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Old 4th June 2012, 12:33 AM   #1
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Default General Power Supply Questions

Hi,

I'm working on a McIntosh C24 pre amp for me.
The power supply is 250V with 20 ma.
It is at the bottom of the attached schematic.

I want to add capacitance and Pi filters to really lower the noise floor.

So I was going to add and replace the existing caps.
1. Change the diodes to the Mur60s.
2. Place film caps around them at .01uf 600V
3. Remove the c43A/C43B dual ceramic.
4. Replace C44 with a CLC filter of
330uf/400V cap + small choke + 330Uf/400V cap
5. R97 W Metal oxide
6. C45/A R99 C45B combo w/ 120Uf/450v cap Metal Oxide, 120uf/450 cap
7. R98 a metal oxide same.

Then I have some other questions regarding feeding the Base of Q102
and the c45C along with the 75V zeener.

Can I make a little Pi filter of C45 with CLC
and add a lot of capacitance? Will it affect the zeener?
or is it better to just add a single cap but with many uf?
200uf, 470uf, 1000uf, 2000uf, or 10,000 uf?
Do the zeeners care?

Then athere is the c45d and c46.
better to add more ufs there? Also
8. Make another Pi filter at c46? at the 10V
supply. I think I have some 470uf, - 20,000uf caps.

I have experience with tube amps but not with
transistors and what makes them sound goood
nor do I have experience with zeners and other
basic SS stuff.

I have a lot of caps that I can use along with bypassing
them with films or PIOs.

I want a rock solid power supply so that when I listen to it first,
then start to lower the niose floor and change the coupling
and other caps along with new resistors and transistors for it.

But I want to listen to it after making the P/S rock solid.

Any thoughts?

Click the image to open in full size.
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Last edited by SyncTronX; 4th June 2012 at 02:51 AM. Reason: Add the Schematic
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Old 5th June 2012, 11:58 AM   #2
DF96 is offline DF96  England
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The thing which can make or break a power supply is correct grounding. Far more important than fancy components or overkill design, assuming the basic design is sound. The circuit diagram you posted does not show the grounding arrangements so it is impossible to tell if they are correct.

What are your ripple calculations? Where does the existing circuit need improvement? You need to understand its current operation in order to determine how to improve it.
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Old 6th June 2012, 07:35 PM   #3
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Quote:
Originally Posted by DF96 View Post
The thing which can make or break a power supply is correct grounding. Far more important than fancy components or overkill design, assuming the basic design is sound. The circuit diagram you posted does not show the grounding arrangements so it is impossible to tell if they are correct.

What are your ripple calculations? Where does the existing circuit need improvement? You need to understand its current operation in order to determine how to improve it.
Here is a pic of the Power Supply Caps.

Click the image to open in full size.

Then, here is the PS board and low level transistgors which feed the
preamp circuit.

The power supply feeds from the right side on the board where the thermally heat
sink is located.

Click the image to open in full size.


Here is the overhead fiew of the preamp.

Click the image to open in full size.

Ripple TBD.

Cheers,

Sync
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Last edited by SyncTronX; 6th June 2012 at 07:39 PM. Reason: Add the pics.
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Old 6th June 2012, 08:40 PM   #4
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Are you sure that the noise is ONLY generated by power supply, and NOT by other source, like tubes/transistor, shooting noise in resistors, bad shieldings, etc.? If not sure, you will work uselessly.
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Old 7th June 2012, 12:17 AM   #5
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Are you sure that the noise is ONLY generated by power supply, and NOT by other source, like tubes/transistor, shooting noise in resistors, bad shieldings, etc.? If not sure, you will work uselessly.
No, I'm not. But the caps were bad and physically leaky. So as long as
I'm in here, I might as well make the P/S rock solid.

The grounding scheme isn't so go as it is that little tab on the multisection
cap. Thinking of making a grounding bus bar from solid Romex 14 awg wire.

Trying to do a little spice model of it, but now realize that I have to
define all the componants into the superspice program. I was just
looking for something simple to model it but I guess it would be
simpler to just rebuild the circuit.

However I did find some other model schematics where McIntosh did
upgrade the power supply. They are shown here.

C26 Power Supply Schematic:

Click the image to open in full size.


C28 Power Supply Schematic:

Click the image to open in full size.
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Old 7th June 2012, 01:21 PM   #6
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Ok, but I think that given you want to manipulate the circuit and do lots of mods, I would start by eliminate the simple pass transistor, and make regulated both supplies, using ICs or discrete components. And respect to the grounding scheme, surely your idea is good. But, i first would try to get working the equipment as is, and then do all mods.
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Old 7th June 2012, 04:44 PM   #7
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Ok,

Will do Osvaldo. Get it working then do the other stuff fiirst.

That must be the consensus.

I"ll need some suggestions picking out which ICs or discretes to
use.

Sync.
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Old 7th June 2012, 04:59 PM   #8
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Q302 may be deleted, and directly replaced by a LM317 if you want exactly 14V, or a easiest LM7815.
D304 in the 100V supply may be wired in series with a TL431, and then you must add the feed back resistors, plus (eventually) a cap between anode and ref pins if it oscillates. The zener voltage must be lowered, by the fact that TL431 only supports 36V maximum, so putting a zener of 24V less than the original, the resting 24V will be easily managed by TL431. And this way, the PS becomes lower noise and lower output impedance than actually. Only my suggestions, may be hundreds of different opinions.

Good luck.
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Old 8th June 2012, 03:36 AM   #9
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Ripple Calc at the first filter:

V = It/C

60 Hz
time = 0.015s

20mA from schematic/Power Supply

470uf cap = .000470

It = 0.015 * .020
------------------------ = .64V (Vripple)
.000470

V ripple
330uf cap = 0.91V
200uf cap = 1.5V
100uf cap = 3.0V
68 uf cap = 4.41v
47 uf cap = 6.38v
22 uf cap = 13.64v

cheers,
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Old 11th June 2012, 01:54 AM   #10
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Thinking along these lines...

inout_ps_layout_web - My Photo Gallery

Quote:
Originally Posted by Originally Posted by CBS240
Also it is important to seperate the return currents from the different capacitor stages. Each capacitor stage should have it's own return path to the star ground point so as not to mix power supply AC input currents with speaker return currents.

[QUOTE=DF96;3048746
First sentence is true. Second sentence is false, as it guarantees the very thing you should be avoiding by injecting charging pulses into the star ground.

The return path for the first cap should not go to the star point but to the bridge/transformer. The return path for the second cap can go to the star point. Then link the ground sides of the two caps together. The result is that the star ground only gets a little smooth ripple current into it instead of big narrow charging pulses.

I am puzzled why people keep making this mistake. There must be a website somewhere which encourages it.
]

That said in my little scheme of things I would do the following:

from the PS with parts on hand put in a 330uf 400V cap,
reference that back to CT AC ground, the run the a little coilcraft 12Uh/3.8A inducter series connected to 68uf/450 then s.sK in series with a 1000/200V cap and the rest of the schematic.

To be continued...
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