Acurus L10 & LS11 Preamplifier performance upgrade kit
R1200S told me the past week that he had posted about the Acurus kits I was offering on Ebay so curiosity got the better of me and I thought I'd check out this thread.
Just a little info. The kits are designed to help those that may only know how to operate a soldering iron but do not understand how to select the parts they need. Probably half of the folks that contact me to do some upgrade work on their Acurus also ask if I could offer them a kit to recap their pre on their own. Due to a lack of free time I've had to stop doing work for others and I thought I'd offer a kit for people to be able to replace parts themselves. This kit is a basic performance kit, there are some other things that can be done to these pre's but the average guy with a soldering iron would probably get into trouble.
As far as what can be done to improve the Acurus pre's further. If you've ever popped the cover on the upper tier siblings the Aragon 18k or Aurum Pre's you'll get some ideas. The areas where Acurus cut corners are, lack of an active current source on the differential input in the Acurus where the Aragon 18k and Aurum got a proper current source on their diff input, the Aragon's also used the more expensive RN-60 style metal film resistors throughout, the Aragon power supply was massively overbuilt using a 30va transformer into a regulated supply in the IPS then fed to a CRC filter before going to the discrete regulators inside of the Aragon pre chassis. The Acurus in comparison uses a 5VA transformer feeding a pair of 3,300uF caps and no other filtering prior to the discrete regulators.
I have an RL11 on the bench and I've done a little work to it so far, primarily in the power supply, I have boosted the supply up with an external supply using a 10VA toroidal transformer and 2A soft recovery diodes and CRCRC prefiltering before the stock regulators. The only other mods inside of the pre are the same Nichicon capacitors and signal path resistors I included in the kits. I plan to eventually retrofit an active current source to the input diff pair and do a full swap of the resistors to the RN-60 style resistors.
I did a quick residual noise check using my old Option 1 equipped HP 339A which has a 100uV measuring range. Base residual noise on my HP 339A is 4.5uV with the meter input shorted with a 1k ohm plug. The residual noise from the modded RL-11 on my bench shows 16.5uV. These measurements were both made using the 80kHz LP filter on the HP339A to approximately simulate the AP 80kHz measuring range that laatsch55 is using. Laatsch55 I'm not sure what the residual of your AP is, but I'd bet being a much newer piece of gear its probably a bit lower than my 339A's 4.5uV. Some day maybe when the kids are all out of college and I've got some spare money I'll get an AP and move out of the stone age.....
Just an FYI for anyone measuring the Acurus or Aragon pre's, no shorting plugs are needed in the input RCA's. When you move the power knob to the "OFF" or "Mute" position you are effectively shorting out the input to the line amp. The knob positions "Off" and "Mute" really are the same thing on these pre's only Mute will leave the green power LED on, or turn it red on the RL11 and 28K. The line stage is always powered on as long as you have it plugged into the wall, so "Off" is not really off, its really "Mute" without the green LED.
