Why is there 5Vdc on my outputs?

BunyipElectronics · 31st January 2010, 11:39 PM

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!

BunyipElectronics · 2nd February 2010, 05:54 AM

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!

Enzo · 2nd February 2010, 06:14 AM

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?

Mooly · 2nd February 2010, 07:14 AM

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.

AndrewT · 2nd February 2010, 11:17 AM

have you shorted the signal input to the signal ground?

bear · 2nd February 2010, 08:20 PM

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...

_-_-bear

nigelwright7557 · 2nd February 2010, 08:29 PM

There is often a preset for setting the output dc offset.

brainf · 3rd February 2010, 01:53 AM

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

Mooly · 3rd February 2010, 07:00 AM

Quote:

Originally Posted by brainf

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

31st January 2010, 11:39 PM	#1
BunyipElectronics diyAudio Member Join Date: Nov 2009	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!

2nd February 2010, 08:20 PM	#6
bear expert in tautology diyAudio Member Join Date: Apr 2002 Location: New York State USA	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... _-_-bear __________________ _-_-bear http://www.bearlabs.com [...2SJ74 Toshiba bogus asian parts - beware! ] -- Btw, I don't actually know anything, FYI --

2nd February 2010, 08:29 PM	#7
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	There is often a preset for setting the output dc offset. __________________ http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software.

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2nd February 2010, 05:54 AM	#2
BunyipElectronics diyAudio Member Join Date: Nov 2009	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!

2nd February 2010, 06:14 AM	#3
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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?

2nd February 2010, 07:14 AM	#4
Mooly diyAudio Moderator Join Date: Sep 2007	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.

2nd February 2010, 11:17 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	have you shorted the signal input to the signal ground?

3rd February 2010, 01:53 AM	#8
brainf diyAudio Member Join Date: Feb 2007 Location: Manila, Philippines	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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