LM1875 Drawing ridiculous current? - diyAudio
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Old 20th July 2011, 07:53 PM   #1
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Default LM1875 Drawing ridiculous current?

Hi there!

First time poster, long time lurker

Before I start, I have had a search on the forums and so far, haven't been able to find anything similar to the problem I'm having currently.

I've previously built a single supply quad 40w/channel amplifier so I'm not entirely new to this soldering malarkey

I managed to get a hold of a few lm1875 free from national and had a 19,0,19 120va transformer I fancied having a play with and am attempting to build a bass guitar amplifier.

I've built a tone control circuit based on the design here: 3 band EQ

using a NE5532 and was hoping to couple it with
http://users.otenet.gr/~athsam%20/bridge_amplifier.

My aim was to build the entire thing on a single piece of veroboard which for the mainstay has worked quite well.
The power supply is included on the board and is based around Building a Gainclone chip amp with snubberized PSU. with a few numbers changed (due to only being what I had to hand)

I had initially an issue with the tone control circuit introducing a hissing noise into the lm1875 which I fixed.
Also, I had an issue with a ground loop which I fixed with a ground loop breaker and it now sounds as good as It's probably going to get considering the high gain from the first stage.

Next the lm1875 pair had started to oscillate when the tone was a certain position so I removed the second lm1875 hoping to return to it later once the first is sorted.

My issue now arises with the first LM1875 itself, I now have an issue with it drawing a high current at turn on, measured nearly 1A on the +ive rail which is of slight concern to me of course.

The preamp stage has its own regulated +-15v supply but the LM1875 is unreg straight off the filter caps. The circuit is built how it is on that diagram and I cannot see any shorts across the tracks or anything untoward which has me baffled.

Any ideas per chance?

Cheers!
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Old 21st July 2011, 12:43 AM   #2
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Booski, can you post schematic and pictures of what you actually built with all the part substitutions? Without more specific info, it's hard to help you.

Mike
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Old 24th July 2011, 01:04 PM   #3
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Hi there, I haven't had a chance to produce a full circuit diagram yet, however, I did in the end de-solder all of the components related to the LM1875 and am now replacing them one by one and testing.

Currently, I've got just he LM chip, +ive and -ive supplies with decoupling 0.1uf ceramics on each attached, and the output voltage is 26v DC currently.
Surely there shouldn't be any voltage on the output even with no inputs?
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Old 24th July 2011, 03:39 PM   #4
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Unfortunately, I've had to re-post as I can't edit my previous posts yet.

I've done a little digging and as it turns out, it was the 1ohm 220nf capacitor combo on the output that was causing the amplifier to draw nearly an 1A, no matter which way around they were.

Question is, is it really necessary? if so, can I get away with using a different value?
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Old 24th July 2011, 03:42 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by booski View Post
Unfortunately, I've had to re-post as I can't edit my previous posts yet.

I've done a little digging and as it turns out, it was the 1ohm 220nf capacitor combo on the output that was causing the amplifier to draw nearly an 1A, no matter which way around they were.

Question is, is it really necessary? if so, can I get away with using a different value?
That's the zobel network across the output of your amp, and the fact that the amplifier is drawing 1A with them connected indicates that the chip is likely oscillating at a few hundred kHz or higher. The network is there to assure stability in the presence of inductive loads, and control the HF phase margin and gain of the output stage, and changing values is ill-advised as the amplifier could become unstable even with no load connected.

The cause of this oscillation is either down to poor layout, inadequate supply decoupling or operating with closed loop gain below the minimum stipulated for stability. (There are ways around that, but beyond the scope of this discussion.)
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Old 24th July 2011, 03:53 PM   #6
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I quite believe that it could potentially be oscillating with it connected, however, I haven't got a scope to connect to it to check.

There is 1 thing however that I haven't included on the board yet is any supply filtering caps next to the chip.
Supply is taken directly from the power supply filter caps which via 2 fuses of which are either side of the board on individual vero tracks. There is however, only 0.1uf ceramics on each of the supply pins.

Would you suggest I include some kind of filter caps next to the supply pins on the amp?
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Old 24th July 2011, 03:59 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by booski View Post
I quite believe that it could potentially be oscillating with it connected, however, I haven't got a scope to connect to it to check.

There is 1 thing however that I haven't included on the board yet is any supply filtering caps next to the chip.
Supply is taken directly from the power supply filter caps which via 2 fuses of which are either side of the board on individual vero tracks. There is however, only 0.1uf ceramics on each of the supply pins.

Would you suggest I include some kind of filter caps next to the supply pins on the amp?
Need to add very good decoupling right next to the chip!! Probably some specific recommendations in the application notes for the LM1875..
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Old 24th July 2011, 04:54 PM   #8
johnnyx is offline johnnyx  United Kingdom
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I had a PCB designed for these from Maplin, it had 100n decoupling caps to gnd on both rails, but it still oscillated. I cured it by using a 100n 100v ceramic cap soldered directly between the +ve and -ve pins on the chip.
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Old 24th July 2011, 05:26 PM   #9
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The large value decoupling caps are necessary, the data sheet showws 100uF but larger is better IMHO, and they should be as close to the chip pins as you can get them. Another thing that's very important is the circuit layout and and how it's all connected. The paths from the low signal parts of the circuit to ground, and the high current paths to ground should be totally separate, if you're using a common conductor for these, it's probably causing trouble. It would be really helpful if you could post a schematic and pictures.

Mike
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Old 25th July 2011, 01:08 AM   #10
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Out of (learning) curiosity, could this be a grounding issue? I have seen something like this too, but it was a bad ground problem. The zobel network resistor also got fried then, and the chip seemed to draw maximum current.
Michael, all the grounds should return to one point though as i understand it?
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