I’m looking to create (or buy) a board/circuit that takes two stereo audio signals (at either line or speaker level) and mixes them 50/50 into one stereo output.
Preferably as simple as possible and with a fixed 50/50 mix. I’ve currently got an amplifier that connects to a MP3 Trigger v2 board but will be purchasing another board shortly. I only got two speakers available for this, and I need stereo so connecting them to one speaker each isn’t going to work.
How about if you re-encode your MP3 files to mono mixes, then just use the left (or right) output of each MP3 Trigger. Doesn’t that get you the same result a lot more easily?
Edit: Sorry, I misread what you are trying to do. Never mind…
I’m looking to create (or buy) a board/circuit that takes two stereo audio signals (at either line or speaker level) and mixes them 50/50 into one stereo output.
Preferably as simple as possible and with a fixed 50/50 mix. I’ve currently got an amplifier that connects to a MP3 Trigger v2 board but will be purchasing another board shortly. I only got two speakers available for this, and I need stereo so connecting them to one speaker each isn’t going to work.
Any ideas?
Thanks for your time!
I’m not sure what you’re trying to do. Do you mean to connect/mix 2 outputs into 1 (now mono) output to send through a single channel amp and then into 2 speakers ?
If you have 2 line levels outputs you could use the input impedance of the amp (probably on the order of tens of k ohms) as the base of a voltage divider. Your line driver outputs (of the MP3 Trigger v2 board) are probably low impedance (< 10 ohms), so add a resistor (say 5k*) in series with each channels output and connect the resulting outputs together @ the amp input. Just like a Y cable but with inline resistors.
You could put the resistors in a tiny enclosure/box. The box could be connectorized to adapt from the 1/8 stereo jack of the MP3 Trigger v2 to whatever cabling/jack(s) (RCA ?) your amp uses. No need for a PCB.
If you mean 2 stereo (L1+R1, L2+R2) line signals into 1 stereo (L+R) signal to feed into a 2 channel amp and then into 2 speakers (which is what I think you mean), then make 2 of the above “circuits” (put all 4 resistors in the same box) and combine (L1 + L2 → L, R1 + R2 → R) and feed the resultant (L+R) into your amp.
resistor should be >= the min specified impedance allowed for the MP3 Trigger v2 boards and <= the input impedance of the amp.
I’m using the “MP3 Trigger v2” board for Arduino which gives a stereo line-out level sound.
I will be using one board for sound effects and one board for music.
I only got one set of speakers (2 speakers @ 8 Ohm).
I want to be able to use stereo for both effects and music independently from each other.
The simplest solution would be to buy a small mixer and plug the line-outs from each board into, but I figure there must be some kind of cheaper/circuitry way.
Another solution would be to play the music through another speaker inside the cabinet, but it will sound muffled.
Footnote:
I’m not sure if this is the right forum, but it’s hardware/circuit electrics related. Feel free to move it elsewhere if needed to.
rasmadrak:
The simplest solution would be to buy a small mixer and plug the line-outs from each board into, but I figure there must be some kind of cheaper/circuitry way.
I agree, there is a simpler & cheaper way. I believe my suggestion above, with the mods below, will suit your purposes.
Combine the 2 stereo (L1+R1, L2+R2) line signals into 1 stereo (L+R) signal to feed into your 2 channel amp and then into 2 speakers by putting a 5k resistor plus a non-polarized 10 uF capacitor in series with each MP3 Trigger output line. Tie the “amp side” of the two left resistors (or caps, whichever is placed the furthest downstream from the MP3 output) together and tie the “amp side” of the 2 rights together (L1 + L2 → L, R1 + R2 → R) and feed the resultant (L+R) into your $7 amp. You’ll get a ~ 6 dB reduction in volume for each channel but since you’re doubling the levels into the amp (assuming equal levels for sounds and FX) you’d have to do that anyway to keep the amp from clipping. Put all the components (4 resistors + 4 caps) in a tiny box, connectorize it to make it simple to plug into and voila … you’re done. You could make a PCB for this but I wouldn’t even bother putting them on any board. I’d just solder lead to lead and lead to connector.
If you want to, try to measure the input impedance of your $7 amp. While I’ll guess it’s about 50k ohm, it could be as low as 10k ohm. If that’s the case you’ll get a little more volume reduction. If you want you could reduce the 5k to perhaps 2.2k to make up for it. From what I can gather the VLSI chip output is capable of driving high impedance headphones (ie - hundreds of ohms) so a 4k load shouldn’t be an issue.
I note from the SF schematic that they bring out the GBUF signal to the apparent ground of the 1/8" output jack (JP2). This is a mistake for your usage ! (fine for free floating headphones) Do not connect to this signal nor tie it to any other ground in your system or amp (regardless of whether you follow my suggestion above). See the cautions noted in [Section 3 of the datasheet. SF has also neglected ([so far as I could see) to include the DC blocking caps so I’ve added them above. Probably a 1 uF value would work just as well for your application. If your amp has them already (dubious @ $7) you don’t need to add them in your box. If you’re unsure … put them in.](http://www.sparkfun.com/datasheets/Widgets/MP3Trigger/mp3-trigger-v21i.pdf)](http://www.sparkfun.com/datasheets/Widgets/MP3Trigger/vs10XXan_output.pdf)
Basically cut out GBUF from the circuit and connect directly to ground instead?
The mp3 triggers got GND connectors and the amplifier has one as well. Is it possible to connect the negative side of the speakers to ground directly, or does it need to return to the amplifier?
As for the capacitors -
I’ve got a couple of yellow wrapper 2.2 uF @ 250 volt axial lead non-polarized metalized polyester capacitors (that’s a mouthful) standing by - will these suffice?
rasmadrak:
I’ve made a little schematics to see if I’ve understood correctly:
Looks correct. To connect your “other components”, do the same for it as for the L+R MP3 line outs and connect them to the amp inputs as well.
rasmadrak:
Basically cut out GBUF from the circuit and connect directly to ground instead?
Correct. THe MP3 runs of a single 3.3V supply and the line outs (L+R) are biased at 1.25V as a result. Sending GBUF (also at 1.25V) to floating headphones effectively removes this DC bias. Alas your other components and amp are no doubt referenced to ground and so can’t use the same trick. Hence the need for DC blocking caps and no connection to GBUF output.
rasmadrak:
The mp3 triggers got GND connectors and the amplifier has one as well. Is it possible to connect the negative side of the speakers to ground directly, or does it need to return to the amplifier?
The speakers probably need to be connected to the amp + and - output terminals.
rasmadrak:
As for the capacitors -
I’ve got a couple of yellow wrapper 2.2 uF @ 250 volt axial lead non-polarized metalized polyester capacitors (that’s a mouthful) standing by - will these suffice?
They should work. You’ll want to use resistors closer to 5k, and not less than 2.2k, so you don’t attenuate your bass too much. As a design tool you might find the link below useful. Keep in mind (2.2u = 2200n) that the R value to use below is half the resistance you put in the circuit due to the 2 inputs on any 1 line.
“Out left” and “Out ground” connects to speaker left
“Out right” and “Out ground” connects to speaker right
(it says in the document on page 3 that ‘the grounds near the IN’s and OUT’s are for the shield of the connecting leads’, which must be GBUF - correct?)
I tried hooking up a single MP3 trigger directly to the small speakers and I get sound, but very faint (as expected). I connected the shielding of each L/R cable to the negative side of the speakers. If this is not correct - how should I connect it? The last attempt fried my amp so I’m quickly running out of components to test this with…
Maybe the creator of the “MP3 Trigger v2” would share some thoughts on this matter?
(it says in the document on page 3 that ‘the grounds near the IN’s and OUT’s are for the shield of the connecting leads’, which must be GBUF - correct?)
The left and right outputs of the MP3 Trigger have a 1.25 V DC component on them as well as any music or sound (AC). I think it very unwise to connect them directly to any amp.
As said in the app notes for the MP3 Trigger … the GBUF output is also at 1.25 V DC. It is not to be connected to any ground or the power supply return. You will damage the MP3 Trigger if you do so. It is not a shield return but rather balances out the DC on the left and right outputs when they are all connected to a floating set of speakers … aka the headphones or earbuds the MP3 Trigger was intended to drive. Perhaps I wasn’t clear enough in my prior post.
EDIT : kinda late here now. I’ll look at your links later. What did you connect to cause the amp to go poof ? What was it the you “connected directly to ground” ? Any speaker line might cause that but the amp in - shouldn’t. Of course the power supply return should be able to be tied to ground. So I’m confused where things went wrong.
Both of the above are preamps and are made to drive other circuitry and not 8 ohms speakers. They won’t work as (power) amps. The second URL lists an M055 stereo amp that is made to drive speakers. Mind you it’s only 3W output but that’s probably not very different from your $7 amp.
rasmadrak:
I connected the shielding of each L/R cable to the negative side of the speakers. If this is not correct - how should I connect it? The last attempt fried my amp so I’m quickly running out of components to test this with…
Is this when the smoke came out ? By “shielding” do you mean to say that you connected the outer conductor of the amp input(s) to the - (negative) output of the amp speaker output ? While the other end was tied to the amp’s ground ? If so then things make sense. If you had the “shield” tied to ground on the amp input side, you basically shorted the speaker - output to ground. Not good ! If you’re using true shielded cable (3 conductors; signal, return and shield) then the shield can be tied to true ground at one end or the other (but not to ground at both). If you’re using co-axial cable (2 conductors; inner and outer) then tie the outer conductor to the power supply return line (which probably is also ground) at both ends.
My guess is that the speaker output - terminal is not ground, but is driven to some AC voltage out of phase with the + terminal. It’s how most automotive amps (and other low $ items) work to maximize power out, given a single +DC voltage supply like you’d have with a battery (vs a + and - DC voltage supply that would be made from the mains AC line) . The speaker + and - outputs should not be connected to anything other than the speaker itself.
Mee_n_Mac:
Both of the above are preamps and are made to drive other circuitry and not 8 ohms speakers. They won’t work as (power) amps. The second URL lists an M055 stereo amp that is made to drive speakers. Mind you it’s only 3W output but that’s probably not very different from your $7 amp.
Ok, I’ll look around for a speaker version. The music and sound doesn’t have to be very loud, I believe the $7 amp was 1W and it was loud “enough”. So 3W would be just fine.
Mee_n_Mac:
Is this when the smoke came out ? By “shielding” do you mean to say that you connected the outer conductor of the amp input(s) to the - (negative) output of the amp speaker output ? While the other end was tied to the amp’s ground ? If so then things make sense. If you had the “shield” tied to ground on the amp input side, you basically shorted the speaker - output to ground. Not good ! If you’re using true shielded cable (3 conductors; signal, return and shield) then the shield can be tied to true ground at one end or the other (but not to ground at both). If you’re using co-axial cable (2 conductors; inner and outer) then tie the outer conductor to the power supply return line (which probably is also ground) at both ends.
I didn’t even get to plug the audio cable in, just power and ground - and the circuit burned. The way it’s connected now (directly to the speakers) is:
From mp3 trigger line-out, leftcable to left speaker positive and shield to negative, and rightcable to right speaker positive and shield to negative. This seem to work, but I’m not relying on it.
rasmadrak:
Ok, I’ll look around for a speaker version. The music and sound doesn’t have to be very loud, I believe the $7 amp was 1W and it was loud “enough”. So 3W would be just fine.
A cheap set of powered speakers, [like these, might work just fine then. Availability depends on where in the world you live.
rasmadrak:
I didn’t even get to plug the audio cable in, just power and ground - and the circuit burned.
I don’t know what to tell you in this case. You’d think applying just power and return (assuming the voltage was correct) wouldn’t be a problem.
rasmadrak:
The way it’s connected now (directly to the speakers) is:
From mp3 trigger line-out, leftcable to left speaker positive and shield to negative, and rightcable to right speaker positive and shield to negative. This seem to work, but I’m not relying on it.
If I understand your meaning of shield above, then this would seem to put a 1.25VDC bias across your speakers. Generally not a good idea. This might heat the speakers (and MP3 Trigger) up a bit and will reduce the volume they can play at w/o distortion. Using the MP3 Trigger to drive speakers directly is the one case where you want to use the GBUF signal. In this case, GBUF to be connected to the speaker - terminal, in lieu of connecting the speaker - terminal to “shield”. I’m not sure the MP3 Trigger was designed to drive 8 ohm speakers but …
The circuit burning could just as easily have been caused by the extremely sloppy soldering that came with the purchase of a cheap amp. Power and GND should generally be ok.
So basically:
I combine the line-outs with resistor and capacitor, plug this into a amp that handles line-in, and connects the speakers to the amp.
I bought a new $7 speaker set, butchered it and did some voodoo.
For those who are curious -
It turns out it was my fault that the circuit burned, since what I connected to wasn’t ground… The ground circuit was isolated in the cheap amp (who knew!?) and it was designed for GBUF “ground”… So all I did was put 10K resistor from each L and R, connected the L resp. R sides and GBUF to amp-GBUF - viola, we got mixed audio!
Also, I can’t know for certain, but it seems to be about the same audio level although it should be a couple of dB lower. But for now this will do!
