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Old 25th November 2008, 04:56 PM   #1
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Default ChipAmp 1875 kit questions

Recently completed a chipamp.com LM1875 kit. All standard/recommended parts, including the Avel PT.
I am driving a pair of small full range speakers of 91db efficiency. Source is an integrated HD sound card on my PC. There's a little hiss when the sound card output and chipamp's volume pot are maxed, but far less than I'd actually expect from such a noisy source. The chipamp itself is dead quiet.

The sound is very impressive, but there is far less gain than I expected. With the program material I've been listening to, (vocal/string jazz, for the most part) I haven't been able to produce a single complaint from my wife, much less the amp/speakers. They just don't play very loud. Is this normal? If not, where should I start looking for the weak spot?

Also, the amp has an annoying habit of blowing the 2A slow blow fuses on start up. I'm guessing that it;s the PS caps that are sucking enough current to blow the fuse. Again, is this normal? Should I be using a different value of fuse, or should I consider something like a CL90 inrush current limiter?
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Old 25th November 2008, 05:14 PM   #2
AndrewT is offline AndrewT  Scotland
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what type and size is your mains transformer?
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Old 25th November 2008, 06:03 PM   #3
Igla is offline Igla  Slovenia
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First... If you have a fuse on primary winding and the transformer is a big toroidal I would say this is normal because toroidals have big inrush current. My 500W toroid was blowing me 6amp fuses and I had to build current limiter.

I suspect that the amp is powerfull enough to drive your loudspeakers.
So...if the amp is not amplifaying the input signal enough I would check the resistor from output to negative input and resistor from negative input to ground. I guess you need more gain. Check LM1875 datasheet how to calculate the gain.
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Old 26th November 2008, 02:49 AM   #4
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Andrew,

It is an Avel 160VA 18V+18V torroidal.

Igla,

Thank you for your input. If understand you (and the National data sheet and Daniel's handy spreadsheet) correctly, my NFB transistor should be a higher value. I'm going to start with 33k and see where that takes me.

Thanks again, to both of you.
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Old 26th November 2008, 08:48 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
a 160VA transformer running on 110Vac will require T4A or T5A fuse.
Less may allow starting but could suffer from nuisance blowing with no apparent faults.

Now that T4A fuse could pass 5 or 6amps, in a continuous fault condition, for hours before rupturing.
How hot is the equipment going to get consuming 500 or 600W?
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Old 26th November 2008, 06:49 PM   #6
Bone is offline Bone  United Kingdom
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Default Lm 1876

Sorry should have started a new Topic

Tony
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Old 28th November 2008, 03:30 PM   #7
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Quote:
Originally posted by AndrewT
Hi,
a 160VA transformer running on 110Vac will require T4A or T5A fuse.
Less may allow starting but could suffer from nuisance blowing with no apparent faults.
Actually, a 3A is working just fine. I'll try a 2.5A as soon as I get some. I suspect those will work as well. The 2.0 did not fail consistently, so I think that was right on the edge.

Also, I've replaced the NFB resistor with a 33k piece. More gain, as expected, but a rather dramatic increase in the authority and precision in the bass, even with my smallish full-range speakers. Thanks again, Igla, for the suggestion.
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Old 28th November 2008, 05:17 PM   #8
Igla is offline Igla  Slovenia
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No problem jrn77478.
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Old 28th November 2008, 05:19 PM   #9
Igla is offline Igla  Slovenia
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By the way jrn77478; you NFB resistor in 33k.
What's the value of the other resistor to ground which also sets the gain?
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Old 28th November 2008, 06:42 PM   #10
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I left it at 1k.
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