Sumo Andromeda III Settings

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deafbykhorns

deafbykhorns

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Joined 2004
I have a blown channel on a Sumo Andromeda III
It uses IRFP240/IRFP9243 combo. The 9243's are no longer available and IR recommends the IRFP9240 as a replacement.
I cant find any data sheets on the 9243 to compare. This is a fully balanced amplifier in which the output gnds. are not grounded to chassis. I have seperate bias & null adjustments for each side of balanced amplifier for one channel. I dont trust the settings on the good channel and looking for recomendations on bias. I believe the null is set 0 to ground on each side and not between + & - since this is fully balanced.
 
dimitri

dimitri

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Joined 2002

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carlmart

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Joined 2001
preciseaudio.com
johnsciolino said:
This is a fully balanced amplifier in which the output gnds. are not grounded to chassis. I have seperate bias & null adjustments for each side of balanced amplifier for one channel. I dont trust the settings on the good channel and looking for recomendations on bias. I believe the null is set 0 to ground on each side and not between + & - since this is fully balanced.
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Several Sumo amps were designed by James Buongiorno. You could trying asking him about the settings:

jamesbongiorno@ampzilla2000.com

In any case his balanced amps were in fact bridged amps, so I don't know if it's accurate to call them balanced amps.

And I think null & bias was carried out separately on each side of the bridge.

There should be some information on the web, but I failed to find it just now.


Carlos
 
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carlmart

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Joined 2001
preciseaudio.com
OK. I could see what was happening.

My firewall (Zone Alarm) was not letting some pdf files from passing on.

I wonder how to set things up for that not to happen.

And now getting back to audio matters.

Apparently the A and B outputs have separate circuits that have to be nulled and biased separately.

Do write to Buongiorno and see if you can get some help for the setup. There should be a proper method and an easy method.



Carlos
 
Steven

Steven

Member
Joined 2003
Hawksford again?

What is the design date of this amplifier? To my surprise it looks like the Andromeda uses the Hawksford error correction circuit in the output stage. Quite some posts in this forum are about this circuit. Details can be found in Hawksford's paper, see fig 5. http://www.essex.ac.uk/ese/research/audio_lab/malcolmspubdocs/J3 Distortion correction PA.pdf
Hawksford's paper is from 1981. Was Bongiorno's design based on Hawksford's?

Anyway, since this is an error correction output stage, the null-setting is not for offset adjustment but for minimization of the output stage distortion. It also adjusts the damping factor to infinity because at the right setting the output impedance becomes zero. This is also the way to adjust it. Connect a load resistor to the output, intermittently. Watch the output level on a scope and adjust for no changes between the loaded and the unloaded situation.

The bias adjustment and the null adjustment are reasonably orthogonal, i.e. do not influence each other much. But better check repeatedly after making changes in one of the settings.

Output offset should be near zero; both outputs have a servo.

Steven
 
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