My BC1’s are now fitted with replacement drivers, which I bought as spares from Spendor in 1996. The originals were Iron - core and power level was pretty paltry. The new drivers; rated at 55 watt, use ferrite cores which are much more substantial. So, after quite a few years unused, I was interested to see how well they performed, This I did by way of my 303, 405 and 909 Quads.
The 303 is still a very nice amplifier. However it is clear when comparing it with the 405 and the 909, how far Quad have advanced over intervening decades.
The 405 - uprated to a 405 - 2, being the next tested, illustrated just how sensitive the BC1’s are. The clarity of vocals remains exceptional, even at relatively low listening levels. Bass is solid, but does not extend down as far as the 909 - though the content is there, it is less pronounced. Treble is lovely - the HF 1300 taking care of everything from 3khz. to 13khz., where the Cole’s 4001G takes over. There is a “Tightness” to everything which comes as quite a surprise given the age of the 405 and the Spendors. They are still a combination I could comfortably live with, although the 405 is now destined to go to my daughter.
Finally, I moved on to the 909. I fitted 12db. attenuation to its input as a precaution.
The 909 was bought purely on speculation as I thought it possible to drive my Stax headphones with it, at such a level that it would remain in Class A - stay below the current dumping threshold. Had there been little difference in performance in this direction, I intended to sell it on and stick with the 405.
Not now however!
Very little power is needed to reach decent sound levels with the Spendors and that I attribute to the nature of the both the 405 and the 909, both being DC coupled amplifiers. Both amplifiers having excellent damping factors, explains the tightness in the music, which is lacking with the 303 and to be fair, with many amplifiers.
The stereo imagery is glorious with the 909 and the cleanliness of both vocals and instruments is outstanding. It is clearly very easy to overdrive the Spendors such that they lose that definition, but within their comfort zone they remain exceptional loudspeakers. Which supports the long held view of many, that Class A amplifiers, by their very nature sound more powerful than their Class A/B equivalents. One noticeable difference being the need for Class A amplifiers to have substantial power supplies, which invariably means expensive and very heavy Mains Transformers.
Which brings me neatly on to my original thinking, surrounding Quad’s Current Dumping Amplifiers.
The attached link illustrates something which I was aware of decades ago with my early Sugden and now with both the 909 and to a lesser degree, 405. The quiescent current of a class A amplifier is set at half peak. If it is driven by a signal which takes it above 1/2 Power, that power is dissipated in the load, whilst that in the output stages remains the same.
However, if my reasoning is is correct, if one is driving a Class A , Directly Coupled Amplifier at a level below 1/2 power, there is less power dissipated in the output stages, as it is transferred to the load (loudspeakers).
For this statement to be true, the temperature of the output stages - transistors mounted on heat sinks, should go down. Which is exactly what happens with the Heatsinks of the 909 when listening at comfortable levels with the Spendors or my Stax Headphones. Had both the Spendors or the Stax been less efficient, or the amplifier A/C coupled, this would not be the case.
So, substantially, I have a power amplifier, the 909 and to a lesser degree, the 405, which, at the kind of levels I need, remains in Class A. I further add, the reproduction of music played through them is indistinguishable - to me - between it and my Sugden Bijou Amplifier. My ideal combination for when we eventually move to a smaller place.