# Audio amplifier limiting output power

What I have:

• A transformer rated at 150VA / 30V (center tapped) / 5A
• Speaker rated at 30W / 8 ohms (measured 6.3 ohms with my multimeter)

Will use class AB amplifier:

Question:

If I send +30V & -30V to this chip, I will get 45W at 1%THD+N:

Is it possible to keep +30V & -30V but limit the maximum power to 30W? Is it by playing with the gain (feedback resistor value)?

I read somewhere "...power amps at lower gains requires more input voltage to get the same power output, with the same speaker impedance".

How to adjust/calculate the gain so that I get a maximum of 30W output with +30V / -30V power supply?

Any advice would be much appreciated.

• When you say that you have a transformer rated at 150 VA, is this a transformer that you plan to use for the power supply? – Dwayne Reid Mar 24 '15 at 16:19
• Does the transformer have two secondary windings or only one? – Dwayne Reid Mar 24 '15 at 16:19
• Yes, I own the transformer already so I plan to use it as the power supply (+30V / -30V). It is center tapped. – psc Mar 24 '15 at 16:33
• I'm still not sure about your transformer. Is it 30-0-30 or 15-0-15? – Dwayne Reid Mar 24 '15 at 16:41
• It is 30V-0V-30V. – psc Mar 24 '15 at 16:44

A 30-0-30 transformer would, when fed into a bridge rectifier and smoothing capacitor produce dc voltages of about +/-40 volts and not +/-30 volts. Here's a picture to help you: -

A 30V AC signal means$^1$ 30V RMS i.e. having the same heating effect as 30V DC but the AC waveform also has a peak and this peak (for a regular sine wave AC power voltage) is $\sqrt2$ higher than the RMS voltage number. A bridge rectifier will charge up the smoothing capacitor to the peak voltage (minus a couple of volts lost in the diodes) hence, 30V RMS becomes 42 volt peak which then becomes about 40 volt DC on the smoothing capacitors.

This is likely to overload the amplifier - it has a maximum supply of 84 volts and a few percent increase in your AC power voltage could cause it to fail or at least not work correctly.

But, assuming you created a dc power supply of +/- 30V then you could limit the peak power into your loadspeakers by adding a series resistance of a few ohm. You could also build a circuit that limited the voltage driving into the input of the amplifier i.e. a "clipper" circuit.

$^1$ without any other terms like peak-to-peak (p-p) or peak (pk), if the stated voltage for a signal is 30V the default meaning is RMS.

• Like you said sending +40V/-40V is almost at the maximum (recommended) value of 84V. Is it possible to use more diode to drop the voltage or any other method (I didn't found any regulator that fit my needs)? Maybe I can send 40-0-40 but then my output power will be around 82W and my speaker is rated 30W. In this case, I can lower the gain to 20db to help (minimum value) and add a resistor before the speaker (ie: 4 ohms, but very high watt)? I guess I could add a potentiometer at the input (A-B) to limit the power sent to my speaker right? – psc Mar 25 '15 at 16:20
• Hum... from the title of the chip, I think I cannot go higher than 60W in stereo: "Overture Audio Power Amplifier Series Stereo 60W". What will happen if sending 40-0-40?! – psc Mar 25 '15 at 17:49
• It's 60 watts per channel – Andy aka Mar 25 '15 at 19:28
• Exactly, if sending 40-0-40 to the amp then the output power per channel will be around 82W. What will happen (an implosion or a black hole)? – psc Mar 25 '15 at 20:11
• The total voltage applied to the amp will be 80 volts under full power load conditions. If your AC supply fluctuated up by 5% this voltage will rise to 84 volts i.e. at the very upper limit of the absolute maximum voltage limit for the device under load. This is a significant cause for concern irrespective of your speakers being damaged. – Andy aka Mar 25 '15 at 21:13

Music is generally rated as having a crest factor of around 6dB (depending on genre, of course - some highly compressed modern music will have something closer to 0dB).

Given that, there is a rule of thumb that the amplifier should be 1.5 - 2 x the power of the speaker in order to avoid clipping the amp. I think that you´ll be fine with the above.

I took the time to simulate the circuit. You can download the ltspice schematic and the LM3886 model (LM4780 is made of 2 LM3886): https://dl.dropboxusercontent.com/u/1455235/stuff/lm3886.zip

My conclusion is to set the gain of the amplifier to output around 30W in my speaker (knowing that the gain have a minimum and maximum value).

Here's a very useful read titled "Audio Power Amplifier Power Rating Mysteries Explained": http://www.rocketroberts.com/techart/powerart_a.htm

Now I need to find how to lower the 40V/-40V entering the amplifier.