Author Topic: Making a sound mixer  (Read 17143 times)

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Offline Vogtinator

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Re: Making a sound mixer
« Reply #15 on: November 04, 2014, 10:55:29 am »
Quote
well i wanted to be able to control that thing without the need of a computer ;)
I can only recommend that you read a bit about MCUs, they're really useful for this kind of stuff ;)
(MCU = Micro Controller Unit = Mikrocontroller, in case that wasn't clear)

Offline Sorunome

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Re: Making a sound mixer
« Reply #16 on: February 04, 2015, 03:47:44 pm »
I have stereo now!
And and better handling of wiring to the potentiometers!
And a cinch port thingy!
Images:

This is the back side of the cinch port thingy:


And now with plugs:


And the top:


Trust me, soldering those headers was a PITA

The boardy-ness:


And the volume regulators integrated into my knex desk stand my monitor is standing on:
« Last Edit: February 04, 2015, 03:50:55 pm by Sorunome »

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Offline Keoni29

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Re: Making a sound mixer
« Reply #17 on: February 09, 2015, 06:36:55 pm »
Cool beans! Those hama cables are fancy :3
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Offline Sorunome

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Re: Making a sound mixer
« Reply #18 on: February 10, 2015, 05:34:06 am »
I got them for free, just like all my other cinch cables :P

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Offline Sorunome

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Re: Making a sound mixer
« Reply #19 on: April 14, 2015, 08:01:33 am »
Sooooo, i just noticed a little issue that makes no sense to me:
Input A is my raspberry pi
Input B is my laptop

If I connect only input A then everything is fine
If I connect only input B then everything is fine

If I connect A and B then I get static noise on B, but not on A

Is it a issue that I just connect all the grounds? But if I split grounds I get even greater static noise, perhaps they need resistors or something?

EDIT: I mean connecting the grounds in such a way, would that help to prevent static noise?


If so, which value should the resistors have? 10k?

EDIT2: Also, when regulating the channels, the static noise also gets louder/quieter, which is actually weird as the volume control only effects the signal.

EDIT3: so now I connected the ground with a power socket ground and the static decreased a lot, but now there is still some, but the weird thing is that the static increases if i decrease the volume, that makes absolutely no sense!
« Last Edit: April 14, 2015, 01:10:03 pm by Sorunome »

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Offline Keoni29

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Re: Making a sound mixer
« Reply #20 on: April 21, 2015, 07:06:56 am »
This is probably caused by a ground loop somewhere in your system.
http://en.wikipedia.org/wiki/Ground_loop_(electricity)

You cannot isolate grounds using resistors. The grounds are still electrically connected this way. Instead you could try using audio transformers or optocouplers to electrically isolate the signals. This technique is commonly used to get rid of ground loop interference.

Instead of isolating grounds you could also use differential inputs for the amplifiers. This requires an extra feedback loop on all input amplifiers.
http://en.wikipedia.org/wiki/Differential_amplifier#Operational_amplifier_as_differential_amplifier
With this method the voltage reference is no longer common ground, but the signal ground. You no longer need to connect all signal grounds together. Edit: They're not completely electrically isolated but you can set a very high input impedance, so they can barely affect eachother.
« Last Edit: April 23, 2015, 02:59:54 am by Keoni29 »
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Offline Sorunome

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Re: Making a sound mixer
« Reply #21 on: April 21, 2015, 09:23:43 am »
Thanks, I found another ground loop - either my laptop or my desktop sound card is designed badly: you can create a static noise on line out if they have a common mic on line in.

EDIT: But how would I merge then? As I have multiple signal grounds instead of one common ground

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Offline Vogtinator

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Re: Making a sound mixer
« Reply #22 on: April 21, 2015, 05:25:27 pm »
Thanks, I found another ground loop - either my laptop or my desktop sound card is designed badly: you can create a static noise on line out if they have a common mic on line in.
So you have essentially a T-connector for your microphone? That's not really a good idea, noise won't be the only issue here. You can try capacitive coupling, around 330n should be enough.

Offline Keoni29

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Re: Making a sound mixer
« Reply #23 on: April 22, 2015, 03:06:45 pm »
Thanks, I found another ground loop - either my laptop or my desktop sound card is designed badly: you can create a static noise on line out if they have a common mic on line in.
So you have essentially a T-connector for your microphone? That's not really a good idea, noise won't be the only issue here. You can try capacitive coupling, around 330n should be enough.
That only reduces any DC offset and other low frequency interference. The signal grounds should be isolated from the power supply's ground and most importantly from eachother.

Here is some more reading material:
http://books.google.nl/books?id=PvKPEFu2PVkC&pg=PA344&redir_esc=y#v=onepage&q&f=false

This differential input also rejects any common mode noise: Static noise introduced in the cable. Because the difference between the two signal wires is amplified any noise common to the wires is not amplified.
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Offline Vogtinator

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Re: Making a sound mixer
« Reply #24 on: April 23, 2015, 01:17:25 pm »
Thanks, I found another ground loop - either my laptop or my desktop sound card is designed badly: you can create a static noise on line out if they have a common mic on line in.
So you have essentially a T-connector for your microphone? That's not really a good idea, noise won't be the only issue here. You can try capacitive coupling, around 330n should be enough.
That only reduces any DC offset and other low frequency interference. The signal grounds should be isolated from the power supply's ground and most importantly from eachother.

Here is some more reading material:
http://books.google.nl/books?id=PvKPEFu2PVkC&pg=PA344&redir_esc=y#v=onepage&q&f=false

This differential input also rejects any common mode noise: Static noise introduced in the cable. Because the difference between the two signal wires is amplified any noise common to the wires is not amplified.

If you design it correctly, you won't need a differential signal for audio. Just make sure that not more than one device is earthed.
You definitely need to connet all signal grounds together, otherwise the shielding becomes useless and will in fact introduce even more noise.
The decoupling is needed to make sure that both input devices don't interfere with each other, to get the best signal out of it you'd even have to calculate some resistors in series with the caps for impedance matching (audio is normally about 200 or 400 Ohms).

Offline Keoni29

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Re: Making a sound mixer
« Reply #25 on: April 26, 2015, 03:41:14 am »
Shielding ground and signal ground are not the same thing. They are typically connected to eachother with an inductor, so the current is limited and high-frequency noise is rejected.
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Offline Sorunome

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Re: Making a sound mixer
« Reply #26 on: June 25, 2015, 12:57:33 pm »
PCB GOGOGO!!!


I know it looks like crap but it is my first PCB.

Also I just noticed that my 10K stereo potentiometers are freaking crap.......anyone know good ones to recommend?

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Offline Keoni29

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Re: Making a sound mixer
« Reply #27 on: June 25, 2015, 04:19:23 pm »
Congrats on soldering your first board!
This looks a lot better than our project group's preamplifier. That thing had long airwires going all over the place.
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Offline Sorunome

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Re: Making a sound mixer
« Reply #28 on: June 25, 2015, 04:24:58 pm »
Congrats on soldering your first board!
This looks a lot better than our project group's preamplifier. That thing had long airwires going all over the place.
Thanks! The back is even more messy, though :P

Edit: here it is!
« Last Edit: June 25, 2015, 05:06:08 pm by Sorunome »

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Offline Keoni29

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Re: Making a sound mixer
« Reply #29 on: June 25, 2015, 05:09:37 pm »
That's not bad for a first. You haven't seen my early work yet :P
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