USB REGEN: Questions and Answers
The process of extracting the most common questions and answers about our products from hundreds of forum posts and e-mail exchanges has begun. Some of those will get posted on these Q & A pages. Please choose the product you are interested in reading about from the F.A.Q. drop-down menu at top.
QUESTION: I lost the little instruction booklet that came with my USB REGEN. Can you download it?
ANSWER: Sure. Here it is. USB REGEN User Guide.
QUESTION: The DC power input jack on the REGEN specifies 6~8 volts. I know that the 7.5V/2.93A/22W SMPS that comes with the REGEN was selected as best "sounding" for the price, but I have a some other small linear power supplies that I would like to try with it.
Can I use a 12 volt DC supply? Can I use a 5 volt supply?
SHORT ANSWER: Yes, but only if you are certain that your DAC's USB input does not draw ANY power from the 5V USB VBUS wires. You can test this (without the REGEN) by taping over USB 5VBUS pin 1 (this is easiest to do at the computer 'A' end of the cable with a narrow cut strip of business card stock inserted and folded over). Or use a cable that you know does not connect 5VBUS line. That way you can determine that your DAC's USB input will or won't function without ANY external 5V—not even for computer "handshake."
Once you are certain that no 5VBUS is needed, then yes, you can run the REGEN from a 5V or a 12V supply. The main function, USB hub chip signal regeneration and impedance match runs off of 3.3V from a separate ultra-low-noise regulator, thus 5V input allows for plenty of drop required for that regulator to do its thing.
While it is true that our own JS-2 LPS makes a slight improvement for the REGEN, it is not a large difference and the REGEN sounds GREAT with just the selected stock supply.
LONG ANSWER: The REGEN has two expensive, ultra-low noise adjustable voltage regulators (the wonderful TI TPS7A4700), one for 3.3V for the USB hub chip, and one for 5V for forwarding clean VBUS to the DAC for those that require it.
Linear regulators require that the input voltage be a little higher than the voltage you want them to regulate to. But the difference between whatever voltage level you feed then and the output voltage they are set for must be dissipated as heat. The REGEN is in a small enclosure and there is not a heat-sink on the regulators—therefore we must consider that. So although the regulator chips we use can take up to 20V, the drops down to 5V or 3.3V are quite large. For the 3.3V to run the hub chip and low jitter clock, it really is not a problem no matter the voltage since that part of the REGEN draws almost no current.
Let's look at some examples and the simple formula that Volts x Amps = Watts:
First keep in mind that the USB 2.0 specification allows for a USB device (be it a webcam, a DAC, or a headphone amp, etc.) to draw a maximum of 500mA (0.5 amps) from the USB bus of a single port of a computer or other host. Thus we plan and specify around that.
Additionally, without a heat sink on the REGEN's 5V regulator, we would not feel comfortable with asking it to dissipate much more that about 2.5~3 watts. It might be fine a little above that, but for reliability it is preferable to not go over that.
Let's say you have a DAC that uses USB VBUS power for its input circuit. (This is quite common as it may allow the DAC designer to avoid providing a separate internal PS for the USB input while allowing them to keep the "dirty" USB input circuitry more isolated from the rest of the DAC.) While it may not draw the allowed full 500mA, let's assume that it does. If you power the REGEN with 12V, then it must drop that by 7V to arrive at 5V. So 7 volts times 0.5 amps tell us the chip and circuit board will have to dissipate 3.5 watts. That is pushing it.
Feeding the REGEN 9 volts requires a drop of just 4V and that of course is just 2 watts to dissipate if the DAC draws full power. No problem.
If you know that your DAC does not need any bus power at all—or for sure only just for initial handshake—then you really don't have to worry. Our USB hub chip and clock together draw about 50mA at 3.3V, so it would be almost impossible to overheat that second regulator. Even 20V dropped to 3.3V will, at 50mA be only about 0.84 watts.
By the way, the only reason we chose such a high-wattage SMPS to include with the REGEN—22 watts is overkill—is that I found that size to be a sweet spot. The quality of the chosen model and and its output capacitance for current delivery, delivered better bass better than from small, cheaper wall-warts. And it is MUCH better, with clearer highs than any of the available inexpensive linear wall warts (all the factory tabletop linears are unregulated units, and those are terrible for this application). Go figure…