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Old 4th May 2011, 01:14 PM   #1
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Default Snubberized set up issues


Setting up a snubberized psu for a dual mono LM1875 amp based on this design:

Click the image to open in full size.

1. The 1W 0.47R resistor just ignited as soon as I switched it on - everything seemed to be connected properly?

2. I replaced the 100uF capacitor on the V+ pin of the LM1875 with a 2200uF, like it says, and I got a terrible hum coming through, that actually sounded virtually non existent with the previous 100uF.

Any ideas how I should be modifying my layout for this chip?

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Old 4th May 2011, 02:12 PM   #2
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sorry i mean on pin 5 - Vcc

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Old 4th May 2011, 03:24 PM   #3
johnr66 is offline johnr66  United States
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You seem to be using a dual supply (+,- voltages and ground) and have a single supply voltage amplifier circuit shown. Use the dual supply schematic for the amplifier. How do you have the power connected to the amp now?

Also, you may be exceeding the supply voltage max with that power supply. What voltage do you measure at the supply output?

The snubber cap is shorted or you wired it wrong. I'd remove the snubber. I doubt you'd here any audible difference.

You lost me with the 2,200 cap on the V+ pin "like it says". I see the cap on the output pin. In dual supply mode, the cap is not needed.

I'd use an transformer with an 18-0-18 secondary max. I don't think these little amp chips should be pushed that hard.

Last edited by johnr66; 4th May 2011 at 03:28 PM.
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Old 4th May 2011, 04:26 PM   #4
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One chip is connected to one rail, the other to the other and they have they're own isolated circuits, with their ground being shared.
I'm using 25-0-25 giving ~40V per supply, which seems fine, considering national semi states they can take a supply up to 60V. I've been running them like that for 2 weeks now and they've coped tremendously, whilst not heating up considerably at all.

I will double check the snubber.

In reference to the 2,200 cap, I am referring to the 1st diagram, where carlosfm says to use a 2,200 and 0.1uf cap on the psu pins. Do you mean to say that as I am using a dual supply, these capacitors are not necessary and can be disconnected? (Because its perfectly silent without them connected, when at full volume!).

Thanks for your help
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Old 4th May 2011, 05:24 PM   #5
johnr66 is offline johnr66  United States
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You must have the bottom part of that supply reversed to provide the second positive rail (+ 0 +) because no way would it work in the + 0 - configuration as shown with the IC connected as you described. If it is + 0 + config with the chip on common ground and each having its own rail, no problem with the supply voltage for the IC.

If the PS is near the IC, I see no benefit to having a 2,200 cap on the Vs pin. It won't hurt. I'd use a 220 uf along with the film cap.

IMO, diminishing returns apply here. The PS is way over designed, but fine if that is what you want. I guess people who come up with this stuff have the deep pockets.
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Old 4th May 2011, 05:36 PM   #6
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Sorry John you're right it is reversed, I forgot to say that.

Yeah I'm getting what you're saying - it's not as cheap as it is in some places buying parts in the UK, which is annoying, but I'm on a gap year atm, and I'm bored out of my mind, so anything to keep the brain ticking is fine by me!! :P

Thanks a lot though
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Old 21st May 2011, 04:07 PM   #7
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
post1 shows two independant power supplies that are commoned to give a centre ground.
The PSU has everything duplicated.

You can easily modify the layout to separate the two PSUs. Each PSU will power it's own single ended Chipamp. This allows both Chipamps to operate completely independantly from each other (isolated).

But you do need to connect any exposed conductive parts to Chassis. This will create a connection between the Audio Grounds of the two channels.
regards Andrew T.
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