[Greg Erskine]

Hi all,

I'm not working on an AKSA amp at the moment but it does have EF topology and uses the same output transistors.

In Randy Slone's "High-Power Audio Amplifier Construction Manual" on page 409 it says " Vbias is adjusted according to Vq, which is the sum of the quiescent voltage drops across the output RE resistors…..In the case of EF OPS designs, Vbias is adjusted for a Vq of 47mV."

This just sounds too easy and to general.

On the amp I am adjusting the values are currently 27.5mV and 23.5mV which apparently are way off.

Any comments before I start adjusting the Vbias on this amp.

Thanks

[AKSA]

Greg,

This sounds right.  A close analysis of SS physics according to the Boltzmann constant tells us (eventually!!) that the ideal bias point is 26mV across EACH emitter resistor on a EF output stage in Class AB.  If you double this figure, you get 52mV, which is close to the Randy Slone figure on his Optimos.  I found the best was 58mA by listening tests, but in truth this is a highly subjective adjustment, and further quiescent current often gives better, more powerful bass at the expense of sound stage.

You can't have it all, where would you put it??

Cheers,

Hugh

[Greg Erskine]

Thanks Hugh,

I'll install a pot and start adjusting and see what happens.

You said "I found the best was 58mA" but that depends on the value of the emitter resistors. It's the 55mV across the both  emittor resistors no matter what value they are that counts.

Does that sound about right or am I confused as normal?

Thanks
Greg

[fajimr]

In Randy Slone's "High-Power Audio Amplifier Construction Manual" on page 409 it says...

just out of curisoity... I have been purusing Amazon.com, looking for books to inform myself better- it's that damn academic side of me I just can't shake:lol:  I just ordered "Tab Electronics Guide to Understanding Electricity and Electronics",  G. Randy Slone and "How to Test Almost Anything Electronic",  Delton T. Horn as primers but ran across Sloan's book (I've also looked through his website).  

Just how technical is this book?  I don't feel I need to know 'how' to build an amp myslef but just want to understand a little better what I am doing when I am building one of Hugh's    

thanks as always in advance for your insights

[AKSA]

Jim,

These books are what I would consider the 'populist' press.  As an academic, I doubt you'd be happy with them because they lack academic structure as a rule, and are often couched in layman's language, with resulting lack of precision.  Fine in it's place, but for serious work where math based understanding is required, probably not sufficient.

I use Boylstad and Nashelsky, 'Electronic Devices and Circuit Theory', Prentice Hall, 1996.  This was a fourth or fifth edition, so it's quite likely it's still available in a current reprint.  The books is magnificently conceived, very well set out, large, worked examples, lucid explanation and problems with answers for working through.  Hell, even I can follow most of it, and I have the math ability of a circus trouper.........  I love this book, and treasure it.  It even has reference pages set out with all the maths pertaining to all the common (and uncommon!) circuit configurations for each device as they are analysed chapter by chapter.  Go through this book and you have a brilliant working understanding of modern analog electronics.  I would stress this is NOT digital electronics, bear in mind!

Cheers,

Hugh

[fajimr]

I had to laugh with your quote that you had the math ability of a circus trouper....  pleeeeeaaaaase!!!!  where oh where would that put me?   I could always get the right answer in statistics but I never knew how I came about them      ....  and besides I like qualitiative research much more.

Boylstad and Nashelsky's book is up to the 9th edition and I just found a good used copy of the 7th ed (1999) for 8.00 USD- so I'll add that to my collection.  Start with the populist reading to figure out North from South and then get into something more substantial... my soldering gun, mulitmeter, and kits should be arriving soon so I can start on that thread, too!

[Greg Erskine]

Hi fajimr,

Well, I was about to say that I found "High-Power Audio Amplifier Construction Manual" an excellent book. My favourite 'populist' press book because of the lack academic structure, often couched in layman's language, with resulting lack of precision.  

I do refer to my electronics texts for time to time but for a good read on the train I prefer Slone's book.  

[AKSA]

Greg,

My favourite 'populist' press book because of the lack academic structure, often couched in layman's language, with resulting lack of precision.  

Ya know, I luv it........  gudonya, mate!!


Cheers,

Hugh

[ginger]

So you want to understand you AKSA Amp better.

For FREE you can go to the Doug Self Web site and download his paper on the "Blameless Amplifier".

Then all you have to remember is that we don't want just another Cold Sterile and Boring "Blameless" Amplifier.

What we want is something with just the right amount of 2nd harmonic distortion with just a tad of 3rd harmonic distortion to give us that gorgeous warm emotionally engaging sound.

Edit: I went looking for the paper BUT could'nt find it - eventually tracked down a copy here:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

The Doug Self Paper is very interesting from the point of view of what of Dougs ideas of a "Perfect" amplifier are incorporated in the AKSA and possibly of even more interest is what of those ideas have been deliberately left out.

In a conversation probably 2 years ago or more Hugh assured me he had tried every circuit variation covered by that paper during the development and fine tuning of the AKSA. As a Professional Design Engineer with a GOOD understanding of the way my AKSA 55N+ works I can confirm Hughs decisions - how, because I basically tried them all again, particularly Current Source Loaded VAS stage (in lieu of the bootstrap VAS load - those of you who have compared the front end of your AKSA 55 or 100 with the front end of the GK-1 will know what I'm talking about here) and on pure listening evaluation I ended up back at the original AKSA - that is my evaluations paralleled Hughs.

More rave:
I've just completed a retest of all of the various options for the bootstrap cap in the 55N+ (BGs, Elna Cerafines, etc etc.) and found the 100uF/50V BG Standard in the N+ Kit to be the second best option available. The best option is a 470uF/50V BG Standard BUT you have to do some serious PCB hacking to make it fit and the improvement was marginal anyway.
Aside: I stated in an old thread here that 2 Super E connected 100uF/50V BG Ns where even better BUT this round of tests contradicted that.

In good engineering tradition (and because I've just finished reading Allen Wright) I am trying one thing new - shunt regulation for the diffamp positive supply. Will let you know if this proves beneficial but I'm not real hopefull because it involves messing with the one bit of the circuit containing some of the AKSA Black Art I've never really got my analytical head around.

Cheers,
Ginger

[andyr]

... I've just completed a retest of all of the various options for the bootstrap cap in the 55N+ (BGs, Elna Cerafines, etc etc.) and found the 100uF/50V BG Standard in the N+ Kit to be the second best option available. The best option is a 470uF/50V BG Standard BUT you have to do some serious PCB hacking to make it fit and the improvement was marginal anyway.
Aside: I stated in an old thread here that 2 Super E connected 100uF/50V BG Ns where even better BUT this round of tests contradicted that.
...

Hi, Ginger,

Oh dear ... just when I had 8 x 100uF/50v BG Ns delivered, in readiness for doing that mod!!

Why wouldn't a 100uF/50v N sound better than a 100uF/50v BG Standard ... according to your previous ranking of the "goodness of sound" of the various types of BG caps, it should??

Yes, it may not be as good as a 470uF/50v BG Standard but you wouldn't have to hack the PCB around.

Regards,

Andy

[ginger]

Andy,
I've had the Super E connected 100uF/50V BG Ns mod in and out about 3 times. Each time you need to allow 2 weeks for it to settle in. The first time I liked it better, the last time I wasn't as convinced. Give it a go and decide for yourself. It may well just be the music I was playing that week.

The problem with these "at the edge" upgrades is that the old do one channel only and see if it sound better doesn't neccessarily show up any benefit - you need to do both channels to get any imaging improvements.

You can always back it out again later if you find its not to your taste.

A second opinion is ALWAYS valuable.

As to why a "known better" cap might not sound better - well suffice to say that part of the "AKSA sound" is the Bootstrapped VAS stage. Certainly if you delete the bootstrap and use a Current Source load for the VAS (which I've tried) it doesn't sound like an AKSA anymore - Too cold sterile and boring. So it might be possible to overdo the bootstrap. Please proceed and let us know your opinion.

After all my opinion on technical stuff might have some "authority" due to my training as an EE but my opinion of the sound is no better than yours and if we're talking about the system in your house then my opinion is irrelevant.
Cheers,
Ginger

[andyr]

Andy,
I've had the Super E connected 100uF/50V BG Ns mod in and out about 3 times. Each time you need to allow 2 weeks for it to settle in. The first time I liked it better, the last time I wasn't as convinced. Give it a go and decide for yourself. It may well just be the music I was playing that week.

The problem with these "at the edge" upgrades is that the old do one channel only and see if it sound better doesn't neccessarily show up any benefit - you need to do both channels to get any imaging impr ...

Mmmm, OK, Ginger,

Can I take you up on a coupla points (ie. can youse please explain more!!  ):

1.  You said: "So it might be possible to overdo the bootstrap".

Yes, I can understand this concept but I would've thought "overdoing it" was associated with the # of uF ... ie. changing the "100uF/50V BG Standard in the N+ Kit" to "a 470uF/50V BG Standard" is certainly "overdoing it"!!??.

I can't understand why changing a "Standard" to an "N" for the same uF value would be "overdoing it", if the "N" is regarded as a better-sounding cap??

2.  To my way of thinking, to "delete the bootstrap and use a Current Source load for the VAS" is a vast difference to merely substituting one type of cap for another ... or even changing 100uF to 470uF.

Regards,

Andy

[ginger]

Andy,
Well here goes - Gingers BS Theory.

The boostrap 1st and formost is POSITIVE feedback. That means it increases distortion AND its only stable because there is a NEGATIVE feedback loop around it. The bootstrap is there to maximise the effective value of the VAS load resistor and hence get maximum gain from the VAS stage. If the bottom of the load resistor (3K3) has say 10V pk-pk signal and the otherside has 9.6 V pk-pk signal coupled from the output via the bootstrap cap THEN the 3K3 has 10 - 9.6 = 0.4 signal across it and its effective value is 3K3/(1 - 9.6/10) = 82.5K. Since the value of the VAS load resistor appears on the top line of the Open Loop Gain Equation the Open Loop Gain has been increased by 25.9 times by the BS action. This mechanism will impart most of the sonic signature of the bootstrap cap on the VAS and hence on the whole amp. If the bootstrap cap is TOO GOOD we won't get that 2nd harmonic rich distortion profile. At very low frequencies the capacitive impedance of the cap increases and the bootstrap action is reduced, thus increasing distortion at low frequencies (c.f. a valve amp). If you want some of this low frequency distortion then its quite possible to use TOO high a value BS Cap. Similarly at the top end, above the self resonant frequency of the BS cap the bootstrap action is reduced and gain drops. This means that you don't need quite as much negative feedback around it too keep the amp stable at high frequencies and this has other sonic benefits in other parts of the circuit.
Personally I think its more important to choose type and value of BS cap to get the high mids and tops clean - the "Super E" connected BG Ns option SHOULD do this the best BUT MY listening tests are NOT conclusive.

In the end its what suits your musical taste and you need to balance quality and value of the BS cap to get the sound you want.

I can tell you from my experiments that the following ALL sound quite different and suit different music and musical tastes:
Hitano 100uF/50V
BG Std 100uF/50V
BG Std 470uF/50V
2 Super E connected 100uF/50V BG N
Nichicon Muse 330uF/63V  

So experiment for yourself and choose what best suits YOU.

Cheers,
Ginger

[andyr]

Andy,
Well here goes - Gingers BS Theory. ...

Hi, Ginger,

Thanks muchly for your detailed response.  I have to admit (it having been a few days since I had posted) that when I clicked on your post and saw "Ginger's BS Theory" ... I interpreted this first as BS = bullshit!!  

Then I read on ... and realised it was the genuine article!

Now you say your experiments show the following suit different music and musical tastes:
Hitano 100uF/50V
BG Std 100uF/50V
BG Std 470uF/50V
2 Super E connected 100uF/50V BG N
Nichicon Muse 330uF/63V.

Let me, just for fun, make an (un)educated guess here!  
Hitano 100uF/50V   -  heavy metal, headbanger type
BG Std 100uF/50V  -  rock
BG Std 470uF/50V  -  orchestral, opera
2 Super E connected 100uF/50V BG N  -  chamber music, guitar
Nichicon Muse 330uF/63V  -  hip-hop/rap!

Whaddya reckon??  

Regards,

Andy

[ginger]

Andy -we seem to have got Off Topic a bit - well not to worry!

Yeh the Gingers BS Theory was meant to be a bit tounge in cheek. Ill leave it others to decide if it meant "Bootstrap" or the more obvious.

Well as a SWEEPING GENERALISATION (which we try to avoid - right?) and based ONLY on my experience -

The bigger value caps seem to tighten up the bottom end a bit BUT cost a little impact and slam.
The "Super E" connected BG Ns did a stunning job on the high mids and tops (gave my 845 SET a serious run for its money) BUT seemed to cost even more in impact and slam - I got that lovely SET sound which does amazing things to female vocals in particular. I used Linda Ronstadt Spanish Folk Music as a test ("Tales from my Father"). I also tried Norah Jones BUT found it sounded stunning on any old amp and is NOT a good test music choice. That old problem of picking suitable test music. In actual fact some of the "head bagging" stuff makes good test music (Spiderbait "Black Betty", Nickleback etc.) because its so full and harmonically complex that intermodulation and harmonic multiplication products stand out like the proberbial dog's ....
The 330uF/63V Nichicon Muse I liked from a spectral/tonal balance point of view but there was a layer of noise? indestictness? over the top of it and I regretfully pulled them out again. I might have to try them again because after I'd finished the tests I found that I had an old experiment lingering in that channel - some 100nF ceramic caps across the main power supply filter caps which were contributing to that sort of sound. High K Ceramics DON'T belong in HiFi Amps - not even in the power supply.

CAUTION to those trying mods to the basic 55 - You need the 55N+ Power Supply Upgrade to get the full benefit of any of these changes. If you haven't done that, then do it first. I'm not sure if you get the upgraded power supply with the Nivarna or ONLY the Nivarna Plus - But its critical for best from your amp. Hugh will be able to advise those interested I'm sure.

Cheers,
Ginger

[SamL]

Hi Ginger,

Have you tried Elna SILMIC II? I got quite interested after reading the comment from this site but find nothing in this forum.
http://www.audio-cube.nl/elna.htm

Sam

[ginger]

Sam,
No - sorry, one for the "To-do" list.
Cheers,
Ginger

[ginger]

FINAL REPORT
55N+ Bootstrap cap (C4) experimental results - NOT suitable for the 100N+ due to cap voltage ratings.

After weeks of careful listening to a huge range of music I finally came to the conclusion that the 2 x 100uF/50V Blackgate N "Super E" (paralleled but one cap reversed with respect to the other i.e giving 200uF total) suited my musical tastes, speakers, listening room etc. the best, and worked best with my GK-1 Manual as the drive.

However the standard 55N+ part - the 100uF/50V Blackgate Standard was a VERY close second and in fact had more impact and slam BUT slightly less high mids and top clarity/definition. I think however that this would be a better option for passive or Solid State Preamp or direct drive from a CD player etc. that is whenever you don't have the tube magic from the GK-1.

Larger Caps (e.g. 330uF or 470uF) were not as good, costing too much impact and slam (not to mention cash). The 330uF have been put back in the bin for other projects - the 470uF/50V Blackgates have already been redeployed as cathode bypass caps for an autobiased EL84 Ultralinear Push Pull Tube Amp.

Cheers,
Ginger

[AKSA]

Ginger,

I agree pretty much lineball with your comments, and thank you for doing the tinkering and auditioning;  it's a lonely game!

However, I should add that while it's generally true that the larger the bootstrap the less is the impact and slam, it should be said that on the 100W AKSA, where VAS current is appreciably larger, this threshold is increased and 470uF is about optimum PROVIDING the bootstrap cap has very low dielectric absorption.  I believe this is the reason the better quality caps work well even though their greater efficiency means there is less euphonic supplement to the signal.......

Ginger, your comment about a bootstrap being positive feedback and holding stability because it's within a negative feedback loop is a moot point.  There are many angles to this.  I see the BS as a CCS mechanism, and as you point out, it actually aids Nyquist stability because its effectiveness quickly diminishes with increasing ESR and frequency.  In this sense I don't feel the NFB loop makes much difference;  but the reduced need for lag comp/phase lead is a huge benefit.

Cheers,

Hugh

Cheers,

Hugh

[ginger]

Hugh,
Your comment about the positive feedback aspect of the bootstrap is reasonable.

What I should perhaps have emphasised about the bootstrap is that it by effectively multiplying the load resistors value (at AC ONLY) it forces the VAS (Voltage Amplifying Stage) into constant current operation and hence linearises it. This action significantly reduces distortion and so any minor increase in distortion due to positive feedback is more than compenstaed for by reducion of distortion due to constant current behaviour.

Explanation for newbies: The signal current swing in the VAS transisitor is reduced by the same factor as the effective load resitor multiplication. i.e. about 26 times - you don't have to swing as much current into the larger effective resistance to generate the same signal voltage swing. That means the operating point on the Vbe (base to emitter voltage) vs Ic (collector current) curve hardly varies at all. This is called constant current operation, the current is not truely constant but the current variation about the operating point is vastly reduced from a normal Common Emitter amplfier stage.
Cheers,
Ginger