Luxman Integrated phono stage improvements

[Kei]

Spent an evening working on replacing all the electrolytic capacitors in my L-220 and noticing the improvement in sound (only minor, but a big reduction in audible noise) it got me thinking whether there were any improvements that i could make to the phono stage, like a better op-amp for instance. (i realise an external stage would be vastly better, but i would prefer to keep the internal one due to space and input limitations)

Phono Stage


The rest of the amp


I can post up schematics of the phono stage, the specs on the original NJM op-amp and component values if it helps. The cartridge i'm using with this is a shure M75ED T2 with jico SAS. (only other cart i'm likely to use is an AT30 MC via an AT630 transformer) The turntable i use is a Thorens TD160B with SME 3009S2 improved.

[Kei]

Found this thread that so far is the only pointer i have for upgrading the original NJM2043D op-amp.

Originally Posted by Chucko

The NJM2043 is roughly equivalent to a National LM833. Its strength is its OK bandwidth and noise, and a very low price.

For the phono stage, the AD712 seems like a poor choice because of its limited GBW and high voltage noise. Either the Burr-Brown OPA2228 or Linear Technology LT1126 should be a good choice. They're both low noise, decompensated bipolar op amps with good GBW and modest current demand. The LT1126 is faster but requires higher closed loop gains for stability.

The current setup with the 2043 i find to be slightly bright, but due to a lack of experience with swapping out op-amps i am unsure what is the best solution.

This is the phono stage section of the schematic.

[h_a]

i find to be slightly bright

I'm not surprised; the unit does not have switchable input capacitors (cartridge loading) but sticks with the ubiquitous 100pF. Depending on your phono leads that might not be enough.

see Shure docs ('optimum load'):
http://es.shure.com/idc/groups/publi...10%28oh%29.pdf

This cartridge appears to need 400-500pF while the amp comes only with 100pF (+another 100pF from the phono leads or so). Increasing this capacitance will remove the brightness as it dampens the high frequencies.

The first and biggest improvement would be to add sockets (IC-socket pins for example) to allow using the proper capacitance recommended for your cartridge. One way to achieve this is by unsoldering the 100pF caps and use the holes for the sockets. If you do not intend to use any other cartridge you can also simply replace the 100pF with the proper value of about 300pF.

Then you can think about other improvements.

Enjoy, Hannes

[sakis]

one thing might worth giving a try is changing the zener based power supply for the phono stage

[Kei]

Measured the cable capacitance and got 0.13nF, which I believe is 130pF. This was measuring at the phono plug with it disconnected from the amp and the cartridge.

[h_a]

Personally, I would not bother with fine details of the circuit at all; after modding it for switchable loading the next candidate would be the opamp itself.

Working on the fine details like the power supply is IMHO not really changing things. This is, after all, a very simple single opamp phono stage. Any decent opamp has in any case huge PSRR.

The NJM2043 is a bipolar opamp (stable for gains greater 20dB) with 14MHz and 6V/us slew rate, running at 18V rails in a dual package. I would try a OPA2134 (check pins!). You can also read the Analog addicts phono stage and the comments on suitable opamps there, it is roughly similar in concept.

The Analogue Addicts Phono Preamplifier 2006 Edition

Hannes

EDIT:

I believe is 130pF

100pF/m is a common value, so this should be right.

[Kei]

So i need to replace c101a/b with approximately 300pF for it to suit the shure. Should replace these with polystyrene like the existing ones or something like silver mica? (which seem to be easier to get hold of)

The pin config on the OPA2134 seems to match the NJM2043 in DIP-8 form.

[Hypnotoady]

I recapped my Luxman L-430 and it had the same lowly JRC op amp, I removed it and put in a socket, I tried a OPA2134 and found it to be a huge improvement.

I don't use that amp anymore though as I have gone to separates, but it's a worthwhile upgrade.

[Kei]

The L-430 should have an NJM4556 which is similar. Hoping to sort the opamp out soon. In the meantime i've been advised that I might get away with removing c104a/b and replacing the gaps with jumpers with the small possibility of cartridge dc leakage. Also swapping C103a/b for film replacements may gain a small improvement. (That is if i can find any in 22uF value that will fit, they are elna silmic II at the moment)

I've also been looking into swapping all the original carbon resistors for metal film. (the whole amp) Is there any disadvantage to using 0.5W over the original 0.25W and fitting them like this... (0.25W takman metal films cost nearly twice as much as 0.5W holco)

[Mooly]

I'd go along with trying an OPA2134 and maybe an OPA2604 too.

Those resistors look like metal oxide to me. I don't really think you would gain much swapping all the resistors... I think if you believe it will sound better then it will.