Hi adauphin
The equipment you will need is a scope (which you have) and a sine wave frequency generator that can be continuously adjusted over the frequency range 5Hz to 32kHz (20Hz to 20kHz is the maximum frequency range of human hearing). Why have a frequency generator with a range wider than the range of human hearing: for two reasons. Firstly, it is not wise to have any equipment running at the extremes of its performance and, secondly, the additional frequency range will be useful for other testing you may wish to do in the future.
The frequency generator would also need to have a continuously adjustable output voltage amplitude of 0V to 1V RMS (2.83V peak to peak) at least. The output voltage from the frequency generator needs to stay constant over the entire range of 5Hz to 32kHz. Make sure you do not get a frequency generator where the output voltage/frequency bounces when you change frequency or output voltage, as this will make the frequency response testing exercise tiresome.
To check the frequency characteristics of your pre amplifier:
Connect the scope to the preamp output.
Set the the signal generator to 0V output at a frequency of around 440Hz and connect the signal generator to the preamp input.
Adjust the signal generator output until you get a sine wave of around 100mV RMS (283mV peak to peak) approximately displayed on the scope. Adjust the scope trigger to lock the sine wave on the scopes screen.
Then adjust the frequency of the oscillator from 20Hz to 20kHz and observe the amplitude of the sine wave on the scope.
To check the frequency response of your power amplifier(PA) do the same thing, but, to avoid damage to your amplifier and speakers (the speakers should be connected to the PA output for the PA frequency response tests), you must observe the following precautions without fail:
- Always ensure that the output voltage from the oscillator is set to 0V, and the frequency is set to around 440Hz, before connecting the oscillator to the PA
- Never adjust the signal generator output voltage so that the output voltage from the PA in more than 1VRMS, give or take a touch, for amplifiers and speakers with an RMS power rating of 10W or more. For lower power amplifiers/speakers scale down accordingly
- As a precaution, keep testing at frequencies above 5kHz to a minimum
To check the frequency response of your Xovers, you will need to get inside the speaker cabinets and do the same test as for the PA, but with the scope connected across each pair of speaker chassis terminals in turn. For the tests the speakers should be fully assembled as they would be for normal listening (this is most important for the bass speaker). The speakers should also be in a normal listening mode: that is not facing a wall or with anything on the speakers to suppress the sound output. If you plan to do a lot of Xover testing it may be advantageous to permanently wire-in a set of terminals connected to the speaker chassis terminals.
This test assumes that the frequency response of your PA is flat, which it should be, except possibly for frequencies above 20kHz and below 20Hz.
By the way, when doing audio frequency response tests it is conventional to use 440Hz (middle A in music) as the reference frequency.
Also, to mimic the human ear's perception of loudness, a logarithmic scale is used with deciBells (dBs) being the working unit. So, if you draw graphs of the various frequency responses, convert the peak to peak reading that you see on the scope to RMS (multiply by 0.35) and convert the RMS value to dBs. Also plot the frequency on a logarithmic scale. An idealized frequency/dB graph for a three-way Xover is attached below as an example.
Finally, remember the warning about damaging your PA and speakers. Why all the drama: PAs and speakers are not necessarily designed for reproducing continuous sine waves (normal music has a surprisingly low RMS value) and, with too higher voltage input, you stand the risk of overheating the speaker chassis voice coils and wrecking the speaker chassis. This is especially the case with tweeters. If you take a typical 100W RMS speaker, the woofer may have an RMS rating of 150W, the squawker 50W, but the tweeter only 5W. Also, by their very nature, tweeters are delicate transducers with minuscule clearances. So the message is beware of high frequency testing.
If there is any doubt about the stability or otherwise of a PA, always connect a 15R or higher, 25W or higher, resistor between the PA output and the speakers for initial, look-see tests.
The above procedure is the low-cost knife-and-fork approach, but if you are doing frequency response testing on a regular basis, you may want to investigate test equipment that automates the frequency sweep and produces a nice frequency response graph for you.
Manual frequency testing may sound like a palava but, with a bit of practice, it is quite easy to do.