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Elektor AXL V2 with Lateral MOSFET's

More then 30 years ago (1985) I build the Elektor AXL class A/AB amplifier, time to upgrade this amplifier using new (= better)components.
My main goal was to replace the power mosfets 2SK134/2SJ49 with Exicon ECF10N20/ECF10P20. Also the medium power transistor BF469/BF470 changed to MJE340G/MJE350G.
Extra power supply filter capacitors where used and placed on the PCB.
The two capacitors for the 15V zener diodes where increased to 100uF 25V
and the capacitor parallel to the quiescent current potentiometer (1k) is increased to 220 µF (Single-Ended Conductive Polymer Aluminum Solid Electrolytic Capacitors)plus extra 47nF WIMA MKP.
The two source resistors (0,22 Ohm) are no longer needed.
The input capacitor (C1,C2,C3-820 nF) is replaced by a high end capacitor 2,7 uF Clarity Cap ESA.
The Exicon Application notes told me to decrease the inductance in the output line to 1µH (1mm Cu wire inner diameter 8 mm, 15 windings). Due to different gate input capacitance of the mosfets the gate resistors are also different, 330 Ohm for ECF10N20 and 220 Ohm for ECF10P20.
The power supply for each channel (2x25V AC 2,4A 120 VA) is filtered by 4 x 10.000 µF rectifier (Ebay) so that results in 2 x 38V DC output voltage.
It is still very important to set the quiescent current potentiometer to zero Ohm before switching on the amplifier for the first time.
I tried to use the same component labels but did not 100 percent succeed.
On my old (same as me) but still working oscilloscope (Philips PM3226) I saw a maximum error free waveform of about 60 Volt peak-peak with a 8 Ohm resistor load.
No oscillation what so ever, 60 Volt peak to peak over 8 Ohm i.e. 30 : 1,4 = 22 Watt output power with a quiescent current 0,1 A.
The calculated damping factor is more then 200.
I am using a Rasberry Pi in combination with the HiFiBerry DAC plus and a home made power supply (5V 3A), controlled by Volumio software, NAS. The loudspeakers: Acoustic Energy AE1.
 

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Thanks for your reply -mlloyd1-.
I checked again my schema with the original one from march 1985 but I asure you there is no difference at all. I also check the pcb layout again and came to the same conclusion. If there is a problem then my amplifier could probably not produce such a clear sound at all I think.
Herr Flick
 
It is always difficult to find the right words to descripe the difference between two amplifiers -jonnego-, escpecially when you are not a native english speaker. The first thing we (my wife and I) said to each other when we listen to my upgraded amplifier was: much more details and a lower bass. A couple of weeks ago I went to a friend and made comparision with his amplifier NAD C352, Bowers and Wilkens 804S loudspeakers and -van den Hul D-101 III interlinks.
All three of us came to the conclusion that the Elektor AXL V2 was slighty "better" i.e. more details. Later on I invited another friend, HiFi freak as well, at home and he was impressed by the sound. Another big improvement was earlier made due to the use of the Raspberry Pi and the HiFiBerry DAC, thatswhy I sold my Arcam Alpha 7 CD player.
I will try to make more comparings with other Hi-End amplifiers and let you know by that time, but it is hard to find people who uses Hi-End stereo equipment nowadays.
 
The 2SK176/2SJ56 is a good replacement for the original 2SK134/2SJ49, but you have to use the old Elektor schema (1985). I.e. you have to insert two source resistors (0,22 Ohm low inductance) and lower the gate resistor R25 330 Ohm in 220 Ohm. The output coil L1 must be changed back in 2 µH. Because of the source resistors you need a new PCB layout and last but not least, I think the the sound of the lateral mosfets ECF10N20/ECF10P20 is better but I cannot confirm that.
 
upgrade AXL

The 2SK176/2SJ56 is a good replacement for the original 2SK134/2SJ49, but you have to use the old Elektor schema (1985). I.e. you have to insert two source resistors (0,22 Ohm low inductance) and lower the gate resistor R25 330 Ohm in 220 Ohm. The output coil L1 must be changed back in 2 µH. Because of the source resistors you need a new PCB layout and last but not least, I think the the sound of the lateral mosfets ECF10N20/ECF10P20 is better but I cannot confirm that.


What was the motivation to replace the mosfets? In your opinion which upgrade improved sound quality most?

I build 8 versions of the AXL back in the 80's, and still have them, stored in the shed (next to fancy also unused electrostatic speakers), as they have been replaced with a modern Yamaha AV-receiver. However I have some time for the Audio hobby again and look at options to use these again.
 
Hi Kay, sorry for my late answer.
I build this amplifier more than 35 years ago,
and as you know a lot of components will deteriorate during time,
especially the electrolytic capacitors and nowadays one can buy much better replacements.
That's why I started to upgrade my old Elektor AXL amplifier.
But before that I only replaced the input capacitors 3x 820nF MKM with a 2,7µF MKP Clarity Cap.
That was a good improvement, much more details. I also wanted to use a better power supply capacitor with low ESR near the power transistor.
So I had to redesign the PCB layout. I read somewhere in this -diyaudio- forum that a good replacement for the BF470/BF469 is MJE340/MJE350.
The power mosfets 2SK134/2SJ49 where replaced by Exicon ECF10N20/ECF10P20 because they have a low distorion, very fast slew rates
and free from secondary breakdown and thermal runaway.
For a long-time stable capacitors I replaced C5/C6/C9 with Single-Ended Conductive Polymer Aluminum Solid Electrolytic Capacitors.
Very critical capacitors are the one in the snubber network C18, the best is a FKP3 (Wima), and last but not least,
the power supply capacitor where increased to 20.000 µF per channel.
I must say it was really worth to do this investment, I invited some HiFi freaks and the where all impressed by the excellent sound quality of
and this elektor AXL V2 amplifier. I am using a Rasberry Pi with a HiFiBerry DAC on top, directly connected to this amplifier.
More info:
Exicon Application Notes
https://www.wima.de/wp-content/uploads/media/WIMA-Audio.pdf
HiFiBerry | Audiophile 24/96, 24/192 sound cards (Audio DAC and SPDIF out) for the Raspberry Pi, audio out, audio cards, audio projects
p.s. you're right, 56 Watt RMS power, enough for an average living room.
 
Hi Kay, sorry for my late answer.
I build this amplifier more than 35 years ago,
and as you know a lot of components will deteriorate during time,
especially the electrolytic capacitors and nowadays one can buy much better replacements.
That's why I started to upgrade my old Elektor AXL amplifier.
But before that I only replaced the input capacitors 3x 820nF MKM with a 2,7µF MKP Clarity Cap.
That was a good improvement, much more details. I also wanted to use a better power supply capacitor with low ESR near the power transistor.
So I had to redesign the PCB layout. I read somewhere in this -diyaudio- forum that a good replacement for the BF470/BF469 is MJE340/MJE350.
The power mosfets 2SK134/2SJ49 where replaced by Exicon ECF10N20/ECF10P20 because they have a low distorion, very fast slew rates
and free from secondary breakdown and thermal runaway.
For a long-time stable capacitors I replaced C5/C6/C9 with Single-Ended Conductive Polymer Aluminum Solid Electrolytic Capacitors.
Very critical capacitors are the one in the snubber network C18, the best is a FKP3 (Wima), and last but not least,
the power supply capacitor where increased to 20.000 µF per channel.
I must say it was really worth to do this investment, I invited some HiFi freaks and the where all impressed by the excellent sound quality of
and this elektor AXL V2 amplifier. I am using a Rasberry Pi with a HiFiBerry DAC on top, directly connected to this amplifier.
More info:
Exicon Application Notes
https://www.wima.de/wp-content/uploads/media/WIMA-Audio.pdf
HiFiBerry | Audiophile 24/96, 24/192 sound cards (Audio DAC and SPDIF out) for the Raspberry Pi, audio out, audio cards, audio projects
p.s. you're right, 56 Watt RMS power, enough for an average living room.


My upgrades which I did many years ago where a bit different.
>After making the circuit boards I put a layer of silver on top of the copper, to reduce risk of corrosion.

>I bypassed the input capacitors C1, 2, 3) completely; thus there is no capacitor directly in the signal pathway. btw I did the same with the output of my pre-amp, thus only cap is at the input pre-amp
> I separated the power supply of the 2 powerfets from the rest. I lifted the power supply with +/-5V (to +/- 45V) for the part which takes care of the voltage amplification, with it's own rectifier and Elco's. While the 2 powerfets were kept at +/- 40 V. Thus increased the headroom of the amp, while keeping the dissipation relative low. This also improves signal to noise ratio. The main reason I did this was because the units were driving an electrostatic speaker with a relative low efficiency. Bias setting is around 0.6 A
> I replaced each of the series resistors for the power mosfets with 5 non inductive film resistors.



My AXL's still play ok with all the old capacitors. Although I do worry a bit about over-voltage as the units were build for dutch 220 V AC, while we often have 250 - 260 V AC here in aussieland mainly due to the popularity of solar panels which push up grid voltage.
 
Well -phri- that's one of the reason I reconstructed my amplifier. My old PCB was homemade, drew with a pencel and was only a copper surface and the soldering tin was old fashioned with lead. My new designed PCB was professional made and I used tin/silver soldering. To bypass the input capacitor is a tricky thing but as long as you use your amplifier with a well known sound source then you have a advantage because the input capacitor got a great influence on the overal sound quality. Thatswhy I used a high end capacitor instead. Yes, to drive a electrostatic loudspeaker you need a lot of power. But the sound quality is excellent but often a little lack of bass. I would like to to tune my elektor AXL V2 for instance by raising the quescent current to 200mA for less distortion. And I have to determining the gate resistor values as well. One method to determine the right gate resistor value is to overdrive the amplifier with a square wave. The shape of the rise and fall curves will indicate a correct value. A very good application note with lot of information is: http://www.ti.com/lit/an/snaa045a/snaa045a.pdf
But first holidays!
In the Netherlands by the way is nowadays the main voltage 230 V 50 Hz.
 
I had a look at the paper on mosfets mentioned in the earlier message, good stuff, although the old mosfets in the original AXL design are not listed; 2SK135 & 2SJ50

I started measuring the setting on my old build. The AXL I build as mono-blocks run at 93 V now and a bias of around 0.6 A, thus a bit over 25 W dissipation for each fet. Elektor stated a max total dissipation of 65W. Datasheets of the mosfets state 100 W each thus I should be fine. The datasheet also mentioned a drain-source voltage of max 160V for the 2SK135 thus below 100V shouldn't be a problem.

btw, the units do get really hot after running for an hour or so; scientifically..... touching a mosfet longer then 2 seconds is uncomfy.

I also got my first build of the AXL back which is a stereo version build unmodified with the orginal elekor prints. This unit can oscillate now and then probably due to the wirewound resistors in series with the mosfets. It runs at 80 V with a bias of 200 mA. If I push this unit; sound is less clean compared to the others. maybe due to working in AB class?? I am not sure. This stereo unit is also extreme reliable as it lived at a friends place running daily for more then 20 years. However always had (and still has) a buzzing power supply; toriodal transformer (300 W & 2 X 51000 uF) which really annoys me.


Overall sound quality of this 35 year old amp design (the AXL of-course) build by a hobbyist (me) still outperforms my modern $2000 yamaha surround sound receiver, and the latter got stellar reviews from the experts...
 
I constructed my AXL in 1989. Its operation is in class A- 1A current without input because quality matters. I have used it to so many parties as a D.J and it has never denied neither quality or strength. Now it is my main equipment for testing the tube preamplifiers I build.My power supply caps are 40000uF/63volts and supply the amplifier through a darlington stabilization circuit, giving it +- 38volts. This amplifier just stays a little bit before my Audio Note class A tube power amplifier and I use it in every party.
 
On/Off switch LED pwr supply

Nice to hear from you -Metanastics- What a pity we live so far from each other. I would like to make a direct compare between your old AXL amplifier
and my upgraded AXL V2 amplifier. I recently made a tiny modification for the ON/OFF switch LED illumination. It was powered by a old fashioned
small powersupply with a 1,8VA transformer and 100mA stablized 12V out. But I realized that it would be more usefull to have a more "intelligence"
one. This signal LED in the ON/OFF switch should go ON when all the four power supply voltages (-38V and + 38V) for the left and right channel are in good
health. If one of them is down the LED signal should be off. So I used a opto-coupler powered by the four amplifier power supply's. It is just a small PCB
which can easily find a place inside the amplifier.
 

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Nice to hear from you -Metanastics- What a pity we live so far from each other. I would like to make a direct compare between your old AXL amplifier
and my upgraded AXL V2 amplifier. I recently made a tiny modification for the ON/OFF switch LED illumination. It was powered by a old fashioned
small powersupply with a 1,8VA transformer and 100mA stablized 12V out. But I realized that it would be more usefull to have a more "intelligence"
one. This signal LED in the ON/OFF switch should go ON when all the four power supply voltages (-38V and + 38V) for the left and right channel are in good
health. If one of them is down the LED signal should be off. So I used a opto-coupler powered by the four amplifier power supply's. It is just a small PCB
which can easily find a place inside the amplifier.

I can assure you that my old AXL is playing superbly so far!!! Of course i haven't listened to yours so I can't compare each other, but, my power supply is not the one given by Elektor. There is a pcb stabilization circuit in darlington connection which kills any ac voltage if there were any!!! Pure dc means clear sound plus the 40000uF capacitors = excellence!! Now i am trying to make a new circuit board using sprint layout 6.0 'cause the old pcbs were been made in an amateur way and they need renovation!! As far as the circuit you have made, it is a very good idea for protecting the speakers. Audio Research and a couple of other companies used to place optocouplers at their tube preamplifiers in late 80's- early 90's. Good job.!!! Personally I don't deal with transistors, just tubes! I have made exact pcb clones of some legends of tube preamplifiers brands as Audio Note L2-M2, Marantz 7C, Audio Research SP8E MKII etc.