Summing mixer questions

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

johnheath

Well-known member
Joined
Jul 31, 2014
Messages
890
Location
Sweden
Hi all...

I have some plans on building a summing mixer with gain make up stages. I have found some "old" schematics from NYD and it seems to be fairly understandable for me but I still have some questions.

According to the attached schematic (all from NYD) I thought that I could use a 10k:10k  inpyt transformer and then a PAN-pot designed by NYD and then the gain stages.

My questions are:

How do I calculate the loss ( and gain make up) according to how many channels I am willing to use (build)?

How do I calculate the Rx depending on how many channels I am willing to use (build)?

I am far from an expert on this so any thoughts are welcome.

Best regards

/John
 

Attachments

  • Summing mixer.png
    Summing mixer.png
    11 KB · Views: 46
  The loss on each channel will be given on the pan pot, the transformer and what you want to call 0dB on your fader.

  The loss in the bus depends on the gain stage configuration you want to use. If you go for an inverting stage (usually called virtual earth summing) you basically have no loss and the gain is just Rf/Rx.
  If you instead go for a non inverting stage (usually called passive summing, good luck taking out the PS from it) The gain you need there is equal to the number of channels.

  To choose Rx for the optimal noise you want the optimal input impedance Zio=Ein/In of the opamp seen by it, so Rx=N*Zio. But you still want not to load too much the previous stage, in this case the pan pot and the fader, so you don't want any lower than 24k, even higher if possible depending on the expected law of the pan pot.

JS
 
Thank you sir

I am sorry for not explaining that I had the intention to use some sort of tube gain stage... depending on how much gain I need.

Using tubes will end up in quite high input impedance so I guess the load from the pan-pot will be in the 1:10 ratio anyway.

Actually I am mostly interested in how much gain I have to produce depending on how many channels I will get. I remember seeing a formula for calculating the loss but I cannot find it again.

Even so with the Rx... have seen a formula somewhere but cannot find it.

I guess I just could say that I will get six or eight channels and let someone just tell me what I need but it would be nice to be able to calculate it by myself :)

Best regards

/John
 
> it would be nice to be able to calculate it by myself

So why don't you start?

The transformer is just a signal source, so close to 1:1 that you may eliminate it. In a later step you may want to call it a 1K series resistance to approximate its loss.

Fader position is operator choice. We'd start by shoving knob full-up, which makes it a 10K to ground and little effect. In a session, we normally start with faders at "nominal", leaving some gain "in hand" (in case the cowbell gets off-mike and needs boost). But start with a full-up condition.

You NEED a value for "Rx". You also probably(?) have a resistance shunting the bus. It is popular to use mike-amps as bus recovery amps. These must "see" a couple hundred Ohm source to keep their input transformers happy.

Pencil all that in.

Then it is simple Voltage Divider arithmetic. Two 10K resistors series, the voltage at the tap is half the total. For 9K and 1K, 1/10th. Since this is audio where 10% is no big deal, 10K+1K is also 1/10th for our purposes.
 
A no-math alternative is to mock it up with spare parts. Put a 1.5V battery in place of the signal source (here, the transformer). See what comes out of the output. Modern DMMs read mV just fine, so this works even for quite large attenuation (more than you want).
 
Thank you sir

I'll have a go at it.

Just a thought: You mention recovery amp and the need for shunt resistors of a couple of hundred ohms... I see that. But if I plan to build a pair of tube gain stages would you recommend to use those as mike-amps with a input transformer or would it be possible to use a higher value resistor as grid resistor for the tube gain stage?

I guess that I am struggling to express myself here but my main thought is to just go from bus to grid of a preamp tube... doable?

Best regards

/John
 
> just go from bus to grid of a preamp tube... doable?

Obviously. See any PA amp of the 1940s to 1960.

 
PRR said:
> just go from bus to grid of a preamp tube... doable?

Obviously. See any PA amp of the 1940s to 1960.

I see, thank you sir... just out of curiousity... what happened after the 60's? Transistors? Or did people change for another reason?


Best regards

/john
 
Back
Top