After a couple of days of using the controller, some thoughts ....
I've used it for two listening sessions of around 4 hours each, the first one with the output set at 220V and the second with the output set to 180V.
I honestly could not detect any difference in sound at 220V, from running on the normal AC supply. This seems to contradict what some others have reported, but this MAY have something to do with how clean (or not) the AC supply is to start with. I live in a rural area, no industrial or commercial premises nearby and I was listening late at night, so the load on the supply generally would be quite low, I would think. I suspect the supply in my house is quite "clean" to start with - I've certainly never had any problems whatsoever with mains-borne interference.
Running on 180V, however, does
seem to make a difference. As we would expect, there is less vibration from the motor. The motor in my L75 is pretty quiet anyway, but it's quieter still on 180V. I'm trying to think of a way of describing the effect on the sound, without resorting to clichés .... but I can't, so - "it's as if a veil has been lifted"
Is it more dynamic? .... No.
Tighter bass? .... No.
Sweeter highs? Yes
It just seems to give greater clarity, more precision
, we could say. Does it make the music more enjoyable? YES - and that's the important thing, after all! What I'm hearing is more music and fewer "artifacts", I think.
Is this purely down to the reduction in motor vibration?
Well, I don't know the answer to that and I don't much care
At the moment, I'm not hearing any downside
to running at reduced voltage, but a few LPs isn't enough to say for sure, so I need to listen some more....
On the technical front, there are a few issues to be sorted out -
1. After about an hour or so, the supply transformer starts to get VERY hot. This is obviously undesirable from the point of view of the transformer lasting a long time, but also because it is heating up the oscillator board, which I separated from the amplifier board in order to prevent it getting heated up! Doh !!
Looking back through my notes, I realise that I measured the total power drain of the controller to be about 40W at full output. Somehow, I had forgotten this at some point and ordered a supply transformer rated at ... 40VA.
Oops! So, it's running right on its limit, which is a BAD idea. I have ordered a 63VA transformer to replace it, which should run a good deal cooler.
2. I also found that the power supply oscillates a little, at very low frequency. The reason for this is pretty obvious, if we think about it. The ripple frequency of the supply will be 100Hz (from the 50Hz AC line frequency) and we are trying to amplify a sine wave of ... 50Hz or thereabouts. So, what I'm seeing is a "beat" frequency. I think the supply is "sagging" too much under load and I thought this might be related to the under-rated tansformer. I tried replacing it temporarily with a 100VA frame-type transformer, but that actually doesn't make any difference, so instead I'm going to try uprating the supply electrolytic caps a little, from 4700uF to 6800uF (which is what I started with, actually
). We'll see if that makes a difference.
It's unlikely that I can completely eliminate this effect, just reduce it and it's also questionnable whether it matters anyway, but I "don't like it".
3. I need to alter a couple of resistor values around the amplitude control. At the moment, the output is about 180V at minimum setting and is already up to 270V at halfway
Clearly I miscalculated something there! I had intended it to be no more than 250V at maximum
setting, so I need to fix that ..