Well I deleted the last post as the writing didn't match up with the photos. I was one out all the way to the bottom, so let's hope I've got it right this time.
Right!
I get bored without something to tinker with, so the other day I dug out my transistor power amp, with the boards generously donated by the late Richard Dunn. So I guess we had a strange hybrid of his design mixed in with my attempts to make a decent power supply.
This amp almost made me give up on valves, but for one bugbear. It blasted the room out on my 98dB speakers on nothing more than a slight turn of the volume pot on the NVA passive preamp I had it connected to. In practical terms, in my tiny living room with those big, efficient speakers and despite its good sound, it was unliveable with long term because basically it was a poor match with my speakers.
Anyway I thought I'd give it another go, and with the previous mismatch to my speakers at the forefront of my thinking, I turned it into an integrated amp with 3 inputs. I used an attenuating potential divider of 15K top and 10K bottom resistor to drop the 2V RMS digital input levels to 800mV before they reached the volume pot. I know this is not regarded as an ideal solution to excessive input levels, but beggars can't be choosers, and it has made the amp usable. 9 0'clock on the volume control gives decent sound levels and it will kick butt at 11 o'clock, with neighbour annoying levels occurring at 2 o'clock. That's a far better usable range.
Selector switch is on the back panel. The volume pot is close to the rear and is actuated by a coupling rod which connects to the knob on the front panel.
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It sounds clean neutral, liquid and plays the music with unflustered authority no matter how busy things get.
I have a broken Phono1 in the loft too. Nothing major, just one of the legs on the reg chip on the negative side of the regulator board was broken (I did it) and I couldn't be arsed to fix it, so bought our Ant's MoFi phono stage. The reason for me not being arsed, was that it picked up mobile phone signals, which was bloody annoying to be honest. I suspect a lack of RF screening caused by the acrylic enclosure. The MoFi suffered none of these issues and sounded fine with my MM carts so that was that.
Anyway looking for something else to tinker with, I ripped out the boards from the Phono1, fitted a new negative regulator chip and mounted the phono boards inside the integrated amp chassis, powering the regulator from the input stage power supply. It runs at plus and minus 22V, which is the recommendation for the op-amps it uses. No RF pickup now either.:
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Close view shows the phono boards mounted on a stiff plastic plate, which is in turn epoxied to an aluminium bar that runs front to rear inside the chassis
So now, the record player can simply be plugged into the back of the amp, eliminating a phono cable and four sockets from the equation
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The sound of this amp is giving me a troubling set of choices to make. If the 45 amp is to compete with it for resolution and clarity, on anything other than girls and acoustic guitars, then it is going to require far better output transformers than the ones I converted from push-pull to SE operation. But then again; back in June last year at Steve's place, Nick Paul and Steve himself
did try to tell me about that issue.
I didn't really understand what they meant, but it's now painfully evident what they were talking about.
Gonna have to save up my pennies by the looks of things.