DC offset issues with GC

[AndrewT]

what a mess turns up when the circuit is not understood.

Far better to download the National datasheet and read it from cover to cover.

[fishski13]

Originally Posted by richie00boy

I suspect your pre-amp is home made too and does not include a DC blocking capacitor on its output. Then when you connect your amp, the DC offsets are all multiplied by the amp which also has no DC blocking or attention to DC conditions whatsoever, and produces an unacceptable output offset.

i'm having a difficult understanding why this would be when there is nil offset at the preamp. The Cavalli-Kan Kumisa III Stereo Headphone Amplifier

AndrewT,
i have read it, more than once. i'm no EE and have been DIYing for a little over a year. i guess i should interpret your post as "implement the chip as it was designed to"?

[richie00boy]

Just re-read your post and I missed the bit where you say you replaced R1 with a cap and it was worse. Also looking at your pre-amp it has an offset servo so should be ok.

I think your problem is the one that many builders of this particular gainclone seem to be having and that is the lack of DC gain roll off capacitor between R3 and ground. Try 100uF.

[fishski13]

Originally Posted by richie00boy

Just re-read your post and I missed the bit where you say you replaced R1 with a cap and it was worse. Also looking at your pre-amp it has an offset servo so should be ok.

I think your problem is the one that many builders of this particular gainclone seem to be having and that is the lack of DC gain roll off capacitor between R3 and ground. Try 100uF.

thank you - that was helpful. i'll give it a shot.

[fishski13]

no bi-polar caps in the parts bin, but i found two 100uF polar caps i soldered together in series (+ to + = 50uF) and soldered these in series from R3 to ground in one channel only. DC offset dropped to 35 mV . i'm placing an order for 47uF and 100uF bi-polars. i'll go with the 100uF if i can get them to fit with a reasonable amount of security with P2P.

thanks !!!

[AndrewT]

polar electrolytics will do the job as DC block in the NFB route.

Add a 1n4*** diode across the cap pins to prevent excessive reverse DC voltage.

If the amplifier is set up correctly there should never be any significant AC voltage across the cap.

The DC voltage will be the input offset voltage if the front end is reasonably well balanced. Check that the cap is oriented correctly to take this permanent DC voltage. Except when the output fails to one rail or the other, then the NFB resistors and the cap/diode share the supply rail voltage.

[fishski13]

Originally Posted by AndrewT

polar electrolytics will do the job as DC block in the NFB route.

Add a 1n4*** diode across the cap pins to prevent excessive reverse DC voltage.

If the amplifier is set up correctly there should never be any significant AC voltage across the cap.

The DC voltage will be the input offset voltage if the front end is reasonably well balanced. Check that the cap is oriented correctly to take this permanent DC voltage. Except when the output fails to one rail or the other, then the NFB resistors and the cap/diode share the supply rail voltage.

sorry, i'm a little confused. are you suggesting i can use a single polarized cap with the (-) to ground as long as i have a diode across the cap with it's cathode tied to ground?

[AndrewT]

the NFB DC blocking cap can be any type of low voltage or high voltage capacitor.

If the amplifier components have been selected correctly that DC blocking cap should never see any significant AC voltage across it. That means a polarised capacitor can be used.

If the amplifier is working properly then there should be a small DC voltage across it. This voltage is approximately equal to the input offset voltage, maybe in the range 5mV to 500mV. This voltage can be of either polarity. The input stage devices determine whether it is +ve or -ve. Check your voltage and orient the cap appropriately.

It is possible for the amplifier in exceptional/fault situations for the DC voltage across the cap to exceed 1V. Using a diode to prevent this reverse voltage removes the possibility of damaging the cap due to reverse voltage.
If you choose to use a capacitor of lower voltage rating than one of the supply rails then it's wise to also add a second diode in inverse parallel to the first. These two diodes prevent voltages across the capacitor exceeding 1V, even in full output offset (failed/shorted output stage).

Use a polarised capacitor sized to pass a wider bandwidth than the input DC blocking capacitor and select 10V to 16V working rating. These are cheap and tiny.
You can experiment by adding an extra capacitor across the electrolytic. This may /maynot improve the sound quality.

[fishski13]

Originally Posted by AndrewT

the NFB DC blocking cap can be any type of low voltage or high voltage capacitor.

If the amplifier components have been selected correctly that DC blocking cap should never see any significant AC voltage across it. That means a polarised capacitor can be used.

If the amplifier is working properly then there should be a small DC voltage across it. This voltage is approximately equal to the input offset voltage, maybe in the range 5mV to 500mV. This voltage can be of either polarity. The input stage devices determine whether it is +ve or -ve. Check your voltage and orient the cap appropriately.

It is possible for the amplifier in exceptional/fault situations for the DC voltage across the cap to exceed 1V. Using a diode to prevent this reverse voltage removes the possibility of damaging the cap due to reverse voltage.
If you choose to use a capacitor of lower voltage rating than one of the supply rails then it's wise to also add a second diode in inverse parallel to the first. These two diodes prevent voltages across the capacitor exceeding 1V, even in full output offset (failed/shorted output stage).

Use a polarised capacitor sized to pass a wider bandwidth than the input DC blocking capacitor and select 10V to 16V working rating. These are cheap and tiny.
You can experiment by adding an extra capacitor across the electrolytic. This may /maynot improve the sound quality.

thanks, i get it now .