Need some advice rebuilding/upgrading 2 power amps.

[SuperKris]

Hey, im new here so i hope this thread is welcome here. I found this site a couple of times when donign some research about a few things. Great site!

I already posted a thread on audiokarma, but a lot of questions remain unawnserd there, so i thought maybe this would be a better forum for a ropic like this.

So here it is...


A few weeks ago i bought 2 big poweramps for a small price. 1 of them was working for 90% and the other one is broken,a nd in parts. They are basicly identical. I'm pretty sure i will be able to fix the broken amp without any big problems, but first i want to rebuild the broken amp and do a few upgrades on it.

They are both Audiolab M480's. These amps were made by the
belgium Rodec and were sold by them as the Rodec HDA 450. The Dutch Audiolab (not the British audiolab most of us know) replaced the frond of the amp and added clipping indication and sold it again as the Audiolab M480. The dutch Audiolab mainly makes speakers and cables now.

I already contacted them for more info but i wasnt able to get chematics or anything else so far. They told me the amp was used in movie theaters and bars, but also "used as a HiFi power amp voor home use, where it preformed fine". I also found a PDF witch specs of the oritional Rodec HDA 450 power amp. You can find it here. http://www.rodec.be/site/leaflet/hda450.pdf I also emailed Rodec for more info but they did not gave me any real information so far.

The amps have been rewired by a retard and were very dirty so i decided to disassemble the working amp to clean it and rewire everything.

Im still a bit of a noob when it comes to hifi electronics but i had my doubts about the power suply. it had new 15.000 uF caps and no "soft start". I did a bit of reading and i decided to build a new PSU with soft start and bigger caps. When calculating a number of diffrent solutions while keeping in mind costs, preformance, and space, i came up with a design with 50.000uF of caps per chanal that had a combined rimple current of 22A. Also i defsigned a simple "soft start" using a nice 4x 20A relay i had laying around. Here is the design:

[imgatt]

I alread started to build it, and i hope i have the time to finish it this weekend. Some opinions about this PSU are very welcome.

Im going rewiring the whole amp with high quality interconnect wire and some nice speaker cable and im taking out all the internal connectors. (where are way to many of them).

Also im replacing all the electrolytic capacitors. The amps are from somewhere in the 80's and probaly where abused a lot, so i gues that wont hurt. Also someone sugested i replace the potiometers on with 10 turns models. I also ordered these.

So, like i said, i still have a few questions. Im still new to working with amps. I heard a couple of things about ajusting Bias current and DC offset. Per channal i have 3 pots (all 1K) that controll these settings i gues. I dont have a service manual or schematics. How can i in a save way find out what pot, does what, and when i fnaly know this, what setting should i go for?

The PDF i posted in the start of this post sias that the amp has a filter that filters out everything out of the 20hz-20Khz range. Is this really a good thing, or should i try to remove this filter or maybe rebuild it for a wider range? My turntable, my current amp (denon PMA-900V) and speakers go far beyond the 20khz, and im also a bit afraid that the filter might kick in before the actual 20khz range to keep the bandwith as small as possible to produce as much power as possible. I did not expirience this so far, but i did not do alot of testing yet. Any opinions on filters as these?

Also i would love to get some advice on other things in this amp (or amps in general) that can be upgraded.

Some more info on how this amp is build.

From the input on the back the input goes straight into a 100K linear pot. From here the signal goes back into a small print in te back that has a few resistors, caps. and a quad opamp IC (a motorola TL074C). Also the print has a "bridged" switch on it than can be operated fromt he back. Right nw im not sure what this print does. Maybe the filter is in here, or maybe its only a buffer and it does the things that are needed for bridge mode. It has a left and right input, and is powered by +63 and -63V. It doesnt has other inputs or exits.

The input PCB is connected to the main PCB that doesnt do much else than holding the other PCBs, house a few fuses, speaker relays, resistors for the power transitors and a few other small things.

The main PCB routes the signal from the input PCB directly to 2 PCB's that control the power transistors. The output of these 2 small PCBs goes into the power transistors and its resistors. They are 250W motorola's that can handle 20 Amp. The amp has a total of 12, so with 6 per chanal, and with both PNP an NPN thats 750W per chanal and 60amps of transistor power. Not to bad i gues. http://www.jaycar.com.au/images_uploaded/MJ150037.PDF

The siganl goes back to the main PCB that has some speaker relays and suff. Also there is another PCB mounted ont the main PCB that must have something to do with power and/or speaker protection.

Well, i hope you guys can give me some advice on what mods i can make, and help me out with my questions. Thanks!!!!

[SuperKris]

I see my thread is posted now, so here are a few pictures.

The amp

[SuperKris]

The back

[SuperKris]

The inside of the working amp as i got it.

[SuperKris]

The input PCB

[SuperKris]

the small PCBs that controll the power transistors

[SuperKris]

The main PCB

[SuperKris]

The PCB small pcb that i gues is used for speaker/power protection

[SuperKris]

The parts for the new PSU im building

[AndrewT]

Originally posted by SuperKris
The PCB small pcb that I guess is used for speaker/power protection

Hi,
does R22 run very hot? The PCB is scorched, but that might be from a previous failure.

If it runs hot enough to scorch the PCB then change it for a very much higher power device and move it further from the PCB.

The adjacent transistor must be getting a roasting!