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Old 2nd February 2015, 04:01 PM   #11
AndrewT is offline AndrewT  Scotland
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I always recommend AC coupled Power Amplifiers to all inexperienced Builders.
Then when the experience allows you to foresee the dangers and you become able to design the various add-ons to protect your speaker and amplifier, you can then take the informed decision of whether to adopt DC coupling.
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Old 3rd February 2015, 07:53 AM   #12
r100 is offline r100
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I have left the amp on for a while and the DC values have shot up to close to around 190mV on both channels

Listening levels where low and heat dissipation on the chips negligible.

I unplugged the amp and left it for the night. Next morning I got the same values.

As I understand it is the source (DAC with preamp) which is injecting the DC. The strange thing though is that with any other amp connected to the DAC there is no more DC on the Loudspeaker terminals than usual.

Could this be linked to a grounding issue (CHG is not connected yet ? The build is still on a piece of wood).

With shorted input terminals the build measures around 27-28mV on one and around 5-6mV on the other channel which seems very reasonable.
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Old 3rd February 2015, 10:42 AM   #13
r100 is offline r100
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Quote:
That's also what I'm doing in all my stereo amps and in the picture below I'm showing trouble free way to connect power grounds: output grounds of both channels (OG) are connected with a piece of thick copper wire. Central point on that wire is power star ground and all 4 ground wires from rectifiers board (PG+, PG+ and PG-, PG-) are connected here. Output grounds are taken directly from the board, signal ground connects to the point on the board marked as SG.
Is this still the way to do proper grounding on the newer blue boards ? please see post 522
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Old 3rd February 2015, 11:13 AM   #14
AndrewT is offline AndrewT  Scotland
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the PG+ and PG- was placed on the PCB to allow a dual secondary transformer to be used.

If one uses a centre tapped transformer, other options become available.
You choose the option that minimises the interference.
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Old 3rd February 2015, 01:19 PM   #15
Peter Daniel is offline Peter Daniel  Canada
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PG+ and PG- was placed on the PCB to allow keep the grounds separate, it has nothing to do with dual secondary or CT transformers.
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Old 3rd February 2015, 01:20 PM   #16
Peter Daniel is offline Peter Daniel  Canada
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Quote:
Originally Posted by r100 View Post
Is this still the way to do proper grounding on the newer blue boards ? please see post 522
If using a single transformer then yes, this is a preferred method of grounding.
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Old 3rd February 2015, 01:24 PM   #17
Peter Daniel is offline Peter Daniel  Canada
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dc offset of 180mv. Send a message via AIM to Peter Daniel
Quote:
Originally Posted by r100 View Post
I have left the amp on for a while and the DC values have shot up to close to around 190mV on both channels

Listening levels where low and heat dissipation on the chips negligible.

I unplugged the amp and left it for the night. Next morning I got the same values.

As I understand it is the source (DAC with preamp) which is injecting the DC. The strange thing though is that with any other amp connected to the DAC there is no more DC on the Loudspeaker terminals than usual.

Could this be linked to a grounding issue (CHG is not connected yet ? The build is still on a piece of wood).

With shorted input terminals the build measures around 27-28mV on one and around 5-6mV on the other channel which seems very reasonable.
It's not a problem of grounding, this amp will amplify any DC present at its input, because there's no capacitor between R3 (680R) and ground.

To avoid any excessive offsets at the amp's output, you have 3 options:

- use a source with no DC offset
- use input coupling capacitor (2.2uF or bigger, can be installed in place of R2)
- or use capacitor between R3 and ground (22uF or bigger)
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Old 3rd February 2015, 01:26 PM   #18
Peter Daniel is offline Peter Daniel  Canada
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dc offset of 180mv. Send a message via AIM to Peter Daniel
Quote:
Originally Posted by r100 View Post
DC offset is around 4 mV on the right side and around 20-25mV on the left side. I've visually checked the soldering and all seems ok.

Although 20-25mV is not much, what could the reason for the difference between the left and right channel be ? Is it a concern ?
As stated before, the offsets will vary with chips: some chips show much bigger offset values than other in this particular circuit

20-25mV is nothing to be concerned and anything less than 80mV or so is still fine.
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Old 3rd February 2015, 03:01 PM   #19
r100 is offline r100
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Quote:
Originally Posted by Peter Daniel View Post
It's not a problem of grounding, this amp will amplify any DC present at its input, because there's no capacitor between R3 (680R) and ground.

To avoid any excessive offsets at the amp's output, you have 3 options:

1 use a source with no DC offset
2 use input coupling capacitor (2.2uF or bigger, can be installed in place of R2)
3 or use capacitor between R3 and ground (22uF or bigger)
Hi and thank's All for the input !

I cannot change the source it is the only one I have

I is a Yulong DA8 DAC i payed a fortune for

So I guess I'll put a input coupling capacitor in R2. What is the advantage of point 2 over 3 ?

Thank's !

Rupert

PS. I still don't understand why the measurements changed so much in the course of 3 or 4 hours of continuous playing music at normal levels ?? Probably my Yulong ? I hope not.

Last edited by r100; 3rd February 2015 at 03:05 PM.
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Old 3rd February 2015, 04:21 PM   #20
Peter Daniel is offline Peter Daniel  Canada
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dc offset of 180mv. Send a message via AIM to Peter Daniel
It's most likely Yulong changing it's output offset.

As to advantage of pint 2 is that you can use much smaller cap, so better quality and you actually protect the amp against any possible offsets from sources.

Point 3 only prevents amplifying of the offsets. The amp has approx 32x gain (or 30dB) so if the the source offset is 5mV you will see 150mV at the amp's output.

The possible advantage of point 3 is that the cap, although much bigger, is not in direct signal path so it's effect may not be that obvious. Although, OTOH, there's no such thing as component not in a signal path, everything actually matters.
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