Design Critique for Basic Mixer (Beginner!!) - diyAudio
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Old 1st April 2013, 10:28 AM   #1
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Default Design Critique for Basic Mixer (Beginner!!)

Hi all,

I am trying (for the first time no less!) to build a very simple two channel active mixer. I have designed a circuit based on a schematic I found on Rod Elliot’s superb website.

I was hoping some people may be kind enough to answer a few questions I have about this design. I have attached the schematic.

1) First, is this design ok? I can’t see any errors but given my inexperience, I probably wouldn’t know if I had made any!

2) If I understand correctly, opamp A1 is inverting the signal. Do I need a second opamp circuit to invert the signal again back to “normal phase”?

3) The intention is to use this mixer on stage to mix a Yamaha Tenori-On signal and the signal from an iPod. Therefore the output will be going into a DI. Is this mixer suitable for use in front of a DI? Will the phantom power supply to the DI cause any issues for this mixer?

(Both the stereo signal from the Tenori-On and the iPod have been summed into their own unbalanced mono signal before they reach this mixer).

4) I have set resistors R1, R2 and R3 at 5k as I understand less resistance equates to less noise but anything lower will overload the opamp. For two channels is this value ok?

5) According to my calculations 1/4W resistors would be ok, is there any reason to use a higher value?

Sorry for the questions, but I would really appreciate any help.

Thanks very much!
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Old 1st April 2013, 10:41 AM   #2
Mooly is online now Mooly  United Kingdom
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1) No errors. Only comments are that the input impedance is very low (5K pot + the low input impedance of the opamp configuration). The 5K resistors R1 and R2 determine the "opamp" input impedance. You could usefully increase these to say 22K and the same for R3. The pots at 5K pots still determine the basic input impedance each source component sees. Do we normally use log pots for a mixer ?

2) It does invert and purists argue it might be detectable on certain material. If you swap polarity of both speaker leads in a stereo set up you invert the polarity in the same way. Can you tell any difference ?

3) No idea what a DI is

4) Answered in 1. Noise is not an issue at line level unless you are chasing numbers. 10K to 47K would be considered a normal range.

5) 0.25w is fine although I think most small metal films are 0.6w

Not familiar with the virtual ground generator IC so I'll take your word its correct. You can use a standard opamp for that function too with the addition of two resistors.
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Old 1st April 2013, 01:17 PM   #3
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The virtual ground is not going to work, Pin 3 should be 1/2 of the 24 volt and pin 2 should be connected to pin 1. See here for some circuits: Virtual Ground Circuits
Gain is, dependent on VR settings, about 1/2. Input and output DC blocking capacitors should be added.
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Old 1st April 2013, 01:28 PM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by mickeymoose View Post
The virtual ground is not going to work, Pin 3 should be 1/2 of the 24 volt and pin 2 should be connected to pin 1. See here for some circuits: Virtual Ground Circuits
Gain is, dependent on VR settings, about 1/2. Input and output DC blocking capacitors should be added.
DI: Direct Injection Box
Have fun! E
Sorry but it *is* going to work with the TLE2426 specified.
Pinout is correct for that chip, according to datashet.
The splitter resistors certainly must be inside.
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Old 1st April 2013, 02:33 PM   #5
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Oops, my bad! I did not check the partslist and assumed (no comments here please!) it was a dual op-amp.
I did not know that such a beast existed, thanks for making me aware of it! E
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Old 1st April 2013, 05:02 PM   #6
JMFahey is offline JMFahey  Argentina
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Don't worry
Exact same happened to me ... only I was intrigued by the part name and checked first.
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Old 1st April 2013, 10:35 PM   #7
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Thank you very much to all for the helpful and encouraging replies.

I will change the value of R1, R2 and R3 to 22k. In doing so can I leave the pots at 5k or should they be closer in value to R1, R2 and R3?

I will also change the pots to the linear type.

Thank you very much again! I really appreciate it.
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