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Old 4th September 2010, 03:31 PM   #1
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Default Circuit Hum -du?

I recently constructed this basic circuit to merge stereo inputs to mono for an old Stromberg Carlson mono amplifier I am retrofiring. It runs on a 9v battery. I hooked it up and it works well but there is a nasty hum. Just wondering if anyone had some hints as to how to knock out the hum. I should know this but can't seem to get rid of it. Thanks, Dub

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Old 4th September 2010, 04:00 PM   #2
DF96 is offline DF96  England
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To merge stereo into mono you just need a couple of resistors. No need for a source follower, or power supply, or introducing a new signal ground (and hum, possibly).

The resistor value is not too critical. Just make them the same, and a few hundred ohms up to 1K. Take the mono from the junction.
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Old 4th September 2010, 04:22 PM   #3
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I thought that a basic resistor setup would result in signal volume loss. I have noticed other designs on the web using a transformer to boost the signal to make up for this. Here is one design.

http://www.jensen-transformers.com/as/as078.pdf
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Old 4th September 2010, 04:39 PM   #4
DF96 is offline DF96  England
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The two resistors do the stereo to mono conversion. The transformer then converts the impedance to match a balanced low impedance input; it can't boost anything, as a transformer has no gain. You only asked about the first part. If you need to do an impedance conversion too then you may need a buffer or transformer.
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Old 4th September 2010, 05:13 PM   #5
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The hum may be caused by poor grounding or poor shielding. Since the power supply is a battery, that eliminates poor filtering.

Your leads are probably too long and the grounding scheme faulty.

However I completely agree that you don't need an active circuit to merge two channels. Make sure the resistors are considerably higher in value than the impedance of the source, yet not so high they will make an audio filter with the input impedance of the load. And you don't need gain controls.

I recently built a passive mixer to merge a microphone and a guitar input to an amplifier. Works great.
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Old 4th September 2010, 06:10 PM   #6
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I am not that proficient with electronics design and normally construct from schematics I find on the net. Do you know of a design that will work for this application? Cheers, Dub
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Old 4th September 2010, 06:25 PM   #7
DF96 is offline DF96  England
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You don't need a design. Take two resistors, equal value (say, 470 ohms or 1K). Connect one to the left channel, and the other to the right channel. Now join the two spare legs, and take your mono from there. Job done! (If the input impedance of your amp is 50K, and you use 1K resistors, then you will lose 1% of your signal voltage. Nobody will notice. If you load this with 500pF of capacitance (cable etc.), your HF rolloff will start at 600kHz. Nobody will notice.)

This assumes that the setup works OK if you simply connect one channel to your amp input i.e. the impedances are compatible. If not, it gets more complicated so you would have to tell us more about what you are trying to do.

If you pick circuits off the internet, then you need to know what you are trying to achieve, what the circuit is trying to achieve, and whether the circuit actually does what it says (some don't!). Or ask someone.
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Old 4th September 2010, 06:25 PM   #8
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I suggest you learn more about the subject rather than rely on random comments from the gallery. You can make circuits to do just what you want and will know what parameters to use.
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Old 4th September 2010, 08:32 PM   #9
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Thank you for the help guys. I think I have enough to work with! Cheers, Dub
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