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Old 21st June 2010, 12:56 PM   #1
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Default Voltage regulators that work ~50V?

I'm wanting to build a carlosfm type regulated supply but my transformer is +-35Vac which after rectification is giving rails of +-48Vdc. I want to regulate this down to about +-40Vdc but the LM338 regulators suggested in (Building a Gainclone chip amp with a regulated power supply (PSU).) have a max input voltage of 40V.

I see that LM317HVT are rated at 60V but have max currents of 3.7A whereas the LM338s have max currents of 5A. Will LM317HVT do the job? Are there others?

Thanks,

Simon.
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Old 21st June 2010, 03:15 PM   #2
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Yes, but it depends on the current required. You also need to consider the power dissapation in the regulator.
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Old 21st June 2010, 03:24 PM   #3
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The LM338 is a floating voltage regulator. The regular version will tolerate a drop of 32 volts across it. You have to arrange the circuit so that this is not exceeded. Then you can regulate a few hundred volts if you want.

Have a look at the datasheet. If you can't figure it out from that, get back.

w
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Old 21st June 2010, 03:27 PM   #4
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Quote:
Originally Posted by simple simon View Post
.. LM338 regulators suggested in Building a Gainclone chip amp with a regulated power supply (PSU) have a max input voltage of 40V.
No 3-terminal adjustable reg that I'm aware of (inc LM338) has a max input voltage spec. That's because none has a ground connection. What they have is a max input-output voltage spec. So long as you don't short circuit or otherwise overload them, they'll work fine on higher voltages. An external voltage limiter can be employed so they don't blow up if they do get overloaded.
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Old 21st June 2010, 03:36 PM   #5
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Originally Posted by wakibaki View Post
The LM338 is a floating voltage regulator. The regular version will tolerate a drop of 32 volts across it. You have to arrange the circuit so that this is not exceeded. Then you can regulate a few hundred volts if you want.

Have a look at the datasheet. If you can't figure it out from that, get back.

w
Thanks wakibacki.

The circuit I'm trying to follow is http://www.decdun.me.uk/gc/snub.reg.psu.png but with the voltage defining resistors R3-R4 set at 120R and 3.7k which I believe (hope) outputs 40V.

The application is for a gainclone using a TDA7293 which has a max voltage limit of +-50V so I'm trying to add a bit of safety margin with the regulator. Output of say 50W rms? but I'm not to picky.

Simon.
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Old 21st June 2010, 03:44 PM   #6
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the 7A limit of the 3886 is already a bit too low for an 8ohm reactive load. I don't have TDA data for max current.
Neither a 1.5A nor a 5A current limited regulated supply is suitable for a real speaker load. Unless you can provide a short term source of transient power after the regulator to satisfy the demand of the power amplifier.
That short term power source can be very big capacitors just like used on an unregulated supply.
But read the datasheet for all the regulators you are considering. Many do not like a big capacitance on their output.
You will need to ensure the regulator you buy matches the datasheet that you have used for your design.
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Last edited by AndrewT; 21st June 2010 at 03:47 PM.
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Old 21st June 2010, 03:55 PM   #7
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Quote:
Originally Posted by AndrewT View Post
the 7A limit of the 3886 is already a bit too low for an 8ohm reactive load. I don't have TDA data for max current.
Neither a 1.5A nor a 5A current limited regulated supply is suitable for a real speaker load. Unless you can provide a short term source of transient power after the regulator to satisfy the demand of the power amplifier.
That short term power source can be very big capacitors just like used on an unregulated supply.
But read the datasheet for all the regulators you are considering. Many do not like a big capacitance on their output.
You will need to ensure the regulator you buy matches the datasheet that you have used for your design.
AndrewT - your expertise is beyond questioning, but what I do not understand is that many claim to have built regulated supplies for gainclone type amps and say they sound very good. So I'm confused when you say they wont work.

Simon.
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Old 21st June 2010, 04:08 PM   #8
AndrewT is online now AndrewT  Scotland
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chipamps do work.
I have listened to quite a few, without regulators.
But the 7A of the 3886 (which is amongst the highest available) is a serious limitation to excellent performance.

I cannot get Medium to High SPL from a chipamp powered 88dB speaker.
I don't believe that my Medium level listening is voltage clipping. I am more inclined to believe it is current clipping.
High level listening is very probably voltage and current clipping and it sounds terrible.
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Old 21st June 2010, 04:13 PM   #9
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Quote:
Originally Posted by AndrewT View Post
I don't believe that my Medium level listening is voltage clipping. I am more inclined to believe it is current clipping.
So this is just a matter of belief what you're propagating about LM3886s? Why not measure and verify?
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Old 21st June 2010, 04:15 PM   #10
AndrewT is online now AndrewT  Scotland
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do you have the gear and expertise to do that for me?

But at least I have stated they are my beliefs.
It is up to the Members to decide to ignore my comments and/or find ways to corroborate rather than simply accept.
I expect the Members to make informed decisions, never to go in blind and be led by the blind.
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Last edited by AndrewT; 21st June 2010 at 04:18 PM.
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