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Old 23rd June 2005, 01:32 PM   #1
dinu is offline dinu  United States
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Question a75 power supply problems

I'm trying to put together an a75 and am truly baffled. To start with, is it just me or is the hookup for the fan printed wrong on the board? R18 is hooked up to drain of Q1 and should be from base/gate to ground. The led seems wrong too.

THE BIG PROBLEM

The big issue is that i'm getting alot of AC ripple on the regulated supply. aprox 250mV SEE THE PICTURES i ATTACHED

IT GETS WEIRDER

The chassis is set up in a way that places the psb and the input board as well as the rectifiers and fuses on a 3mm aluminum plate which sits on the transformer at one end, held down by the transformer bolt, and two 70mm stainless steel pins on the other side. The plate is very firmly grounded to the rest of the chassis with a nive, heavy gauge ground bus wire BUT THERE'S STILL A 250mV AC POTENTIAL FROM THE PLATE TO GROUND. AND THE 70MM PINS GET REALLY HOT

Do I need to keep the plate a good distence away from the transformer? I even grounded the transformer bolt with a wire to earth ground. Is this the same 1/4v I see in the regulated supply? Should I stick to buying SONY?

PLEASE HELP.
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Old 23rd June 2005, 01:45 PM   #2
Magura is offline Magura  Denmark
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You can't place an aluminum plate on top of the transformer and ground the plate. That gives you a shorted turn for the transformer. DO NOT RUN THIS SETUP, as it may blow.


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Old 23rd June 2005, 02:24 PM   #3
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Ditto - use insulated standoffs to prevent the shorted turn if the plate needs the support. the voltage you see on the plate is the result of the current flowing plate-leg-chassis-bolt. This current is probably coupling to your regulated supply, causing the ripple.

and yes, if you are using the Audioexpress boards, there is an error in the fan circuit. It will apply full rail voltage to your fan. Poof! (I ended up with 60 volts on a 12 volt fan that screamed for around 20 seconds) there is an unmarked hole in one of the traces around the fan regulator pass transistor that allows the proper connection to be made.

I assume that you found the silk screen errors on the main board.

I found the LED circuit correct.

EDIT: nice looking amp.
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Old 23rd June 2005, 02:55 PM   #4
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dinu,

In my case I have used 4 CRC's
5 * 5600 MicroF + 0.5 Ohms 30watts + 5 * 5600 MicroF
added some 100 micro F plus 0.1 micro F,
gives 2*40 Volts and the ripple is down to 80 milli volts...

To see the details picture #5
http://www.passdiy.com/gallery/a75-p11.htm

Regards.

PS: For the fan I have made a temperature regulated supply in 12 VDC
and didn't use the original pcb {with the pb}... it's DIY ; but don't hesitate
to ask!

Alain.
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Old 23rd June 2005, 03:06 PM   #5
dinu is offline dinu  United States
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Wow, what a great community of helpful people! Thanks alot guys.
Now I can't wait to get home and rework.

As for the silkscreen errors, a few yanked out hairs later, I think I got them all. The article posted on the passdiy site seems to include the corrections. I just wish audioxpress made that clear with the instructions in the article they send with the boards.

The way the article shows the LED connected right to those tabs gets you + rail and - rail. (100v potential) anyway, I was thinking of just using a high value voltage divider to the unreg side so you can see the LED dim as the caps discharge on shut down. (OPINIONS?)

THANKS AGAIN
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Old 23rd June 2005, 03:14 PM   #6
dinu is offline dinu  United States
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Alain,

Just saw your post. I've been following your A75 and, like everyone else was very impressed. That tap problem got everyone pretty fired up. As for that 12v temp control. I'd love to see it. I was just thinking of just placing a thermal switch on the fan to kick on at about 65c.
Here's another pic of the face plate. Like the meters?
By the way, what do you think of your amp sound wise.
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Old 23rd June 2005, 03:20 PM   #7
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that would work fine - just pick a resistor that will drop the voltage at the 20 mA or so you'll need. Check the power in the resistor and use one rated for at least double the dissipation. You could also go rail to rail. Either way it will take a few minutes to dim out. If it stays lit a long time, you might want to put bleeder resistors across the PS output - maybe 5K 2W. The idea is to bleed it off fast enough that the charge will be gone before you can unscrew the case top and get to it.

the LED on the PSU board is in series with a couple of resistors that limit the current and put the LED pins at close to ground portential. If you measure across the terminals without an LED you will see 100V, but as soon as you put the LED into the circuit, the current flows and voltage drops. I ended up using 15K 1/2W resistors to keep my blue LED intensity reasonable. It's about a 30 second fade out here.

I like my A75s - they aren't as pretty as Alain's but the sure sound nice.

Hi Alain. I figured I'd see you here, too.
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Old 23rd June 2005, 03:31 PM   #8
dinu is offline dinu  United States
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Bob,

I didn't even hook up the led when I saw that 100V. What you're saying makes sense though. I'll try hooking it up as is. Worse case, I'm out a 50cent LED. I did place bleeder resistors on the caps. At 5k they do take awhile.

Thanks for the help, by the way.
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Old 23rd June 2005, 03:50 PM   #9
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dinu,

the A75 is working with a pair of Electrostat Martin Logan Aerius i
and the preamp a CCS-X-Bosoz

I have listened to a lot of Class A amplifiers and others, especialy here in Montreal there is a "Festival du Son" each year ; to listen to the world best Hi-Fi set-up's

I am pretty happy with this, and enjoy every second of music!
To be honnest, the only one that impressed me last year was a pair of Passlabs XA250!!! I think they are not cheap, but sooooo good...

I am working on a Tube SE 6C33C-B 2*20watts and a pair of
back loaded horns with Fostexe's...

PS: BobEllis is a talented builder, but too modest!

For the temperature regulation I'll check my notes.

You have done a great job!

Alain.
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Old 23rd June 2005, 04:08 PM   #10
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dinu,

Welcome to the A75 Club !!!!!!! one of the best DIY designs !
Thanks to the Master.

Hey Bob, pleased to see you here! ; thats a lot of watts gathered here!
Pure Class A ; wouahhhh ! Bob is alway there to help and a real friend.

I can tell that 2 friends have listened the A75 and all were pleased
with the details, the stereo image is wide and in case of big orchestral
pieces it's a marvel! {Compared to Audio Research D100 and other's}

The only drawback in summer ; you must use the clim.

Enjoy!

Alain.
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