lm3875 questions

jimbo1968 · 13th June 2005, 05:42 PM

The datasheet say that there will be no power on/off transients or DC spikes. Can I trust this? eg if I use the chip to power an expensive tweeter in an active system?

One of the graphs on page 10 shows power of up to 75W into 8ohms with 40V supply. Tthe same THD+N spec as used in the performance section which is based on 35Vsupply and gives 56W.

So what is this region of greater than 35V supply? Is it not recommended, just for the brave ?

Nuuk · 13th June 2005, 06:17 PM

I can understand your aprehension about connecting up a tweeter (or woofer for that matter) directly to an amp. I always used speaker protection with my active speakers.

I can only report from my (long ) experiences with the LM3875 chip amps that there is NO noise whatsoever when powering up and only the very feintest of pops a few seconds after powering off.

Of course, if you use a pre-amp or buffer (or active crossovers!), they must be powered up a few seconds (longer in the case of a valve buffer) before powering up the LM3875.

jimbo1968 · 14th June 2005, 01:05 AM

Quote:

Originally posted by Nuuk
Of course, if you use a pre-amp or buffer (or active crossovers!), they must be powered up a few seconds (longer in the case of a valve buffer) before powering up the LM3875.

Are you being serious? Oh well, something else to find room for.

Nuuk · 14th June 2005, 07:43 AM

Quote:

Are you being serious?

This is all documented on Decibel Dungeon.

jimbo1968 · 14th June 2005, 02:59 PM

Give us a clue (which section of DD) ?

I'm guessing a relay on the supply rails?

Fortunately I do have space in the PS compartment to add more components.

Nuuk · 14th June 2005, 03:05 PM

I was referring to the mention of noise at the output of the amps if you turn on everything together. What I meant was that if you read the Gainclone pages on DD, you would already have been aware of that problem.

I am literally about to try a delay circuit for the pre-amp/buffers and will write it up on DD if and when it is working successfully.

jimbo1968 · 14th June 2005, 04:48 PM

Thinking about it, my old av amp very clearly waits 3 or 4 seconds before switching in the power amps.

Haven't noticed it with the new one, but that is probably just because it does it quietly or quicker.

13th June 2005, 05:42 PM	#1
jimbo1968 diyAudio Member Join Date: Nov 2004 Location: UK, bristol	lm3875 questions The datasheet say that there will be no power on/off transients or DC spikes. Can I trust this? eg if I use the chip to power an expensive tweeter in an active system? One of the graphs on page 10 shows power of up to 75W into 8ohms with 40V supply. Tthe same THD+N spec as used in the performance section which is based on 35Vsupply and gives 56W. So what is this region of greater than 35V supply? Is it not recommended, just for the brave ?

13th June 2005, 06:17 PM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	I can understand your aprehension about connecting up a tweeter (or woofer for that matter) directly to an amp. I always used speaker protection with my active speakers. I can only report from my (long ) experiences with the LM3875 chip amps that there is NO noise whatsoever when powering up and only the very feintest of pops a few seconds after powering off. Of course, if you use a pre-amp or buffer (or active crossovers!), they must be powered up a few seconds (longer in the case of a valve buffer) before powering up the LM3875. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

14th June 2005, 03:05 PM	#6
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	I was referring to the mention of noise at the output of the amps if you turn on everything together. What I meant was that if you read the Gainclone pages on DD, you would already have been aware of that problem. I am literally about to try a delay circuit for the pre-amp/buffers and will write it up on DD if and when it is working successfully. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

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14th June 2005, 02:59 PM	#5
jimbo1968 diyAudio Member Join Date: Nov 2004 Location: UK, bristol	Give us a clue (which section of DD) ? I'm guessing a relay on the supply rails? Fortunately I do have space in the PS compartment to add more components.

14th June 2005, 04:48 PM	#7
jimbo1968 diyAudio Member Join Date: Nov 2004 Location: UK, bristol	Thinking about it, my old av amp very clearly waits 3 or 4 seconds before switching in the power amps. Haven't noticed it with the new one, but that is probably just because it does it quietly or quicker.

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