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why Tantalum caps?

Lavane

Lavane

Lunatic Member
Why are Tantalum caps used in some equipment instead of plain old aluminum bi-radial caps? I'm getting ready to recap an old preamp and noticed a handful of Tantalum caps sprinkled across the preamp board. I know this could be a really dumb question, but I need to know why? :scratch2:
 
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Tantalum are usually smaller for a given capacitance & voltage rating. The big advantage is usually the much lower ESR of tantalums. This is an advantage in power supply filtering. I would be dismayed to see one in the audio path, but for power supply filtering, they are better performers than aluminum electrolytics.
Of course the low ESR can actually be a disadvantage. When they first came into widespread use, a lot of them would spontaneously explode into flame. Quite impressive. Part of the cause was that designers that were not familiar with them neglected to account for the tremendous inrush current into them if the source impedance was very low. The inrush could cause localized heating that caused a them to short. Then all heck broke loose. Manufacturers and designers eventually worked through these issues and now they are as reliable (or more so) than electrolytics. But they got a bad name and it sticks with them to this day.
Some circuits rely on the low ESR, so changing to aluminum electrolytic is not a good idea. But this does not include anything you will find in vintage audio equipment. Switcher based power supplies are a different story.

Terry
 
Thank you Terry!! Great info. I'm recapping my Marantz model 33 preamp and while making up a cap list I saw these on the preamp board.
 
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Usually, if I am recapping, I will leave the tantalums alone. They don't exhibit the 'finite lifetime' issue that the electrolytics have. But many others would tell you that they have no place in audio equipment, and I don't mean to argue with them. Depending on where they are used in the circuit, there may be improvements if you change them. For me, as long as they are power supply decoupling (from a power supply rail to ground) they are fine (in fact prefered). That is because their job in this position is to basically attenuate (remove) any AC signal (noise, ripple or even audio signals) and they do this quite well. But anywhere in the signal path where an an audio signal is present across them, then they will introduce distortion. I doubt that you will find them used in such a position in any reasonably well designed equipment.

Terry
 
Thanks, I'll look at the boards closer. There are two tone boards, left and right channel. I'll look at the schematic closer to see if they are in the signal path.
 
But aren't there good Low ESR Al electrolytics now, an outgrowth of the computer industry's needs? Or is that low ESR only at high frequency?
 
mech986 said:
But aren't there good Low ESR Al electrolytics now, an outgrowth of the computer industry's needs? Or is that low ESR only at high frequency?
Click to expand...

A good remark: low ESR for what... and referring to switch mode power supplies, those are indeed switching frequencies around and above 50 KHz, and for the purpose to be able to deal with relatively high ripple currents (current-ESR is proportional to heat development) combined with very small (too small) capacitor size (because everyting must be smaller and thinner these days). This kind of caps have nothing to do with 50/60Hz transformer frequencies, nor audio signal path. They measure pretty high in ESR in the first kHz !
 
Just opinion, to be taken with the usual grain of salt. As modern aluminum electrolytics have become smaller and have lower dissipation factor (I never talk about esr unless talking about switchers), the need for tantalums has shrunk.

Looking at the positive side, tantalum caps have moderately low dissipation factor, but it's also very consistent. Low, but not so low as to cause ringing. A designer knows that every unit will operate within narrow specs, even if the vendors are different. Tantalum caps last near to forever; when recapping you'd never change them for new tantalums unless there were a problem or history of problems for that unit/type.

On the negative side, the tendency for short circuit failures is well documented. IMO, tantalums of any vintage have no place in non-current limited locations. If a short circuit will cause high current consumption and start a fire or char the PCB, it's a bad design.

I'm lucky enough to have a vector impedance meter and can look at caps as high as 110 MHz. As far as I can tell, the HF benefits of tantalum caps are mostly myth. Lead length is the killer up there and any cap with leads is going to perform poorly. SMT is the way to go. At more reasonable frequencies I haven't seen a place where a modern low dissipation factor aluminum cap won't replace a tantalum just fine. Maybe they exist, as National Instruments was always very specific about using a small tantalum or large aluminum electrolytic to bypass their regulators, but I can't confirm it.

If you look at the residual signal across a tantalum cap, it's pretty awful. They make horrible integrators and you'd never put them in the signal path, series, shunted or any other way. Even where the audio signal across the cap is very small, say in a coupling cap, I wouldn't use one.
 
They usually get replaced by Nichicon KL series when I do servicing. I have never heard a sonic deficiency or anything by replacing them this way. I worry about the tants shorting and causing noise or more severe problems. Lower values get film caps.
 
Boy do I feel like a dummy ....
Just did a vintage SS receiver total recap and replaced the 3 tantalum caps in the tuner board with 3 new ones.
They were Vishay, and they were comparatively expensive.
Values were in the pf.
If they are so bad, why do they still make them?
 
They aren't bad. In fact they are better than electrolytics in many important ways. Just not in ways that are important in audio circuits. The main advantage is the smaller size and ESR compared to electrolytics. But now low ESR electrolytics are available, so unless size is a big concern..... They are still probably more reliable (long term) than electrolytics since there is no electrolyte to dry up. As ceramics are becoming available in larger and larger capacitance values, tants are disappearing from new designs.
They are still very commonly used in designs where small size is important enough to justify their higher cost.

Terry
 
Here is an example from the digikey catalog:

10uF at 25V surface mount:
Tant: 0.45 Ohms 3.5 x 2.8 x 2.11 mm, $.329 ea
Electrolytic: 0.85 Ohms 4 x 4 x 5.8 mm, $.124 ea

Tants are available down to 100mOhm ESR in that value / voltage. Electrolytics down to only 850mOhm. Guess which one is used in your cell phone or tablet?
 
When I was recapping my CDP302 back in 1985, there were 10 microfarad electrolytics decoupling all the op amps in the audio section (the reconstruction filter). Replaced them all with tantalums, mainly for the reliability. However, before I replaced them there was an edge to the sound. Afterwards that edge disappeared and the sound was much cleaner. Just goes to show that even though the power supplies were regulated, that extra bit of low impedance due to the caps, particularly at the higher frequencies, made all the difference. Needless to say, I had already replaced the mylar caps in the filter with polystyrene and that made a world of difference. And those caps are still OK after almost 30 years.
 
Tantalum capacitors ARE electrolytic. Conventional electrolytic capacitors are aluminium.
So you can't speak of tantalum or electrolytic. It's tantalum or aluminium.
(Aluminium with two i's, please).
 
steerpike2 said:
Tantalum capacitors ARE electrolytic. Conventional electrolytic capacitors are aluminium.
So you can't speak of tantalum or electrolytic. It's tantalum or aluminium.
(Aluminium with two i's, please).
Click to expand...

mmm....
Digikey sells Aluminum Electrolytics, but Mouser sell Aluminium Electrolytics. I wonder which sound better in audio equipment?
I think I may have actually mixed both types in a single piece of equipment. Is this OK?
:D
Terry
 
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