stereo to mono converter?

[Chris Holmes]

Hi All,

When I?ve wanted to connect a stereo source (cheaply and small-ly) to a mono input, I?ve either used a Y connector with no electronics or just picked up one side of the stereo channels (fine for most pop, rock, etc).

I have seen a circuit for a simple passive mixer (2 or 3 resistors from memory) to do it ?properly? and may even have built one in the past.

I may now want to connect the headphone jack of my mobile to a mono amp (HMV 1251 (works lovely with just the left channel into its Gram input)).

Is there something small and cheap (£5 or less) I can buy for this, and if so, what search term should I use?

Or would I be fine just summing the outputs without any electronics?

[gareth evans]

My guess is that by doing that you would risk blowing either one or both channels as both will be expecting an 8 ohm loading but with both commoned together if at one instant one is at high volts and the other at low volts then a catastrophic current could flow.

Assuming that your amplifier has gain in reserve, then do common both together but with a 47 ohm resistor in each leg.

[Brian Gaff (Sofa]

I'd say just join them, most headphone outputs get abused with odd naff headsets and they go on working. If the level is too high, just use some small resistors in each leg. It really depends on how the mono input is designed. Brian

[Martin Brown]

You can usually get away with it with relatively high impedance signals. Farnell sell suitable bits for this:

The headphone socket is a low impedance source. The two amplifiers will fight each other and at best distort and at worse thermally shutdown or fail. They can tolerate a few seconds shorted out since that happens every time you plug the headphones in or out. Less clear how happy they will be running longer term with their outputs shorted together.

You need a resistor in each leg so that the amplifiers see a load that is within their reasonably expected operating parameters.

L ---r---\____ mono out R ---r---/

I suggest r at about 47 ohm as a starting value. No real power involved here so 1/8W will do fine.

It is asking for trouble. You might get away with it but my instinct is that the headphone drive amplifiers will not be happy and may distort.

[Brian]

I?d recommend something with a few resistors ?just in case?. Do a Google for a circuit - I?m sure someone will have posted one and it will be easier than working from a text diagram etc.

You mention Gram input- I assume you mean record deck? Have you modified the input to remove the equalisation ?

I forget the details, it is ages since I dabbled with phono inputs but not only do the levels differ but some have equalisation circuitry.

[Adrian Caspersz]

Use a cheapie bluetooth receiver instead of wiring direct to the phone?

[Dave Plowman (News]

Chances are a relatively high impedance headphone output has already got series resistors to protect against shorts, and could be paralleled safely.

[Chris Holmes]

I?ve found a commercial device to mono the outputs, Called a SMADT1 from a British Co (not necessarily British manufacture I suppose).

it?s probably only a box, a couple of resistors and a couple of sockets, but I decided I?d support the Co rather than home brewing.

Thinking about it, I am pretty sure I have made stereo 3.5 to mono 6.3 cables to connect my phone to a DI Box in the past and it?s all been fine and sounded good to my ears.

The wireless is sufficiently old that it?s expecting a relatively high voltage at the input and won?t have RIAA equalisation (again, it sounds fine with just one channel of the phone?s output connected even with the phone?s volume set quite high).