I accquired a load of cheap tantalum caps.

I wanted to use them in decoupling the DC from an audio signal.



Some websites are very critical of tantalums in this application but I get the feeling the websites may be a little out of date.

Im just after opinions here?
Is it a good idea? If so what should I watch out for when using tantalum caps in audio decoupling?

__________________
The first rule of mstechca club is do not talk about mstechca club.
http://www.electro-tech-online.com/s...induser&u=1245

 

Decoupling the DC from an audio signal???

DC is decoupled from a power supply. Then any capacitor type can be used.
Audio has coupling capacitors.

Metalized plastic film capacitors have the lowest distortion.
Tantalum and electrolytic coupling capacitors change value with voltage so the low frequencies will have even-harmonics distortion.
If the signal across a tantalum capacitor causes reversed polarity then the capacitor is a rectifier producing severe distortion.

__________________
Uncle $crooge

 

i don't understand that...
capacitance in an electrolytic varies according to voltage?!

also.. do you know if tantalums can be used for decoupling where
high ripple current exists? i'm confused about that because i have read
that they can more easily destroyed than electrolytics.

 

Go to Google and look at Distortion From Capacitors.
People at www.diyaudio.com say that distorting ceramic, electrolytic and tantalum capacitors have no business in an audio chain.

Tantalum capacitors are small so i think their ripple current rating is low.
I have had many tantalum capacitors blow up and disappear so I don't use them anymore.

__________________
Uncle $crooge

 

Hi audioguru, thanks alot for the reply.

Sorry for my bad semantics. I did mean coupling.
(to remove the DC from an audio signal)


I did notice the difference between two circuits I made, (changing 3 ceramic 4.7uf for 1 22uF electrolytic made the sound seem very loose.. arg loose is a bad word.. I hope you get what I mean)

Would that make sense or did I just make the whole thing up?


I found metal film caps (polyester?) but they are such low values, Im using around 20uF right now to decouple audio signals (maximum 2v peak to peak)

Does that sound appropriate?
If so, are there metal film caps available in the size I require? I cant find any.


Cheers
John

__________________
The first rule of mstechca club is do not talk about mstechca club.
http://www.electro-tech-online.com/s...induser&u=1245

 

I have some small 1uF 5% metalized plastic film capacitors. Larger size (really big) and higher value ones are used in good speakers' crossover networks.

If your audio circuit uses fairly high impedances then small metalized plastic film 0.33uf capacitors can be used as coupling capacitors. If the impedance is 120k ohms then a 0.33 coupling capacitor will have a flat response down to 20Hz.

__________________
Uncle $crooge

 

Surely the amount of distortion depends on the current flowing through the capacitor. A JFET amplifier with a 100nF ceramic input capacitor and an input impedance of 1M probably won't suffer from much distortion.

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

Electrolytics as AC coupling capacitors are fine. Just make sure that the DC bias across them is enough. Using a tantalum as a coupling capacitor, that I have never tried - no real reason to when a cheap aluminum electrolytic does the job just fine @ audio frequencies. Keep the f3 well below 10hz - if you don't you'll wind up introducing distortion into the low frequencies. 10uF-100uF is plenty.

Do NOT ever allow the tantalum to see reverse bias in this application. An aluminum electrolytic would handle it OK,but a tantalum might go poof.

Honestly the best place to use these depending on voltage and size is as power supply decoupling capacitors and in voltage regulator circuits.

 

Thanks again for all the information..

I will relegate my tantalums to power supply decoupling etc only.



Prompted by what you guys said, I set up a blind test with some decent headphones and I can definately tell the difference between two circuits I have here. (identical design, but one has bipolar electrolytics and one has multilayer ceramics).

In comparison to the circuit with ceramic, the electrolytic one sounds a bit like its been recorded in a seashell.


I think ive already decided metal film caps are too expensive in the values I need (the price for each component would cost pounds rather than pennies)

But Im really curious if going to metalised film would give a similar step up in quality over the ceramics. If thats even possible.

__________________
The first rule of mstechca club is do not talk about mstechca club.
http://www.electro-tech-online.com/s...induser&u=1245

 

Could you pleas post the test circuit.

Have you tried oversizing the electrolytic capacitor?

The chances are, if it's just a simple push-pull (on-sided not bridged) class A-B output stage then an electrolytic will do since there's no chance of it becomming reversed biased.

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

From what people have posted above, it seems like Im oversizing my components already. Im using 47uF where 2uF would suffice.


Also, since I used bipolar capacitors for the electrolytics, reverse biased wont be an issue here? Or am I missing something?


Ill draw up the schematic for the test circuit and post it up as soon as I can.

John

__________________
The first rule of mstechca club is do not talk about mstechca club.
http://www.electro-tech-online.com/s...induser&u=1245

 

OK heres the test circuit. (showing just 1 channel)


C1 and C2 are the caps im talking about.

The POT is 50K

Attached Thumbnails

 

__________________
The first rule of mstechca club is do not talk about mstechca club.
http://www.electro-tech-online.com/s...induser&u=1245

 

Many analog switch ICs have a pretty high distortion when they use only 5V for a supply voltage. A CD4016 is not even spec'd with a supply voltage as low as only 5V.
A CD4066 has much lower distortion than a CD4016 but it is also not spec'd when its supply is as low as only 5V.

Your C1 feeds 33.8k if the input impedance of the amplifier is high. Then a 1.2uF capacitor will have a response that is flat down to 20Hz.

__________________
Uncle $crooge

 

What are you using as an audio source?

I think that there will probably be more sources of distortion in the audio source and in the analogue switch than the electrolytic capacitor.

I think your reason for preferring the ceramic to the electrolytic is purely physiological. Somewhere you heard that electrolytic are worst than ceramics and believed it to be true so it's no surprise your experiment backs up this theory. Constructing an experiment to prove what you think is true probably won't prove it otherwise, especially if the testing is purely subjective as it is in this case.

I would suggest a blind test, try encasing both of the capacitors in film cases or something that will make them both look the same. Test them both randomly, decide which is the best, then reveil which is the electrolytic and which is the ceramic. You might be very surprised.

__________________

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

One article measured the distortion from coupling capacitors. Ceramic capacitors that were taped in a row by the manufacturer had a range of distortion that was 10 times.

Rod Elliot figures that distortion from "bad" capacitors is lower than 100dB below the music level. So low that the distortion cannot be heard.
http://sound.westhost.com/articles/capacitors.htm

__________________
Uncle $crooge