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ROLLS Promatch Review (XLR RCA Converter)

infinitesymphony

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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?
Going from balanced to unbalanced passively by using a cable that leaves XLR pin 3 (cold) disconnected on the RCA side, as most do, is going to reduce the voltage by half, but that's not necessarily a problem. All that matters is what voltage your subwoofer input can handle and how much voltage your output device can deliver.
 

DonH56

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Obviously it would. It's an unregulated PS section.

Dave.

I saw that, did not look up the opamps they are using to see if they tolerate higher rails. I've no dog in this hunt.
 

Slobmw

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howard416

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Whatever it is, it doesn't have it.
lol'd

I wonder if "1U" means 1 uF, and polar capacitors... what is happening here? If you're going to be using polar electrolytics for audio input decoupling you better have them back-to-back and waaaaay bigger than 1 uF (even if the input impedance is high).
 
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lol'd

I wonder if "1U" means 1 uF, and polar capacitors... what is happening here? If you're going to be using polar electrolytic's for audio input decoupling you better have them back-to-back and waaaaay bigger than 1 uF (even if the input impedance is high).
1uF is an acceptable value for this application. These are biased to about half the applied DC voltage, so an electrolytic polar capacitor is perfectly appropriate. (No back-to-back configuration necessary.)
And, depending upon the brand/type, some polarized electrolytic's have excellent distortion characteristics and can make fine coupling capacitors in line-level circuits.

Dave.
 

Francis Vaughan

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I found this schematic for it. Note this is from 2008,
Well there is nothing actually wrong or broken in the design. It is just, well, a bit basic. One would be hard pressed to design a simpler circuit that did the job. It does what it says on the tin, meets specification, and is cheap. What more could you ask for? (Well, OK, for a bit more money you could improve a great deal, but you are changing the market, requirements and expectations.) I wouldn't buy one, not unless I was in desperate need and had to get something going that day. I would build something. But desperate need often wins. (A desperate drive to Jaycar before they close to buy a horrid adaptor is not unheard of.)
Robust, cheap enough to be disposable and have one kicking about for emergencies, that is probably the main use case. It isn't intended for domestic HiFi use.
Comments above about passive adaptors miss what this is intended for. When you need one of these, a passive adaptor won't work, or will perform so poorly as to be unusable. This device will drive a low or complex impedance load with no issues. You can't do that reliably with a passive-pre adaptor. This is a get you out of trouble device for professionals. (Albeit, cheap professionals.)
 

restorer-john

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Well there is nothing actually wrong or broken in the design. It is just, well, a bit basic. One would be hard pressed to design a simpler circuit that did the job. It does what it says on the tin, meets specification, and is cheap. What more could you ask for?

I disagree, and I can't be bothered picking apart each and every basic flaw, but suffice it to say, a dollar or two could have made a huge difference.
 

howard416

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1uF is an acceptable value for this application. These are biased to about half the applied DC voltage, so an electrolytic polar capacitor is perfectly appropriate. (No back-to-back configuration necessary.)
And, depending upon the brand/type, some polarized electrolytic's have excellent distortion characteristics and can make fine coupling capacitors in line-level circuits.

Dave.
1 uF with 10k input resistance gives you a LPF with corner frequency of 15.9 Hz. That will intrude on the audio band (albeit in a limited fashion) and the distortion of the polar electro will rear its head.

Tests have shown that using series back-to-back polars will significantly reduce the distortion (useful if effected in audio band), though not quite to the level of a bipolar, as will dropping the corner frequency to a low-enough point such that there is no developed voltage across the cap from music-signal to cause cap-related distortion. I suppose if you have a low enough Fc (no impact on audio band) then you can get away with a single polar as long as the reverse voltage stays below 1 V or so.

https://linearaudio.nl/sites/linearaudio.net/files/Bateman EW 12 2002 mar2003 1uF electrolytic or film.pdf
 
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Tests have shown that using series back-to-back polars will significantly reduce the distortion (useful if effected in audio band), though not quite to the level of a bipolar, as will dropping the corner frequency to a low-enough point such that there is no developed voltage across the cap from music to cause cap-related distortion. I suppose if you have a low enough Fc (no impact on audio band) then you can get away with a single polar as long as the reverse voltage stays below 1 V or so.

https://linearaudio.nl/sites/linearaudio.net/files/Bateman EW 12 2002 mar2003 1uF electrolytic or film.pdf
Yet, measured distortion was very low with this device.......until he drove it into clipping. :)

Dave.
 

howard416

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Yet, measured distortion was very low with this device.......until he drove it into clipping. :)

Dave.
It's an easy way to reduce the distortion, still. Then again, with the pennies being pinched it ultimately doesn't matter.
 

Francis Vaughan

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There are no end of improvements that can be made. Sure, an output buffer on the unbalanced output would be nice. But no, it isn't broken without one. It works to specification. As noted above, it will almost always be used with the level set to maximum anyway. There will be a few users that need to attenuate the output and may notice a loss of low frequencies in some circumstances.
This thing sells for $56 US on most sites I see. Assume a 2.5 multiplier over COGS, that means the manufacturer is making them for $22. In the USA. Everything is razor thin. You make money on these devices by saving the cost of a single resistor. Adding an entire dual opamp for what is a marginal gain in only some circumstances, and circumstances where nobody seems to be complaining in actual use, just isn't going to happen. You have to ask what other improvements could be made, improvements that are arguably just as good or even better to make. They are all in competition with one another, and overall none individually will make a huge difference over the basic performance. The next step up in performance is going to require a whole set of improvements to justify itself. Buffered output, better power supply, better line drive, better line receiver design. Each one is a useful but marginal gain. Together you get to a nicer product. But the BOM will rise maybe $10-15, assembly another few dollars. Price goes up, relative price in the market moves, sales may suffer, volumes drop, margins need to go up. You could easily be looking at a $100 US product. Better? Absolutely. But not the same product, and not the same market segment.
A new design, and have the entire thing made in China, you could probably have that better design for a similar retail price. But that isn't what this is.
 

peniku8

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