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Frequency response with generator and meter
#1
Hi Guys

Measuring the frequency response of your amp is pretty straight forward, but can be tedious or simple depending on the test equipment you have.

Most hobbyists have a multi-meter and if they also have a signal generator then it is possible to do a frequency response test. The first thing to know is that when the signal level drops by 30%, you've reached the 3dB-down point, or the accepted bandwidth limit of the unit. 

Set the amplifier to whatever is its "flat" response. If you have a scope you can see this easily, but for now we are hoping there is something close to it that you can set by ear or by knowing the equipment. Connect the generator to the input and the meter to the output. Set the generator to a mid frequency, 400Hz or 1kHz, then adjust the volume for an easy to read value like 1V out.

Now change the generator frequency and watch the meter reading. With an analogue meter-movement type AVO or similar, there is no problem with this sweep but there might be with a DVM (digital volt meter). You might have to sweep the frequency and stop to let the meter get a new reading, then sweep up more and stop, get the new reading, and so on. You will get to a frequency where the meter reading is low and continues to drop as frequency is raised. With the 1V reference, the 3dB point is where the meter shows 700mV. You can do the same towards the low-frequency end to see where the bass roll-off is.

We are assuming that the generator output is constant over the frequency range and most are quite stable in that regard. If you have some doubt, you'll have to check the input signal level to the amp (generator output) as you change the frequency. Certainly at the point where the 3dB limit appears to be, it would be a good idea to verify that the amp is causing the drop rather than the generator.

You might encounter points where the meter reads high or low but you are not at the expected limit of response of the circuit. These indicate peaks or dips in the response that are the result of EQ controls not being at a flat setting, or of frequency shaping built into the circuit, or of points where the circuit might be prone to oscillation (if a peak). The "unstable" peaks tend to be near the 3dB limit or even beyond it. A wideband amplifier that is not compensated correctly could show a rise in response well above the designed roll-off - this is a quirk of some active EQ circuits used for cross-overs and reflects the limitations of the opamps used in the circuit. You are unlikely to encounter this in a tube circuit or a guitar amp.

Of course, now you can do similar tests with a given control set to zero or set to maximum and see its influence.
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