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
That's certainly one way to go. However, I have tried these types of boxes and I cannot tolerate the 12dB Voltage change that comes with most commercially available converter boxes.My box is 1:1 aside from a small insertion loss. This may not be a problem for Pro gear. It is a big problem for my Hi-Fi system. Price is about the same. My box is not as pretty, no doubt. |
Yes there is a volume reduction 1:12 with the Radial JDI Stereo (has Jensen transformer) but that has the benefit of impedence matching for an optimal signal.
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@funnychap That different Jensen transformer is a member of the same family of 1:1 transformers as I use, the JT-11 family. This box is very competitively priced. One off, each JT-11 is $110 from Parts Express. All the rest of the components are cheap. I'd say that my expenses were about the same as the price of the Radial Box.
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I use the Radial JDI Stereo di box for my hifi system and the 12dB drop is not a problem for me. preamp (back of the room) --> 3’ RCA cable --> Radial Stereo JDI di box --> 25’ XLR cable --> amplifier (in front of the room) source=Apple Music lossless |
