Cable to connect a 120 Ohms output mixing desk to a 8 Ohms loudspeaker

[frilance]

Hi everyone,

I want to solder a cable to connect the main output of my mixing desk **broken link removed** (model 802, but anyway the output characteristics are the same) to an 8 Ohms loudspeaker. I used it in the past with headphones (higher impedance) or some other self amplified loudspeakers sets, but this time I'm not sure if it's gonna work with this passive loudspeaker.

I've been reading for a while about the ideal situation (having a similar output impedance in my desk for maximum power transmission) but still, do you think this configuration could work?

Thank you in advance.

Best regards.

 

[jiripolivka]

If your source has the source impedance of 120 Ohms (a line output), connecting an 8-Ohm load will cause it shorted.
The best way to "connect an 8-Ohm speaker' to the line output is to use an amplifier, like LM386 or similar. Its input impedance is high so it will not load the line output, and output can be loaded with 8 Ohm speaker.

 

[FvM]

Audio line outputs are usually operated without impedance matching for lowest distortion. Amplifier and other equipment's line inputs have at least kOhm impedance, so the output is effectively unloaded.

I presume you expect a certain sound level from the loadspeaker, so an amplifier with at least several watts ouput power would be suggested. Probably better quality than a LM386 with it's considerable THD amount.

 

[frilance]

Thank you for the quick reply guys!

So obviously I need some kind of amplifier, and the higher the power of the amp, the louder it can get. THing is that if I want to build something simple, the idea of a high power amp seems unattractive. I have access to different kinds of OpAmps, and I could also borrow a (+- 19V, +- 15V, +- 9V, +-5V) power supply for this hypothetical circuit.

I'm not really concerned about the loudspeaker sound level, nor the quality. I just want to give it a try since I have all these components available. So do you think that using a "high power" OpAmp-based amplifier, with a supply of +19 V maximum, I could get a reasonable sound level out of it? If so, is that LM386 a good option? do you have some other candidates?

Thanks guys! I appreciate a lot the feedback

 

[Audioguru]

An opamp is overloaded if its load is less than 1000 ohms so it will not drive an 8 ohm loudspeaker.
The datasheet for the LM386 shows that its output power is 0.45W into 8 ohms with low distortion when it is powered from 9.0V. If it is powered from 12V then the output power is only slightly more but the IC gets very hot and might burn out.

There used to be 76 amplifier ICs available but today only a few are remaining.
An LM1875 amplifier IC is still available. Its simple circuit produces 8W into 8 ohms when it is powered from 30V.

An amplifier powered from 19V will produce an output of about 14V peak-to-peak which is 5V RMS. Then its output into 8 ohms is (5V squared)/8= 3.1W.