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Building stacked Quads

Jonathan Ribee said:
From the Beeching article on stacking quads...



You've still got that 2 ohm extreme HF issue - which has been suggested as a reason why valve PP amplification is often preferred.

You can pretty much use whatever amp works well with a single pair on a stacked pair. Robert has pointed out some interesting relationships with his amplifier output and the characteristics of the 57.
Click to expand...

Power transfer characteristics.
As amplifier output impedance rises, the power transfer characteristic starts to depart from perfect as you no longer have a pure voltage source.
In effect, the frequency response gets drawn into increasingly mimicking the impedance curve - which is this:

57imp.jpg


EEK!

Generalising, push-pull valve amps have the lowest output impedance of the tube amp family, though in all but a handful of cases still higher than SS amplifiers.
Therefore those have the least effect on frequency response.
SETS and OTLs usually have many Ohms output impedance and the effect on the ESL57 is similar to applying tonal EQ as per the impedance graph above - so it progressively reduces output with frequency. It will sound warmer, with less brilliance to the sound and fatter bass. Same things happen with most conventional speakers but the ESL is more prone to the effect.

What are we talking in terms of numbers?
Well no more than a couple of dB with most push-pulls but considerably more with other valve amps - you will definitely hear it!

Fortunately a fully working ESL57 treble panel actually has a little lift at the very top (on axis).

If you stack 57s on a single amplifier, these effects grow in magnitude, assuming it can drive them.

Despite the fact that the ESL is a nominally 16 ohm load, you can mitigate these effects by using the 4 ohm transformer tap if you have one - reduces output impedance, though also maximum output voltage swing. No free lunch. Thankfully, power demands up above 10kHz are very low which is why amplifiers that appear less than ideal on paper in terms of ability to drive low loads will often work pretty well.

Hope that helps :)
 
Dunno about that but my Chinese valve amp rund EL34s P-P and that sounds a treat with stacked ESL 57s. Or it did until the power supply went pop. I suspect a smoothing capacitor has gone sick.
 
Quad IIs into stacked 57s will work so long as you listen at pretty low SPLs.
It's rather a waste of the very reason you stacked them in the first place though - throwing away the extra SPL capability, plus the amplifier will produce considerably more distortion and that again seems to waste the stunningly low distortion (mid-top) available from the 57.

Buy four IIs - that has to be the best bet for that amp.
 
thanks Robert
I just wanted to clarify If one pair the would drive them safely {without risk of damage to either the amps or 57's} whilst I find either another pair of Quad 2's or a more powerful suitable valve power amp
 
Rob's spot-on with the analysis of the impedance vs amplifier output characteristics.

But the handy addendum is this: the spectrum of most music (unless you like mp3-rips of Metallica) rolls-off the power requirement at very approximately 6db/octave above 500hz.

So the power required is roughly -30dB at the problematic 15khz impedance minimum of stacked 57s . Call it a single watt required here if you are driving the ESLs to max. acoustic output short of arcing with music . And at 1 ohm impedance that only requires the amp to source a single amp of current - which most should withstand very easily.

IOW its not really as bad as it looks -provided you want to listen to music, not square waves :)
 
FWIW, I'm using a little PP EL84 amp giving about 10 wpc and it sounds great. Now I've no doubt that the bass would be a little grippier with about 25 wpc, but at 10wpc it sounds bloody good.
 
I've been playing a bit with my set up. First off, in my case with the EL84 amp, they sound best working off the 8R taps (as opposed to 4R). The other thing that is the rake - it seems to make a pretty big difference. What I don't know is whether this is room dependant or not, so I suppose YMMV.

I want to give them a little more time for me to play with positioning, rake toe in etc etc before I go trying bigger amps. When I get to that stage, the amps that are high on the list are my Pass F5, airtight EL34 and 6L6 amps, and the Naksa amp. That last in particular sounded wonderful with single quads, and at 70WPC should have plenty of grunt too.

Can I ask the other nuts here who have stacked these what have you guys done for rake and toe in?


Cheers,

Fran
 
I'm full of cold but I'll be better tomorrow and giveyou full details. Mine are fantastic, provided you can live with the size of the things, which is just daft.
 
Right, I'm still as rough as soot but in passing I had a look. The bottom speaker is inverted so the power supplies and connectors are nearby. The frames are straight and vertical, and the panels screwed in so that theyare approx vertical. The backs certainly look vertical, though it's difficult to see as they are curved.

For toe in they aren't as critical as single pairs. For space reasons I have one parallel and one toed in about 30 dgerees. It works OK but I'm not precious about the way they are set up. I can't afford to be, I don't have the space as I live in a small semi. Frankly if you can do better than this then you either live in a barn or have a dedicated listening room. Either way I doubt you are married. If you are, I wouldn't worry, that can change!
 
Ta kindly. I'm loving them at vertical but have them toed in pretty much equally. As for the missus, she likes them!
 
Vertical and toed in to varying degrees is fairly conventional for these, I think the sheer size means there's a bigger sweet spot so they are less choosy. If your wife likes them, that's less convenbtional, so well done, think yourself lucky and enjoy the music!
 
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