Component Selection Question

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cpsmusic

Well-known member
Joined
Dec 3, 2013
Messages
292
Location
Melbourne, Australia
Hi Folks,

I have (well, I think I have) beginner-intermediate-level electronics knowledge. I've built several guitar pedals and a tube guitar amp (under the watchful eye of my late-Father).

I have a general question - If I'm faced with a schematic that doesn't have a BOM, how do I determine the component characteristics? I'm primarily referring here to things like resistor wattage and also to capacitor voltage, but I'm also unclear about, say, when to use specific types of components such as metal film resistors, or the various types of capacitors.

Is there any online resource or textbook that discusses things like this?

As a simple example, I'm interested in building the Helios69 EQ which requires a make-up gain amp. I thought I'd try the TL072 version. How do I figure out the wattage/voltage ratings for the various components?

Cheers,

Chris
 
I think there's a lot to what you are asking but, this is something that can be helpful somewhere as far as one "online resources" go. The site is pretty neat....

https://www.youtube.com/watch?v=bHV7FCShdic

I'm interested in hearing the suggestions. OHM's Law, Kirchoff's Law....... I still need to get my head around these.....
 
cpsmusic said:
I have a general question - If I'm faced with a schematic that doesn't have a BOM, how do I determine the component characteristics? I'm primarily referring here to things like resistor wattage and also to capacitor voltage, but I'm also unclear about, say, when to use specific types of components such as metal film resistors, or the various types of capacitors.

Is there any online resource or textbook that discusses things like this?

I haven't seen any resource you want, but in general:

¼ W resistors are fine for most things, except in power amplifiers and perhaps power supplies. And metal film are always preferred to old carbon composition resistors. 1% resistors cost the same as 5%, so there's no point in the looser-tolerance parts.

Capacitor voltage rating is determined by operating conditions, and that means the voltage rails. In a ± 15 V circuit, a 35 V electrolytic cap for decoupling a rail to ground is fine. Most ceramic caps are available in 50 V so that's fine too. For small value (picofarad) generally you want a C0G or NP0 dielectric ceramic cap. Some might argue for film caps for best stability. For standard 0.1 uF decoupling the X7R ceramics are fine.  You can buy ceramic caps in values up to over 100 uF, but the largest values have low working voltages. For most things, Panasonic VS and Nichicon aluminum electrolytics are fine. Do not use tantalum caps: they explode when reverse-biased or when subject to working voltages above their ratings.
 
Andy Peters said:
I haven't seen any resource you want, but in general:

¼ W resistors are fine for most things, except in power amplifiers and perhaps power supplies. And metal film are always preferred to old carbon composition resistors. 1% resistors cost the same as 5%, so there's no point in the looser-tolerance parts.

Capacitor voltage rating is determined by operating conditions, and that means the voltage rails. In a ± 15 V circuit, a 35 V electrolytic cap for decoupling a rail to ground is fine. Most ceramic caps are available in 50 V so that's fine too. For small value (picofarad) generally you want a C0G or NP0 dielectric ceramic cap. Some might argue for film caps for best stability. For standard 0.1 uF decoupling the X7R ceramics are fine.  You can buy ceramic caps in values up to over 100 uF, but the largest values have low working voltages. For most things, Panasonic VS and Nichicon aluminum electrolytics are fine. Do not use tantalum caps: they explode when reverse-biased or when subject to working voltages above their ratings.

All good advice except I have to disagree about not using Tantalum caps.
Just don't reverse bias them and use them within their rated voltage  ;D
Then they are fine and can have advantages over Aluminium Electrolytics wrt ESR, ageing and other specs.
Tants seem to have a bad reputation with some people due to problems with them many moons ago. This no longer applies.
They can be a bit pricey as the capacitance increases.
I use SMT TAJ series Tants in scientific and industrial instrumentation. No problems.

 
Newmarket said:
All good advice except I have to disagree about not using Tantalum caps.
Just don't reverse bias them and use them within their rated voltage  ;D
That's exactly what Andy Peters mentioned. It turns out that in DIY audio circuits, the risk is quite frequent, so I think it's a good recommendation to avoid them unless some of their properties make them unavoidable.


Then they are fine and can have advantages over Aluminium Electrolytics wrt ESR, ageing and other specs.
OTOH Al caps have made so much progress the difference is not so big.  The cases where these advantages are significant in audio applications are so rare they may be tagged as exceptional.

Tants seem to have a bad reputation with some people due to problems with them many moons ago. This no longer applies.
Well, the reasons most audio designers rejected tants have not changed; they still need reverse-voltage protection and increased voltage rating compared to Al types.

I use SMT TAJ series Tants in scientific and industrial instrumentation. No problems.
Are you sure you don't have to take extra measures regarding start-up of bipolar supplies? Are you sure you don't have to take necessary steps to prevent total failure due to a cap shorting out?
Not "problems", but still issues.
 
abbey road d enfer said:
That's exactly what Andy Peters mentioned. It turns out that in DIY audio circuits, the risk is quite frequent, so I think it's a good recommendation to avoid them unless some of their properties make them unavoidable.

Yes - I realise that was what he had mentioned - which was why I referred to it  :)
Other components are going to have problems if they have the wrong voltage value / polarity so I don't see any real reason to avoid Tants on this basis. Just remember that they are marked on the positive side  :)
Admittedly they can make an awful smell and mess when they 'go' - look on it as an effective fault indication system  ;)


abbey road d enfer said:
OTOH Al caps have made so much progress the difference is not so big.  The cases where these advantages are significant in audio applications are so rare they may be tagged as exceptional.
Yeah.  Don't get me wrong - I'm not saying saying that they are especially better for audio work. Just that there's no real need to avoid them. I have designs with electrolytics of course. Tants are sometimes better for form factor eg TAJ height cf electrolytic.
But they can get expensive cf electrolytics as the uF and Volts increase. For higher values I'm looking to migrate to film caps which offer significant advantages at a bit of a cost.
But, to be fair, these are largeish caps for power applications (eg 150uF / 450V) so - yeah - different game. Heat and Ripple Current are the enemy  :)
Last time I looked in detail Tants stiil exhibited better ageing characteristics - but paper specs are always 'subject to interpretation' - main thing is they haven't come back  :)

abbey road d enfer said:
Well, the reasons most audio designers rejected tants have not changed; they still need reverse-voltage protection and increased voltage rating compared to Al types.
Are you sure you don't have to take extra measures regarding start-up of bipolar supplies? Are you sure you don't have to take necessary steps to prevent total failure due to a cap shorting out?
Not "problems", but still issues.

Well (assuming they are put in the right way around) I'd say that all circuits benefit from reverse voltage protection so I look at that on a board / module level.
Increased Voltage Rating - not really found that. Then again they are typically 35V rated for use at 12V - the circuits might also need to run off 24V so standardised to cover that.
Bipolar Supply Start Up - well I use electrolytics for that ( my DIY stuff - fair time since I used bipolar supplies on commercial kit) since I'll be using relatively large values (100uF upwards) so cost...
But I can't say I've found Tants in general to be particularly 'delicate' wrt voltage rating / short term polarity reversal.
Failure on short out - it's going to be a problem if the cap shorts out - elec or tant.  So - permutations of : Local Regulation / fuse / Non-flammable Resistor - to limit extent of failure and protect other system areas.

 
Rob Flinn said:
Rupert Neve never seemed to have an issue using Tants for audio ................

Was this for bypass or signal path.
I could have been clearer that I was discussing in the context of power supply bypass capacitance.
I've no idea of how they spec out for ac coupling.
Although, as it happens, I have a Quad 303 amp that has 1 tant (1uF) IIRC on the input paths.
 
Good luck Chris...  ;D ;D ;D

We now know who likes and dislikes tantalum...

This is what happens when we ask questions on the WWW, we get too much information that we then must filter and sort.

JR
 
His original question

As a simple example, I'm interested in building the Helios69 EQ which requires a make-up gain amp. I thought I'd try the TL072 version. How do I figure out the wattage/voltage ratings for the various components?

was too hard to answer......lol
 
Newmarket said:
Increased Voltage Rating - not really found that.
My comment was in regard to people who put 25V caps in circuits that are powered by bipolar rails, typically +/-15 or +/-17, thinking they have a safety margin, when in fact some locking mechanism an submit the caps to a larger voltage.
 
abbey road d enfer said:
My comment was in regard to people who put 25V caps in circuits that are powered by bipolar rails, typically +/-15 or +/-17, thinking they have a safety margin, when in fact some locking mechanism an submit the caps to a larger voltage.

Right. I see. But does that apply any more to Tants than Al Electrolytics ?
I don't know .
 
abbey road d enfer said:
That's exactly what Andy Peters mentioned. It turns out that in DIY audio circuits, the risk is quite frequent, so I think it's a good recommendation to avoid them unless some of their properties make them unavoidable.

Forget DIY audio circuits -- the reason we stopped using tantalum caps at the day job some dozen years ago is simple. The last thing you want is for an expensive multilayer board stuffed with parts to have to be scrapped because a tantalum cap was inadvertently installed backwards. Because when that cap explodes -- and oh, my, yes, it explodes -- it can dig a nice hole in the PCB.  The first "Oh fsck!" comes from being startled by the explosion. The second "Oh fsck!" comes from when you realize that you just blew up a thousand-dollar PCBA. (Oh, there's a reason why gray-beard engineers never have their head over a new board being powered up for the first time.)

Now that you can buy ceramic caps of sufficient capacitance, for most things you don't need even an aluminum electrolytic.  This is especially true if you're doing mostly digital stuff with 3.3 V and lower rails -- a 10 V cap will do ya fine.  Hell, you can buy a 100 uF 25 V X7R ceramic cap in a 1206 package -- why would you consider a tantalum for that position?

For audio stuff, rarely are we constrained for space, so using an aluminum electrolytic is not a problem. And for things which require low ESR, like a SMPS output cap, Nichicon (and others, I'm sure) have organic-polymer aluminum caps that work well.
 
Rob Flinn said:
Rupert Neve never seemed to have an issue using Tants for audio ................

Rupert Neve has stated that he wished he had modern op-amps and such when he was designing those "classic" products. He used what was available.
 
Rob Flinn said:
Rupert Neve never seemed to have an issue using Tants for audio ................
At the time, tants had a definite advantage in terms of compacity and leakage. This is to be put in perspective with the use of single rails, which proscribed the use of low-volt caps. It was not uncommon at the time to see capacitors cascaded, one that absorbed much of the voltage, and the second (a low-volt type) that would complement; a resistor to ground between them made sure any residual was dumped to ground.
Today, with the generalized use of bipolar rails, coupling capacitors can be low-volt and are much less bulky and leaky than they were in the 70's.
 
80hinhiding said:
I found a small resistor and decoupling cap close to the channel amp I'm powering with a single rail helpful in reducing hum.
That seems like your power rail has to much ripple on it. Inserting a resistor drops the hum current injected into the ground.
 
Andy Peters said:
Rupert Neve has stated that he wished he had modern op-amps and such when he was designing those "classic" products. He used what was available.
Yes - have to consider what were other options at the time and compare to now. Not just copy for 'historical' accuracy like it's a listed building  ;D


Andy Peters said:
Forget DIY audio circuits -- the reason we stopped using tantalum caps at the day job some dozen years ago is simple. The last thing you want is for an expensive multilayer board stuffed with parts to have to be scrapped because a tantalum cap was inadvertently installed backwards. Because when that cap explodes -- and oh, my, yes, it explodes -- it can dig a nice hole in the PCB.  The first "Oh fsck!" comes from being startled by the explosion. The second "Oh fsck!" comes from when you realize that you just blew up a thousand-dollar PCBA. (Oh, there's a reason why gray-beard engineers never have their head over a new board being powered up for the first time.)

Yes - they can 'go with a bang' . Playing Devil's advocate I'd probably say that with an expensive board I'd get the build checked then checked again  :)

Andy Peters said:
Now that you can buy ceramic caps of sufficient capacitance, for most things you don't need even an aluminum electrolytic.  This is especially true if you're doing mostly digital stuff with 3.3 V and lower rails -- a 10 V cap will do ya fine.  Hell, you can buy a 100 uF 25 V X7R ceramic cap in a 1206 package -- why would you consider a tantalum for that position?
Mmmm...100uF/25V/X7R/1206. That's impressive,
Do you have a link or part number for that - I couldn't find it and it could be of real interest.

Andy Peters said:
For audio stuff, rarely are we constrained for space, so using an aluminum electrolytic is not a problem. And for things which require low ESR, like a SMPS output cap, Nichicon (and others, I'm sure) have organic-polymer aluminum caps that work well.
Those organic polymer caps look interesting - well in as much as a cap' can be  interesting ;D - but they haven't lined up with anything I've needed to date.
 

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