2 by 1 passive mixer question

[Coldsnow]

I know this topic has been covered but can't find exactly what I need.  I was planning on a unbalanced 10k pot to 22k resister on each channel.  Balance the ins and out with 1:1 transformers.  Any problems with the design?
Make up gain will just be a line level mixer input

 

[ruffrecords]

I know this topic has been covered but can't find exactly what I need.  I was planning on a unbalanced 10k pot to 22k resister on each channel.  Balance the ins and out with 1:1 transformers.  Any problems with the design?
Make up gain will just be a line level mixer input

Yes, the bus impedance is too high. It basically looks no lower than a pair of 22K resistors in parallel or just over 10K. That is too high a source impedance to feed into a 10K:10K output balancing transformer. Simplest solution is to slug it with 1K.That should be low enough to work well into a 10K:10K transformer.  Means the bus loss is now just over 26dB rather than 6dB so you need a mixer with that much gain on its line input.

Cheers

ian

 

[Coldsnow]

Hi,
For the 1:1 xformer on the output, I was going to use a 600:600 ohm.  Does that change it?

 

[ruffrecords]

Hi,
For the 1:1 xformer on the output, I was going to use a 600:600 ohm.  Does that change it?

Nope. If anything that makes it worse.

Cheers

Ian

 

[Coldsnow]

Thanks for your help.  Would it be best to just leave it unbalanced on the output?

 

[RuudNL]

I would go for an unbalanced output and a higher output level. Or use a buffer amplifier to drive the transformer. Driving a transformer from a high impedance will increase distortion.

 

[ruffrecords]

Thanks for your help.  Would it be best to just leave it unbalanced on the output?

The problem you have is that your output impedance is pretty much 10K. This is bad new for a 10K:10K transformer which actually is designed to be driven from a much lower impedance; like 1K. Going to an unbalanced output does solve this problem.

However, the input impedance of most line inputs is 10K to 20K (some are higher but most are in this region) and if you feed them from a source impedance of around 10K you will at least lose 6dB of level and possibly a little very high frequency response if you use long cable lengths. If your line input is not transformer coupled then you may well get away with it. It is worth a try at least. If the line input you use is transformer isolated then you have exactly the same old 10K:10K transformer problem you had on the output.

Hope that helps.

Cheers

Ian

 

[Coldsnow]

Makes sence thank you for the help.  Would you also use a 1k or even a 10 ohm (I have a few of these) pot instead of the 10k?

 

[ruffrecords]

Makes sence thank you for the help.  Would you also use a 1k or even a 10 ohm (I have a few of these) pot instead of the 10k?

No. The pot broadly sets the input impedance which presumably you want to look like close to 10K so it acts like a regular bridging line input.

Cheers

Ian

 

[Coldsnow]

What about taking the unbalanced output and send it to a transformer Direct box then to a moc pre?

 

[ruffrecords]

What about taking the unbalanced output and send it to a transformer Direct box then to a moc pre?

Yes, that would work!!

Cheers

Ian

 

[Coldsnow]

Still go with the 1K resisters? 

 

[ruffrecords]

Still go with the 1K resisters?

No. Use 10K pots and 22K mix resistors.

Cheers

Ian