These diminutive Dacs pack quite a punch for their price and size. One of their claims to fame is a battery power supply, which is slowly charged by a trickle charger. This Dac came in for a battery replacement.
This interesting amp was in the workshop for hum reduction. There is a lot of correspondence on the internet about the amp and ways to reduce hum, and in this case the hum was as much mechanical as electrical. We swapped out the input wiring for screened cable and fitted screening cans to the Edcor autoformers all with little effect on the hum levels. The next set is to try a DC blocker on the mains to see if that helps the mechanical noise from the transformer.
This amplifier is a clone of the original WAD 300b stereo amplifier. It was ninth workshop of repairs, as valve failure had taken out two cathode resistors. This version had been made with two 6D22 damper diodes for slow start and rectification and the owner was nervous about the top caps, which carry 550v, a potentially lethal voltage.
So we took the opportunity to convert the amp to solid state rectification and I installed a slow start module, to mimic the functioning of the valve rectifiers. We also lowered dissipation as the owner was keen to use Mesh Plate 300b valves, which don’t like to dissipate much more than 24 watts.
So how does the solid state rectification change the sound? I would describe the change as a faster sound with more low level detail apparent, so all in all a small increase in perceived sound quality.
The slow start module can be seen in the middle bottom of the above photo.
These Unison Research pre and power amps are currently in the workshop for servicing. The power amp is the hybrid valve /solid state model. Repairs included the input selector and a choke for correct earthing and hum.
I’m constantly amazed by the results of upgrading Yaqin amps of which I’ve now completed quite a few. I remember my nervousness upgrading the first one as the received wisdom was ‘Chinese transformers are no good’. Yaqin’s transformers are made in house and they claim, are made of Japanese steel.Whatever the ingredients the result is an amplifier that sounds very decent as standard but which can easily be improved with some simple upgrades.
Circuit wise the amp is a fairly standard ultra linear output stage with input and phase splitter. The upgrades this time were to add improved Mundorf coupling caps, to change the grid stopper resistors on the output valves and to add Elna Silmic bypass capacitors to the output valves.
Once installed the amplifier performs at a new level. The light graininess of the sound disappears, vocals become more breathy, and bass and dynamics improve. All these improvements were heard with the stock Chinese valves and of course these too can be swapped for improved Western designs or NOS types.
The Onix Melody is an Australian design made in China and runs 6L6 valves or equivalents. It had a pretty decent sound for the money when released in early 2000 and one of its main claims to fame was the use of a stepped attenuator.
Another feature is its use of Russian military paper in oil capacitors (in the green cans in the picture above). These are not bad in terms of sound quality, but can be bettered with a decent audio grade capacitor. So, after discussion with the owner, we agreed we would upgrade the cathode bypass capacitors with Elna Silmics and the coupling capacitors with Musicaps. I also installed an earth lift system to stop the amplifier interacting with other equipment.
Overall the result was a very decent uplift in performance.
This preamp dates from the early 90s when it had a reputation for great sound quality. It stands the passage of time well and still sounds very decent. It had had various mods and servicing over the years, including main electrolytic capacitors, valve sockets etc.Someone had also replaced the ribbon connector with some Cat cabling.It is a hybrid valve /solid state design, with transistors used for the phono stage and valves for the line stage.
My work was to upgrade the coupling capacitors, and add a choke power supply by adding a choke and additional smoothing capacitor. Once this work was complete it transpired the original main smoothing capacitor was pretty spent, so that was replaced too.That took a while to find as it broke down after around 2 hours use, but was fine up until then.
The result was very worthwhile, performance has been lifted to a very pleasing level. The pre operates with a much quieter background, displays subtleties better and is more dynamic to boot.