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Chip Amps Amplifiers based on integrated circuits

LM3875 Chipamp
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Old 22nd April 2015, 07:13 AM   #31
Philfr is offline Philfr  France
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Hi,
Mounting is completed :

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Phil.
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Old 22nd April 2015, 09:57 AM   #32
Philfr is offline Philfr  France
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Quote:
Originally Posted by kp93300 View Post
Hi Philfr,
What are the sound changes when you add these?
kp93300
About Rf2 and Cf, it appears that high frequencies give more clean details, regardless of the level..
And again, thank's Tom !
Phil.
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Old 22nd April 2015, 02:54 PM   #33
Roscoe Primrose is offline Roscoe Primrose  United States
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Quote:
Originally Posted by johnr66 View Post
No kidding...
If you know that, as you claim, then your comment about four diode drops is nonsensical...
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Old 22nd April 2015, 06:10 PM   #34
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by Roscoe Primrose View Post
If you know that, as you claim, then your comment about four diode drops is nonsensical...
In the OP's diagram there are two bridges. There are two diodes in conduction on peaks of each half cycle to charge the filter cap on each of the supply rails or four diodes conducting in total.

If he bonded the the secondaries together in the middle to create a center tap, he can use one bridge and have a total of two diodes in conduction on each half cycle.

I don't understand the reason to use two bridges. If there is a benefit, someone please explain. The way he has it requires an extra part (diode bridge) and the extra two diode Vf loss.
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Old 22nd April 2015, 07:17 PM   #35
Roscoe Primrose is offline Roscoe Primrose  United States
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Sorry, I misunderstood what you were trying to say. You are indeed correct in that case.

Of course, that begs the question of why we're worried about that extra Vf loss? PS voltage is going to change much more than that over normal line voltage variations. It's not going to have any affect on PS regulation either, since Vf is essentially constant over a diode's normal operating current.

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Old 22nd April 2015, 07:55 PM   #36
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by Roscoe Primrose View Post
Sorry, I misunderstood what you were trying to say. You are indeed correct in that case.

Of course, that begs the question of why we're worried about that extra Vf loss? PS voltage is going to change much more than that over normal line voltage variations. It's not going to have any affect on PS regulation either, since Vf is essentially constant over a diode's normal operating current.

Roscoe
Sure the line voltage will fluctuate but why incur an additional loss on top of that (plus the extra part)? Losing about 3 volts across the rails isn't that significant to the ear, but the amp takes a hit in output power. For example, a 50 watt amp becomes 45 watts if it loses just one volt rms of output swing.

Last edited by johnr66; 22nd April 2015 at 07:58 PM.
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Old 22nd April 2015, 08:04 PM   #37
Roscoe Primrose is offline Roscoe Primrose  United States
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He did say in the original post he only needs 5-10W... Rail-to-rail voltage decrease should be more like 1.5v. He's only going to have about +/- 18v with that transformer. Higher rail voltage will give a higher maximum power out, but does it necessarily sound better? The difference between 50w & 45w is <0.5dB, not particularly meaningful...

Roscoe
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Old 1st May 2015, 11:06 AM   #38
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by johnr66 View Post
I don't understand the reason to use two bridges. If there is a benefit, someone please explain.
It is usually too expensive to bottleneck a very large transformer with a single bridge rectifier; however, somewhat more cost effective to use a more modest size transformer with dual bridge rectifiers.
Shorter answer: More effective transformer utilization; Current.
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Old 1st May 2015, 11:16 AM   #39
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Roscoe Primrose View Post
He did say in the original post he only needs 5-10W... Rail-to-rail voltage decrease should be more like 1.5v. He's only going to have about +/- 18v with that transformer. Higher rail voltage will give a higher maximum power out, but does it necessarily sound better? The difference between 50w & 45w is <0.5dB, not particularly meaningful...

Roscoe
Provided the internal components are not overstresssed by the higher supply voltage, it is my opinion that the higher supply voltage leads directly to better performance.
This is because many of the circuits in an amplifier are ClassA and many of these ClassA stages are single ended. For this type of operation, accuracy is improved by using less of the total available voltage swing. There are many websites that explain this and even recommend using a lesser part of the total available swing just to get that improved performance.

Once this is done, one finds that the NFB has less work to do in trying to reduce the errors at the output.

There is a separate issue that leads to improved performance with higher supply voltages.
LESS CLIPPING of the output signal transients, when the supply voltage is well above that required for average signal levels.
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Old 1st May 2015, 02:31 PM   #40
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by danielwritesbac View Post
It is usually too expensive to bottleneck a very large transformer with a single bridge rectifier; however, somewhat more cost effective to use a more modest size transformer with dual bridge rectifiers.
Shorter answer: More effective transformer utilization; Current.
Nonsense! Just how is the single full wave bridge not utilizing the transformer to the fullest if the two secondaries are joined in the middle to make it like the center tap? Current flows in both secondaries on both half cycles. Current would be the same but with two extra diodes per half cycle with two FWB.

Last edited by johnr66; 1st May 2015 at 02:37 PM.
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