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AKSA 18th June 2011 07:01 AM

Swordfishy/ASPEN FETZILLA power amp
2 Attachment(s)

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.



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

Aspen Amplifiers P/L (Australia)
Aspen Amplifiers

Jens-AT 19th June 2011 01:17 PM

Hi Hugh,

Very interesting news indeed!

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



JoMo 20th June 2011 12:11 AM

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

The thread has been great to follow!

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

Best wishes,

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?



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,

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.



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.


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.

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