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Old 18th February 2010, 04:07 PM   #31
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If you can shield the wires to the pot and keep them reasonably short (to avoid stray pickup and RFI), it's ok, otherwise just omit the pot and jumper it.
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Old 19th February 2010, 11:07 AM   #32
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Hi, I done the mods, but just got a very loud hum. I turned off immediatly to try to save my speakers. Checked everything, but can't see whats wrong.
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Old 19th February 2010, 11:54 AM   #33
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Quote:
Originally Posted by barry.childs View Post
Hi, I done the mods, but just got a very loud hum. I turned off immediatly to try to save my speakers. Checked everything, but can't see whats wrong.
Check all your joints again, look for solder bridges/shorts:

i)The 10-ohm resistor is connected to the ground plane on one end, and to the feedback resistor network on the other - which is the "lifted" ground.
ii)The input bias resistor (47k) is connected between the non-inv input of the opamp and the lifted ground.
iii) The shield ground from the input signal cable is connected to the lifted ground.

This is repeated for the other channel also. Everything else is the same as before on the op-amp circuit.

Check carefully for shorts or bridges if you've soldered bypass caps on the underside of the PCB near the +Vs, -Vs and GND pins of the LM3886.
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Old 19th February 2010, 01:18 PM   #34
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Quote:
Originally Posted by barry.childs View Post
Hi, I done the mods, but just got a very loud hum. I turned off immediatly to try to save my speakers. Checked everything, but can't see whats wrong.
Tried the same mods and got the same result (loud hum), plus a knackered speaker. Have now completely bypassed the 5532 by soldering inputs directly to the second pair of wima's on the board. Now no hum at all!
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Old 19th February 2010, 02:15 PM   #35
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The mystery deepens - the assembled Yuanjing board appears to have subtle differences from the bare board from Jim's Audio on EBay. In particular, the location of the bypass caps for the NE5532 differs, and the electrolytics next to the NE5532 are unpopulated. It's possible that the layout of the resistors in the vicinity of the NE5532 differs between the two boards. Ensure that you have a non-inverting gain of 3.2 stage (22k + 10k), followed by the jumper block, followed by a non-inverting unity-gain buffer before proceeding with the mods.

To repeat: the gain stage has a 10uF, 25V DC-blocking capacitor to ground in the negative feedback network, which I remove and replace with the 10-ohm ground-loop breaker to create the lifted ground. This will increase the DC offset at the output of the gain-stage, but this has no effect beyond the unity gain buffer, which drives the DC-blocking Wima at the LM3886 input. The shield ground from the input connector on the gainclone case is connected to the lifted ground, as is the input bias resistor from the non-inverting input of the gain stage.

On the Jim's Audio bare board which is shown in the picture I posted, the mods I detailed work perfectly. Check the traces in the pre-amp section of the assembled Yuanjing to ensure that it's the same before proceeding. Before plugging in the source, do a touch test on each of the signal inputs of the signal cable and see if you get some mild (not loud or heavy) hum pickup.

Bypassing the op-amp completely is probably the best option, in any event. You may have to give up the volume-control pot option on the gainclone box, but that's no big loss.
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Old 19th February 2010, 02:44 PM   #36
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Well iv'e managed to unmod it and all seems ok. I was too woried about my Linn speakers to mess around anymore. Perhaps there are different variations on the board. This is my second chipamp, and I still think there is something lacking. I have been looking on ebay at a site run by Indrasep in Indonesia. He has a 50w dual mono amp that looks to have a darlington pair output similar to my John Shearne amp. Does any one know about these. It would use the same power supply, but I don't know if it would need a bigger heatsink. I am using a bar of 300cm x 40cm x15cm at the moment. What do you think Linuxguru?
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Old 19th February 2010, 02:57 PM   #37
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But Borninthefifties, I tried one side with and one side without the pre and thought the extra gain and clarity to be superior all be it with a little hum. By bypassing it you could get the gigaworks pair for 15 which most people say is superior to this board. That's what I used on my first gainclone and the results were very good. What about using the pass B1 buffer copy on ebay for about 25? I might try one and let you know. Barry
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Old 19th February 2010, 04:23 PM   #38
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I noticed a Sanken discrete monoblock board on Ebay, from Indonesia (probably from Indrasep). I simulated the schematic shown in the Ebay listing, and the simulation numbers weren't great either for distortion or stability. In general, a darlington output will have worse sonics than a discrete darlington emitter follower.

The best bets at the entry-level are still gainclones, either LM3886 or TDA7294. Another alternative is to go with a hybrid gainclone + discrete, like the LM4702 + discrete output pair boards often seen on EBay.
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Old 19th February 2010, 04:29 PM   #39
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Thanks for the advice, What about the naim nap140 clones. Iv'e heard with a bit of tweeking these are quite good. I'm just looking for the best amp for little outlay.
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Old 19th February 2010, 05:06 PM   #40
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Quote:
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Thanks for the advice, What about the naim nap140 clones. Iv'e heard with a bit of tweeking these are quite good. I'm just looking for the best amp for little outlay.
I haven't tried the Naim clone PCBs from EBay, but I've seen them on EBay and read about them here. There's a lot more room for modding and tuning the Naim clones, but the results can be either much better or much worse than a basic gainclone. Consider also that there seem to be several variants of the NAP-140 clone PCBs, some of them with errors in silkscreen outlines. If you have the patience to work around all these issues and try different components/mods, the Naim clones can potentially give much better results than gainclones.
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