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-   -   Swordfishy/ASPEN FETZILLA power amp (http://www.diyaudio.com/forums/aksa/191053-swordfishy-aspen-fetzilla-power-amp.html)

AKSA 18th June 2011 07:01 AM

Swordfishy/ASPEN FETZILLA power amp
 
2 Attachment(s)
Folks,

I have collaborated with Greg Peters, Lineup and Mikelm to design a new power amp of 50W which uses a jfet front end, a mosfet voltage amp, and lateral fet output devices. This amp runs on 36V rails, has integrated power supply rails, and is configured to lie flat on a flat backed heatsink, screwed directly onto the output devices.

The board is 230mm x 69mm, borrows heavily from the NAKSA layout, and while it does not have anything like the power is a good amp set up specifically for FETs, which many like. It was conceived largely as an intellectual exercise, but has survived 115 pages of intense scrutiny on the SS forum, with varied input from lots of clever people.

I will offer these boards (which aren't cheap as they are premium quality) for sale through my website in due course. In the meantime, would those interested please register their interest, along with the numbers required, so that I can get a feel for how many to order.

This board has taken many tens of hours but because humans are fallible may well have mistake(s). If you see something glaring, please let me know in this forum. If you have strong design issues you wish to bring up, then do so on the SS forum, and be aware that Greg and I will not from this point forward, for obvious reasons, change the design. Greg has done lots of listening and CRO tests and feels this is as good as it gets. For myself, I will be most interested to compare it to my own designs; the bar is high!!

16th July 2010: Strategic design update for the benefit of new visitors

Here are the design approaches of Greg Peters, Mikelm and myself so far. I would point out this has all been the original brainchild of Lineout, the redoutable northern Swede who while he lives in one of the coldest places on the planet has an almost feverish, active mind.

#1 1st stage: singleton input device. After much discussion and a few trials, Greg has opted for a jfet, as this gives very low transconductance, around 20mS, and this keeps loop gain low which seems important to the good sound of this amplifier despite higher THD figures.

#2 IIRC Lineout's original design called for a single ended input and VAS with complementary push pull output stage using DC coupling throughout. Essentially this meant eliminating the feedback shunt cap, a 1000uF beast that Mikelm in particular disliked. The problem is that offset control without a servo - which adds complexity and detracts from the circuit simplicity - is very poor with DC coupling as DC gain is no longer unity, but the reciprocal of the gain. In amp design the DC offset control is not a trivial design problem, as it has the potential (literally!) to destroy speakers if poor. Accordingly, Greg has chosen to continue with AC coupling, so back in goes the cap, and DC offset is now very good. I endorse this choice strongly.

#3 Lineout originally chose the IRFP9610 p mosfet as VAS. Briefly the advantages of a bipolar device such as the 2SA1360 were mooted, but Greg noticed, and Andrew confirmed, that a mosfet VAS required no Miller capacitance for rock solid stability. It also gave a more 'lively' sound with palpable vitality. I went looking for a better mosfet VAS, one with a lower current rating to improve its linearity at very low drain currents, typically around 12mA in this application. Greg had suggested the Zetex ZVP3110A, but it's only a TO92 and limited to 740mW. In this application, and with the original 20mA VAS current selected, this gave very high dissipation, too high, so I felt the 2W smd Zetex ZVP2110G would be superior as it was essentially the same chip in a SOT223 high power package. Eventually we decided that since the Ciss of each output device gate was bootstrapped, 20mA of VAS current was not needed, and ultimately we've settled on just less than 12mA, a good compromise.

#4 Originally Lineout had selected a bootstrap CCS for the VAS, but this seemed to detract from the bass response of the amp, though it does sound very good on vocals. Paul Bysouth, a clever Melbourne engineer and good friend, suggested using the Supertex DN2530 as a CCS/bootstrap hybrid in an attempt to get the best of both worlds - good bass, and engaging vocals. The jury is still out on this clever modification, but it's incorporated into the pcb as T3 and on Monday Greg will be able to tell us how it sounds.

#5 I decided to offer a pcb as a means of getting Aspen's name out there, and thought it might be convenient to put independent power supplies for each rail directly onto the amp pcb. These power supplies use ultra fast soft recovery diodes and RC decoupling between the two filter caps on each rail; this adds some refinement to the sound quality. Since Greg had also found that it sounded better at 300mA than 100mA quiescent, and better again at 1A, I felt that appreciable distance between the output devices was required so that we could install the board directly onto a longish 0.42C/W heatsink. The Fetzilla board is therefore 230mm long and only 69mm deep, with 120mm between the outputs, and this gives useful options to set quiescent current nominally at 500mA with 36V rails, or 1A with 24V rails in cooler climates than Oz, all with a simple convection heatsink and no fans.

#6 Every precaution is taken in the board layout to ensure unconditional stability. This includes loading the amplifier with electrostatic speakers, known to be very difficult loads. Phase margin is high, and unity gain loop frequency is just over 700KHz. There is considerable ground plane, and some separation of input/VAS stages and output. An output Zobel, inductor and parallel damping resistor is fitted to the pcb, and the bias generator is range limited by a 4.7V zener diode to prevent overcurrent blowups. There is also provision for experimentation with lag compensation (Miller cap) and phase lead (the JLH feature), making this design highly suited to a novice learning the ropes of audio amp design.

#7 I have suggested polarising both signal coupling caps, at the input and the fb shunt positions. This approach improves sound quality hugely, particularly if the polarising voltage is greater than the peak AC voltage passing through the cap. Accordingly, the input gate is set at around 2.5V positive to achieve the necessary zero output offset.

#8 This circuit is deliberately low loop gain (around 30dB), and very simple, using only one type of semiconductor, ALL of them FETS. This should draw considerable interest, as it appeals to many to own an amp which is designed for a different style of semiconductor. Most are bipolar.

The pcb is 70um copper, two sides, 2mm FR4, overlay both sides and soldermasked. I will attach the smd VAS prior to despatch.

I have attached the parts list for this amplifier as a Word file, and the very latest pcb as a gif.

Cheers,

Hugh


PS For the very latest, as of 26th March 2012, see here: Latest Schemat March 12

__________________
Aspen Amplifiers P/L (Australia)
Aspen Amplifiers

Jens A. 19th June 2011 01:17 PM

Hi Hugh,

Very interesting news indeed!

Where would you place this new amp in the Aspen 'hierarchy' - qualitywise?

Cheers,

Jens

JoMo 20th June 2011 12:11 AM

Expression of interest
 
Hi,
Sorry if this is a silly question...do I express my interest here or through the Aspen Amplifiers website? I would be after a stereo pair.

The http://www.diyaudio.com/forums/solid...t-perfect.html thread has been great to follow!

My thanks to all those involved especially Lineup, Swordfishy and AKSA!

Best wishes,
Joe

AKSA 20th June 2011 03:04 AM

Hi Jens,

Now here's the fun part, I do not yet know!!

I have indulged myself and watched others develop this amplifier, but because I like the topology and have high hopes for it I have not taken much part, aside from doing the pcb layout. I intend to send the pcb off some time soon for manufacture, but I am waiting for interest to be shown here, on this thread at the Aspen forum, to get an idea of just how many pcbs to order!! In the meantime, I'm waiting and refining the layout.

I would expect this to sound very good. I cannot say if it will pip the NAKSA or not. I would think not because there are many more design tricks in the NAKSA than there are in this JFET circuit. However, who knows? I could be surprised.

JoMo, thanks for your interest, this thread is fine for expressing your interest in a couple of boards. Bear in mind that you will require one pcb per channel, but that the power supplies, one for each rail, are built into each module, and all you need to get it running are then simply a transformer, rated at 25Vac and 160VA rating.

BTW, I will be in Adelaide on 30th June for about a week, staying in Trinity Gardens, so maybe we could meet up?

Cheers,

Hugh

JoMo 20th June 2011 03:38 AM

Hi Hugh,
Excellent, I will officially be up for two then!
I will more than likely go down the monoblock path so these boards sound ideal...I have two rack cases from earlier projects which I plan to reuse.
I have been following the thread with a lot of interest, and was thinking about how to go about getting started but these boards will make it much easier!
I would be very happy to meet when you are in Adelaide, thanks very much!
I am pretty sure I have private messaging and emailing through the forum enabled else I could put my email address here in a post.
I live in Enfield 8 or 9 kms north of Adelaide and work in the city.
Best wishes,
Joe

Ian Finch 21st June 2011 02:48 AM

Expression of interest
 
I'll support this project. I's been an unusually exciting and interesting thread for the very reasons Hugh has outlined. Considering the many other ways it could have resulted, it's a credit to all you guys who got down to the real work and retained much of Lineup's inspiration in this result. Great DIY and a great PCB with some nice touches, thanks AKSA.
Mark me down for 2 please.

AKSA 21st June 2011 04:02 AM

Thanks Ian, Joe,

That's 4 so far:

JoMo Adelaide SA Australia 2
Ian Finch Coffs Harbour NSW Australia 2

Excellent start!

Right now we are waiting for:

1. Final Schemat to be posted by SWF.
2. Report card on the sonics from Mikelm.
3. Further refinement of the pcb by AKSA


If this all seems a bit slow, consider this: normally a full on design commercially takes a long time, anything up to one year. This has been relatively fast, as it's exploited the experience of a number of people. The pcb design is almost done, and with schematic finished, will take about two weeks from date of despatch to taking delivery of the pcbs. I should add that the pcb will be 2mm FR4, gold immersion, silk screened both sides, and 2oz copper - premium quality, better than most commercial boards. They won't be cheap, but they will be the best available, and two sided.

Thanks for the interest, much appreciated, let's see if a few more can place their order! I will add to the list as time goes on.

Please be patient. This looks to be a VERY good amplifier, and at 50W will be an excellent size, not too big to be wasteful, and very high quality because the first couple of watts will be Class A.

Cheers,

Hugh

supernet 21st June 2011 11:34 AM

Nice design...and I like the idea of "Naksa" layout design too :)


If I see correct...there are two separate rectifiers. One for negative and one for positive rail - galvanically separating transformer ground from amp ground. This should defenitely bring some benefits :D

AKSA 21st June 2011 12:09 PM

Yes, no center tap, so amp ground electrically separated from both secondaries and the mains in general. This makes it quieter and enhances resolution.

Hugh

gannaji 21st June 2011 02:29 PM

Expression of interest
 
I had been following the original thread regularly and I am interested in putting my request for two boards, initially.
I am from Hyderabad, India.

klewis 21st June 2011 05:24 PM

Hugh,

You can put me down for a pair. Looks like fun.

Ken

RonR 21st June 2011 06:39 PM

I'd like to put my name down for a pair Hugh.

WRT Mosfet part numbers, I can't find the mentioned ZVP2210 anywhere. Do you mean ZVP2110? :confused:

Cheers,
Ron

keantoken 21st June 2011 07:47 PM

I thought it was the ZVP3310A, not 2210.

- keantoken

AKSA 21st June 2011 11:08 PM

Order List: FETzilla Group Buy
 
2 Attachment(s)
Folks,

Fantastic, we are making good orders already!

That's 10 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK

Moving right along......

I have attached a portion of the pdf for the VAS device, ZVP2110G, the SOT223 version. My apologies Bwana, I got the name wrong. Senior moment! Below this is the latest pcb, growing more artistic with each day now.

This device is about $1.20 from Digikey, and comes in the smd package which will dissipate 2W max. In this application we will be doing around 430mW, so it will get warm, a little too much for the TO92 package which is rated only to 740mW max. This device has very similar specs, with max continuous drain current just over 300mA and Ciss of 100pF, considerably better than the IRF9610 it replaces.

Gaetan, you are 100% correct; there is provision on the pcb for experimentation with a phase lead cap (VAS drain to jfet source), which I would expect to make this amp extremely load tolerant. Two sizes have been accommodated.

Cheers,

Hugh

jkeny 22nd June 2011 12:12 AM

Yes, Hugh, I really appreciate you doing this & was fascinated at the novelty & pace of the thread that gave rise to this design.

I'm in for a pair please!

AKSA 22nd June 2011 03:05 AM

Hi John,

You're in!!

That's 12 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland


This is building steadily!!

Hugh

gaetan8888 22nd June 2011 04:22 AM

Quote:

Originally Posted by AKSA (Post 2614611)
Gaetan, you are 100% correct; there is provision on the pcb for experimentation with a phase lead cap (VAS drain to jfet source), which I would expect to make this amp extremely load tolerant. Two sizes have been accommodated.

Cheers,

Hugh

Hello Hugh

I've read a white paper where it say that using a phase lead cap would reduce the phase intermodulations.

Bye

Gaetan

woodturner-fran 22nd June 2011 07:45 AM

Count me in for a pair too!



Fran

AKSA 22nd June 2011 10:41 AM

Hi Francis,

Thanks for your interest!

14 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland

I'm conferring with Paul Bysouth tomorrow over the models, and we will scrutinize the circuit to be sure. Then, after chatting with Greg, I will publish. This circuit is extremely critical on choice of voltage amplifier and I will admit I'm loathe to post until I know the chosen beast, ZVP2110G, is confirmed good.


Hugh

vitalica 23rd June 2011 12:42 PM

Dear Hugh,

You can put me down for a pair.
Thanks!

Best regards,
Vitali

AKSA 24th June 2011 11:31 PM

Vitalica,

Thank you for your interest, we have a growing list:

16 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania


Greg, Paul, Mike, Lineup and I are all chatting away refining this amp.

Won't be long before it is signed off, I would say around two weeks. Once I have 50 orders, we have 100 boards, a good time to order the manufacture. I believe people will be thrilled with the results.

Cheers,

Hugh

woodturner-fran 24th June 2011 11:32 PM

Hey Hugh,

will it run stacked quads? :)

Fran

acidbrain 25th June 2011 07:20 AM

Hello Hugh, this amp looks promising !
2 boards to add to the list.

Erik.

AKSA 25th June 2011 08:44 AM

Hey Erik,

Nice of you to visit!!

You're on the list, hope I got your town correct..... thank you for your interest.

18 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands

Francis, I don't believe so, but you never know!

Cheers,


Hugh

woodturner-fran 25th June 2011 08:58 AM

Quote:

Francis, I don't believe so, but you never know!
Heh heh - this will be one to keep the smoke in, not out, so better not go there!!

Fran

keith_correa 25th June 2011 12:29 PM

Hey Hugh:

Please put me down for a pair.

\-keith

ipolyakov 25th June 2011 07:27 PM

Hugh,
Please put my name down for a pair.
Thanks,
Igor

AKSA 26th June 2011 12:06 AM

The numbers are growing nicely, thanks Guys!

22 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA

Cheers,

Hugh

srh 26th June 2011 12:28 AM

Hi Hugh,
Please put me down for a pair.
Cheers,
Steve.

JKC 26th June 2011 12:52 AM

I would like to get two boards too.

George

AKSA 26th June 2011 01:00 AM

1 Attachment(s)
Let me give some explanation of progress. Paul Bysouth, a local engineer, and Greg Peters, aka SWF on this forum, have been in discussion at some length.

Firstly a decision has been taken to keep this as simple as possible for exceptional performance, no simpler, but certainly not unnecessarily complex, as this would put off early adopters who should be encouraged to build this lovely amp. This has meant power supplies on the same board as the amp, so it can simply be hooked to input/output and trafo, simple mounting onto a large heatsink to permit choice of quiescent currents without thermal stress, and through hole components wherever possible.

Secondly, there is a lot of design input to the switch on/off transients on this amplifier. This is always a vexed issue as people are very leery of seeing their speaker cones excursing across the room a la Back to the Future when the power switch is thrown. Switching spikes are largely caused by settling currents in the input stage; with zero volts at both gate and source at switch on, we have the jfet moving immediately to Idss; in the case of the 2SK170 this is typically 8-12mA and this current viciously turns on the VAS, which responds immediately with a large voltage spike on its drain. The output stage faithfully follows this spike, and voila, we have a spike on the speaker itself.

The correction is effected when the input cap and the fb shunt cap are charged to the set bias voltages; but this takes time. What is worse is that the 1.5uF input cap charges via the 33K bias resistor (time constant 50mS) and the fb shunt cap, fully 1000uF, charges via the source current of the jfet (which initially is at Idss). This charge pulse also prevents immediate current flow through the series fb resistor, further preventing offset settling.

Mitigating circuit conditions are in the power supply, however. LTSpice calculates switching spikes on the assumption that the rails build immediately to the 36V chosen here; in fact the process is more gradual as the filter caps take finite time to charge. This reduces the output spike. With mosfet (DN2530) input device, LTSpice indicates about a 19V spike, yet with the 2SK170 device SWF says the spike is no louder than the sound of the toggle, and the speaker cone hardly moves, indicating much, much less in practice. [EDIT: Paul has just informed me that with the 2SK170 front end device modelling in LTSpice indicates only a 3.6V output spike, less in practice, so all is well. HD] In order to avoid the complexity of a servo, I feel this is quite adequate. If it proves an issue for more fragile single range drivers like Lowthers or tweeters I would suggest using a three position (double throw), three pole power switch, with 1st position off, 2nd position amp on, and the final switch position connecting the hot output of the amplifier to the speaker. Since settling time is less than 100mS, as long as the user is not sniffing white powder all should be well.

I have made subtle changes to the layout. There are vertical overlay lines demarcating the power supplies from the amp module, so that they can be cut off if desired to permit use of off board supplies; smd provision for the ZVP2110G, the mosfet 2W version of the ZVP3110A VAS; RC decouplers on both rails for quieter operation, and rail/ground diodes to offer DC protection to the speaker should only one rail fuse blow. I have configured the output stage so that source is the center terminal, for both Exicon (Profusion) and Hitachi devices. Note that this is NOT the same pinout as the IRF/Vishay hexfet Mosfets, which are Gate Drain Source, left to right. I have also added a very clever CCS/bootstrap hybrid which combines the best of both approaches, courtesy Paul Bysouth, a highly competent local engineer. C1 is a large (20Lx17D), quality axial component, I have selected an Auricap of 1uF. I have split the bias generator into three 4148 diodes and a 200R trimpot to reduce 'accidents'. If quiescent control in hot conditions is poor, the diodes can be relocated on the underside of the pcb, facing the heatsink base. This will offer some thermal feedback and should throttle back the output stage in very hot operating conditions. I have confined max adjustment with 12mA VAS current to about 4.5V. 2.53V gives a calculated 350mA bias current, so this would give us up to just shy of 1A, sufficient for this design - deep Class AB. I'm assuming 0.1R source resistors and 0.7 Siemen tranconductance on the outputs, about the average. In practice this will vary somewhat, but the range of adjustment is quite adequate and ensures that it will be 'safe' to set up. I've done a general tidy up of the pcb, it's looking quite refined now and will, I can assure readers, be very easy to build.

Throughout this design phase Greg and I have moved away from Lineup's current drain approach on the input device using a CCS. While this approach has strong appeal for biasing reasons it slightly reduces PSRR as the very sensitive fb node, the source of the jfet, is coupled via CCS to the negative rail with its half wave rectified pulses due to output stage operation. It also adds at least one other semiconductor. In my experience a CCS is NOT a firewall for noise; it's actually quite sensitive to noise injection. The decision to pass all jfet current through the series fb resistor is logical from a noise POV as the output is one of the best decoupled nodes in any amplifier. This approach does mean that a bias voltage must now be applied to the input jfet gate; but this actually polarises the input capacitor and greatly improves its sonics. Knowing what can befall amps in the field, I always cap couple the inputs of my amps; it's a safe approach and avoids many mishaps.

Greg is presently on holiday in Singapore, Munich and Paris and will be back in Brisbane on 11th July. On arrival he will verify our choice of ZVP2110G and recheck the turn on transient for the Fetzilla.

I have attached the revised board. All my experience with amp design over 15years tells me this will be an extremely good ampifier.


Cheers,

Hugh

AKSA 26th June 2011 01:19 AM

More orders:

26 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada

Thanks guys, we are rapidly moving towards the magic 100!

Cheers,

Hugh

evette 26th June 2011 01:43 AM

I would like to know the price before I order.

AKSA 26th June 2011 01:52 AM

Evette,

Mentioned before; $AUD50 a pair, including shipping.

Not cheap, but 2mm FR4, gold immersion, overlay and silkscreen both sides, and competent design, which took a long time.

Hugh

Ejam 26th June 2011 02:20 AM

Put me down for a pair of boards
 
Hey Hugh

Long time between chats, must catch up on the phone. I agree whole heartedly about keep the amp as simple as possible. All too often the engineer in us gets lose and destroys a beautiful thing. As Leo d V said "simplicity is the ultimate sophistication". Anyway, $50 is cheap for quality boards and I have many Hitachi laterals, so why not. Any chance for a final circuit after Greg gets back?

Regards

Anthony Mills

AKSA 26th June 2011 03:57 AM

Hi Anthony,

Nice to hear from you, lines open any time!

You're on the list, now at 28 boards:

26 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia

Cheers,

Hugh

chlorofille 26th June 2011 04:08 AM

Hi Hugh,

I'm up for 2 boards.

AKSA 26th June 2011 04:23 AM

Doing well, 30 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand

Cheers,

Hugh

chlorofille 26th June 2011 04:26 AM

Hi Hugh, I'm Dinesh =)

supernet 26th June 2011 10:58 AM

Nice progress :cool:


it is already possible to predict final power (8/4 ohms) or low impedance loads driveability?


Also...what would be recommended heatsink size? Maybe the same like Naksa 100 use for mono channel?

AndrewT 26th June 2011 11:17 AM

The supply voltage and the current ability determine the Power output.

The undistorted volume we hear is solely dependant on the undistorted voltage into our chosen loading. We don't need any more information, particularly if we use the recommended 8ohm speaker.

col_s 26th June 2011 11:18 AM

hi Hugh,

You can put me down for a pair. I'm in NSW.

Colin

AKSA 26th June 2011 12:02 PM

FetZilla Power Output
 
Paul, Greg and I have done extensive Spice modelling and now have a real indication of power output.

This is a single ended voltage gain block and as such will always have an asymmetrical clip. This slightly lengthens the usual SS PP distortion profile, and tends to produce more even than odd at least at clip onset. Using 36V rails (25-0-25Vac trafo) we think it's around +28.3Vp on the positive half cycle at 1KHz into 8R. Negative half cycle clips at just under 2V more.

This means that undistorted power will be exactly 20Vrms (peak x 0.7071). Square this, divide by 8R for power output, and it comes to exactly 50W. This is a very reasonable power output for a medium sized room with 87dB/watt/metre speakers, and more than enough for a moderate sized apartment in a city. Of course there will be headbangers who want more, but for most, and particularly for middle aged people and older, this is more than sufficient for most music, even Hotel California.

The most linear region of operation for the lateral output fets chosen is 1A to 5A. With 28Vp, we would achieve nominally 3.5A into 8R speakers, and max 5A into a reactive load, so this accords very well with strong, linear power output. FETs have a much broader SOAR, so with a 200V rating you would never blow them with a difficult load unless of course it was a short.

Hope this explains things,

Hugh

AKSA 27th June 2011 01:50 AM

FETzilla PCB Group Buy
 
1 Attachment(s)
Keep 'em rolling, now at 32 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia

Acting on advice from Paul, I have added a 1K5 resistor to the drain circuit of the input stage to prevent damage to the VAS gate during switch-on transients. It does not affect loop gain, but does reduce the dissipation in the input device, no bad thing. Now down to just under 80mW with 36V rails, about right for a 400mW package.

One of the two LEDs (the green one) used as voltage reference for the input stage bias generator is now off the board and doubles as the front panel 'on' indicator. It can also be mounted on the pcb if desired.

There is now ground plane separation between 1st and 2nd stages and the high excursion output stage. This should improve intermodulation performance.

Board attached,

Hugh

AKSA 27th June 2011 08:30 AM

Andrej,

Yes, one 300x75 mm heatsink per channel, each rated to 0.4C/W. Each channel (one board) will dissipate around 25W, so that would raise the temperature 10C above ambient at idle, more when pushed hard. This is the same heatsinking I use for the NAKSA 100, you are correct.

I would think 4R loads would push this amp. It would work OK, but strain it at high volume. I would think 6R should be the minimum at highish volume. In this connection it's not dissimilar to the AKSA/Lifeforce 55.

At 350mA idle current with an 8R load, the first half watt would be pure Class A. At 750mA idle current (54W dissipation into the heatsink, 22C temp rise above ambient at idle), we get a full 2.25 watts of Class A. This is surprisingly loud in fact and is essentially a Class A power amplifier.

Thank you for your interest,

Hugh

crispow 28th June 2011 07:15 AM

I will also take a pair Hugh
Chris p

AKSA 28th June 2011 07:40 AM

Done, Chris, thanks for the interest!

Do we have a mutual acquaintance, Ian McP?

Keep 'em rolling, now up to 34 boards. Check the graphic from #44, just been updated.

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia
2 Chris Powell (?) Hong Kong China

Cheers,

Hugh

danny_66 29th June 2011 05:59 PM

Hello Hugh,
I'm also in for a pair.
I need something cooler than my F5 now that it is summer :-)
Regards,
Danny

AKSA 1st July 2011 11:28 AM

HI Danny,

You're in!! Now at 36 boards. Check the graphic from #44 for the PCB update.

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand
2 Colin S. (?) (city?) NSW Australia
2 Chris Powell (?) Hong Kong China
2 Danny (?) Lierde Belgium

Cheers,

Hugh

sonnya 1st July 2011 05:56 PM

Add me as well. - 1 pair.


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