Transformers for synth output

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rbuskov

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Apr 5, 2019
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I have a bunch of synths with -10 dBu unbalanced outs hooked up to a patch bay. I would like to balance these connections and bring them up to ~ +4 dBu at/near the source.

If I understand correctly, I can do this with a passive circuit  by simply running the signal through a transformer that “steps it up”.

Can anyone recommend suitable transformers for this purpose? I would love to experiment with different transformers to see how they affect the sound, so the more better!

Cheers,
Rasmus
 
Can't do it. Going from -10 to +4 is +14dB which is a voltage gain of 5:1 and an impedance ratio of 25:1. So if the output of the synth can handle a load of say 1K (which is not guaranteed), after the transfomer, a 1K load will look like 1K / 25 = 40 ohms to the synth output. Meaning a 1K load will look like 40 ohms to the synth output which will just cause horrible distortion or worse it will burn out the output circuit.

In practice loads are more like 10K but even still that's 10K / 25 = 400 ohms which is still too low. Some loads are only 600 ohms which would look like 24 ohms.

The correct way to do it would be to make an amplifier that steps up the voltage and then use a 1:2 transfomer. That can be driven by just an op amp although one with a little more power like an RC4580 is better because with a 1:2 transformer, a 1K load will look like 1K / 4 = 250 ohms which might distort a little but probably not too bad. And a 10K will look like 2.5K which is a breeze for an op amp.

Or drop the transformer altogether and just use an op-amp to boost +14dBu. Transformers are great for a number of reasons but they do not affect the sound like many folks claim. Not unless you overdrive them in one way or another. And to do that you would have to put a lot of current into it and load the it so that it saturates.

Or find a "keyboard mixer" on Ebay that has +4 dBu outputs.
 
Agreed with squarewave. If you do connect it to a transformer it should be to balance the signal and drop the level (i.e. a passive DI), then get the gain with a preamp.

I run synths into a preamp with a DI - never directly to the interface.

 
Thanks, makes sense. I will use an op amp to boost the signal.

But I'm not sure what kind of transformer I could use to balance the signal though?
 
rbuskov said:
Thanks, makes sense. I will use an op amp to boost the signal.

But I'm not sure what kind of transformer I could use to balance the signal though?

Search the forum and you'll find a lot of info on this.

Basically a low noise opamp set for about 14 dB gain followed by a 1:1 transformer to create the balanced out.  Many different transformers would work. Jensen, Cinemag, Edcor, etc.
 
dmp said:
Basically a low noise opamp set for about 14 dB gain followed by a 1:1 transformer to create the balanced out.  Many different transformers would work.

Such as Carnhill VTB 2281? (see http://www.audiomaintenance.com/downloads/carnhill_design_guide.pdf page 5)
 
The Sowter would work fine but you will be paying more than you need to. You probably won't be able to hear any difference between that and a cheaper, smaller transformer.  And the sowter is physically larger than you need.

That transformer is meant to handle extremely high levels, up to 34 dBu, with little to no distortion. What signal voltage is 34 dBu? 34 Volts. Can you drive your interface with a 34v signal? No, the interface will clip at about 1.4v.
Also,  you won't be able to 'hear' a transformer like that -  you want the opposite (if you want transformer 'color'). That guy is meant to have no distortion.

The Carnhill is a very good transformer that would work fine. If you're in the EU it would make a lot of sense.


 
dmp said:
The Carnhill is a very good transformer that would work fine. If you're in the EU it would make a lot of sense.
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Great, I will try that one.
 
squarewave said:
Transformers are great for a number of reasons but they do not affect the sound like many folks claim. Not unless you overdrive them in one way or another.

So, if I want to be able to add some "transformer flavor" to the signal, would it make sense to add an attenuator after the transformer, so I can drive it harder with the amp? I'm thinking 2 stepped controls, one for amp gain and one for the attenuator?
 
Keep in mind there is a possibility of simplification here. It's almost certain most of those synth -10 dBu outputs have opamps underneath capable of driving something like 100ohm easily. This means you can balance those outputs with common 600:600 transformers or perhaps some rarer step-up configuration to accommodate at least a part of the missing gain.

You gain galvanic isolation and a balanced output with just the passive transformer, but perhaps the receiving end can provide the missing gain?
 
rbuskov said:
So, if I want to be able to add some "transformer flavor" to the signal, would it make sense to add an attenuator after the transformer, so I can drive it harder with the amp? I'm thinking 2 stepped controls, one for amp gain and one for the attenuator?
Consider the relatively small Hammond 107H 600:600 transformer. Here's a plot of distortion from the datasheet:

hammond107h.png


So 10dbm into a 600 ohm load is 10 dBu. This shows that even though distortion is starting to kick in, it's still only at low frequencies. You're not really going to hear it until you get into the maybe 1% at 100 Hz range. And this is a little 125mW transformer. You could pump 20dBu into that VTB 2281 and it would laugh it off.

This is why I say and will continue to say that transformers do not add color. They are great for isolation, blocking DC, noise rejection, impedance conversion and simplicity. Transformers are powerful problem solvers. But unless you're pumping tons of current into them like in the output of a guitar amp (or they're just super small), you're not going to "hear" it.

If you REALLY want to hear transformer "color", get a Hammond 107H or something equivalent and put +20dBu into it. But then yes, you would need to attenuate to get back down to +4dBu. Also Nickel cores saturate at higher levels but the onset of saturation is sharper so it might not sound as pleasant as an iron core.
 
Passive Di was the standard approach , you'll probably have enough signal to get some distortion in the core if you want , loading will be light on the synth output too . Synths do vary a lot , typically you  might expect around 100kohms load from a passive DI ,on ocassions I got better results with an active DI with  a couple of megs input Z.
If you go for the Carnhill , why not add BA283 in front of it , its probably a lowish load at the input but you probably wont need to drive the synth output much cause its the Neve style output stage that will be providing the distortion , you could then use an attenuator to dial back the output .

Id say try a  passive DI with a transformer core suited to a maximum level something below what the synth puts out  , it might add just the colour you need .

Klarke Teknik DI10/20P looks perfect  , quite afforadble too .
Actually maybe thats too good , 15db pad allows input levels up +30 db , so without it handles input levels up + 15 probably quite a bit more than your synth will be capable of , a lot of the cheaper ones only do upto around 0db without attenuation , ideally you want worse performance than that. Try the cheapest nastiest passive DI you can find
 
Tubetec said:
If you go for the Carnhill , why not add BA283 in front of it

Interesting idea. I was actually looking at the 3415 line amplifier, which uses both BA438 for input and BA440 for output :)

Any particular reason you suggest the BA283?

I like the idea of a Neve inspired box... maybe I could do an API after that :)
 
Well its a natural partner to the Carnhill transformer you mentioned , the gapped version at least ,
and its got a particularly nice overload .

I made a Neve Ba283 op stage only in its own small enclosure , I arranged a pot on the feedback to control gain , comes in handy in all kinds of line drive situations , its a nice back end to try out DOA or 'THATS' type balanced mic inputs, kinda like a Di box ,but not so hi-Z, with tons of transformer drive on tap if you want it .
 
Sometimes I use 2x12v  lead acid battery , sometimes Li-ion cells , sometimes a power supply ,OP stage alone requires around 80ma.
 
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