HELP needed!!!! How to bias my Bass Amplifier?? Ashdown ABM 400

[fishdrop]

Hey Guys,

I have this Ashdown ABM 400.
originally it has been a ABM 200, but i turned it into a ABM400 according to the schematics.
I did a complete recapping and changed all the elkos for new ones.
and i also cleaned all those dirty pcb's from solder rests.

Here my question:
How do i bias this thing??
I see that there is a adjustment pot on  the pcb, but i don't know what i have to meassure.


I'd really appreciate some help  ;D

all the best

Bernhard

PS:
schematics of output board on last page:
https://dl.dropboxusercontent.com/u/841029/ashdown_abm500.pdf

 

[pucho812]

have you tried to e-mail ashdown?

 

[ln76d]

Definitely Ashdown service is the first place where you can go if you have some troubles with your amp. One of the most "user friendly" companies ever

Here's the biasing procedure for ABM500 (should be the same as for 400) from Ashdown:

"To set the bias, you need to connect an 8 ohm load to the output and
connect
a 10Khz signal into the input .. turn the preset fully anticlockwise and
turn the amp on..
adjust the input signal so you read 1 volt peak to peak signal on an
oscilloscope connected across the 8 ohm load.

Adjust the Preset slowly clockwise until the step in the waveform just
disappears and no further.."

Works like a charm

Definitely not recommend generating a signal from the computer, unless your interface works with high sample rate.

 

[fishdrop]

Here's the biasing procedure for ABM500 (should be the same as for 400) from Ashdown:

"To set the bias, you need to connect an 8 ohm load to the output and
connect
a 10Khz signal into the input .. turn the preset fully anticlockwise and
turn the amp on..
adjust the input signal so you read 1 volt peak to peak signal on an
oscilloscope connected across the 8 ohm load.

Adjust the Preset slowly clockwise until the step in the waveform just
disappears and no further.."

Thank you very much.

What Sample Rate would you recommend if i want to generate the Sine Wave from the computer??
How "loud" should the output volume pot be set??

Ah took a time to see your post. didN't get a notification

 

[ln76d]

96kHz and higher would be best. All eq/presence etc. to "0".  Check with the scope output from the soundcard to get proper 1V/10kHz. Turn anticlockwise "bias" pot. Connect signal to the input, turn amp on, set gain and volume to get 1V/10kHz at the scope connected to the 8Ω load. Rather more volume than gain (less distortion at high frequency).
"Adjust the Preset slowly clockwise until the step in the waveform just disappears and no further.."