Thursday, 15 December 2016

(Almost) the end of the road

With the arrival in the mail today of the Emergency replacement KT88 output valve to replace the one with the internal short, we now have a milestone: a working amplifier

The amp has been sealed up and moved from the workbench to the living room, and connected to the big floorstanding KEF speakers, wired up and biased up, and some listening tests have been successfully conducted.

The moment I've been looking forward to for months, since I started this project.

What did the listening tests reveal??

First, this amp has plenty of power. The volume control hasn't been advanced beyond about 20% and at that level I was anticipating howls of protest from the Teenager upstairs when the parents' ancient music invaded through her headphones. Such howls were not forthcoming however, but I did keep the high-volume tests to a fairly brief period in the interests of domestic relations.

Second. There's a bit of hum, as completely expected with valves. If you put your ear up to the speaker you can hear it. If you stand about a metre away you can't. I am happy enough with that.

Third. The mains transformer makes a bit of physical hum... I had already noticed this when I loaded it up on the test bench. This is by far more more noticeable than the hum from the speakers.

Fourth. The sound quality is exactly what I was hoping for. In an hour of burn-in listening we were delighted with its imaging and separation and clarity.

Fifth. These KT88s get HOT! They're biased up to 28W idle disspiation and they will readily scorch unsuspecting fingers. The rest of the chassis gets warm, but nowhere near too hot to touch.


The build process is not quite complete however. The brass nameplate is the next item on the agenda, that will be in the centre under the row of indicator lights. Also I might make a few aesthetic changes yet, and the 3D-printed cover for the butt-ugly mains transformer still needs to be designed and made.

But for now, we have an extremely powerful yet easy-on-the-ear home designed and hand-assembled amp to listen to. Win!

A few photos of the usable (I won't say finished yet, until the aesthetics are completed) product:

The unmistakable warm glow of valves 

The mains transformer is the rudest part of this amplifier. It looks ugly and it hums.

Looking pretty :) The orange glow reflected on the turntable is in fact coming from the phono preamp, also valve, on the other side of the turntable

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