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Old 23rd January 2017, 09:41 PM   #1
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Default Help needed on DIY TL074 mixer

Hey guys!
As a part of a small endeavor in the DIY jungle, I'm in need of help when it comes to a schematic of a simple mixer I've found over at musicfromouterspace.com. The schematics looks like this:
Click the image to open in full size.

And I guess I've got a couple of questions.

1. Should I use Log or lin pots for the level? I've ordered both, since I realized that I probably want log ones after I've already ordered linear.. I know it's normally log pots for audio circuits, but I'm uncertain about the effect of the IC.

2. In this circuit, one of the outputs is meant for headphones, but I will use this as line out to record the mix. How do I connect a pot to adjust the level, and what resistance should it be?

3. Also, this part of the circuit is different from the line-level part. How would I modify it to be the same level as the "line-out" part? What I'm uncertain about is what to do with the 1K/2K resistors after the first op-amps. And should i change the value from 100K to 39k over the second op-amps, and remove the 270 Ohm resistors?

4. I will use one of the outputs to feed my amp (active speaker), so I really only need mono for that output. Is there an elegant way to sum the two signals again? I've found this circuit, but it might be unnecessary?
Click the image to open in full size.

Any help is greatly appreciated, been waiting a good while for all components and stuff, so I just want to get started
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Old 23rd January 2017, 10:27 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by rackabajsarn View Post
Should I use Log or lin pots for the level?
It's quite possible that linear pots are intended here, as they are 100k and will have a taper
that is set by the loading of the (much smaller) input resistor following them.

Last edited by rayma; 23rd January 2017 at 10:41 PM.
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Old 23rd January 2017, 10:33 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by rackabajsarn View Post
this part of the circuit is different from the line-level part.
How would I modify it to be the same level as the "line-out" part?
The simplest way is to change the two 2k input resistors to 100k each.
This would make the gain of that stage 0dB (x -1).

Last edited by rayma; 23rd January 2017 at 10:39 PM.
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Old 23rd January 2017, 10:45 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by rackabajsarn View Post
In this circuit, one of the outputs is meant for headphones, but I will use this
as line out to record the mix.
It would be best to use the outputs labeled "line out" for recording.
Then you can use the input pots for setting the gain/output level, as intended.

Then you would use the headphone outputs for monitoring. For one mono output only,
use just one op amp, with both input resistors going to the same inverting input.
You can have a 10k level adjust pot at the output of this op amp.

Last edited by rayma; 23rd January 2017 at 10:51 PM.
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Old 24th January 2017, 06:53 AM   #5
Zero D is offline Zero D  United Kingdom
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Hi,

1 - You don't say how many stereo inputs you need ?

2 - Do you actually need the L/R panning ?

3 - The design shows single Pots for the L & R inputs. Is this what you want, or Dual ones ?

4 - The design shows TL084's, but you want change to TL074's ? If so that's ok.

Here's what i would do in my image. All pots are 10k, Lin should be ok due to loading resistances. They could be single or dual. See if you can return the pots you bought. Caps are non polar types.
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Old 24th January 2017, 07:49 AM   #6
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Thanks for the feedback!
@ZeroD It's actually mono inputs all of them, 8 inputs. I would like to be able to pan them for recording purposes. The mixer is a part of a portable "jamming suitcase" with an active speaker built in to it. I'll have several small mono synths connected to it.
Yeah, I read that the 074's have better noise handling, have both so I'll try out them both and see the difference.

@rayma Great tips, thank you. The main usage is the monitoring part, secondary is the recording. So I would like to use the input pots as "gain" level for the amp, but I'm not sure if the level to the line out will be good, so I'd rather have the possibility to adjust that. Should I still have 100k instead of the 2k? Maybe its better to have some gain there and be able to adjust the level with a pot? (not sure if I have it right here)
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Old 24th January 2017, 08:23 AM   #7
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So, trying to understand the circuit better: when summing the two signals, how does the resistance affect the signal? In the original there is 1k, in Zero D's suggestion they're 10k (R11&R12). And I guess he calculated the parallel resistance for the 5.1k feedback resistor(?)

And also, is there anything more general to be said about the resistors in an audio circuit? When looking at different schematics I found, it seems the values are high and low..
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Old 25th January 2017, 10:33 AM   #8
Zero D is offline Zero D  United Kingdom
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If you had explained ALL your wishes originally, it would have saved us time etc !

As you want 8 inputs, keep the main mixer circuit as is. Replace the headphone part with my Mono Mix circuit.

Yes, TL074's are better for noise than the 84's.

R11 & R12 mix together 2 x voltages, both L & R = twice the voltage. So you need half the gain to remain at the same level as the Main mix outputs. So R13 is approx half of either R11/12. Approx because 5k's don't exist. You won't notice the difference !
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Old 26th January 2017, 07:30 AM   #9
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Sorry I wasn't more clear from the beginning Zero D! I think I've got the hang of it now, didn't think about the addition of the two signals as increase in voltage first. I don't know why I'm having such a hard time understanding audio circuits. I'm used to calculating electric circuits, but there it's fixed frequencies and no IC's and stuff, so that makes me uncertain.. Big thanks for the help to both of you, I'll probably need some more help later on other parts of the PortaJam
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Old 26th January 2017, 10:52 AM   #10
Zero D is offline Zero D  United Kingdom
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Hi, we all have to learn, & keep on learning

Keep us posted
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