I have had an ART SLA-1 (aka the Unika MT1000) amplifier for nearly two years now.. I got it after Col showed me it was heaps better at driving my Alesis Monitor One speakers than my old faithful Rotel 30 watt amp. This is not that the Rotel was a bad amplifier, it just didn’t have the headroom to drive the relatively low efficiency Alesis speakers, they really do need a 100W power amp to get the best from them.
Here I’ll document some minor tweaks and random info on the ART SLA-1 amplifier for future reference..
ART SLA-1 info scattered around the net
Perhaps the most exhaustiveÂ page is Peaks ART SLA-1 page. He obviously got hugely enthusiastic over the ART SLA-1 for a while and gathered a lot of information about it. Some of it very useful, some in the more “crystal gazer” magic power cable audiophile vein, but a useful resource.
As well as that googling will find lots of comments and postive reviews from Audiophile types showing this amp is considered a low cost “secret weapon” amp for audiophiles.. for a while even a tweaked upgraded version was being sold by an after market customiser.
Googling the Unika name mentioned on the PCB board of the ART SLA-1 it becomes apparent that the ART SLA-1 is also sold under the name Unika MT1000 in Asia, or at least Taiwan. Unika appear to make gear under license for Yorkville, who own ART – or perhaps Yorkville/ART rebadge Unika designs.. regardless it appears they are the same unit.
My semi-successful op amp upgrade idea came from these comments by John Lance, where he upgrades an ART SLA-2.
Disable volume controls:
The volume controls are fiddly to use as they are dual mono, so easy to end up with mismatched volume levels, so to shorten the signal path I opened up the amp and solder short snippets of solid copper to short the across the edge connectors that lead up to the front panel PCB where the volume control lives. This cuts the volume controls out of the circuit, as well as about 25 cm of think hookup wire from the signal path.
left: edge connector going to master volume controls
Below: copper jumpers shorting out the volume lines (covered in solder).
After all this I’d like to think it sounds better but it’s hard to say. I don’t have the time or resources to do a full ABX test. I then removed the knobs and covered the holes with black tape.
Upgrading the op-amp:
I orded the LME49740 chip from ebay after reading about its stunningly low THD specs and other people having success upgrading other ART amplifiers with it. I desoldered the AN6554 chip and soldered in a 14 pin socket and plugged in the LME49740. Initially it sounded great, but a day later I noticed a very faint whine/buzz sound in the left channel. It was actually so faint I didnt pick it up over fan noise from the computers and amps in the rooms initially, but sticking my ear against the speaker it was definately there, while the right channel was dead quiet.
Opening up the amp and moving my fingers around the op and nearby capacitors changed the sound of the buzz… looks like the circuit layout caused some oscillations in such highly specced chip. Replacing the LME49740 chip with a TL074, a pin compatible op amp chip that is higher specced than the AN6554 but not as highly specced as the LME49740 eliminated the noise completely. So I’ll call the first stage of the tweak a partial success. Both chips “seem” to sound better with a few hours listening (remember the LME49740 sound was virtually inaudible in normal use, it was just annoying me that it was there at all in an otherwise very silent amp) – but again it could be perception bias from my excitement over the upgrade, I’ll give it a bit of a long term listen to see how it all pans out.
above: socketed op amp. moving fingers around the capacitors caused the left speaker interference to change, at one stage even picking up radio.
Conclusion: I think it sounds better. But I am aware of the biases of my own perception due to doing the tweaks so hard to say. But in theory all of the improvements will have cumulative benefits so will help get the best sound.