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Old 23rd June 2006, 10:30 AM   #1
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Default amplifier bias drifting

Hi,

i have been testinga new power amp i got yesterday, the fostex 600.
It all works fine and i was rebiasing it when i came accross a possible problem, the bias seems to drift.
Its supposed to be between 5mV and 10mV accross 0.47ohms.
It starts at less than a millivolt then creeps up at a constant rate up until 8mV then i turned the amp off thinking somthings wrong.
It does this on both channels from power up over the course of a few minutes to reach 8mV.

I have posted this in the fostex 300 thread but i dont think many people read it.

Heres a link to the schematic post:

Comments on Fostex Lab 600 Amp circuit?


whats going on? is it normal?

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Craig
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Old 23rd June 2006, 11:22 AM   #2
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If it stabilises at some point between 1-10 minutes idle then I wouldn't worry about it, it's just the heatsinks etc coming into equilibrium, and not quite optimal bias implementation.
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Old 23rd June 2006, 11:38 AM   #3
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Thanks for reply, ill see if it does, ill keep the bias below 10mV as it goes and hopefully i can stabilise it within the limits.

I really dont want this amplifier to explode on me, leaving it on with this possible bias problem will not cause any harm will it?


Thanks
Craig
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Old 23rd June 2006, 11:53 AM   #4
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aha, good suggestion thanks! the bias has stopped drifting!
I kept it at less than 1mV per channel whilest it stabilized.

My only other concern with this amplifier is the heat generated by TR606 and TR605
(the ones in the diagram controlling current through the string of 5 diodes)

They get pretty hot, i probably had the amp on for 5 minutes for the bias stabilization and these transistors on both channels became very hot, i could hold my fingers on them but it was uncomfortable.

is this expected? only 10mA flowing through them at +/-85v rails (diagram says 75 but for some reason mine is 85, i guess the mains is dead on 240V here).

Thanks
Craig
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Old 23rd June 2006, 12:11 PM   #5
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With high voltage rails those transistors will be pretty warm, especially with a relatively high 10mA through them. The finger test indicates their heat is acceptable, but small clip on heatink wouldn't hurt.
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Old 23rd June 2006, 12:23 PM   #6
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Thanks again,

They do have clip on heatsinks, i might just look into obtaining some larger ones for them to be on the safe side.
I figured out why my rails are high.
Its because the rails of 75V DC are obtained by running the amplifier off 220V AC, the amplifier is specified between 220 and 240v AC for operation.
I am on 240V from the socket but have been mostly testing at 220 using a variac so when i plug this thing straight in its rails are gonna be +/-85V which is gonna make things even hotter.

I think im going to run this amp from a variac permanently


edit:
Teh data sheet for these transistors specifies Max VCE of 150V so exceeding +/-75V would be putting the transistors under serious risk of exploding


Thanks
Craig
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Old 23rd June 2006, 12:30 PM   #7
Jennice is offline Jennice  Denmark
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They'll only suffer if the voltage across them exceeds 150V.

I don't have the schematic, so I can't comment on your amp, but are the transistors in question really exposed to full rail/rail voltage?

Jennice
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Old 23rd June 2006, 12:35 PM   #8
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Jennice, im still learning electronics (uni hasnt even touched on transistors yet) but at turn on they might be exposed to the full supply temporarily?

The schematic is linked in my first post.

What do you think?

Thanks
Craig
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Old 23rd June 2006, 01:48 PM   #9
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I would personally not run them with more than 75V rails, but due to the swings they will see and the emitter resistors etc they will likely survive with the higher rails.

Does the amp not have a switch on the back for 220/240?
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Old 23rd June 2006, 01:49 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
I think I understand how your circuit works.

You have a voltage (across r612//r614) feeding the base emitter of tr5 and this controls the current through tr6 and the variable resistor as well as the diode string.

There appears to be no constant current control on the voltage feeding the base nor temperature compensation on either the r612 voltage nor the diode+variable resistor.

Are the diodes in contact with some other semiconductor or heatsink?

The current through the VAS (tr6) and the current through R612//r614 both vary with rail voltage.
Increase your rails and the currents increase. So a 13% voltage increase will give rise to a 26% dissipation increase.

With this style of uncompensated VAS current any rail voltage variations will feed through into the output signal causing much increased distortion.

You have a three stage darlington output arrangement. With 3Vbe drops the stage bias is very susceptable to small changes in temperature of the devices. Combined with the variable and possibly uncompensated bias voltage coming from the diodes, then variations in Iq are VERY likely as the amp warms up.

The VAS and the Current source will both feel about 90% of rail to rail voltage when a high level signal at or near clipping is passed through the amp. So your 150V transistors will feel about 0.9*170V across them.

Have a look at the Tc derating for these transistors and at the SOAR at about 85Vce.
Apply both factors to your transistors and compare your permissible to the actual quiescent current.

Without checking the design, I suspect your transistors are in real stress when the rails are above +-75V and may be near their limit even at +-75V.
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