AD1865 - passive i/v stage dc offset

lordearl · 11th May 2011, 05:09 PM

Recently been enjoying a modded ebay ad1865 dac with 560R resistors as the output stage. DC offset was only 3 or 4 mv so I did not bother with a filtering cap.

However, wanting to get some more volume from the dac, I changed the resistors to 1.5k, however the dc has now increased to 8.5mV on the right channel and 6.5mv on the left channel - is this still safe without a capacitor on each channel to block the dc?

I am running the dac directly into my power amp (6v6 push pull with output transformers) so am not concerned about dc reaching the speakers, but can dc offset < 10mV do any damage to the power amp? Apologies as I do not have a schematic for my power amp.

singa · 12th May 2011, 03:18 AM

Hi, I think it's small enough in amplitude to ignor since you have a tube amp
but even if it is a transistor amp I think it would be a concern if the DC offset
is in the 50-100mV region as it will be amplified and the DC will translate
into heat energy for crossover and voice coil of the speaker.

lordearl · 03:38 AM

Thanks - how come it is okay in the tube amp?

singa · 12th May 2011, 03:58 AM

The output transformer only allows AC to pass through or rather works
with AC only.( It's working principle)

CraigBuckingham · 12th May 2011, 05:13 AM

Quote:

Originally Posted by lordearl

Recently been enjoying a modded ebay ad1865 dac with 560R resistors as the output stage. DC offset was only 3 or 4 mv so I did not bother with a filtering cap.

However, wanting to get some more volume from the dac, I changed the resistors to 1.5k, however the dc has now increased to 8.5mV on the right channel and 6.5mv on the left channel - is this still safe without a capacitor on each channel to block the dc?

I am running the dac directly into my power amp (6v6 push pull with output transformers) so am not concerned about dc reaching the speakers, but can dc offset < 10mV do any damage to the power amp? Apologies as I do not have a schematic for my power amp.

I take it you are using the Iout pins? If you are it is best you feed a virtual ground to do the I-V conversion.

If your amp is AC coupled on the input then the DC offset is not going to be amplified. If your amp is DC coupled internally then you need to determine if the internal offsets are going to cause any issues.

lordearl · 12th May 2011, 11:43 AM

The pcb has a slot for the i/v resistors - what would I need to 'feed a virtual ground'?

The resistors I am using currently are 1.5k - I notice most diyers have used between 200R and 300R, however the datasheet states "A load impedance of at least 1.5 kΩ is recommended for best THD+N performance".

Not sure whether the i/v resistors count as "the load" - but I've noticed the output level is fine to drive a power amp directly, whereas the 220R to 500R range seems to require another gain stage in between.

11th May 2011, 05:09 PM	#1
lordearl diyAudio Member Join Date: Jun 2007	AD1865 - passive i/v stage dc offset Recently been enjoying a modded ebay ad1865 dac with 560R resistors as the output stage. DC offset was only 3 or 4 mv so I did not bother with a filtering cap. However, wanting to get some more volume from the dac, I changed the resistors to 1.5k, however the dc has now increased to 8.5mV on the right channel and 6.5mv on the left channel - is this still safe without a capacitor on each channel to block the dc? I am running the dac directly into my power amp (6v6 push pull with output transformers) so am not concerned about dc reaching the speakers, but can dc offset < 10mV do any damage to the power amp? Apologies as I do not have a schematic for my power amp.

12th May 2011, 03:35 AM	#3
lordearl diyAudio Member Join Date: Jun 2007	Thanks - how come it is okay in the tube amp? Last edited by lordearl; 12th May 2011 at 03:38 AM.

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12th May 2011, 03:18 AM	#2
singa diyAudio Member Join Date: Apr 2004 Location: near	Hi, I think it's small enough in amplitude to ignor since you have a tube amp but even if it is a transistor amp I think it would be a concern if the DC offset is in the 50-100mV region as it will be amplified and the DC will translate into heat energy for crossover and voice coil of the speaker.

12th May 2011, 03:58 AM	#4
singa diyAudio Member Join Date: Apr 2004 Location: near	The output transformer only allows AC to pass through or rather works with AC only.( It's working principle)

12th May 2011, 11:43 AM	#6
lordearl diyAudio Member Join Date: Jun 2007	The pcb has a slot for the i/v resistors - what would I need to 'feed a virtual ground'? The resistors I am using currently are 1.5k - I notice most diyers have used between 200R and 300R, however the datasheet states "A load impedance of at least 1.5 kΩ is recommended for best THD+N performance". Not sure whether the i/v resistors count as "the load" - but I've noticed the output level is fine to drive a power amp directly, whereas the 220R to 500R range seems to require another gain stage in between.

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