Progress report - March 2002
I've finally finished the guts of the amplifiers and powered them up for the first time. The job took far longer than I ever expected, mostly because I only had a few hours each week to spend on it. The metalwork was much more involved than an amateur like me ever expected, and just rewinding the two mains transformers took me over three months. The input circuits and the monitoring and protection circuits were a pain to get right, too.
The crucial question - How does it sound?
When I connected up the first of my two amps to my system (this consisted of my passive preamp and ProAc Studio Ones, my new preamp being incomplete and my active speakers mothballed until I finished this project), I was a little disappointed. The amplifier sounded powerful and didn't give any impression that it was even close to running out of steam, but I didn't hear the clarity I was expecting, and there was a slight harshness when I turned the volume up, even after an hour or so when the heatsinks were nice and toasty. Overall there was a blandness and sameness to the sound, which made listening even at moderate volumes rather boring.
After a day or two, however, the amp was definitely starting to run in and sounded much more open and authoritative than the modified Maplin 150W monoblocs that it had replaced. I've never understood how electronics can "run in" - what is the physical process involved? I know I'd been looking forward to hear these amps ever since I started planning them more than two years ago, and I wasn't exactly relaxed when I plugged the speakers into them for the first time, but it can't be just a psychological effect. All the same, I've noticed it many times, and it definitely happened here. My wife certainly complained after the first half hour that the music was giving her a headache, but after a few days was keen to find out how to switch them on and off herself, and indeed told me "this is the best your system has ever sounded"! I certainly don't find the sound fatiguing any more, except when the recording is at fault. If anything, there is a certain "darkness" about the sound, and a lack of "sparkle", which reminds me a little of the Exposure integrated I owned many years ago. I wonder whether this is related to the lack of high-frequency nasties, or perhaps the result of that ridiculously low input impedance (4.7 kilohms) loading down the passive preamp and maybe even the source components. I look forward to hearing the amps with an active preamp.
One of the most impressive strengths of the Croft 5 valve amplifiers that I was using until a year or so ago was their transparency. My favourite test for this is to play one of the free cover CDs from Gramophone Magazine, which contain a wide range of recordings dating from (mostly) high-quality modern recordings back to the 1930s, and with the Crofts I could clearly hear the change in acoustic and instrument placement from track to track. I find that this is a subtle but crucial test of amplifiers, as many fail it, and in most cases it seems to be the fault of the amplifier at least as much as that of the source or loudspeakers. I've never been impressed by the performance of any of the solid-state amps I've owned in this respect, and even the Stochinos don't quite manage this trick as well as the Crofts, although they do the job better than most. Another aspect of transparency is the ability to differentiate different instruments. For instance, different makes of piano sound very different in real life, but this isn't obvious through most hi-fi systems. A second example is the timbre of a viola, which is very different from a violin when heard "live". This is something that valve amplifiers again are typically very good at, and I'm very pleased with the way the Stochinos do it, even through the relatively modest ProAcs.
One subjective aspect of these amplifiers which I'm very pleased with is the bass. This is definitely the most solid and tuneful, and least muddy, bass that I've ever heard in my system, even through the ProAc Studio Ones (which aren't exactly lean in the bass). For the first time I can hear clear and consistent differences between CD and LP in the bass. One of the reasons I bought my Pink Triangle PT TOO all those years ago was that it didn't sound "wrong" next to CD, unlike many other record decks which sound bloated in the upper bass (I won't name the Glasgow-built deck that was the worst offender...). However one may feel about the other attributes of the CD medium, it's impossible to deny that when implemented properly it has a superbly flat frequency response and a very stable and extended bass, and these are (for me at any rate) the hardest things to get right with vinyl. My system is now transparent enough to suggest that my vinyl front end needs tuning, and may indeed be limiting the low frequency performance, as it definitely shows some loss of definition and solidity next to CD. How much of this is due to the Pink and how much to the arm and cartridge (SME Series IV and Ortofon MC30 Supreme) - or even to the vintage of many of my LPs - is hard to know without direct substitution.
A few weeks after I wrote the last paragraph, I realise that things are not quite as they seemed, and that my record-playing setup is better than I thought. In the last week I have listened to Neil Young's "Harvest" and Zubin Mehta's 1969 recording of Schoenberg's Variations - both discs which might lead one to suppose that the vinyl front end is adding some substantial surplus weight to the upper bass. However, I also played Salif Keita's "Amen" and a digitally-remastered release of Frank Zappa's "You Are What You Is", and both sound almost like CD recordings - a very tight, solid bass and plenty of superficial detail, but with the sound apparently focused onto the line between the speakers, and with very little depth and reduced separation and presence of the performers. I am becoming more and more convinced by the method of "comparison by contrast", in which a superior system is defined by its ability to reveal differences between recordings. I think my own system is becoming progressively more discriminating between the LP recordings I have, and certainly shows the superiority of the black disc over CD in this respect. I just hope that I can stay on this path without too many of my favourite recordings becoming unlistenable!
An interesting observation is that vinyl surface noise seems to be significantly quieter with this amp than with the Maplin 150W. I wonder whether this is a consequence of the hugely increased slew rate (300 V/us compared with something like 10 V/us). It's just a supposition, but the speed of the amp may well be ensuring that short transients don't confuse the feedback loop.
At this stage the amps are still improving and I'm still working on them, finishing off the aesthetics, upgrading wires and connections and adding the housekeeping functions. I'm looking forward very much to hearing these amplifiers with my active speakers, which should provide a much wider window.
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