Tantalum decoupling caps

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Or put a new low esr electrolytic?
The importance of low ESR/ESL caps in analog circuits can be debated.
Used in coupling, there is no doubt that ESR/ESL is utterly negligible compared to the impedance of the circuit they are used in.
For example, a stage with an input impeance of 10kohms should use a coupling cap of at least 8uF (for less than 0.5dB attenuation at 20 Hz), but due to their higher non-linearity, electrolytics should be at least 5 times larger. So a 47uF should be used. The typical ESR of a standard lytic 47uF cap is less than 1 ohm (typically about 0.2). It is clear that apart from the marginal attenuation (less than 1milli dB) there is no effect.
Where it makes sense to discuss the subject is decoupling. Because of the impedance of the supply rails, the decoupling caps have the task of maintaining the correct voltages in case of demand.
It is important to distinguish between sudden demands, created by transients, and more steady demand created by high amplitude LF signals.
In order to react quickly to a sudden demand, the supply must have a very low series impedance, that's why it takes low-ESR caps, such as ceramic and short lines, and also why the ceramic caps must be placed close to the stage they are decoupling. In addition, the return current from this stage should be returned as close as possible to the reference point ("ground") of these decoupling caps.
For example, if this stage drives an external load, this load should be returned to the decoupling caps reference point, rather to a convenient (but inadequate) star point.
Regarding low frequencies, the length of supply lines/traces does not matter that much, so it's quite common to have only one point where lytic caps decouple rails. Due to the much different demand (continuous LF) these decoupling caps should be dimensioned according to the output requirements. When a pair of 47uF caps is enough for a circuit that drives a 10kohm input, much larger values are required for driving a 600 ohm transformer. The same rule applies, when driving an external load, it should be returned to the reference point of the decoupling caps, which may not be exactly the stage's reference point.
Back to the point: what about ESR/ESL?
The actual impedance of the rails reflects as how much the supply rail drops when submitted to a current demand.
The consequence is how much this voltage drop reflects on the output signal?
The answer is PSRR (Power Supply Rejection Ratio). When opamps have a typical PSRR of 100dB - meaning that a voltage drop of 1V of the supply rail results in only 10uV at the output - , a basic common-emitter stage has zero PSRR, so using a low-ESR cap makes perfect sense.
The situation with opamps (whether monolithic or discrete) is a little more complex, because the wonderful 100dB PSRR is valid only a LF, falling down to about 50dB at 20kHz.
That summarizes the need for large capacitors accompanied with their inevitable ESR, complemented by local low-ESR caps.

Is there a significant benefit using low-ESR caps for the larger ones? Some think there is, which makes them use large film capacitors for this task.
I have not seen a proof of increased performance resulting from this choice, however there are reports of impairment, due to the combination of trace/wire inductance and capacitance which result in a high-Q LC circuit in series with the supply rail and resonating in the audio band.
Member ricardo has a lot to say about this.
Of course, requirements for high-speed logic and RF circuits are different.
 
Thanks everybody for the knowledge. exactly what i was looking for.

On the lexicon Primetime 93 i ended up replacing all tants, none were shorted but i found a few with unexpectedly high esr, so they could have cause trouble in the future!
thanks!
 
Well, in my situation, it's a choice of what will easily fit in a tightly packed PCB (designed in the 1970's).

In a perfect world, I would use a film cap as a replacement. But a 25 uF @ 25V film cap would be half the size of that PC board! <g>

Bri
I've had good results with the following:

If bypassing a regulated rail next to audio op amps then smaller value film caps would work. I think you can fit 4.7 ufd/50 v from Wima or others. You have better HF ESR and they will probably (disclaimer) never fail. Depends on the load on the op amp(s).

If there is an R-C decoupling scheme you want a larger 'lytic bypassed if desired w/ film. Large load-bigger cap value if not regulated.

Tants are unreliable compared to film and AL lytics.
 
The situation with opamps (whether monolithic or discrete) is a little more complex, because the wonderful 100dB PSRR is valid only a LF, falling down to about 50dB at 20kHz.
That summarizes the need for large capacitors accompanied with their inevitable ESR, complemented by local low-ESR caps.
This assumes R-C decoupling. If the opamps are fed from a stiff low Z regulator nearby then smaller low ESR caps are fine, if film caps don't cause oscillation. If so a few ohms in series restores stability.

I don't like to use ceramic for audio as they are microphonic. One can argue that as bypass caps it doesn't matter but even with bypass everything counts.
 
I had to replace the coupling ones on my 70s Revox because nothing was working anymore, the old ones had too low working voltages
 
Yes, it was a very common failure at the time. First time I saw it I wouldn't believe that the residual capacitance was so small (a few pF).

Can't 'compete' with that and it's not audio...but...I just replaced a couple of 1000uF caps in the psu section of a Samsung Monitor - about 16 years old.
They read around 30u and 50u. The monitor had gradually taken longer and longer to turn on properly and not take ages to start then flash up the screen intermittently for ages. Seems all good now and I have a spare bit of shielding from where I couldn't work out where it fitted. Note to self : Take photos of the mechanical details :rolleyes:
 
Can't 'compete' with that and it's not audio...but...I just replaced a couple of 1000uF caps in the psu section of a Samsung Monitor - about 16 years old.
They read around 30u and 50u. The monitor had gradually taken longer and longer to turn on properly and not take ages to start then flash up the screen intermittently for ages. Seems all good now and I have a spare bit of shielding from where I couldn't work out where it fitted. Note to self : Take photos of the mechanical details :rolleyes:
Note: this anecdote has nothing to do with tantalum. I've shared this before. While working at Peavey last century we encountered an entire batch of 1,000 uF aluminum electrolytic caps with a weak swage connection on one lead. Most of these capacitors measured full capacitance until the weak swage connection broke, only a few in an incoming inspection sample measured tens of nF. A benefit of me spending time in the factory is that the person doing the incoming inspection called me asking if the entire batch was OK because only a few were low capacitance. Sampling theory allows for a tiny percentage to be out of tolerance, but these were not just low capacitance but effectively open circuit.

I said hell no. I dropped everything, drove out to the factory, and proceeded to take a few of the dodgy caps apart. It was immediately obvious that the swage connecting one wire lead to the capacitor foil was faulty. I contacted the capacitor manufacturer and they discovered that their factory swage tooling was broken.

We recalled that entire batch of capacitors, and swapped them out from finished goods, including some FG that were in a container headed for an ocean trip. Luckily we caught this early but there were several thousands of those caps in the one faulty batch. 1,000 uF is a common value used in many SKUs. The capacitor manufacturer valued our business and made us whole. Lucky for them and us we caught this before the faulty caps resulted in field failures. The insidious thing about this failure was that they measured good, until they didn't, like little time bombs.

JR
 
Don't suppose it would be too ethical (?) to mention who they were..?
if I remembered.... 🤔

I do remember it was a major US based cap manufacturer. I think we were one of their larger customers.

They treated us more than fairly, and production tooling breakage like that falls under sh__ happens.

JR
 
Point of interest to any Hammond tone wheel organ owners or repairers - also totally unrelated to tantalum capacitors - I’ve seen many expensive quotes for replacing the motor that drives the tone wheels, when in fact it’s the run/start capacitor that is failing which costs less than ten bucks. To check all you need to do is twirl the motor shaft with the power on and if it runs then it’s the capacitor. Owners tend to not refill the oil bath and it makes the bearings dry out and it becomes hard to start. The old style paper capacitors dry out.
 
There’s another type of capacitor type that is prone to failure over time and that’s the old “liquorice allsorts”, “candy stripe” or “tropical fish” capacitors made by Mullard (C280) or Philips back in the ‘60s and ‘70s. Over time the ends delaminate from the body and because they are soldered in place the separation is not readily apparent but if you have any noisy, crackly old gear with these in place then they are likely failing. The ends will usually fall off dodgy ones when you desolder them. In circuit there will be very fine hairline cracks at the end-posts. I have done a lot of Memory Moogs which use these as a coupling cap to or from the filter ladders on the voice cards and you get wild tuning drift or crackling audio. I’m not sure what the body coating is made of - it looks like some sort of baked clay or ceramic around these polyester film caps and it has probably shrunk and cracked, the only thing holding the ends on being the painted exterior and the soldered to the board legs.
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Imagine my surprise opening several Dolby pro products with tantalums in the signal path. Weren’t they known for generating distortion? Then I read (someone remind me who) that a C large enough to impart no voltage drop at the lowest frequency passed cannot add distortion. So I design for an fc of 1.59Hz and measure negligible THD from 15.9Hz up.
 
Imagine my surprise opening several Dolby pro products with tantalums in the signal path. Weren’t they known for generating distortion?
A number of reverred vintage products use Ta caps. Aficionados swear it's what makes their sonic signature.
Then I read (someone remind me who) that a C large enough to impart no voltage drop at the lowest frequency passed cannot add distortion.
Actually, "no voltage drop at the lowest frequency" is not possible, but yes, making capacitance large enough to reduce significantly the voltage across the cap is a way of decreasing distortion.
Since the voltage decreases linearly with capacitance, distortion halves for every doubling of capacitance.
Wet tantalum caps generally have higher distortion than Al caps, so their value should be scaled accordingly. The main advantage of Ta caps is their smaller size, but considering they have to be sclaed up, there is no significant advantage, and their cost makes it a poor choice.
Solid Ta caps have lower distortion but are $$$.
So I design for an fc of 1.59Hz
Typically, for good LF response @20Hz, fc should be <2Hz, but for good THD with lytic caps, it should be about 0.5Hz.
and measure negligible THD from 15.9Hz up.
Of course, it depends what you consider "negligible".
 
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I am not sure I want to reopen this can of worms but back in the 70s/80s Tantalum caps were criticized for DA (dielectric absorption) by the audiophile press.

The old (good) advice to minimized terminal voltage is addressing voltage coefficient, or how much the capacitance changes wrt terminal voltage. Quality tantalum caps are probably no worse than aluminum electrolytic in this regard.

DA is a somewhat different mechanism and argued by some (like me) as insignificant for audio DC blocking applications.

JR
 
Imagine my surprise opening several Dolby pro products with tantalums in the signal path. Weren’t they known for generating distortion? Then I read (someone remind me who) that a C large enough to impart no voltage drop at the lowest frequency passed cannot add distortion. So I design for an fc of 1.59Hz and measure negligible THD from 15.9Hz up.
As I recall, Rubert Neve actually preferred the sound (euphonic distortion?) of tants in the signal path.
 
As I recall, Rubert Neve actually preferred the sound (euphonic distortion?) of tants in the signal path.
well there you go... 🤔

The only time I used tantalum on purpose was in an audio decoder side chain time constant circuit (where DA could matter), to hopefully match the encoder that used tantalum to set their time constant.

JR
 
well there you go... 🤔

The only time I used tantalum on purpose was in an audio decoder side chain time constant circuit (where DA could matter), to hopefully match the encoder that used tantalum to set their time constant.

JR
Of course he was more about 'color' than 'straight wire with gain' ; - )
 

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