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Old 20th March 2010, 02:52 PM   #111
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my second unit has arrived.wima caps have differnet layout of printing but still say 1.8uf (not listed on wima website!)it has the non isloated chips on this time complete with screw, tape and plastic bush.
do i still use thermal paste?
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Old 20th March 2010, 03:22 PM   #112
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Quote:
Originally Posted by skodaboy View Post
linuxguru reckons performance is best with all the mods and op amp intact. im going to try carbon resistor mod and second lifted ground mod.ill then try opa2164 opamps again.
Sorry - preoccupied with other matters. Actually, I'm not sure if it's better with or without the opamp in the signal path - I'd guess that the latter is better. However, if you do want to keep the op-amp, then both lifted ground mods are recommended if you want to use a fast, high-performance op-amp like the LM4562 or LT1208. Without the second lifted-ground mod, the ground-return currents from the speaker will contaminate the ground reference for the signal/feedback networks near the LM3886. The downside of the second lifted-ground mod is that the output offset increases from mVs to ~100 mV.

Again, regarding the carbon resistor - it might well be the increased value (27k vs. 22k), rather than the type of resistor, that is relevant to improving the sonics. It's probably worth experimenting with different values (up to say 47k) and types (Dale brown MF, Allen-Bradley CC, and maybe boutique types like Shinkoh, Caddock, etc.) at these locations. I used Allen-Bradley 27k because I just happened to have them lying around.
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Old 20th March 2010, 03:30 PM   #113
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Quote:
Originally Posted by skodaboy View Post
my second unit has arrived.wima caps have differnet layout of printing but still say 1.8uf (not listed on wima website!)it has the non isloated chips on this time complete with screw, tape and plastic bush.
do i still use thermal paste?
Regarding the Wimas, they could be counterfeit. It's not difficult to remove and upgrade them with known good ones. The LM3886TF (isolated case) are the preferred ones, because it simplifies heatsink mounting - you can use a single heatsink and also bolt it to the chassis easily. With the metal-tab LM3886, the paste is optional if it has the soft gray or pink rubbery isolating tape.
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Old 20th March 2010, 09:19 PM   #114
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It looks like the Goldmund/Symasym clone kit is back on EBay:

COMPLETE STEREO AUDIO POWER AMPLIFIER KIT! - eBay (item 380144028687 end time Mar-26-10 08:20:45 PDT)

The price has gone up to $69.99 + $20 s&h, but it now includes the NJW0281 and NJW0302 power BJTs. I think the earlier offer at $42.99 + $6.99 s&h without the power BJTs was the better deal.
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Old 20th March 2010, 11:11 PM   #115
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Linuxgurru sorry if misquoting re opamp bypass improvement.
I may well try bypass in future when I have ESP electronic crossover as they have an output buffer.IT may be unnecessary to have the output buffer running straight into another buffer.
what do you think?

Last edited by skodaboy; 20th March 2010 at 11:12 PM. Reason: spelling!!
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Old 21st March 2010, 07:52 AM   #116
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Got my second board with full mods up and running sounds great! but when i touch the metal of of the volume pot (alps blue) i get mains hum?!
Hey linuxguru i notice on the second mod photo for the jumper lead it also connects to the gnd pin of the input connecter but this is not mentioned in the text is this something else to be done?
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Old 21st March 2010, 08:02 AM   #117
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also the hum stops when i touch the chassis.maybe i need to change gnd arrangement.i currently just have a wire from 0v in to the chassis.this didnt seem to be a proble with previous mod.
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Old 21st March 2010, 09:41 AM   #118
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Quote:
Originally Posted by skodaboy View Post
when i touch the metal of of the volume pot (alps blue) i get mains hum?!
It's some kind of introduced ground-loop, with your body being part of the pick-up loop. You may have to isolate the power ground from the chassis ground to get rid of it.

Quote:
Hey linuxguru i notice on the second mod photo for the jumper lead it also connects to the gnd pin of the input connecter but this is not mentioned in the text is this something else to be done?
Good catch! Yes, on one of my boards I isolated the ground pad of the input connector by using an exacto knife on the copper on both sides (component and solder side). I then bridged the isolated ground pad to the lifted-ground. This mod is purely optional, but makes it more convenient to connect and disconnect the input shield ground without soldering/desoldering the shield wire at the upper lead of the 10 ohm ground-loop breaking resistor.

Last edited by linuxguru; 21st March 2010 at 09:44 AM. Reason: typo
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Old 22nd March 2010, 12:02 AM   #119
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I bypassed the op amp by connecting the input to the second Wima, and I removed the op amp alltogether. I left the other channel stock. There was a little improvement in the noise floor, but not enough to make it usable (in my opinion). Running the output from my mp3 player instead of the input forom my preamp improved the noise floor a lot but didn't clear it up completely (suggesting the noise floor is a grounding issue????).

When I did the second ground mod and used 27k resistors, the noise floor got worse, and subjectively the sound quality got worse as well. Of course, without being an expert at this stuff, I don't know if I ran it all the best possible way. Just desoldered the two resistors, put the two new ones in and scraped off the tracings to the required grounds and soldered jumpers to a spot near the op amp.

I'd really love to get this thing to a usable sound quality level as a couple of them would be great for a project I'm working on that is cost sensitive.
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Old 22nd March 2010, 05:57 AM   #120
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Quote:
Originally Posted by bcarpman View Post
When I did the second ground mod and used 27k resistors, the noise floor got worse, and subjectively the sound quality got worse as well. Of course, without being an expert at this stuff, I don't know if I ran it all the best possible way. Just desoldered the two resistors, put the two new ones in and scraped off the tracings to the required grounds and soldered jumpers to a spot near the op amp.
OK - I guess you removed the DC blocking cap and bridged the resistor to the adjacent resistor as well? You need to isolate both the points where the resistive network near the LM3886 is grounded. For me, it improved the sonics audibly, but I've retained the op-amp pre/buffer.

The mods can only go so far - the original ground-plane based design is nonsense. There's no way to avoid ground-bounce in the original design. There's now an improved version of this board on EBay, with blue solder-mask and an LM833 buffer. It has better grounding, from the EBay pictures. However, it's priced at $45 assembled, which makes it less interesting - for that price, a discrete design like the Symasym/Goldmund is preferable.
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