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How do I test an audio amplifier using a signal generator? For example, I would like to input a 440 Hz sine wave and

  • listen to the speaker output at various configurations of volume, etc.
  • watch the speaker output signal on an oscilloscope

and measure the frequency at which the subwoofer filter takes effect.

Specifically, what voltage/amperage is appropriate for typical stereo hifi gear input? I plan to feed the input via the 1/8'' stereo input jack.

Context: this is primarily for my own education and understanding. The only practical aspect will be verifying the subwoofer low pass filter setting, which is controlled by a potentiometer.

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    \$\begingroup\$ Related: electronics.stackexchange.com/a/594177/2028 \$\endgroup\$
    – JYelton
    Jun 27, 2022 at 17:14
  • \$\begingroup\$ Also be aware that there are many amplifier designs. Most of these will "play nice" with oscilloscopes and signal generators (are Earth-referenced), but some percent will not. Once encountered a musical instrument amp which tied the input ground to the -60V rail.. Not a problem for (an isolated) musician and a speaker, but loads of trouble for a (ground-referenced) signal generator and oscilloscope. \$\endgroup\$
    – rdtsc
    Jun 27, 2022 at 18:08
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    \$\begingroup\$ Be aware that while a good speaker can handle a large amount of power spread over many frequencies, too much power at a single frequency could damage the speaker. \$\endgroup\$
    – Mattman944
    Jun 27, 2022 at 18:36
  • \$\begingroup\$ The subwoofer will not jump along just at a certain frequency when the signal frequency is gradually decreased. Somewhere in the system - today it's before the amplifiers, in old systems it's just between the subwoofer element and the amp - there's a filter. A practical filter for a subwoofer starts gradually pass more power to the subwoofer as the frequency decreases, but no sharp frequency treshold exists. It's common to say that the limit frequency (a.k.a. -3dB frequency) is where the voltage is 71% of the maximum passed voltage, when the input voltage to the filter stays constant \$\endgroup\$
    – user136077
    Jun 27, 2022 at 20:28
  • \$\begingroup\$ Also please note that a signal generator may be unnecessary. Likely you have a PC or smartphone and they both have audio processing or signal generation software applications available and they both have audio output ports, so unless you have a specific need, you really don't need the signal generator. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 21:43

2 Answers 2

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I will assume since your stereo is using an 1/8" input jack that it's a consumer product and thus 300 mV would be my recommendation.

Pro audio gear will typically use 1/4" or XLR inputs and in that case, you would instead use up to 1200 mV.

Sometimes inputs will have a trimmer or gain control, and in some cases a button to select between -10 dBV (consumer) or +4 dBu (pro). In such scenarios, your equipment is likely able to handle any value in between those listed.

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  • \$\begingroup\$ Out of curiosity, why do you recommend 300mV and 1200mV? Where do these values come from? Are they Vpeak, Vpeak-peak, or Vrms? Or nominal related to some specific alignment level? \$\endgroup\$
    – Justme
    Jun 27, 2022 at 18:17
  • \$\begingroup\$ They are Vrms, and based on various manuals for audio gear I own (from Focusrite, Roland, etc.) as well as online searches. I have some sources given in the answer I linked to under the OP question. \$\endgroup\$
    – JYelton
    Jun 27, 2022 at 18:28
  • \$\begingroup\$ Actually those voltages even according to the source are nominal levels compared to some reference level. Which is not mentioned. For example, a random Focusrite DAC has 15.5dBu full scale 0dBFS output level. And common audio DAC chips have 2Vrms full scale output level. Typically in a studio 4 dBu would equal maybe -18 or -20 dBFS. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 18:52
  • \$\begingroup\$ @Justme So if I'm understanding you correctly, you (or the OP) are looking specifically for a maximum or full scale level as opposed to nominal. Wouldn't this value necessarily vary by device, and require either that the manufacturer state it, or knowledge of the input circuitry used? \$\endgroup\$
    – JYelton
    Jun 27, 2022 at 21:10
  • \$\begingroup\$ Exactly. So for example if you make a device that only works with up to -10dBV or +4dBu it is pretty useless because peaks of audio are higher than the nominal level. That's why the Focusrite ADC I found can take in up to +22dBu input levels for 0dBFS. Which means, the reference for the +4dBu is -18 dbFS which is a common reference alignment level in broadcast studios depending on which parts of the world you live. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 21:22
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I generated this data quite a few years back, I haven't checked for its more recent accuracy.

LINE LEVEL

0dBu = 0.775V RMS.

0dBV = 1V RMS.

Typical line level for consumer equipment = 0.316V RMS = -10dBV = -7.8dBU.

Typical line level for professional equipment = 1.23V RMS = +4dBu = +1.8dBV.

To convert from dBu to dBV just subtract 2.2dB. To convert from dBV to dBu just add 2.2dB.

0dBV = 2.2dBu. 0dBu = -2.2dBV.

Another typical level often quoted as line level is 0.775V RMS = 0dBu.

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  • \$\begingroup\$ Yes those are typical alignment levels for consumer and professional equipment. Unfortunately the alignment level is omitted so it's impossible to determine what the maximum level is. At which level I should play a digital sine wave or how much should I attenuate the maximum analog sine wave level, to get -10dBV or +4dBu alignment level? Is it 0dBFS, -18dBFS or -20dBFS? \$\endgroup\$
    – Justme
    Jun 27, 2022 at 19:58
  • \$\begingroup\$ @Justme I've read, read and reread your comment and I'm struggling to understand the point which you're trying to make or the potential problem which you're trying to highlight. Sorry if I'm appearing to be a bit slow! Can't the op just twiddle the amplitude knob on the sig' gen' to get the required signal amplitude? \$\endgroup\$
    – user173271
    Jun 27, 2022 at 20:35
  • \$\begingroup\$ The point is that you say what the typical voltages for alignment levels are without defining how much the alignment levels are compared to the maximum level. For example, in a radio studio, the -10dBV or +4dBu alignment level could be -20dB from the maximum allowable signal level, which means the maximum voltages are +10dBV or +24dBu respectively. Common audio DAC chips you can buy have 2 Vrms full scale output level. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 20:56
  • \$\begingroup\$ @Justme So, if the op is thinking about using -10dBV or +4dBu for his experimental signal level, isn't the important thing just to make sure that these levels aren't overdriving the input and saturating the output? \$\endgroup\$
    – user173271
    Jun 27, 2022 at 21:12
  • \$\begingroup\$ Yes, but it also means that if the designed equipment only goes up to -10dBV or +4dBu, it will be useless, as peaks of audio are much higher than the nominal level. A typical consumer equipment can easily output a 2Vrms maximum signal. \$\endgroup\$
    – Justme
    Jun 27, 2022 at 21:34

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