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Cambridge Audio 340A SE preout conversion and other mods.

mikesnowdon said:
Has anyone ever done this and if so was it beneficial?
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I ended up doing this in one of my amps, starting out of necessity as I'd knackered some pcb tracks. I ended up doing it with loads of the tracks to see. I now have a rats nest of wire in there and no real benefit. Some crosstalk was removed by doing the tracks from input to selector but that's about it. It's a fair amount of hassle so I wouldn't bother doing it again.
 
James Evans said:
Mike - do go out and buy a meter. I haven't fully read this thread but it sounds like you've bought some of the NCC200 clones? If so, you really will need a meter for that... I wouldn't remove dc blocking caps without checking the dc that they're blocking. For instance, I had lm4562 opamps in my cdp output stage which gave next to zero dc on their output so I removed the dc blocking caps. Last night I thought I'd do some opamp rolling to look at what dc I get from different ones - lm6172 gave me over a volt, ad826 about 0.5 volts and opa2604 (the original amps I removed) gave exactly zero. Lesson being - measure...



I don't think that can be done without a redesign of the power amp can it? (again havent read the whole thread so apologies if this has been covered.
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I dont have the NCC200's but I'd like to get some. The main reason for the thread is that Im considering modding the amp so it can be used as a reasonable pre for the NCC200's. I worked out where to tap in to get a preout (Balance pot output) but It seems I'd need a buffer after it. One option I was really into was to fit a Tube buffer module I found on ebay. This idea died when I tried to get impeidance data and PSU requirements from the seller on. HE couldnt (or wouldnt) help.

Another poster gave a simple recipie for a opamp based buffer (see above) which seems a good option.

Other that the above Im looking foir ideas to improve the sound quality of the amp as I'll be using it as my main amp for quite a while longer, and it would need to do the NCC200's justice later. I think the main problem with it (soudwise) is that its a little thin sounding. I'd like to get a warmer sound from it so installing OPA2107 opamp in the buffer stage seems the way to go.

Im my CDP I have LM4562's so the DC blocking caps were removed. I dont have a meter but plan to get one soon. I was just wondering what were the best 'failsafe' mods I could carryout on the Amp to get me started. Probably to begin with I'll be fitting the OPA2107 opamp and upgrading a few caps in the signal path, maybe I can remove all but the last pair? One thing which isnt shown in the schematics is a row of caps near the phono inputs. I suspect these are dc blocking caps, If I can confirm this I'll see about removing or upgrading the ones on the inputs im using.

Haave a good read through and tell me what you think, I have schematics if your interested.

RE: Replacing the PCB tracks with wire. I think I'll leave this alone then.
 
Mike, if you want to send the schematics through I'm at *removed*.

I'll take a look (for my interest as well!). I'd probably start where you say, by upgrading coupling caps (you'll need to fit "flying leads" from the pcb to use film caps) and the opamps. Once the caps are in you can easily measure dc on the flying leads. If safe you can then just take the cap out and solder the 2 leads together. That then allows you to do other component rolling and easily put the cap(s) back in if needed.

opa2604 and opa2134 would be good opamps to try as well - I've put them in a CA amp before and they've rounded the sound out nicely (preference for opa2604)

feedback cap is definitely worth playing with as well as it has a definite influence on sound. nichicon muse kz are my current faves of the electrolytics. mmks I find a bit grey sounding and polypropylene were just too damn big to get the case back on (but sounded great)

PS, from reading the above posts it sounds like you've got solid input/ideas already based on the schematics.
 
Got my opa2107 now. My mate reckons replaceing the local decoupling caps with 220uf BG's, as I have a pair spare Is it worth it?
 
Fitted the new opamp and changed the 2 decoupling caps to BG 220uf. Now im getting hum from one speaker. Its fairly quiet and is there regardless of the input seleted. Could it be the BG 220uf's causing the problem, should I replace the originals?
 
mikesnowdon said:
.....Im also tempted by the NAIM sound although I've only read about it.

Mike
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Mike , the last sentence that i have quoted from you causes me a certain amount of concern, before you go out on this particular DIY Hi-Fi journey i would recommend that you either seek out a friendly local dealer who stocks Naim and will let you have a good listen, or alternatively and even better, seek out a friendly audiophile/Hi-Fi geek/PFM member nearby with Naim amps who will let you pop around to their house and have a listen over a coffee and a chat.

The Naim brand has an incredibly good reputation for providing top quality equipment, it is a path that i considered and aspired to for quite a few years, however on the few occasions that i have heard Naim systems being 'showcased' , mostly at local Hi-Fi shows I was more than a bit disappointed at what i heard, i've never heard a Naim system that has 'blown me away' regradless of its cost , but i have heard sytems from 'lesser' manufacturers that certainly have..


good luck.
 
I'd say retrace your steps and put the old components back in and see if it's still there, if not, do the work again etc...

Good luck finding it
 
Ok will do. Apart from the hum Im really happy with the sound of the OPA2107. I hope its a simple fix to remove the hum.
 
Before you do that Mike check and double check all your connections and joints. Doesn't matter how experienced you are, it's often dumb simple stuff that catches you out.

If there's still a problem, then replace the caps.

If there's still a problem, replace the opamp.

Dan
 
James Evans said:
One question Mike - have you soldered sockets in for the opamps, or are you soldering/desoldering them each time? If the latter then check the soldering around the opamp pins -they can be fiddly sods. If you haven't got sockets in then I'd recommend them. I've bought from here previously: http://www.net-audio.co.uk/connectopamps.html
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No sockets, I know its bad. I couldnt wait so I decided to solder the chip direct. My gut feeling is that ive lifted/broke a pad somewhere. I'll check everything with a magnifying glass (still dont own a meter, doh!).

I dont think I mentioned this: When I first fired her up after installing the opamp/caps there was quite a loud hum/buzz from the right speaker. I immediately switched her off. I turned vol to 0 and switched on again, still buzzing. On the third attempt everything sounded fine so I assumed it was the circuit settling down or something. It was only when I listened late at night that I realized the noise was still there, but fairly quiet. If I have the amp on with no input the noise is audible from the right speaker, as I turn the vol pot up the noise gradually shifts across to the left but remains at a constant volume. Does this indicate anything to any of you?

Cheers, Mike
 
On e of the Blackgates I fitted for the decoupling was swollen. Took them out and replaced the original opamp decopling caps. Checked for broken pads and tracks, repaired anything which looked dodgy. Slight buzzing remains on one channel, very faint though cant be heard when music is playing. It might have been there all along maybe I didnt notice it? I also fitted 2 x OSCONS after the +/- 15v regs and replacd the original speaker terminals (I'd fitted some chunky WBT fakes). Its possible that the opamp is dodgy, I will try a new one.
 
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