Best way to do Aux Send for tube mixer

[lassoharp]

Been looking at ways to incorporate an aux send for an 8 channel line mixer. Not really practical for use of PC boards/chips here.  The schematic below is my best hack attempt to come up with something simple that does the job and I wanted to get some opinions/suggestions on it before proceeding to the test bench.

The 8 input sources will be 600r and are mixed in parallel with pots for send level on each channel.  The amp is pretty much straight out of the RCA tube manual.  I don't have much experience using cathode follower circuits so bear with me.  Expected mixing loss looks like 17 to 18db for 8 channels.  Load will be the inputs of digital delays/reverbs.

Am also in question as to what the best value of the mix resistors/pots should be - 5K or 500K?  The incoming line will already be terminated with 600r resistor.

 

[dustbro]

check the gmail account. there is a schematic for the UA610 that incorporates the echo send. Should be good to use as a guide

 

[emrr]

Off the cuff I think vintage console style would go with 5K or 10K R1/R2 values.  Check the tube Neve schematic, it has an aux along these lines. 

 

[lassoharp]

Good call.  5K pot on 10K mix works very well.  May just go with transformer coupled out and be done with it, or even standard output stage with high terminating resistor would probably drive the 100K input on my Lexicon just fine for aux purposes.

Looking now at maybe doing the whole mixer as passive only and using outboard amps for line, aux and program duties.

 

[lassoharp]

check the gmail account. there is a schematic for the UA610 that incorporates the echo send. Should be good to use as a guide

Thanks.  I'd seen that before and used the plate tap for several other projects.  It works fairly well but one of the drawbacks is that it changes the sound of the amp from loading effects.  With FB it's less noticeable but still seemed to be there on the circuits I tried.  The arrangement for the aux feed I was planning to use involved splitting the signal after the output transformer via a standard resistive dividing network.  There's a small 5 to 6 db loss but it doesn't affect the amp sound and the loss isn't critical for most purposes.

 

[ruffrecords]

Could you clarify a couple of points for me please?

You say the feeds are 600R - are these via a transformer of are they unbalanced? If they are from channel direct balanced outputs then wiring an unbalanced pot across each one loses the balance and adds a whole bunch of potential local ground loop problems. It miht be better to take the feed before the transformer unbalanced.

I notice you don't have a master Aux Send control. The easiest way with passive mixing is to fit it at the mix point and set its value to the bus resistance. Then it is just like adding one extra source for gain make up calcs.

You don't say whether this is a pre or post fade Aux Send?

Another point to watch is cable capacitance from each channel to the mix point as this can affect the HF response.

Cheers

Ian

 

[lassoharp]

Could you clarify a couple of points for me please?

You say the feeds are 600R - are these via a transformer of are they unbalanced? If they are from channel direct balanced outputs then wiring an unbalanced pot across each one loses the balance and adds a whole bunch of potential local ground loop problems. It miht be better to take the feed before the transformer unbalanced.

I notice you don't have a master Aux Send control. The easiest way with passive mixing is to fit it at the mix point and set its value to the bus resistance. Then it is just like adding one extra source for gain make up calcs.

You don't say whether this is a pre or post fade Aux Send?

Another point to watch is cable capacitance from each channel to the mix point as this can affect the HF response.

Cheers

Ian

Hi Ian,

The mixer is going to be this passive circuit from NewYorkDave:

http://www.groupdiy.com/index.php?topic=1298.0 

The idea being any external amp with 500/600r output can be used.  So the 600r sources will be symmetrical(floating) - but not balanced as would be with a CT on sec to ground, and would also be pre fader.

My addition of an aux send involved adding a simple resistive dividing network before the mixing circuit at the input of each channel.

As far as a master aux control goes I doubt I'd need one for my purposes - just a small amount of fixed gain. I've actually decided to omit the CF amp and just use an outboard line amp for this duty anyways so it will serve the same function as a master control.

99% of the time my aux is running into a digital reverb which always has a compressor in front of it for ultimate level control.

 

[ruffrecords]

OK, now you pointed out the NYD circuit is makes a lot more sense. One thing to watch out for though. NYDs circuit is cleverly subtle and maintains an input  load pretty near to 600R all the time - notice how the 1K pot with the 1.5K slug makes exactly 600R when the pot is fully off. I have not worked out the details but I expect it does not vary a lot from there to fully open.

So.... you either need to tweak all the values of NYDs circuit so that when your Aux Send is added the 600R load is maintained or...

make the values of your Aux pots 10 times bigger - say 10 or 22K pots so as not to add an additional load.

Cheers

Ian

 

[lassoharp]

The dividing network is set for "600r" impedance and requires termination on all branches (2 in this case) which is why the NYDave circuit worked out well.  As you pointed out the input trim pot is designed to keep a near constant impedance through its rotation. The aux line will have a permanent resistive termination to preserve the impedance relationship.

As far as I can tell, at the end of each aux feed - where it connects to each aux send pot - can be treated like a typical amp output to other input interface. In this case a 600r terminated output into 5K pot. 680r resistor should do the trick.