Fast and slow

April 8, 2022
 by Paul McGowan

After 50 years of working with audio gear, it's easy for me to forget not everyone knows the basics. I'll do my best to help remedy that.

A few days ago we got into fuses, a subject that helped me realize not everyone knows there are two basic types. Fast and slow.

A fast fuse saves circuitry. A slow fuse saves lives.

First, let's talk fast blo fuses. You don't see these a lot anymore. They are typically used in the DC "rails" (the power supplies) that feed circuitry. They need to be quick because they are there to stop the voltage feeding circuitry before something bad happens to that circuitry. Their most common use is to protect power transistors in the output of a power amplifier. Where small signal circuitry like that of a preamplifier doesn't have enough "juice" behind it to cause damage to the silicon, a power amplifier surely does. The fuse must die faster than the transistor it is protecting.

A fast blo fuse is basically a whisker-thin wire inside a glass (or ceramic) enclosure. Here's a picture of one.

Fuses are designed into the power supplies that feed the output transistors of amplifiers. They are chosen by their rating of how much power they can pass before the little wire inside heats up and vaporizes—thus breaking the connection to the power supply.

Most amplifiers don't have these because there are now more modern means of providing the quick shut off (other solid state devices).

A slow blo fuse has the same characteristic as the fast blo, meaning it too is chosen by the maximum amount of current (power) that is allowed to pass through it. The difference is time.

When we first plug in a product to the AC wall socket, or flick on the power switch, a surge of power flows into the unit. Perhaps you've noticed the room lights dimming on first turn on of a power hungry something. This inrush of current (power) needed to fill empty power supply capacitors (or start a motor or heater) is only for a brief moment. Thus, what we want is a fuse that will tolerate that momentary inrush of power until the device settles down.

That's why it's called a slow blo. Here's a picture of one:

Instead of the whisker-thin wire of the fast blo, we now have what looks kind of like a spring. Pass enough current for long enough through the spring and it too heats up and vaporizes.

Slow blo fuses are found at the very input of products. Before the power supply. They exist to make sure that your product doesn't light on fire, or that the wires in your home don't light on fire.

Of course, modern homes are doubly protected. An electromechanical version of the venerable fuse is required for safety. It is called a circuit breaker.

Hope that helps.

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37 comments on “Fast and slow”

  1. Yup.
    However, I have to say that in 46 years of home-audio gear
    ownership, I have never experienced a blown a fuse in any
    of my electronics...blown a few tweeters & midrange drivers though.

    1. In the late 70s I used an integrated amp from Pioneer (Modell A9) in my main stereo system which randomly activated the circuit breaker in the breaker box when being switched on. I never had to replace a fuse and the amp is still in good condition and running.I recently replaced the internal e-caps.

        1. After having let install a dedicated power line from the breaker box to my P10 power regenerator and a “hifi-grade” breaker module with exchangeable fuse (now 20 A instead of 16 A) I never saw this phenomenon. I guess the P10 already helped a lot.
          And I trust my electronic fuse (e-fuse from Germany) installed in my actual power amp which reacts most sensible when loading the caps in the amp’s power supply after a regular powering off every six months.

  2. Of course it helps. 🙂

    Thank you, Paul.

    I have just one question. If a solid state amplifier uses a fast blow fuse and that fast blow fuse is swapped out with a higher quality ceramic fuse would that be a benefit to greater sonic performance/reproduction?

    Reason I ask is that I had in my head that slow blow fuses because of how they work inside an amp would mostly benefit from a sonic upgrade. Fast blow fuses work purely as protection and I think may not effect sound whatsoever? Of course I’d love your input on this and your advice.
    Thank you again in advance. 🙂

    1. I have not experienced any sonic benefits from fast blo fuses. I think that's likely because they are normally in the DC rails (that's what we nerds call the output of a power supply. It's also referred to as B+ and B-). The steady state nature of the DC is likely unaffected by the type of fuse. That said, and to be honest, I haven't yet played around much with this.

  3. So based on your well written and detailed summary of fuses, how can they possibly improve the overall sound of your rig? It simply serves a purpose (like safety) other than sonic fidelity.

    1. I think the theory is that input fuses must handle large peak currents as they charge power supply capacitors in short "gulps" to fill them up at each half cycle of the mains sinusoid. Fuses have a nonlinear resistance as a function of current passed through them so again in theory a "fancy fuse" may optimize (minimize?) this nonlinear resistance. I would be very concerned about boutique fuses without any UL/ETL rating to provide that necessary safety function. Try to explain to your home insurance company that your $200 fuse had no agency approvals and see how happy they'll be to pay your claim due to fire damage.

      NOTE: This also assumes the most typical non-power factor corrected type power supply input; full-wave rectifier and large filter capacitors for a PF ~0.7. Although most cringe at the idea of switching power supplies they can resolve this important issue of poor power factor and large harmonic distortion of AC input current.

    2. I can only speculate as to your answer. I have never heard a rails fuse make a difference. I have heard a mains fuse making a sonic difference. In this case I imagine it has something to do with the tiny bit of thin wire used on the AC input. Logically, if a power cable matters then placing a tiny wire in series with the power cable would seem to have an impact as well.

  4. It is also common to see fast blow fuses right at the output of an amplifier to protect your precious speaker drivers. When playing music especially loud I've had a fuse blow which is far better than having a speaker (or driver) go up in smoke! Also protects the amplifier power devices too.

    And from the inter-webs:
    "Fast Acting Fuses (Fast Blow, General Purpose or General Application Fuses) Fast Acting fuses are designed to protect less sensitive components and are also used to protect cabling from melting or catching fire when an overload occurs. Fast acting fuses are the most common type of fuse."

    1. Though it once was common to have a fast blow fuse at the output of the power amplifier I would argue that today that's very uncommon. Most modern amps have a protection circuit that either uses a relay to disconnect the speakers or simple shuts down the power amplifier.

      1. This will again point out what an old geezer I am. My original stereo system ( circa 1971 ) had a Heathkit 100W / channel receiver ( that I built ) which came with two fast blow fuses that you were instructed to solder to the red lead of your zip cord speaker wire to protect your speakers! In spite of such crudeness I still remember my delight at the sound when I got the thing built and fired up my system consisting of a Thorens TD150 MkII with a Shure M93 cartridge, the Heathkit receiver and a pair of JBL L88 two way speakers. It seems like a million years ago.

    1. As I have mentioned before we're not brand specific with fuses. We are type-specific. We switched from those glass envelope/spring types I showed in the article to all ceramics. We don't spec a vendor. Just a type.

      On some parts, we are vendor-specific. Resistors and capacitors are a good example (though not all). Resistors in the signal path are vendor and type specific while resistors in, say, the logic or protection circuits are not.

      1. Paul, could you explain why your BHK power amps have four accessible fuses on the back, please. Perhaps a "dual-mono" construction might explain why you have at least two fuses...but four? Thanks,

              1. Ok, now I see 6 fuse holders on the back of each amp. But would it be necessary to put designer fuses on the rails? I would think just putting them on the AC inputs would get you most of the benefit of designer fuses. Do you have any experience with that to share?

  5. Fuses can be found in fast-acting, medium-acting as well as slow-acting, and are generally selected by reason of it's characteristics in the intended circuit. As Paul points out, slow-blow are used in cases where high inrush currents are experienced, like charging the tank circuit in the power supply. What most don't know, is that glass slow-blow do in fact change as they melt slightly in each time they protect against inrush and really should be changed periodically.

    Which brings us to the HRC "ceramic" fuses which has properties of much longer life, and the fuse element is encased in a ceramic outer shell, with sand inside. These can be found in the same action categories as described above, but yield longer life.

    This brings up the last point, and that is the fuses resistance. Depending on the type, manufacturer and rating this can vary for a given amperage value. Some aftermarket fuses are intentionally de-rated at a given value to improve the insertion loss, but this can be misleading to the consumer as many aftermarket companies do not submit their product for UL, CSA or other international certifications to ensure their product is safe as advertised.

    For me, I will not use a fuse that has not been approved by UL here in the US. My P10 came with a ceramic Schurter fuse. It has been replaced with a Littelfuse 285 ceramic Rhodium plated which yielded lower impedance than the original.

    1. I like using Littelfuse devices (fuses, TVS diodes, etc) for low amperage and Bussmann fuses for Hi amperage.
      Littelfuse has come a long way in the last 20-30 years. They put a lot of money in acquisitions and product development. I've used some Schurter fuses but in my SWAG opinion the 20mm cartridge product line looked and felt cheap. I bought them because they were lower price than the Littelfuse. I buy in thousands to tens of thousands.

  6. Paul, you and I are of the same generation, and this was a good review and reminder of the purpose of fuses (and circuit breakers). Yes, it did help. Thank you.

  7. My dads house burned down due to a faulty circuit breaker and improper installation of aluminum wiring.
    Dad, being a full-fledged electrical engineering scientist should of knew better, but growing up in a German community during the depression - I think he helped invent copper wire by fighting over a penny… He fell for cheaper to many times.
    If aluminum wire is used to bring power into your service box (not recommended) special paste must be applied to the wire to prevent oxidization.
    Apparently resistance at the service box connection went up slowly over time. This slowly cooked the breakers. The main 200 amp breaker did not trip.
    At some point the service box burst into flames and the fire got into the walls where it built up heat until it burned a hole and got the needed oxygen to burst in flames. I saw the windows blow out.
    The house went down in 20 minutes with two fire districts responding. I don’t know the name or type of breakers but they were still intact. They looked like a charcoal briquette (were not tripped). Touch them an the cooked thermoplastic shell crumbled.
    The wire inside the romex had bubbles on it indicating the copper wire inside the service box got so hot it “boiled” and caught the insulation on fire.
    I mention this so anyone that has aluminum wiring going to the service box should probably have it checked by a qualified electrician.
    Its easy to spot the white grease that is supposed to be on the connection. AC service should be installed by qualified people with quality components.
    The irony of it dad knew power. He developed a 2 million watt AM transmitter for VOA. It had its own substation!

    1. Tim, I have no idea where you live, but here in Westchester county of NY all of the wire used to connect home service to the electrical grid is aluminum ( both overhead and underground ). I have no idea what precautions the utility company ( Con Ed ) takes, but it must be working because houses are not burning down due to poor service connections. I had to have my underground connection repaired and I questioned the Con Ed service man who did the repair about the aluminum wire and he said aluminum was the standard wire used.

      1. Utility company only connects to your weather mast here in the mid-west. They don't have anything to do from the mast to the service box. The run from the transformer to the weather mast is indeed aluminum.

        1. Tim, I miss understood what you meant. Since my service comes in underground when it comes out it runs through metal conduit to the meter which obviously is from the utility company. The run from the meter to my service box was probably done by the electrical contractor that wired the house when it was built. I pretty sure it is aluminum ( I will check tomorrow ), but whatever it is it had to pass inspection when the house was built.

  8. I replaced all the square D Bakelite (yup - Bakelite just like my vintage viewmaster) breakers in my panel about 15 years ago as one was heated, warped and starting to smell of that burning electrical odour - yet HADN’T tripped… so fuelling the concern in my ooooold house I coaxed an old electrician to come & check out my house wiring. As he looked up at the green cloth coated wiring in the ceiling of my basement he just said ‘oh my…”
    “That bad eh?” I responded. “Nope, not at all, don’t TOUCH IT!! I just wired 5 new houses & there’s more pure copper in this wiring than all 5 of those new houses combined”
    So my fine audio system in its dilapidated old Dr Suess farm shack is at least currently (so to speak) getting well fed… (I added a hospital grade outlet & Connex tri-plex power centre)
    Level floors and 90 degree angles are grossly overrated. Cooking an omelette with a sloped floor works all too well as everything naturally slides to one side of the pan…. G’head, laugh all you want - I’ve got 5 acres & a backhoe…they’ll never find you OR your car…
    My abode is warm, dry, houses all my toys but is just ratty enough that women refuse to live there. So I’m pretty dang okay with it.

    1. Sweet!
      I agree with plumb and square being overrated. I got back from one of the best resorts in Punta Canta last week. I don't think Dominicans know what a plum line or a level is!
      I don't have a back-hoe but my back yard is the Mississippi River.
      There are slews and backwaters that would make a person disappear for a thousand years! (A commercial fisherman gave me this warning only once) LoL

  9. One very important property of a fuse is the current breaking capability. For a given size of 5x20 mm it can vary between 50 to max.1500 Ampere. High rate fuses has sand filling to purge the electric arc inside during activation.
    Years ago I used a cheap china style multimeter to measure a 400V AC voltage. Due to some short inside the internal glass fuse couldn't break the enormous current that followed. The result was a huge flash transforming the meter into chlorine tasting charcoal.
    Pure aluminum tends to migrate under pressure. Therefore screw terminals have to be tightened periodically. If possible use cage clamp terminals.

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