ROTEL RB-981 Modification

Phase 1 - changing all electrolytes (Aug. 2020) 

The 981 series was built with a very solid foundation, considering the components alone. Just look at the toroidal transformer rated at 1kW, a rectifier bridge for each channel, the BHC slit foil filter capacitors, the Rubycon Black Gates, the Sanken power transistors and so on...add the conservatively rated output power of 2x 130W/8 ohms and you get a machine that can handle just about any load.    

There is a drawback however. The amp is old enough to remember the millennium, which means that some of the parts might not have the same parameters as they had when they were new. These parts i am referring to, are the electrolytic capacitors. During their lifetime, they tend to  lose the capacity, increase their ESR, as well as the current leakage. This deterioration depends on the working hours and operating conditions, under which they develop unwanted behavior of a circuit they are used in. While it is very common to use the electrolytes in the power supply circuit, Rotel used them also as the input coupling caps, where i believe they should be omitted.  

The first two images below show the original circuit board of the 981.         

Fortunately, there are not many electrolytes and the most expensive, were those large power filter caps. In this case, i stayed with the same values (10000μF/63V) with the slit foil technology and replaced the BHCs with Supertech caps. (80 Euro for the Supertechs, order from the U.K.). 

It is very unfortunate though, that the design comes with four caps in total. It would be very nice to see at least 30000μF, or better 40000μF/channel. On the other hand, there is an unused place, next to the transformer on the right side, where it is possible to place a second PCB with the extra power cans and run a solid core wire to hook them up. Although this could be a reasonable solution, it also brings into the circuit an additional resistance of the wire, as well as the susceptibility to pick-up unwanted high frequency noise (if the chassis is not RFI/EMI immune). 

While increasing the power filter capacitance is good in terms of minimizing the peak to peak ripple voltage, we have to consider also the generated stress by the charging caps onto the bridge rectifiers. The more caps we use, the more stress is generated during the charging time, since the caps act as a short to the circuit (although in milliseconds, but still). Now you see why there is the 1kW toroidal transformer. This thing can handle a lot of abuse while preventing any significant voltage drop. No surprise, there are people that recognize the design advantages and upgrade the components of this well engineered power amp. For the piece of my mind, I also changed the rectifier bridges with a little more “powerful”, but same – soft recovery types. 

The highly acclaimed Rubycons, were swapped with the ELNA Silmic II caps and the input coupling Rubies were changed to Audyn MKP Plus series polypropylene white beauties. 

See below:

Now, somebody familiar with the design topology might be asking, why the new input coupling Audyn cap has lower capacitance in comparison with the original Rubycon (3,3μF vs. 4,7μF). The input coupling capacitor connected in series, acts like a high pass filter.  Because the input impedance of this amp is 33k ohms, with the 3,3μF capacitance we get the -3db rolloff at approx. 1,5Hz. Plenty of reserve.

The total recap cost reached 140 Euro. Considering the purchase price of this unit (300 Euro in 2018), it seems to be a lot. However, i can honestly say, that for this amp, it is well worth it. This piece of equipment deserves much more, no doubt about it. This is definitely a keeper that will perform another 20+ years regardless of speakers being used. 

When i first played a CD (Sabaton) after the recap changes, i noticed the high frequencies being more open and airy. After several hours of playing i felt, that the whole sound became more fluid, as if there is nothing to hold it from being released and there are no barriers in the signal path. I realized that this amp is more transparent than before and it became obvious, that the only thing that is holding it on a leash, is the preamp (at that time, i used the Creek EVO). So, for the recap upgrade i can only say, that it was a success and I believe, that the greatest sonic benefit has been made by the input Audyn caps. Just grab one 981 piece, try and hear it for yourself.       

Phase 2 - chassis vibration control + connectors upgrade - certainly one day, because this area, needs it urgently.