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Cambridge Audio 340A SE preout conversion and other mods.

All done with no smoke or burst capacitors and playing music rather well indeed. Initail listening is very promising, David Gilmour - Live in Gadansk sounding very natural and refined. Bass sounds better defined and detail is improved, the slightly muddled mid is cured now also. The overall tone is quite warm yet retaining good detail seperation and dynamics. Top end is clean and extended.

Ive not done anything with the row of caps directly behind the phono sockets. I tried to trace the PCB traks to verify if they were signal decoupling caps but I wasnt certain so left them alone. I know the most obvious purpose for these caps would be decoupling but untill I can be 100% certain I'll leave them alone. I have a spare pair of Mundorfs so I can use these but as there are allready quite a few decouplers in the signal path I might be ok to link them out.

Apparently LM4562 give next to no DC output so when I get a meter I'll check this and may be able to remove a few caps from the signal path.

I left the caps before the opamp/selector-chip regs alone as Im not sure if replacing them with big 4700uF caps is the right thing to do. I also need to find out why the + and - rails have different value caps first. 1000uF on pos rail and 470uF on negative. ???????

Obviously replacing the regs with Teddy's would be good but I need help making a pair really. The assembly is no problem but Im not sure about setting the voltage?

Here's some piccys:

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Winny approves :)
 
Unfortunately I have a small problem. There is a buzzing sound on the left channel. Its not audible while music is playing, though it might be during late night listening. I unplugged the CDP so there was no input and the buzzing reamins. When the vol knob is at zero the buzz is there but as the pot is rotated the buzz shifts. At the 12 oclock position its is audible in both channels and shifts to the right with the vol knob at max.

When I did the first mods (fitting BB opamp and blackgate decouplers) I had trouble diassemling the amp. For some reason I couldnt get the vol knob off. I forced it a bit but it wouldnt budge so I thiunk Ive damaged the volume pot.

Any ideas please?
 
Yes, I plan to use an ALPS blue motorized. If the buzz reamains after that, have you any ideas what else could cause it?

Actually I have a 20K alps blue in another project, its not motorized but it would serve for fault finding purposes. Also, I will be able to evaluate the sound of the alps while its in and decide whether or not to buy a new motorized one.
 
Been busy tonight trying to solve the problem.

One thing I know was definately wrong was that the supply decoupling caps were the wrong way round so I've corrected that with new caps.

I also checked the tracks thorougly against the schematics and repaired anything I felt looked dodgy. Obviously when I get a meter I can check this properly and be certain the circuit is ok.

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I checked the solder pads for the volume pot and they are fine.

I noticed a cap just before the +15v reg had leaked so replaced that and also fitted new +/- regs to be sure it wasnt a bad regulator.

STILL BUZZING!:confused::mad:

At least I have eliminated a few possibilitys. I used the headphones to see if the buzz was still there on my BOSE TP-1's and it was. Im prettey sure this means the problem dosent lie in the power stages. I think its got to be either the pot's or someting wrong with the input buffer circuit.

I WILL be getting a meter very soon.:D

Any other ideas as to what it might be?
 
Checked for continuity around the opamp - all fine.

Checked voltages on opamp and input selector - all fine.

Checked voltage to regulators - fine 24v

Checked decoupling caps to ground continuity - all fine.

Checked resistors around opamp - all reding correct.

Im a bit lost really, never done this before. Im not sure what to do next and i'm uncertain if ive checked the continuity properly.

Any advice please?
 
I have started a new thread for the troubleshooting so this one can be just for mod ideas and results.

I plan to mod this amp further and the next area I want to attack is the power stages and their supply. There is a feedback resistor on the power chip's which if reduced will lower the gain. That will apparently (this has been computer moddeled) reduce PSSR by -6db and show an easier load to the input buffer. For this mod can someone advise of any good quality resistors? Also on the power stages are +/- supply decoupling caps. Are these really worth upgrading and if so which caps would be good here?

Next up is the PSU:

The power stage has some very large diodes. I've read good things about Hexfreds. Are these an good choice or are there any better ones for a power amp PSU? The current caps are 10'000uF 50v per rail. It it worth upping the capacitance when I upgrade them? Also I would like advice as to which caps to upgrasde to? I know there are some favorites that most of you use in your Naim gear.

Preamp PSU:

Obviously teddy regs is a no brainer. As for caps, most seem to go for Panasonic FC but again I have a question on whether its worth upping the value a bit? Currently I have 1000uF (before reg) on positive rail, and 470uF (before reg) on negative rail (stock set-up). Both regs have 220uF Blackgates after them. I think a combination of Blackgate and Pana might work well, anyone?

For the diodes am I right that Shottkey types would be good here? (supply is +/- 15v) Before the rectifier the supply voltage is 24/25v AC.

Please, you advice is very much wanted and needed.

Thanks,

Mike.
 
I SMELL COFFEE!

Decided to end this 340 mods malarky. See my last post on the 'HELP PLEASE' thread.

Thanks to all who chipped in though, I appreciate your paitence.

Rgds.

Mike.
 
Just realized I need to recind my last post, I am modding this amp still and I have a bunch of new caps and diodes waiting to go in once I've de-bugged the buzzing problem....

Mike.
 
Just to follow up on my experiences modding this amp:

I had an email from a guy who wants to mod his 340A. In hindsight of what I learned from modding mine and the exellent information found on PFM I wrote this:

Hi Will,

I made a typo. The opamp you want id the OPA2107 :) but your welcome to try any other. Search DIYaudio.com for the usual suspects. For me the OPA2107 gave the right balance combined with the LM3886 sound adding a little warmth and smoothness with sweeter treble and improved detail.

In hindsight I would have not done as much as I did to my amp. There are mods that make a definite improvement like changing the opamps and using a film capacitor in the feedback position. Past that stage you chasing the ever diminishing improvement while spending more and more for tiny little gains in sound quality.

Bear in mind that there are a lot of caps in the signal path. As long as you have a cap on the opamp input and poweramp (LM3886) input you should be fine replacing the rest with a wire link. Even if you still use the tone circuit any DC produced there should get blocked by the poweramp input caps. By the way if you are still going to use the tone circuit I would suggest you upgrade the opamp there too. I wouldn't be tempted to use big MKP (metalized polypropylene) type caps here. Get some wima polyester films (*MKS2) instead. They fit the board properly and sound really good! Go for at least 3.3uF. One thing I never tried but would if I could start again is to use a Tantalum cap for feedback. From my experience with Naim amps (they use Tants) you might get a surprising result! I used a 22uF film cap which was a big upgrade but 22uF is a little bit small. Try a 47/68uF Tant or if your using big speakers use a 100uF. Tants have a round punchy sound with excellent dynamics and great bass detail that gives a reall sense of timing and rhythm. The feedback capacitor has a massive influence on the sound so I would expect a 'naim-a-like' sound if you try this. While your at it with the Tantalums I would use them on the 7815/7915 regulator outputs and also for the local decoupling (+/- power supply decoupling) on the LM3886 chips. Make sure you use the correct voltage rating and keep the same values. Tants are quite cheap and have much lower ESR than electrolytic caps, they also filter high frequencies much better. After that consider low noise regulation for the preamp.

I wouldnt bother with the little things like fancy resistors and such. Just the opamps and a few critical cap upgrades should do the trick. Its a great little amp and these few select modifications will make it into a real giant killer!

Good luck!

Rgds

Mike.
Click to expand...

I actually think if I had done what I advised above I would have got a better amp than I ended up with (for a lot less money!). My modded 340A was amazingly detailed but also overbearing and difficult to listen to. The new ZAP250 is a revelation!
 
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