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Old 11th July 2006, 05:16 PM   #1
folkeb is offline folkeb  Norway
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Default Aleph 1.2 questions (soon done)

I will soon have completed my Aleph 1.2 project. I will at a later stage post images and so on, but for now I have some questions.

1. Each PSU concists of the following. I have 2x625VA trannies with 45v secondaries. This goes into a capasitor bank of 8x66000uF (the psu is based on KK's design). My first question is: Will a thermistor do the job as an inrush limiter? I currently have a couple of CL-10s that I got from my supplier that I was thinking of using. If these thermistors are far from enough (my worry), can someone please help me with (direct me to) a sufficient softstarter or something that will work?

2. The capacitors are all Mallory cans. Original I was told. As they only are rated for 60VDC with a surge of 75VDC I was thinking of taking the voltage down to 58VDC by the use of power resistors. First of all, is this safe? ( I don't really get understand the rated versus the surge value) Secondly should I try something else? I used a variac and got 58 VDC out of one capacitor and one 625VA, the primary side was then 215V. As the secondaries are ca. 46 volts I was thinking of using a 1.7-1.8 ohm resistor to bring down the voltage to 58-59VDC. Any comments will be appreciated!

It may seem that I have taken on a too difficult project. This is true, very true. But the last 6 months have been spent worrying, waiting, building and more waiting, and in the not to distant future I will hopefully have succeded. (I have some helpers), so please don't be too frustrated with these questions...

Thanks for all your help!

Folke
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Old 12th July 2006, 07:03 PM   #2
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Folkeb

Well I have not completed my 1.2 yet but have spent a lot of sleepless nights going though the various aspects of the amp. Like you, I am learning as I progress through the project, as such please seek a second opinion of what ever I suggest below.

Anyway first off, Nelson uses a single CL-60 thermistor per channel on the production 1.2s. So that would be a good place to start. (However, I am using 2x1000VA with 6x100000uf per channel, so I may decide to use 2 CL-60s in sequence. I have blindly purchased some softstart modules from http://eshop.diyclub.biz/ and I am not sure if they are rated for my purposes!!! So go with the thermistors as they have been tried and tested before)


Secondly, 60Vdc caps for the 1.2 does sound incredibly low, especially since the 1.2 is supposed to work off 60Vdc rails. But I noticed you are using 2x45vac secondaries so that should net you lower rail voltage, lets say you are prudent, the we should give the caps an allowance of +/- 5%, so the target rail should be 57vdc. Using Duncan Amp's PSU designer, with a CRC using a R=0.39ohm, I got just under 56Vdc rails. (With safety in mind, I have decided to use 80Vdc caps because my trafos have 2x50Vac secondaries.)

I'll be interested to learn of other opinions.
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Old 12th July 2006, 07:27 PM   #3
folkeb is offline folkeb  Norway
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Hi,

I have just ordered 10 more CL-60, now I can really experiment with thermistors! With regard to the capacitors all the other DIY 1.2s I have seen use 63volt capacitors. However, these have not had any surge capability stated. Mine do, 75volts. But I'm unsure of what surge realy means. I had this idea that I could bring the voltage down to 58 volts before the first caps. This is slighty under 5% margin for normal operational conditions. I will monitor the mains voltage to see if there is any major changes in the AC, and make sure that I design for the upper value and then add some more.

Here is the PSU and the principle design of my attempt.

http://i40.photobucket.com/albums/e2...leph/Aleph.jpg
http://i40.photobucket.com/albums/e2...eph/Aleph1.jpg
http://i40.photobucket.com/albums/e2...h/DSC_0128.jpg
http://i40.photobucket.com/albums/e2...h/DSC_0127.jpg

Could you post some photos of your approach?

Best regards,
Folke
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Old 13th July 2006, 04:44 AM   #4
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Since this is DIY, I am taking the overkill approach.

I am building the 1.2 with a seperate PSU, so thats 4 chassis. Each PSU will house 2 1000VA trafos in parallel, each secondary with its own bridge rectifier, the secondaries are connected after the bridges to the filter section with a CRCL configuration, values are 200000uf > 0.39ohm/25watt > 200000uf > 2.2mh. This is then connected via 1 metre shielded umbelicals to the main amp chassis where the Final C section of 200000uf is housed along with the pcbs, transistors etc.

With so much trafo and capacitance(total of 1.2 farad), I am also very worried about inrush. Why this overkill? This PSU is giving me ZERO ripple.

I have only started to design the layout and dimensions of the chassis, I will show some pictures/drawing when I have them completed.

Your PSU is coming along very nicely, post more pictures. I have a couple of questions:

1. What heat sinks are you using?
2. What gounding scheme will you be using?
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Old 13th July 2006, 10:43 AM   #5
folkeb is offline folkeb  Norway
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Hi,

The heatsinks are fischer SK-535 and have a C/W of 0.12 each. This will give me a temp rise of about 30 degrees over idle. They have saw fashioned fins and look very cool, but are somewhat expensive (ca 1000 euros for four). I did a lot of research and could not find any with as good specs for natural convection for the price.

With regards to grounding scheme I will follow Nelsons design, or KK's version of it. Use a thermistor between ground and chassis.

What will you be doing?

Your project makes my amps look like a S O N Y - damn!

I have big enough WAF issues as it is.... Even if she is wonderfully patient with this hobby of mine, they are somewhat big! They measure 50x40x25 cm each.
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Old 14th July 2006, 09:26 AM   #6
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Default Grounding

This seperate PSU along with the shielded mains, shielded umbellicals and shielded interconnects has been giving me a headache with regards to the grounding scheme. Where do I start?

1. Shield of the mains cable is connected to the neutral/earth of the wall plug only.

2. Chassis of both PSU and Amp are connected to earth. In order to ground the chassis on the Amp's side, there is a dedicated 'chassis ground' wire from Amp to PSU. This keeps the Chassis seperate from the other grounds in both the PSU and amp sides

3. Shield of the umbellicals are connected to the a central ground point located in the PSU chassis only, umbellical shields on the Amp side are unconnected

4. Shields from interconnects are connected to PCB as instructed by KK

5. Ground from the speakers, power supply, RCA, XLR and main PCB are connected to the central ground. As these are located in the Amp chassis, there is another 'ground' wire from Amp to PSU.

6. Central Ground is connected to earth through a CL-30 thermistor.

My umbellicals therefore have the DC+, DC-, ground and chassis ground. Hope this covers everything... holler if I missed anything.

The other complication that I am facing is the implementation of the thermostatic switch. The thrmostatic swithc should be placed between AC mains and the transformers, both of which is in the PSU. However the thermostatic switch will be placed on the heatsink located in the Amp chassis. How can I avoid running a the live AC wire from PSU to AMP and back!!

Any ideas?
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Old 14th July 2006, 09:56 AM   #7
folkeb is offline folkeb  Norway
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hmm - seems like you thought of the ground issues somewhat more than I have. I was just planning on follwing KK's suggestion to use the thermistor from the central ground on the PCB. I will also use shielded cable for internal AC connected to ground on the mains cable (connector plug) only. This may or may not shield the rest from the AC. The rest of the power supply is not shielded at present - I may change this a later stage. I will use shielded cable from the PCB to the transistors, but I am not certain of where to connect the shield to ground.

The thermal protection should cut of the power to the transistors, right? It is installed to save them from burning up if the heatsinks get to hot, or so I thought. Will it not then be sufficient to connect it after the PSU? If you want to cut power to the transistors this should do the trick. This is just me thinking/guessing.

Maybe someone else can comment on this?

Today is the day when I connect the monoblock to the mains. A stupid question: It is alright to use a variac and slowly adjust the voltage to operating levels?

Just to easy everyone - I have called in some more experienced help to aid me through the process!
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Old 14th July 2006, 05:38 PM   #8
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Hi Folke,

Our first concern is to be about the "inrush current."

The inrush current happens at the moment when we turn on the
power. And, it will run thru the primary coil of the transformer,
thru the secondary coil, and thru the bridge diode.

Meanwhile, as PSU caps have low ESR, the caps are hardly able
to stop the inrush current running thru the bridge diode so that
we have to use a big bridge diode (25-35A). Of course, the PSU
caps will see high current during the first charging.

We could reduce this inrush current, effectively using NTC
thermistor on the primary side. We could use one, two, or three
of them in series, or as many as we finally feel comfortable.

Regarding the caps, I think the 60VDC caps are still useful with
the slightly higher rail voltages. However, the thing we have to
consider is the ratio of operating/rated temperatures as the life
of the electrolytic caps is depending mainly on it.

By the way, 8*66,000uF are considered overkill. Wow. . .

I hope you will see soon the nice sound and the happy smile of
your wife's.

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Old 14th July 2006, 07:44 PM   #9
folkeb is offline folkeb  Norway
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I have now successfully started the PSU with a small load (a light bulb). The variac I use is a BIG tranny in its self so the fuse goes the first time. But after the fuse is switched back on the PSU works. No smoke or flames or anything too worrying. However there was one thing that concerns me:

After about 5 to 10 minutes the aluminium plates witch the PSU is mounted got very hot. I would guess about 50 to 70 degrees. I was able to touch the alu. but it wasn't pleasent. My friend and co- or lead builder /assistent thought it may be that the rectifier brigdes generally get hot. But then again? Can someone please enlighten me as to what may be going on?

As the light bulb is only a small one it does not draw enough amps to let the resistors have any affect on the end voltage. This I hope will improve when the transistors are connected.

Just to give some more info. The images show (earlier on in this thread) how the PSU design is. In short the trannies and the recifier and resistors are mounted directly onto the bottom alu. plate. On this a second alu. plate connected with a M10 screw/bolt are the capacitors and the coils. Both plates got hot and more so closer to the bolt that was tightened hardest. This was the forward one, and is not to far away from the rectifier.

Whatever the problem is, what would be the best solution to fix it?

So, the end line just moved a bit further away - again!

Have a nice evening!
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Old 14th July 2006, 07:58 PM   #10
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Quote:
Originally posted by folkeb

Whatever the problem is, what would be the best solution to fix it?

[/B]
I believe that the problem is a high current (= high power).
Solution must be finding the reason of the high current.

Good luck.
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