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Old 21st April 2013, 01:31 PM   #1
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Default 4 Channel Mixer Design Critique Please?

Hi all,

Following the simple two channel mixer, I have now drawn up a design for a basic four channel mixer. This version has two balanced XLR inputs with preamps (channels one and two) and two unbalanced stereo inputs (channels three and four). All channels need to go to an unbalanced output. As this will be used live on stage, a “popless” mute switch is required for all four channels.

I have attached the schematic and I was hoping people would be kind enough to critique the design.

The preamp stage for channels one and two is based upon the Dantimax ThatMic design with a few changes to the variable gain stage, the addition of an OPA137 opamp and the removal of the phantom power supply circuit (dynamic mics will be used). The OPA137 is to drive down the DC output to 0V as per the recommended design for the INA217 (as seen here – page 7).

For channels three and four, 0V DC output is achieved using C303/R306 and C403/R406 and is taken from Douglas Self’s Small Signal Audio Design (p. 341).

Are these methods the best to get DC output to 0V – thus avoiding pops and clicks when the channel is muted? If not, what changes should I make?

Are there any other goofs I have made (quite likely being me!)?

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

Thanks very much!
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File Type: png 20130420 4 Channel Mixer Full Schematic v1.00.png (106.5 KB, 127 views)
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Old 23rd April 2013, 07:48 PM   #2
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Hi all,

I just edited the attachment – I had missed off a resistor part code. The file attached to this post is the correct one.

If anyone has any feedback on my design I’d really appreciate it.

Thanks very much again.
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File Type: png 20130423 4 Channel Mixer Full Schematic v1.01.png (106.3 KB, 118 views)
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Old 27th April 2013, 09:21 AM   #3
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Hi all,

I was hoping to begin a PCB design this weekend. Is there anyone who can give any advice at all on my design? Even the smallest of hints would be appreciated!

Thanks very much again.
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Old 27th April 2013, 03:19 PM   #4
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I had a quick look at your schematic:

1) Unless you plan to add phantom power, the input caps C104, 105, 204 and 205 (and also the output caps C303, 403 and 501) should not be polarized.
2) Why are ther 2 pots on the line-level outputs (VR301, 401)?
3) Why C103, 203?
4) A601 could be replaced with an inexpensive op-amp and I would add a capacitor from pin 3 to ground.
Cheers, E
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Old 27th April 2013, 09:02 PM   #5
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Hi Mickeymoose,

Thanks very much for looking at my schematic, I really appreciate it.

Quote:
Originally Posted by mickeymoose View Post
1) Unless you plan to add phantom power, the input caps C104, 105, 204 and 205 (and also the output caps C303, 403 and 501) should not be polarized.
I am certain phantom power will not be added in the future as this project will be used only with dynamic mics. I have therefore changed these caps to bi-polarized versions as per your recommendation (please see new schematic attached).

Quote:
Originally Posted by mickeymoose View Post
2) Why are ther 2 pots on the line-level outputs (VR301, 401)?
I have probably drawn this poorly! VR301 is one pot as is VR401. These are dual gang pots and I have tried to show both “gangs” on the schematic. I selected to use a dual gang linear pot to get closer to a true logarithmic control. I have read that using a linear pot in this way provides superior performance to that of a standard “log” pot. I read this information from Rod Elliott’s “Better Volume (and Balance) Controls” page (see figure 5).

Quote:
Originally Posted by mickeymoose View Post
3) Why C103, 203?
This was a design feature of the Dantimax ThatMic pre-amp design on which this is based. I must say I did not understand this feature but as I have plenty of space for this inside the enclosure I plan to use for this project, I decided to retain it the design.

Quote:
Originally Posted by mickeymoose View Post
4) A601 could be replaced with an inexpensive op-amp and I would add a capacitor from pin 3 to ground.
I have added a 100uF cap from pin 3 to ground (please see new schematic). Is there a particular opamp you would recommend as a replacement for the LT1206? Budget concerns are not really an issue with this project so I don’t mind paying for the LT1206.

Thanks very much again for looking at my design. I really appreciate the help.
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File Type: png 20130427 4 Channel Mixer Full Schematic v1.02.png (109.0 KB, 99 views)
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Old 27th April 2013, 10:52 PM   #6
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Hi Dufflefan:

I had a quick look at Elliott's website. I believe he writes about hifi systems and a balance control married to a level control. I'll have another look when time permits, but I believe a log pot would serve you better. Or use a linear pot in the feedback loop of the amplifier (goto Shure.com | Global Home | Choose Your Region and serch for the M267 schematic and look at the U201 circuit)

Also, you will have some level shifting (and propably some popping noises) when SW101 and 201 are activated. This than becomes a active selector, not a mixer. All 4 circuits should be permanently attached to the mixing bus. If you need switches, do not use toggle switches, but self-cleaning slide switches!

Cheers, E
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Old 27th April 2013, 11:00 PM   #7
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When you get to laying out the pcb take great care with grounding.
The audio ground and power supply grounds should meet at one place at the power connector on the pcb.

I mixed power supply and audio grounding on a pcb and ended up with massive hum.
Despite lots of cuts and links I couldn't get the hum to stop.
So I designed a new pcb with power and audio grounds connected once at the power connector.
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Old 28th April 2013, 12:11 AM   #8
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Hi Mickeymoose,

Thanks very much for the heads up on volume controls. I’ll check out the M267 schematic and see what I can do. If, for whatever reason, that doesn’t provide a solution I’ll use a log pot for simplicity.

As for the level shifting, do you think it would be possible, rather than muting the signals and causing pops, to use a low pass filter to essentially keep the signal, but render it inaudible (filtering to below 1Hz for example)? Would this avoid popping? Unfortunately the application of this project requires instant muting for each individual channel. Am I correct in thinking popping is not a problem with SW301 & SW401 as it eliminated by C303/R306 and C403/R406 respectively?

Sorry for all the questions!

Hi Nigelwright,

Thanks for the warning about grounding. I’ll keep this mind when I get to the PCB layout.

Thank you both again - I greatly appreciate you taking the time to help me out.
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Old 28th April 2013, 03:01 PM   #9
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Sometimes I get lazy! Put a 100 ohm resistor on the outputs of A301 and A401. Than the 4 switches to ground to short the outputs for muting. The op-amps should be able to handle that and you can use cheap toggle switches for this. Alternately, use analog IC switches especially designed for this. Level shifts will be minimal. E
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Old 28th April 2013, 03:10 PM   #10
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I'm not sure what all those circuit ground symbols are doing on the left hand side of the drawing?
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