Transformer suggestions for summing line in. Good, to exotic, to ridiculous

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Active member
Oct 26, 2013
Hello, first off thanks for taking a look at the thread. I am looking to build a 16 channel tube summing mixer, functionally not too unlike Holger’s recent rackmount unit (but in a larger housing). I would really appreciate some advice and discussion about input transformers for this type of circuit. At the bottom of my post is my proposed setup, and some talk of transformers I already have.

I’ve tried to do a bit of reading on the difference between a 10k-10k, and a 600:600 transformer for the purpose. I’ve learned that, it’s complicated. Suffice to say, in a modern commercially available product, a 10k:10k transformer would be the bet. I’m leaning towards something like.

Jensen JT-11-FLCF
JT-112P-2HPC  (ultra low frequency)

1. is the ultra low frequency transformer the better Jensen offering for my purpose?
2. Do you have a favorite that is preferred to the Jensen? Can you explain that preference?

OK, here’s where my question gets interesting. What would be some high quality, vintage pieces that would rival the performance of modern offerings? Something like the UTC LS etc. I’m thinking the LS-30 would be the best? Now, I don’t think I could find, nor afford those on each channel, MAYBE a couple channels. So, here’s the question.

1. Can you folks suggest any other UTCLS series that would be good for this purpose? Would the ls31 work (200:200)
2. Any performance issues that would arise from using these old 600 ohm units?
3. How about a western electric 111C/119c? Anyone got a couple of those laying around they’d trade for a kidney?
4. Lastly any suggestions for the supersecret equivalent of these popular old models?

Output transformers - While I have you folks here, I might as well ask about the output transformer too. I already have one of the nice Jensen tube output transformers. That would save me half the cost of nice output transformers. That being said, would anyone care to comment on what the best UTC LS or other vintage make/model would be for that purpose? Let’s assume I’m using Ian’s twin amp circuit.

What I have - A while back I owned a really nice MCI console. I ran into a gentleman selling spares, and I grabbed several of the transformers from the motherboard. These are all 1 to 1 transformers, capable of putting out at least 28 DB, and they sound good. I’ve tested these with my set up, running out of the Apogee into the transformer, and then back in at line level. they performed perfectly, the frequency response was perfect and they can take everything the converter could give it without wincing. I wish I could pick up a few more of these, but alas they seem to be gone from the Internet once I find I need them. I have six of these, that leaves me 10 transformers I need to purchase.

My set up will be as follows, out of an apogee ensemble FireWire, into potential outboard gear (let’s say Manley vari mu), into the mixer, transformer.....summing... ls10x mic preamp for makeup gain, Jensen output transformers, to apogee in.

Thanks again for your time, have a great day.


Well-known member
Nov 10, 2006
Norfolk - UK
If you are feeding line level into a tube stage then you are right, 10K:10K is the way to go. There are plenty of current production transformers of this type to choose from at varying price levels, specifications and price/performance ratios not to mention esoteric vintage types. Which you chose depends a lot on what you think is important about this component but any of them will work with a tube stage.

Output transformers for tube circuits are a whole different ball game. In the semiconductor world, amplifiers with very low output impedance are the norm which means once again there is a large range of possible output transformers to choose from. However, tube  circuits do not have very low output impedances so the choice of transformers is more limited and the specifications tend to be different. Bottom line is a whole range of 1:1 transformers that will work fine with semiconductor circuits are unsuitable for use with tubes. Unfortunately, all transformer manufacturers both past and present, are rather shy when it comes to publishing the parameters you need to know in order to decide if a transformer is a suitable candidate for use with tubes, It is not too hard to measure key parameters in current production output transformers in order to determine if they are any good for tube circuits but this can be a rather expensive process if you are after the rarer vintage types. To help with this task I published a paper  a few years ago with a comparison of some of the more popular transformers. Quite a few other members of this group contributed to the results. I have attached a copy of it to this post.




  • TransformerInductancev0.3.pdf
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Well-known member
Oct 14, 2010
California, US
It would also depend on the summing topology.  A passive summer with make up  gain would usually use a step up  transformer.  The JT 115K is a good 1:10  for tube pres.

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