Rotel RB-1050 Mods

1210

Member
2015-06-24 5:09 pm
I'm not qualified for this, so take my advice with a grain of salt:

You could balance the DC load on the input transistor pairs by reducing R603/R604 from 33K to 12K to match R651/R652 and up R605/R606 from 470 to 560 ohms to match R613/R614. This might give less DC offset at the outputs and lowers the gain from 70x to 21x.
You can definitly change C601/C602 for a fancy MKP or at least an MKT/MKS of your choice. Anything above 4.7µF should work good enough.
You could lower C603/604 down to ~150pF styroflex or polypropylene foils to extend the upper frequency response above the audible range and solder them parallel over R603/R604 instead. This might help with RF ingress after the volume knobs.
You can replace the main power supply caps C909-C912 with the biggest caps you can fit in their place. Voltage rating may go up, but not down. Depending on how much bigger you make them, you might consider upgrading the rectifiers D901/D902 to >25A models, just to stay safe.
You could put in low-leakage or bipolar C607/C608 and bypass those with a small foil cap (1nF-1µF) of your choice.

If you want to dive further into modding, you can match the input pairs Q601-Q604 and Q607-Q601 or at least thermally couple them (e.g. by glueing them together).

HiFi-fuses on the mains will not change anything other than the contents of your wallet. Changing F601-F604 might improve damping factor somewhat, but I would advise against it, since it has a really bad bang-for-the-buck-factor. Unless you believe in the sound of speaker cables and are a voodoo-priest, that is...
If you just want to throw fancy and expensive parts in, I'd rather change the volume control potentiometers VR001/VR002 for stepped attenuators (for 25K, logarithmic) or put in a buffer, instead. If you use a pre-amp, you might even leave them out completely, always running maximum gain on the RB-1050 and control the volume with the pre-amp.

Hope I could help. Maybe someone more qualified could chime in and comment on my ideas...
Here's the link to the schematics I used.
 

1210

Member
2015-06-24 5:09 pm
Sorry for posting again, but the forum only allows edits up to 30min after posting...

I can't recommend reading up on this thread enough. Depending on how hot your part of the US gets and keeping a very close eye/hand on the heatsink-temperatures, you might adjust the bias to 5mV at the test points (instead of the 4mV described in the manual) to stay more in Class-A at listening levels. But this also raises any main-hum at idle and might cause damage to the amplifier when driven at higher output levels (while the measured benefit is miniscule)...

If you go through the hassle of replacing resistors, use metal film types as replacements. If you believe in resistor-sound beyond this, the places to put in fancy ones would be the feedback/input ones R651/R652, R613/R614, and R601-R606.
Also, if you decide to switch out the potentiometers for stepped attenuators with discrete resistors, you might put a jumper in place of R601/R602.
If you want to get creative and improve the power supply more than other fuses do, you might want to put in foil-capacitors (63V rating or better and 100nF or bigger) after the fuses to ground or bypass the big power supply caps with smaller foil types. Solid-polymer types should also work great here, if you can find them in a high enough voltage rating.
Having +-48V rails, I'd replace the main caps C901-904 with 63V ones, if you can fit them - just to stay on the safe side, should your mains voltage rise above 119,7V for whatever reason...wich might already be the case... 50V main caps only operate within spec up to a little more than 4% over-voltage on the mains.

edit: don't forget to at least clean the speaker relais. You could also substitute suitable better ones in hopes of a better damping factor (e.g. Omron G2R series), as the ones currently in there are only rated for 3 amps...

Greetings from another ape-with-soldering-iron and welcome to Rotel-modland :cheers:
 
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1210

Member
2015-06-24 5:09 pm
Sorry to bump this thread up once again...but another error crept in after editing time was over: the main supply reservoir caps I was talking about are of course C909-C912, not the small C901-C908s around the rectifiers (you can/should keep those in, if possible).
Also, looking at the datasheets for the µPC1237HA that's driving the speaker relais, you might even use something like this relais and adjust R905 down to about 750ohms in case the relais doesn't close, but make shure to use one that's at least a 1W-type as the substitute. 2W wouldn't hurt here and it can be a wirewound type resistor. This will cost about the same as a single "HiFi-fuse", but would raise the 3A limit that the relais puts on your output current to 10A. A worthwhile mod, if you plan on using more than 36W on 4 ohm speakers while staying in spec of the relais connecting the amplifier to the speakers...
If you still want to change fuses for the sake of changing fuses, go with the ceramic body type as those have a breaking capacity of up to 1500A instead of the 35A of the glass ones. On the primary side this might even make a lot of sense...in any case I'd rather have a proper rated fuse than an "audiophile" one without approvals that protects my gear...the saved money can always be put into fancier input capacitors (here C601/C602) or resistors (see previous post) to get the same approval from the voodoo-people or, better yet, bigger main capacitors to woo both believers and engineers. Everyone is a fan of big, heavy cans ;)