Integrated valve amp with phono stage

[A.N. Beal]

I'd like to suggest a low cost valve integrated amplifier (like the KEL84) but with a built in (MM) phono stage. Why? I know that phono stages are hard to do with valves and there are advantages of having them separate from the power amplifier but:
(a) most valve enthusiasts are also vinyl enthusiasts;
(b) a low cost valve amplifier without a phono stage means either using a transistor (Goldring/Project etc.) phono amp, which rather negates the point of 'going valve', or else spending several hundred pounds on a separate valve phono preamp - which rather negates the point of the amplifier being 'low cost;
(c) there are several moderate cost intergrated valve amplifiers and kits on the market but nobody else seems to do an integrated with built in phono section - so there is a gap in the market.

Thoughts on how to do it? I assume it would be 15 watts or so (EL84s or the like). The 'on board' phono stage would take power from the main power supply, saving costs there as well as in casework compared with a separate phono stage. Either the relevant transformer taps, circuit board space etc. could be provided to accomodate the phono stage, which could be offered as an extra cost option, or else (probably cheaper overall) just go for it and design a fully integrated arrangement. (My old Rogers HG88 has an interesting circuit where the phono is the main input and radio, tape etc. are fed in through resistors to step down their voltages, rather than there being a separate stage to add gain for phono.)

If you fancy giving the circuit a novel twist, how about a new kind of rumble filter? Most rumble filters roll off the low frequency response. Is it possible to do it another way and produce a rumble filter which kills rumble and cone flap but maintains full low frequency output and improves bass power handling? It is well known that frequencies below 100Hz contain very little directional information - hence the success of subwoofers. It is also well known that most rumble and disturbance caused by record warps is vertically modulated. A filter which rolls the left and right channels together to mono (like a subwoofer) at low frequencies would filter out this undesirable muck, while letting the full (laterally modulated) music signal through. In theory the result should be (i) greatly reduced rumble and speaker flap but (ii) minimal loss of stereo effects, (iii) fully maintained bass response down to (say) 20Hz, and (iv) with less of the amplifier's precious watts going on spurious low frequency signals it should have more left to produce music. If it is feasible to do this without too many components it should not be expensive and it could be an interesting novel feature to catch the eye of potential buyers.

What do you think?

Alasdair Beal