Alesis RA-100 Amp Measurements and Mods

[Andybot]

Old thread i know but did anyone ever track down a schematic
for the ra100 amp. I am looking for one.
Thx

 

[jensenmann]

I thought the schematic is in the manual, isn´t it?

 

[Andybot]

No, not in my manual. This is a tricky schematic to find,
its kept close to the chest for some reason.
Thx

 

[Andybot]

Hi,
im interested in trying some of the mods outlined above but i have no
objective way of testing my results after except perhaps leaving one channel
untouched and seeing which i prefer.

Id would like to try anything that makes the amps response flatter and anything that i could do to the power supply that may need doing due to its age.

I dont hate the amp but im sure it can be improved.

Any suggestions as to how i could proceed.

Thx

 

[smallbutfine]

RA-100:
Finally, somebody traced the RA-100, so get schem and instructions for a guitar amp mod here_

http://www.kylheku.com/~kaz/ra100/ra100-mod.html

Best regards,
Martin

 

[tchgtr]

Look what the cat dragged back from the archives.
I'm about to attempt some of these upgrades, and have a few questions. Don't even know if the OP is around these parts these days, but here goes:

The two compensation caps (C202, 204) on the one I have seem to be 100pF ceramics. Is there something I'm missing? Am I mistaking a cap that looks like a ceramic for something else, or maybe they changed the value of the caps in later models?

Is it possible to just remove the core from the inductors, instead of winding new ones?

Why two 1uf caps on the input (C215) in parallel? The original seems to be a 1uf electro, so why not match it with a film cap? Just personal taste?


Thanks to Charlie for the original post, and anyone else who chimes in.

 

[Andybot]

Most excellent, i was just about to look into modifying my ra100 again. Thats all for now...a good schematics would be nice though.
Thx

 

[tchgtr]

Just to add to the info here:

I did indeed unwind the original inductors that were already in the amp, and rewind them around the core after loosening the small amount of glue that was holding them, so the core could be removed when done. Seems to have worked fine. These inductors had exactly 20 turns on them.

Also replaced the compensation caps with some that I was sure were ceramic, and used 1uf orange drops for the input caps... just what I had available.

Tho I agree with Charlie about the original speaker connectors, I left them alone, and for the PSU rail caps I used 330uf electros.

The bass is a bit more focused and high end is better, tho not overly bright. Not bad for an investment of less than $75!

 

[12afael]

I know this is an old post but maybe the people is still here. I found the service manual with the schematic  of the RA100. I did some simulations and the high frequency cut of the output RF filter is way over the audio spectrum (100kHz with a resistive 2ohm load). could a saturated core produce a high frequency roll off? I think it is crucial to maintain the DC output voltage low. someday I will have to plot the responce of this amp but sadly not in the near future.

here is the shematic
http://groupdiy.com/index.php?action=dlattach;topic=45697.0;attach=23209

 

[problypropylene]

Bringing this back from the dead. Is anyone still using one of these? I'll be picking one up soon to run some passive monitors (paul carmody hitmakers) and would like to do a few of the modifications to remove the high end roll off. Have any improvements been made past this?

 

[Twilly]

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:
Charlie

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.

 

[Outlawsound Studios]

very informative thread! I have 2 RA 100's I acquired for cheap to use with some NS10s. Changing out the disc caps to silver mica alone brought out more "air" in the mix over the stock ones, I did one channel and left the other side stock to A/B it. I noticed my earlier build unit used 100pf caps in C202 and C204 while my new unit had 220pf caps there. Next steps is unwinding the inductor and take that core out. Its sounding better already. Thanks Fellas!

 

[Euthymia]

Forgive me if this is a necropost, but I have more information for owners of RA-100's who wish to improve the sound. And I mean drastically.

I had been using one to power my studio monitors for years, and then swapped it out for an ancient Crown D60. The difference was amazing. Way more detail, sounded much "larger." Not even a subtle difference.

This inspired me to examine the RA-100 schematic at length. I could find nothing in the signal path that would explain the difference.

I tried a few of the mods described earlier in this topic, increasing the value of the power rail stabilizer cap on the power amp board, decreasing the value of C02 and C04 and changing them to silver mica. I removed the core of the inductor. None of this made much perceptible difference. I tried it on only one channel so as to be able to compare it.

Then I ran it past a friend of mine who is more solid state savvy. He looked at the schematic and pointed out that there is an elaborate protection circuit in the output section that could be responsible for nuking the transients. Sure enough, I took my cutters to it and removed the protection circuit and the amp turned into the best-sounding amp in the house. I mean the kind of sound that makes me want to go through my music collection and hear all the little details that I missed before.

I'm sorry that I can't explain the theory better, but I can give the recipe. It's easy enough to do with a pair of flush cutters. I have no fancy measuring equipment, just my ears.

The most important thing is to remove D01, 02, 04, 05, 06, 07 (this is the big step, the amp will start to sound great after this).

For form's sake, I also removed Q02, Q06, R03, R17, R06, R14 (this will isolate the remaining components from the power supply and bases of the driver transistors).

Note: there is a photo floating around where the person claims to have identified the components. I found this photo to be mostly incorrect. The diodes and transistors are well-labeled, but the resistors are identified underneath their bodies. If you want to be sure, you can carefully nudge the resistor to one side to read the reference designator.

Refer to the service manual: https://audio-circuit.dk/downloads/alesis/Alesis-RA100-pwr-sm.pdf

The amp already has a thermal protection scheme and clipping indicators, so as long as you don't drive the output into a short or ignore the clipping lights you should be fine. It's also a good idea to check (and adjust if necessary) the bias as described in the service manual. Before the mods, one side of mine was in spec and the other 10X too hot. It didn't seem to affect either the sound or the thermals. These things are about 30% heatsink anyway. Even in PA duty, mine have never been anything but cool to the touch.

If you read the designer's circuit theory, you'll see that while the thermal protection circuit is described, the protection circuit that I cut out is not referred to at all.

If your C02 and 04 are still at 220pF, I do think that it's worthwhile to swap them out for 120-100pF. It appears that Alesis did this in later builds. I haven't done this yet on the other side, but I will. The power supply rail cap value increase can't hurt either, while you've got the module out for modding. As for the inductor....I left the core in on the other side and it sounds great.

Anyone who tries this, please let me know if you think it's an improvement. For me, it was a drastic improvement that took the amp from being unusable for monitoring to being my favorite.

 

[JohnRoberts]

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.

I did a deep dive into amp protection schemes for my DIY 4x250W amplifier back in the early 70s. I used a variant on the Phase linear VxI protection scheme and my calculations suggested the protection was inadequate to protect those transistors (perhaps that is why those amps were called Flame Linears). My amp protection worked for decades until I retired that old amp.

Amplifier protection should not be triggered by transients (slew rate) unless driving a capacitive load.

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.

Good luck

JR

 

[Euthymia]

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.

Thanks for the reply, JR. It's good advice for others who might attempt this.

Truth be told, I was getting ready to just trash the power amp boards and replace them with eBay chip amps. I have two of these things, I think I paid $50 for one and $35 for the other, and in their stock state, they were useless to me except for occasional PA use. This was a last ditch attempt to make the things listenable before I modded them even more drastically by replacing everything but the power supply.

I didn't go into it entirely blind, I consulted with a friend who is a professional solid state amp repairman and is good with theory. My understanding of SS amp theory only applies to the audio path, I usually can't make head or tails of protection schemes. I understand the thermal protection circuit, and I left it in. My "short protection" from here on in will consist of not crossing bare speaker wires and turning up the volume. The amp stays connected in my studio. If I somehow butterfinger it and blow the power transistors, well, then I deserve to spend an hour or two on the bench replacing them.

The RA-100, for all its sonic flaws, was designed for really easy servicing. The amp modules and the heatsinks they're mounted on pop right off, and use a connector to the rest of the amp. This is part of why I wanted to salvage them, I had much respect for designs that take servicing into account.

Before putting it back into service, I dutifully checked the bias, looked for DC on the output, looked for instability and oscillations on the scope, cranked it up to just before clipping on my 1000W dummy load. It behaved beautifully. I suspect that the designer came up with a really nice-sounding audio circuit, and was then asked to put the output clamper on it. I've never seen anything like it. It's 25 extra components, and the equivalent protection circuit on the D60 uses only 6.

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.

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 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.

Good luck

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.

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.

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.

 

[JohnRoberts]

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.

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.

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.

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.

not rocket science but requires a basic understanding of discrete transistor circuits.

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 am not familiar with those speakers. It might be useful to look at their impedance plot vs frequency.

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.

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.

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.

fixing some old soldiers is fun and rewarding.

Design engineers didn't routinely add a lot of extra components for protection circuitry that wasn't useful. If you are willing to forgo the short circuit protection and take your chances you are certainly free to do so. For modest power old hifi amps used fuses in series with speaker loads and/or rails.

Peavey invested a lot of design effort into making amplifiers hard to kill. But Peavey customers were good at breaking stuff.

JR

 

[Euthymia]

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'll go with "poor design," then, in this case.

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.

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.

I'm not savvy enough with the terms to use them to describe the differences. I have to rely on more subjective terms. With the protection circuit in place, the amp sounded constricted and lacking detail. As if there was something wrong with the transients. It was like the difference between bit perfect audio playback vs. going through poor resampling.

 

[JohnRoberts]

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.

JR

 

[Euthymia]

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.

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).

In no way was I accusing you of using a logical fallacy, rather people who precede the "all sound the same" statement with "properly designed." Because there obviously exist amplifiers that are not properly designed, and of course it can't be assumed that just because something was designed by an employee of a company and built at a factory that it was properly designed.

I don't doubt that the effect of the protection circuit could be measured by someone with the proper equipment and skills. I'm just not that person. I just stumbled upon a modification that makes the amp sound great. I don't understand the theory behind it, but I also don't need to. It sounds great and the amp is stable. That's all I need to know.

I originally swapped this amp out for my Crown D60 because I wanted to test the newly-repaired Crown. I was expecting it to sound just like the RA-100. When I heard the difference, I was gobsmacked. I'd always just handwaved solid state power amps as sounding pretty much the same (until clipping, anyway), and that's the way I'd certainly prefer it.

I demonstrated/confirmed the difference with other musician friends of mine and they agreed that side-by-side, it wasn't even subtle. With the RA-100 in stock form, the stereo image is constrained to just the width of the speakers. With the Crowns, and the modded RA-100, the stereo image extends past the speakers, and sounds panned center sound what I call "holographic," where they sound not only centered, but also seem to have a vertical dimension. All subjective, I know, but I can't deny what I hear. We even tried it in mono with a single speaker and the difference was obvious. This all went counter to any previous understanding and assumptions I had. Based on empirical observation.

I lack the knowledge and experience to be able to say exactly what I'm hearing in more technical terms, so all I have to go on is subjective ones. If anyone else wishes to analyze the protection circuit to figure out why it messes up the sound, the service manual is readily available. I'd love to see an explanation, but I'm not the person to give one.

To my thinking, it's just a poorly-designed protection circuit that has unintended sonic consequences. Maybe it can serve as a cautionary for other amp designers. As I said, Crown appear to have implemented their protection circuit using about 20% of the components.

The mods are up there in case anyone else has one of these sitting around and wants to experiment. The usual caveats apply, and it does probably make the circuit susceptible to output transistor damage if you accidentally operate it into a short.

 

[Andybot]

I did the mod Euthymia outlined above with excellent results .....a very simple mod too.
Non of the mods ive done on any of my amps have had such an obvious and pleasing effect.
Just dont short the outputs.
Cheers