what do you guys think of this phono preamp ? i know nothing really - dont even know if its in this diagram?
It's not a phono preamp, it's a power amplifier, with possibly a phono stage built in. On closer inspection (pretty low rez image), that does appear to be the correct schematic, at least for the amplifier stage.
jeff
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It won't be a tube based phono stage if that's what you're looking for. This appears to be the amp in question. FWIW, the internet says it's a cheap copy of a Dynaco ST70.
jeff
It's a tube integrated amplifier (with a passive preamp), with apparently a solid state phono stage.
I made a few modification on my VR70e integrated amplifier, and one of the first has been the removal of the phono preamp. It is a small op-amp based circuit on a dedicated circuit board, powered by a dedicated auxiliary winding of the power transformer. It works but it is a basic circuit with consumer grade parts, the kind of board you find on Aliexpress in the 15-20 EUR price range.
Not related to the phono preamplifier, but I wonder if the reverse voltage spec of power-on LED DX isn't exceeded in the schematic of post #1.
It is exceeded, obviously, but on my amplifier it was still working for some reason when I got it. This is another part I've modified, and not the worst design choice of the stock amplifier. It was also sold with the Audio Institute brand name.
I bought mine from the Amazon resale section, and from the labels on the box it was sent back to Amazon multiple times before because it did not work out of the box. This way I don't know if the condition was original, but it is likely that no one had opened it before. The circuit board was meant for AC filament supply for the 6U8 driver/splitter tubes, but they converted it to DC by adding a bridge rectifier and a capacitor in a point to point fashion. The result was a higher than required filament voltage and a very bad internal wiring. They tried to dress-up the mess with some squirts of hot glue, but it failed and the wires were all over the place. I converted back to AC supply, routed/secured the wires properly, and installed NOS ECF82 tubes to get rid of the hum. The phase splitter anode and cathode resistors had different values, 33K and 36K, maybe to "voice" the amplifier with asymmetrical clipping at high volume. I replaced the 36K resistors with 33k resistors according to the original ST70 schematic. This modification has been discussed in a different thread on this forum. I also replaced a few capacitors with better ones (because I already had them in the drawer - not worth doing it if expense is required) and crucially I replaced the original source selector switch, volume potentiometer and input RCA connectors, the stock ones were of the worst possible quality and already failing. The removal of the NE5534 phono board was done because I need 3 line level inputs. I also replaced the cheap plastic volume and input selector knobs.