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Unbalanced to Balanced Converter Circuit with 0dB Gain

I have created an unbalanced to balanced converter using a Jensen transformer - JT-11P-1. The circuit was designed by Jensen Transformers. It is the right hand circuit in the Jensen AS060 drawing, which is available online.

I assembled it into a Hammond enclosure. 

Performance is as I expected.  It reduced my already low 60, 120, 180 Hz noise peaks by an additional 10dB. These peaks now are at 90dBV, which is exemplary for my SP-6B (AR refurbed) tube preamp of early 80s vintage. My normal signal level (music) is about -10dBV, highly variable of course. Baseline RMS noise is unaffected as this noise is stochastic (random).

There is a 2.7 dB insertion loss (0.73x). The XLR output is fully balanced with pins 2 and 3 equal in magnitude but 180 deg apart.

This circuit is notable as it does not increase the signal. Commercially available converters of this type are usually -10dBV (unbal) to +4dBr (XLR), which is a "gain" of 4 (12 dB). I can't tolerate this high a signal as the SP-6B gain is already a bit high for my system at the optimum 12 o'clock position on the volume knob. One can obtain a converter box with gain. This uses an active transistor or opamp circiut for gain. The box I made is passive all the way. 

The frequency response is very flat from 20-20KHz, with only a 2dB drop between 19 and 20 KHz.

The transformers are a bit expensive - $120 each. Resistors, box and panel mounts are inexpensive. I believe I paid about $300 for everything.

Assembly is quite straightforward and not difficult if you can solder.

Finally, I should point out that my system did not suffer badly from noise problems before I added this circuit, even though I use a mix of RCA and XLR cables. I've been careful to connect all components into the same wall plug. I made sure all components are grounded properly. The box I made had a -10dB effect on the signal. I expect noisier systems will see more reduction

kevemaher

Showing 1 response by pindac

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pindac
3,273 posts
 

@kevemaher It is interesting to see a design put in place that is seemingly similar to something I have had put in place early in 2025. 

 I have had a circuit change made to my 845 Monoblocks that has now got them performing in a Balanced Design, which I believe shares similarities to your own design.

The conversion in my own estimation was not overly expensive and the End Sound produced was to my perception much improved, I could detect the increase in quietness, which certainly had a discernible impact on how very well known tracks were presenting the embedded data on both Vinyl and CD.

The End Sound also made such an impression that a well established design to be a Commission Build to produce a Mono Block Pre Amp' design. Which has been  selected to be used a long time ago and where most parts are purchased is now superseded as a design / build.

The impression left by the new experience of the Power Amp's has been the influence on a new design for a Mono Block Pre Amp.

A New Pre Amp' is now designed and is at the place where all components are now purchased and Bespoke Built Stepped Attenuators for the earlier build are to be reassigned to this build.

This design is also produced as a Commission Build, which is purposely designed to enable the Power Amp's to receive a signal (current - gain ) that is optimised to work with them. 

A Prototype of this Pre Amp as the SS section only, has proved itself in use to be able to produce an End Sound that is very very attractive. Especially when used in comparison to other owned / loaned Pre Amp's able to be used, which as Pre Amp's used prior to and after the Balanced conversion were always very well thought of. 

I too have been expressing an interest in having a Balanced design for the Analogue and Digital Sources. I'm keen to learn more on the variety of methods that can be used to produce this. 

For myself in relation to one Phonostage I am thinking of purchasing a second model? 

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