Saturday, 14 March 2009
After listening to this amp and the Fostex combination for around 50 or so hours, here are some additional thoughts. The setup has changed, and the amp is directly fed by my Macbook Pro using standard analogue output, the source material is Apple Lossless through iTunes. Eventually, a proper DAC will be made to feed, along with a proper volume control.
The speakers are dead quiet. No hum. The only hum is the mechanical hum on the power transformer, which can be heard from around 1m distance. Not too annoying, but will try to add some damping to the chassis to reduce this somewhat. The tubes do run hot. Being my first all tube power amp, I did not realise that it was such a case. One can easily feel the heat rising around the tubes, especially on the top. It appears that the orange glow of the heaters is confined to the heaters (I think). I will make some more enquiries and also some testing of the circuit condition to make sure its all okay. The top panel (black painted MDF) gets quite hot to touch after a few hours of playing. At this stage, I don't think it is too bad, especially with Winter coming, it might be well to have all that extra heat in the room!
The driver tube I have now changed to a Phillips PCC88. I was initially running with the recommended Russian tube, but changed to this one because of my concerns regarding the heater voltage. The 6.3V spec'd seems to be running a bit high, and I may use some dropper resistors to bring that down. From memory, the tested voltage was around 6.9V or higher. This may also explain the heat from the tube.
Finally, listening to various music in my collection has really revealed this setup as really easy on the ears, even at loud volumes. I would not say this amp combination extends to the very highs or very lows (due to the speaker combination), but the sound is something that I find very soothing. This is especially true on female vocals (e.g., Missy Higgins, Dido, etc). This combo seems to have no problems with the kinds of music that I play, but it definitely is not great to very hard rock with heavy bass lines and crazy drumming.
All in all, this combo is very good, and in the future, I plan to upgrade the speakers by the addition of a super tweeter, building of a decent DAC and Phono stage, minor circuit upgrades to parts and eliminating the transformer hum, and finally upgrading the coupling capacitor from the current Premium DIY Hifisupply Obbligato caps to V-cap Teflon or Audionote Silver foils. But the top priority at this stage is to remove transformer hum, check and change (if required) heater voltages and add super tweeter.