Hello, bit late to this party but, I have a untouched 909 I would like to service. I think I can handle to power supply caps but, would like to bypass the Quadlink board as I would never use it and heard this will improve the sound. Can anyone tell me exactly how to do this and if anyone has an answer to gilwe question re the LED, please?
I bought Mundorf boutique caps and noticed a big improvement, if it was the caps themselves or the fact that the old ones might have been tired (been there for 15 years).
I have also removed the Quadlink board, don't know if removing an opamp did make much difference. IIRC I took the LED supply from the main board but it needs another resistor for a suitable voltage drop. Maybe Dada has a note about it as they recommend to remove the board.
I have also removed the Quadlink board, don't know if removing an opamp did make much difference. IIRC I took the LED supply from the main board but it needs another resistor for a suitable voltage drop. Maybe Dada has a note about it as they recommend to remove the board.
If you are going to play with the 909 / 707 out of anything other than necessity then there is some logic in targeting the ‘Quadlink’ circuitry - because the choice of OP275s does produce relatively high levels of distortion at higher audio frequencies (source - Doug Self’s Small Signal Audio design book). The balanced inputs via Quadlink will also be noisier than the unbalanced inputs. I’m always quite intrigued about what people mean when they say ‘bypass’ it. As both the unbalanced and balanced inputs are all driven through the OP275s and associated control circuitry. A direct rewire of the phono plugs into the input of the power amp PCB is perhaps what is meant - this stage gain is set for a 0.5V input for full power in the old Quad way, so any direct feed that isn’t from an old Quad preamp source or similar is going to require sensitivity on the volume control 
Hello Dave S, thank you for your input. That is interesting to note and probably will save me the bother of figuring out how to bypass the Quadlink. Especially as I find the 909 (as it is now), a bit heavy on the bass. So, for me its down to power supply caps. Not opened it up yet to see what kind of state they are in. I can report that the 909 runs rather hotter than I would expect (too hot to hold your hand palm down on the top).
Might be these, but you must check pins and sizes: https://www.hificollective.co.uk/components/mundorf_mlytic_ag.html
No need, there are several predrilled holes on the PCB for different pin configurations.
Write about your impressions when the job is done
I chose these because of the 125 C spec, not because of "audio grade" but they made a significant difference and surprised me
The biggest improvement that can be made to the 909 is to replace the input coupling cap (C2) with a polypropylene and to replace C7/C9/C11 with something like Nichicon FG or KZ. Doing this will FAR exceed any benefit from bypassing the Quadlink board, but there is no harm in bypassing the Quadlink board.
Unless the original caps were electrolytics or paper ones, they surely didn't age or get tired. Your »big improvement« almost certainly was expectation biased.
Best regards!
The 4 main capacitors are indeed electrolytic, and after 12 years of use you may notice some difference. But what kind of difference are you looking for? Replacing old caps with new ones can certainly make a change, but also so can the brand of capacitor you use. Different brands of capacitors have different sonic characteristics. Assuming your 909 was made in China (which most of them were) they used Capxon electrolytic capacitors which have a relatively neutral sonic character. Both BHC and Mundorf are good replacements and probably more reliable, but the sonic signature of these is slightly more laid back. Mundorf I feel is overpriced and very similar in sound to the BHC, and BHC are also a top quality capacitor, so either BHC or Mundorf is a god replacement. Quad used MKT for C2 which is cheap and crappy, and switching to Polypropylene here will help with crispness and dynamics, as will changing C7/C9/C11. The 909 is rather smooth and laid back sounding with recessed highs, and following the recommendations I have laid out should liven the amp up a bit.
I replaced the big PSU elcos. I had no expectations as I wrote, I just changed them because they were 15 years old (and very close to the tolerance limit) and 125 C spec. Maybe the BHC had made the same improvement. FWIW my 909 is made in the UK and also seriously modded (not by me) and has allegedly the lowest distorsion of all 909s
As for C2 I doubt a MKP film cap would make such a big difference. Also difficult to find low voltages.