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Old 6th October 2012, 04:38 PM   #31
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Ok i rebuild the amp and i include Cin & Ci.

I build four channels using point to point, dc offset on all channels is approximately -6mV dc ?????

Is this acceptable or there is some connection problem?
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Old 6th October 2012, 11:21 PM   #32
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Quote:
Originally Posted by zakman35 View Post
Ok i rebuild the amp and i include Cin & Ci. I build four channels using point to point, dc offset on all channels is approximately -6mV dc ????? Is this acceptable or there is some connection problem?
Maybe your rails are unequal dc voltage or have more noise (ac content) on one rail than the other?
Question:
Did you measure DC offset with a load connected, such as a ~6 ohm resistor in the speaker jack? The idle load of the amp would increase, symmetrically and possibly swamp the offset.
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Old 7th October 2012, 04:15 AM   #33
Marc Vi is offline Marc Vi  Netherlands
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Andrew, I completely agree with you.

Zakman, a DC offset of - 6 mV is very good and fully acceptable. Can you give the schematics of your amplifiers?

Marc.
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Old 7th October 2012, 04:45 AM   #34
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Originally Posted by Bigun View Post
Full marks - I love this idea. The matching of the input sections on the i.c. are probably pretty good because it's a monolithic device and they likely track well with temperature. I think this approach is going to become de rigueur for chip amps! Makes me consider building one (Daniel has been trying to persuade for awhile).
The bass on LM3886 is great, but the mids can be a pest, and without a totally thorough answer for getting rid of harmonic distortion. SO, I would suggest to start with LM1875, which is very easy, or TDA7294, which is quite good when the gain current and amount is set within the limitations of the inbuilt miller comp (aka, you'll need a preamp).
Quote:
Originally Posted by zakman35 View Post
With the Cm 100uf installed and no other changes Dc offset = 20mV. I try some smaller values for Cm and found that 22uf gave the best results of Dc offset = 4mV. (Rm was 39K). Further degrease of Cm to 10uf result in Dc offset = 30mV. So i kept the Cm to 22uf and increase the value of Rm from 39K to 49K and Dc offset reduced to 1.7mV!!!!!. . . All the above tests was done with and without Ci = 220uf. Dc offset was the same with and without Ci installed.
Noise on the rails is a low current voltage source and easily felled by a load on the rails, or the RC that you've got powering the mute circuit will remove some noise from just one of the rails.

We probably need to examine the power supply and scrape off some of that noise. A nice head start could be RC's across the transformer secondaries.
What bridge rectifier do you have, and do you have a CRC power supply?
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Old 7th October 2012, 08:32 AM   #35
Marc Vi is offline Marc Vi  Netherlands
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Daniel,

The title of this thread is "LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage".

Your post has another subject. Can you do the same with the LM1875 as I did with the LM3886?

Marc.
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Old 7th October 2012, 10:23 AM   #36
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Quote:
Did you measure DC offset with a load connected, such as a ~6 ohm resistor in the speaker jack? The idle load of the amp would increase, symmetrically and possibly swamp the offset.
Yes, I measure with a 8,5ohm resistor i had in hand. Offset with the resistor was -5,4mV.
I also change the input resistors from 330 & 10K to 680 & 22K, the offset remain unaffected.
It looks that the change of values in components or even the installation of Ci or Cin has little or no effect at the dc output, which remains negative on all the channels (4).

I also measure noise output of 0.1mV AC, as Andrew suggested in another thread
Quote:
Set the DMM is 200.0mVac.
If the noise exceeds 10mVac, then something is seriously wrong.
If the noise is in the range 2mVac to 9.9mVac, then the amp is bad.
If the noise is in the range 0.3mVac to 1.9mVac them is is moderate. (-80dB to -90dB ref 20Vac).
If it is <0.3mVac then it is good. (better than -98dB ref 20Vac)
If it is <0.1mVac, then it is bordering on excellent. (better than -112dB ref. 20Vac)
Quote:
Zakman, a DC offset of - 6 mV is very good and fully acceptable.
Thanks Marc and sorry for highjacking your thread, dut i still dont like the negative offset.


I assume Daniel is wright, it has to be the psu responsible for this.
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Old 7th October 2012, 11:24 AM   #37
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Up to a +/- 100mV offset shouldn't really be a problem at the output of the amp. Maybe this is all you will get without even matching the input stage resistors.
While experimenting on a circuit I have had up to 500mV offset on the speakers with no audible change in sound AND it certainly is perfectly safe for the speaker.
I should pull out my 3886 board and see what the offsets were.
I do remember trying with no nfb cap and making no other adjustments. There was a film cap at the input. If the source is dc decoupled already then an input cap isn't required either.
However if you use a volume pot at the input of the chip amp it's varying resistance will cause the offset to change depending on position but might still be within acceptable limits. But dc through the volume pot isn't recommended.
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Old 7th October 2012, 09:46 PM   #38
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Originally Posted by Marc Vi. View Post
Daniel, The title of this thread is "LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage". Your post has another subject. Can you do the same with the LM1875 as I did with the LM3886? Marc.
Yes, Marc. The mixed coupled LM1875 was a large group project some years ago. Here's the basic details:
Chip LM1875, amp power caps 470u//100n, power supply board 8x 2200u per rail, transformer 18,0,18vac 4a, input load resistor 10k, feedback resistor 27k, feedback-shunt resistor 820R, input cap 2u and small signal groundlift 2R. It has a high gain of 34, usually 0mv offset and a level frequency response.

I have not made another DC coupled or mixed coupled amplifier after discovering that the dynamics are inferior.
The projects that followed, all had something very much like: 10k input load, 330R stopper, 2u input cap, 2k2 feedback-shunt resistor, 68k feedback resistor, 220u or 330u nfb cap, 2R or greater small signal groundlift.

For amplifier board power caps, TDA7294's want 220u~330u and no mute cap, LM1875's want 330u~470u, and and LM3886's want 680u~1000u and 47u mute cap. Lower figures for the amp board power caps, are facilitated by selecting a just right rail to rail cap power filter, else use the higher figures. Rail2Rail cap for LM1875, LM3886 is Mallory SEK 4.7u 250v from Allied Electronics, and just one per each amp board. TDA7293 and TDA7294 wants a 2u bargain grade high loss polyester for its rail2rail cap. That filter dumps power noise and makes more laid back mids, which eases selection efforts for all other caps. Cleaner power (and cooler amp heatsink) is always worth something.

The NFB cap took some selection effort and usually some bypass cap work to restore a level frequency response; but the extra effort there, paid off in more power, longer life, speaker protection and bigger dynamics. NFB caps are easiest to select when they're big enough for 5hz and when the input cap is not big enough for more than 10hz. Refer to AndrewT on the specifics.

However, a NFB cap does not necessarily balance the load evenly for IN+ and IN-. See the following schematic with 10k-470R load at IN+ (Q15) and 10k//4u7-470R load at IN- (Q14):
Circlophone 2012 Remix (this newer edition not yet proofed)
Both in+ and in- are balanced equally. You've got an offset trimmer circuit on the in+ side. On the in- side the 10k//4u7 has been added to the feedback loop. For sure it wasn't easier than simply using the NFB cap.

P.S.
For the LM3886 that won't stop doing DC offset even if you have it fully ac coupled (have input cap and have NFB cap), then I would suspect a ground loop as the offset causing culprit. Try making the amplifier board power umbilical cables and the small signal ground cables all exactly the same length to minimize differences that cause voltage differential. Also, try to retrofit small signal groundlift resistors to decrease the current of ground loops. A retrofit isn't as seamless or effective as strict adherence to star grounding on new builds; however, the goal of minimizing differences is the same. It is even better to decrease the number of grounds, because if there was just one ground then there's no ground loop.
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Old 8th October 2012, 09:16 PM   #39
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Quote:
Maybe your rails are unequal dc voltage or have more noise (ac content) on one rail than the other?
I measure dc voltage directly on the pins of the chip, both rails are exactly the same (+36.2V, -36.2V), i also change the input resistors and the mute resistor and cap.
Offset remains unaffected.

My psu has a dual bridge and 47.0000uF per rail,
i install two bypass caps of 100uf & 100nf, and an RC network after that (R=1ohm, C=680nf) Cs caps directly on the chip are 1000uf.

Offset remains the same -6mV.
I also try to play some music, it sounded loud and clear.

So to summarize this i started this trying to achieve similar to Marc resulst but with a different way and i end up with a negative figure that i cant get rid of it.

Although this is acceptable i want to find the cause of it. Any advise?
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Old 8th October 2012, 09:35 PM   #40
Marc Vi is offline Marc Vi  Netherlands
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Hallo Zakman35,

Why don't you like the negative offset? Is there any difference between a positive and a negative offset? I don't think so. And 6 mV is a very good result.

But if you post the schematic of your amplifier, maybe I can help.

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