No ESR caps in audio circuits?

[Doomguy42]

Hi,
I've work on some boards with a 2 channel Audio amp IC on the board.
There's big 2200uf caps after the amp just before the output pins. I pulled 2 off for performance testing and noticed there was no ESR measurement. Just wondering what the use of no ESR caps here?

Also in these style of audio circuits (game, arcade etc). What is the larger audio caps just before the output pins doing? I've found and repaired faults in this area but can't fully understand it yet.

In the pic this one should have been 470uf (same purpose cap but a different board). The audio was very very very quiet?

 

[Nigel Goodwin]

There's no such thing as 'no ESR' electrolytics - the one you have in the picture is seriously faulty, and reads so low for capacitance that ESR would be pointless, hence it doesn't offer a reading.

Sounds like they are speaker coupling capacitors, used back when single supply rails were the norm - and really makes little difference. A single supply amp uses two capacitors, one for the PSU, one for the speaker. A dual supply amp uses two capacitors for the supply. In either case the audio travels through all the capacitors.

 

[unclejed613]

those caps are so dried out that they no longer work.
good 2200uF caps should read between 1800 and 2600uF ,and close to 0 ohms on an ESR test unless the cap is drying out... anything over 2 ohms would be bad... the caps isolate the speaker voice coils from the DC present at the amplifier output.

 

[Nigel Goodwin]

those caps are so dried out that they no longer work.
good 2200uF caps should read between 1800 and 2600uF ,and close to 0 ohms on an ESR test unless the cap is drying out... anything over 2 ohms would be bad... the caps isolate the speaker voice coils from the DC present at the amplifier output.

Anything anywhere near 2 ohms would be a complete disaster - a 2200uF should have EXTREMELY low ESR - 0.1 ohm would probably be the upper limit.

 

[unclejed613]

thanks... i seem to have missed a decimal place somewhere....

 

[DerStrom8]

Nobody has mentioned that cheap multipurpose meters like those are often very unreliable anyway. Even if the capacitor is perfectly good, the meter may not display it properly.

 

[Nigel Goodwin]

Nobody has mentioned that cheap multipurpose meters like those are often very unreliable anyway. Even if the capacitor is perfectly good, the meter may not display it properly.

I've never heard any such suggestion? - I don't imagine accuracy is wonderful, but there's no need for it to be. They use perfectly standard techniques for the tests, so no problem.

 

[DerStrom8]

I have seen several reviews of these types of testers (EEVblog, for example) and they often mis-identify the component, or at least do not know how to measure it correctly.

 

[Nigel Goodwin]

I have seen several reviews of these types of testers (EEVblog, for example) and they often mis-identify the component, or at least do not know how to measure it correctly.

The only misidentification I've come across is with relabelled counterfeit parts - with supposed LM35's testing as small fets and small bipolar

I wouldn't normally try and test an LM35, but as NONE of them worked I thought I'd just see what they might be - I later tested some 'proper' ones (from RS) and obviously got no sensible results.

 

[gophert]

I have seen several reviews of these types of testers (EEVblog, for example) and they often mis-identify the component, or at least do not know how to measure it correctly.

@derstrom,
You haven't read the memo?
If Nigel has never seen it, there is no way your suggestion is possible. And stop using references - Nigel hasn't seen those either.