Using two synth speakers

[sarcher1228]

I created a stereo synth by starting with a microsequencer, then splitting the outputs into two paths (left and right channel), each with its own oscilator, envelope, filter, and speaker. I then usied the audio jacks to send the results to my studio mixer.

The concept works, but I've noticed that I get high frequency interference when both speakers are attached to the circuit. If I temporarily disconnect one or the other channel, the remaining signal sounds clean. When I have them both connected, I get the interference.

(I even simplified the circuit, so it started with power>button>split! and each path was simply one oscilator and one speaker, but still had the same interference problem, even before pressing the button.)

Any idea what would cause this?

[chris101]

Hi Steven (@sarcher1228),

I have read about some folks getting noise coming through the power line to their synth speaker. I have built a 'bit' to use a separate power source for part of a circuit, here. For this application, the "First Prototype" that does not use an op-amp may work fine. All you need for that is a proto module, a wire that you don't mind cutting in half, and a power module.

This way the power lines to the two speakers will not be in common, and your squeal may be attenuated.

[sarcher1228]

Thanks, @chris101 ! I'll give that a try. just ordered the proto module and wire.

In your photo, I'm not quite sure I can tell what changes you made to the wire. Looks like you cut off half, then grafted a different set of wires to the original, and capped one lead. I don't recognize the rectangle things at your wire ends. Can you clarify what I will need to do? (Any chance I can remove the bitsnap at one end and use the original wire ends?)

[chris101]

I cut a wire in half, and also cut three (red, blue & black) 'jumper pins' in half. I soldered the red one to the power (white) wire, blue to the signal (orange?) wire and black to the ground(brown?) wire. Then I insulated the joints with heat shrink and self vulcanizing rubber tape (really handy stuff!).

Since the signal is not used, I put a wire nut on it's jumper, so it wouldn't be dangling around waiting to short something out. This 'half wire' can now be used to connect a littleBits output to a breadboard, or in this case the 'out' side of a proto bit.

Set the jumpers on the proto so the signal goes from the input bitSnap to the output bitSnap (put the middle jumper on the pins.) The +5V comes from the upper proto connector to the output bitSnap (remove the right - out side - jumper. Connect the grounds together by placing the left jumper on the pins of the proto bit. Like this:

You can now power a speaker (or oscillator-speaker combo?) from a second battery or power supply. I find those "phone charger" batteries to be excellent power sources which can be used with a usb power module.

...(Any chance I can remove the bitsnap at one end and use the original wire ends?)

If you leave the JST connector on the wire (and just pull it out of the bitSnap) you would still need some way to break out the power and ground lines. A second proto would do this and you would not need a half wire at all.

[sarcher1228]

Thanks for all the detail! I'll give it a try!

[sarcher1228]

First the good news, @chris101... Your instructions were good, and even though I'm new to this, I managed to create two custom wires and hook them up to proto modules. I think they came out well, and as far as I can tell they work just right!

I started with the synth/sequencer circuit running off the main battery. Got music. I then inserted one of these new wire/protos just in front of the first speaker. The wire/proto was connected to a second power/battery in the "off" position, and the speaker appropriately turned off for lack of power. I flipped on the second battery's switch and the music returned. Meaning, the speaker was on independent power, but still receiving the signal. High-five!

The bad news, I don't seem to have solved the original problem. : I still hear the high-pitch interference in the background. I first tried using one custom wire/proto/battery in front of one speaker. Didn't help. Tried inserting the second one in front of the second speaker. Still didn't help. Although, I did find it fascinating that the pitch tuning changed a little bit when I added or removed the extra batteries. (The only thing that eliminates the high-pitch interference is to disconnect one speaker and return to mono.)

So, anyway... I had fun trying out this little soldering project, and appreciate the help. Meanwhile, I am still open to suggestions, otherwise, I am assuming I have to live with the noise!

[chris101]

Well noise in music isn't something one should have to live with! I only have a single speaker, but I may be able to borrow one in a couple weeks, so I will try out your circuit and see if I can, at least, replicate the noise. Perhaps @alexpikkert has some thoughts, as I believe he had a similar issue but with a single speaker (and I don't know the rest of the situation, nor the outcome.)

Oh, and your 'cut wires' look great! Better than my hack jobs...

[alexpikkert]

Hello Stephen @sarcher1228 and Chris @chris101,
My first encounter with noise from the synth speaker was about external noise signals that entered the speaker bit through the 5 Volt VCC connection, and it was caused by a servo, powered in the same circuit.
My main conclusion was that the synth speaker is extremely sensitive for noises...
But your circuit is something different...

I copied your circuit with 2 synth speakers and discovered a few interesting things.....

I also heard this funny squeeking noise when I connected both speakers to an amplifier via the output connectors from both synth speakers...
I connected speaker one to the left channel and speaker two to the right channel.
All GND connections connected together.

Disconnecting one speaker and this noise was gone indeed..
Next I hooked up my oscilloscope and did some tests.
This is the funny noise that should not be there:

Then I connected the amplifier directly to the oscillators via a proto bit.
Both GND connections to GND from the amplifier and the signals to the left and right channel.
Result:
A very clear sound without any noises.

Next I connected the amplifier via both synth speakers and saw the same signal, fully disturbed by high frequency noises:

Conclusion:
The noises are fully caused by the synth speaker bits.
This is the circuit of the synth speaker bit:

On the output side you can see an amplifier IC type SSM 2305.
I think the output signal (out+ and out -) might be floating from the GND connection, so funny things will happen if both output - connections from both synth speakers will be connected via the amplifier.
I connected my oscilloscope between both "output -" connections and yes , there I could see the noise signal again...

Then I tried a few trics to eliminate the noise and only two things really helped:

Adding a 1 MegaOhm resistor in each signal connection from the synth speakers to the amplifier weakened the signal, but the noise was completely gone.......

Connecting the amplifier directly to the oscillators via a proto bit solved the issue also.

Hope this helps,

[sarcher1228]

Thanks, @chris101, ! I appreciate the help!

[sarcher1228]

OMG, @alexpikkert! You nailed it! That was some amazing diagnostics there! I am relieved to know the noises weren't just in my head! Ha!

You and @chris101 have both been a huge help!

So, it appears that when I want to send stereo signals into my studio mixer, I should use two proto modules in place of the two speakers. (Curious how strong the audio signal is, as compared with running through the speaker, which has an amplifier.)

Can you give me an idea of how to wire the audio cable to the proto? I am assuming that were I to slice open a mono patch cable, I would find three wires inside that each need to be soldered to a jumper pin (like Chris had used in the example,above)? Any idea which wire is which?! Ha!

(FYI I'm connecting into a MOTU 1248 audio interface)

[alexpikkert]

Hi Stephen @sarcher1228,
No noises in your head indeed ! (Would be much worse.... )

This is the wiring diagram, I suppose you need a stereo cable to connect the left and right channel and the ground connection to your audio device..
I added a capacitor of 1 uF in each channel connection to block any possible DC level entering your audio interface, the oscillator output is a direct connection to an opamp output and might have a certain DC level. I think you only need the AC signal...

Let me know if this solves your question..

PS
If the signal is too strong for your audio device, you can add a series resistor in each channel left & right, let's say 100 kOhm or more. Just try it....

.

[sarcher1228]

Thanks, Alex @alexpikkert!

You ROCK!

[sarcher1228]

Great news, Alex @alexpikkert and Chris @chris101 !! It works! I didn't even need to add the capacitor All I needed to do was use the proto bit and an adapter I found on adafruit.

The noise is gone, and the stereo synth sounds great!

Couldn't have done it without your help! Thanks!