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Old 4th September 2006, 02:16 PM   #1
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Default Dan's Aleph Build Thread

Hello Everyone,

I am about to start my Aleph project and this will be the thread where I document my progress.

I'd first like to start by thanking James Dumesnil, a fellow member, for sending me some surplus PCBs to get me moving. Mind you, he drives a hard bargain - I quote from a message from him to me, "if you are certain that you will BUILD one and post pictures. I will part with them free of charge." Now that's dedication to supporting fellow members. He posted PCBs across the world within days - thank you James!

Now the question is, what am I going to build? I'm hoping to get some good feedback from you guys to help me work this out. The original plan was to build an Aleph 5 stereo amp as a rehearsal for building a high powered Aleph X down the track. I'm now considering building Aleph 2 monoblocks after much reading on the Aleph amps.

The amps will be driving the Acorn from ER Audio, a full range electrostatic speaker:

http://www.eraudio.com.au/

They're not a particularly difficult load to drive (I'll see if I can get more info on the speaker impedance curve). Let me say that my NAD C320BEE drives them with no trouble at all, which really surprised me.

I've read mixed opinions about pairing Alephs with ESLs - please, make you comments heard here as I'm most interested! Negative opinions are not going to stop me building the Alephs for two reasons - 1) The Alephs can alway be re-purposed, driving other speakers, or acting a a boat anchor. 2) If I don't, James will hunt me down.

I'll see if Rob Mackinlay from ER Audio wants to chime in as he planned to build Aleph 4's once upon a time. I wonder if he ever got round to it.

Anyway, I'm likely to take a while to ramp this up, sourcing time, money and parts.

Cheers,

Dan
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Old 11th September 2006, 07:19 AM   #2
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It's decided - I going to build Aleph 2 monoblocks.

I will be increasing the bias to help drive the difficult electrostatic load. I have a couple of questions about this:

1) If I remove R19 altogether, what will the bias current be (I've read that it approaches 3A), and what does this mean to heatsinking requirements over the original Aleph 2 design?

2) Has anyone experimented with the effect of varying the AC current gain when driving reactive loads like electrostatic speakers?

3) Will the removal of R19 give a high enough bias, or should I lower the value source resistors too?

I've settled on Conrad heatsinks as all feedback I can find is positive and they're prices are very reasonable. Now I just need to settle on a bias setting to work out how much heat I'm going to be dissipating.

I'm looking into transformers now - can anyone recommend an Australian manufacturer? Even better if they're Perth based!

Cheers,

Dan
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Old 11th September 2006, 10:05 AM   #3
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According to my collection of information through this forum, removing R19 makes Q5 a voltage source of about 0.66V to the R40-45 on idling condition. I.e. 0.66/R40*6 = about 4A bias. And as you say, reducing R40-45 also has the effect of bias-increasing.

And, read through this:
Aleph-2 50W@8ohm 20KHz
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Old 11th September 2006, 08:42 PM   #4
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Try varying R19 first to get a >3A bias; increasing its value increases the npn ref voltage by a fraction -as it approaches >>high resistance or infinity the ref voltage nears a Silicon device's G-S voltage drop value of about 0.6V.

Doing this has the advantage of not messing with the original AC current gain set by NP at 50% (best sound), as opposed to lowering the Source resistances.
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Old 12th September 2006, 12:06 AM   #5
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Quote:
Originally posted by Blues


. . . not messing with the original AC current gain set by NP at 50%

Ahh . . . right, Blues . . .

I should have said that too . . .
If R40-45 are reduced, R22-27 should be reduced at the same rate.
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Old 12th September 2006, 01:03 AM   #6
Blues is offline Blues  United States
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Ni hao are you, JH? That's if you are in Shanghai.
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Old 12th September 2006, 01:49 AM   #7
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Wo hen hao, xie xie ni.
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Old 12th September 2006, 12:29 PM   #8
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Default Keeping it cool

I've been thinking about forced air cooling - fans! Here's my maths transfered from the back of a napkin...

I'll assume that I'll need to dissipate around 400W as heat once I raise the bias and that I'm shooting for a 25 degree temperature rise above ambient. This leads to heatsinks that have a thermal resistance of 0.0625 deg/W.

Conrad have a suitable heatsink, the MF18-151.5 in their Fine Fin Spacing range:

http://www.conradheatsinks.com/products/flat_hd.html

Lets assume I can quitely push 27 cfm of air over two of them, I'm down to 0.077deg/W (0.11*1.4/2.0 = 0.077). This includes the correction factor of 1.4 to get 25 deg rise, not 80 as quoted on the Conrad site.

Working backwards, that'll take me to 31 degrees rise above ambient. Close enough for me.

If you put two of those heatsinks face-to-face, you'll end up with a 124mmx125mm tunnel, ready to mount a 120mm fan. Which fan? This one (NF-S12-800 from Noctua):

http://www.noctua.at/main.php?show=p...ts_id=4&lng=en

It pushes 19 cfm at <5dB(A) and 34 cfm at 8dB(A). Bingo! Quite and powerful.

Include therm-control of the fan speed and we're done. Fast warm up time, extra cooling capacity, small and light heat sinks. All this and the heatsink/fan assembly will be about 12x12x17.5cm overall in size.

Cost wise, this will be very cheap, saving on heatsink and shipping costs!

Now, am I dreaming, or does that all sound right?
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Old 12th September 2006, 01:16 PM   #9
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monarodan,

Just a piece of advise....

I tryed 3 different fans in my A75
and it's difficult not to get noise, even with silicon suspensions
thermo regulation etc ; even lowering the speed {1/3 rd}.

It has been very difficult to shut-up the fan noise...
It induces vibrations into the chassis, think special suspension plate
isolated from the chassis to fix the heatsinks and fan.
Avoid open air in and output for the air ; otherwise the noise will be there...

Fans are noisy by nature... so search and find the right fan before
starting anything!

I pushed 2*80 watts with 2*6 pieces of Therma Flo, EH 1126 B long 8"
and the temp reached 53 degrees about 0.06deg/W

http://www.passdiy.com/gallery/a75-p11.htm

Good luck with your project.

Regards.

Alain.
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Old 12th September 2006, 03:20 PM   #10
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Points taken - these fans are very, very quiet and would run at 500 to 800 RPM to achieve the desired air flow. Not to mention their 150,000 hour MTBF! Very reliable.

I might shell out for one fan and play with it first. I can always stick it in a computer (the intended use for it anyway).

Cosidering that a bog standard 12cm computer case fan will emmit around 28dB, and these run at around 5dB, that's pretty quiet.

From their marketing blurb:

http://www.noctua.at/main.php?show=s...sign&setlng=en

"Sound-optimised blade geometry and profile: The NF-S12 is the first fan employing a novel rotor which was developed by the Austrian Institute for Heat-Transmission and Fan Technology (Österreichisches Institut für Wärmeübertragung und Ventilatorentechnik, ÖIWV). Thanks to the special optimisation of the blade geometry towards ultra low noise application, the NF-S12 is up to 10% more quiet than current high-end fans at the same flow rate."

I've only read good reviews about these fans. There is a local supplier who has these in stock, so I should be able to get hold of one quickly and let you know how it goes!
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