And just to finish this run of table models from Roberts, here's the RM50.
I admit that this one is better than I'd remembered. It was one of the first radios I bought, but as I was anxious to make a start on my new Hackers, this one was put aside for a rainy day. It should restore nicely, if I ever find the time
This uses a 6 by 4" speaker, which looks very similar to the other types seen in the RM30/33 and RCM1. The power amplifier is fairly meaty, using the AD161/162 pair to deliver some 5 watts. The bass and treble controls use the active Baxandall arrangement - like the R707 - but unlike the R707 there's no loudness tapping.
There are two plots - the first (black) shows the response with the tone controls in their centre position, and the blue plot is taken after they have been adjusted for the best sound (bass at 4 o'clock, treble at 3).
In common with the other table models, the treble is very limited. But the bass is better, peaking at the resonant frequency of ~100Hz before falling away rapidly. Careful with the bass control, as like the R707, it's easy to make this set go boomy! But unfortunately, the treble control brings up the midrange hump at 3kHz, which makes this set somewhat painful to listen to. Ideally, it needs a lift starting at 6 or 7kHz to bring up the high treble region. Looking at the schematic, I see that they've split the emitter resistor of the preamp into two (R35 and R36) for no apparent reason, and I'm wondering about adding a capacitor in parallel with R36 to get some upper HF lift. That'll have to wait for another day...
This set is a bit more spacious than the others, so a drive unit upgrade is quite feasible.