Designing a versatile pre with balanced and unbalanced in/out

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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 ?

Thanks,

Davide
 
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.

;)


Patrick
 
Hi,

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
 
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.

D.
 
Hi,

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
 
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) ;) ).
 
Hi,

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.

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.

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
 
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).

http://www.thatcorp.com/1600-series_Balanced_Line_Driver_ICs.shtml
http://www.thatcorp.com/1200-series_High_CMRR_Balanced_Line_Receiver_ICs.shtml

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

Btw, The Wire is this.

You may be better off with one of Walt Jung's compound Op-Amps.

I see little to even recommend the above project, the circuitry shows nothing past 1980's levels. Much better options exist for the balanced input with improved CMRR. I can also one or two possible stability nightmares lurking there.

In your application I think you can dispense with the input followers, as I expect your preamp's balanced outputs will have low output impedance, so they are redundant.

Better power supplies are available and will improve this "not much of a wire" circuit as well.

Ciao T
 
I think that the choice of a specific circuit or device at this stage is premature. Personally I use "The Wire" as I have it, and I bought it mostly to get wet with SMD work. So I am not married to that.

Anyway, there is a good point in the discussion: granted that the "Black Box" will have both balanced and unbalanced, inputs and outputs, all the "processing" like input switching, volume control etc., should be done in SE or with Balanced signals ?

For sure I see the complication of keeping everything balanced internally, but I also see the "beauty" of having the balanced signals reaching the balanced amp without a conversion back and forward to SE, just for volume control.

I wouldn't like to sound too philosophical, but I would be tempted to keep the BAL signals as intact as possible. Like mixing the liquids, increase entropy and some informative content get lost. (Here maybe I am mixing things. Sorry I try to fill my electronic knowledge gaps with what I know better, so I could be very wrong)

Actually modifying the sources to have both type of outputs, and build two pre-amps, one serving SE amps and the other one BAL amps, could ease the task a lot. Beside the fact that I already have a SE pre-amp.

Thorsten, no worries about the suggestions. I think people can recognize genuine advice from commercial and I think your follow in the first categories. It is obvious that you like what you designed, otherwise you would have done it differently :)
We all know you like transformers :). I don't have much experience with them, except few OPT for tube I use, but I like their apparent simplicity.

D.
 
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Look at the topology in Nelson Pass's Bride of Son of Zen. Should be able to find the write up on PassDIY. It does exactly as you want. Not bad sounding either. WHile my gear is all valve today, I use that preamp to drive my Aleph5, Zen4, and 100W AlephX mono blocks in my other systems. Have SE and Balanced inputs into it.
 
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Anyway, there is a good point in the discussion: granted that the "Black Box" will have both balanced and unbalanced, inputs and outputs, all the "processing" like input switching, volume control etc., should be done in SE or with Balanced signals ?
For BAL/SE -> BAL/SE versatility you want, make it balanced internally. Conversion is needed only for SE -> BAL, it could be done with LTP – differential amplifier – as input stage and shorting BAL– input to GND when source is SE.
 
I see where it says about the conversion, but I do not understand how it works. If I have an SE signal in, where do I get the antiphase?
Additionally, why not doing the supplies using linear regulators? Is there any mosfet that can be used, with lower voltage?

Thanks,
D.

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