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Old 7th February 2007, 08:01 AM   #1
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Default 41Hz amp6 problem

I have finished my first 41Hz kit (an amp6 basic) and everything works brilliantly - a very good sounding small amp.

The problem is that whenever I switch off my TV (sitting on the same Isotek mains distribution unit as the amp) I get a big thump through my speakers and the drive units 'suck in' and rest at maximum excursion 'inwards'. This remains the situation until I disconnect the power supply to the amp, at which point the cones return to the normal resting position.

I am aware that it requires DC to keep a cone sitting at maximum excursion, so I am worried about this damaging my speakers.

When I initially noticed this problem I was using an SMPS to power the amp, and thought that this might be due to a lack of grounding in my AV system (TV and DVD are double insulated, dac and amp6 powered by single rail SMPS). I switched to a linear power supply with a big cap between the PSU and the amp, and ran an additional ground wire from the - side of the cap to the Isoteks grounding post. This initially prevented the problem occurring, but the problem came back this morning.

Does anyone have any suggestions for solving this?

Thanks in advance :-)
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Old 7th February 2007, 10:22 AM   #2
Pano is offline Pano  United States
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Hmmmm... That is certainly a strange one. Will have to think about that.

The DC offset normally comes from a problem at the inputs. Is there room for you to get to the input caps to measure the DC there when this happens? Also measure the DC out, for reference.

Another question. Does this happen if you disconnect the inputs to the AMP6?
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Old 7th February 2007, 10:37 AM   #3
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Thanks Pano,

I will measure the DC at the input caps later on and see if anything strange is happening.

I will also try running the dac from the same linear PSU as the amp6 is now using, to tie these grounds together at PSU level. The dac is still using a SMPS (am I correct in thinking that the 0V rail is floating and separate from ground in switch mode PSUs?) so removing this SMPS may prevent 'ground plane offsets' from occuring.

I will also try disconnecting the inputs, and report back.

My previous two gainclones and two modified t-amps didn't suffer from this! (one gainclone now in use in my bedroom, the other in my flatmate's bedroom, both 'pimped' t-amps sold to grateful friends!)

Thanks again for the useful pointers :-)
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