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Old 15th May 2003, 01:07 PM   #1
diyAudio Member
Join Date: May 2003
Location: Pretoria
Default Transformer choice for LM3875

I want to try my hand at a LM3875 based amplifer. I have a 30V 400VA transformer sitting on my shelf, which will give me +- 43V out of my power supply.

Is this too much for the LM3875 to handle? Looking at the datasheet it seems okay (supply V < 94V) , but looking at the designs on the web most people appear to supply these devices with +-30V.


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Old 16th May 2003, 12:48 AM   #2
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Location: De Winton, Alberta
Hey there,

Are you looking at a single supply +43V or a split 43-0-43?
If single I think you'd be fine, but a little low on power. Plus you need a coupling cap.
If your transformer is 30-0-30 for approx 43-0-43 DC you're pushing the limits, but from what I understand it would work. Stay away from 4 ohm loads with that methinks!

Good luck.
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Old 16th May 2003, 06:44 AM   #3
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Join Date: May 2003
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It's a 30-0-30 transformer. The situation actually gets a little worse - by calculation I should get a 43V rail, however given that the local electricity supply runs somewhat higher than it should the real figure is more like 45V.

At this point, I'm more interested in getting the project up and running. I can order a more suitable transformer later.

Initial tests would most likely be with a cheapo 8 ohm speaker, then going to a Celestion 23 which is rated at 6 ohms.

Thanks for the input!
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Old 16th May 2003, 06:53 AM   #4
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Apologies to all. I just read through the other thread on transformers and realised my mistake. I read the supply voltage in the datasheet to be 80V max, so thought that a 43V odd rail would be fine.

This is of course a mistake, because the supply voltage to the chip is relative to the + and - rails, not relative to ground. So a 43V rail effectively gives an 86 V supply voltage.

I understand now why the transformer should be somewhere around 24 - 0 - 24 V.
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Old 16th May 2003, 09:27 AM   #5
Paulr is offline Paulr  United Kingdom
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Join Date: Nov 2002
Location: S.Yorks, UK
Hi Taliesin,

Hoe gaan dit?

Just to test you could use a half wave rectifier. That would not increase the voltages by any great amout after rectification. Probably around 30 volts under load.

This method is definitely not Hi-Fi but will give you a useable voltage for testing. My own semi-built gainclone with 3886's uses a single secondary transformer from a UPS transformer (serious budget version costing under 12 so far) The CMRR of these chips will allow you to get away with a much noisier supply that most - mine even sounded fairly quiet, sight ground loop type hum.

Tot siens

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Old 16th May 2003, 02:25 PM   #6
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Join Date: May 2003
Location: Pretoria
Goed dankie

That's about the limit of my spoken Afrikaans

Good idea ... as it happens I can't get the chips for the weekend, they're on back order, so I'll probably pick up a smallish 22-0-22 transformer and work with that given that I've got to wait a while in any case.

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Old 18th May 2003, 11:40 PM   #7
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I just built a prototype amp with the LM3876. I used a 28-0-28 Signal Transformer for the supply. The tranny is rated for 115 on the primary. I got about 120 line voltage here, so it gives +/- 44 volts after the bridge rectifier. This worked alright with 6 and 8 ohms loads. However, when I increaced the gain to calculated 35, it actually turned out to be around 40 maybe because of the higher voltage, and used a 4 ohm load, I managed to destroy two of the "pratically indestructable" chips.

I spoke with an engineer with Nationals audio division, and he recommended a maximum of +/- 35 volts on the rails.
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