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Old 23rd February 2011, 12:37 AM   #1
g(f(e)) is offline g(f(e))  United States
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Default Help with stereo to mono for project

Hello,

I have been assembling and getting parts for a breadboard project for awhile now. I have searched all of the usual forums and internet for the simplest solution to coming out of a stereo cd player to a one channel breadboard amp, but I have not found clear answers for this specific application.

I have read using a simple y splitter is good. Others say no.

A transformer type like the sm2 by Edcor. Would it create other problems?

A mixer circuit.

Adding resistors. Here again, I have seen many different types of circuits with much disagreement on where to hook them up and values.

I am mainly interested in learning to use tubes with simple one channel circuits for now to keep my costs down and to make things easier. The input tube will be a 6SN7, output will be a 6V6 in triode mode.

Thanks

Gary
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Old 23rd February 2011, 12:51 AM   #2
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Assuming your source is a normal solid state device like CD player, tape or tuner just connect both source grounds to the ground of the amp input and connect each positive (signal) source through a 10K to 20K resistor to the amp signal input.

The signal from the left channel goes through one resistor to the amp input and the right channel goes through the other resistor. It is like a Y cable but with resistance instead of short circuit. This will keep one side of the source from trying to "drive" the other side.
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Old 23rd February 2011, 05:56 AM   #3
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Actually, the outputs from a cd player, tuner, tape etc contains no power output, but just a low millivolt signal, it is ok to just use a Y adaptor from the cd player. Integrated amplifiers, receivers, etc., that have mono/stereo switches, short the positive inputs together when they are switched to mono.

Last edited by century tek; 23rd February 2011 at 06:00 AM.
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Old 23rd February 2011, 10:27 AM   #4
DF96 is offline DF96  England
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Shorting outputs together is only OK if they have a small series resistance to stop one output driving the other. Almost all competently designed units will have such a resistance but to be on the safe side include them yourself. 1-10K is fine.
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Old 23rd February 2011, 10:35 AM   #5
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I know I am going to get flamed for this. But in my experience any resister net work I have ever tried doesn't work at best and at worst will break something. Only 2 trans will do the job right.Or 2 windings on the one trans. Check out direct heating. Sakuma will set you straight on the mono....If you dont believe me Try resisters. Then pull one lead while you listening to it and it get louder and the midrange appears. Only one Chanel though. The only other way is a RCA handbook style tube based mixer. Which would be cool too.
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Old 23rd February 2011, 11:46 AM   #6
DF96 is offline DF96  England
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Break something? an exaggeration?

If you don't trust resistors then you need to stick to live acoustic music only.

One thing I forgot to say: if possible put the resistors at the input to the mono amp, not the output from the stereo source. This avoids any cable capacitance issues, although these will be minor anyway unless you have very long cables.
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Old 23rd February 2011, 01:17 PM   #7
g(f(e)) is offline g(f(e))  United States
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I do not understand putting the resistors at the mono amp. Wouldn't I already be past the Y? I could see inserting them in the Y cable and heat shrinking them.

I also agree a mixer circuit would be a fun project and I may look into that someday. But for now wanting to keep things real simple I do not want to add another tube and heater.

Thanks
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Old 23rd February 2011, 01:34 PM   #8
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What DF means to say is this: use a stereo cabling from the stereo source right up to the entrance of the mono amp, then insert the resistor network and attach that directly to the amp. Since the amp is a breadboard project, that's the easiest way to do it and you won't need a Y-splitter cable.

But as century tek pointed out, there is no realistic chance that you'll damage your source. The resistor network suggested by DF96 eliminates any chance of problems. This is not true if you're using a power amplifier as stereo source, obviously.

Quote:
But in my experience any resister net work I have ever tried doesn't work at best and at worst will break something.
A practical example of your experience could help finding out what you did wrong.
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Old 23rd February 2011, 06:07 PM   #9
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Quote:
But in my experience any resister net work I have ever tried doesn't work at best and at worst will break something.
Every professional audio mixing console I have ever worked on (and that's a lot of consoles) has used resistive mixing. Some of it balanced, admittedly, with transformers on the mix amp inputs, but the signal mixing has always been on resistors. After all, they're at theoretical noise levels, have a perfect frequency response and only distort when they're ho enough to smoke. Plus they're cheap.
Transformers, on the other hand, much as I love them for their isolative properties, have to be shielded from electromagnetic hum, have hysteresis distortion at ultra low levels, and magnetic core saturation at ultra high levels at low frequencies, require careful damping to avoid frequency response variations, are frequently microphonic, and decent ones are anything but cheap. And they don't do the job (of mixing two audio signals together) any better; the low impedance from the output of one side of the CD player is reflected across to load the other side, unless there is a lot of resistance in the windings.
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Old 23rd February 2011, 10:45 PM   #10
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Let's assume that this isn't professional equipment, or mixing studio equipment, the basic home audio audiophile isn't going to blow or smoke anything using a Y adaptor to get a mono signal from a stereo signal such as a home CD player. This will work just fine! If you are using a stereo turntable using a Y adaptor to convert the stereo signal to mono signal, this will rendor the same results and work just fine.
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