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 Post subject: Stereo to mono transformer question
PostPosted: Aug Wed 14, 2019 3:33 am 
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Location: Warner Robins, GA
How exactly does this stereo to mono transformer work?

https://www.edcorusa.com/wsm6400

I cannot seem to wrap my head around it.


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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 7:53 am 
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Location: Powell River BC Canada
Think of a mono phono that has a pickup with a 1 mil needle. Two wires.

Now you want to play stereo records but the needle has to be 0.7 mil or you
ruin your vinyl.

Maybe you connect the two + wires and the two - wires together back to
mono amplifies.


That transformer mixes R+L without shorting out the channels to each other,
and does so without the insertion loss that two resistors would have.

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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 1:51 pm 
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I'm thinking I may see how it works.

Given both inputs have a common connection and the impedance is 10K when the secondary is loaded with the specified impedance it is acting like I used resistors to keep one channel from driving the other.


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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 4:55 pm 
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Location: Lexington, KY USA
To work correctly, the transformer needs to present a high impedance between the L and R channel inputs, It is not clear how it would do this. There is no specification.

No doubt leakage inductance and winding resistance result in some impedance between the inputs, but who knows what it might be.

Perhaps the primary is wound with resistance wire. But two resistors mixing to a single winding would work just as well.

You could buy one and measure it.

Ted


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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 6:22 pm 
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I suspect the impedance of the windings is what does it.

You would have both primary windings so it would be 10K impedance of one winding then the 10K impedance of the other winding.

In much the same way as if you had two 10K resistors I think only minus the voltage loss the resistors cause.


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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 7:41 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
I think that 10k is the expected source impedance, not the winding's own impedance.

However, if each primary winding has a DCR of 500 to 1000 ohms, that would work pretty well in most cases.

This sort of impedance between channels is pretty low for home HiFi tube equipment, might be OK in a 600 ohm system, and would look like infinity to an output designed for 32 ohm earphones.

Ted


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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 7:41 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11701
Location: Powell River BC Canada
The big danger is trying to combine the R and L outputs (i.e. speaker) by
just bridging them. Big time smoke if they are high power. (very low source Z)

However you can make a stereo amplifier into a mono amp, by inverting one
channel at the input, and drawing 2X the power off the R and L + terminals.

Another is the mono output from the FM detector of a tuner is not the same
as bridging the L+R outputs of the MPX decoder. For same reasons as a colour TV
needs a delay line.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Last edited by radiotechnician on Aug Thu 15, 2019 7:45 pm, edited 1 time in total.

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 Post subject: Re: Stereo to mono transformer question
PostPosted: Aug Thu 15, 2019 7:45 pm 
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Posts: 20272
Location: Warner Robins, GA
Exactly.

I have one phono preamp where it specifically says not to short the outputs to each other.

I noticed as a teen when I'd short the left and right channels of a source producing a line level signal together that the audio didn't sound quite right. Wasn't sure why at the time just that I knew it wasn't good to do so.


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