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willy 26th September 2003 04:36 PM

DC Offset Question
 
Could someone please explain, or point me to a tutorial on DC offset. Or tell me in simple terms "what it is". I've searched the forum and the net. Lot of articles. But none just come out and says it simple enough for me. I'm not exactly a beginner, I've built a lot of amps. I know and use ohms law but I only have a volt-ohm meter and just use the trial and error method. Is the DC offset the reason for putting a resistor directly to ground on the input? Is there a formula for this resistor? How do I measure DC offset - just put my volt meter on the amp input with no input device connected? Is this DC offset also a problem on the output? I've never blown a speaker, so surely I'm doing something right.

Have read this forum word for word for a year now, and literally having a blast. Thanks so much for all the info.

DrG 26th September 2003 05:09 PM

DC offset is a net, no-signal DC voltage measured at the amplifier output, which should be zero. DC is unhealthy for speakers...

The input resistor is not strictly for determining DCO, although it does establish a zero reference for the input signal. It determines input impedance.

It's possible for a small input-offset (eg due to DCO of source) to become amplified at the output. This is commonly avoided by using a series capacitor at the amp input or between ground and the second feedback resistor. A separate servo for DC feedback can also be used.

willy 27th September 2003 06:38 AM

Does this mean that my volt meter should not register any DC voltage on the output to the speakers? Is that not what the output capacitor is for - to block the DC?

weeghel 27th September 2003 11:01 AM

Quote:

Originally posted by willy
Does this mean that my volt meter should not register any DC voltage on the output to the speakers? Is that not what the output capacitor is for - to block the DC?
Indeed, your voltmeter shouldn't register a DC voltage on the output.

And yes, the output capacitor is also for blocking DC. However, output capacitors are not normally used to block small (<100mV) DC offsets. The whole point of a direct coupled (no output cap) is that you don't have/need a output capacitor.
But some amps (with a single supply, say from 0 to 20V) have the output of the output transistor at half the supply voltage. Output caps are necessary for those amplifiers (else, you would have a 10V dc voltage at the output and no way to get rid of it)

dutch diy 28th September 2003 01:21 AM

DC-offset
 
To add to earlier posts. It's virtually impossible to achieve exactly 0V DC offset over the entire temp / power range of an output stage without capacitor.
A DC offset within a range of +/- 50 mV is acceptable, lesser values are preferred.

EG: My DIY JLH amp changes from +40mV (after an initial peak of 150mV) at startup to somewhat in the region of +/- 3mV over an hour warming up depending on ambiant temp.

willy 30th September 2003 05:42 AM

Thanks so much members for those simple, easy to understand explanations. One of the things that was throwing me off was that all the amps I have built so far have single supplies. So I''ve always got about half the supply voltage on the output.

Shaun 30th September 2003 07:00 AM

Willy,

One drawback of using that topology is that the output cap has to be very large if you want your amp to work down to very low frequencies. Direct-coupled designs generally don't have this problem. It is also best to avoid having the signal pass through a capacitor if possible, to avoid introduction of capacitor-induced distortion.

Most designs nowadays are direct coupled.

Regards

peranders 30th September 2003 07:33 AM

Shaun, you are a bit unclear. If you have an amp with fed only with single supply, you must have a capacitor at the output, (unless it is bridge connected) and if you have dual supply as most amps have you never use an output cap.

Shaun 30th September 2003 08:09 AM

Yes, of course. Sorry about that. Some designs need the output cap.


Quote:

DrG:
The input resistor is not strictly for determining DCO, although it does establish a zero reference for the input signal. It determines input impedance.

I always believed that the main purpose of this resistor was to establish DC refence to ground, and that the input impedance was a consequence of the resistor value.

DrG 30th September 2003 08:49 PM

chicken, egg... egg, chicken... whatever bends your boerewors, Shaun


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