I haven't really listened to music properly in a few years due to disillusionment with a lot of (expensive) equipment and sound quality. One of my loudspeakers gave up the ghost a while back and I've had some temporary replacements that are not great. Rather than spend the money one new ones, a couple of weeks ago, I decided to try some headphones first and see if it was my hearing or the equipment that was the issue! So after some searching I came across this thread and decided to build the amplifier. I also purchased some Sennheiser 650s secondhand.
I bought the components from Farnell, nothing fancy or expensive, a pair of JJ Tesla 12AU7s from Watford valves and wired everything point to point.
I must say I was awestruck at the sound quality! It was spectacular. Deathly quiet and totally engaging. I thought the gain was a little low at first but really I just had it too loud. It is perfectly adequate at around 75% volume.
So I decided to add de_equalizers CCS last week. I got the components as listed and soldered them up, again point to point, turned it on and.......nothing. Well not nothing but very low volume. So I measured up and found the voltage supplying the CCS had dropped to 30V. The addition of the CCS added 7mA across the power supply resistor. I couldn't figure it out as the CCS itself was designed to deliver 1mA only. I eventually realised it was the 10K collector load. I measured 3mA through this. I figured I could just up it so I put in a 220K I had left from the build. This seemed to sort it all out. The voltage came back up to 41.8V and the amp played.
The sound gains were as described. The gain increased, the bass is much tighter and more impressive and I think the upper mid range is much smoother but I need to listen to it a little more to be sure! In any case it is pretty awesome. I would recommend anybody to build it and to add the extra €5 components for the CCS. The BC327s seem to be fine!
That being said I would appreciate any comments on the CCS.
- Is the 220K resistor a bad thing?
- Would it be better to stay at 10K but connect the CCS to the unfiltered power rail instead?
- How did de_equalizer get the measurements with the 10K in place?
- I also put in the trim pots and trimmed the plate voltage to half the supply (20.9V and 41.8V respectively). However, this left me with a cathode voltage of 0.3V and not 0.9V as described in the post. I left it at that rationalising that the plate voltage was more important than the cathode voltage. Is this a bad thing or just a function of my valves?
I guess I will have to get the school books out and make the calculations but I'd prefer just to be listening to it!