Unbalanced to Balanced Converter Build?

[bob p]

I'd like to build a box to convert a preamplified unbalanced audio signal to a balanced audio signal. I need to link-up a preamplifier that has unbalanced outputs with an amplifier that has balanced/unbalanced inputs. I'm trying to avoid the 6dB loss that goes with the unbalanced inputs.

Rather than re-inventing the wheel I thought I'd ask around to see if anyone can help to locate a suitable schematic. I've been looking, but I haven't had any luck so far. Some of the pages that looked promising on the Google search have failed to load.

So I thought I'd ask if anyone here has a good schematic, or a link to a good schematic. I'm looking for something that fits the following criteria:

1. It has to be Hgh Fidelity. Its going to be used with an AudioFool grade amplifier.
2. No op-amps.
3. All tube design.
4. Actually, the only requirement is all tubes in the signal path; SS components would be fine if they are confined to voltage regulation and other support applications.
5. I would also entertain any HiFi designs using JFET if all tube designs aren't available.

Anybody?

[Steve Conner]

Stick some dummy tubes on top and TI's DRV134 chip in there.

[Giaime]

Build a differential amplifier (think of a guitar amp LTP), then direct-couple to each anode a cathode follower stage. You can use SS CCS under the LTP and under each cathode follower. You get stereo with 4 double triodes: if it's too much, use MOSFETs to do the cathode (source) follower.

Uh Broskie just lend us a hand:
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[Enzo]

First off, is 6db REALLY a problem, or is this just some philosophical issue?

There are no shortages of direct boxes in this industry - they do exactly what you describe.

And there are these things that come in various shapes
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And if they are picky, make your own box with a couple high prices nice transformers from Jensen or whoever the darling of line matching transformers is these days.

[J M Fahey]

That amp's balanced front end, takes a balanced input signal and turns it into an unbalanced one, suitable to drive the amp itself.
Personal opinion, I'd just pull that bal>unbal stage, and wire the input connector so the naturally unbal preamp out reaches the naturally unbal amp input.
It's the most organic , ecologic and transparent solution you'll have.
NO transformer, tube, any component in the World will be as natural, flat, transparent, faithful, reliable, everything, than a very short piece of wire.
Think about the Phylosophical implications too.

[Steve Conner]

Well, if we're going to debate philosophical implications...

I agree, in fact this is how I made my own hi-fi amps. They're minimal integrated amps with just one pair of balanced inputs on XLRs, and a volume control. The tube one uses input transformers, and the solid-state one uses balanced line receiver chips with a gain of 1/2. The output from the receiver stage goes to the volume control, and then to the power amp.

For unbalanced inputs, I wire the signal and ground to the hot and cold input pins. I also add a 10 ohm resistor between cold and chassis ground inside the plug, in case the signal source isn't grounded.

My "preamp" is just a balanced line switch box from Hosa.

The unbalanced sources are 6dB quieter than the balanced ones, and a low output device might not drive the amp fully, but this has never been a problem in practice.

Some high-end audiophile amps really are balanced all the way through. Manley warn against applying large common-mode signals to the inputs of their tube hi-fi amps, because it can blow the HT fuse. :O

[dai h.]

no tubes, and apparently you need to select FETs. Design goals given as:

-uncomplicated, relatively simple circuit
-can be configured w/discrete components
-is for all practical purposes low distortion, wide bandwidth, low noise, and stable in operation
-input Z greater than 20kohms, output Z around 100ohms, max. output voltage better than 5V
-sounds good

Balanced Project

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(input also accepts a balanced signal)

[bob p]

That amp's balanced front end, takes a balanced input signal and turns it into an unbalanced one, suitable to drive the amp itself.

Actually, the amp has the opposite configuration of what you've described. The amp has balanced and unbalanced inputs. if the amp is given a balanced input signal then the signal path within the amp is fully balanced, like Steve mentioned.

Personal opinion, I'd just pull that bal>unbal stage, and wire the input connector so the naturally unbal preamp out reaches the naturally unbal amp input.
It's the most organic , ecologic and transparent solution you'll have.
NO transformer, tube, any component in the World will be as natural, flat, transparent, faithful, reliable, everything, than a very short piece of wire.

I have to say that I'm a bit confused by the appeal of the bare wire Maybe I'm just not the typical audiophool

I don't have the amp on hand right now, I'll have it next week and I'll know more then. But let's consider this -- if the 6dB loss creates a problem that cannot be solved by adding a little gain to the input signal, then the 6dB loss amounts to converting a 100W tube amp into a 25W tube amp. I doubt that anyone would be too keen on the idea of spending a lot of money on a 100W tube amp and then cutting their output's signal power by 6dB. A more direct route to that result would have been to save a lot of money and just buy the 25W amp.

I'm hoping that the simple short term solution will be to drive the amp's unbalanced input with a little more input voltage, and let the amp generate the balanced signal internally. I'm hoping that its not too presumptuous to assume that the amp's designers would have considered this solution, and designed the first amplification stage to have enough headroom to accept a little more voltage when dealing with unbalanced input signals. Time will tell.

Of course, the other option would be to convert my preamp's output from unbalanced to unbalanced & balanced. If I need to I could rescale the preamp's Vout.

[bob p]

Dai, you're making me feel illiterate.

[Enzo]

I leave the high end audio folks to themselves.

But the other side of that 6db coin is that no one in hifi gets a 100w amp to crank 100 watts out of. Quite likely they listen at 10-20 watts, so the question becomes whether ther is headroom within the amp as used for the 6db loss. And that money saving 25w amp still wouldn;t solve the connection issue. But I am not trying to sell the 6db idea.

But a nice full range matching transformer can boost the signal amplitude if it needs to.

The appeal of the bare wire is evidenced in such things as the stright through bypass switch for the CD player input on modern hifi amps. Wouldn;t want our sound runnning through any of that nasty EQ section .

[J M Fahey]

Hi bob p .
Now I feel intrigued by that power amplifier.
My personal experience with end-to-end balanced equipment has only been with Recording and Radio Studio Equipment, Tube Line Amplifiers and Compressors , both 50's and early 60's technology, sporting input and output transformers.
I've never seen a 100W end-to-end balanced amplified, although I know some like that were made to be used as distributed sound sources in Stadiums and Racetracks, where the balanced input was *really* needed, being up to a mile or even more from the audio source.
But on a Home Audio setup, it really baffles me.
Would you be so kind as to provide some schematic?
I'd be grateful.
(Even if it's in Japanese, my Son can help me with that)
EDIT: I forgot :
1) What preamp do *they* suggest to drive their power amp?
2) You do not *really* need to build any "circuit" at all, simply hook up a good quality 1:1 transformer there; primary hot to ground, secondary +hot to -hot. Keep everything in phase.

[bob p]

I leave the high end audio folks to themselves.

but we all like to poke fun at them every now and then.

But the other side of that 6db coin is that no one in hifi gets a 100w amp to crank 100 watts out of. Quite likely they listen at 10-20 watts, so the question becomes whether ther is headroom within the amp as used for the 6db loss. And that money saving 25w amp still wouldn;t solve the connection issue. But I am not trying to sell the 6db idea.

i kind of mis spoke. my allegory should have compared the 100W balanced input amp to the 25W unbalanced input amp. but i think we all know what i meant.

The appeal of the bare wire is evidenced in such things as the stright through bypass switch for the CD player input on modern hifi amps. Wouldn;t want our sound runnning through any of that nasty EQ section .

i really do understand the "straight wire" concept, but what I don't get is the fact that some audiophile types get hung up on straight wire / minimalist approaches when the straight wire isn't always better. i think this is one of those cases where the added complexity of a balanced circuit makes the complexity worth the while.

although i do appreciate the simplicity of a power amp that has nothing but an on/off switch, and i do like minimalist design, i'm not one to obsess on minimalist designs when they're not the best solution. i don't have a purpose built "listening room" that's designed to provide flat response and to minimize reflections. my main stereo is in my living room, and we really do live in there. there are a lot of things about that room that make it a disaster as a listening environment -- like a highly reflective 6' x 18' picture window along one wall, and open doorways and arched hallways on the opposite wall. We've got big fluffy couches, heavy drapes, and carpeting. my "listening" room is an EQ nightmare. don't tell any of my audiophile friends, or i'll lose my secret audiophile decoder ring: instead of building a purpose built listening room, i broke the rules, and i used a real time analyzer and a 1/3 octave EQ to try to tame the room's acoustics. i'm not too proud to use pro audio solutions if i have to.

[bob p]

JM - to answer your preamp question -- the preamp they recommend for you to use is one of their preamps!. since i'm shopping in the used market, my ability to make a purchase is subject to pricing and availability. i was able to find the amp but not the matching preamp, so i'm either going to have to wait to find one of their matching products with balanced outputs, or do some DIY to add balanced outputs to my existing gear.

on the subject of a balanced output conversion, i have looked at the Jensen line transformers, as well as modifying my preamp. there's a relevant white paper on the jensen site:

Interconnection of Balanced and Unbalanced Equipment

At this time I don't have the full schematic for the amp. Maybe next week.

[Enzo]

And frankly, I don't recall ever in my life just sitting there actively "listening" to the music, unless i was trying to pick out the lyrics. My listening was always an adjunct to relaxing. No sitting in perfect posture with my head in the "sweet spot." No listening room for me either.


To the audiophiles, my objection to the whole straight from CD to speaker thing is the false notion that that somehow would sound "just like real life." They never watched recordings being made, never watched the recording engineer adding a little this, correcting a little that, compressing some of this, and adding a little pitch tweak here, and panned delay there. Sorry, but it won;t sound like the real thing, it will sound like what the engineer wanted to hear.

[tedmich]

And frankly, I don't recall ever in my life just sitting there actively "listening" to the music, unless i was trying to pick out the lyrics. My listening was always an adjunct to relaxing. No sitting in perfect posture with my head in the "sweet spot." No listening room for me either.
.

I am having one of these installed to give me more perfect stereo imaging: