Based on Hugh Dean's AKSA 55

[Greg Erskine]

A few years ago Destroyer X (with Hugh's permission) published the conceptual schematic of one of Hugh's commercial AKSA 55 kitset products.

Here is the thread for reference:
Attention AKSA 55, Hugh is thinking in let us see schematics on forum

I was learning EAGLE at the time so I drew the schematic and designed a PCB as an exercise. At the time the AKSA 55 was still a commercial product, so I didn't think it was appropriate to publish the PCB. But now, the AKSA 55 is no longer for sale, so I sent the PCB files off to a PCB manufacturer and have now built a few of these amplifiers.

My schematic is a little different from the one that was shown in the reference thread in a couple of ways. I changed a couple of values that were obviously wrong, I used BD139 / BD140 as drivers and I used a BD139 for the VAS. When testing the PCBs I used the recommended transistors mentioned in the quote below.

Here is a quote from Destroyer X's original thread:

• Differential transistors (Q1, Q2) are matched 2N5401
• Voltage bias multiplier (Q4) can use BD139
• Voltage amplifier (Q3) can not inform, a lot of them works there (I used a KSA3505)
• Drivers (Q5, Q6) are 2SC4793 and 2SA1837
• Output (Q7, Q8) are 2SC5200 and 2SA1943
• VBE multiplier transistor (Q4) can be mounted over 2SC5200 (Q7)
• Bias is 50 mA, and re-adjust again some minutes after first adjustment
• 100 Ohms resistor connected to Voltage Amplifier transistor is a trimpot, small variable potentiometer
• If you do not reach the bias current, change R12 value
• Voltage is plus and minus 36 Volts and power will be 55 Watts RMS over 8 ohms with 0.65 Volts RMS input.
  

I have actually built one amplifier using BD139 / BD140 transistors and will report on that at a latter date.

[metal]

Good luck Greg

I am waiting for your results.

[Greg Erskine]

I really like this basic design and believe it is a good starting point for anyone wanting to get into DIY audio. I intent to publish enough information for anyone with a little DIY experience to build these amplifiers.

regards

[metal]

Some say, keeping things simple is a bless, tell me Greg, how does it sound? I am dying to know. Are you going to tweak the values to obtain the best performance?

I like this Zobel module, very attractive indeed. Output terminal is in the perfect location, placement of Q4 is also perfect to me. The row of resistors gives it a nice touch. Well done Greg.

[AKSA]

Greg,

My thanks for keeping the old AKSA alive.

Metal,

This amp is utterly conventional in its engineering. It has simply been tweaked down to the last hertz to extract the best possible sound for the human ear. It wouldn't be much good for bats, for example, or Martians, and the magic sound qualities are purely dimensioning and component choice.

It sold by the hundreds around the world, quite a successful amplifier. Worth a good listen.

Hugh

[destroyer X]

regards,

Carlos

[Greg Erskine]

I have had a few inquires, so....

I should emphasize that the schematic I have published is based on the "conceptual" schematic Carlos published in his thread (see reference in post #1). It is not identical to any of the production AKSA 55's I am aware of. This means, it will not sound exactly like Hugh's production AKSA 55.

regards

[Greg Erskine]

Originally Posted by metal

Some say, keeping things simple is a bless, tell me Greg, how does it sound? I am dying to know. Are you going to tweak the values to obtain the best performance?.

To early to comment on the sound, it is only working on my work bench. I plan to test various tweaks to experience them for myself. So far, the amp on the left has BD139/BD140 for drivers and a BD139 for the VAS while the other 2 have the recommended drivers and a KSC3503 for the VAS.

Originally Posted by metal

I like this Zobel module, very attractive indeed. Output terminal is in the perfect location, placement of Q4 is also perfect to me. The row of resistors gives it a nice touch. Well done Greg.

I got sick of trying to squeeze the zobel and output inductor on the amp PCBs. This way I can built a half dozen of them suitable for testing the amps. This amp PCB has room for the zobel, but I am not using it. Thanks for the compliment.

I plan to build 2 mono blocks with 3 amps per heat sink. This way I can quickly compare 3 options in quick succession. I might automate the switching between them one day.

regards

[Jay]

There are theoretical based experts, there are empirical based experts, that's if you can separate the two. I think Hugh started as an expert empirically (don't ask my words please) that's why he's so knowledgeable. It's different with those who started as theoretical experts and more or less remain there.

I think you can get good sound from simple circuits, but there are things that you have to do it specific in your implementation. Simulation program doesn't work here. Measuring equipment is better but listening test is also has the same function. I don't think you can easily copy the Aksa even if you have the complete schematic.

Reading this thread I thought you would finally update this schematic you posted long ago (because of the out-of-production issue you mentioned). But it turned out I'm wrong. As far as how I can understand it, many Aksa's ideas are not implemented in your schematic. Your schematic is very old and I don't think the schematic itself is the complete idea of this Aksa amplifier. If you follow all the available information about Aksa, I believe you can duplicate it because Hugh imo had explained all, tho not in the same place and not explicitly.

[Bigun]

AKSA - the amplifier that has inspired and launched many a DIY attempt. It's what got me into this hobby. I started with the same schematic posted at the start of this thread for my TGM1 amplifier. It's good to see that the AKSA amplifier still has the same power to inspire.

I think Hugh knows it would be a disservice to reveal the secrets now, far better to keep the legend....