Very simple Audio Mixer - battery operated - Page 1
Forum section: electronics / projects

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 14, 2020, 03:43:05 pm
content: HiI am trying to make a very simple audio mixer running on battery. It's for a hobby project.I have tried a few design and cannot quite get what I want:* two channels summed* sent to one headphone (one only)* possibility to adjust the gain on each channel* only two opamps (in one package)* Ideally with a slithly different gain on one channel than the otherI attached my designs. What do you think, and how would you recommend to do it?

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 14, 2020, 03:50:50 pm
content: Take a look an the Uher 500 Mix schematic.Opamps with suitable bandwidths consume too much power.Its hard to beat power-wise a circuit with discrete transistorslike in the Uher 500 Mix.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 14, 2020, 04:11:54 pm
content: Thanks for the reply but I have never tried using transistors as amplifiers before so I will keep that in mind for the future when I have learnt and played with all of them.For now I just want to make a very simple mixer, with few components. It does not have to be perfect, neither have the best sound but it has to work, i.e. mixing the channels and adjusting the volumeEDIT: unless you meant a battery operated circuit with an opamp would never work because they use too much power?

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 14, 2020, 06:40:32 pm
content: I came up with a 3rd design which seems to workInstead of having volume control for each channel it has a separate volume control for the guitar input (which is slightly lower anyways) and an output volume control. No ideal but it works

author: Renate
author_rank: Super Contributor
timestamp: September 15, 2020, 01:40:22 pm
content: The -40 design you have two opamp outputs slamming into each other.The -26 design you're going to have weird interaction between the two volume controls.The last design will generally work.Are the two inputs low impedance?If you hook a high impedance electric guitar directly to the input the low (and variable) input resistance could affect the sound.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 15, 2020, 02:48:21 pm
content: Thanks RenateThe _26 design was kinda working but yeah like you said the volume controls affect each other. I made the math and ended up with this (attached - Y axis is the gain and X is the potentiometer). But testing it in real shows the math is wrong because I definitely did not get a gain of 15 when testing.Anyways - the new design works - it's a good point about the impedance. The piano is definitely low impedance but the (acoustic) guitar has a passive pickup which probably has a relatively high impedance. I tested it and it sounds ok, but I can see that the gain affects the input slightly.How can I make the input suitable for high impedance, then?

author: Renate
author_rank: Super Contributor
timestamp: September 15, 2020, 03:09:34 pm
content: Quote from: simonlasnier on September 15, 2020, 02:48:21 pmIf it's a piezo under-the bridge pickup it's very high impedance.It could be any number of other things too.My Taylor has a piezo bridge pickup but it goes to an internal preamp with an input resistance of 50 k.To my mind that's a bit low and I'm sure that they have to boost the bass.I've always wondered about making the piezo preamp a charge amplifier, a logical way to do piezo.You could put in a unity gain op amp on the input.You'd need to put a load/bias resistor from the + input to V/2.I'd start with 100 k?

author: Hideki
author_rank: Frequent Contributor
timestamp: September 15, 2020, 05:34:39 pm
content: Throwing out some ideas for a more conventional virtual earth mixer. You might want to tweak the component values a bit.

author: Renate
author_rank: Super Contributor
timestamp: September 15, 2020, 06:28:31 pm
content: Looks nice, but surely you meant "A" taper on those pots.C6 is a good idea too, we had no high rolloff.

author: Hideki
author_rank: Frequent Contributor
timestamp: September 15, 2020, 07:00:37 pm
content: No, definitely linear taper, since R1 and R2 will load them down and create a half-reasonable curve.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 15, 2020, 09:34:01 pm
content: Alright thanks guys this helps a lot!I have been googling opamps and input impedance a bit and I think I understand it better now. Looking at your design Hideki I have a few questions is you do not mind1) The first opamp is non inverting in order to have a higher input impedance I understrand that, but then why adding R10 - to reduce the input impedance?2) Wouldn't R12 influence the bias voltage? (and R10 but 1M is not much influence anyways).3) Why are C2 and C3 needed? Isn't the signal already biased at this point?Thank you both

author: Hideki
author_rank: Frequent Contributor
timestamp: September 15, 2020, 10:17:17 pm
content: 1) The super high input impedance of an opamp is not what you need, so a resistor should be present. 1Mohm is a completely normal input impedance for a guitar with magnetic pickups. If it's a piezo without a preamp, and you feel the high frequencies are rolled off, you may even want something higher, like 2.2 or 3.3M.2) R10 sets the + input to the bias voltage. The opamp will then change its output to make the - input the same as the + input. The resistors set the gain for any signal different from the bias voltage. 1+R11/R12 = 2x gain.3) C2/C3 are there to remove the bias DC voltage around the pot, which will otherwise sound really scratchy when you rotate it with a DC offset present.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 16, 2020, 09:01:33 am
content: Ok I understand! A bit confused about 2) though - I asked about R12But do you mean that R10 is actually necessary to make sure the bias is aplied to the + input? (which answers my question 1) actually)And about C2 and C3 I get it, I guess one way to avoid them would be to connect the pin 1 of the pots to the bias voltage, but that would require putting an opamp on the bias voltage (like an LMV321) otherwise its level will be changed as the pot moves. Anyways I will stick to having C2/C3One last question: I have seen small caps like C6 before, I heard a few things about it - how would you call it, i.e. what should I write in google if I want more information about it?Thank you!!

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 16, 2020, 09:35:20 am
content: Quote from: Hideki on September 15, 2020, 07:00:37 pmI am trying to calculate the second opamp's gain, on let's say the piano signal.The gain will beA=-R3/(input impedance)But that is applied on the signal after the voltage divider created by the pot. If we call p the pot position (from 0 to 1), then the voltage divider simply multiplies by p.Also the input impedance changes with the pot it actually isR1+(1-p)VR1If all of this is correct then the gain isA=-(p*R3/(R1+(1-p)VR1))that gives-A=1/((1.22/p) - 1)which is definitely not linear!(see attached plot)Are my calculations right?If they are then I will definitely go with this design, I might change R1 & R3 to 10k to have more gain (even if it means it will distord when pushing it too hard), and R12 to 10k as well to have more gain on the guitar input which is slightly lower.

author: Saimoun
author_rank: Frequent Contributor
timestamp: September 16, 2020, 11:04:53 am
content: So I just tested it on the breadboard - it worksI cannot really tell if the volume controls are linear or not... To the ear it does feel a little like it is "dropping" all of a sudden at the end (i.e. pot towards GND).Also it is really noisy - clearly because of the breadboard, I will solder a version later and let you know

author: Hideki
author_rank: Frequent Contributor
timestamp: September 16, 2020, 08:03:54 pm
content: Yes, R10 is necessary to bias the input. Then R12 and R10 set the gain. None of this changes the bias voltage.You can experiment with connecting the pots to the bias voltage, but then it may be a good idea to stiffen it a bit more (decrease R6/R7 and increase C4). Doing that will increase the current draw a bit.Hmm C6.. Bandwidth limiting cap? Does it have an official name?When using a linear pot, the resulting gain versus rotation will not be linear because the mixing resistors load it down, thereby creating the curve. You can certainly tweak the R1/R2 values to get a different curve.If you allowed using more than two opamps you could use log pots and buffer them, but it's hard to make every parameter perfect when having limited resources.R3 would be much lower in a normal mixer, which also helps lowering the noise.