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Cambridge Audio CXA81 Review (Sample 2)

audio_tony

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Dear All,

after all such interesting discussions it would be useful to have some hints on what are scenario works better, replying to some questions and doubts.
I report here for anyone who wants to elaborate on those helping to choose best scenarios.

Here I have sketched my two possible scenario based on my setup:

View attachment 212332

The solution 1 uses internal DAC of CXA 81 starting from an USB power supply (that has no ground pin) and uses the internal CXA 81 DAC over USB input.
1) In this case I positioned the USB switch on LIFT. It make sense to set USB input as lift even if raspberry pi has no ground pin? I would consider it equivalent since there is no ground pin in the raspberry input.
2) Does the fact that Rpi4 doesn't have a ground pin impact the noise source highlighted in post # 169?

The solution 2 uses my Topping E50 external DAC and hence only XLR inputs are used for CXA81. In this case both Rpi4 and Topping E50 have their own usb power supply and there is no ground pins (and no switch to choose for input).

I tested both these solutions qualitatively from a listening point of view without relevant audible differences. Maybe the second one feels more well-balanced but honestly we are speaking of nuances and I can not say how much these are objective.
Having said that I would like, from a signal integrity point of view, a hint on which solution you think is better and why, just to draw some conclusions on how use this unit at its best.

Thank you!
Assuming there is little to no leakage from either power supply, this should not make any difference, as neither the Pi nor the power supplies provide an additional path to mains ground.

Additionally, having the USB grounded may in some cases actually be advantageous in this scenario, as it will provide a ground reference for the Pi and the USB PSU.
 

Kosimo

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Assuming there is little to no leakage from either power supply, this should not make any difference, as neither the Pi nor the power supplies provide an additional path to mains ground.

Additionally, having the USB grounded may in some cases actually be advantageous in this scenario, as it will provide a ground reference for the Pi and the USB PSU.
 

Kosimo

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So if we want to summarize the best guideline to use CXA81 internal DAC is to drive it with a source with no ground pin.
But if we want to drive it with an external DAC, is, even in this case, better to opt for DAC with no ground pin when using balanced XLR connection as input?

Furthermore, forgive the naive question, but what is not clear to me is this: if the problem is the ground loop that can be generated as exemplified in post #169 by the green path , why USB ground lift is not enough to break the ground loop?
 
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jasonhanjk

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So if we want to summarize the best guideline to use CXA81 internal DAC is to drive it with a source with no ground pin.
But if we want to drive it with an external DAC, is, even in this case, better to opt for DAC with no ground pin when using balanced XLR connection as input?

Furthermore, forgive the naive question, but what is not clear to me is this: if the problem is the ground loop that can be generated as exemplified in post #169 by the green path , why USB ground lift is not enough to break the ground loop?

Yes via a laptop with battery powered (a likely possibility but again need to be measure) or just use it's optical.

The USB D+ and D- requires a return ground to the PC.
If you break that loop, it will use the mains ground and thru the air for return (increase radiation).
 

Kosimo

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Thank you @jasonhanjk.

Furthermore, forgive the naive question, but what is not clear to me is this: if the problem is the ground loop that can be generated as exemplified in post #169 by the green path , why USB ground lift is not enough to break the ground loop?

@KSTR could you elaborate a bit on this? I would like to know your opinion to clear my understanding of the issue.
 

KSTR

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@KSTR could you elaborate a bit on this? I would like to know your opinion to clear my understanding of the issue.
The "GND lift" is not a real ground lift -- which would not work anyway (the USB data lines need a ground reference to work). Rather, a 100Ohms resistor is inserted into the USB GND pin path to chassis ground. It is still paralleled with a 100nF capacitor (short for high frequencies). The shield pin is always connected to chassis... but we don't know if that connection is local or or not. I'm assuming the GND lift scheme that was posted (for another Cambridge device).
Normally it's done the other way round: USB ground is directly connected to circuit ground while the shield is connected to chassis via a capacitor. On the USB source USB GND and shield may or may not have direct connection. Lot's of variables...

Without the amp on my desk I cannot make any clear statement what's really going on with GND lift vs normal.
 

Kosimo

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The "GND lift" is not a real ground lift -- which would not work anyway (the USB data lines need a ground reference to work). Rather, a 100Ohms resistor is inserted into the USB GND pin path to chassis ground. It is still paralleled with a 100nF capacitor (short for high frequencies). The shield pin is always connected to chassis... but we don't know if that connection is local or or not. I'm assuming the GND lift scheme that was posted (for another Cambridge device).
Normally it's done the other way round: USB ground is directly connected to circuit ground while the shield is connected to chassis via a capacitor. On the USB source USB GND and shield may or may not have direct connection. Lot's of variables...

Without the amp on my desk I cannot make any clear statement what's really going on with GND lift vs normal.

Thank you for the clarification. Having said that I prefer to use an external balanced DAC with XLR inputs over CXA81, leaving USB input disconnected.
 

Kosimo

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Hello,

I have a question about CXA81 and Bi-Amping, I was thinking of enabling both A and B set of speakers outputs.
Has anyone experienced bi-amping on this unit ?
Do you think is a good idea and could improve sound on specific aspects or, with this unit, I only have one pair of cables left?
 

HarmonicTHD

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Hello,

I have a question about CXA81 and Bi-Amping, I was thinking of enabling both A and B set of speakers outputs.
Has anyone experienced bi-amping on this unit ?
Do you think is a good idea and could improve sound on specific aspects or, with this unit, I only have one pair of cables left?
I don’t see any reason whatsoever why this or any other unit would benefit from (passive) biamping.

 

Kosimo

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I don’t see any reason whatsoever why this or any other unit would benefit from (passive) biamping.


Thank you for the post, very informative.
 

Kosimo

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As a follow-up to the discussion, I have managed a test using the 4 channels of CXA81 instead of 2, in a passive bi-amping configuration.

With passive bi-amping configuration I hear subtle but always present differences: bass is almost the same but for mids and highs there is more separation and more
refinement than with standard two channels. This additional refinement is absolutely audible and palpable in voices for instance.

I had to teach my wife to connect and disconnect binding posts to make a blind test.
In 15 listening sessions (each one made of three song each - a jazz, a rock and a classics) I mistaken only one time (when I was in doubt and slightly inclined
towards single wire but was bi-amping).

I realize that active bi-amping certainly produces more benefits (damping factor, no crossover network between output and speaker, etc. etc.) but it also requires more effort and investment (not to mention that the speakers must be modified, which for me is not to be taken into consideration).
However with the CXA81 doing passive bi-amping is almost zero cost and effort (just a couple of decent cables) and you get some results.

Now I have to bring my wife out for a cool dinner (which should be added to the overall cost of the operation), but I think it was absolutely worth it :)
 
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Kosimo

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It's bi-wiring. From Cambridge Audio: Bi-amping/Bi-wiring

This is a two-channel amp, not four.

Sorry but no.

1657304870970.png


Using 4 channels (enabling both A and B speakers) and removing the binding post on the speaker is passive bi-amping (whatever CA is calling it).
If there are separate powered channels that's passive bi-amping.

The only difference here is using two different amplifiers with 2 channels or a single one with 2 set of 2 channels (like in this case) but it is still (passive) bi-amping.

This is bi-wiring: (from their site)
bi_wire_setup.png

and is not what I have done.
 
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Snoopy

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Absolutely not.
There aren't 2 amps in the cxa81. There are 2 sets of outputs. By using both at the same time you are decreasing the output power by 50% and doubling the impedance.
 

Kosimo

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There aren't 2 amps in the cxa81. There are 2 sets of outputs. By using both at the same time you are decreasing the output power by 50% and doubling the impedance.
Of course not, but you should read the specs better: it is rated as an 80W *per channel* into 8 ohms amplifier (120W/channel into 4 ohms)
Using two different amplifiers with different PS is surely better from isolation point of view, but this aspect is secondary.

From efficiency and thermal points of view passive bi-amping is surely a waste, but it is not relevant for my use case.
I liked the points 6 and 8 of the @DonH56 post:
6. There is no longer electrical interaction among drivers with passive (or active) bi-amping. (There may still be mechanical coupling if the drivers are not isolated from each other.) That is, if the woofer starts to distort the input signal through electromechanical forces, it no longer modulates the HF amp’s output. One plus for bi-amping, active or passive.
8. There may be some distortion reduction since power output is lessened in the amps. I suspect this is not significant, but it should happen due to the lower current draw. The catch is that the voltage swing of each amp is unchanged, so any distortion related to voltage swing is not changed. Only distortion components depending on output current may be reduced. That is design-dependent, but since most amps are primarily voltage-mode amps, I suspect any distortion reduction is small.


Having said that, in my opinion, your sentence on doubling the impedance and reducing the power is pure fantasy....
 
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audio_tony

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As a follow-up to the discussion, I have managed a test using the 4 channels of CXA81 instead of 2, in a passive bi-amping configuration.

With passive bi-amping configuration I hear subtle but always present differences: bass is almost the same but for mids and highs there is more separation and more
refinement than with standard two channels. This additional refinement is absolutely audible and palpable in voices for instance.

You are not bi-amping, you are bi-wiring.

The CX-A81 is a 2 channel amp, with switching for A & B speakers (hence the 4x binding posts).

So you have left channel A only or A & B / right channel A only or A & B (depending on switch position).

Connecting the "A" terminals (L & R) to the woofers and the "B" terminals (L & R) to the tweeters is bi-wiring.

As the amp only has one amplifier per channel - you can only bi-wire and not bi-amp.

The difference between passive bi-amping and active bi-amping is that passive bi-amping will use the internal crossovers in the speakers.

Active bi-amping requires separate active crossovers.

There is a good explanation of bi-amping on this site --> here and I have borrowed a couple of their visuals to illustrate bi-wiring.

Bi-wiring:
Bi-Wiring-Setup72dpi-01-1024x1024.png


Bi-amping (note that TWO stereo amplifiers are required).

Active-Bi-Amping-Wiring-Setup-1024x1024.png
 

Jimbob54

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Of course not, but you should read the specs better: it is rated as an 80W *per channel* into 8 ohms amplifier (120W/channel into 4 ohms)
Using two different amplifiers with different PS is surely better from isolation point of view, but this aspect is secondary.

From efficiency and thermal points of view passive bi-amping is surely a waste, but it is not relevant for my use case.
I liked the points 6 and 8 of the @DonH56 post:
6. There is no longer electrical interaction among drivers with passive (or active) bi-amping. (There may still be mechanical coupling if the drivers are not isolated from each other.) That is, if the woofer starts to distort the input signal through electromechanical forces, it no longer modulates the HF amp’s output. One plus for bi-amping, active or passive.
8. There may be some distortion reduction since power output is lessened in the amps. I suspect this is not significant, but it should happen due to the lower current draw. The catch is that the voltage swing of each amp is unchanged, so any distortion related to voltage swing is not changed. Only distortion components depending on output current may be reduced. That is design-dependent, but since most amps are primarily voltage-mode amps, I suspect any distortion reduction is small.


Having said that, in my opinion, your sentence on doubling the impedance and reducing the power is pure fantasy....
I think you might need to revisit that thinking.

Richer sounds (owner of CA) says :https://blog.richersounds.com/product-review-cambridge-audio-cxa81-amplifier/

"Like the original, the CXA81 extracts 80 watts of power per channel and is capable of powering two pairs of speakers or bi-wiring a single pair."

You can call it whatever you like but you are biwiring not passive biamping.
 

Kosimo

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@Jimbob54 @Snoopy @audio_tony

In my opinion you have a wrong understanding of the matter, you rely on the lexicon instead of going into the merits of the concepts.

Bi-wiring uses *the same cable with 1 end on amplifier and 2-ends on speaker* to drive the speaker crossovers at the same time. This is why the speaker crossovers are electrically coupled. This is bi-wiring and basically a useless practice since it is not different from what is done with binding posts. In my opinion bi-wiring is a media hoax, an urban legend. A lot of people does bi-wiring using binding posts without even realizing it.

Active or passive bi-amping eliminate binding posts and *decouples* crossovers using independent powered channels. This is something useful. Surely it would be better to use active crossovers at line level and connect the amplifier output channel directly to the speaker without the crossover (i.e., active bi-amping) but it is a much more invasive intervention. It produces the best results with more effort. Passive bi-amping gives less benefits and wastes more power but requires much less effort with the right units. Just to avoid misunderstanding: I would never buy another amplifier to do passive bi-amping because the benefits are not coherent with the investiment.
But with a unit with four independently powered channels it is a almost zero-cost investiment and it is worth it.

Coming to the CXA81, you can cite popular articles here or there but the fact remains that they are 4 independent channels (hopefully well decoupled and with low inter-modulation distortion, judging by the crosstalk measurements made by @amirm not exciting but not too bad) of 80W or 120W (based on impedance).
So technically it's passive bi-amping.
Finally, I don't think that using the four channels instead of the 2 the power is halved. I invite you to post measurements showing this since they would violates the specifications. I myself tested the CXA81 with all four active channels, obviously qualitatively not having specific equipment to carry out quantitative tests, and I confirm that, when listening, the power conferred does not vary. Happy to be proven wrong by quantitative charts.
 
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