simple mixer, master section question

[Dimitree]

I'm building a mini line mixer to monitor my synthesizers, and I have a question about how I implemented the master section. It is a really simple master section, just a pair of virtual earth summing opamps, an additional mono summing opamp for PFL signals, a relay activated by PFL detect and a master stereo fader followed by +10dB gain stage.
My question is, would there be audible clicks when switching due to DC differences between the two paths? should I add DC blocking caps before (or/and after) the relay? or should I wire the relay after the inverting opamps (and so add another inverting opamp for the PFL line) ?

thank you!

 

[JohnRoberts]

I didn't inspect your schematic but will answer in general. It is OK to have small clicks in the PFL output from switching but you want the two mix output to always remain pristine. So you can PFL individual stems during a mix without stepping on the mix.

JR

 

[ruffrecords]

It looks like you have not separated monitoring from the main stereo bus output. Normally a mixer would have a stereo bus output much like yours but without the relay. A feed from the main output would go to a monitor level pot which would feed a separate monitor output - to a loudspeaker or headphones. Your PFL relay would be in this latter section so PFL only goes to the monitor section/output.

Cheers

Ian

 

[Dimitree]

thanks, but indeed this is a simple monitor mixer, so only a single pair of outputs going into the monitors. There is no actual "stereo bus output" to be used for recording, I don't need that.
I'm just wondering how I can make the switching as silent as possible

 

[ruffrecords]

thanks, but indeed this is a simple monitor mixer, so only a single pair of outputs going into the monitors. There is no actual "stereo bus output" to be used for recording, I don't need that.
I'm just wondering how I can make the switching as silent as possible

OK, understood. To avoid clicks you should aim to ensure here is never any dc on the switching contacts. This usually means adding dc blocking capacitors.

Cheers

Ian

 

[gyraf]

..but there will off course always be clicks whenever you switch in a waveform..

 

[Dimitree]

OK, understood. To avoid clicks you should aim to ensure here is never any dc on the switching contacts. This usually means adding dc blocking capacitors.

Cheers

Ian

thanks! do you think I should only put the dc blocking capacitors before the relay, or also after, to also block DC bias current coming from the opamp inputs?

 

[ruffrecords]

thanks! do you think I should only put the dc blocking capacitors before the relay, or also after, to also block DC bias current coming from the opamp inputs?

Personally I would put them on both sides and make sure they have drain resistors to ground to ensure they remain discharged. I would eliminate the unity gain stage directly after the relay. It is also not a good idea to have bias current flowing through the wipers of pots - they can easily causes scratchiness. In fact, as this is just a simple monitor mixer you also could probably eliminate the output amplifier altogether mixer. Just take the output straight from the pot which is connected straight to the relay

Cheers

Ian

 

[Dimitree]

but if I remove both the output amp and the unity gain stage after the relay, how would I correct the phase after the summing amp? you mean I don't need to correct the phase because it is a simple monitor mixer?

 

[JohnRoberts]

Polarity is important.

Thumpy clicks come from DC voltage differences. Interrupting musical waveforms will also generate high frequency clicks.

There are strategies to reduce even those normal clicks but perhaps too much effort for a simple mixer.

1- Pre/de-emphasis. Applying some HF boost before the switch and some complementary HF rolloff after the switch will reduce the HF energy from the click. You want the EQ to be applied precisely and this will eat into HF headroom/

2- zero crossing synchronization. If you switch only at zero crossings there is less energy to click. To be even more sophisticated you want to coordinate rising or falling waveform segments to not introduce DC from say half a sine wave cycle.

I don't think I have ever seen anybody do this in a mixer master section, I never did. Maybe check out some commercial master section schematics for ideas. There are different ways to skin this cat.

JR

 

[ruffrecords]

but if I remove both the output amp and the unity gain stage after the relay, how would I correct the phase after the summing amp? you mean I don't need to correct the phase because it is a simple monitor mixer?

It is debatable if absolute phase is a thing that needs to be maintained but as you say it is also a simple monitor mixer so probably unnecessary.

Are the inputs balanced? If they are then just swap hot an cold to restore absolute phase.

Alternatively, as this is a simple mixer for synths, presumably all the input are stereo so there is no need to use virtual earth mixing with its unwanted phase inversion. Just use passive mixing into a non-inverting gain make up stage. Even if you have some mono inputs, as long as you don't need -70dB crosstalk you will be fine with passive mixing.

Hope that helps.

Cheers

Ian

 

[Dimitree]

thanks Ian! actually almost all the synths are mono (those are analog synths) but I'm really considering your advices on passive mixing

 

[JohnRoberts]

It is debatable if absolute phase is a thing that needs to be maintained but as you say it is also a simple monitor mixer so probably unnecessary.

Apparently there is even debate about the nomenclature... . There is valid debate about the audibility of "absolute polarity" by itself, but it is important to maintain in system design because mixing together stems with opposite polarity can cause significant cancellations. Those mixing interferences can occur inside the box or outside.

Are the inputs balanced? If they are then just swap hot an cold to restore absolute phase.

yup. When in doubt do it right.

Alternatively, as this is a simple mixer for synths, presumably all the input are stereo so there is no need to use virtual earth mixing with its unwanted phase inversion. Just use passive mixing into a non-inverting gain make up stage. Even if you have some mono inputs, as long as you don't need -70dB crosstalk you will be fine with passive mixing.

KISS keep it simple.

JR

Hope that helps.

Cheers

Ian