No. It calculates to 60 Vpp²/(8 x 8 Ω) = 56.25 W.
Congrats and best regards!
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.
Exicon Application Notes
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.
I forgot another important internet site, it is al about highend capacitors. A very extensive capacitor test with a lot of information:
Humble Homemade Hifi - Cap Test
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...
Wow, 2 x 51000 µF that a lot capacity. Did you use a rubber slice under and on top of the toroidal transformer ? Nice to read about the outperforming Yamaha receiver. My AXL also had a slighty better sound compared to the NAD C 352 stereo amplifier.
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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