Bus summer, oscillator & output

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carlmart
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Bus summer, oscillator & output

Post by carlmart » Thu Jul 24, 2014 8:26 am

One thing I wonder is if bus summer and output functions can be done on the same stage, or if it's not advised to do so.

As it's only two channels we are talking about, not 3 or more, I think there wouldn't be any problems, and a bus summer shouldn't be absolutely necessary.

At the same time I would be adding a switch to activate or not the reference oscillator.

THAT's support did mention that leaving the oscillator generating 1KHz continuously might seep into the audio tracks through contiguous tracks or through the switch, and they suggested to power the oscillator on and off instead. That would require an extra switch, of course, which takes more space.

Well, in any case I would appreciate your opinions on these matters.
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emrr
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Re: Bus summer, oscillator & output

Post by emrr » Thu Jul 24, 2014 12:11 pm

I have rarely seen the case where a 1kHz tone DIDN'T bleed into everything if connected physically in any way and running.
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carlmart
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Re: Bus summer, oscillator & output

Post by carlmart » Thu Jul 24, 2014 12:38 pm

What happens if the switch is grounded when not connecting?

Like on the design?
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JR.
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Re: Bus summer, oscillator & output

Post by JR. » Thu Jul 24, 2014 5:27 pm

carlmart wrote:What happens if the switch is grounded when not connecting?

Like on the design?
That will keep the switch clean... :lol:

If you do it right it could be silent when switched off, but if you do one of several things wrong, it can leak into the primary path.

Do you feel lucky or smarter than THAT support staff?

JR
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carlmart
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Re: Bus summer, oscillator & output

Post by carlmart » Thu Jul 24, 2014 6:47 pm

Well, I'm not feeling lucky or anything, but I do remember Shure used a very similar method on their famous FP33 mixer, and nobody seemed to ever complaint about it.

I only know that because I had the chance to see the schematics. I'm not sure about the FP32, that preceded it, because I also had the schematics but I can't remember where I put them.

On the FP33 there was a switch in the signal path that selected that or the oscillator in channel one.

What about the other question I asked?

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JR.
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Re: Bus summer, oscillator & output

Post by JR. » Fri Jul 25, 2014 10:14 am

carlmart wrote:Well, I'm not feeling lucky or anything, but I do remember Shure used a very similar method on their famous FP33 mixer, and nobody seemed to ever complaint about it.

I only know that because I had the chance to see the schematics. I'm not sure about the FP32, that preceded it, because I also had the schematics but I can't remember where I put them.

On the FP33 there was a switch in the signal path that selected that or the oscillator in channel one.

What about the other question I asked?
Just to be clear It sounds like you are dismissing my advice that there are multiple vectors involved in signal crosstalk, besides the switch, but are asking for more advice. :?

OK, yes the sum amp can also be an output as long as it can drive low impedance. In fact if not combining many stems you may be better off starting with an output amp, instead of a sum amp.

JR
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carlmart
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Re: Bus summer, oscillator & output

Post by carlmart » Fri Jul 25, 2014 12:33 pm

It would be disrespectful to dismiss your advice on the problems with signal crosstalk. I am not doing so.

I just commented something about a very respected commercial product that was doing exactly what shouldn't be done.

I may not have all the experience you have in electronics, I certainly don't, and I became interested in electronics after I started working in location film sound recording about 40 years ago. At first soldering my own cables and knowing the tricks on how to do things properly. Crosstalk is one of the first things you have to be careful with, particularly when you are dealing with unbalanced circuits.

Later on I started to learn more because I didn't want studio people telling me something could not be done because of some technical reason, and things then inverted, as I started using new techniques that worked when everybody said they wouldn't.

Another thing is that you learn empirically there's a limit for things that is not necessarily as strict as theory says, or that there are other variables to be considered that make things work.

Maybe this is one of those cases. That's all, no dismissal of anything.

Great thing about the sum amp also working as output. In this case the opamp would be an NJM4556.

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mediatechnology
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Re: Bus summer, oscillator & output

Post by mediatechnology » Fri Jul 25, 2014 6:31 pm

I recall the SSL 4K and 6K also switching off the oscillator power.
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