Passive direct box

[mi-fu]

Just saw this thing on sale at B&H. For 50 bucks, is it useful for converting unbalanced RCA signals to balanced XLR signals? And is it good for eliminating hum noise?

https://www.bhphotovideo.com/c/product/1340342-REG/kopul_sdi_ppc_stereo_direct_box_passive.html

 

[Blumlein 88]

It probably would do what you want. Though knowing more about your precise needs would help. It is mainly for putting other inputs into mic inputs.
If you need to just go RCA to XLR, there is a good chance some shielded cables with the appropriate connector is all you need. The ground lift feature on the box in your link might be handy.

So exactly what pieces are you wishing to connect?

I've done a few long runs where I had RCA outputs feeding balanced inputs. Old AVR using XLR input active speakers for surround. Some cables for that can pick up hum or noise. When I used nicely shielded cables that wasn't a problem and everything was quiet. I've used the Monoprice Premium RCA to XLR adapters in both directions at different times. Quiet whenever I've used those.

 

[mi-fu]

Thanks @Blumlein 88 for your help

I would like to connect my DAC or pre-amp with RCA output to an amp / active speakers with XLR input.

This box probably should serve my need? Or it will make little difference from simply using a shielded RCA -> XLR cables?

 

[mi-fu]

One thing I don't understand from the product description is, "The high-performance transformers are custom-wound to meet the needs of converting high-impedance unbalanced signals to low-impedance balanced audio. "

What does that mean?

 

[Blumlein 88]

It will be a few hrs before I can look into it further. But I've used the adapter cable for the same use as you need. But I will see if I can find out more.

 

[DonH56]

I would try RCA to XLR cables first to see if you have a noise/ground loop problem.

The transformer provides a high-impedance input so it does not load the source and low-impedance balanced output to drive the load. Same sort of thing any preamp or DI box does.

Curious that the specs include 48 V phantom power requirement; I posted a question about that. I suspect it is a mistake.

 

[gvl]

While using transformers is likely the only correct approach for a full passive SE to balanced conversion, aren't transformers inherently non-linear to some extent and may have less than ideal frequency response, so basically you're trading better hum/noise rejection for higher THD? And if there is no hum/noise in the first place and especially if there are SE inputs present, why bother at all?

 

[sergeauckland]

If it's a passive DI box, then it can only provide a high impedance input by a significant step-down of voltage. If it needs phantom power, then it's an active box which can have any sensible input impedance and gain or attenuation. I use Behringer DI box D120 which costs around £20, when I need such a thing for live recording.

For the OP's application, a well screened coax cable (like RG6) from the DAC's RCA output to the 'speaker's XLR input is all that's needed. I've used active 'speakers that way for years. Just join XLR Pin 3 to pin 1 and to the cable's screen, and the cable's inner to XLR pin 2.

S.

 

[sergeauckland]

While using transformers is likely the only correct approach for a full passive SE to balanced conversion, aren't transformers inherently non-linear to some extent so basically you're trading better hum/noise rejection for higher THD? And if there is no hum/noise in the first place and there are SE inputs, why bother at all?

Good quality transformers have very low THD, so are perfectly usable in that application. The issue is more that good quality transformers are expensive, and you won't get two of them for $50, more like $100 each. Cheaper transformers will have issues at LF, and getting, say, 0.01% THD at 50 Hz or below and at 0dBu or above is unlikely unless the load impedance is kept high.

When I need galvanic isolation and unbalance-balance conversion, I use Sowter transformers, and there are other manufacturers like Lundahl in Sweden and Jensen and Triad in the US.

S.

 

[gvl]

For the OP's application, a well screened coax cable (like RG6) from the DAC's RCA output to the 'speaker's XLR input is all that's needed. I've used active 'speakers that way for years. Just join XLR Pin 3 to pin 1 and to the cable's screen, and the cable's inner to XLR pin 2.

Me thinks this gives half the voltage, which may be a problem if you don't have enough gain in the amp.

 

[DonH56]

Oops, I missed reading the spec closely enough. The impedance ratio from their specs is 67k ohms in to 600 ohms out, about 112:1. For transformer the impedance ratio is related to the turns ratio squared, so Z1 = a^2 * Z2 implies the turns ratio (a) is about 10. The voltage ratio is directly related to the turns ratio so about a 10:1 reduction in voltage. This is probably OK for the stated purpose, turning a line-level input into a mic-level input, but is probably not right for the OPs application. I would look for a DI box (active or passive) with closer to 1:1 input/output levels -- there are plenty of those around as well.

But would still just try the adapter or RCA to XLR cable first.

 

[sergeauckland]

Me thinks this gives half the voltage, which may be a problem if you don't have enough gain in the amp.

Yes possibly, but pretty much all preamps these days seem to have excessive gain, and all can manage a couple of volts output, so should load an active speaker adequately. I used an unbalanced passive preamp, so no gain at all, with my balanced Meridian M2s and the 2v output from a digital player was quite sufficient.

S

 

[mi-fu]

Thanks a lot for you guys help. Very helpful!

 

[rwortman]

Oops, I missed reading the spec closely enough. The impedance ratio from their specs is 67k ohms in to 600 ohms out, about 112:1. For transformer the impedance ratio is related to the turns ratio squared, so Z1 = a^2 * Z2 implies the turns ratio (a) is about 10. The voltage ratio is directly related to the turns ratio so about a 10:1 reduction in voltage. This is probably OK for the stated purpose, turning a line-level input into a mic-level input, but is probably not right for the OPs application. I would look for a DI box (active or passive) with closer to 1:1 input/output levels -- there are plenty of those around as well.

But would still just try the adapter or RCA to XLR cable first.

This is exactly correct. I do live sound on the weekends. This box is for reducing a line level signal to microphone level for connecting to the microphone input of a mixer. The signal will be much to small for a powered speaker. If you have ground loop hum and want a transformer line level isolator search for that. Such things exist. If you just need to make a connection from unbalanced to balances over a short distance a properly wired cable will work. P.S most pro powered speakers have a combo connector that will take either an XLR or 1/4 TS/TRS. RCA to 1/4TS cables are easier to make and cheaper to buy and will work the same.