LM3900 mixer question

[KE0USX]

I am trying to figure out how to hook up 2 radios to one external speaker in my truck. After a bit of searching I found a pinout for the LM3900. I came up with a simple circuit that I think might work but I wanted to run it by some folks to see if it seems right. I might add pots to the input lines for mixing. I would be building this into the speaker housing.

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[bharvey2]

I think you're going to run into a few issues. I'm not real familiar with the LM3900 but have included a sample schematic below. It can be used for audio mixing but it seems to be for mic or line levels. Your going to need to address the impedance mismatch on the inputs (and too much power supplied by the speaker out. Also, I don't see this as being able to drive a speaker directly. You'll need an external audio amp. Lastly, it looks like it has a wide supply voltage range so you could use 12VDC without any problems.

http://www.idc-online.com/technical...ineering/LM3900_multi_channel_audio_mixer.pdf

 

[RFI-EMI-GUY]

Buy three 70 volt audio transformers. wire them all up with the 70V sides in parallel with each other. Feed two RX into two of the transformers on their 8 ohm sides, attach a speaker to the remaining transformer. Easy Peasey.

 

[KE0USX]

Buy three 70 volt audio transformers. wire them all up with the 70V sides in parallel with each other. Feed two RX into two of the transformers on their 8 ohm sides, attach a speaker to the remaining transformer. Easy Peasey.

So something lie this?

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[RFI-EMI-GUY]

So something lie this?

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Yup; that pretty much would do it. Get transformers rated for the radio output power x 2.

 

[RFI-EMI-GUY]

Clarification.

Radio AUDIO output power x 2!

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[KE0USX]

So here is what I have come up with but do I match the output to the high or low

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[RFI-EMI-GUY]

The 70V wires get bridged like your drawing. The radios connect to the 8 or 4 ohm windings. If your radio is rated into a 4 ohm impedance, use that winding if available. It is not critical. What you don't want is for the radios speaker output wires to touch ground or A+. They must go to the separate windings like your drawing.

If your transformer has multiple 70V windings for power setting. Bridge the highest power windings together, leave the others free.

 

[needairtime]

I suppose this is the easiest way to mix the outputs together, certainly wouldn't be the way I'd think of first, but at least you don't need:
-additional power source
-need ICs or calculate components
-worry about isolation

I guess I'll go along with the transformer solution provided, it's simple and works... just large, heavy, and expensive... If isolation isn't an issue, a few resistors, capacitors, and an LM386 would do the job...

 

[RFI-EMI-GUY]

I suppose this is the easiest way to mix the outputs together, certainly wouldn't be the way I'd think of first, but at least you don't need:
-additional power source
-need ICs or calculate components
-worry about isolation

I guess I'll go along with the transformer solution provided, it's simple and works... just large, heavy, and expensive... If isolation isn't an issue, a few resistors, capacitors, and an LM386 would do the job...

The LM386 would need a power amplifier after it, as it can only produce 100 or 200 milliwats which is useless in a car, so the parts count and size would grow.

There used to be a commercial product similar to my transformer solution to convert a stereo car radio into mono dash mount speaker for antique cars. It works same way, That product was a bit smaller. Unfortunately no longer made. I see a niche market for a passive mixer. Most new scanners and LMR radios have a floating audio amplifier that will be damaged if you happen to ground one pin , so transformer isolation is a given requirement in nearly every case.

 

[slicerwizard]

The LM386 would need a power amplifier after it, as it can only produce 100 or 200 milliwats which is useless in a car

More like 500 to 1000 mW, although the distortion levels might be an issue.

 

[needairtime]

Well, transformers are no longer cheap, they've gotten much much more expensive nowadays and is the bane of modern design.

I'm surprised that new radios have differential/bridge outputs. Usually don't need this unless you need power, and usually 5 watts is more than enough for low fidelity communication radio (no need to emphasize CTCSS tones to the speaker!) - and that is readily attainable from a single ended 12V powered amplifier.

 

[RFI-EMI-GUY]

More like 500 to 1000 mW, although the distortion levels might be an issue.

It would certainly distort quite atrociously.
The specs are here.
Distortion: 0.2%, RL = 8Ω, PO = 125 mW, f = 1 kHz

Figure 6 THD is frightening

http://www.ti.com/lit/ds/symlink/lm386.pdf

Might work for a Pitiful Plastic Picnic Player, but not much else.

 

[RFI-EMI-GUY]

Well, transformers are no longer cheap, they've gotten much much more expensive nowadays and is the bane of modern design.

I'm surprised that new radios have differential/bridge outputs. Usually don't need this unless you need power, and usually 5 watts is more than enough for low fidelity communication radio (no need to emphasize CTCSS tones to the speaker!) - and that is readily attainable from a single ended 12V powered amplifier.

Yes some parts are getting scarce and they are important for integration of equipment. There might be a 10 watt 4/8 ohm to 600 ohm transformer out there in the wild that would suffice. The secondary impedance is not critical.

I think modern receiver design has given way to selecting cheapest parts from a catalog. That BTL amp is probably pennies in bulk. Your car radio probably has four of them.

I wonder how many who are putting feeds on Broadcastify are just plugging a wire from (BTL) speaker jack to soundcard. I would guess many because there are a lot of bad sounding channels out there. I was on a thread with some newbie 2 years ago who was trying to get the hum out of his feed and it took over a week back and forth to explain how to isolate the feed with a simple Radio Shack isolation transformer. I think he might have had a blown amp in his scanner.

 

[RFI-EMI-GUY]

4 PA System 70 Volt NOS Line Matching Transformers 4 8 16 Ohms | eBay

Qty 4 10 watt transformers CHEAP.

 

[bharvey2]

Yes some parts are getting scarce and they are important for integration of equipment. There might be a 10 watt 4/8 ohm to 600 ohm transformer out there in the wild that would suffice. The secondary impedance is not critical.

I think modern receiver design has given way to selecting cheapest parts from a catalog. That BTL amp is probably pennies in bulk. Your car radio probably has four of them.

I wonder how many who are putting feeds on Broadcastify are just plugging a wire from (BTL) speaker jack to soundcard. I would guess many because there are a lot of bad sounding channels out there. I was on a thread with some newbie 2 years ago who was trying to get the hum out of his feed and it took over a week back and forth to explain how to isolate the feed with a simple Radio Shack isolation transformer. I think he might have had a blown amp in his scanner.

That should do the OP well. It might likely be bulky but it follows the KISS principle quite nicely.

I think one problem people have with computer interfacing is that the mic input often has a 5VDC to power mics. It'd be handy to turn that off in software or problems can rear their head. Need to AC couple other to resolve the issue.

 

[prcguy]

Why not get a used amplified speaker (Motorola) then make a simple passive mixer that includes a load resistor for the radio and series dropping resistor to a pair of cheap telephone 600 ohm isolation transformers? You could use the volume control on each radio to adjust the balance or install pots to control the gain of each channel. If both radios use common ground on their speaker outputs you would not need the isolation transformers, but they only cost a few$$ each anyway.

Surplus amplified Motorola speakers are cheap and of much better sound quality than most internal radio speakers. You could easily build the parts into the amplified speaker housing with just a couple of RCA input jacks on the back.

 

[wrath]

Why not get a used amplified speaker (Motorola) then make a simple passive mixer that includes a load resistor for the radio and series dropping resistor to a pair of cheap telephone 600 ohm isolation transformers? You could use the volume control on each radio to adjust the balance or install pots to control the gain of each channel. If both radios use common ground on their speaker outputs you would not need the isolation transformers, but they only cost a few$$ each anyway.

Surplus amplified Motorola speakers are cheap and of much better sound quality than most internal radio speakers. You could easily build the parts into the amplified speaker housing with just a couple of RCA input jacks on the back.

I got a bunch of the mother "M" and the older but bullet proof Amplified Standard before they became mixed up with horizon or bought out by Yaesu ,i can literally turn my mobile up so loud in the garage ,that hearing it in the other side of the house is not a problem .i bought them dirt cheap($8 a piece) on ebay from a ham that decommissioned Emergency vehicles ,some of these speakers served a full career on the job and are still going strong 25 years later .

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