DIY Audio Projects Forum
http://diyaudioprojects.com/Forum/

need help for constant volume active mixer circuit
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=6216
Page 1 of 1

Author:  sarv [ 12 May 2018, 23:01 ]
Post subject:  need help for constant volume active mixer circuit

Hi all,

I'm looking for some help on a mixer circuit design. To summarize, I've got two audio (AC) signals that I'm mixing and want to be able to mix them with a pot and have constant perceived volume.

I found this great source which gives the circuit for the job: http://music-electronics-forum.com/t32035/

I've got a single-supply configuration so I had to slightly modify things by putting the virtual ground on the non-inverting input and AC coupling the pot wiper to GND.

Both the 'wet' and 'dry' inputs are coming from other op amps so are low impedance.

The circuit works exactly as it should other than a couple issues:

a) when I have a very loud input, I get a kind of 'scratching' at the extremity of the pot, as it's turned from its max up to about the first 5% of the turn or so
b) the other signal isn't completely killed at either extreme, it comes through albeit very faintly

Both of these issues seem to diminish proportionally as the pot value increases (scaling up the resistors along with it to keep all the ratios the same). So you're probably wondering why I'm using a 1K pot in the schematic then... well this is an 11th hour fix for a project that's meant to be finished already, and unfortunately the pot and mixer circuit are located on the PCB next to a very noisy part of the circuit as far as EMI goes. So if I go any higher than a 1K pot I'm getting a clock signal bleeding into my audio output. Barring a complete rerouting of the board, which at this point is basically out of the question, it seems I'm stuck using the 1K pot.

I found that by increasing the value of the capacitor on the pot taper I can at least diminish issue (b), but it doesn't seem to make much difference for the scratching. Also it seems to only diminish issue (b) up to a certain point, i.e. 1000uF isn't much different than 470uF.

I suspected that the scratching may have been related to some small amount of DC on the pot and so tried using a 'plastic conductive' pot instead of the carbon types that I would normally use (I've read that they can be more forgiving if you have some DC present), but I found it made no difference.

I also tried AC coupling pretty much everywhere possible, a cap in series with each of the four 1.5k resistors. No difference.

I'm thinking about maybe trying a different op-amp (currently using a TL072), but not sure which would be the crucial parameter here if this solution is worth investigating?

Or of course open to any other suggestions anyone might have.

Thanks in advance for your help!

Author:  ILoveHiFi [ 13 May 2018, 04:20 ]
Post subject:  Re: need help for constant volume active mixer circuit

Instead of using the current schematic, direcly have two seperate pots from R3 in series with capacitor to ground and another in series from R4 to ground in series with capacitor.
This is equiviliant to your current schematic.

Author:  ILoveHiFi [ 13 May 2018, 05:43 ]
Post subject:  Re: need help for constant volume active mixer circuit

If you want to fully kill the signal, try capacitor couppling the input. Then directly insert two seperate pots at input, I think log pots.
Then just use both as a seperate volume control as you would in a standard audio pot.

A circuit that sums voltages is called a summing amplifier. Thats what you have built in the schematic provided.
I don't think your prolbem is with the op amp but more with a poor volume mixer schmeatic design. I have zero exprence with eq and mixers

Author:  sarv [ 13 May 2018, 07:00 ]
Post subject:  Re: need help for constant volume active mixer circuit

KochiyaYamato wrote:
Instead of using the current schematic, direcly have two seperate pots from R3 in series with capacitor to ground and another in series from R4 to ground in series with capacitor.
This is equiviliant to your current schematic.


I'm afraid I can't make such a significant design modification at this point, this is for a packaged product which already has the design set and therefore requires a single wet/dry pot control

Author:  ILoveHiFi [ 13 May 2018, 16:03 ]
Post subject:  Re: need help for constant volume active mixer circuit

The input shouldn't have any dc voltage, therefore you can directly short the middle of 1k pot to ground.
It is fet input so zero voltage

Theres zero need for a cap to ground the middle of pot.

At zero volume, the pot is at low resistance, the series capacitor has a high resistance at low resistance, therefore the input is not directly shorted to ground. This causes your plorbem where
some sound passes when pot is at zero resistance.
This is likely all of where your prolbem is comming from as theres zero reason on why this circuit not work therotically

https://www.electronicproducts.com/RC_F ... lator.aspx
At 0.1ohms and 470uF the cut off frequeqncy is 3.38khz. this means frequences at 3.38khz and lower will not be shorted directly to ground. it will pass straight thorugh
Your circuit is equivlant to two of these boys at the input acting as a voltage divider to adjust volume

Author:  sarv [ 13 May 2018, 19:34 ]
Post subject:  Re: need help for constant volume active mixer circuit

My audio signal is coming in referenced to a virtual ground (the midway point of the supply voltage), so I have to have AC coupling somewhere. I can either do it with the one cap on the pot wiper, as shown in the schematic, or with four caps in series with R1-R4 and directly grounding the pot wiper. Practically speaking both give the same results (tested both options), and so I went with the fewer components solution.

Author:  ILoveHiFi [ 14 May 2018, 01:55 ]
Post subject:  Re: need help for constant volume active mixer circuit

try increasing r3 r4 r1 r2 to 2k7
increase r5 to 27k
leave the pot as 1k.

This gives you a gain of arround 1.5 times input

500/(2700+500)=0.15625
input voltage seen at op amp 0.15625 times input

to get gain of 1.5 total
1.5/0.15625=9.6
The op amp has to have a gain of 9.6

Using 2k7 and 27k gives you gain of 10

Author:  sarv [ 14 May 2018, 12:41 ]
Post subject:  Re: need help for constant volume active mixer circuit

It's a good idea but those values give the correct gain in the middle of the pot but too much gain on either end.

Author:  ILoveHiFi [ 15 May 2018, 04:12 ]
Post subject:  Re: need help for constant volume active mixer circuit

I think the only fix is to make the 1k pot larger resistance.
Is your thing close to transfomers or 220v ac cables picking up noise?

If its not close then likely its a bad groudning scheme or power supply noise in the circuit.
You need to be extra careful and fine tune your ground wiering if your using swtich mode supplies, the switching ripple current at caps passing through ground has large effect on noise.

Author:  Yungman [ 15 May 2018, 22:55 ]
Post subject:  Re: need help for constant volume active mixer circuit

You should talk to people in Music Electronics forum. Those are very knowledgeable people. When I was designing my guitar amps, I talk to them. They have a lot to offer. Those guys know their stuffs.

Page 1 of 1 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/