WTB: help fixing a humming F5 in MA

[Zen Mod]

offset of 100mV is sort of max allowable for long term use

off course that smaller offset is better , from many reasons

anyway - that amount of offset which will influence output power in greater amount , will influence health of your speakers much faster than you'll be able to spot lack of power ;

let just say that - DC offset of 10V will influence both coils and your ears

any lesser DC offset is pretty irrelevant , speaking strictly about decreasing p-p output wave

speaker coil doesn't like DC

 

[sts9fan]

I did notice that jumpered I get the mV offset but connected to a cd player it jumps to 4V. Is that normal?

 

[Zen Mod]

did you measure offset on output of your CD , unloaded ?

if you have offset there , F5 will amplify it , divided by momentary position/ratio of attenuator in between

I presume you have some sort of passive attenuator in between , because you're mentioning CD , not preamp

 

[sts9fan]

Well it's actually a squeezebox with digital volume control.
In the main system there is a pre with 18db of gain.

 

[Zen Mod]

Well it's actually a squeezebox with digital volume control.
....

well - that's not telling me anything , just because I'm not using sqbox ;

....
In the main system there is a pre with 18db of gain.

I resume that's irrelevant for this case ..... same as neighbor's system

 

[sts9fan]

The relevance is that I noticed the lack of
power using a pre. Not just a source.

 

[Zen Mod]

OK

AMP:
-short inputs - measure offset

-open inputs - measure offset ;

sqbox - measure offset , unloaded

did you checked amp for oscillations ?

if you don't have a scope , this circ connected instead of speaker ( or in same time with speaker ) can be handy

if osc. is present , led will lit

 

[sts9fan]

With no input it is ~18V with SB ~4V(does not need to be on). Shorted ~80mV

 

[Zen Mod]

you messed something , big time

so - either put/link some biiiig pictures here ( and plenty of them ) , or find someone to check that for you in vivo

 

[sts9fan]

Thanks, Gotcha

 

[sts9fan]

I took a few measurements.
At the node between the 1k and 100k resistor with the input grounded I get 1vdc. Ungrounded it raises to 7.2vdc. I also tested the drains of the input FETs which is 16.2vdc.
Any suggestions?

 

[Zen Mod]

we need pictures and schematic with written measurement on it