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Old 5th May 2011, 02:29 PM   #1
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Default Classic amp, Chinese or scratch-build for Quad ESL?

I picked up a set of Quad ESLs and I have spent the last 2 weeks swapping tube my amps into them. I've used an EL34-based PP made from a kit (Assemblage ST40) and a 300B SET (scratch built JELabs). I was surprised how well the SET works with the Quad, although the bass is tubby and the high frquency not as extended relative to the EL34. The SET actually sounds louder with my passive linestage than the 40 WPC EL34!

Anyways, I'm thinking about a new project amp for the ESL. My first thought would be to get a pair of Quad 2s, replace the drifting resistors and old caps. The cost for this would probably be in the ~$1500 range. I can certainly build a pretty good amp (SET or PP) for this sort of money. Another possibility would be to get a Chinese 845 amp and replace the counterfit components. the Mingda 845 would be my choice here. While I am interested in the Quad amps as a "historical" thing, I'm really more interested in the sound. From a sonic/value for money perspective, does a classic Quad amp offer anything that a scratchbuilt or Chinese amp can't deliver?
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Old 5th May 2011, 04:39 PM   #2
Loren42 is offline Loren42  United States
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The best choice would be scratch build. You could do that for the budget of $1500.

The best amp would be something that has enough power reserves to drive those speakers. What is the sensitivity for those speakers?

I was thinking it was 86 dB•m, which is going to require some oomph to get the level up to reasonable levels and have enough headroom left over.

I built a 60 WPC amp from scratch for about $1200 in parts. It is clean and transparent, which is exactly what I wanted, using KT88s in ultralinear PP. I have since upgraded to Gold Lion KT88s.

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Old 5th May 2011, 04:48 PM   #3
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That's a nice looking amp. I like the contrasting bell ends.

Quads are 86 dB sensitive, and KT88 would be my first choice if I build a PP. I was frankly startled that my 300B SET could drive the Quads at all. It's not a great overall combination but the midrange sure is sweet and it does play pretty loud. That's why I was thinking that an 845 SET could provide the necessary power to drive the ESL and give me a nice sweet SET sound.
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Old 5th May 2011, 04:55 PM   #4
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If you are going to use an 845 SET for your Quads, I would do a class A2 design, with a mosfet source follower, like tubelab power drive.
PowerDrive
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Old 5th May 2011, 05:07 PM   #5
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by summilux View Post
That's a nice looking amp. I like the contrasting bell ends.

Quads are 86 dB sensitive, and KT88 would be my first choice if I build a PP. I was frankly startled that my 300B SET could drive the Quads at all. It's not a great overall combination but the midrange sure is sweet and it does play pretty loud. That's why I was thinking that an 845 SET could provide the necessary power to drive the ESL and give me a nice sweet SET sound.
1 Watt gives you 86 dB at 1 meter. You probably are about 1.5 to 2 meters away and that power will drop with the square of the distance.

At 1.5 meters that speaker will deliver 82.5 dB with 1 Watt.

A normal symphony orchestra peaks at over 110 dB, but let's use 100 as a goal. This is peak dB, but you still need to deliver the power to reach it without distortion.

Every 3 dB of increase requires doubling your amp power:

1W = 82.5 dB (at 1.5 meters)
2W = 85.5
4W = 88.5
8W = 91.5
16W = 94.5
32W = 97.5
64W = 100.5 dB

My speakers are 94 dBm efficient, so I have a lot of headroom and my amp loafs a lot and that is why the distortion levels are low and I get nice clean sound.

A SET will never deliver enough output to drive those speakers to a respectable level and the distortion will be very bad long before you get to that point. On the flip side, many SET owners like the distortion or coloration that they get from those amps, so you may find that it fits your desires better.
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Old 5th May 2011, 06:25 PM   #6
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You can get 45W out of an 845 SET with less than 2.5% 3rd harmonic distortion and 40W less than 1.2% 3rd and 4.5% 2nd. Those are pretty low distortion numbers for a SET.
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Old 5th May 2011, 11:06 PM   #7
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Quad II. Morgan Jones describes it a circuit that's not supposed to work but does; in the same way 15 watts shouldn't be able to drive 86db electrostatics but they do (and have since 1957). I was expecting them to be a money pit but there was surprisingly little work to be done to make them functional.
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Old 5th May 2011, 11:37 PM   #8
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by carbondated View Post
Quad II. Morgan Jones describes it a circuit that's not supposed to work but does; in the same way 15 watts shouldn't be able to drive 86db electrostatics but they do (and have since 1957). I was expecting them to be a money pit but there was surprisingly little work to be done to make them functional.
Please cite the page number of that. I would like to read it.

15 Watts will drive a 86 dBm speaker, but it will output slightly less than 98 dBm, which is still a lot of audio!
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Old 6th May 2011, 12:07 AM   #9
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It's in the section where he describes some classic designs - p.419 in my edition. Another source that describes the design nicely (in terms that a layman like myself can understand) is Stereophile's review of the new Quad IIs: Quad II Classic monoblock power amplifier | Stereophile.com
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Old 6th May 2011, 12:47 AM   #10
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by carbondated View Post
It's in the section where he describes some classic designs - p.419 in my edition. Another source that describes the design nicely (in terms that a layman like myself can understand) is Stereophile's review of the new Quad IIs: Quad II Classic monoblock power amplifier | Stereophile.com
Thanks. Both good reads.

The Stereophile review actually did not say that 15 Watts sounded louder than 15 Watts, just that it performed well with the electrostatics and any clipping was a soft clip without heavy distortion.

Morgan also did not say the Quad was not supposed to work, just that it was an unorthodox approach for the input and phase splitter drive that worked very well rejecting noise and hum. Only the Williamson amp was quieter due to the 6SN7 triode. The Quad II was a good amp then and it is a good amp now with its second release. I am glad they made it almost exactly as they did when it first came out. That was a nice touch.
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