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AC or DC coupling on input of gainclone
AC or DC coupling on input of gainclone
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Old 6th January 2012, 03:05 AM   #1
john dozier is offline john dozier  United States
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Default AC or DC coupling on input of gainclone

I am building a pair of gainclones. My DAC will drive them directly, having an on board pre-amp. The manufacturer lists offset at less than 5mv. Do I need to AC couple the input on the gainclones? My calculations show a 30x factor in the gainclones thus a DC offset of 150mv at the speaker output. Is this too much? I am using tube amps and there is no offset. Thanks
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Old 6th January 2012, 10:35 AM   #2
AndrewT is offline AndrewT  Scotland
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The fact you are asking the question suggests to me, that you should follow the general advice and always include input filters, that attenuate, or remove non audio signals and that you should adopt an AC coupled amplifier design.
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Old 7th January 2012, 08:15 AM   #3
pacificblue is offline pacificblue  Germany
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Offset is always bad. Good amps have 2 mV or less, mediocre amps have 10 mV or less.

One thing you should never do is to AC-couple the input and DC-couple the NFB-loop. The output offset will soar, if you do.

The first thing you should do is to include offset control in the amp, i.e. either add an NFB cap or a DC servo. The LM3875 and LM3886 have worst case input offsets of 10 mV of their own. That alone would lead to 300 mV output offset, without even adding more offset from the source.

Then you should decide, whether you need to AC-couple the input as well. It is always wise to AC-couple, because the DC-offset strains all components unnecessarily. You may come to the conclusion that the strain is small enough to be acceptable, but strain is not the only aspect.

Since your DAC seems to be DC-coupled, you should absoutely AC-couple the amp input, because a fault in the DAC's output stage will lead to rail voltage at its output. That means your amp will get the DAC's supply voltage as input and will try to amplify that 30 times. The output to your speakers will be DC at maximum output swing.

If you use passive multi-way speakers with small woofers, you should also AC-couple the amp. The DC offset will maintain the woofer off its center position, while the crossover will filter the DC component out for the midrange and tweeter. The woofer will be misaligned in relation to the other drivers, and that will affect the speaker's phase and frequency response around the crossing frequency between woofer and midrange/tweeter. It will become more rippled. Parts of the music will either be swamped or exaggerated or you may even get both effects at different frequencies.
The smaller the woofer and the higher the crossing frequency, the worse is the effect. E.g. a 2-way speaker with 4" woofer will be more affected than a 15" subwoofer crossed to high-pass filtered satellites.
You may now think a fullrange speaker won't be affected at all. Nevertheless the sound quality of a fullrange speaker may also be affected, because THD usually rises, as the membrane is offset from its center position.

You should of course also wonder, why the DAC should have DC output offset to begin with. If its output is not AC-coupled, it was obviously not designed for stable DC working conditions, so it takes a lucky match to get the best sound from it. It also has no protection against DC coming in from the amp. So it would be good to provide that protection in the amp by AC-coupling the input.
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Old 7th January 2012, 12:37 PM   #4
john dozier is offline john dozier  United States
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Thanks for all the input. I am concerned about the effect of capacitors on SQ. I do have some 1uf Polystyrene caps and I suppose two of those on each channel, plus .22uf Teflons should be fairly transparent. I will try that. Thanks again and kindest regards
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Old 7th January 2012, 01:33 PM   #5
Th3 uN1Qu3 is offline Th3 uN1Qu3  Romania
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The effect of capacitors on quality has been way overblown. Unless it's a really bad electrolytic, a cap will not produce any audible distortion. The only subtle changes that they can bring about is due to their tolerance - but they make high quality plastic caps in lower tolerances as well.
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Old 7th January 2012, 01:56 PM   #6
john dozier is offline john dozier  United States
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I will not get into an audibility of caps wrangle here. I will only say that I respectfully disagree with the statement and have proven it to my satisfaction. Perhaps your system is just not revealing everything it should.. Regards
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Old 7th January 2012, 02:13 PM   #7
Peter Daniel is offline Peter Daniel  Canada
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Quote:
Originally Posted by john dozier View Post
I am building a pair of gainclones. My DAC will drive them directly, having an on board pre-amp. The manufacturer lists offset at less than 5mv. Do I need to AC couple the input on the gainclones? My calculations show a 30x factor in the gainclones thus a DC offset of 150mv at the speaker output. Is this too much? I am using tube amps and there is no offset. Thanks
First of all, if it's less than 5mV you may be also getting less than 150mV actual offset at the speakers. However, the amp by itself generates some offset, usually 50-80mV if chips are not selected. I measured offset of the Gaincard amp, which is AC coupled and has NFB cap, and it was still approx 80mV.

Is 150mV too much? It all depends on speakers; with small 2 ways, I would be concerned.

As a general rule, if the amp is DC coupled without NFB cap, you should always make sure that the source shows zero offset, otherwise coupling caps are really needed.
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Old 8th January 2012, 03:23 AM   #8
john dozier is offline john dozier  United States
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A stupid question, but should the AC coupling cap be in series with the input (in line in positive lead ) or across the input (connected between the positive and ground ) of the input? I ask this because all of my experience has been with tube amps. Regards and thanks
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Old 8th January 2012, 07:15 AM   #9
pacificblue is offline pacificblue  Germany
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The AC coupling cap is in series, the same for tube as for semiconductor amps.

If you haven't used such a cap in your tube amps, that is because people tend to rely on the fact that tube amp outputs are usually AC-coupled and will not let DC pass to the speakers anyhow. They don't care about source protection, because they assume that the source has an AC-coupled output anyhow. And the DC offset leads to asymmetric clipping, which produces those even order harmonics tube amp fans love so much. However clipping in a chipamp is not so desirable.
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