Hi!
Five years ago I built a ghetto blaster based on an Amp6 from 41Hz and IMG Stage Line SP-6/100PRO speakers, and some accompanying tweeters. I'm still very pleased with the sound! Recently I bought my first Arduino and found a nice synthesizer project.

My plan is to add the synthesizer to the ghetto blaster and since I'd like to both play music from my cellphone and the synthesizer I'll need to build a mixer. I also thought I might add a preamp for each input and a graphic equalizer.
I've found pre-amp and mixer circuits based on LM833 http://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer that seems nice. That site also has two equalizer circuits, one 3 band
http://www.circuitlib.com/index.php/projects/product/25-3-way-tone-control/category_pathway-29
and one with modular amount, both based on NE5532. I think I can manage these circuits and perhaps even tweek them with the knowledge I already have.

My main concern is the power supply. The ghetto blaster is now equipped with a 7.5Ah 12.8V LiFePO4 battery. For now it only supplies the amplifier, without any converter. To power the arduino I'll need 5V and for the op-amp:s I need, or at least I think i need, +-15V.

I've found this circuit for the 5V https://circuits.io/circuits/895267#schematic based on MC34063, should be more than sufficient for the arduino i guess.

For the dual rail power supply for the op-amps however I'm not sure what to look for. I've found this circuit:
http://www.discovercircuits.com/H-Corner/battery-power-15v.htm
but it might not supply enough current, for all the OP-amps, and I cant find the particular dc-dc converter. Can i swap it for an TMR 6-1223, or were can I read about what changes needs to be done in order for the circuit to fit my needs?

Best regards, Axel!

 

Voltage converters oscillate to convert the voltages. The oscillation causes interference to audio circuits.
Use a linear regulator to make 5V from the battery.
The NE5532 opamps work fine from a single 12.8V supply even when it runs down to 6V, if you bias them at half the supply voltage and use coupling capacitors with the correct polarity then the negative supply is not needed. Use NE5532 opamps in the mixer too.

 

Voltage converters oscillate to convert the voltages. The oscillation causes interference to audio circuits.
Use a linear regulator to make 5V from the battery.
The NE5532 opamps work fine from a single 12.8V supply even when it runs down to 6V, if you bias them at half the supply voltage and use coupling capacitors with the correct polarity then the negative supply is not needed. Use NE5532 opamps in the mixer too.

AudioGuru is correct, that is the way to go.

 

Thanks for good answers
I think I understand what to do and will dig a little deeper into the circuits. But I have one short question regarding the coupling capacitors. Will they be needed only at the end and start of my circuits(before preamp and after equalizer) or do I need them for every part of the circuit(both ends of preamp etc.)?

 

An audio circuit needs a capacitor at the input to block any DC from a signal source. If an audio circuit is to feed another audio circuit then it should have an output capacitor to block any DC on what it feeds.
Use common sense if you have an opamp feeding another opamp in a circuit. If the output of the first has the same DC voltage as the input of the second then a coupling capacitor is not needed and if it is installed then the second opamp needs to be biased and the capacitor might cause droopy bass sounds.

Here is the difference in biasing an opamp with a dual supply and with a single supply. Since the inputs of an opamp have a very high impedance then the biasing resistors can have a high value like 100k or more.

 

Ok, so I've dug a little deeper and think I got the hang on some of the stuf!

I've downloaded ltspice to try and see how the different components affect the transferfunction. My circuit is similar to the the one at the bottom right of your figure Audioguru. I've however not included the capacitors at the bias voltage resistors since I haven't got the hold of their purpose.

Unfortunately I can't manage to get a decent crossover with components that have reasonable values, eg gain resistors below 1M and input capacitor below a few mikroF. I tried to derive a transfer function and ended up with C1/C2*(1+S*C2*R3)/(1+S*C1*R4), is that correct? When doing this I neglected the bias voltage. In my ltspice simulation I have used a nobrand OP-amp since I couldnt find the NE5532.

Since the boombox is battery powered I'd like to minimize the power consumption. How much higher than 100K can I use for the biasing resistors, will 1M work?

After each preamp I'll have a volume pot for each channel. Since they don't like DC I'll need capacitors after them, thus before the mixer?

When I'll start to solder everything onto circuitbords, is there any gain in soldering everything on the same board or can I simply have one board for each preamp, mixer, arduino etc.?

 

The opamp circuit you show is inverting with a gain of only 2 times (slightly more than a piece of wire). Its resistor values are too high for an old NE5532 opamp because its input resistance is low (a minimum of 30k ohms) and its bias current is high (almost 1uA). Instead if you use a modern opamp then its input resistance will be very high and its input bias current would be almost nothing. I use TL07x opamps for audio circuits.

Your inverting circuit has a much lower input resistance than a non-inverting circuit. Why make it inverting?
The filter capacitor for the bias voltage is important to prevent the opamp from amplifying and oscillating from supply voltage fluctuations caused by the power amplifier's high current.
I do not understand your formula for the coupling capacitor C1 that produces a loss of -3dB at 160Hz and much more loss at lower frequencies so there are no bass sounds as shown on your graph.

I do not know why you need a preamp that does nothing but reduce bass sounds. The mixer can have the inputs and be built together with the equalizer. The Arduino can be built on its own board.

Why do you need an equalizer? If the amplifier, speakers and your hearing work properly then an equalizer is not needed.

 

The figure in my previous post was only to illustrate were I've placed the capacitors and resistors. C1=500nF and R_in=20K gives me a quite ok response. I dont see that C2 affects the output, except for blocking DC, could 1nF be a good size?
I didn't know the NE5532 was that bad, I'll look into the TL072

I've made it inverting because the circuits I linked in my original post were inverting and they had a thourogh explanation. But I dont mind changing to a noniverting if it gives me better performance. Ok, then I'll add them, could 200 and 100 nF be good for the ias voltage capacitors?

I added the pre amps to make sure that all inputs into the mixer had the same amplitude but of course I can to some extent fix that with the input resistors on the mixer.

I dont really need the qualizer and the pre amp. The pre amp was both to adjust in levels but mostly to learn about circuits. The equalizer is also mostly to learn but also because it looks cool and retro with a graphic equalizer .

 

A capacitor has reactance that blocks DC and low frequencies. Your previous schematic had C1 as 10nF and the R4 input resistor as 100k so it cut frequencies at 160Hz and lower. Now you have 500nF and 20k which cuts 16Hz and lower so it passes bass sounds fairly well.
If the output capacitor is only 1nF and if its load is 10k then it cuts 16kHz and lower which is almost all audio frequencies, a squeaky sound. You must decide the minimum load resistance then calculate the value of a capacitor that passes the frequencies you want. If the load is not less than 10k then the output capacitor should be 800nF (use 1uF) to pass 16Hz and lower.