Upgrading a Croft 7 Power Amp to a 7R - How does this help?

[Wakefield Turntables]

You really need to speak to Glenn Croft. I asked for a list of mods for my Croft Epoch and suffice to say they completely changed the amp into a stellar performer. It still has the classic Croft sound and it's the only piece of kit in my valve setup that will not be sold/swapped or replaced.

[Firebottle]

They might drive them 'well', but they won't drive them optimally. Is that 55W into 8 Ohms or 4 Ohms?

That's 55W into 8 ohms and 90W into 4 ohms. There was no problem with driving the maggies

[loonytunes]

Sorry I didn't say this but - I'm not using the Croft's with the Maggies right now "just in case" I may cause an issues with old panels like this causing damage to the Croft. I am playing safe and using an integrated Musical Fidelity M3si (specified to deliver 25A but is only 85 Watts p/c).

There are no specs online showing what both the Croft's can do when presented with a 4 or 2 ohms load, or what the MF can do over 4/2 ohms either - so - it's the best I can do for now.

[Firebottle]

I found that the maggies had a fixed 4.5 ohm load, so maybe not such a difficulty after all.

[Sherwood]

I tried many power amps with my Maggies before finding something that really drove them well. That included powerful amps from Exposure, Parasound, Audiolabs, Musical Fidelity, and QUAD, none of which really gelled. Yes they can sound fine with lesser amps, but they seem to soak up power (current) like a sponge. I even tried them with my Art Audio Quintet valve power amp in pentode mode. They sounded very nice with spectacular imaging and a very sweet midrange but only at low volumes. Dynamics were constricted and bass very soft.

Once you get the amplification right, the difference is startling! At that point I think they are really one of the audiophile bargains (even with the outrageous UK mark-ups). I agree, it is not just about brute force power and low impedance stability, but matching is a challenge.

Geoff

[Yomanze]

It's not that they are a 'difficult' load, it's a non-reactive resistive load, but they eat watts for breakfast & thrive on high current delivery amps.

[loonytunes]

I wonder if you techies could read the following article from top to bottom by a chap in the States that designs and sells Elecrostatic loudspeakers, who has also designed an amplifier that can control his speakers and the Maggies (and possibly every speaker on the Planet for that matter) - it should do being 500 Watts per channel - but if you read his white paper it would seem that everyone and his dog should have a powerful amplifier, even for convential loudspeaker design...

http://sanderssoundsystems.com/techn...vs-transistors

Those with the necessary instruments (Firebottle for example) can you run the 'Scope' test as explained in the article and tell me your results?

[JohnJo]

I wonder if you techies could read the following article from top to bottom by a chap in the States that designs and sells Elecrostatic loudspeakers, who has also designed an amplifier that can control his speakers and the Maggies (and possibly every speaker on the Planet for that matter) - it should do being 500 Watts per channel - but if you read his white paper it would seem that everyone and his dog should have a powerful amplifier, even for convential loudspeaker design...

http://sanderssoundsystems.com/techn...vs-transistors

Those with the necessary instruments (Firebottle for example) can you run the 'Scope' test as explained in the article and tell me your results?

Just had a quick flick through this white paper and found this
"Resistance is measured in ohms. The term "resistance" is used in DC (direct current) circuits. "Impedance" is the same thing as resistance. But it is used when discussing AC (alternating current) circuits because the impedance often varies with the frequency of the AC"

I'll be first to admit I know very little about audio engineering but in my opinion, from a simple electrical point of view, this is wrong. Impedance refers to the out of phase relationship of current and voltage due to reactive components i.e. capacitance and inductance in AC circuits. The only guy I ever heard referring to this on AoS is Alan (Firebottle) with regard to the capacitive nature of Quad ESLs. I suspect Tannoy HPDs are similarly awkward at the other end of the spectrum, a highly inductive load but I've never seen any graphs other than simple resistive plots. An amp's ability to drive lowish resistances at awkward phase angles may be the key to getting the most out of these speakers.

If Maggies are a reasonably constant >4 ohm load and not reactive I can't understand how they need high current unless they are just massively inefficient. What's the typical efficiency of a pair of these?

[JohnJo]

Here's a bit more info: http://www.audioholics.com/loudspeak...l-phase/page-2

As can be seen from the table at a 60 degree phase angle 1/6th of the current being drawn from the amplifier's power supply is actually going to the speaker. If I remember correctly Alan's graphs for Quad 57s showed a phase angle of something like 55 degrees at the same time as a pretty low impedance at certain frequencies. This is a massive ask of an amplifier and is probably why electrostatics are sometimes accused of sounding undynamic when actually it's the amp that's incapable of keeping up. I imagine Maggie's to be similar in this respect to the Quads.
I also suspect that valve amplifier's are much less susceptible sagging under difficult phase angles than most solid state amps hence the development of the term "valve watts" or "it's only a 20W valve amp but it sounds louder than my 50W SS amp.

[Yomanze]

86db at 2.83v, but the speaker is 4 Ohms, so this is 2W not 1W power, so at the more common 1W / 1M sensitivity measurement they are 83db, and below...