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Musiland Monitor 01 USD - late to the party, but...

Posted by Thorsten (M) on December 17, 2009 at 10:35:40

Folks,

I have been aware of these Musiland units for a while (and had some samples around). While doing some work for diyhifisupply we also included one of them in a heavily modded version as USB2SPDIF, this project may or may not go into full commercial mode, if interested anyone can read about at the diyhifisupply section on thishere site.

Meanwhile, having these little Musilands lying about (together with loads of other eval and demo boards, experiments and so on) encourages their use.

One of the reasons is the really good and solid software support, easy to get to work and rock steady (more than I can say about some other stuff even from big names), work bitperfect, isolated SPDIF outputs etc.

They generally get used as impromptu signal generators (with some software) and the Monitor 01 US I occasionally attempt to use as portable headamp to go with my Laptop (not really seriously, my windows smartphone always sounded better!!!).

Okay, so much for background.

After finding major problems to have a very nice SPDIF receiver lock reliably onto the Musiland Monitor 01 USD SPDIF signals I decided to look seriously inside either of the "Monitors" to see what is going on.

Looking closely I was VERY disappointed to find many of what I would consider serious and very unnecessary design flaws.

Fred has already touched on the Monitor 01 USD output level. On the unit in my hands I measured a little over 200mV peak-peak of actual signal and extra 100mV spikes of overswing at the transitions. That is the picture on the left.

(please note the "grass" to the two sides of the cetral slope is how my digital 'scope reacts to SPDIF data, it cannot really trigger that well on this signal or I'm too daft to make it trigger well, it is not actually in the signal)

Measurement was using 1.5m Belden 75R Video Cable (the good plenum stuff, forgot the #) with either decent RCA's that measure not too far off 75R (cheap stuff made for video) or 75R BNC's with the far end terminated with a socket to which two 0805 SMD resistors of 150 ohm where soldered with minimum lead length as load, measured at the output of the Musiland with a calibrated 10:1 probe, sample rate 96KHz.

Well, it seams that with the cable length so short the transmission line effects are not very pronounced, but the output transformers leakage inductance together with the cable capacitance ring like mad.

In order to get the trace on right side I removed the voltage divider Musiland included, which is wrong on all levels. The output transformer is driven from 90 Ohm (not 75) and the level is correct if the load is NOT 75 Ohm terminated but less than halve of the IEC Standard minimum if it is.

The combination of these two can give several recent designs of SPDIF receivers major problems to lock, only "old faithful" CS8412/14 did fine, but the output was so jittery, you could see the jitter on the 'scope with a naked eye!

When I had tested the Musiland in my system at home it had done sonically much worse than the modified motherboard SPDIF output from dedicated PC Player, sounding gritty, forward and spatially challenged.

I first tried to simply drive the transformer directly from the buffer with just a cap and find a resistive divider on the secondary that would match the transformer impedance and give about correct levels, no luck, probably way too much parasitic inductance to squash using this methode.

So I replaced them with parts (sorry, you can't buy these anyway, so don't ask) that I know work under this kind of application and fitted the PCB (I did not want to waste more time) and applied a suitable divider after the transformer that presented the correct load to the transformer AND the correct 75 Ohm output impedance (well, close, I did not much about with the snubber to offset the rather low leakage inductance). This ended up with around 800mV peak-peak output, pretty much smack bang in the middle of the range specified in IEC standard.

This gave something close to the right trace and my SPDIF circuit finally got a solid lock. But the traces where very furry. The same "fur" appeared on the power supplies. The power supply filtering in the Musiland is vestigial at best and noisy USB Power rides straight across the input filtering and through the regulators to appear on the rails and eventually on the output. I measured nearly 150mV peak-peak on the various chips, all stuff above around 60KHz or so and really wideband. I need to get a faster scope (mine is only 150MHz) to measure just high it went. As a result I would expect different USB cables and USB ports and the like to make a disproportionately large difference in sound quality.

Anyway, Musiland has a place for a filter choke but elected not to fit it and the capacitors they use in the power supply do not well at high frequencies to filter out incoming crud.

Okay, I fitted a simple filter using an SMD choke, some ceramic SMD caps and the biggest Os-Con I had around. This finally cleaned the fur up pretty well, noise on supplies all across the board (meaning the circuit board) is now in the region of a few mV, showing that Musiland really did an excellent job with board layout and decoupling the chip supply lines.

Given all this technical excellence in the difficult part of the hardware and the software the oversight of such extremely basic stuff as I had to fix and the way in which it cripples a potentially excellent and affordable product is really disappointing, especially as fixing these issues at the design stage costs pennies (well, okay, except the Os-Con(s).

If you use a Musiland Monitor 01 USD and you have problems with your DC not locking or major jitter issues, try to find someone who knows what they are doing to fix these issues, sadly after such a modification job the Musiland may not be so good value anymore.

The Powersupply noise issues can be fixed using an external supply (easily attached actually) but the problems with SPDIF outs are much more serious and not easily fixed.

Ciao T

PS, a quick and dirty fix for the output problems of the Musiland is to remove the transformer and the (SMD) resistor divider and to then short the series resistor (tiny SMD soldering required, sorry) and to use a 220 Ohm Series resistor plus a 110 Ohm resistor to ground soldered in instead of the transformers (can use 1/8W types and you can actually see the resistors when soldering).

This gives around 0.55V peak-peak and 75 Ohm impedance, but no isolated output. Depending on the computer used this lack of isolation may be a deal breaker. But the traces look quite clean and in practice I found this working well, much better than the original output anyway, not that this is saying much.
Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

Edits: 12/17/09

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Kimber Kable

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