Transformers in passive summing box output

[marconine]

Hello, my name is Marco and this is my first post here.

I have realized a passive summing box with 10 unbalanced stereo buss (20 inputs).

For each input I used a 47k Ohm resistor.

Now I’d like to balance the outputs of the summing box with transformers.

Can you suggest to me the right transformer specs for this circuit?

I have a pair of Lundahl LL1539 and a pair of Gardner MU7544 but I don’t know if they are the right ones .

Many thanks in advance

 

[ruffrecords]

Your 47K resistors are probably a little on the high side. The bus impedance is effectively the bus resistor divided by the number of inputs which ins your case is 10 so the bus impedance is about 4700 ohms. Both the transformers you mention appear to be 600:600 types so they are not designed to work at 4700 ohms. To fix this you would normally slug the bus with a 600 ohm resistor so it matched the transformer. However, this causes additional loss in the bus - in your case the bus loss becomes nearly 40dB. This needs extra gain make up and increases the noise.

If you used 10K bus resistors instead and slugged the bus with 600 ohms the bus loss would drop to less than 29dB which needs at least 10dB less gain make up and improves noise by 10dB.

I am not familiar with the Lundahl type you mentioned but the Gardners ones will certainly work.

Cheers

ian

 

[marconine]

Hello Ian.

Thank you for your help.

Put two 5k:600 tranformers instead of 600 ohm resistors can reduce the extra loss?

Eventually I can consider to use an unbalanced to balanced opamp circuit with high impedance input. It can be a good solution considering I’d like to have the output signal as transparent as possible to add colour with external preamplifiers.

What do you think?

 

[ruffrecords]

Yes, using a 5K:600 transformer with 47K resistors will reduce the overall loss to about 29dB so much the same as changing the resistors to 10K. Is there any reason you want to use 47K bus resistors?

If you are going to include an op amp you might as well configure it as a virtual earth mixer and gain the benefit of reduced interaction between controls.

Cheers

Ian

 

[marconine]

The only reason is that I have already done the circuit and I don't want to build another one

 

[Newmarket]

The only reason is that I have already done the circuit and I don't want to build another one

Depending on how it's built - could you leave the existing 47Ks in place and fix 12K7s in parallel with them to give a value near 10K ?

 

[marconine]

This can be a good solution

 

[Sam0311]

To fix this you would normally slug the bus with a 600 ohm resistor so it matched the transformer.

Hey Ian,

I’m trying to better my understanding of transformers, in particular about how input and output impedances work and if it’s possible to manipulate them so this is quite interesting to me. I can’t find an explanation for what slugging something with a resistor means, could you help me please?