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Old 10th September 2004, 03:45 AM   #1
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Default DC Offset circuit for Threshold

What would be an easy way to adjust DC offset in this circuit?
I know the typical way with most AB amps is a 100k pot at base of inverter, series 300k resistor at base and + & - 15v at each side of pot. Heres the schematic
http://www.munsonandbryan.com/400a.tif[/IMG]

Would this dc offset schematic work in circuit?
http://www.munsonandbryan.com/dc offset.jpg[/IMG] [IMG]
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Old 10th September 2004, 10:41 PM   #2
anatech is offline anatech  Canada
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Hi John,
How much DC offset are you trying to get rid of?
-Chris
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Old 11th September 2004, 04:02 AM   #3
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The offset is 76mv on one channel.
Offset on other channel is about 45mv
This is not acceptable with high efficiency speakers.
I know matching the outputs will help but that could be expensive and time consuming.
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Old 11th September 2004, 03:22 PM   #4
anatech is offline anatech  Canada
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Hi John,
Okay, don't modify the circuit. Match the 6571 pair. The Hfe circuit on small digital meters will work fine for this. This will drop your DC offset to a low value and improve the sound. While you're in there, replace the 470uF 16V cap unless it's new. You can calculate the value of DC offset closely by dividing the tail current by two. Divide the figure by the Hfe of a 6571 transistor. Multiply this by 9400 plus the output DC resistance of the preamp. This unit does not have a DC reference to ground for the input. Not nice! You could hang a resistor from somewhere on the input to true ground (not the RCA jack ground).
If you were to unplug the source from the amp. the DC output voltage will rise from X mV to some undesireable level.
-Chris
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Old 11th September 2004, 03:22 PM   #5
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DJK says to replace 300ohm with 10turn 500ohm.
I experimented with paralleling pot, it took about 280ohm to increase offset by 10mv. If I do the opposite by increasing the resistance, it seems that it would take about 400-500ohms to get 0 offset. Would this not decrease the current to diff pair and change the general performance of the circuit.
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Old 11th September 2004, 03:25 PM   #6
anatech is offline anatech  Canada
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Oh yeah, a bit. Why not correct the problem at the source?
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Old 11th September 2004, 10:25 PM   #7
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The offset is due to the difference in base currents in the 6571 pair. By increasing the 300 ohm resistance, you lower the tail current, and thus lower the base current for both transistors, and offset goes down. This isn't the way to solve the problem, as output drive for this stage decreases with decreased tail current.

Put in a pair of high-gain transistors, matched as closely as you can get them. High-gain NPN's are fairly easy to locate...perhaps a 2SC1775 or a Zetex ZTX694B (I've used the Zetex in this capacity with very good results).

With a tail current of about 2 1/2mA, and no provisions for adjustment, you will be lucky to get offset down below 20mV no matter how close you match transistors. But it is certainly about your only option besides radically changing circuit parameters (not recommended) or adding a servo (not a bad idea, but not as easy as maching transistors).
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Old 11th September 2004, 10:41 PM   #8
anatech is offline anatech  Canada
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As long as the DC offset is below 50mV, who cares? You won't damage the speakers until the offset gets much higher.

Echowars, with matched transistors, I routinely get the DC offset within a few mV of my calculated values for a given circuit design. It really isn't that difficult to do.

John, what is happening that causes you to want to reduce the DC offset? If you get a turn on/off thump, then there are transients that are causing this. You will not get a sound from a steady DC offset across your speakers at these magnitudes. If you plug and unplug your speakers withthe amp running (no sound), you may hear a faint click. That's all this does. Any change in the input impedance of this amp will cause a bigger noise than this offset. Did you calculate the expected DC offset of this amplifier?
-Chris
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Old 12th September 2004, 06:15 AM   #9
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"This unit does not have a DC reference to ground for the input. "

The schematic shows 4K7 + 10K + 390R = 15K09 to ground.

The AC input impedance will be much higher than 15K due to the bootstap action.
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Old 12th September 2004, 08:39 AM   #10
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Quote:
Originally posted by anatech
As long as the DC offset is below 50mV, who cares? You won't damage the speakers until the offset gets much higher.
But you're not receiving the full benefit of the distortion cancelation abilities of the diff pair. We're tweakers, remember? 0.00V offset is da goal.
Quote:
Echowars, with matched transistors, I routinely get the DC offset within a few mV of my calculated values for a given circuit design. It really isn't that difficult to do.
Your meter is better than my meter. I bow to you Sir. (thermal parameters also make this difficult, but U know that)

Getting offset down as low as possible just happens to be a fetish with some of us. Don't necessarily have to have a reason, other than to know that 1mV is better than 100mV.
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