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Old 9th October 2008, 04:58 PM   #41
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Thank you for that. I know some say the LM4562 is the best but I havent personally tried many others. Your list above I will keep for reference.
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Old 9th October 2008, 04:59 PM   #42
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So has anyone actually modded a 640A then?
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Old 10th October 2008, 07:01 AM   #43
jaycee is offline jaycee  United Kingdom
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Havent modded but what I would do is swap out the cheap capacitors in the thing, in favour of decent makes eg Panasonic FC. There should also be room on the PCB to replace any electrolytic capacitors in the audio path with polyester ones. It looks like the PCB was designed with this in mind, and then they fitted cheap rubbish.

Swapping out the volume pot with a good one might be an idea too, but this might be hassle.

I wouldn't bother replacing the opamps - at least not with anything excessive. They usually have fairly primitive 7815/7915 power supplies around them. OPA2134 is as far as I would go.
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Old 11th October 2008, 03:52 PM   #44
Stream is offline Stream  Russian Federation
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My 340A have the input dec. Caps on input stage, after the RCA-terminals (before the resistors).
What for they are placed there? if it good idea to place a peace of wire Instead of caps? If it's bad idea, then say somethng about placing there some MKP cap...
P.s. As i can see, 340SE also have them, but there are nothing there in service manual for 340a WTF ?
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Old 11th October 2008, 04:14 PM   #45
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Quote:
Originally posted by Stream
My 340A have the input dec. Caps on input stage, after the RCA-terminals (before the resistors).
What for they are placed there? if it good idea to place a peace of wire Instead of caps? If it's bad idea, then say somethng about placing there some MKP cap...
P.s. As i can see, 340SE also have them, but there are nothing there in service manual for 340a WTF ?
The 340A service manual seems slightly different in the layout, but its good as a guide. Those input caps are worth upgrading. However, as your source components allready have decoupling caps on their outputs (you should check this) I would say your ok to link them out. The 340 has alot of decoupling caps at different stages of the signal path. Just before the input buffer, in the tone/tone bypass circuit, and just before the power stages. Something I plan to do is check for DC on the output of the input selector chip and if its low enough link out the caps. Again, if the output of the input buffer is free from DC, remove the decoupling caps. ( I changed to input buffer opamp toi LM4562 which gives out nearly no DC.)

If your not using the tone circuit then leave it alone. You can then just remove decoupling caps in the tone bypass. You should only do this if you have a meter to check for DC at various points of the signal path. Im not certain what is an acceptable level of DC but I would say at a guess .0005v would be great.

I would say IMHO to keep at leat one cap per channel to be safe. Just find a good place to put it (where DC is highest - maybe after the selector chip).

I am a beginner so you should verify this with someone more experienced.

Good luck!
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Old 29th October 2008, 01:22 PM   #46
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340A Power amp and PSU modding:

CA 340A SE LM3886 based amp - Upgrade advice please.
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Old 16th April 2009, 08:04 PM   #47
skidude is offline skidude  United Kingdom
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I have just found a full service manual for this amp if anyone's interested?

http://www.eserviceinfo.com/download.php?fileid=40495

Towards the bottom of the page it says " Download >> To download the file, please, click here ! << Download "

Sounds obvious but was quite tricky to spot where the link was on the page!
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Old 27th April 2009, 06:27 PM   #48
skidude is offline skidude  United Kingdom
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Having got the service manual for the 640A I've devised a plan for upgrades.

1. Replace 8 x 2200uF smoothing caps in the power supply with Panasonic 3300uF M series (couldn't find any larger that would fit in the standard positions)

2. Replace 8 x 220uF decoupling caps in power amp with Panasonic 220uF M series.

3. Replace rubbish volume pot for Alps Blue Velvet motorised 10k log. Might need to add a 10K resistor in series to bring it to the original 20K value and this will also usefully reduce the range of the pot.

4. Remove headphone wiring and speaker B wiring. (never used anyway!)

5. Rewire speaker and power supply cabling with something thicker.

That's for starters. In the future I might bypass the long signal PCB traces with screened cable and also bypass the tone control/balance circuitry.

I bought the amp non-working on eBay for 55, lucky me all it needed was a fuse! It sounds pretty good but a bit weak in the bass and slight sibilance in the treble. Could the NP input caps be responsible for the lack of bass? I've read they tend to deteriorate quite quickly.

Any comments or suggestions for further mods?
Cheers,
Matt
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Old 27th April 2009, 06:49 PM   #49
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Sound like a good place to start.

-Don't forget the feedback caps on the poweramps, makes quite alot of difference. Try Elna silmic or even Wima 22uf MKS2-XL.

-The opamp in the preamp section, try some better alternatives and be sure to experiment with the local supply decoupling caps when doing this so you get the best out of each opamp you try. I'm using LM4562 in my 340A with 1uF BG N fro local decoupling, good results.

-Another important area to consider is the DC blocking caps in the signal path. Usually there will be several cheap electrolytic caps, many of which can be bypassed completely with a wire link. You need to keep at least 1 pair at the preamp input and the poweramp input. Use wima polyester film caps for neatness, they will fit the board nicely and sound way better. You could also use MKP's but they will be awkward to fit and could pick up RF interference, but any film capo will beat the the stock lytic caps.
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Old 27th April 2009, 09:43 PM   #50
skidude is offline skidude  United Kingdom
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Thanks very much for your suggestions.

The feedback cap is currently a 100uF NP. I've been doing a bit of reading of this thread and now I'm not sure what to try!

http://www.pinkfishmedia.net/forum/s...ad.php?t=61242

Regarding the input caps, I guess I will need to stick to the original 10uF so I don't loose low frequency response? A poly cap of this value is physically pretty big and there's very little room on the pcb for it! maybe there's more room in the 340A?
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