fuses impact on sonics of a supply

[moray james]

Any comments on different fuse types and how they affect the sound of a supply? Glass vs ceramic and or sand filled. Is this strictly an issue of microphonics? Is there such a thing as a low impedance fuse? What are the mechanics behind fuses impacting the sound of a supply. Are there recommended types of fuses for audio or ultra low noise measurement equipment? Not referring to cryo treated fuses here but fuse style types. Thanks best regards Moray James.

[darkfenriz]

I'm not very much into changing one component and hearing drastic difference, but I believe it's mainly because non linear impedance of a fuse, the main reason for not putting one directly at the output of an amp. Better than trying different fuses is to make an amp less prone to supply non linear impedance distortions. Simple RC for VAS will be helpful. Best to use IC regulated supply for everything but last output follower stage. The unregulated part of supply has than virtually no impact on quality and for the rest IC has its in built current limiter. Sorry for not answering your question directly but I had to say this. Maybe ask some 'gainclone' guys, they'll tell you the pricier the component the better. Fuses too I suspect.

[beppe61]

Dear Sirs,


I would like to get anyone opinion on this problem.
I have a power amp that has 4A/250V glass fuses in series with the + and - power rails between the filter caps of the PS and the amplification circuit (for me an unelegant protection method).
I wonder which kind and value of impedance these fuses present to the current flowing in the supply rails.
Could they be a bottleneck ?

Thank you so much.
Kind regards,

beppe

[peranders]

Originally posted by moray james
Are there recommended types of fuses for audio or ultra low noise measurement equipment?

When you have this type of circuits, the fuses are pretty far away and besides those circuits consume rather little power, therefore fusing is not a big problem.

[Eva]

Originally posted by beppe61
Dear Sirs,
...
Could they be a bottleneck ?
...
beppe

There is not such a thing as "current bottlenecks" in electronics. I strongly recommend you to study some papers on electronics basics (use google) in order to understand concepts like current flow, electric potential, resistance and impedance.

A fuse has an impedance, mainly resistive (it works like a resistor most of the time). The power lost on the wire is I^2*R and it will manifest itself as a temperature increase, and ultimately as glowing. The resistance of the fuse gets higher and higher as its temperature increases in response to higher currents flowing though it, until a point is reached where thermal runaway happens and the filament starts glowing. Looking carefully, the filament may be observed dilating and bending before that happens.

The fuse only works as a current limiter when it enters thermal runaway, as both temperature and resistance will increase dramatically in response to any current increase reached this point. However, it will fuse almost inmediately. That's why fuses work as resistors during normal operation.

In other words: If you can't see the filaments of your fuses bending or glowing with the music, then they are acting as harmless and linear resistors, whose value would be probably several dozens of miliohms (nothing to worry about). Otherwise, try fuses of a slightly higher current rating.

[beppe61]

[QUOTE]Originally posted by Eva
[B]
Dear Miss Eva,
thank you so much for your kind and very valuable reply.
First of all I would like to state that I am very ignorant in electronics.
I am just trying to understand more.
I tried all by myself, but failed miserably.

> There is not such a thing as "current bottlenecks" in electronics.
- If I say high impedance devices would it be better ?

> I strongly recommend you to study some papers on electronics basics (use google) in order to understand concepts like current flow, electric potential, resistance and impedance.
A fuse has an impedance, mainly resistive (it works like a resistor most of the time).
...
The fuse only works as a current limiter when it enters thermal runaway,
...
In other words: If you can't see the filaments of your fuses bending or glowing with the music, then they are acting as harmless and linear resistors, whose value would be probably several dozens of miliohms (nothing to worry about).
....
- Very interesting.
So a fuse acts like a more or less 0,1 ohm resistor?
Actually I measured with the tester the resistance and found a short-circuit (beep).
The actual resistance must be very low indeed.
So nothing to worry about.

In the end I cleaned the contacts with isopropil alchol and left the original 4A fuses in their places.

Thank you very much indeed.
Kind regards,

beppe

[AndrewT]

Hi Beppe,
Try measuring the voltage drop across the amplifier fuses.
I have found 8mV to 30mV across them in quiescent condition. With care you can also measure the mains fuse on the AC mV scale.

A good clean often removes much of this volts drop. This seems to indicate that the corrosion and dirt that builds up on the recepticle and fuse cause much of the resistance.

For mine, converting the volts drop of a cleaned F2A fuse and receptacle to resistance came out at 9mohm. Bigger fuse ratings will have lower resistance. Undisturbed fuses that have been used for a long time will almost always be much worse.
That's why vendors pedal the contact cleaning fluids and fine abrasives (brasso in UK) can also be used to really polish the surfaces.

[beppe61]

[QUOTE]Originally posted by AndrewT
[B]> Hi Beppe, Try measuring the voltage drop across the amplifier fuses.
I have found 8mV to 30mV across them in quiescent condition. With care you can also measure the mains fuse on the AC mV scale.
- Dear Mr. Andrew,
thank you so much for your always kind and valuable advice.
I will try to do it without frying anything, expecially myself (eh, eh)

> A good clean often removes much of this volts drop.
This seems to indicate that the corrosion and dirt that builds up on the recepticle and fuse cause much of the resistance.
- A disclaimer first: maybe I am me hearing things.
Nevertheless I can happily report that after cleaning the fuses ends and the holders ends with isopropyl alchol (the ends of the fuses came out shining) I hear some benefits: less noise, more power !
The ends were not corroded at all, just dirty.
Nice improvements in sound ! Good move.
I wonder if it would be the case to bypass them straight away, anyway, as only a cleaning can give such effects.

> For mine, converting the volts drop of a cleaned F2A fuse and receptacle to resistance came out at 9mohm.
Bigger fuse ratings will have lower resistance.
Undisturbed fuses that have been used for a long time will almost always be much worse.
That's why vendors pedal the contact cleaning fluids and fine abrasives (brasso in UK) can also be used to really polish the surfaces.
- Thank you Mr. Andrew for your very valuble directions.
Anyway I am thinking to solder a nice zero resistance piece of copper in the place of fuses more than ever.
I found it disturbing to have anything in series with the voltage rails.
Looking at other power amps schematics I cannot see these fuses and they could harm the sound eventually.
Thank you very very much.
You are very patient teacher, not like Miss. Eva above.
If it were not for the national flag of Spain I would have said a German Miss .

Kind regards,

beppe

[john curl]

I just received an E-mail from an associate concerning fuses. Apparently, they changed the sound in one of my designs. I also spoke with a German guy who made exotic audio fuses, at CES, and he made sense to me. They are pretty expensive though, and I would think that other things should come first.

[AndrewT]

Hi Beppe,
if your amp was designed to include fuses then it would be rash of you to change the values.

You are inviting disaster if you bypass them.

The amp should be designed to reject some power supply ripple.
A small resistance in the supply leads gives a small increase in this PSU ripple. A well designed amp should not produce bad sound with clean fuses. But, I will reluctantly accept that dirty and/or corroded fuses may affect the sound, particularly if the volts loss across them is substantial and not linear.