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Old 20th March 2012, 03:48 AM   #1581
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Quote:
Originally Posted by idjoel2000 View Post
is there any way i can fix this small (yet noticeable) flaw?
am I asking too much out of this little thing?
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Old 20th March 2012, 01:02 PM   #1582
udailey is offline udailey  United States
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I'm not saying it cant do it. Maybe it can. My speakers are full range so while the bass is 'visible' at 11hz.. I get no punch to very low bass notes with my speakers. I added a few subs and a plate amp. Very happy with the sound. I am in the Houston Audiophile Society and more than a few people have commented that out of all the members systems they have heard, mine sounds best for the money invested. Using a chipamp that is not very different from what you are using. LM3886 vs LM3875. I dont think the chip is the problem here. Probably that punch you desire needs higher voltage rails to push more current and ultimately higher power into your speakers in those bass notes. To sustain that high current you need bigger PSU caps.
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Old 21st March 2012, 04:25 AM   #1583
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Quote:
Originally Posted by udailey View Post
I'm not saying it cant do it. Maybe it can. My speakers are full range so while the bass is 'visible' at 11hz.. I get no punch to very low bass notes with my speakers. I added a few subs and a plate amp. Very happy with the sound. I am in the Houston Audiophile Society and more than a few people have commented that out of all the members systems they have heard, mine sounds best for the money invested. Using a chipamp that is not very different from what you are using. LM3886 vs LM3875. I dont think the chip is the problem here. Probably that punch you desire needs higher voltage rails to push more current and ultimately higher power into your speakers in those bass notes. To sustain that high current you need bigger PSU caps.
i had bad experience with adding capacitors to the existing psu caps. while they give you extra mid bass punch, the low bass punch increment is not as significant as the mid bass. this results in worse tonality overall
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Old 18th April 2012, 05:16 PM   #1584
jimchan is offline jimchan  Norway
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Hi Peter, just building your kit now.

I only have a 40v-0-40v transformer at the moment, and notice this is the top of the chip's recommended voltage, with warnings about .

Any concerns/recommendations here ?

Thx,
Jimchan
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Old 18th April 2012, 06:31 PM   #1585
AndrewT is offline AndrewT  Scotland
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Is that 40Vac from the transformer, or 40Vdc after the smoothing?
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Old 18th April 2012, 08:32 PM   #1586
jimchan is offline jimchan  Norway
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This is chassis mounted laminated core centre tapped secondary giving 40v-0-40v ac direct from the transformer - perhaps too high ? (used in a previous amp)
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Old 19th April 2012, 03:34 AM   #1587
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Quote:
Originally Posted by jimchan View Post
Hi Peter, just building your kit now.

I only have a 40v-0-40v transformer at the moment, and notice this is the top of the chip's recommended voltage, with warnings about .

Any concerns/recommendations here ?

Thx,
Jimchan
you can even use 50-0-50V CT transformer, and you still get away with it. see the datasheet LM3875 - High-Performance 56W Audio Power Amplifier the maximum voltage swing is 84V

but remember, you have to adjust the value of the supply capacitors voltage accordingly
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Old 19th April 2012, 09:31 AM   #1588
AndrewT is offline AndrewT  Scotland
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The absolute maximum supply voltage |V+| + |V-| is 84Vdc when signal is present.

This will be exceeded using a 0,30,0,30Vac mains transformer.
A 30Vac or 35Vac or 40Vac or 50Vac transformer cannot be used with a normal PSU to power any of these National Chipamps.
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Old 19th April 2012, 09:57 AM   #1589
jimchan is offline jimchan  Norway
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Thx Andrew.... will have to leave the existing transformer on the shelf.

I have found the thread regarding power supplies for Gainclones, so will read up :-)
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Old 24th April 2012, 12:29 AM   #1590
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Default LiFePO4 power for chip amp?

I built Peter's LM3875 kit and have been enjoying it ever since. Now, I have an idea to build it into a battery powered integrated amp with a low voltage tube preamp stage. I am thinking of using a 25.6v LiFePO4 battery pack to power both the chip amp and tube stage directly, without any voltage regulation. The battery pack would hit 28.8V peak when fully charged, then drop to around 25.6V during the majority of its discharge cycle. The pack will incorporate a simple circuit that shuts off the power at 16V.

Does anyone see a problem with doing this with the LM3875?
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