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Old 10th Aug 2020, 9:07 pm   #1
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Default Isolating between stereo speakers and producing mono.

I have broadcast radio on the 'bike. The audio amplifers are not common ground so I have to isolate them from each other and also from the ground of the 'bike in order to feed them to my headset via an intercom. For the previous radio I had taken the audio from the rear speakers via a pair of no brand isolation transformers. That seemed to work reasonably well until I updated the radio. The new one only outputs telephone audio via the front speakers and not all four so I had to rewire to the front. I left thevrear speakers and isolator in place to feed the passenger's headset in stereo. One issue I always had is that only one side of my headset is used and so stereo sounds are sometimes very strange sounding!

My solution for the new radio was to use a commercial "ground loop isolator" modded to break the ground at the input side, the same circuit as I used in my homebrew isolator. Then I decided to make it mono output and so strapped the secondaries in parallel. That I think was where I shot myself in the foot? The isolator feeds a motorcycle intercom, a Starcom1 digital, which expects the output from an MP3 player or other headphone output to be presented to it.

The audio from the radio via the isolator sounds quite thin and no amount of bass boost helps. The audio level is also quite low, in order to have a comfortable headset level, the speakers are intrusively loud and probably annoying to others waiting in traffic etc. That and unintentionally broadcasting any telephone call in progress!

I am pretty sure that I shouldn't be strapping transformer secondaries in parallel so would appreciate some comments about how best to configure the isolator output to give me as high a level as possible in mono. It takes a good hour to strip all the bodywork off to get to the isolator so I would like to get this right next time.

I also have some queries about DAB and FM splitting for the 'bike radio, an Alpine CDE-205DAB. I have now bought two commercial splitters which contain amplifiers for some reason, producing 10dB on FM and 20bB on DAB according to adverts. The first one worked on DAB but nothing on FM, the second was the same but attenuated FM signals appeared when I dropped the power off the splitter. The aerial is a conventional whip which doesn't contain a pre-amp. I see many reviews saying that others have the same issues. None of the devices come with any sort of information.


My RF experience tells me that a splitter should be a simple inductive device with acceptable losses of around 3.5dB and an amplifier of 20dB is just going to destroy the S/N ratio. After having this trouble, I have looked for a passive splitter but not found one as yet. I could use a two way F splitter if I can adapt the connections though? It is wideband after all. The outputs don't really need to be filtered unless the radio tuners are really nasty barn doors surely?

20 years ago I would have had all the answers but I am forgetting as much as I ever knew so would appreciate some brain refreshment please?
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Old 11th Aug 2020, 7:11 am   #2
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Default Re: Isolating between stereo speakers and producing mono.

All the ground loop isolators I have are intended for input stage use so probably expect to see 10k at their output. I think you need something intended for speaker impedance level use. Have you any audio output transformers spare you could re-purpose?
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Old 11th Aug 2020, 8:31 am   #3
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Default Re: Isolating between stereo speakers and producing mono.

Can't you just 'mix' the 2 (original) transformer-isolated outputs by connecting them together via a couple of resistors, hence restoring a mono input to your intercom?
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Old 11th Aug 2020, 10:11 am   #4
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Default Re: Isolating between stereo speakers and producing mono.

Just a quick thought but are the 2 transformer secondaries phased the right way round? They might be out of phase and are trying to cancel each other out. Otherwise maybe a small transformer with 2 matched seperate primary or secondary windings connected to the amp outputs and a single third winding as a mono output.
Another idea might be to join left and right together at the input of one amp and use this combined signal output as mono speaker drive. The second amp could be disconnected or perhaps be used for another purpose.
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Old 12th Aug 2020, 5:03 pm   #5
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Default Re: Isolating between stereo speakers and producing mono.

Quote:
I also have some queries about DAB and FM splitting for the 'bike radio, an Alpine CDE-205DAB
Could you not use either
a) a simple 3dB splitter of the type used in 75 ohm CATV systems? E.g. https://cpc.farnell.com/blake-uk/pro...ort/dp/AP03579

(These are normally rated 5MHz to > 1GHz, and are cheap).
Or
b) an actual passive diplexer: the outlets which are used in domestic wall boxes sometimes have TV and FM outputs, and the TV outlet should extend down to include Band III. I have an example of this.

For the amp / loudspeaker/headset, could you sketch a simple diagram to show the topology? I can't quite imagine how it's all arranged. I presume the amplifier outputs are bridge configuration?
John
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Old 12th Aug 2020, 6:25 pm   #6
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Default Re: Isolating between stereo speakers and producing mono.

Yes, the PA is bridged, in common with most radios nowadays, hence needing to break the common at the input to the transformers. See if this helps...

https://www.vintage-radio.net/forum/...1&d=1597253077
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Old 13th Aug 2020, 1:14 pm   #7
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Default Re: Isolating between stereo speakers and producing mono.

OK: so, how much signal does the intercom input require? Is that input a high-impedance (i.e. 600 ohms or more)? What's the spec on those audio transformers?



I assume that the front and rear outputs from the radio are both "left and right" audio, so you really only need to connect one to derive a proper mono signal ??

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Old 13th Aug 2020, 2:10 pm   #8
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Default Re: Isolating between stereo speakers and producing mono.

Quote:
Originally Posted by John_BS View Post
OK: so, how much signal does the intercom input require? Is that input a high-impedance (i.e. 600 ohms or more)? What's the spec on those audio transformers?

I have no spec on the intercom unit, bar that it accepts a typical headphone signal from a 'phone or music player etc. Previously the rear isolator was fed to both rider and pillion. Apart from the mono/stereo issue, it worked very well.

Quote:
Originally Posted by John_BS View Post
I assume that the front and rear outputs from the radio are both "left and right" audio, so you really only need to connect one to derive a proper mono signal ??
Yes, the front speakers feed the rider and the rear speakers feed the pillion. Both have isolation transformers. Mono is only required for the rider, so it makes sense to convert to mono after the isolation transformers. I am pretty sure that the low and feeble audio is down to strapping the secondaries together like that. I'm tempted to just go for a pair of 100R resistors to sum the outputs next time it comes apart.

I have no spec on the transformers, they came from a commercial product that I took apart. It had nothing inside but a pair of transformers on a PCB with one side of each commoned. I broke the common on the input side and fed each transformer seperately. The device was very similar to this:

https://www.ebay.co.uk/itm/Ground-Lo...8AAOSwWxNYqr-j

https://i.ebayimg.com/images/g/Ze8AA...-j/s-l1600.jpg
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Old 13th Aug 2020, 2:53 pm   #9
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Default Re: Isolating between stereo speakers and producing mono.

You can get a low power ground referenced output from a bridge amp, just put a cap in series with one terminal.
 
Old 14th Aug 2020, 12:31 pm   #10
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Default Re: Isolating between stereo speakers and producing mono.

This is a commercial interface by Autocom that I pulled out of the innards when I bought the 'bike. It is correctly configured to do the 4 in, 3 out business but although I have found the pairs of primaries, I have no idea "which way up" they are. DC resistance of the primaries is just over 1K.

Without cutting the sleeve off, is there any way to work out the phasing?

Then we still have the issue of turning the output into mono, but in the absence of any other advice, I am going to try the 100R resistors to sum the output.
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