examples of the kind of graphs I refer to:

RT-707, LPR 35, 7.5 IPS, 0VU relative to

This graph shows the combined record/play frequency response of my personal Pioneer RT-707 reel-to-reel at 7.5 inches per second tape speed, measured with RTM LPR35 tape, at 250nW/m (0 VU). The blue line represents the left channel, and the red line the right. On a graph produced for a client, the heading of the graph (the text in the green area just over the upper left corner of the graph) would include the serial number of the unit and the date.

Here is a printout of a wow and flutter test of my RT-1050. This test was made before I repaired the power supply, as mentioned below, and shows the hum problem the unit used to have.

RT-1050 15 IPS wow and flutter.jpg

The text in red at the bottom of the graph tells the story: There are actually two simultaneous wow and flutter measurements, one for the left channel and one for the right, and you simply average them together. In this case, the figure comes out to about 0.023%, which is outstanding. I can choose between several weighting options for the measurement as well. The "drift" measurement shows how far off from perfect the speed is; in this case it is a very tight 0.03%.

Here is a distortion test:

THD @120WPCh, 1KHz.jpg

This is the harmonic distortion created by a Luxman M-120 amplifier I recently recapped and adjusted for a client. It is being run with a 1kHz sine wave at its rated power of 120 watts per channel RMS, both channels at once, into eight-ohm dummy loads. As you can see from the text at the bottom, the left channel is running at a very clean 0.0107% THD, while the right is even cleaner, at 0.0047%. This analysis excludes noise and only measures distortion, although if you prefer an old-fashioned THD+N (total harmonic distortion plus noise) readout, I can do that as well. Here are the results of a similar test of a Marantz Model 240 power amp, after I rebuilt its blown left channel:

Marantz full power THD.jpg

Sometimes, this kind of analysis is good for verifying that a problem exists, diagnosis and repair of the problem, and verification that the problem has been solved. On another page of this site I mention a hum in my Pioneer RT-1050, which became audible after making improvements to the electronics to reduce hiss. You can see it in the wow and flutter analysis shown above. Here is the spectrum analysis of that hum after I first discovered it:

RT-1050 Hum.jpg

Check out the peaks at 120 Hz and 240 Hz. They're both higher than the (less audible) 60 Hz fundamental. Now look at it after I replaced part of the power supply (the graph mentions wow and flutter because I was about to measure that):

RT-1050 Hum fixed.jpg

The harmonics of the 60 Hz power line frequency are now about 15dB lower than before, and are even lower than the 60Hz fundamental. Those peaks very nearly blend into the hiss created by the electronics near those frequencies, and are no longer audible above the hiss. Problem solved. Here is the new noise floor of that machine at 7.5 IPS, with NAB equalization, relative to 840nW/m (+10.5 VU), which is the level where LPR35 tape reaches 1% THD:

RT-1050 Noise Floor LPR-35 Tape at 7.5 I

Pretty impressive, huh?

I can produce a complete report on the performance of your amp, preamp, tape deck, turntable, etc., for a reasonable fee. Contact me for details.

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