An illustrated guide to building an F5

[sts9fan]

My F5 is up and running great. Dead quiet and sounds fantastic.
One thing I noticed on the test bench is I get 18v DC at the output if the input is not shorted or a source attached. I doubt this is normal. Any clue what could cause this? Shorted I get perfect bias with 0mvdc at the output.

 

[Zen Mod]

missing gnd somewhere - on input (gate to gnd resistors ) ?

 

[AudioSan]

fake J-fets?

 

[sts9fan]

I got the parts from techDIY so I hope the Jfets are not fake. Would it work with fakes?

 

[Itsmee]

...One thing I noticed on the test bench is I get 18v DC at the output if the input is not shorted...

Is that on both channels?

 

[6L6]

Take some photos. Some people can see the smallest errors.

 

[AudioSan]

I got the parts from techDIY so I hope the Jfets are not fake. Would it work with fakes?

okei. no doubt those are fakes

 

[sts9fan]

Yes it's both channels. I will try to take some pictures later. Thanks for the ideas.

 

[6L6]

okei. no doubt those are fakes

?????

TechDIY stuff is genuine. The Jfets might be BL grade, but not fake.

 

[bobodioulasso]

missing gnd somewhere - on input (gate to gnd resistors ) ?

Sts9fan, if that happens only when inputs are open, you should follow Zen Mod advice.
The ground is made from the device you plug in.
Nothing plugged in : no ground.

 

[AudioSan]

?????

TechDIY stuff is genuine. The Jfets might be BL grade, but not fake.

oh ****it was missing a not

no doubt those fets are NOT fake

you meen they migth be GR i guess

 

[sts9fan]

I am sure it helps to have a wire from the board to gnd. I will attach one and see if that does it.

 

[AudioSan]

I am sure it helps to have a wire from the board to gnd. I will attach one and see if that does it.

what do you meen?
from the board to GND? are you saying that you don't have a wire from board's GND to PSU's Gnd?? that will for sure make a offsett of 18V++.

 

[6L6]

And without it, the amplifier was finding ground through the source/preamp...


If that's the only error, it will fix it.

 

[AudioSan]

exatly 6L6

 

[sts9fan]

Yup. Is that really bad?

 

[6L6]

The short story is that if you disconnected your preamp or source (the RCA inputs) you would fry your speakers. 18v of offset is not good at all for the health of the speakers.

 

[AudioSan]

you can also fry your amp

 

[sts9fan]

Fixed

 

[AudioSan]

so how is it now? around 0mV without a source?