Cambridge Audio 340A SE preout conversion and other mods.

Thanks for reading.

I would like to build up a pair of NAP140 clones so I need a pre amp.

The idea is to convert my CA 340SE integrated to a decent pre amp. This means first I need to find out where the pre amp section is and work out how to run a pair of preouts to a new set of phonos. The amp already has a 'Tape monitor' output but im not sure if this is a proper preout (ie. Balance, tone controls, volume still work on this output).

Secondly I'd need to upgrade some of the components to improve the sound quality so the amp will do justice to the NAP140's. Having seen inside I see a few NE5532 opamps. Cheap pots. and various cheap capacitors. I have limeted knowledge of amps but I can solder and I know that pots are worth upgrading particularly the volume pot. But also a balance control pot would improve things. (Im aware that stepped attenuators are the best soloution but I havent been able to find one that is motor controlled, I want to use the remote functions and input swithcing etc. This is why I decided not to build a pre amp from scratch.) opamps are another thing I can change for better quality items. Obvoiusly power supply caps are worth upgrading and voltage regs too. The input DC coupling caps are worth changing too.

The problem I have is that I cant see where the pre amp section is. And Ive never modded an amp before. You should also know that im not an electronics expert but I do have an aptitude for learnig. Im looking for advice on how to do the above and any other suggestions which apply.

I'll be seeking out a service manual soon so I can study the schematics.

Below are some pictures of the internals. Please take a look and see if you can shed some light as to what each area is doing and where to do mods.











Thanks for looking.

Hello Mike,

On the first picture, the powers supply for the preamp stuff is top center. Schottky diodes, caps and regs if you can afford them.

One the right hand side, there are two opamps. I'm pretty sure that the top one is for the phone stage only, the lower one is for all inputs. Will you be sticking with LM4562s, or will you try something else?

Bottom left are the tone controls, but obviously if you don't use them you can just bypass them.

Good luck,
Dan

p.s. If you get the diagrams from CA, it may well be possible to reduce the gain by changing a couple of resistors in the preamp - save you making up an extra cable!

Sparts155 said:

Hello Mike,

On the first picture, the powers supply for the preamp stuff is top center. Schottky diodes, caps and regs if you can afford them.

One the right hand side, there are two opamps. I'm pretty sure that the top one is for the phone stage only, the lower one is for all inputs. Will you be sticking with LM4562s, or will you try something else?

Bottom left are the tone controls, but obviously if you don't use them you can just bypass them.

Good luck,
Dan

p.s. If you get the diagrams from CA, it may well be possible to reduce the gain by changing a couple of resistors in the preamp - save you making up an extra cable!

Click to expand...

Sticking with the LM4562's depends on how they sound with a decent power supply. Once I fit the new regs in the CDP I'll re-evaluate. Until then the NE5532's willl stay in place.

I always have the tone controls bypassed out with the amps 'source direct' button exept on the odd occasion when Im watching a movie or listening to poorly balanced recording.

Any idea's about the preouts 'tape monitor'? [ actually its labeled 'Rec out' ] I should really find a way to test them. I have a headphone amp so I'll nock up a test lead and see what's coming out. I hope it is a preout.

Can you tell where the preamp section is?

The preamp is what I've described above, plus of course the volume control and the input section at the top right. What part of the preamp are you thinking of?

I'm not sure about whether the tape monitor will function as a preamp out. I doubt it, but I may be wrong. If not, we can have a look at the schematic and find a place to tap the signal and route it to some sockets at the back.

Any idea where to get a sevice manual/schematic?

I was thinking of the preamp in general. Swaping out some components for better sounding ones. Nothing too technical, I dont want to change the circuit or anything like that. (apart from adding preouts)

I have the service manual if anyone would like to take a look:

email me @ mikesnowdon 'AT' YAHOO 'DOT' CO 'DOT' UK

Looking at the schematic, I think the signal goes straight from the balance pot into the poweramps. I think I can just whip out R81,82 and tap in there. May need to disconnect power amps from supply also.

Be careful with the automatic assumptions that NAP140 board clones are going to sound better - looks like a very tidy amp 'as is' to me and it will have been designed by Mike Creek or Doug Self who aren't exactly daft when it comes to amp design.

If I can be so bold as to offer some general advice, don't assume 'cheap' is bad.
The 5534 has many parameters that make it well suite to audio work. You can pay 10x the price for a faster OP amp but what if that speed is completely unnecessary for the job required and the OP amp requires additional circuitry to keep it stable?

If I were building a Naim clone I'd be inclined to also build a simple buffer/pot stage to drive it.

Robert said:

Be careful with the automatic assumptions that NAP140 board clones are going to sound better - looks like a very tidy amp 'as is' to me and it will have been designed by Mike Creek or Doug Self who aren't exactly daft when it comes to amp design.

If I can be so bold as to offer some general advice, don't assume 'cheap' is bad.
The 5534 has many parameters that make it well suite to audio work. You can pay 10x the price for a faster OP amp but what if that speed is completely unnecessary for the job required and the OP amp requires additional circuitry to keep it stable?

If I were building a Naim clone I'd be inclined to also build a simple buffer/pot stage to drive it.

Click to expand...

Why build a 'buffer/pot stage' when I already have one? Besides, I want to use the input switching, balance, tone controls etc and have the remote function. [I should also say im more interested in modding that building from scratch.]

It might be the case that the preamp section is indeed good enough to drive the NAP140 clones. It will inevatably perform better when Its running of a decent supply and the pots are upgraded. (Changing opamps seems to be a contientious issue here on fish, maybe I am better off leaving the opamps alone and just giving them a better supply.) Anyway, this is what I intend to do so any advice which is positive toward me acheiving that is what Im looking for.

Whether the power amps are as good as the NAP140 clone is hard to believe (bear in mind this is a £180 Amp). However, if anyone can suggest how to improve them markedly id love to know.

Looking forward to your positive comments.

Here's what I suggest the preamp section of the CA:

1. Bypass capacitors C14, C25, C19, C29 with a wire link. This will take them out of the circuit.

2. Replace opamp U1. With something better e.g. LM4562 if you want an analytical sound or OPA2107 if you want warmth. Better still, fit a socket and try a few 'till you find which you like best. There is no universal best.

3. If you want to reduce gain to give better volume control range, remove R33, R34, R35, R36, C12 and C13. Fit a wire link across R33 and R35.

4. Upgrade D5 - D8 with schottky equivalents.

5. Upgrade C45 and C46 with a low impedance cap e.g. Panasonic FC 2,200uf.

6. Fit better regs at U8 and U9.

7. If you want to upgrade the amp section, replace C3 and C37 with something better such as Blackgates, and link across C22 and C38

Out of these, replacing the opamp will make the biggest difference, in my opinion.

I agree with Robert, I wouldn't assume the Naim clones will be better. The Cambridge uses LM3886s for the power amp section, which are pretty good. There are better circuits than the NAP140 out there.

Dan

Wow! Youve really studied the circuit. Thank you.

I think I understand why youve slected those changes. Were mainly looking at improving the power supply. And changing opamps will always alter the sound.

A couple of questions:

Im still uncertain about how to get a suitable pre out. Am I on the right lines as ive mentioned above?

NAP140 clones: I have to be honest and admit I dont know much about the various DIY power amps out there. Maybe there are better alternatives? The NAP 140 clone is certainly a good price and offers more power with the added benefiet that I can build them into a pair of monoblocks. Im also tempted by the NAIM sound although I've only read about it. I also have a chassis handy and some other bits, Tx's heatsink, various caps etc. All salvaged from a 'Opus' (mordaunt short) multiroom amp/controller. The main reasons Im looking at this is that, A: I can use what Ive allready got so reducing the Technical aspect of starting from scratch. And B: I have alimited budget so upgrading to 'off the shelf' ready built amps is not an option. I would only get ca. £100 for my amp (if I sold it secondhand) which would leave me having to find £700 odd buy (my most wanted choice) a NAIM NAIT 5i.

I Feel converting the CA 340 into a decent preamp, and adding a good DIY poweramp, will bring greater benefits for much less cash.

Feel free to advise any better DIY power amps, and thanks for looking at the schematics.

Regards,

Mike

Are those power resistors in the poweramp section near the coils worth upgrading? And the row of caps by the inputs, they appear to be input decoupling caps so I should upgrade those too?

Also Im not sure how usefull the service manual is as the circuits seem really different to the 340A SE? The 340A service manual shows no input decoupling caps and the input driver seems different, also the 3450A manual dosent show the mute driver?

Hello Mike,

Actually I shouldn't recommend against the Naim clones - they are technically not the best, but you might really like them.

I wouldn't bother with the power resistors. I'd wire link all DC blocking caps, then measure DC offset at the output.

You will be able to take the pre-out from where you say, but you'll need to build a buffer stage into your power amp for best results.

By the way, if you get into building your own stuff, you'll have to become proficient at doing some basic measurements with a meter .... are you up for that?

Dan

I guess so. Ive seen some coplete kits for amps but I guess even those require some set up before you can run them. RE buffer stages: How about tubes!

http://cgi.ebay.com/Tube-buffer-amplifier-PCB-w-parts-Musical-Fidelity-LS29_W0QQitemZ250248281825QQihZ015QQcategoryZ12050QQcmdZViewItemQQ_trksidZp1742.m153.l1262

Would need to make holes in the top of the chassis for the tubes maybe. How cool would that be! Converting a budget amp into a tube preamp!

Whaddaya reckon?

PS: This ebay shop has some cool stuff, power supplies, power amps, preamps, regs, even chassis...Nice.

Or maybe I could replace the whole preamp section with this:

http://cgi.ebay.co.uk/STEREO-6DJ8-ECC88-6922-E88CC-SRPP-TUBE-PREAMPLIFIER-KIT_W0QQitemZ120241504415QQihZ002QQcategoryZ12050QQcmdZViewItemQQ_trksidZp1742.m153.l1262

I like the idea of the tube buffer stage but I'd like to look at incorporating it into the CA. If possible I could get it set up so I't runs the built in power amps and also provides a good quality preout.

The blend of LM4562's and Tubes could give a really nice sound.......

It looks prettey easy to do and I love the idea of a pair of tubes sticking out the top of a AZUR 340A. Sound to me like a seriously cool mod!!!!

If I were you Mike, I'd finish off modding your CD player and amp to get the best performance out of both. I can't comment on valves as I've never used them. Fitting a tube buffer into the CA is possible, but my gut feeling is it will require significant DIY skills to do it properly.

I think project management skills are important in these situations. It's easy to end up going in several different directions at once and end up going nowhere fast - I'm talking from personal experience!

Fair comment and a good point to raise. The parts arrived for the CDP today. The spowers are tiny! To be honest I cant see much else being done to the CDP for a while now besides trying out some altenative opamps.

I really want to focus on the amp and the idea of fitting the tube buffer in there feeding the CA's poweramps and providing a preout is very tempting. I need to find out more, but from what I can tell I just need a simple psu. Then It gets fed from the CA's balance pot and feeds the power amps. Seems simple but Im sure you guys will tell me theres complications.

Whats the best way to install this? (Bear in mind I want it running into the 340's poweramps and providing a preout)

A, After the balance pot feeding direct into the poweramps? (bypassing the buffer stage thats already there)

B, After the balance pot feeding into the solid state buffer stage?

C, From the CA's buffer stage output to the power amps?

Im waiting for a response from the seller on ebay to find out what kind of supply the tube module needs. All I know so far is that it runs on 12VAC 3Amps but I dont know if that needs to be regulated or not. Its feasable that if I go for option A I can run it off the supply for the original buffer stage.

I am a bigginer so feel free to comment. Im here because I need your help.

Mike

There isn't a buffer stage between the balance pot and the power amp, so this would be the place to put one.

The opamp in the photo nearest the input phonos is marked input buffer?

Sorry I what I should have asked is should I bypass the 'input buffer' (top right of picture) with the tube stage or do as you mention and run it from the pots to the poweramps?

Another thing Ive considered:

Would it be beneficial to give certain areas of the board their own dedicated supply in doing so freeing up the main psu for the poweramps - basically seperating the 'pre' from the 'power' sections so each has its own supply? (obviously if/when the tube buffer goes in it'll have its own supply too)