how to calculate unbalanced summing bus

[kambo]

plenty of docs here for balanced summing calculation.
but, i couldnt find any info for calculating unbalanced summing bus for mic level....
it seems simple voltage divider, but i dont know where to start from,
besides a 150-220r_ish shunt resistor going to ground to keep the mic transformer happy!

 

[joaquins]

Whyy this again??

Noise performance of such device will be really bad, start with that.

  If you really need to do so I guess it would depend on the kind of mic you're talking about. I'd connect 2 identical dynamics in series to sum them and loss as little as possible. It would depend so much on the kind of mics and levels that other than specific application doesn't seem viable, mics likes to see ~2k impedance, that shunted to form a 150 to 200 Ω is 20dB loss, unless you start with pretty hot mics and sources you have a serious noise compromise.

  Also, I don't get why unbalanced at mic level...

JS

 

[kambo]

Whyy this again??

i guess i missed the topic  :

Noise performance of such device will be really bad, start with that.

ohh, got it!

  Also, I don't get why unbalanced at mic level...

JS

may be it doesnt have to be....

i have 8  tube line_amps,  need to sum...
they have unbalanced outs, and i am trying to avoid balancing them, (ton of weight, and wiring)
and i was thinking of using unbalanced passive mix bus and micpre for the gain makeup!

shall i just skip the micpre input transformer, and hit the grid with resistor network ? (edit: for summing)

 

[ruffrecords]

So it sounds like you have 8 unbalanced line level sources. I think some people thought uou meant they were mic sources but I guess you are thinking of using a mic pre as a gain make up amp for the bus. Is that correct?

Cheers

Ian

 

[kambo]

So it sounds like you have 8 unbalanced line level sources. I think some people thought uou meant they were mic sources but I guess you are thinking of using a mic pre as a gain make up amp for the bus. Is that correct?

Cheers

Ian

yes sir

 

[ruffrecords]

OK. Balanced summers tend to use external mic pres for gain make up simply for convenience and this demands the summing bus impedance be similar to that of a mic pre. If you remove the restraint of having to use a mic pre the problem becomes much simpler. First of all, you no longer need to make the bus impedance around 150 ohms. It can be anything you like.

For any unbalanced passive mix bus, the bus impedance is simply the value of the bus feed resistor divided by the number of feeds. So for example, if you were to used 10K bus feed resistors, the bus impedance is 10K/8 = 1.25K. This is too high to feed into a mic pre for gain make up but, as we shall see, you probably don't need to use a mic pre.

For any unbalanced mix bus, the bus loss is simply one divided by the number of feeds. It does not matter what the value of the bus feed resistors are, the loss is the same. So for your 8 buses, the loss is 1/8 = 18.06 dB. With such a small loss, you don't really need a mic pre for gain make up. You could feed it straight into most 10K line inputs as long as it can provide 18dB of gain. There should be no noise problems with such a small amount of gain.

The big question for you is what value of bus feed resistor to use. This depends on the load the tube line amps can drive. Can you post a schematic?

Cheers

ian

 

[kambo]

thank you Ian, that explains well
its based on NYD 6SN7 pre for makeup gain. i made some changes on second stage...
i believe R1 and R11/R12 for his simulation purposes... i omitted R1 anyway!

 

[kambo]

and input modules

 

[ruffrecords]

Hmm, I will have to explore this to understand why.

I that the same for balanced summing?

Yes and no. If you have no slugging resistor then it is the same for a balanced summing bus. However, most balanced summers assume the gain make up amp will be a mic pre and they work best if fed from a source impedance in the region of 150 to 200 ohms. In most cases, the bus impedance turns out to be higher than this so a bus slugging resistor has to be added across the bus to reduce its impedance. This also increase the loss and worsens the noise.

Cheers

Ian

 

[ruffrecords]

and input modules

Am I right in thinking you have 8 of these input modules whose  outputs you wish to sum? What is the tube?

Cheers

Ian

 

[kambo]

Am I right in thinking you have 8 of these input modules whose  outputs you wish to sum? What is the tube?

Cheers

Ian

yes, 8 in total :  4 for L  - 4 for R channel,
tubes are all 6SN7

 

[kambo]

btw, for testing with a single channel,
i was using 15K from line out with 220r shunt resistor to feed the micpre TRX,
and it was working great. i was able to keep the harmonics the way i wanted...
more gain loss, obviously...
i was thinking of using a simple voltage divider formula to calculate resistor value for 4 inputs to keep the same ratio...
(referring to PRR's help at  https://groupdiy.com/index.php?topic=65047.msg824228#msg824228)

but avoiding micpre trx would be a bonus too

 

[joaquins]

Yep, I understood mic level signals being summed, line levels signals is another story and you perfectly know your sources.

I guess you are on your way now, sorry for the confusion, let's avoid summing at mic level!

JS

 

[kambo]

Yep, I understood mic level signals being summed, line levels signals is another story and you perfectly know your sources.

I guess you are on your way now, sorry for the confusion, let's avoid summing at mic level!

JS

all good 
i should have described better at first place  8)

 

[ruffrecords]

yes, 8 in total :  4 for L  - 4 for R channel,
tubes are all 6SN7

In which case ny 10K bus feed resistor assumption is OK. I see from a later post you have been using 15K. That will also be fine.

You say you have 4L and 4R channels in which case the loss on each bus will be only  12dB. You could use another input module circuit for this but omitting the transformer. Change R1 to a 10K log audio pot and you have a master gain control. Make it a dual gang type and you can use it as a L/R master fader.

Cheers

Ian

 

[ruffrecords]

Thanks Ian,
So no db loss across a single resistor per channel, because there's only one and all voltage makes it through? Just want to make sure I have that concept.

No, the loss is 1 divided by the number of channels. For one channel, imagine all the other bus resistors have their inputs connected to ground. The channel we are considering now feeds a pot divider consisting of one bus resistor and 1-n bus resistors in parallel where n is the number of channels. A little math shows the attenuation is n times.

The assumption that all the other bus feed resistor inputs are connected to ground is not always true. But, each should be fed from a source impedance that is low compared with the bus feed resistor (otherwise the source will be overloaded) so for all practical purposes it is true. When you mute a source in a passive mix bus sytem you do it by connecting its bus feed resistor input to ground.

Cheers

ian

 

[kambo]

thanks Ian,
doing some crimping, will post a picture or so later in the day... coming along nice 

 

[kambo]

here is couple of pics

almost finished odd channels.... now to start wiring even channels, or vise verse...

 

[kambo]

PSU is still waiting to be touched ! and waiting for Cinemag trx for ins and outs, they are busy...
i actually should start another thread ....
for now, thanks to PRR, Ian, Andy, NYD, John, and many more ...

 

[kambo]

In which case ny 10K bus feed resistor assumption is OK. I see from a later post you have been using 15K. That will also be fine.

You say you have 4L and 4R channels in which case the loss on each bus will be only  12dB. You could use another input module circuit for this but omitting the transformer. Change R1 to a 10K log audio pot and you have a master gain control. Make it a dual gang type and you can use it as a L/R master fader.

Cheers

Ian

i am not sure if i am following you correctly!
change R1 on NYD pre to 10k log audio for master fader ?

i am totally lost on omitting the mic pre transformer... can i just omit the micpre trx with my current 15k bus resistors, and also omit the 220 ohm slug resistor...

what i am really asking is, how should i decide for the resistor value, according to tube stages...
is it depends on output Impedances of my line inputs?
ie: lets say my output Impedance  is 4K  on my line inputs, unbalanced bus is 10k/4 = 2k5  ! ?