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MegatronUK 13th December 2011 06:41 PM

Stereo and Mono from Stereo source

I'm currently feeding a stereo line level signal to a 25w ST540 based Sparkfun stereo amplifier (running the main mid range drivers) and a 18w HA13118 based mono amp (powering a single bass driver).

At the moment I'm using a simple resister circuit to combine the left and right channels into a single mono feed for the mono amp, although this does work I do lose some volume and there is a noticeable drop in audio quality from the stereo circuit.

Is there a simple circuit or IC that will let me combine the stereo channels to mono but still feed the full, unaltered stereo signal to the stereo amplifier stage?

Basically, I need to run a pair of stereo speakers and a mono low frequency driver from a single stereo source.


johnyradio 3rd February 2012 04:34 AM

i'd like to know how to do this too.

would it be better to use a crossover based around an op-amp or transistor, to isolate the mono bass signal from the stereo signal?

here's one that might work:
monoXover 2-way pre-amp audio crossover


DF96 3rd February 2012 09:36 AM

What resisor values are you using? If too low they will add too much load to the system. Something in the region of 1k-10k will probably be OK.

Speedskater 3rd February 2012 02:04 PM

This is the Rane note on how to do it with resistors:
Why Not Wye?

I think that Rod Elliott has an active circuit.
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johnyradio 3rd February 2012 03:42 PM


here's another method, referenced elsewhere on this forum. I'm not sure if this is considered active or passive, since it's not powered, and does not use any transistors or op-amps. It uses an inductor, resistors, and capacitors:
monoXover 2-way pre-amp audio crossover

love that wye article, tho i'm confused about the resistors going from hot to ground. i thought the resistors are supposed to be series?

DF96 3rd February 2012 07:53 PM

I think the resistor to ground is just to maintain DC bias when nothing is plugged in. May reduce clicks.

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