Cambridge Audio SR10 Receiver (sleeper amp mods)

[John3655]

Cambridge Audio SR10, New Capacitors.



The SR10 shows a lot of promise, tons, its brilliant as it is, but you know a few better capacitors might make it even better.

Pros,
Very inexpensive, rumour has it that these were being sold off at £150 back in 2015. Originally £230.
Powerful at 85 watts.
Low flux torroidal transformer.
Good, op amps NJM2068D
As a receiver it has a dab tuner built in.
Solid state volume and tone controls, so no noisy potentiometers.
The heat sink is a work of art (just need to get Tate approval), really chunky.
Second hand available at about ~£50.
Also SR20 model at 100 watts if you need more.
Remote controls still available new for £15. (SR20rc does operate the SR10 for volume)

Cons,
Has a fan but it's quiet and only comes on when really really being pushed. I could only get the heat sink temperature up to 36c and it doesn't even come on until 40-50c.
No source direct but also no potentially inductive potentiometers.

Tweeking
Well the signal goes through two op amps with generic caps for coupling, can be improved.
The op amps and power stages have generic caps, can be improved.
Note the below is just suggestions for the technically able at your own risk.

Service manual available from Cambridge Audio, they were great.

Parts list, this covers the capacitors in the signal path excluding Phono, Sub and Record out and some of the main caps in the power amp stage.
For the coupling caps, the circuit diagram specifies 10uf/16v, but what was fitted was 10uf/25v. I went with 10uf/25v but might be worth trying 22uf.
IC2 Power
C37, C38 100uf/16v
Signal
C39,C40 10/25

IC3 Selector
C35,C36 47/25

IC4
Power C26,41 47/16
Signal C43,44 10/25

IC7 C128,129 10/25 Signal
Power C126,127 100/16

C55,72 10/25 Signal into power amp, there are two caps with opposing polarity making a bipolar going into the main power amp stage. C128,55 and C129,72.

C62,83 47/50 Amp Power

C61,80 220/16 PSU

C65, C86 Non Polar 4.7uf 50v (eg Nichicon ES) Central to the power amp, possibly feedback idk?

C108 220/16 PSU

C100,101 220/25 preamp PSU
----
Pick your favorite caps! (I used Elna Silmic2 on signal and Nichicon KZ and FG on power)



Couple of low ripple Rubycon 100uf caps on the mid right, difficult to see bypassing the main reservoir caps (improvement for any amp). Make sure the big caps are safely discharged before doing anything.
Notice short paths of thick, tinned tracks as they go to the outputs. Circuit board has labelling both sides which helps a lot, it's good quality only a slight lift on one solder pad which was my fault entirely.


Results, well way beyond anything really after 5 hours of run in, the noise floor is far lower than stock and can really hear texture in vocals and hi hats, sounds wide open.

Case Disassembly to get access to the circuit board:-

There are three screws on each side underneath and a few at the back to release the case top.
Then undo the screws securing the speaker binding posts and the rca inputs, to the back
undo the two screws holding the iec board to the bottom.
Unplug the tuner silver box, ribbon cable.
Undo the back panel and move it alongside with iec to transformer cables still attached.
Undo the screws at the front edge of the pcb.
Undo the six (3x2) screws that hold the heatsink down to the bottom panel.
At this point the pcb should be lift-up-able for soldering with heatsink still attached.

Further steps,
No need for me right now but the ceramic bypass caps on all the power supplies could possibly be improved.
Could go c0g if they're not already, c0g is the best dielectric for ceramic caps having no microphony, or film.

 

[John3655]

Cambridge Audio SR10 receiver further modifications.

Couple of caps I missed last time, C23 and C24 important here in the conversion of the +-15v supply to +-7v for the selector and tone chips. Replace with better.


Couple of nice Nichicon low ESR pm/pw after the 15v regulators for the pre amp. Increased from 220 to 330uf, C100,101. (Subsequently thinking go larger)
Love changes like this as there's a lot downstream that might be improved.


Not sure about the value of this change as it's a fiddly job for potentially no or little improvement, it could be worse, but replaced the single layer feedback ceramics on the op amps for TDK mlcc c0gs(small blue caps either side of the opamp chips). C0g is the best dielectric available for ceramics as it is non microphonic and also has a linear capacity to voltage ratio. Better suggestions welcome. Ceramics often sound scratchy for a time as they settle and recover from the heat of soldering.


Pair of Panasonic FMs pre regulator c95,c96.


Overall this amp is revealing things in familiar recordings that I have never heard before. Listening to Sheryl Crows, Live at Budokan, audience noises are clear, can even hear slight murmurs of amusement that ripple through the crowd that would be buried in noise previously.

{This amp the SR10, is the predecessor of the current axr85.}

 

[Mike_S]

Good work, have you thought about replacing all the normal resistors in the sound path with metal film? That will lower the noise floor, and make the highs more crisp.

 

[John3655]

Thanks Mike, interesting, there's a few candidates, 100, 1k, 1.5k and 820 ohms all 1/8th w 5%.

 

[John3655]

The TDK feedback capacitors fitted last week, seemed to improve every day last week.

Today the resistors that Mike kindly suggested arrived. Just Vishay 1 & 2 watt metal film, they dwarf the stock 1/8 watt ones, plus some prp ones that were in the parts bin. Fitted into the signal or feedback path. I wasn't expecting that much but definitely worthwhile, the noise floor is somewhere in Australia I reckon and the singers are in the room!

 

[John3655]

100nf c0g mlcc between the power supply pins 4 and 8 of ic2 and 7, the main opamps , more detail. Some debate in audio circles about whether to remove the ceramics on the opamp electrolytics but for now I haven't.
Also removed the 100uf Nichicon KZs on the power and replaced with 220uf Nichicon PW just because they're a bigger capacity, more dynamic bigger sound, try it.

 

[John3655]

Put some ferrite torroids on the resistors feeding the opamps hopefully to reduce the level of Radio Frequency Interference reaching the opamps. The resistors are slightly better than stock too so there should be less noise of resistance.



Post your SR10/20 experiences here...

 

[John3655]

Three months on, the binding posts are ferrous! Here they are with a magnet attached. The issue is that ferrous materials in the signal path will dull/distort the sound.



The curved thing stuck to the posts is a very strong magnet from an old disk drive. The metal risers are not magnetic just the screw in post and nut, so if a banana plug is inserted the signal has to go through the ferrous bit, but if a cable is inserted it can make contact with the non ferrous bit but it's clamped by the ferrous nut so I don't know. In any case given my experiences of the dramatic change removing ferrous in speakers, they have to go... Tbc

Green highlighted non ferrous, red highlighted ferrous stuck to the magnet.

 

[John3655]

Installed new binding posts today, when they first arrived from a hifi supplier the first thing I did was apply a magnet and they stuck, damn. Luckily it was just the securing ring nut that was magnetic not the body. Bit of a pain but ok can ditch the nut and glue them in. Had to resort to mounting them in wood and epoxy.




The grub screw that clamps the cable was weakly magnetic but just have to live with that as the thermal mass of the posts are too high for soldering.

The change is obvious, not vast but worthwhile, the amp is playing at surprising high volume levels but it doesn't feel like it, my senses are not telling me to turn it down which indicates less distortion. It's incredibly fast and hard hitting. There is a lower noise floor and greater separation, far improved ringing sounds, percussion and snare, even a very sharp sibilance on S sounds which isn't so good but it's revealed.

Why is there so much ferrous in the signal paths of equipment (amps and speakers) well I was looking for solder tags, copper/brass ones (non ferrous) were several times more expensive than steel (ferrous) and of course it's all invisible to the consumer, the cheaper component wins.

 

[John3655]

Something I missed, C115 is a 12v supply to the early stages of the power amp so makes a huge difference, replace with a better audio grade capacitor and bypass with a 100nf film cap or similar. It's near the centre of the power stage.


Also I initially bypassed the main reservoir capacitors with small electrolytics, it's worthwhile changing those for films eg 330nf >=63v, or adding them. Either or both are good but possibly the films have the greatest benefit. Maybe it's a sad reflection on the quality of the mains here but it's amazing how good amplifiers become with a few film caps on the various power supplies.

Adding 100nf films on the 15v supply is also worthwhile.

Bias adjustment.
There's nothing in the v1 service guide about bias, just the presence of a variable resistor vr1&2 on each channel. Measuring the mv at the speaker terminals one was 26mv the other 50mv but adjusting the variable resistors did not change that, so carefully put them back to the original position. However measuring the voltage across the big white ceramic resistors r140 and r139 the variable resistors do change the reading. Both were about 26mv so adjusted them down to 8mv about a 1/8th turn. Really flying blind here, just assuming less across the resistor bridge is better and I think it's sounding better certainly goes very loud easily but both are now at 50mv at the speaker terminals. After the resistor bridge there's a 22k ohm per side bridge between channels(!) and the connections to the speakers are strange in that one side is connected to the positive terminal and the other to the negative. Something clever going on maybe with rail balancing or something. Theres some circuitry on the right doing something to the mid point with some large capacitors that are next for replacement. I further adjusted the bias down to 2mv but wasn't happy with it so backed off to 5mv. If anyone knows what's going on with this design please post.

 

[John3655]

Found that a polymer cap works really well for C115, mentioned above, for clarity and detail. It works so well for this amp I'd mark it as the single standout change for transforming this amp. Any better cap here will improve matters but the polymer improves even on the Nichicon ukz and film I had initially tried.

R90 the big grey 3 watt resistor in the middle of the shot is constantly feeding c115 the silver can polymer cap to the right from the main supply, which builds up a reserve for all the transistors in the initial stages of the power amp. Polymer electrolytic capacitors have very low equivalent series resistance and very good response across frequencies so absorb ripple and can provide power quickly and easily. Something like that anyway. As always give it an hour or two to run in.




Polymer capacitors are now available up to 63 volts and beyond and in the capacities often seen in mid range amps eg 330uf and 470uf albeit with SMT legs but just need legs tacked on in thru hole situations. Expensive though often ~£2 each for high values. But maybe that works out comparable to an electrolytic and film capacitor bypass.