Modding a Kenwood L 1000 M

[Crossy]

Good morning to all,
I recently obtained an interesting PowerAmp - Kenwood L-1000 M - manufactured around 1990. Concurrently I was able to locate a service manual as well. I got the Amp since it is a member of Kenwood's famed "L-Series" which was manufactured by Kensonic Labs (also known as Accuphase) for sister company Kenwood.

http://www.thevintageknob.org/KENWOO...0M/L1000M.html

Specs are as follows:
http://www.thevintageknob.org/KENWOO...00M-block.html

2x150W @8Ohm - 2x250W @4Ohm
Damping factor : 130
THD 0.006% (20Hz-20KhZ, 130W, 8Ohm)
THD 0.0004% (1kHz, 130W, 8 Ohm)
Frequency response 5Hz-100kHz +0dB -3dB
Signal/Noise Ratio : 115dB (Unbalanced), 125dB (Balanced)

What I'm looking for are ideas how to modify this nice power amplifier to make it even better. It has excellent specs but due to some reason, it didn't receive top grades in German higher end audio journals. (Like Stereoplay, Audio and Hifi-Vision). Maybe because it was too cheap for their preference ? I think the amp originally costed around $3000, that's way below the $10K many good Accuphase poweramps sell for.

It also is full of high quality components. Maybe there might be some "easy tweaks" to unlock the potential of this design, which shouldn't be far too different from Accuphase's own.
http://www.thevintageknob.org/KENWOO...00M-edwin.html

The design has some oddities / goodies
1) Full symmetrical layout
2) 2x 270VA transformers
3) 47000µF caps in the power unit
4) Point to point wiring

Aspects that I found questionable (starting points to modding ?)
1) Variable resistors (potis) in the input path, one per channel - is this necessary ? does it introduce noise

2) Maybe try to get caps to tighter tolerances - replace elkos with regular caps where possible

3) Get metal film resistors of smallest tolerances (Holco 0.5%) to replace resistors in audio path, at the extreme: use bulk foil types

4) The "input" board - uses 2 Opamps : 2x Dual Opamp NJM4580 from JRC / New Japan Radio. While these are ok and relatively low noise, there is still room for improvement in noise and especially slew rate (Contemplating AD797 for example)

5) There's a switch from XLR to Chinch Input at the back of the Amp. Well, it would make sense to "migrate" the switch to the front and feed it from 2 seperate preamps: one analog and one digital "cross connect" (with highest quality DA/AD) for example..

Just to give the full picture about my endeavour, it wouldn't be me who would do the modding, a german modding shop would carry out the project. However I'd like to be able to provide useful inputs to them, ultimately in the form of a "checklist" and for me as a a "project supervision" tool. Being familiar with most concepts, I know more than basics in electronics, however I've been at odds with the soldering iron whenever I tried it <g>

any comments and ideas welcome. Even more so because I know this is a rather "strange" and "exotic" amp - manufactured by a company with more than a good reputation..

best rgrds from Vienna/Austria
CROSSY

[mikeks]

I would love a softcopy of this pre/power amp's service manual or schematic please; send me mail. Cheers.

[Crossy]

Mikeks,
a block schematic is here..
http://www.thevintageknob.org/KENWOO...00M-block.html

The service manual ist bigger than A4.. scanning would be cut and paste - difficult to engineer.. If you email me your location, I might send you a photocopy by snail mail ...

rgrds
CROSSY

[mikeks]

Hi Crossy,

I have no means of e-mailing you, as you've not enabled your e-mail facility on the forum.

Send me an e-mail by left-clicking on my e-mail 'button' below, and you'll obtain my address from my response.

Cheers.

[wilhelm krejci]

got the same amp.succesfull with the mod? who has done it?

regards

[miklos]

Did you got the service manuals with tha amp?

[wilhelm krejci]

no,but i learned its similar to old accuphases

[morrosco42]

manuals for this amp and matching preamp in HiFi Engine | Download Free User/ Service Manuals, Amplifier, Receiver, CD, Tape, Tuner, Video

[his047]

This set was the subject of a series of modification articles in the Danish magazine "High Fidelity". The Power amp had only a few upgradable parts and mostly only for when you use it in unbalanced mode: 1) replace the balanced pots with better types or an input attenuator. 2) replace ICs with OP275. 3) Add capacitance to powersupply.

The Control amp is much more fun: there are an amazing number of coupling Capacitors which can all be bypassed. It gives an enormous improvement (and is free)!. In addition all the ICs can be replaced with OP275. Very worthwhile and lifts the unit into another class altogether.

[morrosco42]

Are the Op 275 &/or Ad797 fully pin compatible with the original NJM4580?
Are also compatible at power supply level?
In the web some people recommnd OPA 2132 and/or 2134 as better tha OP 275
Any info about?
Thanks in advance