wow those hyperlinks to your pix are all highjacked according to Malwarebytes, IF by chance you still have the pix could you attach them? I'm about to crack mine for my first ever amplifier repair, used mine from '91 - 2011 bit somewhere after that the right channel died (probably from overuse/abuse) and I'm hoping to fix it myself instead of having a pro friend do it for me.Alrighty Mr. chingon, (and anyone else who might be curious...)
Here are some internal pics of the RA-100 mods.
Back Panel modified for banana jack outputs, eliminate the useless 1/4 inch o/p jacks, attenuator pots moved to the back to shorten the wires and make for a simplified front panel, yet to be completed.
Back Panel Inside View shows the fiberglass board that the bananas are mounted to.
Back Panel Inside View 2 shows the input jacks (the stock ones removed from the original connector PCB, reused because they are isolated from the chassis) and the input attenuator pots.
Channel PCB Right Side shows one modded rail cap and the rewound output inductor.
Channel PCB center shows the larger electro cap on the bias (dead center near the heatsink), the modded compensation caps on either side. In the foreground you can kinda see one of the two film input caps.
Channel PCB Left Side shows the 2nd rail cap that was replaced and both film input caps. The 2nd one is peeking out from underneath!
Channel PCB Bottom View reveals the location of the film bypass on the bias electro and the second input cap. This photo is actually the PCB of the opposite side of the amp than what is shown for all the other PCB pics. This explains the different orientation of this input cap.
I have looked for the original test data and graphs, but so far have been unable to locate it amongst my "piling" system.
Any questions? :thumb:
Thanks for the reply, JR. It's good advice for others who might attempt this.In general I would advise against blindly removing protection circuitry, that said it could be educational to study the protection circuitry to fully understand it, then review if you find any errors.
Power amp protection circuits are generally current limiting to protect power transistors from short circuits. More advanced protection schemes sense for voltage and current to protect against secondary breakdown. Thermal protections are usually breakers attached to the heat sink that open up to prevent overheating.
Recording engineer friends of mine have expressed their loathing of the RA-100's sonic qualities. The combo of Alesis Monitor Ones and RA-100 power amp were everywhere in project studios in the early 90's. I like the sound of my Monitor Ones, although my main monitors are passive Event 20/20's. Montor ones got a bad rep likely to do with the poor-sounding amp they were paired with.I don't recall the RA100 or the D60 being very highly regarded by audiophiles but they were popular for small studios because they didn't have noisy fans.
It doesn't require a deep dive... if you can hear the protection circuitry in normal operation it is a poor design, or problematic load.I would love for someone to do a deep dive into why removing that protection circuitry causes such an improvement in sonic detail. I suspect that somewhere in there, audio transients are getting blunted or slew rate is getting messed up. I'm not the person to do it. Now that it works, I'm only peripherally interested, not practically.
not rocket science but requires a basic understanding of discrete transistor circuits.If taking off the protection circuit causes trouble down the road, I will report it immediately. Ideally, this protection circuit would itself be modded to still provide protection, but not mess up the sound, but again, I'm not the person to do that.
I am not familiar with those speakers. It might be useful to look at their impedance plot vs frequency.Recording engineer friends of mine have expressed their loathing of the RA-100's sonic qualities. The combo of Alesis Monitor Ones and RA-100 power amp were everywhere in project studios in the early 90's. I like the sound of my Monitor Ones, although my main monitors are passive Event 20/20's. Montor ones got a bad rep likely to do with the poor-sounding amp they were paired with.
Worst case for heat dissipation in class A/B amplifiers is 1/3 power, full power is easier on heatsinks. Heatsinks running too cool could suggest under biased class A current (typical amplifiers use about 15-20 mA of class A current at idle). Under biased amps can suffer from crossover distortion during low level HF signals.Yes, the RA-100 has HUMONGOUS heat sinks! It looks like one big heatsink from the outside. No matter how hard I've driven them, they never get more than barely warm to the touch. Maybe some of that was due to the clamping circuit, but it's still true post-mod with 100+ watts of sine wave going into a 4 ohm dummy load.
fixing some old soldiers is fun and rewarding.My Crowns are both radio station rescues. I also have a D150. Yes, very popular in broadcast production rooms. I also have a great-sounding Symetrix A220 from a radio station. These little 20 and 30W amps can drive efficient nearfield monitors like my Events into painfully loud territory. They all had various issues when I got them, but were not that difficult to repair. The D150 had about 1/8" of dust and general crud caked on its circuit board! Yuck.
I'll go with "poor design," then, in this case.It doesn't require a deep dive... if you can hear the protection circuitry in normal operation it is a poor design, or problematic load.
I'm a non-subscriber. An amplifier can have things like tone shaping circuits that induce phase shifts, poor power supply design, etc. I know that some add "properly designed" to that statement which turns it into a "no true Scotsman" fallacy.A general statement (that not all subscribe to), all amplifiers sound pretty much alike when operated in their linear region. Sonic differences are caused by nonlinear behaviors like current limiting. Slew rate limiting is not a protection consideration, but a measure of how fast the amplifier is. An audible dynamic distortion would most likely be current limiting occurring too soon.
Not opposing or arguing with anything you've said! To the contrary, I agree and appreciate your insight and clarification. I prefer objective measurements too. I just don't have the equipment or the understanding of solid state electronics to figure out why this particular amp sounded bad before I cut the short protection circuit out. It sounds amazing now, and that's what counts (to me).sorry I'm an objective guy... I can measure things I hear, I can even measure things I can't hear... I don't hear things I can't measure.
Thanks for throwing in logical fallacies, I prefer objective measurements.
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