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Old 13th October 2004, 05:38 AM   #1
Fragen is offline Fragen  Australia
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Default LM1875 battery question

Hi, Im building a LM1875 chipamp (exact copy of the National datasheet typical operation circuit) however I am powering it with two SLA 12V batteries. So far it sounds good but...

I plan on also building a ESP automatic battery charger for it, does anybody know how much current the LM1875 draws when it is idle? Or how to measure this?

Basically, I want to set the circuit up so that the automatic charger turns itself on when the amp is idle (but not switched off) and off again when the amp is playing music, has anybody else done something like this? On another note, would I have to switch the pre-amp off to get this drop in current drawn or just the CD player? and would this drop be greater than the 10mA needed to activate the circuit?

Oh yeah, one final question, Should I put any big capacitors on the battery power supply, or is the 100uf by the chip enough for battery supply, I did put two 1250uf caps on the battery terminals when testing the circuit but couldn't hear a difference?

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Old 13th October 2004, 08:17 AM   #2
Nuuk is offline Nuuk  United Kingdom
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[QUOTE][Oh yeah, one final question, Should I put any big capacitors on the battery power supply, or is the 100uf by the chip enough for battery supply, I did put two 1250uf caps on the battery terminals when testing the circuit but couldn't hear a difference?
/QUOTE]

Long-time battery user, Ron Clarke uses something like 6 uF on the rails of his battery driven Gainclones and finds it quite adequate (even preferable).

I have answered for Ron as he is in the process of moving home and may not see your post for a while.

Ron also states that he leaves his charger in circuit and charging 24/7 and does not notice any difference. This saves a lot of complication in designing an 'intelligent' charging system such as you have outlined above.
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Old 13th October 2004, 04:49 PM   #3
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Default Re: LM1875 battery question

Quote:
Originally posted by Fragen
Hi, Im building a LM1875 chipamp (exact copy of the National datasheet typical operation circuit) however I am powering it with two SLA 12V batteries. So far it sounds good but...
I've started looking at this chip lately because CarlosFM has said it has particularly good sounding high frequency response. Also, my speakers are 98dB/8ohm, so I really only need a few good watts.

Quote:
I plan on also building a ESP automatic battery charger
I'm glad to hear this because I've been eyeing that circuit for over a year. Unfortunately, I'm a complete newbie so I was waiting for someone with more experience to build it first.

Quote:
Basically, I want to set the circuit up so that the automatic charger turns itself on when the amp is idle (but not switched off) and off again when the amp is playing music, has anybody else done something like this?
You know, that was one of my biggest concerns. I could see my girlfriend turning on the amp(or system) and then going shopping after an hour or so. Or, listenign to some music before bed and forgettign turn it off. This would result in the amp drawing off the batteries until it reached it's minimum charge threshold.

Considering that this probably happen on a daily basis, I'd be going through batteries on a yearly basis - not cheap.

I wonder is there's a way to attatch the sensing circuit to amp's input signal w/o adding any noise? That way, when the amp receives an audio signal from the preamp or cd player, it would disconnect the charger.

Quote:
On another note, would I have to switch the pre-amp off to get this drop in current drawn or just the CD player?
That's the big question! Does the current drop enough when the cd stops spinning and no signal is sent or when the entire unit is turned off? The same would apply for a preamp, if used.

I'm guessing here, but couldn't we add a capacitor to store "x" amount of minutes worth of current? After "x" minutes of no signal, the relays would trip and the charger re-connect?

Perhaps I'm just a newbie freaming of things which would seriously complicate the already simple design.

Please let me know how things go with this charger,

victor
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Old 13th October 2004, 05:32 PM   #4
Fragen is offline Fragen  Australia
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Quote:
I'm glad to hear this because I've been eyeing that circuit for over a year. Unfortunately, I'm a complete newbie so I was waiting for someone with more experience to build it first.
Sorry, I have almost no experience So far I've built the ESP P88 preamp (with OPA627's instead of 2134's) and run it into the LM1875 circuit (I run the amp off batteries but have not completed the charger)

The ESP site http://sound.westhost.com/project98.htm has a table for minimum current and resistor values, If I knew the value of what the LM1875 draws when idle and what it draws playing I could select a resistor that is above the minimum current of idle and below current drawn when playing. This would save me from having to add a power switch to each power amp, (which would ruin the look of my amps ) not to mention I hate having to turn equipment on and off.

The LM1875's do sound great, I don't have very loud speakers (87db efficient bookshelf monitors) but when I hooked it up, it was loud and clear, best of all I only have two little 12V 1.3AH SLA batteries, wired to give -12V-0V-12V, Im very happy with the sound. Also, the 1875 chip only has 5-pins, a real bonus for careless people like me who don't check stuff before they solder.

I'll make the circuit charger tommorrow and post back if it works, trial and error should get the right resistor value.

This preamp/amp combo sounds great, I put the preamp into an old art-deco style silver cigar box, The amps are going into little die-cast boxes painted hamerite grey, the batteries and charger will also go into a bigger diecast box painted hamerite grey (it is also quite small though.)
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Old 13th October 2004, 05:33 PM   #5
Fragen is offline Fragen  Australia
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Here's the amp
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Old 13th October 2004, 06:13 PM   #6
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Quote:
Originally posted by Fragen
If I knew the value of what the LM1875 draws when idle and what it draws playing I could select a resistor ...

Why can't you just use an ammeter or DMM in the circuit and work it out yourself?
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Old 13th October 2004, 07:28 PM   #7
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The LM1875 is a really good sounding chip.
Those who have very efficient and easy to drive speakers should give it a try.

The batteries can be charging permanently (24/7), at a current of up to 10x less than their nominal capacity.
With 7A batteries I would leave them permanently charging with a small 300~500ma trafo and LM317/337 adjusted for +/-13.8V.
You may switch the PSU off when listening, if you notice any difference in the sound.
No maintenance, no worries, easy, great.
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Old 13th October 2004, 07:37 PM   #8
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It seems like batteries act as sort of buffer, preventing PS connected to it from degrading the sonics in big way. I noticed that in a DAC, and I have similar experience with GC.

Although alone (without charger connected), batteries still sound better, the influence on the sound of the charging circuit is not that pronounced (in my system it's more bright, not as smooth and coherent, with Velleman charger) and for less critical listening I don't bother to disconnect charger.
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Old 13th October 2004, 10:34 PM   #9
Pan is offline Pan  Sweden
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My major issues with a LM3886 GC I build for a friend was some low level "grainyness" and slightly hazy top octaves...

what improvements can I expect using batteries instead of standard PS?

/Peter
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Old 13th October 2004, 10:57 PM   #10
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Quote:
Originally posted by Pan
My major issues with a LM3886 GC I build for a friend was some low level "grainyness" and slightly hazy top octaves...
What speakers does your friend use?
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