This Graaf GM20 came along for a health check and adjustment of bias and DC offset. It’s an interesting design that use the massive Russian 6C33 valves and is output-transformerless, thus avoiding any issues of transformer limitations, weight and cost. It sounds great but care needs to be taken with the 6C33 valves. Reputedly used in cruise missiles, Mig fighters and trawlers, these valves run very hot and can cause problems with heir valve sockets. Graaf use excellent quality sockets and overall build quality is excellent, as you would expect from a top Italian brand.
The 6C33 valve needs very careful burn in to perform reliably, and otherwise bias can drift alarmingly leading to short valve life. Therefore, it’s best to buy valves from a source that does this careful burn in process for you or do it yourself.
Once adjusted and checked over this amplifier was sounding excellent again.
This amp came in for repair after the client stated his girlfriend had been connecting up the speakers and had managed to short the speaker cables. There’s at least a couple of morals in that tale, the most important being ‘switch off ‘ before you connect or disconnect speakers-and inputs in fact as the resulting transient can easily damage speakers too.
Despite sophisticated protection circuitry the amplifier had blown its output transistors. Once replaced the amplifier is singing away sweetly again.
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.
Having reviewed the costs of restoration and options with the owner we are going to:
- Turn two into streamers using Google Chromecast Streamers and Class D amplification. Two of the controls will be used for on/off and volume and the tuning dial will be backlit by LEDs.
- The third, the Bush VHF, will be fully restored and a Chromecast model built in and connected to the Aux input.
All three will use the original speakers, in the case of the Bush that’s two 6ins bass /mid units and an electrostatic tweeter.
Just completed building the new Tent Dac board and mainboard for a DDDac commission. I’m excited to compare it to the previous version once completed!
There’s been a lot of highly positive feedback of my work publicly posted on the Web. Here’s a selection of recommendations and feedback:
‘The upgrade work carried out to the WD Power, Pre and Phono has lifted the system beyond what I had wished for. David gives a reliable, knowledgable cost effective service with good communication and is very accommodating…. What more can you ask for?
The Custom built DACs, hand built and professionally finished in high spec casings are also of a very high standard and sounded utterly wonderful. A great alternative to inferior price point products available in the commercial HIFi world. No Middleman, no BS, No advertising and marketing…It just makes sense!
General Recommendations 2
General Recommendations 3
Ear Headphone Amp Upgrade
Goodmans Magister Restoration
DDDac see post 8 of thread