I was commissioned to build a high quality step up amplifier of moving coil cartridges and produced this little silver box. Using top quality Toshiba JFETs this step up amplifier offers silent non coloured amplification for moving coil cartridges that compares with the very expensive step up transformers. This version uses battery power (2 x 9v batteries) but it can also be supplied with a matching mains power supply.
It’s so good I now offer it as a standard product at £175 Moving Coil Step Up amplifier
Another Yaqin 13s on the workbench for the usual upgrades: upgraded coupling capacitors, upgraded grid stopper resistors and fitting of cathode bypass capacitors. These mods worked their usual charms and gave a very decent performance, even with the stock tubes. This time tho, I tried the amp with negative feedback removed. The difference was dramatic, and so a small switch was installed on a plate to replace the mains voltage switch which was re-mounted inside the chassis. In my set up, I never switched the feedback back on and both myself and my regular ‘golden-eared’ mates considered it way better. Here’s what the owner had to say:
‘Just collected my MC13S following David’s mods and I’m stunned! In addition to the mods described above David also fitted a feedback bypass switch and with the negative feedback switched off the resolution of this amp is incredible. I keep wanting to try another track to test the improvements but find myself having to hear the whole track before switching, which is not something that usually happens when I buy new kit. All in all I’m very pleased with these mods….’
The venerable old preamp came in to have the balance control reinstated which someone had bypassed. Still sounds very decent despite its age!
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.