It's not a return to valves exclusively, but it does give me an opportunity to use both valve and solid state amps, choosing whatever I fancy at the time. The amp itself is not really a 'new' device as such. In fact it is 14 years old. It was an SE feedback less amp I built for a mate for his 50th birthday. It has now come back home after it packed in on one channel after 14 years of daily 16 hours use.
Like everyone else in this God-forsaken country, he became conscious of the electricity it was using and bought himself a class D something or other, to connect his TV to his Bose Acoustimass system and to stream AppleTV and Amazon through, so I'm left here with this anachronism, which given today's energy crisis, is becoming increasingly difficult to justify using on a regular basis. Sad really.
Anyway, here we are, and you're right Stephen: I was getting seriously bored - not with the system per se, which sounds the best it has ever sounded in over forty years of messing about. The main reason I was bored was because DIY amps get into your blood and It is difficult to wean yourself off them. They keep the brain working. In terms of the actual system, it's all a bit ironic really, because a pair of 30 year old Mission bookshelf speakers has restored the listening pleasure I had with the big Metronomes (Greg, I accept you were spot on, telling me I had veered off into an unproductive speaker backwater) You have to make the mistakes to make the progress I suppose.
Back to the amp which is the subject of this thread, I had spent weeks reading about negative feedback as applied to both solid and vacuum state situations. This was far more challenging than anything I've ever done in the past. But finding out a bit about it enabled me to improve the sound of my NVA based solid state amp, and quite substantially too, though I say so myself.
One of the main 'eureka' moments was the gain question. Like many dilly-dalliers in DIY electronics I had assumed that negative feedback reduces the gain, of the system except it doesn't, well not in the sense we think it would
Another one was the idea that the feedback signal from the output stage goes back to the input then keeps circulating round and round the amp, slugging everything in its path...er no. Something else that very quickly became apparent, was that if one is going to attempt to design a valve amp that uses global negative feedback, then one is going to have to get to grips with the engineering maths required, otherwise success is not going to happen. Out of band oscillation, shrieky destruction of tweeters and various other bits of electronic mayhem are worryingly easy to produce, and decent sound without the dreaded 7th and higher is not as easy to achieve as the intrepid tyro would believe. As the more experienced among us certainly know, global feedback and valves is an area where tweak, listen and hope is absolutely
not the right thing to be doing.
Easy listening: high fidelity, low anxiety.
so I’m going to have to get a few in. They’re not expensive at the values envisaged. No ceramics goes without saying.
