Hey guys, after reading up a few things about Isonoe regarding restoring Bozak mixers and their EQ's they make I got the hint these guys knew what they were doing and also specialise in electronics as well as their usual turntable isolation products.
So I sent them an email regarding possible modifications to the Urei1620LE
I found the reply rather interesting and thought of sharing it with you.
Although the LE is based around the original 1620; for what is, I assume, reasons of production, the PCB layout is substantially different. Because Harman companies exercise tight control over their service centres (many other OEMs are similar in this regard), I have found it impossible to get a schematic diagram… Without a schematic, you’re shooting in the dark. You can trace the board and make your own schematic, but this is likely to take days of work, and to be honest, not to mention potential errors that anyone who traces a board will experience.
I have extensive experience with American 1620s and have tried various modifications. However, the consensus so far has been that, if a client wants a more ‘boutique’ mixer, it makes more sense to buy a Bozak. There are 3 (not including the original D, which is different, but so rare it doesn’t bear discussion) types of Bozak circuitry – and you can interchange between transistors and various other options such as caps and I/O features freely. This means that the mixer can be customised exactly to a configuration that suits your ears – some people prefer the ‘hairier’ sound of the early Bozak, and some prefer the ‘sweeter’ sound of the later models – you can even have an interim version which isn’t as quiet as the last DLA-onwards version, but is just as smooth – for example.
With the US 1620, so far, my clients have kept them near-original (bar new caps and basic mods such as EQ bypass switches) and bought a Bozak to satisfy their ‘boutique’ tastes. I know many people who own both.
You can extensively modify a 1620… New op-amps, new caps, specialist resistors, etc – however, the end result won’t be a 1620! With a Bozak, you can mod it in many directions, but the end result tallies with something the factory would have put out, rather than a one-off to your taste.
IMHO, the vast amount of the 1620’s character (namely its ‘big bottom’) comes from its US-made Cinemag transformers. I would contact Cinemag and find out if the LE has original Cinemag, or replica transformers. If they’re replicas, I suspect fitting original UREi-spec Cinemags would provide benefits – they would also be very easy to fit. Cloning a transformer is near-impossible, and it’s a big reason why original seventies studio gear fetches crazy prices.
BTW – I always listen before using the analyser. I do a basic test to see if it’s working, then listen in an acoustically-treated room. Once I’ve listened I make notes of the observations. I then run the analyser and see where the test data correlates with what my ears have told me. If you analyse before listening, it can bias your ears.
Many thanks for the kind words! Don’t hesitate to get in contact if there’s anything you think I can assist with.