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Nikon1975 16th December 2011 02:11 AM

Designing a versatile pre with balanced and unbalanced in/out
Dear All,

My electronic knowledge does not allow me to design cutting edge circuits, but I understand enough to combine and adapt other people designs to my needs.

So, the situation is the following:
1) I have some SE and some Balanced sources.
2) I have some SE and some Balanced amps.

I'd like to have the possibility to connect them all, switch between them without sacrifice in term of quality. I have various options:

a) install both SE and balanced output to all the sources, use two different pre amps, one for the balanced amps and one for the SE amps.

b) assemble a preamp that can combine both, but here I am not sure it is possible without double conversions, that I want to avoid.

Of course I can take shortcuts, and use only one phase of the balanced, but I feel like it is a waste.

I am sure I am not the first one to have this problem, and I see the high-end commercial amps (even HT) have these features, but this does not mean it is implemented in an optimum way.

Any hint ?



EUVL 16th December 2011 03:56 AM

Assuming the rest of your chain is fully differential, and you only have single ended at the source, then IMHO the easiest solution is to have a SE to BAL converter to do that job.

You can use a differential preamp to run single ended signals by grounding one input (-Vin), BUT you have to make sure your balanced pre has a diff pair at the input. If you, e.g., use something like the Grounded F5X, the -ve side will just amplify 0V and you do not get balanced output.

So how to do SE->BAL ?
Give me another 2 months. I am trying some recipes.



EUVL 16th December 2011 03:58 AM

PS for SE out, just use one phase of the BAL out.


ThorstenL 16th December 2011 04:45 AM



Originally Posted by Nikon1975 (Post 2822778)
Any hint ?

A TVC can handle both balanced and unbalanced sources and the output can be either balanced or SE (but not at the same time).

Adding input transformers (like Jensen JT-11P-1) to a standard SE Preamplifier gives it the ability to handle SE and Balanced sources, adding an output transformer (like Jensen JT-11-BMCF) allows the output to be connected SE or Balanced.

If you want to use Balanced and SE Outputs at the same time you need multiple output transformers, however these can be driven in parallel...

So for example a simple preamp could be:

Input Bal/SE ->
Bal Input Switch ->
Bal Input Transformer ->
SE 10K Attenuator (DACT or the like) ->
SE Gainstage (maybe AD815, OPA627 etc.)->
Optional SE High Current Buffer (for Op-Amp's with weedy outputs)->
Output Transformer(s) per needed output (with Load Isolator) ->
Outputs Bal/SE

If you need no gain you could just some kind of buffer and cut the gainstage.

Hot is connected to XLR Pin 2 and RCA Pin, cold to XLR Pin 3 and RCA Ring.

This would actually create a true balanced preamp, unlike these so-called balanced designs that prevail in High End Audio and are anything but.

Some acquaintances I have in Japan BTW use external Transformer Boxes to add balanced capabilities to SE Preamp's, this is an option to keep any SE Preamp that you have in place already...

Ciao T

Nikon1975 16th December 2011 05:27 AM

Yes and no...

With the ES9018 I could noticed that there is some distortion or artifact cancellation between the two phases, at least in pseudo differential mode.

At the moment I am using "The Wire" as a Bal to SE converter, that may be an overkill, but for sure has drive capabilities.

I saw you recommended DRV134 for se to bal. This I may do: at least I have a way to produce a bal signal with the generator of the Picoscope.

Additionally, I have few Tube SE amps, that I may use from time to time. At this stage I am just looking for an macro approach, then I am sure I will find, on the net or on diyaudio plenty of solution. Nice also to keep things modular and experiment. The chassis I build for my projects allow for that.


Nikon1975 16th December 2011 05:51 AM

Thorsten, we posted at the same time, I was referring to EUVL post.

I thought about transformers too. The external box, is what I am using now, but with OPAMP.

BTW, what do you mean by TCV ? (Wikipedia did not give me a clue :-) )


ThorstenL 16th December 2011 06:12 AM



Originally Posted by Nikon1975 (Post 2822929)
BTW, what do you mean by TCV ? (Wikipedia did not give me a clue :-) )

TVC = Transformer Volume Control.

In essence it is a special line transformer with taps on the secondary to adjust the volume.

It can even have "passive gain", but it is neccesary to make sure the source has low output impedance.

As it uses in effect a (heavily modified) 1:1 Line Transformer (10K:10K) you can connect the input with either balanced or unbalanced sources, just like a line transformer.

Equally, the output may be connected balanced (floating secondary) or unbalanced (but as remarked, not at the same time).

I hope this will not raise the ire of the Moderators for shameless commercial self-promotion, but here a link to a commercial Preamplifier that uses a TVC at the core. Note that I had a hand in designing the thing...

Music First Audio : : Reference Preamplifier

Ciao T

TheShaman 16th December 2011 10:14 AM

Also have a look at the THAT chips for BAL<->SE conversion.

I'm in the same position as you are.
I plan on keeping a balanced format internally (aka at the preamp and volume control functions) and will just have the required SE->BAL converters at a couple of the inputs.

Regarding SE output I will, too, use The Wire for BAL->SE.
It might be an overkill but it will be connected to headphone output as well (so double task - it's intended use AND BAL-SE conversion) ;) ).

ThorstenL 16th December 2011 10:47 AM



Originally Posted by TheShaman (Post 2823107)
I plan on keeping a balanced format internally (aka at the preamp and volume control functions)

This makes in most cases quite little sense (there are exceptions, granted), I would suggest that for example the unavoidable tolerances in teh volume control will any possible benefit as CMRR will be shot to heck.


Originally Posted by TheShaman (Post 2823107)
and will just have the required SE->BAL converters at a couple of the inputs.

If you have a preamp with truly balanced inputs with good CMRR there is no such converter needed whatsoever.

Just connect the ring of the RCA to pin 3 of the XLR and the tip to pin 2 of the XLR and optionally bridge XLR pin 1/3 with a switch or wire, I tend to use a switch and a 100R resistor in parallel with 100n between pin 3 and pin 1.

Of course, given that most supposed "balanced" pre-amplifiers are actually not balanced and have rubbish CMRR I suspect this may work badly, but then I would question the benefit of attempting to make a balanced circuit.


Originally Posted by TheShaman (Post 2823107)
Regarding SE output I will, too, use The Wire for BAL->SE.

I suspect that "The Wire" is a tongue in cheek name for something that is nothing much like a wire in actuality (otherwise it would be poorely suited to Bal - > SE conversion).

For the line-output I would suggest to consider a separate device from the headphone output, but it's your project.

In fact, if "the wire" is decent, you may be better off just using this as the whole preamp (in SE) and use transformers on the input and output, this will at least ensure you get some benefit from balanced connections, which would be unlikely with a supposedly balanced circuit...

Ciao T

TheShaman 16th December 2011 11:18 AM

Thanks for the tips, Thorsten.
I would like to think I have addressed most of the valid points you raise but I wouldn't want to hijack Nikon's thread.

Btw, The Wire is this.

I also mentioned THAT because they are newer and way better than DRV (at least on paper - and I think in some cases pin-compatible).

I'm mostly trying them out to compare them to my CineMags.

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