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ROLLS Promatch Review (XLR RCA Converter)

amirm

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This is a review and technical measurements of the ROLLS Promatch two-way XLR to/from RCA converter with gain control. It is on kind loan from a member and costs US $56 to $69 from sellers on Amazon.

The box is descriptive enough:

Rolls Promatch Review RCA to XLR converter.jpg


The positions you see on the controls is what I had to set them to in order to have 2 volts on RCA and 4 volts on XLR.

An external 15 volt, 500 milliamp switching power supply gives it the juice to operate:
Rolls Promatch Review RCA to XLR converter two-way.jpg


ROLLS Promatch Review (RCA to XLR)
We often have a need to feed a balanced amplifier using unbalanced RCA output from our sources. Let's see how transparent the Promatch is in this configuration:
Rolls Promatch Measurement RCA to XLR.png


Well, it is not broken but nothing to write home about either. SINAD was jumping up and down from 98 to 101 dB for some unknown reason (noise?). Distortion is quite low though at -120 dB. Channel match is poor though as you can also see in frequency response measurement:
Rolls Promatch Measurement RCA to XLR frequency response.png


I would have wanted ruler flat response to 20 kHz at least but we don't get that.

Signal to noise ratio is sufficient for 16 bit sources:
Rolls Promatch Measurement RCA to XLR SNR.png


Crosstalk was especially poor:
Rolls Promatch Measurement RCA to XLR crosstalk.png


Promatch XLR to RCA Conversion Measurements
Switching the device around and feeding it 4 volts to get 2 volts produced horrible results:
Rolls Promatch Measurement XLR to RCA.png


That's because the top of the waveform is clipped. Sweeping the input voltage we see that it can't handle more than 3.8 volts or so:
Rolls Promatch Measurement XLR to RCA distortion versus input level.png


Shame as our desktop hi-fi DACs output 4 volts and some even more. If you do use them, be sure to turn them down 0.5 to 1 dB either in software or hardware volume control.

Frequency response is just as bad as before except now it droops in low frequencies as well:

Rolls Promatch Measurement XLR to RCA Frequency Response.png


SNR is the same:
Rolls Promatch Measurement XLR to RCA SNR.png


Conclusions
As with other ROLLS products we have tested, these are "problem solver" boxes and not meant for great performance. For that use they are fine. For our use where we want transparency, I am afraid you are not going to get that.

Overall, I can't recommend the Promatch but depending on your application, it may be fine and quite a bargain.

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BrentW

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I have one of these boxes and I remember cracking it open and swapping the op amps and measuring better distortion/noise performance afterwards (the op amps were just sitting in those "push in" type connectors whatever they are called). I'll try to find the measurements and will post if I do find them
 

LTig

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Some thoughts coming to my mind:
  • Isn't the "poor" channel match a result of the volume pot settings?
  • Years ago people would have been very happy with 100 dB SINAD and 120 dB S/N in expensive equipment.
  • XLR to RCA is broken but who needs this anyway? Just use a suitable adapter or cable, it's cheaper as well.
 
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amirm

amirm

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Isn't the "poor" channel match a result of the volume pot settings?
Likely so but since there is no other way to set it, you are stuck with the mismatch.

Years ago people would have been very happy with 100 dB SINAD and 120 dB S/N in expensive equipment.
For equipment that does something significant, sure. But for a buffer, I like to see flat frequency response, much better crosstalk which are also lacking here.
 

gvl

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XLR to RCA is broken but who needs this anyway? Just use a suitable adapter or cable, it's cheaper as

Some may need to run a long cable where CMR can help, considering this box actually rejects common mode noise.
 

restorer-john

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How they can make that in the US and be able to sell it at that price is amazing.

I found this schematic for it. Note this is from 2008, the unit reviewed may be different inside, or not...

rolls.JPG


I just shook my head. No commentary required.
 

dezza

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So, does made in a particular country mean that the PCB must be made in that country.
What are the rules regarding this.
 

milosz

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In your review, one of the graphs is labeled "Signal to noise ratio is sufficient for 16 bit sources: " written on the graph itself is the phrase "18 dB of dynamic range"

Ummmm.... I bet you meant 18 BITS of dynamic range. 18 dB would be pretty broken.
 

YSC

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Seems like a pair of xlr to rca cables with a passive preamp is a better solution, unless the source really needs a preamp to make it to working level of the main amp
 

MediumRare

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If it doesn’t do what it says (frequency response change would be audible and crosstalk reduction would be audible) then it’s BROKEN.
 

Ron Texas

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I used an RCA to 1/4" unbalanced cable to drive LSR305 II's with an unbalanced DAC and it worked with enough volume.
 

m8o

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Passable distortion as long as your using a mediocre/middle'ing source but poor noise I guess, in the unbal>bal direction; tho that FR, ugh. Just as I thought, if someone wanted to get from a 4V output balanced DAC to say 5.4V so the the SMSL SP400 can reach max deflection, doing so cheaply with something like this is completely a waste of time. (Assuming it can get to 5.4v; I didnt waste the time to read its specs to find its ouput capabilities.)

Yuk in the bal>unbal direction. You'd figure that would be the slam-dunk easy thing to accomplish! Saving grace I guess if you could call it that is hitting max deflection (or very near it) 'can' be protected against, as long as the owner knows it is necessary (thanx to this review) and knows how to set it up properly with a generator and meter.

I always buy old used APHEX 124A units off eBay to accomplish the same need; that have probably spent a decade or more (more likely two or three) in a studio rack before I bought it. The defacto 'buffer' used in studio settings, it seems. They are practically a dime a dozen. I have 4 I used to use where/when needed. But none are in use now.

@amirm , do you have any interest in me sending you one or two? (Two sets of two are connected together in a 19" rack bracket kit, so I would leave the two together and send it as is. Could be of interest or use to have two in any case, in the event one displays degradation from age/use.) Given the state of the art of its age, and this was not designed to approach that, I do _not_ expect spectacular results. Just that seeing this here, piqued my curiosity to see what was "good enough to use in pretty much every studio the last few decades". Not hard for bal>unbal to be better than this Rolls tho; what a low bar to surpass.
 
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infinitesymphony

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Seems like a pair of xlr to rca cables with a passive preamp is a better solution, unless the source really needs a preamp to make it to working level of the main amp
Yeah, within the same price range you can get the JBL Nano Patch+ balanced passive attenuator and use XLR-RCA cables (pin 3 cold disconnected on the RCA side) if that's the direction you're going. For RCA-XLR this isn't so bad, but you could use a cable there too.
 

phoenixdogfan

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I guess my question is whether it would be better to use a XLX to RCA cable instead--particularly where you might be doing something like feeding a balance output from a DAC to a subwoofer rca input?
Would gain mismatch be a problem, particularly if the sub had its own gain control?
 

DonH56

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How they can make that in the US and be able to sell it at that price is amazing.

I found this schematic for it. Note this is from 2008, the unit reviewed may be different inside, or not...

View attachment 108964

I just shook my head. No commentary required.

Some "interesting" design choices... Wonder if you used a 15 V or 18 V supply if it would handle 4 Vrms?

What is the "USB function"?
 
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