Feedback on the XO-2001 Crossover

Those who bought crossovers from the first series have all taken a risk, the decision to buy is built on faith in my construction capability and ability to sort out what makes a difference in sound production. For those who have not been as brave as my first customers, here are a couple of feedback e-mails on the design that I have received. The good result with ESL-63/SW-63/XO-2001 is not only because of the crossover, the speakers themselves are both excellent constructions and the crossover merely helps to bring the best out of them.

Feedback #1
From Luc Victor in Belgium I got this nice e-mail:

Hi Mats,

I wanted to give you some feedback on the XO-2001 as soon as possible because I suppose that you are waiting for the first reactions. You have done a wonderful job!!! I am really amazed about the degree of improvement in my system (no kidding). Now I realise how much the sound of my system was coloured by the Gradient filter. Before I was listening to two systems, the ESL-63 and the subwoofers. The bass was very boomy and slow.

With your filter all of a sudden both systems blend together seamlessly and you really don't have the impression anymore of listening to two different speakers - now it is one speaker! The bass is very fast now and instead of an undefinable mix in the low frequencies you can now distinct very easy all instruments. As a consequence you get a lot more of details that you didn't know of before. (Before a lot of details were masked by the filter I guess).

The mid and high range has improved considerably too and the whole soundscape is larger with more details than ever before.
I cannot say that I find it easy to do the delay settings with your CD. I use the setting you recommended (between 2 and 2'30) and the result is very good. I will certainly have to do some more experiments.

To sum it up the whole sound is crisp and clear from top to bottom. This is very apparent with all kinds of percussion which show a lot more details. The sound is not longer masked by a bad filter. The investment in your system is worth every Euro (or dollar). You are right when you are saying on your web-site: "you have no idea how good your system really is"...

I hope the sale of your products runs very good in the future. If you want some help to convince future customers to buy your system I would be happy to help you. I really appreciate what you have done because it would be impossible to find such a good product in the hifi store. I find it difficult to believe that this speaker combination after all these years still beats the best systems on the market. There has always been a lot of sceptism to use the Quads with a subwoofer but I think that you have proved that a perfect marriage between an electrostat and a subwoofer is possible.

Thanks a lot for your good work and I wish you a lot of success with this and future projects,

Best regards,

a happy customer

Feedback #2
From Renze de Vries in the Netherlands I got this also nice e-mail:

Hello Mats,

This letter is a bit long, but as it concerns your first (?) design and product on the market, I suspect you like to read about it. So here we go.

When I first contemplated adding a subwoofer to an already excellent loudspeaker, the Quad ESL63, I knew I was entering trouble. Friends, well respected audio journalists and retailers alike, told me: if you want some good advice, DON'T!! Your speaker is very good as it is, and you can get more out of it still. Chance is you will be very disappointed, never getting your sound right and ending up wasting a lot of money. But there were other advisers too. Not too many, but they were there. Especially a guy in Sweden by the name of Mats Törnqvist encouraged me to take up the challenge.

So there I went. I bought the Gradient subwoofers for the ESL63 and what do you know, trouble indeed. Lots of it. What had I been doing? I learned I had undertaken the task to build a whole new speaker, all by myself. Contrary to what I was thinking beforehand, the Quad-Gradient combination wasn't a good speaker with something extra added, but a whole new thing that should be treated as such. Usually you buy a speaker in a shop and the designer has done all the work for you: selecting components, choosing the enclosure, designing electronic networks, all this to arrive at a nice, satisfying tonal balance. Speaker buying is really very simple, you compare different designs and in the end you pay for the one you like most. In my case, alas, I couldn't choose between different designs. I was stuck with two pieces, a device for the low end and one for mid and treble, which I had to put together in some satisfying way.

This idea to think about my speaker-to be as a whole, instead of as two parts, or even a combination, was reinforced very much by my experience with amplification and cabling. I had to drive the subwoofer with an extra amplifier, a filter network, speaker cables and interconnects. So I went shopping for these things, expecting it should be really very simple: as the subwoofer was only operating below 110 Herz, amplification and cabling wouldn't be too critical. Well, I couldn't be farther from the truth. Different amplifiers resulted in very different sounds, interconnects did their part and the subwoofer-speaker cable proved to be equally important as the one for the main speaker. I had a particular revealing experience with a speaker cable I had tried earlier, some 10 months before, for use with the main speaker. I hooked it up between subwoofer amp and the Gradients and to my great surprise it sounded exactly like I remembered. The character of the cable, it's specific colour, it was all there. I instantly knew why I had returned this cable 10 months earlier, why it wasn't to my liking. Strange, a cable acting below 110 Herz contributing to sound aspects (in my case undesirable) in the midrange and treble area.

Of course I had to make up my mind about it, and came to the conclusion that what I had been doing was really very basic stuff: if you meddle with the low register of your speaker, the whole frequency spectrum changes. If you alter the foundation of a speaker, you alter everything. Because the whole sound is built upon it.

Of course, this is what subwoofer technology is all about. This is what subwoofer trouble is all about. But also, this is what subwoofer success is all about. And by now Mats, at last, I have arrived at the cross-over filter you designed. What should this filter do, ideally? It should integrate the low range of the speaker with the midrange and treble as good as possible. The bass shouldn't stand out, it should be as liquid and natural as the high or midrange. The cross-over unit shouldn't draw attention to itself in any way. Once installed and trimmed it shouldn't be there. The ideal cross-over filter is a device you forget about from the moment the music starts coming out of the speaker.

Of course, you gave XO-2001 the looks it deserves. It's really a very nice thing. But to be honest, I have forgotten about it in less than two days. Because the sound from it is really very, very good. The bass is dry and tuneful and detailed like never before. It goes very deep, but if there's nothing there, it's simply dead quiet down below. I play lots of old 78-recordings on CD, and mostly there's some rumble. Well, this rumble is there with pinpoint precision. Even my standing waves in the room are enjoyable (at least for the moment): feed the CD a 100 Herz sinus wave and you can walk through the knots, they're very precisely located. And for the midrange and treble, it's all very well too of course. Indeed, like you promised, transparency is increased and there are lots of new details. And all this without being etchy or bright - my sound is now just as softly textured as it was before.

I put on a grand piano and it's there, with a beautiful firm left hand. I play a wonderful sounding contrabass, recorded in a small church and not too closely miked (exceptional!), and the sounds from strings and fingers and nails are in just the right proportion to the sound from the body of the instrument. Yes, a contrabass is a rich sounding instrument, and one hears that. So what can I say more?

Summing it up, I think you made a cross-over filter that does everything right. It can disappear from between the speakers, hooray. And it's worth every penny.

bye bye,

PS. Speaking about cabling the filter and subwoofer, in my experience as a rule of thumb identical interconnects and speaker cable for the ESL and the Gradient give the best results. Usually one will have carefully chosen these particular connections for the ESL63. Bringing in other cables (i.e. colours) on the subs is entering trouble area.

Feedback #3
From Ulrich in Germany I got a document with extensive measurements and comments:

XO-2001 Measurements by Ulrich (pdf-file, 395k)

Feedback #4
From Harald Klomann in Germany who has built the DIY dipole subwoofer with original Gradient elements I got this e-mail:

Today is a great day, because I got all my stuff for testing the XO-2001 and Sub together. Assembly of the sub´s is finished and I started testing my "new" system.

First, I was very disappointed, because the sub´s didn´t work as expected. I checked my wiring connections, and after thinking about, I found out that I had to wire the subs internally one speaker normal and one speaker reverse. After doing that, the bass was there. WOW !

After adjusting the switches, level and delay and listening some hours, I found that my system was never sounding as good as today. My congratulation to your XO-2001. First, I was impressed of the new quality and amount of bass I never could hear with only the ESL´s. But after listening a while, I became aware, that also the Hi and Mid section gained an improvement which I never thought to be possible with the Quad´s. And of course the possibility to play louder is great.

So all in all, I can say that I´m happy to bought your XO-2000 device, although I never heard it before. I just had a good feeling about it when I so often visited your famous website with all that ESL related information and so gave it a try. I would recommend it to others if I was asked about. Thanks for this good work, Mats !

Thanks a lot
and greetings from Germany


Feedback #5
From Steven Hill in Singapore, who has a modified XO-2001 crossover with transformers from Sowter for balanced operation, I got this e-mail:

I received the last unit of the first batch of the XO-2001 crossover and so others have been able to report earlier on its excellence.

I can only endorse their comments.  Based on my own listening tests, the upper pass filter is inaudible which, given the resolution of the ESL63 is amazing (I have ESL63 Pros with Mats' recommended modification of the correction network:  Solen 200uF polypropylene capacitor and Caddock 1R5 resistor).  This transparency alone confirms the brilliance of the design. In addition, however, to have a seamlessly integrated SW63 which plays down to well below 20Hz, is nothing more than astounding.  Bass information which was never audible before will be clearly heard.

If you have the ESL63/SW63 combination with the original crossover provided with the SW63s, you will be justified in being very annoyed with Gradient for having built an excellent subwoofer but, by fault of their crossover design, never having allowed you to hear how the combination of ESL63 and SW63 can really sound.  The XO-2001 is worth every dollar, euro or whatever you have to pay to get one.

Of course, with everything there is a downside.  If, like me, you listen to recordings of opera or concerts live, you might be disconcerted by the very low bass information now audible:  movements of performers on stage, or changes of scene.  An excellent (?) example of this problem is the set change between scenes 4 and 5 in Act 3 of Berg's Wozzeck from the Staatsoper Wien (DG 423 587-2).  Don't look out of the window for the forthcoming storm:  you are hearing the rumble picked up by the stage microphones as the set is changed!  Fortunately, Mats has provided a switchable low pass filter which can be used to deal with such occurrences. You will also hear in the upper regions information which was obscured by the Gradient-supplied crossover which might cause you to reassess some of your recordings.

If you have the ESL63/SW63 combination and there is somebody not too far away that has Mats' XO-2001, go and hear what this combination can do (if you can get the lucky owner to allow you 30 minutes away from his/her system to listen to your choice of music!).  You really have no idea of how good your system is until you have heard it through the XO-2001.  I live in
Singapore and, if there are any readers of this report in the area, Mats will give you my email contact so that you can come and hear the astoundingly musical experience which the ESL63+SW63+XO-2001 combination provides.

Steven Hill

Additional information from Steven Hill in Singapore:

Further listening tests have confirmed to me that Ulrich's method of setup is the absolutely correct one.  What we lucky owners now have is a full range speaker system of astounding clarity, purity and musicality.  There is no transducer audible, just beautiful music hanging in the air.  Just what we were always searching for.


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