Mod Parasound HCA-855A w/UCD 400? - diyAudio
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Old 15th March 2005, 08:12 PM   #1
JohnH is offline JohnH  United States
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Location: San Francisco
Default Mod Parasound HCA-855A w/UCD 400?

I am new to this site and, and as a complete beginner, am interested in exploring a potential DIY solution for a multichannel digital amp that will perform at the level of the pricey offerings from Rowland, Bel Canto, Audio Research, etc. Being a total novice, I was looking for a potential shortcut on the DIY path. I currently have a spare Parasound HCA-855A (5x85 at 8 ohms), that could be gutted and then use the power supply and chassis, with the uCd400 or other modules. Specs on the power supply (from Parasound site) are:

- Direct coupled - no capacitors or inductors in signal path
- DC Servo and relay protection circuits
- High bias Class A/AB operation
- 1 kVa Toroid power transformer with independent secondary - windings for each channel, 68,000 F power supply filter capacitance
- Independent power supplies for each channel
- Input stages use hand matched complementary JFETs

Since I do not have the skills and knowledge to do this myself, I would likely find a pro to do it for me. At this point, I was just curious about the feasibility of such an approach.

I do love this whole DIY way, and would like to learn more for future projects. Can anyone recommend some good books, websites (with lots of photos) in addition to this site that a total beginner could learn from the basics up?

Thanks.
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Old 16th March 2005, 12:28 PM   #2
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This is (kind of) what I did while waiting for my transformer and case (I've put myself on a budget audio-wise)

I've modded an old Akai AM-55 integrated amplifier ('93) after I found the voltage on the secondaries are at a good voltage for UcD use.

It's rated at 220V, 650W, so I believe it's got a good VA rating (I don't think it's 650VA but looking at the size, maybe 400VA)

Disconnected the whole power stage on that oldie, drilled a few new holes in the heatsink, installed the 2 UcD400 modules. Found some place to put my BC 22.000uF/63V caps and have put in a new bridge rectifier. Connected the secondaries to the new parts.

Next up is drilling holes in the backplate for RCA and speaker terminals.

For the moment i'm not using the pre-amp - i'm using it as a seperate amplifier, it works like a charm.

I do believe the transformer is current limited though, I'll post an image soon to let the true DIYers in here to give their opinion.

If you have a real nice piece of equipment, you could upgrade it with UcD's. Higher efficiency means that more of the available power is used for audio, and less heat is created.

So for any given VA rating, you should increase your quality and output with adding UcD's.

I had to replace the caps though, the existing caps were 55V, 8200uF (custom made I think...)

Yves
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Old 17th March 2005, 05:54 PM   #3
JohnH is offline JohnH  United States
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Yves - thanks for sharing your project. It sounds like modding provides a reasonable shortcut, but I may have insufficient power for 5x UCD400 modules in the Parasound. Are you happy with the sound of your modded unit? How much do you expect to gain in SQ with the new chassis/transformer? Thanks again.
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Old 17th March 2005, 08:13 PM   #4
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Well the layout is going to be better, and the transformer a lot bigger, so I guess yes.

However, I did a short test earlier with another (larger) tranformer and other caps (chemicon(?), 2x28.000uF, now I'm running BC computer grade 2x22.000uF), and it sounded... different... probably it is just my ears playing tricks on me.

It's a little more "constrained" than I heard in my first test - I'm almost certain it's the transformer size limiting the sound somewhat. Lacking a little bass, that should be due to not enough current on tap.

Personally, I think 800VA-1000VA is about right for 2 modules running at max power in extreme conditions ("hot" recorded disco bass, in a non-stop mix)

Depending on your speakers I'd say 2 channels or maybe 3 on the 1000VA, and add another transformer for rear channels. If stereo is your main thing, put center and rears in a different box.

You can also put all five channels on the 1000VA but that depends on your speaker setup - if you run "small" center and surrounds you could get away with it ("small" speaker setup cuts bass away from the speakers so you get away with less power) and add a switch so you can turn off 3 channels when running stereo mode. The amps in standby just consume a very little bit of power.

If you run a large center, remember that most sound in a surround setup comes from the center!

Yves
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