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Review and Measurements of Cambridge Audio Duo Phono Preamp

amirm

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#1
This is a review and detailed measurements of the Cambridge Audio Alva Duo Phono Pre-amplifier. It is on kind loan from a member who purchased it and drop shipped it to me. The Duo costs USD $299 and includes Amazon Prime or free shipping from the company itself.

Despite its budget price, first impressions are positive as far as look at the unit:

Cambridge Audio Duo Phono Pre-amp Review.jpg

A momentary switch turns power on/off and controls whether it is amplifying the movie magnet or moving coil input.

The volume control is only there to vary the level of the 1/4 inch headphone jack. It has no effect on the pre-out in the back.

Oddly, the unit came with two identical IEC cords. There is even dedicate space for each in the packaging. Wonder if someone is getting paid by how many power cords they move every day. :)

Yes, there is a Solo version of it with just moving magnet input and no headphone jack. Wonder if that one comes with two power cords two! :D

Rear panel connectivity is as you suspect plus a balance control which you don't:
Cambridge Audio Duo Phono Pre-amp rear panel review.png.jpg

Note how thoughtful someone was to put a set of labels upside down so that you could read them when leaning over the unit!

The overall packaging left a positive impression. Let's get into measurements and see if that feeling fades out or gets stronger.

Measurements
This is my first attempt at standardizing my phono preamp measurements and making them more comprehensive. So please provide feedback on them as once there, it should make it easier to compare products from different companies/models.

Here is the dashboard view using MM input:
Cambridge Audio Duo Phono Pre-amp Dashboard MM Measurements.png


I am standardizing on 1 volt output since one of the specs for Duo was stated as such. But also because it reduces the effect of hum which is the main limiting factor in phono pre-amplifiers. Even with that high output (requiring 11.3 mvolts of input) THD+N is limited by the 60 Hz mains leakage. Initially the level of hum was much higher with the floating inputs of Audio Precision analyzer. I grounded the input to the Duo and it made a 10+ dB improvement. Grounding to the chassis screw on the Duo made things worse so I left that alone. Using this configuration, my measurements agree more or less with company's THD+N spec of 0.002%. We will analyze this performance more in later measurements.

Frequency sweep shows a rumble/subsonic filter permanently engaged:
Cambridge Audio Duo Phono Pre-amp Frequency Response MM Measurements.png


This measurement is with the Audio Precision generator EQ for inverse RIAA engaged. As such, it shows the accuracy of RIAA implementation in Duo which seems very good. No coloration from this unit.

Using the same 1 volt output, we get 90 dB signal to noise ratio which confirms our finding in the dashboard:

Cambridge Audio Duo Phono Pre-amp SNR MM Measurements.png


Once again, we have measurements which match the spec which is reassuring.

Running a 1 kHz FFT with high resolution shows the spectrum of any distortion or noise:

Cambridge Audio Duo Phono Pre-amp 1 kHz FFT MM Measurements.png


As with the dashboard we see the mains/power supply harmonics and an odd peak at about 19 kHz. The latter is strange in an analog unit we read that it has a switching power supply. I wonder if that is the switching frequency???

Note that there appears to be no harmonic distortion of any kind! All the spikes are mains driven. We can confirm this by turning off the generator and just examining the spectrum of what is there:

Cambridge Audio Duo Phono Preamp MC Noise Spectrum Measurement.png


I have shown the noise spectrum using log display now to show more detail in low frequencies. And also included that of moving coil input (in blue). We clearly see the 60 Hz mains noise plus other harmonics going on and on. And our 19 kHz or so spurious tones.

In an attempt to make the graph easier to relate to audibility, I have overlaid the threshold of hearing assuming a peak playback of 120 dB SPL. Research shows that live concerts can reach such levels in typical seating position. Using that as the baseline, then we can properly overlay the hearing threshold (in pink) over our graph. Once there we see that none of the measured noise components reach audibility.

Of course this is with my setup. Yours will vary and may very well have far higher hum. If you are into LP playback, you are probably used to this challenge. :)

Here is the intermodulation distortion (SMPTE signals) versus output level which levels the playing field between the two inputs (and other products measured in the future):

Cambridge Audio Duo Phono Pre-amp IMD MM Measurements.png


The MC input (blue) has more gain so it gets pushed into clipping with 20 millivolt input level. Note that I had to lower the input level for MC to get the full graph (was too lazy to document it in the graph). Bottom line, you have to use a lot more gain with MC input and that translates into nearly 20 dB higher noise level.

We get the same view if we measure THD+N versus level:


Cambridge Audio Duo Phono Pre-amp THD vs Level MM Measurements.png


You have to push the Duo to more than 6 volts output to generate any distortion. Since most power amplifiers need less 2 volts (or even 1 volt), you can compute your headroom using this figure.

Switching to headphone amplifier, the outcome is disappointing. Here it is with 300 Ohm load:

Cambridge Audio Duo Phono Pre-amp  Headphone Power at 300 Ohm Measurements.png


Even when you force the unit with far higher levels than any cartridge provides, you only get 10 milliwatts of power. My standard is 100 milliwatts here. With typical cartridges, you are likely to get single digit output power in milliwatts. Better have a sensitive headphone to get much value here.

Kind of odd is how quickly the graph flattens. Seems to be high level of noise there to dominate that way.

The situation is not much better with 33 ohm load:
Cambridge Audio Duo Phono Pre-amp  Headphone Power at 33 Ohm Measurements.png


You will be lucky to get 10 milliwatt output with any real cartridge.

There is good news however in the form of low output impedance:

Cambridge Audio Duo Phono Pre-amp headphone output Measurements.png


Conclusions
The Cambridge Audio Duo phono preamp comes in an attractive package despite its budget price and branded label. Its amplifier is essentially distortion-free and performance is mainly limited by power supply hum/mains leakage using MM input. MC incurs a penalty of 20 dB so you may hear some hiss there that would not with MM.

Sadly the headphone stage is a throw-away design. You can use it in a bind but don't be surprised if it has too little power/too high of a noise floor.

The Duo seems to be competently designed and attractively priced so I am happy to put it on my recommended list.

------------
As always, questions, comments, recommendations, etc. are welcome.

Have craving for some good Dim Sum. Nearest place is 35 miles away so I need gas money for that. Please consider donating money using:
Patreon: https://www.patreon.com/audiosciencereview), or
upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
 

Veri

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#4
Hey amir, in the output impedance graph you seem to have highlighted(in red) the "lyr GE tube" rather than the Cambridge duo :)

Great set of measurements! Nice to see Cambridge being confirmed as an excellent brand as far as their engineering skills go!
 

amirm

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#6
Hey amir, in the output impedance graph you seem to have highlighted(in red) the "lyr GE tube" rather than the Cambridge duo :)

Great set of measurements! Nice to see Cambridge being confirmed as an excellent brand as far as their engineering skills go!
Thanks. Too lazy to fix it right now. :)

But yes, a nice trend is developing for Cambridge Audio.
 

JJB70

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#7
In the UK they are the house brand of one of our major hifi retailers, Richer Sounds, with shared ownership. The hifi brand seems to reflect the shop ethos of sensible down to earth but good products and services. They seem to specialise in the entry to mid level but do it very well. I have used the shop quite a bit (they have a branch five minutes drive from my house) and have always been very satisfied.
 

graz_lag

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#8
The upside-down labeling on the back panel is a kind of "artist touch" !!!
Very well done @ Cambridge Audio, and to the whole British's audio gear industries as well !
 

Frank Dernie

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#9
Cambridge Audio have a reputation for making good value well engineered and solid performing equipment, the reputation seems to be well earned. They seem to have occupied the space that was once held by Arcam.
The original company had that reputation before Arcam existed, so it is appropriate that the new owners are “keeping up appearances”
I had a Cambridge Audio P60 and T55 tuner for years! Probably bought it around 1973 or 4, it was a big improvement in driving real speaker loads over the good measuring (into a power resistor) nominally more powerful amp it replaced.
 

gvl

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#10
Oddly, the unit came with two identical IEC cords. There is even dedicate space for each in the packaging. Wonder if someone is getting paid by how many power cords they move every day. :)
Is there a sticker on the box "Free audiophile upgraded power cord included!"?
 

sergeauckland

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#11
The original company had that reputation before Arcam existed, so it is appropriate that the new owners are “keeping up appearances”
I had a Cambridge Audio P60 and T55 tuner for years! Probably bought it around 1973 or 4, it was a big improvement in driving real speaker loads over the good measuring (into a power resistor) nominally more powerful amp it replaced.
I still have a P50! I recently repaired a P60 belonging to the son of the Cambridge amps' mechanical designer.

S
 

JJB70

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#12
I remember some of the older Cambridge Audio equipment from the 80's, it was very nice gear. I think they are one of the companies that occupy the sweet spot in audio equipment, the performance is a significant step up from cheap all in one systems and such like and much of their gear is well past the point of diminishing returns (well, in my opinion) but prices are realistic and very affordable. And you have the surety of a local warranty and service provider.
 
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#13
Some interesting info below. Would be interested in Amir's thoughts on this. I have personally experienced this phenomenon on hotly cut records when using a high output cartridge with a phono preamp that had an insufficient overload margin @ the lowest gain setting for that particular cartridge.

MM and MC Don't Mix!
Moving magnet and moving coil cartridges have two things in common: they are both magnetic giving rise to their constant velocity outputs (rising with frequency), and they can both be used to play records. After that all similarities end.
There are a number of phono preamps available which claim to do both, and also claim to be high-fidelity. To the layman that may look OK, but to an experienced electronics circuit designer of advanced years (like me), claiming high-fidelity for both is questionable. If the phono preamp were to use separate circuits for each cartridge, then I could be in agreement. The type of circuit that simply alters its voltage gain by a switch cannot do both justice.
Leaving high output moving coil to one side, the conventional moving coil cartridge has little output. The output is measured in micro-volts, and is approximately one-tenth of the output of a moving magnet (which is measured in milli-volts). The lowest output moving coil I have come across outputs just 100uV (0.1mV). The output is measured at a standardised centre frequency of 1kHz because, due to the constant velocity characteristic common to all magnetic cartridges, its output at other frequencies will differ.
At 20Hz when playing an RIAA record its output will be one-tenth of 100uV which is 10uV (0.01mV). To prevent circuit noise, which is mainly a function of the silicon transistor that will be used to amplify the signal, "coming up" to meet that tiny 10uV signal; the amplifier has to be extremely low-noise. To get that low-noise the amplifier's transconductance needs to be very high indeed. This is the reason why you cannot use valves to amplify a moving coil cartridge directly - their transconductance is too low. In addition, most silicon amplifier 1/f noise tends to rise at frequencies below 100Hz. It presents a serious difficulty which is hard to overcome whether the amplifier uses discrete transistor circuitry or op-amps.
Another problem for low-output moving coil is that high transconductance equals small slew-rate and reduced bandwidth. In high-fidelity audio, a slew rate below 4V/uS results in much transient intermodulation distortion - it becomes audible.
The numbers of options open to the audio circuit designer are very few indeed. Add to that the international requirements regarding immunity to spurious noise sources needed (mobile phones being the worst domestic culprits) to comply with EMC (which is mandatory), and there is possibly only one or two op-amps which just "scrape home". Forget discrete circuits - they will be wide-open to today's interference.
Having been successful in implementing solid-state pre-amplification for low-output moving coil by using one of the rather limited methods, can that same input stage be used for moving magnet? The short answer is no.
Whereas the output from moving coil is sufficiently low as not to exceed the linear input region of a silicon bipolar transistor (the type that has the highest transconductance - discrete or inside an op-amp), the output from a moving magnet cartridge can easily exceed it. Remember, all magnetic cartridges are constant velocity, and at 20kHz (percussion and high harmonics) the output of a 5mV @ 1kHz moving magnet cartridge is going to be 50mV. The peak to peak signal the input sees is nearly three times that size, and 150mV is over twice the linear region of the device. The result is incurable distortion, but because distortion simply adds to the percussion sounds and harmonics, some like the sound - but it is hardly high-fidelity.
A moving magnet cartridge therefore requires a different input circuit. Silicon bipolar transistors can still be used, but with emitter degeneration, which reduces transconductance and "stretches" the input linearity. J-FET inputs work in a similar manner. Noise is less of a concern, and the lower transconductance makes it possible for valves to also do the job. In the case of active EQ, where the gain of the stage includes the equalisation, switching gain sends the frequency response off target - another reason why a switched gain "all-in-one-box" MM/MC phono preamp is a poor choice.
The best choice for low-output moving coil is a step-up transformer (S.U.T.) used in combination with a dedicated moving magnet sensitivity phono preamp - either that or use an electronic version of the S.U.T., such as the Elevator EXP step-up amplifier. Another choice would be a dedicated MC phono preamp - which does low output moving coil only.


https://www.hifisystemcomponents.com/blog/all-about-the-phono-preamp.html#mix
 

SIY

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#16
Right conclusion (a MC has very different requirements besides gain than an MM and vice versa), wrong reasoning.

Big differences besides output are load sensitivity (MMs are very critical, MCs far less so) and tradeoffs between current noise and voltage noise and how that affects the actual S/N with an actual cartridge attached.

The best choice for low-output moving coil is a step-up transformer (S.U.T.) used in combination with a dedicated moving magnet sensitivity phono preamp -
I would agree, but that just transfers the problem of optimal loading to the transformer-preamp interface. That's almost never done right, but it can be if the designer isn't lazy. The complication is that the transformer secondary loading has to be adjusted according to the cartridge source resistance.
 
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#17
Cambridge Audio have a reputation for making good value well engineered and solid performing equipment, the reputation seems to be well earned.
Good "value to performance" for sure. Not so sure about their durability. I owned their flagship units at the time. The 851A amp and 851C DAC/CD Player. Bought in 2015 (1600€ each) and issues started few month later. The 851A got VERY warm and sometimes suddenly turned off due to the heat... I wasn't able to listen to it at high volumes for more than an hour. Then the 851C showed serious problems. I started to notice a very audible noise in right chanel (both XLR and RCA) and it gone worse weeks after weeks. I wanted to use some warranty but neither Cambridge or PPL Audio (Cambridge retailer in France) accepted to fix this for free. And asked me 300€ to repair the output stage... I ended by sold both units at a broken price to a guy who was saying he could fix the issue himself.
If this is the kind a quality control (and support) they provide for their flagship units... Then I'm not going to buy anything from them anytime soon...
 

JJB70

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#19
To be honest I think you need more than a sample size of four reviews (one of which is actually very good) to form conclusions about a companies quality standards.

Oddly, the after sales support is precisely why I'd look favourably at Cambridge Audio as if I did have issues the shop is almost on my door step as these things go, and has a very good reputation for customer care.
 
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#20
Oddly, the after sales support is precisely why I'd look favourably at Cambridge Audio as if I did have issues the shop is almost on my door step as these things go, and has a very good reputation for customer care.
Maybe you can get some support from Cambridge once you leave in UK. Great. But it seems that "support" simply doesn't exist once the product is sold in the "so far away" country that France is... And this appart from such a poor QC on two 1600€ "flagship" units bought in two different stores. ;)
 
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