Best Tube Amp/Preamp combination with Quad 2805s

Ralph, this comment is obviously coming over a year since the last post. Nevertheless, if you catch my comment, perhaps you could respond to my question.

You mentioned that the Quad ESLs have impedance curves that vary a lot as a function of frequency. So, rhetorically speaking, why is that diffrent than most speakers?

My question goes to a point that you and Al (Almarg) have made on numerous occassions. Specifically, in order to know what type of amp matches best with a speaker, be it conventional, electrostat, etc., one should try to determine what the designer had in mind when he/she voiced the speaker in question.

So ... in the case of Quad ESLs ... do we know whether the designer intended that the model in question be driven by a Power Paradigm amp or a Voltage Power amp? My sense is that your posts above permit the inference the answer is Power Paradign amps -- not Voltage Paradigm amps. Is that correct??

Perhaps a match involving an ARC type tube amp, which as you observed uses negative feedback to lower output impedance, might be a better match if the Quad ESL is hooked up to the 16 ohm tap (higher output impedance and higher gain too). However, you might point out in response that using the 16 ohm tap might achieve a smoother sonic presentation, but using the 16 ohm tap might be a non-optimal impedance match. That is the back impedance presented to the output tubes off the primary windings of output transformer might be outside the optimal range of the output stage, possibly causing increased distortion?

Your thoughts??

Thanks

Bruce

Bruce, The answer is that the impedance of most loudspeakers relates to how the speaker behaves inside a box, along with its crossover components.

An ESL OTOH has its impedance curve based on a capacitor (IOW, a high impedance at bass frequencies, low impedances at high frequencies). The early Quads had a fairly high impedance at bass frequencies- over 45 ohms. In an attempt to get a little more power out of transistor amps, the later Quads like this one have a lower impedance in the bass, but the shape of the impedance curve vs. frequency remains the same.

As a result many tube amplifiers work better with the newer Quads when a set of ZEROs are employed, due to the lower impedance at high frequencies.

With your ARC, its likely that you would use the 4 ohm tap, but you would have to try it and see- its not always that predictable and you may be better off with the 8 ohm tap.

ESLs, because they are based on a capacitor rather than box resonance, have the same efficiency regardless of frequency so they are by definition a Power Paradigm device. If you drive them with a Voltage Paradigm amplifier, they can wind up being bass-shy and bright because of the amplifier's response to the impedance cuve (which, in most ESLs, varies by a good 10:1 change from lows to highs).

In a box speaker, the peaks in the bass are caused by resonance, meaning that the speaker is more efficient at those resonant points. The amplifier has to throttle back its power into those peaks to prevent tonal colorations (one note bass- bass boominess). As you can see, the principle of operation is different and is why matching the amp and speaker is so important.

Thanks Ralph. Your explanation is pretty clear. Just curious -- while ESLs behave like a giant capaciter, do they also have capacitive negative phase angles?

Also, as I asked above, does the amp/ESL match again come back to what the designer had in mind when he/she voiced the ESL in question? Working with your example, if the Quad ESL has an impedance curve that looks like it was a giant cap (i.e., high impedance in the bass and tappering off to low impedance as frequency increased), and if the ESL was voiced to be driven by a SS amp, plugging a Power Paradigm tube amp would make it sound like a tuba. Conversely, as you posited, if the ESL was voiced to be driven by a Power Paradigm tube amp, driving it with a SS amp or low output impedance tube amp like my ARC amp would make it sound like Mickey Mouse.

So my point is ... don't we gotta know what Quad or any ESL company had in mind. Otherwise, you're gonna have a great ESL product that sounds like a tuba or Mickey Mouse on estrogen if the wrong amp is used.

Do you know in fact what Quad has in mind with its current ESL offerings? I surmise that since you think one of your OTL amps would be a match made in heaven, an inference is permitted that my tube amp would not sound very good - (like Mickey Mouse on estrogen).

Bruce, Its not quite as extreme as that :)

You do get some capacitive phase angle. But most ESLs have a matching transformer too, so you have some inductive issues as well. Overall though other than impedance, all ESLs tend to be fairly benign in that regard.

I don't know of an ESL that is 'voiced'. When I look at the older Quads and the Sound Labs, they certainly aren't. In my book, 'voicing' is some sort of frequency compensation that has been introduced to make the device work a certain way.

I do know of an ESL that has tweeter and woofer sections, and is actually equipped with two woofers (where a Quad has only one). This particular speaker (which is not well known and is not an ESL that comes up when people talk about ESLs) only works with amplifiers that have a very low output impedance.

Martin-Logan loudspeakers have a low impedance and for that reason can be a difficult load for tube amps, but even they are not 'voiced' so far as I can tell- if you get a set of ZEROs you can drive them with a zero-feedback tube amp no problem (as long as the amp has the power). So I think the 'voicing' thing isn't, with regards to ESLs.

In recent years (since the ESL63 and ESL57) Quad has been going down the path of lower impedance, much like Martin Logan, in an effort to get transistors to be happier with their speakers. They have so far been more moderate about it than Martin Logan (whose speakers are typically rated at 4 ohms although they are only about 0.5 ohms at 20KHz), I think the newer Quads have been about 8 ohms (meaning their 20KHz impedance is about 1 ohm), as opposed to the old days when they were rated more like 16 ohms. FWIW the Sound Labs seem to come in around 30 ohms by the same measure.

I ran a pair of Quad 2905s with Conrad Johnson CT 5 and LP 70S for over 7 years. Amazing combination and available used in the OP's budget. Basically any CJ, ARC or similar tube amp with around 50 wpc should mate well with the Quads. Congrats on your selection of the Quads. Brilliant speakers.
Cheers
Pradeep