LCR Assignment with Slide Switch (+ Mixer Development)

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If the bus impedance is low enough then it should work. There will be some bleed to the River channel when panned London via the two 15k resistors in series but this will be equal to twice the bus loss. A typical passive bus loss using a mic pretty for gain make up will have about 40dB bus loss so the bleed will be around -46dB. If the bus is virtual earth it should be OK.

Cheers

Ian
 
80hinhiding said:
I guess I should query a bit more.. :)

Would 10K be considered low bus impedance?  Additional resistors after the DP3T network I've drawn.
No, 10k is high,but remember,  for a virtual earth mixer the bus impedance is virtually zero. For a passive mix bus is is the parallel combination of all the bus feed resisters.  So if you have 10 channels with 10k bus feed resistors, the bus impedance is 1k.
I am doing virtual earth yes.  Would it be better to use a non inverting buffer right after the pan/LCR assignment rather than just another resistor?

A
With virtual earth, a buffer after the pan is not necessary.

Cheers

Ian
 
That is another one of my designs. The Neve trick was commonly used in monitor controls when I was there in the 70s. It works fine  for that application. I tried extending the technique to pan pots. The pans work OK but the limitation of this circuit depends on the quality of the 1K pot and in particular its off resistance.  When building this circuit I found it very hard to turn the signal right off. There was always a small audible bleed with the pot fully off - ok for a monitor level but not for a mixer.

Cheers

Ian
 
80hinhiding said:
Hmm.. just curious, is there a way to fake/or create a lower minimum resistance for the pot?

I guess now I'm looking at a few options for balanced panning: a) continuing with the DP3T slide switch idea, b) looking into 4 gang linear pots with buffers c) hard wiring panning for now and switching out xlr cables if I want to change position in the stereo image.

I've been doing things balanced because it seems to yield a better sound, so far in my experiments.

Adam
I don't know of any way to artificially lower the off resistance  of the pot.

Cheers

Ian
 
ruffrecords said:
I don't know of any way to artificially lower the off resistance  of the pot.

Would something like this work, assuming the rest of the circuit is set up similar to what I drew, with a convenient low Z out-of-polarity source?

The concept being to "tickle" the bottom end of the pot with an out of polarity signal from a low Z source, so the virtual ground is now up into the carbon track of the pot.

Run a hot signal with the pot set to "zero", and then null out any remaining signal with the trim pot to silence. Might require readjusting as the pot wears.

The resistor values were pulled out of thin air, adjust them to suit.

 
If you are using a 10K pot then you don't really have the problem. The off resistance of most pots is a few ohms. With a 1K pot this makes an attenuator of around 50dB or so which can leave an audible residue.. With a 10K pot it is around 70dB.

Cheers

Ian
 
80hinhiding said:
So, if a 10K pot would be more attenuation, and less residue - I am curious about the design choice for a 1K pot in the schematic.

If I swap in a 10K pot in that schematic I should be good to go?  I have 10K audio taper pots here for level at the moment.  If this would work together with a dual gang for panning I might go that route.

Adam

The reason for the 1K pot is to ensure it is not loaded too much by the pan pot. Worst case the pan pot looks like about 1.4K when you work out the parallel combination of the two 10K pots and the 2K2 slug resistors and the bus resistors. That is an OK load for a 1K pot but not for a 10K pot.

Cheers

Ian
 
80hinhiding said:
Ahh okay.  I knew there would be a reason for that, and wanted to understand why.

What would be your recommended solution for balanced panning?  4 gang pot, followed by something? :)

Adam

I would recommend a twin gang level pot followed by a dual  buffer followed by a quad gang an pot followed by bus resistors feeding a balanced bus.

Cheers

Ian
 
80hinhiding said:
Thanks Ian.  I bet that would be an awesome setup.  Time to price it up.

I went back and did a unbalanced vs balanced sound test last night and even though I had configured it for very close to the same level of amplification the unbalanced test was thinner sounding and resonated differently.  Probably doing something wrong in the unbalancing.  I would likely need an input transformer, or to electronically unbalance it better with more components, but then there's added cost to that front section of the line input.

So, maybe I am better off maintaining the balanced connection and potentially putting money into the fancy balanced level and panning.

Adam

You need to be very careful with tests like this. There is absolutely no reason why and unbalanced  system should sound any different to a balanced one. If it does then it is more likely there is something wrong with it.

Cheers

Ian
 
If you are using simple transistor circuits without negative feedback there will be distortion. Unlike tubes, simple transistor circuits produce a lot of distortion. Different circuits will give different distortions. This is what you can hear. If your balanced circuit has a push pull output it will largely cancel even order harmonics (2nd, 4th etc) and leave mostly odd ones.

Cheers

Ian
 
80hinhiding said:
What's a predictable way to keep the entire frequency response while going unbalanced?
A half-decent understanding of the circuit and the use of mathematics.

Failing that, there is always circuit simulation, but it can get tricky when transformers are involved (since you don't know all the parameters).

Measuring the circuit is also way more objective than only going by ear.
 
80hinhiding said:
Hmm.. everything I try unbalanced loses bass response.

Just tried transformer coupled output with a transistor and there's plenty of signal but its low end is filtered a lot.

I've tried other things too but always ends up thinner.  Weird

A

Can you post the balanced and unbalanced circuits. Maybe there is something obvious that determines why they are different.

Cheers

Ian
 
Looking further at the schematics, none of those circuits is really suitable for driving a transformer. Their output impedance is far too high and they have insufficient drive capability due to low quiescent currents. I think what you are comparing is two poor designs.

Cheers

ian
 
80hinhiding said:
Fair enough.  Don't forget I also compared this to a textbook opamp configuration and variations on that based on tips.  Would love to know what a good design sounds and looks like.

Check out Neve and API schematics on line for starters.

Cheers

ian
 
80hinhiding said:
Looking.. for the first time.  I see some similarities, and some things I could try.  I know the obvious thing would be to copy the whole thing but... where's the fun in that?

I'm hoping to keep it pretty simple for now.

Adam

I have an article from the 70s written by a guy working for one of the pre moxer companies in which he describes a bunch of circuits they were using at that time. (It is where I got the idea for the 6dB passive EQ). Most of them are pretty simple using  just three transistors yet they are capable of excellent performance. I will scan the article and add it to my web site. I am sure there is a three transistor line amp in there that will do what you want.

Cheers

Ian
 
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