Problem with DC offset
- Thread starter ToTo Man
- Start date
Arkless Electronics
Trade: Amp design and repairs.
Not necessarily. Many factors come into it, such as the amount of gain, the temperature stability, input bias current and voltage offset of the amplifier (op-amp generally) plus what limits are placed on the amount of offset.
The NFB of course works as the main thing to eliminate output offset and deals with most of it. Often input and output are direct coupled but there is a capacitor in the feedback ground leg giving unity DC gain.
ToTo Man
the band not the dog
What I wrote here is bullsh@t, isn't it? The amount of DC offset from the DAC that makes it through to the speakers will depend on the position of my amp's volume control.
Arkless Electronics
Trade: Amp design and repairs.
If you have a pre amp then its likely to have capacitor coupling.
Most power amps have unity gain at DC so even if there is no input cap it doesn't amplify DC, just passes it on as it were.
In passive speakers generally only the bass unit will see DC.
ToTo Man
the band not the dog
I tested this on my 'bench-testing amp', a cheap Cambridge Audio A1 integrated, and my main listening Yamaha A-S3000 integrated.
The Yamaha must be capacitor-coupled as the DAC's 1.6V DC output is reduced to just 4mV at the Yamaha's speaker outputs (or 3mV with a 6Ω load connected), which is likely the Yamaha's own offset, and the DC does not increase as I turn up the volume pot.
The DAC's DC output at the Cambridge's speaker outputs, on the other hand, does increase as I turn up the volume pot. Fortunately, the highest I have the pot set for my speaker testing is the point at which I'm measuring around 140mV DC, which is likely still quite some way below the threshold at which voice coil damage occurs.
The DAC is going off for repair and while its away I'm also having it upgraded to the latest spec USB board, - two birds with the one stone and all that!...
Arkless Electronics
Trade: Amp design and repairs.
If the Cambridge is entirely DC coupled then like I said it will give 1.6V at full output
ToTo Man
the band not the dog
This DAC has always made a 'pop' / 'flup' noise through the SE output whenever it's locked onto and unlocked from a sampling rate, but has never made this noise through the BAL output. Does this suggest the DC at the SE output has been there from new and I've only just become aware of it? 
I guess I'll find out when it comes back if the sample rate changes on the SE output are silenced....
I guess I'll find out when it comes back if the sample rate changes on the SE output are silenced....