Bias instruction for khartago stereo amplifier

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Tinear2

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Bias instruction for khartago stereo amplifier
« on: 8 Mar 2018, 08:06 pm »
Hi, I am looking for instructions for the Odyssey Khartago stereo amplifier. I had them a while back and they seem to have vanished. I would like it if any members could help me with the procedure. I see Germany has been involved on many requests. Thank You.

Phil A

Re: Bias instruction for khartago stereo amplifier
« Reply #1 on: 9 Mar 2018, 03:50 pm »
Welcome.  If one does a search (do an advanced search and choose the circle for Odyssey only) for the word 'bias' on the Odyssey Audio Circle, there should be some hits - http://www.audiocircle.com/index.php?action=search2

germay0653

Re: Bias instruction for khartago stereo amplifier
« Reply #2 on: 11 Mar 2018, 07:35 pm »
The amplifier should be at operating temperature. If you've been playing music, wait perhaps 10-15 minutes for everything to "settle down".

The board is basically two channels, bridged into a monophonic amp, and essentially there are two "sides" to the board corresponding to the left and right channels in the stereo versions.

First check the output offset, which is the voltage between pads 6 (+) and 5 (-). The (+) wire going to the speaker cable terminal on the back of the amp is on pad 6. Set the DVM to 200 mV DC, put the positive lead of a DVM onto 6, negative lead onto 5, and adjust the trim pot way at the opposite end of the board (not the one in the middle) to zero out the offset. Don’t worry about small offsets, say 1 mV or less.

Repeat for the other channel, with the negative lead on pad 5, positive on 7.

Now, go back to the other side, negative lead on pad 5 or other convenient signal ground, and check TP 5 and 6 to see which one has the highest value (they will be slightly different). This time, use the trim pot in close to TP 6 to set the bias, using the TP point that had the higher voltage, to be about -21 mV. Repeat on the other side, checking TP 1 and 2, and use the trim pot near TP 2. Be careful not to short anything to the power transistor case which I think is grounded.

Then check the offset values again as they may have changed slightly, recheck the bias, etc.

The bias controls the "voicing" of the amp. -18 will be colder, -24 mV will be warmer. Klaus recommends -21 mV as a starting point.

Per Klaus himself:

Make sure that you have the red probe on the speaker out, and only use that part of the board for biasing,  as the board is true mirror image...then move the probe into the other post and bias that part of the board...for offset,  probe - goes to opening 7 or 8 (ground of the unused speaker outs on the board)...offset pots are the 2 close to the face plate,  get the offset close to 0 mV....then for the actual bias go only into TP's 3, 4, 7, and 8...ignore the others, and use the one on each side with the higher value....pots are next to the transistors.......20 mV's....again,  PLEASE  make sure that you move the red probe from side to side,  as the 2 sides of the board right through the middle act independently !!!!!!!!!!!!!!

walk away from it, come back after 10 minutes, check, and repeat if necessary until it is stabile........


Tinear2

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  • Posts: 2
Re: Bias instruction for khartago stereo amplifier
« Reply #3 on: 14 Mar 2018, 04:11 am »
Thanks to Germany and all who responded with the information I needed. I really appreciated the quick detailed information. Regards....Tinear2

germay0653

Re: Bias instruction for khartago stereo amplifier
« Reply #4 on: 14 Mar 2018, 08:57 pm »
For the record, I'm not German or from Germany, like Klaus.  I live in the USA like Klaus but ethnically, I'm Irish, English, Polish and Scandanavian and I'm old as dirt.   :thumb: