IC and cap upgrades in IC150?

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#1 dsrance

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Posted 12 October 2011 - 04:38 AM

Sorry to bother you all again, but since I may have the IC150 volume pot replacement issue under control (with luck), I'd like to go further, as apparently there are genuine gains to be made with updating the IC's and replacing the tant and electrolytics inside.

I've trawled the net regarding updates for this preamp and haven't really come up with anything definitive. On the capacitor side, I understand the 1uF tants on the main board and the 4.7uF beads on the phono board could be changed now, but what of the electrolytics, some of which are odd values in today's market? For the power supply, I've found some 1000uF long life ones which may be suitable, but what of the 5uF and 25uF ones?

Next question - the IC's themselves. I've read much about OPA604's, LM833N's? and the LME49710N, most or all of which claiming to offer lower noise and distortion, as well as superior sonics. The IC's fitted seem to be 3532's and not LM301's.

The serial number of this preamp sample is IC 6684, so a middle production period one? I'm only making a fuss about it because it has been inherited, I sold these when current (and have some loyalty to them) and in the secondary system I'd like to use it in, the facilities (including the tone controls and "loudness feature") would be useful. The "upgrades" suggested above would hopefully help low level listening, as it seems that this was the preamps main weakness (few it seems followed instructions to back off the power amp gain and run the preamp quite hard).

PLEASE can someone take pity on me and suggest which IC's I could try, whether they'd simply "drop in" the sockets with little to no other changes/mods to the circuit, and also possible value and type replacements for the caps (I have some WIMA 1uF to replace the main board tants if they're ok to use and have seen some bead-electrolytics from RS-Online at 4.7uF that may be good for the phono board). Perhaps Crown's service dept still have the capacitor update kits?????

Thanks in advance for any assistance smile.gif


#2 dsrance

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Posted 14 November 2011 - 06:19 AM

A shame that no-one could help here, but just in case anyone lurks here and wishes to have a go for themselves, AT THEIR OWN RISK, I've found that the OPA134 IC is a straight drop-in replacement, doesn't have a race-tuned bandwidth so hopefully shouldn't cause any instability. The +/- 18V rail voltage of the IC150 is at the top of the range for this chip, so since it can be reduced a little, this might be a good idea? I've checked the rails on mine and left them at 18V each side for now. I was able to find a "bitser" which has donated a perfect volume pot and better top and bottom covers too smile.gif

I know sonics are a tricky thing and fraught with opinion, rather than cold hard facts, but my impression is that the above chips offer a far better sound at very low volume levels (9 o'clock rather than the recommended 2 - 3 o'clock setting) and my impressions are of a better reproduction of reverb and atmosphere in the recordings played through it. I have played around with the 1uF and 25uF coupling-cap replacement in the audio path but the difference, if anything, wasn't really worth the aggravation compared to the fundamental sonic refinement made by the IC change above.

Hope it's ok to post this, as I know that engineers prefer cold hard facts, rather than opinions and "feelings" on these sort of things. This product has been long discontinued though (1976?) and in my view, to get such a wonderful all-round performance from it for such a small amount of money was worth every penny..