Balanced Inputs for Truepath

[anomalous]

my truepath amp is nearly complete, but I have only just woken up to the fact that it doesn't have balanced inputs. I just assumed that since it was touted as high end, better than the UcD modules, that it would. but no.

the rest of my setup is balanced, so what do people suggest is the best way of converting the truepath to a balanced frontend?

previously, I would have used a balanced line receiver like http://sound.westhost.com/project87.htm designed by Rod Elliot, but I cant bear the thought of having to build yet another powersupply into the amp (making a total of four! +/-60, VN10, V5 and now +/-15 for the balanced receivers) and a friend suggested trying something like the Neutrik NTE1 audio transformers to provide the balance.

has anyone had experience using transformers to provide balanced inputs to truepaths? (or any other amps?)

I read some application notes from Rane and Jensen about this, and in the Jensen note i read this:

quote:
"In the example above [3.1], CMRR will be about 60 dB at 60 Hz, but decrease at 6 dB per octave, making it effective for hum but not for buzz. The same transformer capacitance will unbalance the balanced line at high frequencies, which won't affect the interface itself, but may affect the CMRR of other inputs "bridging" the line."

which was about using a 1:1 transformer in exactly this situation, and makes me wonder whether it will be effective. My friend, however, says he has had good results with them, and they are certainly an attractive (and not too expensive) option since they dont require any additional circuitry or power supply, and I can get them locally.

so any info or advice from people who have experience here would be very much welcome.

thanks,

Ben

[BddB]

Hi Ben,

Lundahl makes high quality audio transformers! (http://www.lundahl.se/)
Their line level transformers are widely used in pro audio gear.

Cheers
/Benn

[thomaseliot]

Me too have balanced sources and was thinking on how to get a balanced input for Truepath. In current setup I go active: a small board for balanced=>SE conversion, but I think that the opamp before the Truepath affects the sound.

Lundahl or Jantzen transformers are the way to go (Belcanto EVO series used these). The problem is that quality transformers (Neutrik NT1 looks like too ordinary) are more expensive than one Truepath board.

So another solution is to use TWO Truepath boards instead of one and feed each one with +/- balanced signals instead of stereo

The drawback, or the advantage, is that we get bridged amps, a lot of power and perfect channel separation.

[dweekie]

The Lundahl LL1690 looks very appealing. It looks like it can be used to serve as input cap replacements, balanced to unbalanced converter, and a phase inverter to minimize power supply pumping. Or am I overthinking its possibilities?

I played with a cheap $50 balanced to unbalanced transformer (pair). They really hurt the overall sound quality; I don't expect the $20 pair of NTE transformers to work all that great, but it is certeinly worth trying I suppose.

[Sean]

As mentioned, for the price of even a single decent transformer you can buy another truepath kit and have a true dual mono balanced setup. This is the way I would go.

[chris4916]

I don't know how much these Lundahl transformers cost but one should not compare with cost of Truepath board "only" because you will need power supplies (and for Truepath, that's not a matter of only one trafo and some caps...) and maybe some casing as well.

Does it require some additional AOP to get the inverting mode ? I think so. Thus one more power supply (+/-15V) except it you accept to derive from +/-45V... At the end (again, I don't know how much Lundahl LL1690 are) you can not compare with Truepath board only.

On the other hand, I do agree on Thomaseliot's comment: what you get with mono blocks is very diferent. More power, totally isolated channels etc...

Christian

[thomaseliot]

Lundahl LL1690 is at 94 euro in this site:
http://www.tripleaudio.nl/webshop/pd...st_lundahl.pdf
I don't know if it is the best, or if the one at 25 euro or 50 euro are enough. I'm shure, though, that after listening you'll desire the most expensive model

The additional Truepath would use the existing transformers and power supplies while taking just a bit more room in the enclosure.

In this respect I'd ask to Benn if there are EMI issues if mounting two Truepath boards too close, or one on top of the other, or vertically, face to face.

[Sean]

If there are any noise issues from two truepath next to each other it can probably be resolved by adjusting the values for Cfb on the second board.

The amp board in a Motorola DCP-501 has three TA3020 right next to each other on the same PCB with no problems. I should dig one of the boards I have out of the scrap pile and see if they used different Cfb values for each IC.

[thomaseliot]

Thanks Sean, this would be useful. Just a pity you don't fix and use those five channels.

[BddB]

quote:Originally posted by thomaseliot


In this respect I'd ask to Benn if there are EMI issues if mounting two Truepath boards too close, or one on top of the other, or vertically, face to face.

I'd shield the boards from each other, in the chassis. (Like boxes in the box)
Or perhaps make the design a true "dual mono" (and double the psu's).
/B