finishing a simple 2 chanels circuit

[jeanpierrecedro]

Hello,
I am new here, I spend my time on musical structures & instruments making and repair.
For a project, I was looking for a small onboard 2 chanels mixer for Two Piezzo preamp.
The preamps where custom made Few weeksh ago but the circuit maker passed away sudenly and the shematics are not finished.
I already have made the preamps, they work very well but I need this extra mixer for them.
Here is the schematic of 1 pre (the other one is exactly the same): 3preEQ.png
what I need is a small mixing circuit like this (in Two chanel) : mixer.png
But i have no idea about the value of each resistors/pots . My first idea is 100K pots and 1k resistors around the IC but does it feet my circuits (impedence), do i need a condenser between the pre and the mixer ??
I hop you see what i mean ;-)

last info, the circuit is powered with 9v battery ;-)

Any help welcome

 

[JohnRoberts]

Good start but you need to capacitor couple the audio going to and from the pots, then bias the op amp up at v/2.

JR

 

[jeanpierrecedro]

Thanks JR
Yes there will be a 4.5v on IC's + input.
by coupling you mean a capacitor at each input before the pot and another at each pot output? Non polarized about 10nF or something like this?
But on the preamp schematic, at the output there is already a polarized capacitor (and a strange 100ohms resistor) so do I need a capacitor at pot inputs?

JP

 

[RuudNL]

About the mixer circuit: the inverting input of the OpAmp is a virtual ground. (Zero ohms.)
This means that the resistor(s) from the wiper of the potentiometer(s) should have a higher value than the potentiometer itself, in order to produce a not too heavy load. (influencing the control behaviour of the potentiometer(s).
Also: you need a resistor in series with every wiper, because as the circuit is now drawn, there will be no output if one of the potentiometers is turned completely to 'zero'. (Wiper to ground.)

10 nF at the output will be too low. You need a bigger (electrolytic) capacitor, '+' at the IC output. Try 10 µF or so.

You will also need a capacitor at the inverting input of the OpAmp, because with a single supply voltage, the setting of the potentiometers will influence the DC operating point of the OpAmp.

 

[abbey road d enfer]

But on the preamp schematic, at the output there is already a polarized capacitor (and a strange 100ohms resistor) so do I need a capacitor at pot inputs?

First, you should delete the "mixer" schematic, because it is only a block diagram.
Most of the comments from JR and RuudNL seem to pertain to this schemo.

Most of the values in the Eagle schemo are almost correct, except the 470nF should be about 5-10uF.
The 1Meg resistors in the inputs should be increased because the actual input impedance is 500k, which is considered by many as too low for piezos. Me, I'm satisfied with 1Meg input impedance, so I would put 2.2Meg resistors instead, but some insist on having much higher input impedance like 5 or 10Meg, which would imply 10Meg or 20 Meg resistors there. I won't argue about that.
I see all the stages have their own V/2 bias. It is often preferred to have a common V/2 that is distributed to all the stages. I wouldn't recommend it since it implies a complete change in the PCB lay-out.
In theory R24 and R25 should be identical, but I don't think it's that important.
I think the midrange pot could be increased to 20k or even 50k but it would require changing the capacitor and resistor values around it.
Anyway, when the prototype is finished you will need to adjust some values in accordance with the auditive evaluation.

 

[jeanpierrecedro]

thanks everyone,

RuudNL: you're wright for the wiper resistor!

abbey road denfer: the preamp section has been delivered finished and fully working with great performance for my hears. whatever I fully understand the impedance input to be raised up to 1M and I'll try it on one of the 2 preamps I have.
for the V/2 bias you are also wright and I'll will change it on the schematic for another project.

Here is a new version of the output schemo for my 2 pres
I used my instinct because my theory is too far and I don't remember how to calculate it.

 

[RuudNL]

Personally I would raise the value of R4-R5-R6 to 33K. I use as a rule of thumb: 3 times the potentiometer value.
The value of C2-1 and C2-3 is too low, you will lose the low frequencies. (Almost everything below ~ 150 Hz.)
Make it 1 µF or higher.
It might be a good idea to add a small series resistor in the output line (100 ohm or so), to prevent oscillation when driving a capacitive load. (As: a longer cable.)

 

[JohnRoberts]

I'd be tempted to make R4,R5 and R6 larger say 20k-50k. Then instead of capacitors in series with each pot wiper you could use one only cap between the op amp - input and junction of R5 and R6... Making the Rs larger will also improve the LF roll off caused by the 100nF caps.

JR

 

[abbey road d enfer]

abbey road denfer: the preamp section has been delivered finished and fully working

OK, I had not got it.

Here is a new version of the output schemo for my 2 pres

Although not strictly necessary, I would add a capacitor across R31-F1, something like 100nt to 10uF.

 

[jeanpierrecedro]

Thanks 'cause Ive learned with you ;-)
I have set output resistor at 220ohms because I have lot of them (but no 100ohms)
I have replaced the 2 wiper's capacitors with only one.
Questiion: what is the influence of bigger (say 1M) or smaller (10K) resistors in VC/2 circuit? curent consomption?

 

[JohnRoberts]

Thanks 'cause Ive learned with you ;-)
I have set output resistor at 220ohms because I have lot of them (but no 100ohms)

OK, some consumer gear uses even higher values.

I have replaced the 2 wiper's capacitors with only one.

could be polar electrolytic capacitor with + end toward the op amp.

Questiion: what is the influence of bigger (say 1M) or smaller (10K) resistors in VC/2 circuit? curent consomption?

One consideration is op amp input bias current. The Bifet op amp have extremely low current so 1M resistors would work.

The electrolytic capacitor from v/2 to ground helps reduce impedance and noise.

JR

 

[jeanpierrecedro]

about the Output resistor R3, is usefull even on small (50cm) coax wire connection?

 

[abbey road d enfer]

about the Output resistor R3, is usefull even on small (50cm) coax wire connection?

220r should be good, even if you drive a low-impedance microphone input. This resistor is necessary, because you don't know what capacitance has the input it is connected to (which adds to the cable's capacitance).

 

[jeanpierrecedro]

you where all of a precious help for me! I'll be back after I made it.

Thanks again
JP