Celestra RA475x Review

        Here are some internal pictures of the amp while I had one on my hands as it came in for repair (using exact parts as original) from a friend. I did not have a decent source/speaker set on the workshop to evaluate sound quality so this mini-review will be more of the parts and build quality of the unit.

        I modded this amp recently and you can find the details here.

        These are the specs I got from the internet:
* 4x 75W rms nominal power;
* built as "DUAL 2 channels" (fully separate 2 main boards);
* "audiophile grade" audio quality - Zero Feedback Ratio Design;
* 2x 240W rms bridged mono;
* tri-mode operation (mono + stereo without any switch or adapter);
* bypassable low-pass and high-pass internal cross-over fully independent in tuning (50-3.5K)Hz;
* perfect for filtering points as sub & low-mid; low-mid & high-mid; high-mid & tweeter;
* two independent 500W DC (@12V) high frequency switching power supplies;
* no one aluminum capacitor in the signal path;
* 200W complementary power Mosfet (each) on audio output stage;
* intelligent electronic protection design capable to put out high impulsive currents;
* one of the most compact 4ch. amp in the market in its category;
* fine chromed and polished finish.

        The top cover is highly polished chrome steel and is quite thick and sturdy. The bottom plate is matte brushed aluminum with clear anodizing and black silkscreened labels. (exterior pics here)

        The SRP for the RA475x is around Php60,000 over here (approx US$1370) if I'm not mistaken. The amp is a "dual stereo" configuration. Meaning, it is two independent 2 channel amps in one chassis. There are two identical and completely separate boards containing one power supply and two channels of amplification. The other thing they share besides the chassis is a common heatsink in the middle.

        The amp is somewhat tricky to open up. Unless you are careful, you risk bending or breaking some components on the sub boards. Once the top cover is removed, the six screws holding the heatsink to the bottom plate is removed. The nuts holding the PCB to the base plate should also be taken off (all of them). You cannot remove the screws through the bottom as nuts are used as spacers between the base plate and PCB.

        From what I was told, the damage was caused by a shorted speaker terminal. The only thing that was damaged was one output transistor but I decided to replace the complementary pair to ensure reliability. The power MOSFETs used were IRF3710 and IRF5210 (both no suffix). The output stage should test 3kOhm from gate to source on both MOSFETs if the output stage and driver transistors are good.

        I had a bit of head scratching while testing and after a few minutes, figured out that the base plate completes the ground connections between the power supply, amplifier channels and speaker terminals. So during testing, keep in mind that you have to install the boards back into the base plate before powering up.

        Here are a few pics and some things I noticed while working on this amp:

Made in Italy

The power input on this side is for the amplifier channels on the other side of the chassis. During testing, I only powered up one side at a time. The boards are totally independent so it makes no difference if I power one set or both.

The fans are only hot glued on to the top cover. There are methods to weld bolts onto sheet metal without studs showing on the other side (I'm not sure what it is called) I don't know why they didn't go that route. I wonder what would happen if the amp overheats and the glue softens.

The fan connections are soldered to the PCB! no plugs were included so that you could disconnect and separate the top cover and put it in a safe place while working on the main board. How much additional cost would it take to add connectors? I don't think budget is a concern as this is not a low cost amp!

The transformers and input inductors are supported by their own leads. Nothing wrong with that as most, if not all, manufacturers do that with their amps but here, the leads are very long and there is no glue or some sort of additional mechanical support. With the vibrations encountered in a vehicle, I doubt this is a reliable way of building amps.

To save space, the power supply PWM controller is mounted under the transformer. It appears to work fine but I would like to see more space from the magnetic field and switching noises of the transformer messing with the sensitive feedback pins of the PWM regulator controller.

The diodes used are too small for my liking. TO-220 diodes attached to the heatsink should have at least been used. The output filter inductors are a little too big for the PCB footprint so they don't sit flat on the PCB. Also, they are drum core types which aren't magnetically shielded so they radiate noisy magnetic fields. I wonder how they claim the power supplies are rated at 500watts each. The transformers I would comfortably use up to 300, maybe 400 watts but the diodes, not more than a 200watt powersupply.

The input and crossover op amps are general purpose TL082. They should have at least used TL072 which is the low noise variant and not much more expensive.

No name brand electrolytic capacitors all around. Even a general purpose cap from Elna, Nichicon, Nippon Chemicon, etc would have been better.

Six pin header and a small portion of the sub board holds it to the main board. No other form of mechanical support was included. Again, With the vibrations encountered in a vehicle, I doubt this is a reliable way of building amps.

What would you do if the cap is too big diameter wise to fit into the PCB? You squeeze it with a pair of pliers! Although not clearly shown in the pic, there is a stain around the cap, it appears to be dried electrolyte from the cap leaking because the rubber seal is ruined.

Here's one good thing that I found (besides the PCB being actual FR4 and not phenolic). Wima caps for the crossover filters!

Sub board holding switches and pots are attached using nylon snap in stand offs. If someone pushes a little too hard on the pot shafts or switch buttons, these things will break. And again, I doubt this is a reliable way of building amps.

The top cover is tricky to assemble/disassemble. If you're not careful, you'll end up breaking parts.

        Based on what is posted here, You probably know where I stand regarding this amp. Knowing the SRP, I was hoping for something better than this.

Page updated and copyright R.Quan © 17 Dec 2011.