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Old 22nd April 2010, 04:10 AM   #1
bushy is offline bushy  Australia
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Default dbx 386 upgrade

Hi all,
I'd like to officially welcome me to the forum

With that out of the way...

I've got a DBX 386 that is currently up on the blocks, due to faulty coupling capacitors. While looking for some info on it, I found that several people have said that while the design of this amp is top notch, they let themselves down by using el cheapo caps, op amps, and other components. So I'm looking at doing some surgery to see what I can't do with this pre.

First things first: I'd love, absolutely without reservation LOVE a schematic. Believe me, I've searched long and heard for one and come up blank all round. DBX won't give me one, their australian distributor and repair agents won't give me one, all my favourite schematic sites turned up a zero, and my prospects of finding one are looking grim. If I can get a schematic, it means (a) it's a heckuva lot easier to work out which components do what, and whether they need replacing, and (b) that means I can do one channel first, and do an A/B test to see if it was actually worth doing. If anyone can get me a schematic of the 386, next time im in your area I'll buy you a beer. Heck, make that a carton.

Second. I want to upgrade the caps. I've picked elna as a decent replacement to the standard yageo's. What I'd like to know is, which caps are the most critical to replace? i.e., how much benefit will I get from upgrading the power supply caps, as opposed to the filter caps?

Third. A well as all the standard can-shaped caps, there are a heap of little blue caps, similar in shape to the standard brown ceramics but less round and more fat. Can anybody enlighten me as to what these might be, and if they're worth replacing?

Fourth. Coupling caps - at the moment they're (broken) MTPT film RF TA capacitors. Or so DBX tell me, I don't really understand what that means I'm afraid can anyone suggest what sort of capacitors (e.g. electrolytic, tantalum, ceramic, oil...) would be best suited to this application if I were to spend some money and upgrade them? They come from the anode of the 12AU7, one goes to ground and the other to the output pot. They're .1uF 400V and .33uF 250V.

Fifth. Op amps - At the moment I've got a 5532A on each channel, a whole heap of JRC 4580's, and an LF353 on each channel. They're all surface mount unfortunately. That should be fun. Without schematics I can't tell what each one does, but by layout it would appear that the incoming signal goes first to the 5532A. I'm looking at burr brown's to replace them with, and the options I've come up with from my own research is OPA2604's, OPA2132's and OPA2134's. Can anyone suggest to me which BB might be best for which application?

I really appreciate any advice I can get. If I can get some decent results I'll happily post info, before/after and whatever else might help someone else out. So we all know, I'm an industrial electronics technician by trade, so I'm comfortable playing around with electricity, and I have a fair bit of hobby experience with fixing/playing around with sound gear and PCB's. Although this would be my most ambitious project to date.

Thanks in advance! Right, where's my soldering iron...
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Old 24th April 2010, 03:32 AM   #2
bushy is offline bushy  Australia
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anybody? I don't expect all the answers to be forthcoming, but if anyone has ideas on one or two of the things mentioned that'd be sweet
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Old 25th April 2010, 10:28 PM   #3
SHiFTY is offline SHiFTY  New Zealand
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Thoughts:

1) hopefully you shouldn't need a schema just to repair the unit- just replace the broken bits. Replacing passive components with better ones should be fine as long as you use the same values.

2) In my experience of repairing rack mount guitar pre's if the high voltage power supply caps need replacing you will hear it- hummmmmm! Or if the power supply caps are bulging or leaking then yeah they're toast. The low voltage stuff is generally ok, however if you've got new replacements of the same values then chuck them in a few key spots, can't hurt. The critical places are anywhere in the signal path, like anywhere near the output of op amps, or on the cathodes of valves etc. Also test resistors to make sure the values are within specs.

3) can you supply a pic?

4) You will most likely be limited by space as to what you can use. You want some kind of film cap, maybe use solens or something similar. There's so much hype about caps it's ridiculous. At a pinch, Jaycar sells generic mylar 630V coupling caps which will at least get the unit working so you can do some experimenting. The voltage rating is v important- must be the same or higher.

5) You're brave- I'd fix the unit first before doing radical sugery! The risk is oscillation if the op-amp isn't in working in the correct conditions. Once you get it working you should replace the valves with decent NOS ones before any other "improvements"- the existing ones are likely to be flogged out and will this will have a far bigger influence on the sound anyway. Can't beat the sound of 1950s-60s tubes.

Last edited by SHiFTY; 25th April 2010 at 10:57 PM.
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Old 25th April 2010, 10:51 PM   #4
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Shouldn't this be in the solid state forum?
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Old 25th April 2010, 11:14 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by CharleyW View Post
Shouldn't this be in the solid state forum?
The 386 actually has a couple of vacuum tubes in it, so technically it probably straddles the fence..
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Old 26th April 2010, 12:34 AM   #6
bushy is offline bushy  Australia
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yeah I wasn't sure where it should go so i picked here

Thanks shifty. you're right, i AM going to first of all just make it work again before doing anything else, so I don't end up chasing my tail. But anyway. Found these caps which are pretty pricey, but for a critical part of the signal chain, I figure I'm probably better off spending a bit rather than throwing 50c caps back in...
Dynamicaps
Sonicaps Gen II
Sonicap platinums
anyone had any experience with these caps? or know if they'd be a good idea for anode coupling caps? they're not the same package type, but I've got a bit of height I can utilise to make them fit

I already did upgrade the tubes, although not with anything 50s-60s, I just put two JJ tesla ECC802S's in. But I'm looking at getting some mullards or similar as well when I do the upgrades
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Old 17th August 2010, 03:10 AM   #7
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bushy,
have you made any progress?
I have a 386 - working fine - just wanna clean it up...
sending my Motu 386 to Black Lion Audio for upgrades -
debating on taking this 'mid-low line' pre amp out of the loop?

thanks for the links
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