Naimish style power amp

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Paul Barker
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Post by Paul Barker » Thu Jul 26, 2012 1:48 pm

Well it seems very cheap and painless for something which can satisfy one of our respected members with a good sounding SE amp which sounds fine to that degree.

I have the F4 (Is it an F4 or some other no. Nick?) that Nick built me. It is very satisfactory for common use, and not significantly worst from my mediocre SE valve amps at everything they do.

I can do a lot better than the F4 with valves but I need a whole room to spread my construction across.

First I need to hear one of these things. I don't really need another unstarted unfinished project right now. But I am open minded to what you are finding.

Not saying the following to begin a never ending play on words, but just so you know I haven't a clue what this forward sound it. The valve amps I build don't send anything forward except the voice of the singer which is exactly where I want her, in that space between me and the middle of the speakers.
I can do it right or I can do it now, but I can't do it right now.

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Post by Mike H » Thu Jul 26, 2012 1:49 pm

That:

http://www.neilmcbride.co.uk/jknamps.html ('Click here for pdf file of the amp circuit diagram')

And this:
richardcooper2k wrote:Naim schematic here:
http://www.acoustica.org.uk/t/naim/powe ... ematic.jpg
.... are very traditional circuits actually, like something straight out of the mid 1970's. But then, how many different ways can you do it TBH .. :D

Using a CCS in the collector for the driver (hitherto a boot-strap) and over-current clamps for the o/p transistors, and an extra transistor to set the bias with temp compensation, were new-fangled gizmos ATT, also the reversed diodes across the supply rails was something that had me baffled for a while first time I saw it. Very shortly after, long-tailed pair input with its own CCS in the tail put in an appearance.

I'm guessing the 'greyness' comes from the global NFB, although that would keep it tight and controlled; put some metal films in or different caps, it might sparkle a bit more. Or not.... :D
 
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Post by simon » Thu Jul 26, 2012 5:22 pm

That's the chap!

Paul, if I was being unkind I might say that forward was an alternative (but positive) way of saying aggressive! It's in the eye of the beholder as with everything. I suspect you wouldn't like the old Naim sound at all, it was like marmite but worse ;-). But it's a perfectly valid way of making music, if it floats ones boat. It's potentially a lot of performance for the money though, if it does indeed float your boat.

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Post by al newall » Thu Jul 26, 2012 5:41 pm

Nice one Richard.
It's good to explore other avenues sometimes, and keep an open mind.
I don't think it's easy to pronounce something better than something else.
Best to regard different amps as useful alternatives.
Much to learn there is.

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Post by Paul Barker » Thu Jul 26, 2012 6:05 pm

simon wrote:That's the chap!

Paul, if I was being unkind I might say that forward was an alternative (but positive) way of saying aggressive! It's in the eye of the beholder as with everything. I suspect you wouldn't like the old Naim sound at all, it was like marmite but worse ;-). But it's a perfectly valid way of making music, if it floats ones boat. It's potentially a lot of performance for the money though, if it does indeed float your boat.
think I'll not venture any further than the Nelson Pass into solid state then. It doesn't suffer the afforementioned difference from a good SE valve amp. It is much the same as an SE valve amp, but not as perfect. Close enough though in many circumstances. I don't however find anything at all which I don't like about it, yet nothing at all that is better than a valve amp OTOH.
I can do it right or I can do it now, but I can't do it right now.

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Post by richardcooper2k » Thu Jul 26, 2012 10:55 pm

Mike H wrote:That:

http://www.neilmcbride.co.uk/jknamps.html ('Click here for pdf file of the amp circuit diagram')

And this:
richardcooper2k wrote:Naim schematic here:
http://www.acoustica.org.uk/t/naim/powe ... ematic.jpg
.... are very traditional circuits actually, like something straight out of the mid 1970's. But then, how many different ways can you do it TBH .. :D

Using a CCS in the collector for the driver (hitherto a boot-strap) and over-current clamps for the o/p transistors, and an extra transistor to set the bias with temp compensation, were new-fangled gizmos ATT, also the reversed diodes across the supply rails was something that had me baffled for a while first time I saw it. Very shortly after, long-tailed pair input with its own CCS in the tail put in an appearance.

I'm guessing the 'greyness' comes from the global NFB, although that would keep it tight and controlled; put some metal films in or different caps, it might sparkle a bit more. Or not.... :D
Very interesting comments for me Mike. I now have a vague idea of what the various transistors are doing. The fact it is a traditional circuit which had been published elsewhere makes it difficult for Naim to complain about others publicly using it i guess.
Also your comment about the greyness being a product of global feedback. This would perhaps explain why the global feedback cap seems to be the one which modders consider has the greatest effect. Also that Naim use tantalum caps which have a reputation for brightness.
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Post by richardcooper2k » Thu Jul 26, 2012 11:29 pm

So once i had a working amp i decided to see what difference some mods would make(i had wound output inductors as per the instructions on the Avondale NCC200 schematic). First up i replaced the input coupling cap from 10uF electrolytic to a polyester 10uF film cap i had. I also bypassed the onboard power supply decoupling caps at the font end with 100nF polyester. This seemed to brighten things up a bit.
What i really wanted to do was replace the feedback capacitor but film caps in the 47 to 68uF range seemed had to come by, expensive and very large. Film caps wee recommended on Acoustica and Hackernap project. Casting about i noticed that Les Wolstenholme recommended "wet tantalum" caps for the Avondale NCC200 on the circuit diagram. I reasoned that after many years of experimenting with these circuits as part of his business he would have made good choices so i looked into these. They are very expensive new but i found these on ebay:
http://www.ebay.co.uk/itm/3off-47uF-50V ... _500wt_970
Bargain !
I also decided to try his recommendations for the 10uF input cap and the 68uF cap that bypasses what i think from Mike's description is the bias setting transistor. Here he recommends BC 128 caps which are solid electolytics. I couldn't find any new ones but did find something on Ebay which looked like they might be the beasts:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 1068wt_906
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 1257wt_906
Ordered these and they duly arrived.
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Post by richardcooper2k » Thu Jul 26, 2012 11:55 pm

Adding these bits, particularly the wet tants made great inpovements to the sound. Things became much crisper and more solid. The amp changed from an interesting experiment which had some characteristics i liked to a contender. All much to my surprise. I suspect over the years Les at Avondale has discovered a mix of components, circuit and layout that give a good synergy together. It was about this time that i realised that Avondale Audio is about 5 minutes walk from where i work - coincidence or what ? I keep wondering about giving Les a call to see if he's up for me popping round for a chat and a listen.
The final mod i made to the boards is No 7 on Acoustica's list of power amp mods
http://www.acoustica.org.uk/
This involves decoupling the power rails with 470uF caps direct from the legs of the output transistors to the power supply caps. I just used normal caps and only went as far as where the earth wire joined the amp board. None-the -less this gave an audible change for the better i thought. Tightening things further and adding a little welly.

This is pretty much how things stand at the moment

Next up is building a cheap version of the hackernap power supply to see what that does
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Post by richardcooper2k » Fri Jul 27, 2012 12:10 am

And here's the beast !
One board is upside down as otherwise i couldn't get them side by side on the Denon heatsink
On the back of the top board you can see the 2 power supply decoupling caps connected direct to output transistors. The red thing between them is the output inductor so i don't need naim speaker wire to maintain stability.
Attachments
naimish.jpg
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Post by Mike H » Fri Jul 27, 2012 3:23 pm

richardcooper2k wrote:Very interesting comments for me Mike. I now have a vague idea of what the various transistors are doing. The fact it is a traditional circuit which had been published elsewhere makes it difficult for Naim to complain about others publicly using it i guess.
I guess not. :lol:

BTW re the above linked NAP250 schematic, it's very spookily similar to a 100W design in Practical Electronics from about late 1970's, wot I built, worked OK.

Also, where you got 100R emitter resistor from TR10 going to R13 (not direct to output rail as otherwise), I remember being very particular about how I drew that (to get the layout just right) when I used to use Rapidograph pens to make a neat 'final' circuit diagram.
This would perhaps explain why the global feedback cap seems to be the one which modders consider has the greatest effect.
I guess it would, in the same sort of way a cathode bypass cap would. By way of comparison.
Also that Naim use tantalum caps which have a reputation for brightness.
That is interesting, 'cause wherever else I've read (so far) reckon tants are hopeless for audio. It's a fundamental problem of tantalum IIRC, the 'K' factor is non-linear with frequency, or somesuch. There used to be a good web page mentioned it IIRC, all about various cap comparisons, with tests and performace graphs, but long since disappeared unfortunately.
 
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Post by Mike H » Fri Jul 27, 2012 3:31 pm

richardcooper2k wrote:The final mod i made to the boards is No 7 on Acoustica's list of power amp mods
http://www.acoustica.org.uk/
This involves decoupling the power rails with 470uF caps direct from the legs of the output transistors to the power supply caps.
Ah yes I'll go with that. :D

Funny innit we used to have big PSU caps but then long wires connecting them to where the actual amps were, and think that was 'normal'.
 
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Post by IDM » Tue Oct 16, 2012 9:31 pm

I am sure Les would be more than happy for you to come round. I first came across Avondale and Les several years ago whilst Googling around for a cheap alternative to the Naim HiCap.

I ended up giving Les a call and he was really helpful and encouraged me to have ago at modifying my Naim pre-amp boards. In fact he sent me a whole bunch of components including small gyrator PCBS free of charge.

Thinking about it he is responsible for me getting into DIY hifi and the near constant smell of solder fumes in my spare room!

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Post by pre65 » Wed May 07, 2014 7:28 pm

Any updates on this Richard ? :)
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