My first module schematic-a mixer. Feedback?

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GKMendenhall
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My first module schematic-a mixer. Feedback?

Post by GKMendenhall » Mon Feb 10, 2020 10:20 am

Hello! I just started my journey into building DIY Eurorack a few months ago. I've got this schematic I made that is based off of the AI Synthesis mixer module and I want to order a couple of PCBs for my system. I was hoping to get some feedback on it if anyone wants to offer some. This will be the first powered module I've ordered a PCB for. Thanks in advance!
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Troubleshooter
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Re: My first module schematic-a mixer. Feedback?

Post by Troubleshooter » Mon Feb 10, 2020 10:33 am

Maby add some 10 Ohm resistors in series with the power rails close to the power connector. Just for good measures....

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GKMendenhall
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Re: My first module schematic-a mixer. Feedback?

Post by GKMendenhall » Mon Feb 10, 2020 11:00 am

Thanks!

So, put the 10 Ohm resistors between pins 10 and 1 then 6 and 5?

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Re: My first module schematic-a mixer. Feedback?

Post by guest » Mon Feb 10, 2020 12:42 pm

looks good. i would go with some 47uF or 100uF caps at the input (along with the 0.1uF caps - but those can be closer to the opamp), and some reverse protection diodes. at the current draw of this module, the impedance of the diodes will be around 10ohms, so you might not need to do both.
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Re: My first module schematic-a mixer. Feedback?

Post by Synthiq » Mon Feb 10, 2020 1:27 pm

GKMendenhall wrote:
Mon Feb 10, 2020 11:00 am
Thanks!

So, put the 10 Ohm resistors between pins 10 and 1 then 6 and 5?
No pin 1 and 10 are still connected and the resistor, or diode, is placed after the two pins but before the capacitor(s).

As a precaution I would also connect the third pin on the level potentiometer to the wiper so there will still be a feedback path even if the potentiometer get worn and you get poor contact between the wiper and the resistor element.

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GKMendenhall
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Re: My first module schematic-a mixer. Feedback?

Post by GKMendenhall » Mon Feb 10, 2020 1:45 pm

Thanks a lot for your feedback! I'll let you all know how it goes!

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Re: My first module schematic-a mixer. Feedback?

Post by MikeDB » Mon Feb 10, 2020 4:27 pm

I would replace MIX_LEVEL with a fixed resistor and then use it in a similar arrangement to the input pots. This will reduce any noise due to DC offset voltages.

Also is 330 Ohm a usual output impedance for synths - I was expecting 600 Ohms.

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Re: My first module schematic-a mixer. Feedback?

Post by GKMendenhall » Tue Feb 11, 2020 11:46 am

MikeDB wrote:
Mon Feb 10, 2020 4:27 pm
I would replace MIX_LEVEL with a fixed resistor and then use it in a similar arrangement to the input pots. This will reduce any noise due to DC offset voltages.

Also is 330 Ohm a usual output impedance for synths - I was expecting 600 Ohms.
Thanks for the tip! I have no idea how to answer your question about the 330Ohm resistor. I'm trying my own version of someone else's schematic and I'm a total newbie when it comes to the electrical engineering part of building eurorack (I need to start learning more...).

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Re: My first module schematic-a mixer. Feedback?

Post by guest » Tue Feb 11, 2020 1:02 pm

ive seen anything from 100ohm to 1k as an output impedance, there really isnt a standard for modules. that resistor serves 2 purposes: 1. limit current flow in case of over-voltage or plugging into ground/anohter output. 2. keep oscillations down when driving a long cable (opamps and other devices sometimes become unstable when driving a capacitive load, like a long cable). most opamps are internally protected against short circuit, and 100ohm is enough to damp oscillations.
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GKMendenhall
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Re: My first module schematic-a mixer. Feedback?

Post by GKMendenhall » Thu Feb 13, 2020 1:14 pm

I'll keep that in mind, thanks!

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Re: My first module schematic-a mixer. Feedback?

Post by KSS » Thu Feb 13, 2020 8:26 pm

guest wrote:
Tue Feb 11, 2020 1:02 pm
ive seen anything from 100ohm to 1k as an output impedance, there really isnt a standard for modules. that resistor serves 2 purposes: 1. limit current flow in case of over-voltage or plugging into ground/anohter output. 2. keep oscillations down when driving a long cable (opamps and other devices sometimes become unstable when driving a capacitive load, like a long cable). most opamps are internally protected against short circuit, and 100ohm is enough to damp oscillations.
3. Allow passive mixing of outputs. (covered under 1. above, but really is a separate item, since this is an intention, and 1. is protection against unintentional action.)

ARP put 1K output resistors on their 2500 -to support the matrix switches, and also the 2600 -to protect outputs of early OPAs. Bernie Hutchins adopted it for the Electronotes DIY specs of the 70s. Since that time, many other synth designers have used it. Until the last several years, when the groing size and new younger membership of the communitiy along with sharing levels afforded by the internet started to probe more deeply into the why's of output resistors beyond "that's the way it's been done since forever". When the passive mixing as a standard is diminished, the reasons and choices for use and value change.

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