CA 340A SE LM3886 based amp - Upgrade advice please. - diyAudio
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Old 23rd October 2008, 09:53 PM   #1
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Default CA 340A SE LM3886 based amp - Upgrade advice please.

Hi.

I own a Cambridge audio 340A SE Integrated amplifier. I have allready modded the Preamp stage and apart from a minor buzz problem (this will be fixed soon) the sound is getting really good. I want to turn my attention to the power stages but I'm afraid this is where my knowledge and experience runs out.

As I understand it the poweramp stage is basically a Gainclone, it uses the LM3886 Chip. I have read good things about the DIY Gainclone and thought it would be a good idea to see what mods could be made to improve mine. It looks fairly simple with just a few resistors, power rail decoupling capacitors, and a feedback capacitor. So far I have replaced the input caps with Mundorf MKP's. I would like to know what are the 'tried and tested' caps to use for the decoupling and feedback, and if there are any gains to be had with replacing resistors. Also some power supply modifaications are planned. Currently the amp has +/- 10'000uF 50v smoothing caps. Im thinking of replacing these with Mundorf M-Lytics of the same value (possibly slightly higher capacitance?). The Diodes are 1N5402 which I plan to upgrade also. Are the Hexfreds a good choice, if so which?

This is the schematic: (C23 is not fitted as standard)


Click the image to open in full size.

Any ideas?
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Old 24th October 2008, 09:21 AM   #2
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I could probably fit one of these in he chassis:

http://www.chipamp.com/supply.shtml

Click the image to open in full size.
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Old 24th October 2008, 10:00 AM   #3
AndrewT is offline AndrewT  Scotland
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C3 is far too big,
C22 is unmatched to C3 and is too small.

Set your input MKPs and the 22k to give 80 to 100mS RC time constant. Any Polypropylene here is better than any electrolytic. Try the cheapest PP you can find before spending big money on a Mundorf. 3u6F to 4u7F would do.

Set C22 and the 1k0 to give 140 to 200mS RC time constant. 150uF would do.

Does your preamp have a DC blocking cap at it's output? Probably not since it is integrated with the power amp. If it did you have to take account of both caps to calculate the RC time constant of the high pass power amp input filter.

Add a RF attenuating filter to the power amp input.
Check there is a RF attenuating filter at the RCA inputs or at the preamp input.
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Old 24th October 2008, 10:23 AM   #4
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Many thanks for that . Im afraid however that I dont understand this time constant stuff? Im well capable of replacing components and understand most of your post though. The preamp (LM4562 +6dB gain stage) has DC bolcking caps on its input, upgraded to Mundorf 2.2uF MKP's. The output feeds into the tone/Volume circuit via another 2.2uf MKP. Then the feed from the pot goes on to the powestage (LM3886) via another pair of Mundorf MKP's. Someone advised dropping R53 from 270k to 22k for an improvement in PSSR. Again, I dont know what PSSR is/means?

To be honest Im hoping to get some simple advice, I know its kind of lazy but I havent got time to study amplifier basics.
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Old 24th October 2008, 10:46 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by mikesnowdon
I know its kind of lazy but I haven't got time to study amplifier basics.
well, Forumites know my standard response, so I'll not repeat it.
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Old 24th October 2008, 11:02 AM   #6
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While I appreciate your knowledge, I find it unhelpful that you cannot explain you earlier post in laymans terms. Im sure its a very good post and I would be quite certain of the improvements it would bering to my amp if only I knew what it meant. I assume you are talking about oscilloscope readings and the like. I do have a scope but I havent got around to learning how to use it. To be honest Im only looking to upgrade caps and resistors. Its interesting however that you suggest tweaking the values....

If I can ask one more question to you sir: Would you suggestions above bring a definate improvent in sound quality?
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Old 24th October 2008, 11:32 AM   #7
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The RC time constant is really exactly what it sounds like. The input cap and resistor to ground make a high pass filter. The time constant here is the resistance multiplied by the capacitance - RC. The corner frequency of the filter is 1/(2*pi*R*C), and you want this to be far away from any audible frequencies.

A 10uF polypropylene cap is prohibitively large and expensive, but fortunately, a smaller 3.6uF will do fine (I would even use 2u2 if it's all i had). For instance 1/(2*pi*22000*0.0000022) = 3.3Hz which is fine, 1/(2*pi*22000*0.0000036) = 2Hz which is better, but neither should cause any problems
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Old 24th October 2008, 11:48 AM   #8
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Quote:
Originally posted by valleyman
The RC time constant is really exactly what it sounds like. The input cap and resistor to ground make a high pass filter. The time constant here is the resistance multiplied by the capacitance - RC. The corner frequency of the filter is 1/(2*pi*R*C), and you want this to be far away from any audible frequencies.

A 10uF polypropylene cap is prohibitively large and expensive, but fortunately, a smaller 3.6uF will do fine (I would even use 2u2 if it's all i had). For instance 1/(2*pi*22000*0.0000022) = 3.3Hz which is fine, 1/(2*pi*22000*0.0000036) = 2Hz which is better, but neither should cause any problems

Thats good then. As the DC offset is so low on the LM4562 output I'll be removing a few of the siganl decoupling caps. Possibly I can get away with just a single 2.2 MKP on the Poweramp input.

Im very interested on the PSU above. It seems to be ideal in terms of size and It's dual mono configuration combined with the high quality parts should be good. I need to confirm its suitable first.
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Old 24th October 2008, 12:57 PM   #9
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Quote:
Originally posted by mikesnowdon
I could probably fit one of these in he chassis:

http://www.chipamp.com/supply.shtml

Click the image to open in full size.

Could this PSU be run of a single 23v transformer winding and still give +/- 28v into each output, or would it divide that to 2 x +/- 14v?

The standard Cambridge Audio toroidial has 23 - 0 - 23 windings for he power amp supply.
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Old 24th October 2008, 01:01 PM   #10
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Default Lacks a lot.

I am surprised how close this is to the National data sheet circuit. All the time constant info is correct, a 2.2 to 3.3 ufd input cap and change the C22 to 150 - 220 ufd.
It would really benefit from a more robust power supply. Something like 10K - 20k ufd per rail. Then maybe smaller value caps on the board, 100 - 330 ufd for less inductance.
But in my experiance, a small low impedance (Pana FM is perfect) electro soldered right on the bottom of the board. 10 -33 ufd works great. Solder from 1-5 to 7 and 4 to 7.
I do not know if it is the improved decoupling, or the faster current delivery of a small cap with very short leads. But the LM3886 ALWAYS sound better to me afterwards.
If people like these stock, the potential is there to get much more.


George
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