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Building an audio mixer, help with some small parts

post #1 of 12
Thread Starter 

hi everyone, 

 

first post here so hopefully i am in the correct place.

for my first project into the audio realm i decided to build an audio mixer, as it is something i needed for my room.

 

The mixer consists of 4 TOSLINK inputs, being fed into a DAC. The DAC i am building is the one on this project page but changed for the 4 TOSLINKS

http://djuke.nl/en/projects/3-digital-audio/32-cs4398-dac

 

This allows me to select only 1 digital input to one analog output which is what i wanted.

I then am going to have 4 3.5mm audio jack inputs, the input from those 4 and the DAC output goes to a summing circuit

Then the summing circuit goes to 4 3.5mm jack outputs

 

this principle is rather easy to my knowledge. now all the analog inputs are going to have linear pots on them to control the volume, and the outputs will all have the same for output volume control. This is where i am getting stuck.

 

The DAC has volume control which is controlled using i2c communication from an arduino.

So here is where i need some help, and here are my questions;

 

1) What size linear pots should i use? i was looking at some dual pot sliders, so i have sliding volume control like on a dj mixer

 

2) I am going to be running headphones from one of the outputs, my headphones are Beyerdynamic DT770 80ohm's

    Will an opamp summing circuit be able to power them directly out? or will i need a headphone amplifier ic? if so do you have any suggestions?

 

my other 3 outputs will be powering speakers or cheaper headphones.

I hope that i have given enough information to get some help, and i am sorry if the title is misleading or i didn't explain this enough, the audio realm is still extremely new to me and i don't understand it much other then the basics i am using here

 

Regards,

Jamie

post #2 of 12

It's not easy to disentangle all your questions and I'm not entirely confident that you've got a firm grasp on what you're doing. A diagram (yours, not somebody else's), or better still a schematic would make things a lot easier. Anyway I'll chop out the clearly stated questions and answer those.

 

Quote:

Originally Posted by bigjme View Post
 

1) What size linear pots should i use? i was looking at some dual pot sliders, so i have sliding volume control like on a dj mixer

 

You should not use linear pots. You should use logarithmic or audio taper pots. 10k is a de facto standard for solid-state volume controls. I'd steer clear of sliders, they tend to be unreliable and they are more difficult to mount, requiring slots. With a regular pot you can often get away with just a drill-hole.

 

Quote:

2) I am going to be running headphones from one of the outputs, my headphones are Beyerdynamic DT770 80ohm's

    Will an opamp summing circuit be able to power them directly out? or will i need a headphone amplifier ic? if so do you have any suggestions? my other 3 outputs will be powering speakers or cheaper headphones.

 

Don't depend on the summing opamp to drive transducers. That's just pushing your luck. Many professional mixers use multiple NE5532s, which cost only U$0.25 each. Use an opamp per output, minimum in a symmetrical arrangement. An opamp is unlikely to have sufficient output to drive speakers, so if you have speakers then you need a dedicated channel with sufficient drive, or you need to use powered speakers. The DT770s are quite sensitive, but 80 ohms is a big load for an opamp, most are rated to drive 600 ohms (read the datasheet), so a buffer such as the LME496000 would be required for orthodoxy, or you can parallel opamps. The phones will probably run fine directly from an opamp, being so sensitive the phones will not draw much current at sensible volume levels, but it all depends how picky you want to be. The LME buffer will drive small speakers. You can use a discrete buffer, but better to go with something with built-in short-circuit protection.

 

Quote:

The DAC has volume control which is controlled using i2c communication from an arduino.

 

This is potential trouble. Digital circuits and analog circuits cohabit uncomfortably. Audio performance can easily be degraded by digital (clock) noise, and designing a board to accommodate both is probably not for a beginner. A standalone DAC is probably enough to deal with, without adding a microcontroller. I would recommend learning to lay out an analog-only board for a first attempt, but I've ignored quite a lot of sensible advice myself.

 

Post your circuit and layout.

 

w

 
post #3 of 12
Thread Starter 
I will have to get a copy of the dac layout tomorrow as my dad is designing it.
The enclosure I am using is actually most likely to be a custom built aluminium one, but if log dials would be easier I could use those.

Now the microcontroller part, I can control the receiver chip by sending 2 pins high or low, this can be done with the microprocessor flicking some switches. Then having the analog output from the opamp on the dac circuit sent to a pot for analog volume control. The pcb design is the exact same as the one I posted however

For now I am just working on the dac and getting some ideas before starting on the rest. As the dac is already drawn up from one thats working I am just waiting on parts to arrive to finish off the pcb.

I do not understand the using an opamp for each output however. Then again I hardly understand the summing circuit. Now I have an opamp on the output of my dac.

Then that and the inputs from the jacks go through resistors of the same value, merge and then go through another opamp which outputs the final value. Each output will send out the exact same thing, I did read about using a buffer to drive my headphones but didnt understand them at the time.

So I now have the following answer to my questions and some that I didnt ask;
1) use 10k log pots (slider if I can fit them, normal if not)
2) avoid using software control, so I will attempt to fully control my ics with switches
3) use a buffer on my headphone output to run the extra power needed for 80 ohms

An I grasping this a little more? Im trying to follow what you say as much as possible

Thanks,
Jamie.

EDIT: I forgot to say, my current speakers are powered
Edited by bigjme - 2/5/14 at 5:57pm
post #4 of 12
Quote:
Originally Posted by bigjme View Post

I do not understand the using an opamp for each output however. Then again I hardly understand the summing circuit. Now I have an opamp on the output of my dac.

Then that and the inputs from the jacks go through resistors of the same value, merge and then go through another opamp which outputs the final value. Each output will send out the exact same thing, I did read about using a buffer to drive my headphones but didnt understand them at the time.

Thanks,
Jamie.

EDIT: I forgot to say, my current speakers are powered

 

OK, a headphone may be too big a load for an opamp, so 2 headphones will definitely be too big a load for an opamp.

 

Here's a 4-way CMoy. You could redraw U2:A and U2:B to be summing amplifiers.

 

 

 

w

post #5 of 12
Thread Starter 
Thank you. What if I put a buffer on every output? Enough to power headphones but would it cause issues with powerd speakers?
post #6 of 12
Quote:
Originally Posted by bigjme View Post

...would it cause issues with powerd speakers?

No.

 

w

post #7 of 12
Thread Starter 
Ok great. I will have to have a look through some buffers so I know how what they do properly
post #8 of 12

A buffer (an ideal buffer) increases the current available and leaves the voltage unchanged.

 

w

post #9 of 12
Thread Starter 

ok great, so as my buffers i am going to use NE5532 and will use one on each output

 

one question i do still have is, what resistance log pot would you recommend?

would you stick to the 10k as before or get larger if i can get some? as a quick look on ebay and most seem to be 500k

 

and as i want one for each input im going to need to find some dual log pots

would these seem ok to you or would you steer clear? And if possible i would prefer the slider just for knowing volume levels due to using hardware mode, so what out of the 3 would you recommend?

 

Pot 1

Pot 2

Pot 3

 

Regards,

Jamie


Edited by bigjme - 2/10/14 at 11:59am
post #10 of 12
Thread Starter 

ok so as i have everything ordered for my dac and the board built ready to be finished i though i would start work preparing my summing circuit,

if i am correct this would be the basics for a summing circuit? (sorry for the crude drawings, most the information for this is from the internet so may be totally wrong)

 

 

and i believe for volume control instead of using set 10k resistors i would use 10k pots?

but keep the 10k's on the opamp part the same

 

 

and if so i believe this would be it with the buffer for each output channel, without volume control yet

 

 

Anymore help would be great.

 

Regards,

Jamie


Edited by bigjme - 2/11/14 at 5:05am
post #11 of 12

Alps have a reasonable name for pots.

 

The arrangement you have shown is inverting, it's the style these days to arrange your designs so that overall they're non-inverting.

 

You have to post more complete schematics, the devil is in the details. It's not as simple as 'instead of using set 10k resistors i would use 10k pots', there are at least 2 ways you could wire the pots which would be unsuitable. You have to think about how the mixer will load the inputs and how (if) the inputs will interact. The simplest way to do this with reliable and predictable results is to wire each input with it's own volume (10k) control, buffer it, wire each buffer into the summer and then buffer each output, probably off a master volume control. This keeps the inputs isolated from each other and prevents interaction of the controls at the summer. 

 

Your buffer doesn't need the cap and 2M2 resistors to bias it. You can leave the whole string DC coupled. The summer is already biassed to 0V output, although the inputs have not been arranged to optimize the DC offset. AC coupling at some point may be a good idea though, and if you do use it, it gives you the freedom to use a single rail supply.

 

w

post #12 of 12
Thread Starter 
Ok I will have to read through that a few times to make sure I have my head wrapped around it then try and rework my drawing
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