Pass Labs amp combination for Thiel speakers, need Mr Pass' advice ! And everyone...

[JulienG]

I live in Beijing, China. My speakers are the Thiel CS2.4 floorstanders, they are I believe 4 ohm and 87db. Need power. Love them. I have a Bel Canto analogue pre 6 multichannel preamp, use its balanced in and outs (not full balanced circuits though, but I know someone at Bel Canto who told me that it didn't mess up the signal). The preamp/speakers combo sounded great with a Simaudio W5 stereo power amp, and sounds awful and uncontrolled with my Audionet AMPII G2 monos and I have no idea why.
Anyway, my friends' experience led me to guess that a Pass power amp could not sound bad at all with my Thiels, but now I have a few possibilities and don't know what to do in order to get the best possible sound without spending too much.
1) Choosing between the X150.5 and the X250.5. Don't know if the extra class A operation is useful or good for 4 ohm 87db speakers, I'd be grateful if you explain this to me or give me a link, because I'm a little lost...
2) Getting rid of the Bel Canto, and instead buying an X2.5 pre with an X150.5.
3) Going the simple way, and buy the INT-150. (Back in the day my Thiels sounded splendid with my Plinius 9200 integrated)
4) If I see the old X250 at a good price, would you favor it over the X150.5 ?

One basic question : do you guess I'd get better sound from a Bel Canto/X250.5 combo, or a X2.5/X150.5 combo ? (I think can get these two for the price of the X250.5 from a second hand seller)

Thank you very much in advance !

Julien Gaudfroy.

[nar]

Originally Posted by JulienG

I live in Beijing, China. My speakers are the Thiel CS2.4 floorstanders, they are I believe 4 ohm and 87db. Need power. Love them. I have a Bel Canto analogue pre 6 multichannel preamp, use its balanced in and outs (not full balanced circuits though, but I know someone at Bel Canto who told me that it didn't mess up the signal). The preamp/speakers combo sounded great with a Simaudio W5 stereo power amp, and sounds awful and uncontrolled with my Audionet AMPII G2 monos and I have no idea why.
Anyway, my friends' experience led me to guess that a Pass power amp could not sound bad at all with my Thiels, but now I have a few possibilities and don't know what to do in order to get the best possible sound without spending too much.
1) Choosing between the X150.5 and the X250.5. Don't know if the extra class A operation is useful or good for 4 ohm 87db speakers, I'd be grateful if you explain this to me or give me a link, because I'm a little lost...
2) Getting rid of the Bel Canto, and instead buying an X2.5 pre with an X150.5.
3) Going the simple way, and buy the INT-150. (Back in the day my Thiels sounded splendid with my Plinius 9200 integrated)
4) If I see the old X250 at a good price, would you favor it over the X150.5 ?

One basic question : do you guess I'd get better sound from a Bel Canto/X250.5 combo, or a X2.5/X150.5 combo ? (I think can get these two for the price of the X250.5 from a second hand seller)

Thank you very much in advance !

Julien Gaudfroy.

Hello Julien,

Welcome to PassLabs forums

Possibly are you a french ex cellist working and living now in China ? I may be wrong

And for your question, I'm sure you'll get good answers very soon the Thiel are fairly nice speakers I guess. As their efficiency is low and so is the impedance curve, but very linear though in general; plus they can withstand good power handling; you'll need some very good power amps if you want to get other than nice confidential sound levels. I guess the 250W Xamps could work very nicely as they can output exceptionnal currents without distorting the musical signals.

And the Pass preamps are very nice sounding also they really fit the power amps philosophy so they are -to my humble opinion - a better match than the Bel Cantos'- which are very good "high-end" electronics too.

Take care,

nAr

[JulienG]

Thank you nar. I am also afraid of standing waves in my room, maybe the 250 might control the bass better than the 150 ?

[nar]

Originally Posted by JulienG

Thank you nar. I am also afraid of standing waves in my room, maybe the 250 might control the bass better than the 150 ?

Hello,

With your speaker choice and "realistic" listening levels, if your voisinage can cope with it, I would guess the X250.5 will do better at bass control
The X amps all feature SuperSymmetry which is a true balanced topology, and that basically means that your speaker is between two amps that push and pull into it, thus it's a bridge version; those 2 amps been clerverly interlinked for minimizing unwanted harmonics (e.g. distortion) by the SuperSymmetry scheme.
I think that out of the box, the X250.5 outputs more class A power than the X150.5 ( been biased higher ) so it might be a wanted feature in your case.

Standing waves are another problem, acoustics inherent not electric, even if sometimes one can feel the bass are not well controlled when the power amp sucks. Try to populate the room in order to minimize reflexions ( especially on parallel walls ) and also try to damp the corners of the room.

If you're more versed in true acoustic sounds and a little less into raw power, the XA.5s could be a good answer too. Look here:

Pass Labs: XA.5 Series Amplifiers

If your financial condition is good, XA200.5 in pair could be a very good choice - with a XP-20 of course

Luckily if you want buy those, the dealer should normally take time to bring them at your home for a test with your speakers

Best,

Anaël

[JulienG]

Thanks. By the way indeed if you googled me I am that guy. I see you're a cellist too, very nice.
With the budget I want to spend right now, the X250.5 seems about right. I never understood what pure class A was doing with 4 ohms speakers, I still have a pair of Audionet Amp II G2 (http://www.audionet.de/fileadmin/tem...ws_11-2002.pdf) that should sound better than the X250.5 if you judge by the price, but they can't seem to control my Thiel CS2.4. The bass is all over the place ! ( no problem with other speakers or amp combinations) Maybe it's the 87db sensitivity that needs more power ? Or maybe my Bel Canto preamp's impedance doesn't match that of the Audionet monos ? Can anyone explain this to me ?
I used to think that if they operate in pure class A most of the power is lost into 4 ohm speakers. Also I have no idea, with low sensitivity speakers like mine maybe the X250.5 would hardly be able to operate in pure class A at all no ?

[nar]

Originally Posted by JulienG

Thanks. By the way indeed if you googled me I am that guy. I see you're a cellist too, very nice.

No, I didn't google you, but you did for me ? Souvenirs could be better than googling (CNSM) lol

Originally Posted by JulienG;2346069With the budget I want to spend right now, the X250.5 seems about right. I never understood what pure class A was doing with 4 ohms speakers, I still have a pair of Audionet Amp II G2 ([url

http://www.audionet.de/fileadmin/template/media/testberichte/audionet_AMP-II-G2_highendnews_11-2002.pdf[/url]) that should sound better than the X250.5 if you judge by the price, but they can't seem to control my Thiel CS2.4. The bass is all over the place !

Price is not everything
are you sure speakers are in electric phase when you test the audionet ? Also, I see this:
Their gainbandwidth-product is higher than 1 GHz that refers to the input stage
the bias current is 0.6 A that refers to the output stage
For sure bias is low for a 200W/8ohm beast. That means the amp can out 600 mA in pure class A only. Usually, this is not an issue though - but I run a single ended Aleph power amp biased at 2,4A for 100W so I'm not used to class AB amps anymore

Output power 200 Watts into 8 ohms
o 350 Watts into 4 ohms
o 550 Watts into 2 ohms
o 750 Watts into 1 ohm

Means the amp is mostly a class AB push-pull but not bridged, is not biaised so high and that in 8 ohms, you can expect the amp to leave class A beyond 25 W or so, and logically on 4 ohms you could just have only 12,5W left for class A operation, which is very few compared to the 87 dB sensivity and if your listening room is bigger than 25 m2

• Frequency range 0 - 300.000 Hz (-3 dB)
• Damping factor
o > 1800 at 10 kHz
o 10.000 at 100 Hz

The extended frq. range can often lead to stability problems when connecting to capacitive loads. The damping factor is very high, and not constant over the audio band. Means 1°) CR behaves differently on the bass and the rest of the spectrum. This can vary for sure but to this extent I wonder what the conception goal was 2°) The amps use a very large amount of negative feedback. Which means open loop gain is also very high.

One thing is sure: it doesn't seem to help in your case.

18 kg / 200W : not heavy enough for sure

Originally Posted by JulienG

( no problem with other speakers or amp combinations) Maybe it's the 87db sensitivity that needs more power ? Or maybe my Bel Canto preamp's impedance doesn't match that of the Audionet monos ? Can anyone explain this to me ?

Do you run the Bel Canto with symmetric outs ( XLR ) ?
If you run your power amp unbalanced in, you should match the - in of the XLR input with a resistor equal to the impedance of the preamp output, for best performance. One can solder a resistor into an empty XLR that way easily. Means, if the Z out of the pre is 50 ohms, then you run the power amp unbal. in and minus in of the XLR must be terminated with a 50R or so. Perhaps it can achieve a better operation on the power amps, perhaps it'll do nothing. The amp is micro-controller driven so the unused input could be muted while using the other.

Originally Posted by JulienG

I used to think that if they operate in pure class A most of the power is lost into 4 ohm speakers. Also I have no idea, with low sensitivity speakers like mine maybe the X250.5 would hardly be able to operate in pure class A at all no ?

Nelson could answer this better than me I guess the X250.5 can output significative quality class A before going beyond class A. At least, it deserves a try

Hope I wasn't too rude - not my will in any ways -. But for sure I think you should try the PASS on the Thiels, you should have interesting results

Brds,


nAr

[jacco vermeulen]

Originally Posted by JulienG

Amp II G2

Good power amps, but not for tricky loudspeakers, the CS2.4 are tricky in the bass department.

(c'est Zhu Li-an, no ?)

[JulienG]

Yes my name is Zhu Li An in Chinese phonetic. Thank you guys, it is all very helpful. How much pure class A operation can I expect into my Thiels from the X250.5 ? 40 watts ?

[JulienG]

Originally Posted by jacco vermeulen

Good power amps, but not for tricky loudspeakers, the CS2.4 are tricky in the bass department.

So my Thiels obviously need power. Would you choose XA30.5 against the X250.5 in a 40 square meter room ? The XA30.5 switches to AB in the high power and has been measured to clip at 192 watts in 4 ohm. (Stereophile: Pass Labs XA30.5 power amplifier)

Maybe it's not enough to control the bass of the CS2.4s in this room. Which one would you guys choose ?

By the way, this seems helpful (http://www.sorasound.com/products/pa...ss-labs-chats/). My speakers seem to be at the limit for the XA30.5...

[nar]

Originally Posted by JulienG

So my Thiels obviously need power. Would you choose XA30.5 against the X250.5 in a 40 square meter room ? The XA30.5 switches to AB in the high power and has been measured to clip at 192 watts in 4 ohm. (Stereophile: Pass Labs XA30.5 power amplifier)

Maybe it's not enough to control the bass of the CS2.4s in this room. Which one would you guys choose ?

By the way, this seems helpful (Pass Labs Layman Talk | SORAsound). My speakers seem to be at the limit for the XA30.5...

Yes, maybe in a 40 square meter, but would probably be fine in a 20 square meter. Why not try a XA100.5 then ?

Regards,

nAr