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AKSA 55 amp

davidsrsb said:
But this is enough to cause some bass distortion and poor supply rejection at low frequencies, which is also a Rotel feature.
The thing is that changing the tail to something closer to a true current source and therefore closer to a "blameless" design is not liked
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Yes, you may well be right.

It really needs the PSRR plotting out versus frequency, either by measurement or simulation. Poor rejection at low frequency would cause LF distortion, as you say, and could also result in some 'nice' sounding reverberation.

Chris.
 
The diode in the +ve line is going to lead to some very unusual intermods with 100Hz bridge ripple and bass signal derived ripple
 
stevec67 said:
Well, given that I wouldn't have a 250 in the house, I'm afraid you can guess at the answer. Not quite as good, I'd say.:p
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I hope you have your flame suit on ... there's a swag of Naim fanbois on PFM whom you will have incited to cause you damage! :p

stevec67 said:
My valve amp is a simple enough circuit, after all.
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Yes, valve amps generally are simple circuits ... but they often sound very good! :D But these (good ones) have, I suggest:
a) a good PS, and
b) a good (read expensive!) output transformer that delivers an HF roll-off point of 80-100KHz ... not just 20-30KHz.

Regards,

Andy
 
Many people like a little deep bass distortion, which good valve amps and some transistor designs do. A small amount of distortion below 100Hz can give some doubling and make the bass sound "faster".
HF distortion is not ok, sometimes I see reviews of valve amps with poor transformers which are slewing. The review praises detail and then complaints about fatiguing sound appear further down the page
 
Speculating and simulating will get you nowhere. I could never measure any perturbations on the C3 rail. The diode is not that unusual, IR's mosfet amp from decades ago used it. What is a bit unusual is to hang the bootstrap resistor off the same rail - normally it would be supplied from the output rail.
However my experiments lead me to believe I can hear significant differences in sound with modest changes to component values and types at/around C3.

WRT to chip amps - this one is rather good:
http://www.akitika.com/
 
al2002 said:
I had seen this but I don't believe this site has the actual circuit.
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There are a couple of "AKSA 55" circuits generally available on the internet.

#1. The ESP61 that was available on Rod Elliott's website for a short period before Hugh decided to go commercial
#2. DestroyerX's conceptual schematic (contains errors)
#3. The BAKSA (or Baby AKSA) which is a redo of #2 with a couple of obvious incorrect values fixed
#4. Hugh conceptual AKSA schematic.

I have purchased an AKSA 55 amp so I do have 3 real schematics for reference, the AKSA 55, AKSA 55 Nirvana and AKSA 55 Nirvana+

Forget about #1. It is a very early version of the AKSA.
Forget about #2. It has errors on it.

Circuit #4, Hugh's conceptual schematics is very close to the real thing. The interesting thing is the omission of some values. By inference, Hugh must think these values are important in tuning the amp. Good to know.

Circuit #3, the BAKSA. A complete schematic with values. One or two components missing from circuit #2 but the values give you a starting point to the AKSA 55.

It was discussed in some of the AKSA threads the suck-out cap on the output transistors. Pretty much a standard fixture on most amps but missing on all the schematics.

Tune the values for the right currents and select the right components and you will be very close to the real thing.

A couple of years ago Hugh stated:

You are essentially correct, there are some issues not covered. A quick email to Greg will clear most of them up, however. I do not wish to put this valuable circuit in full detail on a public forum, because I know some inconsiderate fellow will start to make them in quantity, and since it's my IP I feel I should make it tough for him.

I agree with NP. A good amp is one third topology, one third layout, and one third parts choice. When it's all weighed up the topology is only a small part of it.
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regards
 
Thanks for the suggestions Greg. I am not really interested in building the AKSA - as stated in an earlier message, my interest was purely academic - so I don't want to bother Hugh.
 
TDI said:
I'm about to start on an active loudspeaker project, and need to decide on which power amps to use.

Has anyone here had any experience of the original AKSA 55 amps compared to Naim clones for sound quality ? Hugh Dean of Aspen has made the AKSA 55 circuit available to the DIY audio community, now that this model is no longer sold.
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Did you make a decision?

As a result of this thread I recommissioned my old AKSA 55N to have another listen. For the purists - they are built on P61 pcbs and incorporate all the Nirvana upgrades as well as a few tweaks to implement the AKSA 55 circuit instead of the original P61. These were the boards that I did all my experimenting with as I wanted to keep my AKSA 100Ns as the maker intended.
No iteration of my 55s ever sounded as good as the official 100N, however I sold that amp a few years ago.
Originally my 55 was dual mono. I've just rebuilt it as a stereo amp with shared PS.

Since I last heard my little AKSA my system has changed from 2-way to semi-active 3-way, so now the main amp only has to do duties above ~100Hz.

How does it sound?....
Very good indeed and not really how I remember it. My memory was of a characterful and colourful amp that excelled with simple (acoustic) music but lost it's grip somewhat when the music got busy - the 100N was rather better in this respect. It now sounds much better than I remember and has a remarkable ability to extract musical phrases and interplay between instruments and voices.
I oiked my Leach low TIM out of the loft for a comparison, in the past this was preferred to my AKSA due to its unflappable control at any frequency and power output (courtesy of its 1KVA toroid). Now that the amps don't have to control the uncontrollable (ports!!) the AKSA leaves the Leach sounding somewhat bland and mechanical.
So it looks like I did the AKSA a disservice earlier in the thread and I would now say it is highly recommendable.

Time will tell if it stays in the system long term and displaces my KT88PP amp, but it's the closest SS amp I've heard (except perhaps the Nytech Obelisk-like amp I built - really must stop dismantling the best ones!).

Now, why did I sell the 100N? :(
 
S-Man said:
Since I last heard my little AKSA my system has changed from 2-way to semi-active 3-way, so now the main amp only has to do duties above ~100Hz.

How does it sound?....
Very good indeed and not really how I remember it. My memory was of a characterful and colourful amp that excelled with simple (acoustic) music but lost it's grip somewhat when the music got busy - the 100N was rather better in this respect. It now sounds much better than I remember and has a remarkable ability to extract musical phrases and interplay between instruments and voices.

So it looks like I did the AKSA a disservice earlier in the thread and I would now say it is highly recommendable.

Time will tell if it stays in the system long term and displaces my KT88PP amp, but it's the closest SS amp I've heard (except perhaps the Nytech Obelisk-like amp I built - really must stop dismantling the best ones!).

Now, why did I sell the 100N? :(
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So why don't you couple it with a NAKSA 100 on the bass? ;)

And I can assure you, having done the switch about 6 years ago, that a Lifeforce 55 sounds better than the 55N ... and the NAKSA 80 sounds better still. :D This is my next upgrade - replace 2 Lifeforce 55s with 2 NAKSA 80s.


Regards,

Andy
 
andyr said:
So why don't you couple it with a NAKSA 100 on the bass? ;)
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The NAKSA is not suitable. My bass amp needs some special characteristics and the ability to drive 1.9 Ohms. Also, no need for "audiophile" quality when it does very little above 200Hz.
 
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