| Linear measurements in audio gear [message #98644] |
Thu, 29 May 2025 22:17  |
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gofar99
Messages: 2015
Registered: May 2010
Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi Everyone, In my addled mind today I wondered if there was a good or easy (or both) way to measure how linear the gain is in an amplifier. Not the frequency response as I can do that easily on one of my PC scopes (Bode plots are great). But more like if you put in one unit of voltage and get out say 3, will you get 6 out if you put in 2. That is simplistic as a scope trace over a good percentage of the ability of the amplifier is what I would like to see. Related is what effect would phase shifts have on the linearity. It would seem like some sort of saw tooth signal of the right magnitude and frequency might work. Something that the ramp would seem like a signal to the amp and if it started at essentially zero and went to something that would drive the amp to nearly full power and was of a frequency (think low) that the ramp would be treated not as DC but a slow raising AC voltage and the duration of the ramp would be suitable for a scope to display. Something like a saw tooth wave. The straightness of the ramp would indicate how linear the amp was. SS gear might not have an issue with this but tube gear might. Any thoughts?
Good Listening
Bruce
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| Re: Linear measurements in audio gear [message #99305 is a reply to message #98644] |
Mon, 23 March 2026 15:42  |
positron
Messages: 159
Registered: May 2020
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Master |
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I know of two different ways. I know most have never
used an oscilloscope before and I would like to help
if I may. I mean no harm to those who can.
I use a Tektronix 535A dual trace scope with
CA plug in, which I will describe use below as the first
way. The second way uses a volt meter.
1. First we want to compare the two channels at what ever test
frequency (ies) and signal levels.
A. Set switch to alternate. Next
B. Set one channel switch to "polarity" + and the other channel to
polarity -. Next
C. Set Voltage level of each channel to the same setting, example, 5 volts.
D. Connect the test input signal to both channels. They should match to zero,
resulting in a flat line across the screen. If the channels do not match,
the line will Not be straight.
E. Each channel has a variable gain control set at maximum. Adjust the
variable control of the higher gain channel until the wiggly line
flattens out. (Adjusting the wrong variable control will simply increase
the wiggly line. No harm done, just max it again and adjust the other
variable control for less gain, a straight line.) Check at different
signal levels and at frequency (ies).
If all is well, feed input to device under test and check. The amp
or preamplifier may or may not be linear vs different signal levels
or outputs. Not super accurate but probably within 5% or better.
2. A less precise method is to use a volt meter, again check at
different signal levels, and maybe frequencies. Since one is
simply checking for, say, does double input signal produce
double output, one should obtain a fairly reasonable conclusion.
Differences may vary with amplifier brand, model, and power output etc.
I hope this helps all. All the best.
pos
ps. 99 I found a gain vs phase shift graph. But it is not
down to the nearest degree.
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