Curious About an Amp's Design . . .

[tomman888]

Hey guys. I have a Cambridge Audio amplifier which I would like to know more about. It's the 640A version 2. All I know at this point is that it's a class-AB which does 75wpc into 8 ohms and 120wpc into 4; frequency response 10-30kHz +/-1dB and it can slew 30V/uS.

But I've read contrasting reviews on the net about this amp. (Before anyone chimes in about the original 640A, I realize they weren't so hot, and yes I searched here and read the review about it being a gainclone in disguise....)

The manufacturer provides very little info on the design, and I would like to know more.

It does sound quite nice with my Vandersteen 2C's and Grado 325i cans. (got a small power amp for cans built in, rather than dropdown resistors)

But my concern is that it uses 2200uF times 4 per channel. While the toroidal PSU is very hefty, and has dual secondaries for a pseudo-dual mono config, I think 8800uF per side is chintzy. What' the deal with that, besides cost-cutting? Is this thing 'tuned' or 'voiced' somehow?

If you can't tell already, I'm really interested in audio and would love any info you guys have on this amp.

[sreten]

chintz⋅y
   /ˈtʃɪntsi/ Show Spelled Pronunciation [chint-see] Show IPA
–adjective, chintz⋅i⋅er, chintz⋅i⋅est.
1. of, like, or decorated with chintz.
2. cheap, inferior, or gaudy.
3. stingy; miserly: a chintzy way to entertain guests.

Hi,

It is not at all. 4,700uF to 10,000uF is a typical class aB range.
4,400uF per rail per channel is not stingy, splitting the capacitors
can give higher ripple ratings and lower hf impedance.

/sreten.

[tomman888]

Well that's good to know, since it does sound good indeed.... Just that my vintage Onkyo amp was rated for 45wpc at 8 ohm and it had 14,000uF per side. Class-AB, too.

[AndrewT]

with ONLY +-4400uF on each channel it will not sound like a -1dB amplifier at 10Hz, even if it measures as that on a constant sinewave test signal @~1W.

A full bodied amplifier capable of driving a 4ohm speaker to extended and strong bass (but not exaggerated bass) requires ~+-40000uF per channel. That gives excellent response down to below 20Hz by having a -1dB ~=3Hz. reducing the capacitance to 4400uF raises the -1dB frequency to ~27Hz and this will be easily audible if you were to compare PSUs on like amplifiers.

[homemodder]

I have self drawn schematic of the amp section only, nothing wrong with the design, not sure if its version 2. Its not a gainclone, and its based on the Self designs. Its a simple design and has a vas design Ive never seen used before in any amp, its good and linear. Its a cheap amp and a lot of cost cutting and cheap components will most probably have been used. Good candidate for upgrading by just using better quality parts, those supply caps would be the first Id replace, 4 X 2200f is lowish if one is looking for better sound quality.

Parts quality is usually the reason a diy design of even simpler design most times outperform this kind of amp although this amp is well designed and could be made to sound very good indeed. It would have pushed the price into a higher bracket with better quality parts.

[tomman888]

I would LOVE to see that schematic, if at all possible.

Regarding the capacitance, and the rolloff at roughly 27Hz, this isn't much of a concern since my speakers are only flat to 30Hz anyway.

[homemodder]

Send me an Email via the link, and Ill email a copy.

AndrewT has a good point, 30Khz might be the rolloff of your speakers but things dont quite function that way when it comes to sonics. This amp is said to sound somewhat lightweight with poor dinamics compared its price competion and even to cheaper units. I know some of the cheaper units that this amp is compaired to and they all have somewhat bigger caps.

[tomman888]

yeah I figured as much......Dynamic response isn't too bad but at high volume the clipping indicator begins to light here and there, and the gain reduction system turns back the volume pot a bit.

My email is soundofcloud21@gmail.com

Couldn't send you and email because I am 'under moderation' (??) But just want to say thanks!

[AndrewT]

Originally posted by AndrewT
raises the -1dB frequency to ~27Hz and this will be easily audible if you were to compare PSUs on like amplifiers.

my speakers start to roll off at ~55Hz and I can hear the difference.
I know which I prefer.

[sreten]

Originally posted by AndrewT

with ONLY +-4400uF on each channel it will not sound like
a -1dB amplifier at 10Hz, even if it measures as that on a
constant sinewave test signal @~1W.

A full bodied amplifier capable of driving a 4ohm speaker
to extended and strong bass (but not exaggerated bass)
requires ~+-40000uF per channel. That gives excellent
response down to below 20Hz by having a -1dB ~=3Hz.
reducing the capacitance to 4400uF raises the -1dB
frequency to ~27Hz and this will be easily audible if
you were to compare PSUs on like amplifiers.

Hmmm.......

you seem to be bandying around irrelevant numbers.
As if you can hear -1dB at 10Hz or 3Hz ...

Your implying the supply capacitance is 10 times too low,
and this somehow affects the amplifiers bass response,
well it affects the response but not in the above manner.

Your design criteria seems obscure and ill-defined.
It seems an excuse for overdoing it, which cannot be "bad",
but is not realistic for "budget" commercial amplifier design.

/sreten.