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Old 7th December 2008, 05:50 AM   #1
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Default Help modifying HCA-3500

Hello,

I would like to do some mods on a Parasound HCA3500, any suggestions would be appreciated. I have read that upgrading the main feedback resistors and checking the polarity is a good place to start, the amp sounds good but can be a bit etchy on the top end. Where might I find schematics for the amp?

Thanks for any help,

Matt
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Old 7th December 2008, 06:18 AM   #2
mlloyd1 is offline mlloyd1  United States
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anytime i have emailed parasound and asked for a schematic, they sent a pdf within a day.

mlloyd1
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Old 8th December 2008, 03:04 AM   #3
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Thanks, I'll email them tomorrow. I understand that John Curl did a splendid job designing this amp but Parasound used some parts that weren't quite what he would have wanted, and it can be an outstanding amp with a little work.

I wish I could get John to elaborate but he must be bored with this one.
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Old 9th December 2008, 10:41 PM   #4
PH104 is offline PH104  United States
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Matt --

You touched on the immediate tweaks -- replace the feedback R's with 1/2W Holco's and check the AC phasing. Most of the hundred or so of bypass caps in that amp can be cut-out. Replace the bridge rectifiers with Harris Hyperfast diodes. These hints and many others are over in the diy section of Audio Asylum. Bob Crump posted several tweaks there.

Phil
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Old 10th December 2008, 01:28 AM   #5
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Thanks Phil,

I searched all over for those tips and only found the stuff about the feedback resistors and polarity, are the tips specific to the HCA3500? I'll search through Bob Crump's posts.

Matt
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Old 10th December 2008, 01:31 AM   #6
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Thanks mlloyd1 I got the schematics from Parasound in one day.

Matt
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Old 10th December 2008, 03:59 AM   #7
mlloyd1 is offline mlloyd1  United States
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tony at parasound is a great guy. john curl trained him well ...


mlloyd1

Quote:
Originally posted by ironhardware
Thanks mlloyd1 I got the schematics from Parasound in one day.
Matt
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Old 10th December 2008, 04:23 PM   #8
PH104 is offline PH104  United States
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Matt -- Somewhere I have a list of posted mods for the HCA-3500's plus some of my own (like removing that auto-bias thing). I'll dig them out and post. Also, if you aren't using the balanced input, you can remove that little input board.

But do the feedback resistors and AC polarity first. The change is pretty dramatic.

Phil
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Old 10th December 2008, 05:03 PM   #9
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Bob Crump and I modified the HCA-3500 about 10 years ago, after listening to it was not what we first expected. The 47Kohm feedback resistor should be changed to a Resista or an OLD Holco resistor. There ARE too many bypass caps in the power lines. They were not part of the original schematic that I submitted, and the feedback resistor brand was changed as well, to my dismay. In the end, we got a very nice product, that we resold for $5000, and it was considered one of the best in its time, before the JC-1, which is better yet. However, what we learned from modifying the HCA-3500 proved to Parasound that there is more to good sound than just measurement, because the first HCA-3500 that we modified, (my own personal unit) was tested by me extensively, before modification, and I thought is was virtually perfect, UNTIL we listened to it. This is what separates audio from industrial designs. If many here would just try some to the changes that we have suggested over the years, almost any product could be made to sound better, even if it did not necessarily measure better.
I don't think that I have met Tony more than once or twice, but he is a very good repair tech, and he knows more about the individual products, outside the JC-1 and JC-2 than I do.
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Old 10th December 2008, 08:43 PM   #10
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Thanks guys, I have copied every post I could find on the subject, and would like to find more, especially specifics on things like which caps to eliminate.

John, I do like the HCA2200ii, I've been listening to it for a few days and save for one fuzzy blip it sounds real fine.

Matt
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