What type AC coupling capcitors do you use for input and output stage?

[AndrewT]

.............. Of course, this is easy with a balanced output as there's no DC through the tranny, single ended operation may be more difficult.

Not really "of course".

The DC voltage in each half of the balanced output varies. If you allow random variation of these DC voltage levels then you are guananteed to have DC current passing through your very DC intolerant transformer Primary. Yes, there will be no DC at the output of the transformer. But DC across it's input will kill performance !!!!!!

You must design to ensure there is no DC imbalance in the feeds to the transformer Primary.

"of course" is in effect completely wrong !!

 

[analog_sa]

You must design to ensure there is no DC imbalance in the feeds to the transformer Primary.

Absolutely. But what makes you think the circuit hasn't been designed for minimal dc imbalance?

 

[AndrewT]

He said as if it was a matter of fact.

Of course, this is easy with a balanced output as there's no DC through the tranny,

It is not "fact" that there is no DC from a balanced source. It must be designed to minimise or eliminate the DC through the transformer primary.

 

[hazard500]

Not really "of course".

The DC voltage in each half of the balanced output varies. If you allow random variation of these DC voltage levels then you are guananteed to have DC current passing through your very DC intolerant transformer Primary.

Where do random variations in DC value come from? I have built a long tail pair with a ccs in the tail, the load resistors are 1% and I have hand matched the active devices to less than 0.5%. Theoretically, the DC voltage across the drains is zero, if devices are not perfectly matched there can be DC offset but this should be constant not random.

So you are right its not an absolute "of course". I did consider putting a pot between the sources of the j-fets (say 5 ohm) to balance out any DC offset but when I measured the actual off set it was around 1mV - so I have ignored it. In practice I don't hear any bass distortion or roll off due to saturation of the core. All is good. Thanks for pointing out an issue that I over looked.