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 Post subject: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 4:46 pm 
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Hello-

Does anyone have a good schematic for a circuit to combine a stereo signal into a monophonic signal? I'd like to build one for use when testing or listening to mono amps, as I have no mono source. It really does sound strange to use just one half of a stereo signal.

I think it would also be useful with the small, legal-power level AM transmitters.

Thank you and best regards-

Troy
Edmonds, WA


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 5:25 pm 
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Several of the Part 15 transmitter designs I've seen have a 10K resistor with the input ends tied to each channel on the input jack and the output ends tied together where they connect to the single channel transmitter.

Others have used a transformer.

I would think one would have to know the source and input impedances for a given use.


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 6:09 pm 
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Agree.
A simple resistor network can be used to combine L and R channels into a composite mono output, but it is best to know the source and output impedance of the devices you are working with to build the right network.

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 6:18 pm 
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Hello-

Well I have two sources, but do not know any values for them. One is a 5 disc CD changer and the other is an old Microsoft Zune MP3 player. I have used both of these many times with the "AUX" input on a variety of stereo amps, so I guess I just thought that there was some sort of universal standard value for devices like these to make them compatible with so many different amplifiers.

Thank you and best regards-

Troy
Edmonds, WA


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 9:19 pm 
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soloapollo wrote:
Hello-

Well I have two sources, but do not know any values for them. One is a 5 disc CD changer and the other is an old Microsoft Zune MP3 player. I have used both of these many times with the "AUX" input on a variety of stereo amps, so I guess I just thought that there was some sort of universal standard value for devices like these to make them compatible with so many different amplifiers.

Thank you and best regards-

Troy
Edmonds, WA


CD changer probably has two options, line out and headphone jack. Line out impedance is 100-600 ohms; newer devices use the lower impedances. Headphone jack should support 8 ohm headphones like the 40 year old ones I have. Newer headphones like the ones designed for portable devices are often 64 ohms, actually 32 ohms/channel. Those work fine on CD changers, so that tells you that 32 ohms per channel is probably the minimum you'd want for a one-size-fits-all cable that connects to the headphone jack. In my experience 100 ohms is enough of a load. And for the line out jack, probably a 600 ohm resistor if you have an older unit.

Zune probably wants to see 32 ohms/channel.

When I've hooked either my mp3 players or my phone to my Part 15 transmitters that had 10K inputs, I could hear distortion unless I connected a 100 ohm resistor from each output leg to ground. Seems my portable devices didn't like that 10K input so much.

I made a cable with 3.5mm stereo plug one end with RCA jacks on the other. Across each RCA jack is a 100 ohm resistor. So my portable device sees 100 ohms, which it's happy with. I can then connect a RCA to 3.5mm stereo wye to plug into the 3.5mm input of my transmitter, which has the two 10K resistors at the input as described in my previous posting.


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 9:44 pm 
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Here are a couple of circuits to play with. Any of them should work.


Attachments:
iPod input circuit to AA5 a.jpg
iPod input circuit to AA5 a.jpg [ 180.03 KiB | Viewed 1519 times ]
iPodAdaptorSchematic.jpg
iPodAdaptorSchematic.jpg [ 60.04 KiB | Viewed 1519 times ]
Revised Ipod Adaptor.jpg
Revised Ipod Adaptor.jpg [ 65.86 KiB | Viewed 1519 times ]

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Sun 15, 2021 10:37 pm 
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I would add that if you use the low value resistor network when using this with a line level output, it will most likely load down the audio level somewhat.

Use the low value resistors if connected to a source designed to be used with headphones or ear buds. Use the higher value resistors if connecting through RCA line level jacks or a DIN plug.

Richard, I am going to have my lawyer contact you about copyright infringement. :wink:

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 5:10 am 
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processhead wrote:

Richard, I am going to have my lawyer contact you about copyright infringement. :wink:

Our member Mr. Newton did the first layout. Which of the bottom two did you do? I can have it or them labelled as "A Processhead Creation"... :wink:

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 11:49 am 
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fifties wrote:
processhead wrote:

Richard, I am going to have my lawyer contact you about copyright infringement. :wink:

Our member Mr. Newton did the first layout. Which of the bottom two did you do? I can have it or them labelled as "A Processhead Creation"... :wink:


I did the last two drawings although I can't claim to be the absolute originator of either circuit.

The bottom circuit is the only one I use anymore. It is simpler, uses one less transformer, and performs just as well as the middle circuit.

No labelling or attribution necessary. :wink:

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 3:03 pm 
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You can't just tie the outputs together as one? I think thats the way I transmit from my CD player or stereo cassette to my transmitter. I notice one channel weaker, but what am I missing on this that would be the issue.
Thanks
John


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 3:12 pm 
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hardluck54 wrote:
You can't just tie the outputs together as one? I think thats the way I transmit from my CD player or stereo cassette to my transmitter. I notice one channel weaker, but what am I missing on this that would be the issue.
Thanks
John

Tying the outputs together basically short circuits the left channel and right channels output amplifier stages into each other.
Some output amps may tolerate this, but ideally you want some isolation between them that simulates the load impedance of whatever they would normally feed into. That is what the resistors do.

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 3:38 pm 
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Thanks Paul!
Then I may need to study those circuits!!!!!!! I'm using a line out on the CD and the phone out on the battery cassette! My cable input is sterio at both ends going into my FM transmitter. Am cable combines. I will look into these circuits. thanks again.

John


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 6:35 pm 
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Joined: Jun Fri 26, 2020 9:35 pm
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Thank you all for the suggestions. I've tried looking up the Radio Shack transformer to get a couple to use for building the circuit, and they seem to be either discontinued completely or else long-term "temporarily" out of stock. Does anyone have an idea where I could buy a couple?

Thank you and best regards-

Troy
Edmonds, WA


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 6:38 pm 
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There have been periodic threads on this same topic here. People have found substitute audio transformers that perform as well or better than the radio shack ones.

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 6:50 pm 
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You could probably find one of these that would work for you.

http://www.mouser.com/ProductDetail/Xicon/42TM013-RC/?qs=h3Xc2LqAealCgSV7Tuf2oQ%3D%3D&gclid=Cj0KEQjw5ti3BRD89aDFnb3SxPcBEiQAssnp0hbiCbmM8nppI56k3Vz5Je1ZKOiKub5itnrhBoOOsGYaApBY8P8HAQ

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 10:06 pm 
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processhead wrote:
fifties wrote:
processhead wrote:

Richard, I am going to have my lawyer contact you about copyright infringement. :wink:

Our member Mr. Newton did the first layout. Which of the bottom two did you do? I can have it or them labelled as "A Processhead Creation"... :wink:


I did the last two drawings although I can't claim to be the absolute originator of either circuit.

The bottom circuit is the only one I use anymore. It is simpler, uses one less transformer, and performs just as well as the middle circuit.

No labelling or attribution necessary. :wink:

If we ever get together, the beers will be on me! :wink:

processhead wrote:

That won't do him any good, as they're apparently out of stock.

Try here; https://www.tzsupplies.com/new-radio-sh ... -i3809230/

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Mon 16, 2021 11:29 pm 
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Joined: Jun Fri 26, 2020 9:35 pm
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Location: 98020
What are the transformers used in the schematic posted earlier, with a circuit showing two individual transformers being used? I don't see any identification on those; perhaps they are still readily available?

Thank you-

Troy
Edmonds WA


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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Tue 17, 2021 2:38 am 
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soloapollo wrote:
What are the transformers used in the schematic posted earlier, with a circuit showing two individual transformers being used? I don't see any identification on those; perhaps they are still readily available?

Thank you-

Troy
Edmonds WA


That circuit uses the same radio shack transformer

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 Post subject: Re: Stereo to monophonic conversion circuit to build?
PostPosted: Aug Tue 17, 2021 12:43 pm 
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It is better to avoid 1) shorting the two channels together or 2) using transformers.The former can cause distortion with the two outputs fighting each other and the latter can have issues with the frequency response profile due to the properties of the transformer.

This leaves two other options. One is simple resistive mixing. Basically you terminate the L and R channel signals with a resistor of a value that represents the correct or intended load impedance for the audio signal. That could be 32 Ohms for a system intended to drive headphones or 1k to 2k Ohms for some line level signals. Then simply mix the two signals with some higher value resistors to a common point for buffering.

However, the resistive mixing still results in some channel interaction to one extent or another.

The correct (or ideal) way to mix the L & R signals is via a resistor for each, to the negative input of an OP amp, with feedback around the amp to limit its gain to say <10 , 2 is a suitable value, but by changing the feedback resistor you can set the OP amps gain to suit the required level.

Due to the feedback, the negative input of the OP amp behaves as a "Virtual Earth", its voltage being stable with signal, so the L & R channels do not interfere with each other, and are correctly "mixed".

This is the way it is done in studio audio mixers, that require to mix many signals with each other, without one signal altering the the load impedance for the other. There are many notes about this on the net if you are unfamiliar with the principles:

https://www.electronics-notes.com/artic ... lifier.php

http://cad.eecs.umich.edu/multisim_tutorials1/4_4.php


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