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Musical Fidelity XCans Review

charleski

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This is a review and measurement of the venerable Musical Fidelity XCans (V1) headphone amplifier. This was first released in 1998 for the surprisingly-affordable price of £129 (I've attached a flyer and pricelist from 1998 below). The '90s were marked by the renaissance of pricey valve electronics in hifiland and this product offered audiophiles a chance to experience the magic of chubes without having to fork out $60k in 1990 dollars for an Audio Note Ongaku. I suspect I bought the unit on test in 2000. Since I don't have any panthers my dog lent a squeaky toy for the picture.

Main.jpgBoard1.jpgBoard2.jpg


At the time the XCans was the subject of a bit of a modding fad, and you can still find a site selling capacitor kits here. As can be seen from the pictures, I tried my hand at this, switching the electrolytics for fancier 'audio' grade versions, bypassing them with polystyrene caps and changing the ceramic feedback compensation caps for polystyrene. Oddly enough, I couldn't hear the slightest difference after making this 'upgrade' ... When I exhumed the device from storage last year I noticed that one of the capacitors in the power supply was bulging, so replaced them all with 105C Panasonics. I added clip-on heatsinks to the BD139s (TR102) as there were signs of heat-stress on the PCB and drilled some holes in the casing to allow a small amount of airflow (as sold there was no ventilation at all).

XCans Schematic - NoWM.jpg

The amp's transformer is in a wall-wart which feeds it 12VAC via a barrel connector. The valves are run as cathode-followers with an active transistor load and an opamp gain stage. It seems to be a unique hybrid design and I can't find anything similar in the resources on valve amplifier design. It's certainly very different to a White cathode-follower, which is the usual starting point for active-load CFs. One flaw that I found while tinkering with it is that if the valves aren't conducting it places 30VDC across the output. This also happens when it's powered up, as the PSU capacitors charge up before the heaters have had time to raise the cathodes to the correct temperature. So I added a timer relay to the input to the main diode bridge which turns it on 40 seconds after the heaters have received power. Fixing this properly would require a thorough redesign, but this works as a stopgap.

The output impedance is 49ohms, largely as a result of R115, the 47ohm current-limiting resistor on the output. When it was introduced it was championed as a good match for 300ohm Sennheiser HD600s, but I've found it works quite well with my ultra-low impedance (18ohm) planars, though this may be because they don't present a reactive load. Obviously with low-impedance headphones the majority of the power produced is actually being dissipated in the output resistor.

Measurements

I recently resurrected my 15-yr-old EMU 1616m audio interface running off an antique laptop with the requisite PCMCIA slot. While old, this boasted class-leading specs back in the day (dynamic range 120dB, THD+N -110dB), and still seems to be working reasonably well, though it's clearly no AP.

XCans noload 1.998Vrms.png
This shows the output at 1.998Vrms (since that's what Amir uses for his dashboard) without any load other than the 10kohm input impedance of the 1616m. Obviously this isn't going to win any prizes. The unregulated high-voltage and heater supplies are letting through mains ripple which shows up at -93dB @50Hz. While all connections are single-ended, the mains signal pollution through my Heresy is over 10dB lower, so this is really just a problem with PSU filtering, but it's good enough that I have never heard any trace of hum. I have no idea what's causing the strange noise hump at 7kHz, but at -116dB it's not a great concern. There's a long tail of harmonics, with the highest at around -85dB.

Here is the result from RMAA when unloaded, with the Heresy for comparison. I include the Heresy results to establish a baseline of the 1616m's accuracy.
RMAA Unloaded.jpg

The frequency response shows a mild (~0.6dB) roll-off at both ends of the spectrum. There is also a 0.6dB imbalance between channels, possibly because I didn't get a matched set of valves when I replaced them.
XCans Frequency Response.png

I was most interested in seeing how it performed under load, particularly as I use low-impedance headphones. Here are the power graphs at 997Hz using REW's new Level Step function into 19ohms and 314 ohms (the odd numbers are a result of me only having 0.25W resistors, so I paralleled a bunch of them together to fabricate the test loads). Voltages were calibrated using a fairly cheap RMS multimeter on a -6dB 120Hz signal which manages an acceptable amount of precision, but I can't guarantee the accuracy. Ignore the crosstalk figures in the RMAA results, as I only had one channel connected for these tests.

Xcans power into 19ohms.pngXcans power into 314ohms.png
RMAA under load.jpg

This is clearly happier with high impedances, being able to produce a healthy 170mW into 314ohms versus only 128mW into 19ohms. Distortion appears in three phases: first a ramp up from the noise floor followed by a plateau phase and then finally a sharp climb as the device saturates and goes into clipping. At the higher impedance the initial rise is steeper and the plateau phase lasts longer. I was intrigued to notice what appear to be periodic dips in the levels of the higher harmonics, so decided to investigate further at a range of frequencies in 0.5dB steps. These are animated gifs, which seem the clearest way to present these 3-dimensional data.

XCansDistortion19Ohms.gifXCansDistortion314Ohms.gif

Here we can see that the dips, or 'distortion flicker' appears to be quite a robust effect. These can span a range of over 30dB, especially at higher frequencies, going from -80 to -110dB. While changing the load impedance alters the absolute levels at which they occur, the relative ordering of the dips appears unchanged. In order to be sure this wasn't some bizarre processing effect, I took screenshots of the RTA spectrum as the output level was moved across one of these dips and you can clearly see the 4th and 7th harmonics flickering down and then back up in line with the plot on the graph.

XCans-19ohms-2560Hz.gif

It's still possible that this was just some odd effect of the DAC and ADC in the 1616m, so I hooked up my old MacbookPro and ran a sweep using a TempoTec Sonata for output and the built-in line input for the ADC. This is unsurprisingly a lot noisier, but you can see the flicker effect happening to the same harmonics at roughly the same output levels. The only common factors here were the use of REW and the short cable I used to attach the test load.

XCans-Flicker-Control.gif

With the output level held constant and distortion measured by stepping the frequency in 1/3rd octave increments we can see that the harmonics show a steady rise into higher frequencies with constant slope, apart from the 3rd harmonic, which has a negative slope in the sub-bass region.

XCans into 314ohms Distortion by Stepped Frequency.png

Here are plots of the IMD and multi-tone responses included for the sake of completeness.
XCans 19ohms IMD.png
XCans Multi 1.657Vrms 19ohms -9dB.png

Sound

"But, but," I can hear the 'trust your ears' crowd asking, "how does it sound?" To be honest, for many months I had difficulty noticing any real difference between the XCans and my Heresy, apart from the fact that turning up the volume to ear-splitting levels (far higher than used for usual listening) would result in clipping. I've never attempted a proper double-blind test, largely because I don't have a couple of assistants willing to put up with the hours of tedium it would involve, so all comparisons were sighted. I believe that subjective impressions are strongly coloured by alterations in mood and mental state, and it can be difficult if not impossible to separate these from a genuine effect of the equipment.

But after living with this for a year I get the impression that bass through the XCans feels more 'lush', the rolling bass lines from LCD Soundsystem feel a little fuller and more satisfying through the XCans than the Heresy. On the flipside, the Heresy gives a greater impression of clarity and precision that's most apparent on complex classical music. Even though the Heresy is, from an objective standpoint, several orders of magnitude superior it must be stressed that the subjective differences are quite subtle. It's tempting to try to draw parallels between these subjective impressions and the rise in harmonic distortion at higher frequencies, but that would be speculation.

The most interesting result here is the distortion flicker effect shown above. Although the result from my MacbookPro goes some way to providing a control, this would have to be replicated in an entirely different setup to be certain it is a genuine effect. If it is genuine it seems unlikely that this is a truly novel finding, but my reading in Electronic Engineering doesn't go much further than a few basic textbooks. So it may well have been described before, but just not have received much attention.

If distortion flicker is genuine, then it has implications for attempts to replicate 'tube sound' using DSP. There are several plugins available that attempt to replicate valve amplifiers, and most of them seem to simply add in a fixed pattern of harmonics that's invariant with level. This also seems true of Paul K.'s Distort program, though it does show accelerated roll-off of higher harmonics as the level declines. The results shown here suggest that modelling these amps may require a more dynamic approach.

There are several examples of internet tests showing that most people have difficulty discerning added distortion even at quite high levels. Our sensory systems in general are much better at detecting changes in stimuli, as shown in this celebrated example from astronomy. It's certainly tempting to speculate that similar flicker effects in the level of distortion components may be detectable at far lower amounts than when the distortion level is static and thus easier to correct by our auditory cortex.
 

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Kevinfc

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Ah the Xcans, forgot that I even had one once. I’m embarrassed to admit I didn’t know it had tubes as well.
 

Jimmy

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Thanks for the review and measurements, I have one (modded with bigger PSU capacitance) that I use occasionally.

Regarding DC on the output, I left headphones plugged in when powering the amp on/off during years and it has never damaged them, but I no longer do that, It's not a good idea with many amps.

Subjectively, compared to a good solid state amp, it sounds warmer and rolled off (veiled), it's a good match for bright headphones like the AKG K702, but it certainly is far from neutral and transparent.

However, take the above with a grain of salt, because I've found the sound to vary wildly when changing tubes, so your mileage may vary.

Anyway in 2021 I wouldn't recommend buying one of these unless for tinkering, or as a collector's item.
 
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charleski

charleski

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Thanks for the review and measurements, I have one (modded with bigger PSU capacitance) that I use occasionally.

Regarding DC on the output, I left headphones plugged in when powering the amp on/off during years and it has never damaged them, but I no longer do that, It's not a good idea with many amps.

Subjectively, compared to a good solid state amp, it sounds warmer and rolled off (veiled), it's a good match for bright headphones like the AKG K702, but it certainly is far from neutral and transparent.

However, take the above with a grain of salt, because I've found the sound to vary wildly when changing tubes, so your mileage may vary.

Anyway in 2021 I wouldn't recommend buying one of these unless for tinkering, or as a collector's item.
As far as valves go, one of the original JAN 6922s had gone bad when I took it out of storage (there were faint spots on the getter, so i suspect it had sprung a small leak). After reading a thread on headfi I tried a couple of 1970s Russian 6N23Ps were apparently the dog's generative organs. That'll teach me for believing anything I read on headfi, they were awful and went into clipping at moderate volumes. So I decided to be sensible and bought a pair of new JJ E88CCs, which are what's measured here.

I think it's instructive to compare this old design with more modern attempts at valve headphone amps like the Little Dot or Woo Audio WA7. The XCans was a cheap design and really nothing special at all, but it kicks the butts of both these models, with comparable power into 300ohms, but around 20dB lower distortion. (It's clearly not that good for lower impedances, though.)

This confirms an impression I've had for some time now, that modern tube amp designs are deliberately poor. It's perfectly possible to design a tube amp with performance acceptable enough that it would be very hard to distinguish from a solid-state design on an ABX. But that's not what the 'audiophile' public is looking for, they want an effects box and are prepared to pay silly money, so that's what they get.

As far as the DC offset goes, I don't think it's that much of an issue, especially if you use 300ohm 'phones, since they'll probably be able to take 100mA of DC for a short period. But with my low-impedance planars I thought it would be wise to put in a bit more protection so I didn't have to keep remembering to unplug them before turning it on.

If you're just looking for a headphone amp these days, then you're perfectly right, of course. Just buy a Heresy or L30 and stop worrying about all this nonsense. :)
 

Jimmy

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Musical Fidelity originally shipped cheap (by the time) Philips 6922 tubes, that were pretty decent, but the russian 6N23P variants are no slouch, the problem is that many 6N1P are sold as equivalents and I woudn't be surprised if some were relabeled as 6N23Ps. I have Sovtek/Reflector tubes and they are pretty good.

This amp is pretty special in the sense that it's an OTL topology (with a pretty strange/weird design), so the tubes are used as output devices with a series resistor for stability, not like it happens with many other so called tube amps, where the tubes are there just to add coloration (as buffers for example). The drawback is that in this amp the tubes used must have good transconductance (6N1P < 6N23P / 6922 / 6DJ8), and the combination of high impedance (can be lowered changing the value of the output resistors, but probably it's not a good idea) and relatively low current output compared to other designs make it more suited for high impedance headphones.

Probably the reason that this amp has relatively good measured performance is because it uses lots of global negative feedback (NFB), while other designs don't (because many people blindly believe that less NFB is better, which hasn't been scientifically proved, quite the opposite). Anyway, any modern well designed solid state design (discrete or opamp based) will perform better.

I don't know who could have been the original designer of this amp, but back in the day, Musical Fidelity outsourced design work to people like for example Tim De Paravicini, and they had pretty good products at relatively affordable prices, cost cutting was made, for example, using cheap components (capacitors, resistors, etc.), because MF founder believed that using boutique or expensive components didn't have an impact on the final result.
 
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charleski

charleski

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Musical Fidelity originally shipped cheap (by the time) Philips 6922 tubes, that were pretty decent, but the russian 6N23P variants are no slouch, the problem is that many 6N1P are sold as equivalents and I woudn't be surprised if some were relabeled as 6N23Ps. I have Sovtek/Reflector tubes and they are pretty good.
If you look at the spec sheets the only difference between the 6DJ8/6N23P/E88CC and 6922/ECC88 models is that the latter can handle a higher plate voltage and supposedly offer better longevity. But in this design I found the 6N23Ps weren’t a happy match and could easily be pushed into clipping whereas the new JJ ECC88s have never shown any sign of strain. Of course with all old tubes you can never really be certain of the provenance.
 

Jimmy

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Yes, tubes can be a nightmare, and buying used or NOS is a hit or miss, my Sovtek's were brand new back in the day (I bought a few for a relatively good price, nine years ago, I'm still in the first pair since I don't use the amp that much), and they sound pretty neutral/flat and undistorted (the original Philips were brighter), but I suppose that finding lemons when buying tubes on ebay is pretty easy, just luck of the draw. It has to be noted that I'm not using the wall wart power supply that came with the unit, but a X-PSU, and I've increased the capacitance when recapping, so maybe there's a relation.

Anyway, there are certainly better tubes out there, but not many are still in production (at least these kind of small signal tubes), and the good ones are pretty expensive. The JJ ECC88's that you mention seem pretty affordable and can be bought brand new, I may follow your advice and test them in the future, but I'm no longer a tube guy.
 
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