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 Post subject: Amplifier question
PostPosted: Mar Fri 01, 2019 4:12 pm 
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If I wanted to use a stereo amp to drive a single speaker from a stereo source and both speaker negatives are grounded would using a stereo unbalanced to mono balanced transformer like this work.

https://www.edcorusa.com/s2m

The source will be the record out of a Fisher 202 Futura Series receiver.

The input impedance of the transformer is 10K

Best I can tell the record output is right after the source selector switch and I think the 10K impedance would be too low for things like the phono or tuner without affecting bass response.

Is there any way I can create a balanced signal with solid state and a high input impedance if the transformer will not work adequately?


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 4:25 pm 
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Tube Radio wrote:
If I wanted to use a stereo amp to drive a single speaker from a stereo source and both speaker negatives are grounded would using a stereo unbalanced to mono balanced transformer like this work.

No---it has a mono "line" output, and is not intended to drive a speaker.

Why not just use a resistor in each leg? Maybe 8 ohms in each leg, driving an 8-ohm speaker?

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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 4:47 pm 
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I should have been more clear.

Meant to say the transformer would be driving the separate stereo amplifier from the record out of the Fisher.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 4:51 pm 
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Same answer---2 series resistors (but more like 50 - 100K each)

You can do the analysis using the output and input impedances of the apparati--or just trial and erro

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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 5:00 pm 
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Not quite sure what you are driving, balanced or single sided? I would probably do it with op-amps though rather than a transformer. That would be more forgiving of various loading effects.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 5:08 pm 
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What kind of problem are you visualizing? If each stereo output sees 50K R + the parallel combo of the mono input and the source impedance of the other channel in series with another 50K, what bad thing is going to happen? I have used 2 series resistors in all manner of similar situation--never a problem.

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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 5:21 pm 
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The OP states he wanted a balanced signal. I read that as a 600 ohm balanced line audio where common mode is a concern. Two resistors are fine for a single sided mono audio but will not create the complimentary signal needed for a balanced line if that is indeed what he is wanting.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 5:52 pm 
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Yes I need a balanced signal as I want to drive the amp in bridged mode where the speaker connects across the positive terminals of the amp.

I'm thinking the impedance the Fisher sees needs to be relatively high so that requires either the transformer option with an OP-AMP or JFET buffer or go with an OP-AMP phase splitter with an OP-AMP buffer since the inverting OP-AMP will have a lower impedance on its input.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 7:51 pm 
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Now I am REALLY confused...

Driving something like a stereo or mono receiver with the "record" output of another similar thing involves UNbalanced inputs and outputs---to wit the standard shielded cables with RCA plugs. I'm not sure I have ever seen audio system interconnects done with balanced lines, 600-ohm or otherwise.

I read "balanced" to mean that the two stereo channels would have the same contribution to the mono output.

So, Tube*----let's have one more clarification.....:)

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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 7:56 pm 
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:roll:

Basically I want an unbalanced stereo source to be turned into a balanced mono source.

The balanced mono source will drive a stereo amp with the positive wire going to one input and the negative wire going to the other input with the shield going to ground.

The output will be across both speaker positive terminals.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 8:37 pm 
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Allow me to translate:

A stereo source with conventional unbalanced** outputs feed a circuit which produces a balanced** output which is the sum (or should it be difference?) of the 2 stereo channel outputs (L+R). This is then fed to a stereo amplifier (or receiver), which has conventional unbalanced inputs. The idea is that the difference between the two signals will now appear across the two outputs (acting as a balanced output).

Here's the problem: A conventional stereo amplifier responds to input signals that are referenced to its local ground. This means that any inputs must also be referenced to the same ground. If this is done, It appears that you can get the differential output as you describe.

This is sounding like a Rube Goldberg creation---perhaps if you can share the end goal, we can give better help.


**"balanced" is used here to mean a signal that is not referenced to the "ground" of the source---it would typically come from the secondary of a transformer--e.g. the typical 600 ohm output of an audio oscillator.

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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 01, 2019 10:01 pm 
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This would depend on the Futura's output impedance - it's intended to drive a high impedance load (recorder) so may not drive a transformer very well. The transformer ideally would be 1:2 ratio with secondary center-tapped to drive the bridged amplifier. The speaker impedance must be be doubled when bridging (don't go below 8 Ohms). Do you REALLY need the doubled power? Single channel may be fine, and MUCH simpler.

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 Post subject: Re: Amplifier question
PostPosted: Mar Sat 02, 2019 2:19 am 
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It's more or less a way to use a stereo amp I have on hand.

Also the distance to the amp will be 25 or so feet and I don't like running unbalanced cable that long of a distance.

I'm tempted to use a high value series resistor for each channel to an OP-AMP buffer with a lower value resistor to ground and use the output to either feed the transformer or because it will be a low impedance I could do away with the transformer and run an unbalanced line and use one amp channel or get a small mono amp and use that.


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 Post subject: Re: Amplifier question
PostPosted: Mar Mon 18, 2019 2:33 pm 
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Here's what I am thinking.

I may build a box with an OP AMP and use resistors to combine the two channels to mono.

Given the low output impedance OP-AMPs have I could run an unbalanced signal to the amp and not worry about picking up any hum or other interference over the 35' run of cable. To get the signal into the screen room where the amp and speaker will be will require two adapters as I am going to use one of the existing feed through connectors that is currently not used for anything so I'd need an adapter to go from RCA to BNC and BNC to a PL-259 like connector and the same on the other side of the screen room.

Now I don't need it to be full HI-FI far as bass response is concerned and I don't need the volume to be very loud so I would only need a smaller speaker and a small amp.


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 Post subject: Re: Amplifier question
PostPosted: Mar Tue 19, 2019 3:23 am 
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Sounds to me like you've stated two issues. The first is getting the signal down 25 or 35 feet of cable and then the other is driving a stereo amp which you would like to drive one speaker off of and to do that you intend to bridge the output.

To the second issue, bridging the speaker output will get you more power but it'll only be a 3dB increase in perceived volume. That doesn't seem to be worth the effort to Rube Goldberg it.


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 Post subject: Re: Amplifier question
PostPosted: Mar Tue 19, 2019 3:38 am 
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True plus running a balanced line wouldn't work with the unbalanced connector I have to use to feed the signal into the screen room properly which is why I suggested just using a mono amp and an unbalanced line given the impedance will be low due to the OP-AMP used as a buffer.


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 Post subject: Re: Amplifier question
PostPosted: Mar Tue 19, 2019 4:33 am 
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you need a inverting matching transformer with a center tapped or dual secondaries, but they are not cheap.


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 Post subject: Re: Amplifier question
PostPosted: Mar Tue 19, 2019 6:09 am 
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Edcor does make a transformer that will combine a stereo signal to mono balanced, but the impedance is too low for the record output jacks of the Fisher.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 22, 2019 8:35 pm 
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If you are looking at buying a transformer, then I assume you are willing to work with more than just the parts you have on hand. If so, why not look at buying a speaker with dual voice coils? I believe these are used in car audio for the same purpose, driving a single bass speaker from a stereo source.


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 Post subject: Re: Amplifier question
PostPosted: Mar Fri 22, 2019 10:25 pm 
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Perhaps I could, but would then still need a buffer unless I find a small solid state amp with high input impedance and running a high impedance line that far is not a good idea.


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