Dumb question. What is DC offset in an amp?

[lewiswhitebass]

Had my EBS Fafner serviced recently and the tech said there was 1.5v DC offset present.

 

I have an ok understanding of electronics but it's something I've never really understood.

 

Amp sounds much cleaner now it's below 100mV

 

 

[bremen]

The output of an amp is AC, on average zero - that is, it spends as much time positive as negative.

 

A DC offset is a DC voltage permanently added to the signal, so the amp is sending power to the speaker and heating it up even when there's no input signal, though only about a quarter of a watt with a 1.5V offset. The cone is always going to be pushed out or pulled in (depending on the polarity of the offset) from its resting place.

Edited July 27, 2024 by bremen
clarity

[lewiswhitebass]

53 minutes ago, bremen said:

The output of an amp is AC, on average zero - that is, it spends as much time positive as negative.

 

A DC offset is a DC voltage permanently added to the signal, so the amp is sending power to the speaker and heating it up even when there's no input signal, though only about a quarter of a watt with a 1.5V offset. The cone is always going to be pushed out or pulled in (depending on the polarity of the offset) from its resting place.

Thank you that makes sense.

 

My rig sounds much better now!

 

Shout-out to sickamps.com in Harlow 

[paul_5]

DC offset also reduces headroom, as the wave isn’t centralised at 0v, so instead of a +/- 10v (for example) each side, one would only have 8.5v of wiggle room with a 1.5v DC offset.

[Chienmortbb]

Oh and it is not a dumb question.

[basstone]

A small DC offset will not significantly affect  amp headroom but is not desirable as it will increase the possibility of noise and hum on the output as well as a small increase in heat dissipation and a thump or click when the amp is switched on and off or the speaker connected. Also worrying is the possibility of a leaky capacitor in the feedback circuit which if left will gradually allow the DC offset to increase to the point where the amp and or speaker will be fried through thermal overload - I know from personal experience! Glad to hear it's now fixed and sounding better.

[bremen]

18 minutes ago, basstone said:

Also worrying is the possibility of a leaky capacitor in the feedback circuit which if left will gradually allow the DC offset to increase to the point where the amp and or speaker will be fried through thermal overload - I know from personal experience! Glad to hear it's now fixed and sounding better.

This is definitely worth investigating

[lewiswhitebass]

Thanks for the responses guys. On recent gigs I haven't had any issues with the amp. In regards to a leaky cap hopefully this is the sort of thing a decent amp tech would check during a service. I will enquire with sickamps

[basstone]

8 minutes ago, lewiswhitebass said:

Thanks for the responses guys. On recent gigs I haven't had any issues with the amp. In regards to a leaky cap hopefully this is the sort of thing a decent amp tech would check during a service. I will enquire with sickamps

The cap in question is the dc blocking capacitor in the power amp feedback ensuring that the power amp has unity gain at dc. If this fails any small dc offset in the circuit will be amplified and presented to the output. It will depend on the particular amp design and is not a common failure but worth checking!