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Old 22nd February 2016, 01:56 AM   #1
fwt1712 is offline fwt1712  Australia
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Default cl60 thermistor in BA3 preamp

Newby question here.

I am starting to build a ba3 preamp and am wanting to clarify the use of cl-60 thermistor as inrush protector and as last link to earth from power supply ground.

I am using block 50va 2 x 24 transformer and want to know if it makes sense to use a cl-60 as an inrush protector by connecting between the two primary windings (I am using a tubecad 24v regulated supply).

Just wanting to know if its a good, bad or unnecessary step. I presume slowing inrush doesn't hurt, but there is not much of a capacitance bank in a preamp so maybe it is not needed.

From the earthing perspective I understand the CL60 gives a useful last link to earth, offering some resistance in normal (no short etc.) situations but gives a reliable path to earth if needed. In the case of not having access to a cl60 I have seen high wattage 10ohm resistors used as this last step from power supply 'ground' to earth, is one preferred over the other.

Thanks in anticipation of your assistance,
Nigel.
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Old 22nd February 2016, 02:19 AM   #2
Zen Mod is offline Zen Mod  Serbia
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no need for any sort of soft start for so small PSU

you still need NTC between audio gnd and case/safety gnd
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Old 22nd February 2016, 02:19 AM   #3
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For preamp using low VA transformer and small CRC supply the inrush protection is not manatory.

Definitely use a thermistor for chassis ground. Buy a stack of them.
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Old 22nd February 2016, 02:20 AM   #4
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Beat me at the starting line.
Hahahaha
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Old 23rd February 2016, 04:31 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by fwt1712 View Post
....................From the earthing perspective I understand the CL60 gives a useful last link to earth, offering some resistance in normal (no short etc.) situations but gives a reliable path to earth if needed. In the case of not having access to a cl60 I have seen high wattage 10ohm resistors used as this last step from power supply 'ground' to earth,..............
Connect the Protective Earth (PE) wire direct to the Chassis.

Connect the Main Audio Ground (MAG) to the Chassis. This can be a direct wire connection, or via a Disconnecting Network.
This link from MAG to Chassis may have to pass Fault Current approaching or even exceeding kA

NEVER use a "high wattage 10ohm resistor" between your MAG and Chassis.
It will blow up and could leave your faulty equipment with a live connection waiting to be "touched" by the operator wondering why the equipment has suddenly gone quiet !
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Last edited by AndrewT; 23rd February 2016 at 04:34 PM.
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Old 23rd February 2016, 04:33 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by 2 picoDumbs View Post
..................

Definitely use a thermistor for chassis ground. ..........
Do you know whether this is safe?
Have you tested your proposal?
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Old 23rd February 2016, 04:39 PM   #7
AndrewT is offline AndrewT  Scotland
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Originally Posted by fwt1712 View Post
.................I am using block 50va 2 x 24 transformer................
The normal fuse one would use for this transformer would be
Fuse rating = 3times 50VA/230Vac ~ 652mA
A T615mA, or T800mA, or T1A could be used to start this transformer direct on line.

If you want to try a close rated fuse, then use Fuse rating = 50VA/230Vac ~ 217mA
Try a T200mA, or T250mA, fuse but you may need to use a soft start.
Would it be worth the effort?

I use a few 3VA, 5VA or 6VA to power auxiliary circuits in my equipment. I use an F500mA to start these direct on line.
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Old 23rd February 2016, 06:59 PM   #8
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Quote:
Originally Posted by AndrewT View Post
Do you know whether this is safe?
Have you tested your proposal?
I just talked to a friend about this issue. He is using the CL60 between chassis and PSU GND.
In a certain faulth condition the CL60 blows before the fuse.

Last edited by palstanturhin; 23rd February 2016 at 07:02 PM.
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Old 24th February 2016, 09:04 AM   #9
AndrewT is offline AndrewT  Scotland
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Originally Posted by palstanturhin View Post
I just talked to a friend about this issue. He is using the CL60 between chassis and PSU GND.
In a certain faulth condition the CL60 blows before the fuse.
Could you go back to Pico and ask him to ask the Moderators to remove his "unsafe" recommendation.
or
prove to himself that it is "safe".
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