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Old 31st January 2010, 11:39 PM   #1
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Default Why is there 5Vdc on my outputs?

Hi All,

I have just built my first generic mono 3 stage amp on a couple of bread boards and was wondering why I am getting approx 5Vdc on my outputs (measured with respect to mains cap ground). A quick description of the amp:
Input: differential pair (bc377's) with simple current source and resistors on the tail.
Vas: Just one TIP42C with 33p Cdom, standard Vbe and current source.
Output: driver & output emitter followers(TIP41C's & TIP42C's).
VCC: +-24.5V
The main thing I don't understand is how the DC conditions are analysed on the outputs and Vas. I understand that adjusting my Vbe from 1-3V sets the quiesant current and I have seen the voltage accross my emitter resistors when I do this indicating that at least that part is working. But why would I still have 5Vdc on the outputs? Am I supposed to be setting something somewhere else for DC offset? Can maybe someone give me a link to the theory I'm talking about? There is probably something quite trival or perhaps fundmental that I am missing here......

Oh well at least I'm not getting smoke!
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Old 2nd February 2010, 05:54 AM   #2
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OK I'm guessing no replies means the problem is not immediately obvious.

I will put the schematic up in a few days with measured voltages to see if that helps!
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Old 2nd February 2010, 06:14 AM   #3
Enzo is offline Enzo  United States
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Yes, we are more likely to pitch in if there is a circuit to look at.

If there is a steady DC offset at the output, then it is likely being driven there. Does it matter if there is a load?
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Old 2nd February 2010, 07:14 AM   #4
Mooly is offline Mooly  United Kingdom
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It may be oscillating at HF... have you added a zobel network at the output ?

The tail current for the LTP may be wildly inapropriate giving this effect.
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Old 2nd February 2010, 11:17 AM   #5
AndrewT is offline AndrewT  Scotland
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have you shorted the signal input to the signal ground?
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Old 2nd February 2010, 08:20 PM   #6
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Well the obvious thing is that one rail is conducting more than the other... thus DC offset.

There must be a means of adjusting or setting the current through the output stage, not just the bias, for each rail - of course one can assume that the negative feedback is going to try set the centerpoint... so maybe you should check the *wiring*? Most problems with a circuit that is known to work (is it known to work) are due to wiring errors, including miswires, and components that look like the right one, but put in the wrong place...

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Old 2nd February 2010, 08:29 PM   #7
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There is often a preset for setting the output dc offset.
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Old 3rd February 2010, 01:53 AM   #8
brainf is offline brainf  Philippines
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Not to hijack this thread but I'm experiencing the exact same symptoms on a kit off eBay and so I'll be very interested in the advise offered here. (Mine has a 5vdc offset ONLY when a dummy load is connected).

cheers!
leslie
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Old 3rd February 2010, 07:00 AM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by brainf View Post
Not to hijack this thread but I'm experiencing the exact same symptoms on a kit off eBay and so I'll be very interested in the advise offered here. (Mine has a 5vdc offset ONLY when a dummy load is connected).

cheers!
leslie
Maybe start a new thread and post a circuit
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