Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: punkykatt on January 30, 2020, 04:52:04 pm

Title: Too much signal output on line out
Post by: punkykatt on January 30, 2020, 04:52:04 pm

Hey Guys, got a Fender Stage 160 s/s amp on the bench, the owner says every sound guy tells him the line out puts out too much signal when they hook the amp to the mixing console. I checked R118, R119 both 470r 2W and R120 15r 1/2W  all three are spot on.  Can I increase these resistors to drop the signal without causing problems. The circuit is not like a line out in the tube amps I have seen.  Any help is much appreciated.  Thank you in advance.


http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Inne/Stage_100,_160_Service_Manual.pdf (http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Inne/Stage_100,_160_Service_Manual.pdf)

Title: Re: Too much signal output on line out
Post by: sluckey on January 30, 2020, 05:11:42 pm

I would inject a proper signal into the input, set the knobs as stated, and check the voltage at the line out jack. If it's all proper according to the schematic notes, I'd tell the sound guy to turn it down on his end.

Title: Re: Too much signal output on line out
Post by: PRR on January 30, 2020, 10:39:49 pm

Well it is a Line level, nominally -6dBV (0.5V) and some sound operators may not have line ins, only mike ins. Any half-assed sound guy carries attenuators, but some are only quarter-assed, and also pads are not a stock item any more. (And never were sexy.)

A 1 Ohm resistor across the 15r will give -30dBV which on a R&R stage is rather mike-level.

BUT, there is a design screw-up. The XLR jack rides at half the speaker voltage, or up to 22V away from ground. Transformer inputs do not mind. But most mike and many line inputs are Transformer-less. With +/-15V rails and often only +/-5V of input common-mode range. Things may have been different in 1999, or Fender may not have done all testing with the latest sound gear. So this may distort bad no matter what the soundperson does (it may not even be clear why it distorts at any gain setting on the board).

A couple 47r resistors to XLR ground will reduce the common-mode voltage without much drop of level. These (and the 1r) may logically be less than 1/2W, but I would use full 1/2W parts for robustness.

The 47rs may actually be mounted ON the XLR rather than round-about as I have drawn.

Title: Re: Too much signal output on line out
Post by: punkykatt on January 31, 2020, 05:30:59 am

PRR, Thank you very much for the lesson and the schematic mod. I will give this a go today. Thanks again.