Swiss Digital Fuse Box - "What headroom sounds like"

Swiss Digital Fuse Box (SDFB) is non-sacrificial overcurrent protection device that serves as a replacement for fuses in audio equipment.

There are four devices in my sound system that I’ve replaced fuses with the SDFB, and its transformative capacity to upgrade sound quality beyond that of fuses has compelled me to write a review. I’m not associated with Verafi Audio, the company that sells them, nor did I receive anything from them as "review units". There’s another thread on Audiogon about them that an audiophile friend referred me to, which got me interested enough to buy them. I am now enamored with the sound of my system in its current state, so I wanted to share my take on one of the more significant steps in how I arrived here.

Before getting into details about its form and function, I want to share my impression of what the Swiss Digital Fuse Box (SDFB) does for sound quality. It makes my components sound like they are operating without any limitation of power. It sounds open and unrestricted across the audio frequency spectrum. The sound is dynamic, detailed, defined, and there a sense of harmony and completeness about it. It’s like my components can now output their full frequency and harmonic potential.

A few things about me... I’m more of an artist than anything. I’ve been a music lover for 35 years and an electric bass player for 15 of those years playing in two bands, with which I was the bass player on one full LP recording and one EP. I’ve recorded, mixed and mastered my own multi-layered solo bass recordings and their backing tracks from sampled percussion using digital audio workstation software. I’ve also fine-tuned two of my custom car audio systems using DSP software and built, tweaked, and tuned a high end home stereo system (yes I said *tuned*, without using DSP) . Given this experience and the resultant development of a keenly perceptive and informed "ear", yet having no formal electrical theory or engineering background, I feel comfortable suggesting that the results of using SDFB can be likened to how audio sounds when the device producing it is operating with ample headroom. What does headroom sound like? The term headroom has different meanings in its use within pro audio recording/engineering and electrical circuitry operation, but they are related in a way that the end result sounds uncompressed, undistorted, fully dynamic, and expresses the sense of the effortlessness of unrestricted flow.

Does this sound like embellishment? It probably does. And I haven’t even mentioned the typical audiophile terms like "inner detail", "layering", or "rock solid imaging", nor have I even mentioned soundstaging attributes yet -- even though all of these qualities have also gone through upgrades due to the SDFB’s being installed. Am I merely in an irrational, excited state because my whole system now sounds much more expensive than it is? I don’t think so. I’ve been using all four SDFB units for three weeks consistently, and the initial excitement phase I was experiencing settled at least a week ago. I also think that the more components a system has which have replaceable fuses, the greater the potential upgrade from replacing each of those fuses with SDFBs. Like I said, I replaced all fuses in four of my audio components (six fuses in total), and there were notable step-up improvements in sonics as I progressively installed each of them.

Now I’ll describe the physicality of the device and how to use it. Then, I’ll try to describe specifics about why my previous fuse setup, which was a combination of Synergistic Research Purple and Master fuses, was completely replaced by SDFBs. These SR fuses were already a major upgrade in sound relative to the stock, generic fuses, and the SDFBs transcended the SR fuses in every discernible way.

These things have two separate parts that work together: 1) a small box that is inserted as the middle of a chain created between an audio component and the electrical outlet from which it draws power, and 2) a solid, cylindrical metal slug (referred to as a "Sluggo") which is the same size as the typical fuse. To install a SDFB, first, with your component off, plug its power cable into the AC socket on one end of the box, and on the other end of the box there is a male IEC connector (C14) which you connect to an outlet using another power cable or an adapter. I’m using a combination of two short, homemade mini-cables, and two generic adapters with my four SDFBs for the best sound (details shared in my Audiogon virtual system). Once the box is connected to a live electrical line, it will go through a brief setup period, and after maybe 8-10 seconds, you’ll hear a clicking sound and the small green LED will stay lit, indicating that charge is now allowed to flow through the box. The second and final step, with the component still off, is to replace the fuse(s) with a Sluggo. The device comes with both copper and brass Sluggos. You can then turn the component on. Don’t replace a fuse with a Sluggo without the SDFB in place because you’ll have no overcurrent protection and you’ll incur the risk of severely damaging your component and/or having a catastrophic fire in the even of a short circuit or other overcurrent scenario.

These units monitor current magnetically, and are calibrated to whatever fuse rating is needed when you order them. They are also calibrated to operate as either slow blow, or fast blow, like a normal fuse. When the set parameters are exceeded (too much current), a relay is switched to the closed position and charge is halted from flowing. Being non-sacrificial, you don’t need to buy a new one, you just unplug it from the outlet for about 30 seconds and it will reset the state. Then it can be plugged back in and reused. There’s no damage done to the device due to the overcurrent condition, unlike a fuse which melts due to high heat. Currently, devices being produced are to be calibrated at 10 amps max, so if there is some crazy high current event, perhaps then it could be damaged. I don’t know. I believe that’s pretty unlikely though. I think I remember reading that there’s a 15 amp version in the works.

There is some inconvenience involved with transitioning to SDFB due to the extra weight of the box (not that heavy, really) and extra length added to the power chain, as well as potentially requiring additional investment in more power cables. I feel that I achieved an optimal result for only about $200 of additional investment by using some DYI cable materials I had available, some high quality plugs to terminate the cable with, and some cheapo adapters from Amazon. For me, dealing with the extra weight and length to the cabling and putting in the work to create the best solution for connecting the SDFBs to my power conditioners has not been a big deal compared to the profound jump in sound quality. Totally worth it.

Before I went all SDFBs, the best configuration I found with SR fuses in my system was one small Master fuse in the LPS that powers the modem and router (I have an all-digital streaming system), two small Master fuses in the DAC, one small Purple fuse in the preamplifier, and two large Purple fuses in the amplifier. It’s worth mentioning that total retail cost of this setup is about 33% more expensive than the retail cost of my final SDFB setup. However, if you choose to buy a bunch of new, expensive power cables to connect your SDFBs, that would quickly become more the more expensive option.

I thought the SR setup sounded great at the time. I was impressed with the top end detail that a few of the Master fuses added to the fuller midrange and mid-bass sound of the Purple fuses which I already had. I’ve seen comments from others on this site in agreement about this. This combination is getting some praise. However, when comparing that sound to the SDFB sound, it was like the SR fuses are stuck at a level of trying to boost certain frequency ranges to make up for how restrictive a fuse really essentially sounds.

How do you improve on a small, low resolution, blurry, drab looking photo? Well, you manipulate it in Photoshop, of course! You try to crank up values of various visual (light-based) metrics to make it more attractive. However, that process will never produce something as close to the original subject as when you start with an ultra-high resolution, high-dynamic range photograph. You can’t "add resolution" to something that is intrinsically underpinned to a state of reduced resolution. To me, this is analogous to the task of starting with the tiny, resistive piece of wire in a fuse and trying to add crystals and various substances of specific resonant frequencies inside and around it to end up with something representative of the innate completeness of the source material.

Comparatively speaking, I was surprised to switch back to the SR setup and find that the soundstage was compressed towards the center. It was like there was a somewhat spherical haziness in center stage from which the sound was straining to emanate from, even with the Master fuses in play. I attribute this sense of "haziness" to a combination of reduced dynamic range, and a distortion of the frequency response coming from the system’s components. If the hypothetical ideal response for a natural sound (assuming the important aspects of room acoustics and one’s hearing quality are held constant) is essentially a linear response from top to bottom in both amplitude and purity, then the sound of the SR setup was now perceived as distorted and a deviation from linearity.

The sound of the SDFB setup is far more natural, far more detailed, and imparts a sense of ease while listening. I’m using all copper Sluggos, as their tonality is more natural to me than the brass versions. The soundstage has opened up with more dimensionality and all of that perceived haziness and limitation of full expression is (seemingly) completely gone. Images became more defined and image positioning is on a more advanced level. I can now perceive the two singers positioned near center and side-by-side in the mix, with a gap of about about a head’s-width between their mouths. This wasn’t perceivable with the SR fuses. The positioning of cymbals on a well recorded drumset are precisely locateable in space. Listening to Russion choral music, I can now hear individual tenor and bass voices and their unique tonal qualities. On one excellent recording of a solo harp, there’s beautiful overtones resonating that I’ve never been able to hear before. The sound is descriptively harmonious.

Over the course of a couple weeks, the sound of these units opened up. To begin, there was some minor coloration of the sound, but I’m not sure I can hear it any more. I think most of it has gone away as the units have burned in. Even with the minor coloration in the beginning, the immediate leap above the SR fuses in sound quality was obvious and highly desirable.

One last thing, I did a rough test of the overcurrent protection functionality, as this is obviously a major thing to get right and have working properly. I’ve been told that most refrigerators pull about 1 amp of current, so I used that as a basis for testing since I don’t have any more sophisticated method (I could use my desktop computer PSU which has a wattage display to achieve more accurate testing, but I’d rather not have its power suddenly cut and risk problems). I have two SDFBs calibrated to trip at levels below 1 amp, and two units calibrated at significantly above 1 amp. With each of the two sub-1A units inline with the fridge’s power cable (doing two test rounds for each unit), they immediately tripped and the fridge’s power was disconnected when I plugged in the power chain into the outlet. For the above-1A units (also did two test rounds each), the units did not cut power, and the fridge turned on and operated normally. I feel like this testing demonstrates enough for me to have a boost in confidence in the overcurrent protection operational integrity.

Being able to safely use solid metal slugs in place of fuses is wholly a paradigm shift in a high end audio system’s sound quality potential. These things deserve attention and I’m grateful to have been pointed to them.


@wig Thank you sir! I do feel that my closing comments are true.

Looking forward to hearing from other who are using them.

Thanks for sharing @gladmo ! Excellent and informative review! I don’t think I will ever try these, as I am too risk averse to experiment with these kind of power things , but I enjoyed your review. Thanks for sharing 

@thyname Always nice to hear about people enjoying my writings. I’m a financial markets trader and investor, so identifying risk/reward, knowing one’s own tolerance for risk, and making moves is the one thing I do all the time. I've done well being an early adopter. Seeing some detailed descriptions about what testing went on during the R&D phase would certainly add to credence regarding reliability. In lieu of this, so far anyway, my decision was therefore based on general evaluation of the likelihood that everything that’s been claimed is actually true, including thoroughness of testing. In other words, going with my gut.

Time will tell.

Thanks for the great writeup review. Enjoyed it!   Love the concept and technology and it sounds low risk to damaging components.  If you want to start out and not purchase one for all components what would be the order of priority?  DAC, Preamp, Streamer, Amp and so on. 

@gladmo I am so impressed with how you’ve written the review that I had to post my enthusiasm immediately. Now back to the review.

@gladmo Having finished your review, I’m pretty sure I will order a SDFB.

Like you I am all-digital. I stream Qobuz via an Innuos Zenth Mk 3 streamer/server which has had the amazing purple fuse in it for a log time. However, unlike you, the fuse in my streamer is the only replaceable fuse in my system. My integrated amp has no replaceable fuse and I can’t quite get excited about replacing my one purple fuse with a SR master fuse, despite the no-risk return policy.  I suspect it will sound somewhat bright or forward without a purple fuse elsewhere in the rig to balance it out.

You were smart to address the ‘excitement phase”. I’ve become very aware of the phenomenon’s effects on my judgement lately and now I have a term for it.

Although my use of "headroom" to describe the sound quality still seems apt to me, I also think it might be something closer to "legroom" that needs a mention here as a way of looking at the mechanics behind this. Haha! Legroom 🙂. I mean like all of the inner electronic parts being lifted up off the floor from an operational energy deficiency vs having been relieved from pressing up against a metaphorical ceiling in a state of energy excess, where clipping and distortion occur. In either extreme state, there's distortion. I can't say with any technical certainty about any of this because I'm not an electricity expert, and that's fine with me. It's just the audiophile buzzwords that count, right? 😁



Has the OP written a review somewhere else for this product or maybe my eyes deceive me? Read this description and review. Similarities are undeniable.

These are around $400 ,now like with my amplifier it has 5 fuses inside ,therefore somethings don’t  apply , and in some dacs no fuses ,or some have thermisters 

which are better-then a fuse , but if you only have 1 fuse these have a microprocessor and a Copper element ,my only concern there is itshould be plated gold or silver for Copper oxydizes what’s the answer to that ?

Good Morning


Thanks for the wonderful review. I think you’ve said it in fine fashion.

SDFB is catching on quickly, but there hasn’t been a lot written really except by a few others. I am really happy reading this again this morning.

I sent this to several friends last night after it was published. Lots of feedback this morning.

I’ll do my best to stay out of this and answer only when necessary


No need to worry – very safe

Many of you have my phone number – call or text is fine – email too



Works everywhere, but on my two rigs amps made a game changing experience happen


I shared this with Walter last night.

Most of you know that I lead the Underwood DEV TEAM (going on 11 years) re LSA, CORE POWER and EMERALD PHYSICS – lots of new products shipping this month and next for The Wally…

The new Bob Carver products as well for Jim and Bob

Wally has his way of doing things. He’s pretty dang good at it


You are spot on.  I leave the B+ rail fuses alone and head for the Power Side of the Fuse requirement.

New Sluggos are in-bound (FINALLY) now from my supplier in China – but we also have some very special ones being made in New England by my buddy Dana B with C101 Copper – SUPER HIGH PURITY – 99.99% PURE COPPER that has 0.0005% oxygen content.

As well, thanks to groovy sauce over at Decware forums I’m bring in some Super Coated Plated Sluggos with Rhodium Plating etc.

And yes, polishing too.

Now that the new Carver Amp is in production, I can get back to listening more – this is pure enjoyment for this old man.

Thanks – Mark



Thanks for your e-mail JCA – our 30 days No Questions Asked Guarantee is in place for this product just as we offer for Puron etc.



Thanks for sharing and very informative write up indeed. I am with @thyname, as promising this thing sounds or performs, it isn’t something I would pursue either.

@tksteingraber Generally, I would say amp first. A little hard to say what order would come next, but I’d guess DAC, then streamer, then preamp. But my preamp has a patented power supply technology that runs on only a tiny fraction of the power that typically preamps do, so I think it’s performance was less restricted by fuses to begin with. So maybe preamp before streamer if it has a higher rated fuse amperage?

@gnu I only posted the review here. If I were running that website, I would place the product description above a long review like this on the product webpage. Just my opinion though.

@audioman58 I can only share that the user manual has a warning not to use the slugs+device in place of an amplifier’s DC rail fuse if there is one. Regarding oxidation, after polishing my copper Sluggos with a gold jewelry polishing cloth they sound different, more neutral and less warm, and at the time it seemed like an little increase in detail retrieval too, but you can’t go back to A/B. I wouldn’t mind having backups of each slug which haven’t been polished because I’m a tonehead. The sound will probably very slowly shift to warmth as oxidation occurs.

@verafiaudio Good morning, you’ve got my attention with those new Sluggos 👀



@verafiaudio Mark, I’ve got a question. It’s been awhile since I’ve read all of your comments on other forums, so I’d like to clear something up about what I recall less than perfectly.

If, for some unexpected reason, the active parts on the circuit board fail (microcomputer, whatever else could potentially fail due to defect or something) while it’s in use, can charge still flow through it? Or is the relay method one that is passively closed and blocking charge from flowing if the active electronics were to malfunction?

Do you mean they just lifted what you posted here without asking for your permission first? if so, that's really tacky. And fishy too, frankly.


In between lunch and a business meeting but I can share this -

The default condition for the relay is OPEN, so if there is any failure no current can flow to the attached device.



That’s kind of a Wally-thing. I long ago let this go and I just don’t enjoy arguing it out with him or others that copy Reviews for their web sites or Promo Materials.

Thanks - Mark



PS - when the new Sluggos arrive from various sources I will gladly let you (or others) test... 

@verafiaudio I really appreciate all three of those responses. Your answer to my question is brief, but that statement is what I was looking for. I admit that I don’t understand much about relays and the related terms.

I just found that I actually have a spare pair of unpolished large copper Sluggos for my amp, so I’m going to see how the system sounds with those in place of the polished ones that are in the amp now. A normal type of daily activity for me 🙂

Switching to unpolished copper slugs in the amp and keeping polished copper slugs in the other three components is really nice. Gonna keep it this way. A little more body, warmth, and low end, and slightly less relative energy in the top end, as expected. There's still the same presence of subtle ambient cues and reverb that I was checking for. Very cool.

Post removed 


That’s kind of a Wally-thing. I long ago let this go and I just don’t enjoy arguing it out with him or others that copy Reviews ...

Theft is theft. I'd never do business with this guy.

@cleeds : yes I thought the same when I read that link. But to be fair, there is this reference at the beginning, right after description:

Read the long detailed AudiogoN Consumer review:

Swiss Digital Fuse Box - "What headroom sounds like"

There is also the link to this thread when you scroll down. IMO, as long as proper credit is given to the original writer, and permission to post (not sure if @gladmo gave permission), all fair game. IMO


I didn’t have any communication with Walter at Underwood HiFi, but nothing about him copy/pasting my review bothers me. I don’t feel cheated, stolen from, or anything like that. He’s selling this device as a type of partner with Mark of Vera-Fi Audio. I’ll allow it, no worries. Linking here was a good move.

I did speak with Mark on the phone a couple times while I was trying to form a buying decision because I had questions about the two different fuse rating in my DAC, etc. He was super helpful and I got an honest and cordial vibe.

I did speak Mark on the phone a couple times while I was trying to form a buying decision because I had questions about the two different fuse rating in my DAC, etc. He was super helpful and I got an honest and cordial vibe.

Totally agree with that statement on Mark. Great guy to deal with. I have dealt with him a few times, EQUI-CORE, and most recently for the Purons.


Very interesting ,Walter said in my case where some amps have 4 fuses inside the unit they would not be a ideal fit.


I also have 4 fuse rails inside of my KT88 amp and the FB made a big improvement!

Wig 😁

@wig what do you mean? Can you please elaborate? How did you replace the 4 fuses inside your KT88 amp with FB (what is FB?)? FB is the Swiss Fuse Box discussed here? I sincerely hope FB does not mean Facebook 😂🤔🤷‍♂️🤦‍♂️😂😂

Ok good to know ,does that mean you just leave stock buzz fuse under theIEC ?

My BHK 250 amp has 6 fuses in it.  That would be an expensive upgrade to do my entire system plus I would have more power cords snaking all over the place, that just doesn't sound like something I could or would do.

You can just use stubby little Nema 5-15p to IEC C13 adapters to connect the fuse box to an outlet. They sound good in my application. You may or may not need some extra support at the outlet with this method because of the leverage the weight of the device will put on the adapter’s connecting spades. Vera-Fi Audio also makes 1-ft mini-cables you can try using.

It’s a $395 upgrade for each component. You’ll get enough Sluggos included for the fuses that are replaced. My DAC and amp both use two Sluggos. I wasn’t charged extra for those.

@vandy357 given Wig’s post just above yours, I’m not sure what your concern about the amp is. Just leave the DC rail fuses where they are.

@gladmo I guess I didn't read that post.  I assume I need one fuse box for each component?  I also assume from what I can find on the interweb that I replace my 2 fuses near the IEC on the amp with the sluggos, then insert the fuse box somewhere between the component and the wall outlet or Power Re-generator.  So this basically just adds what would be a circuit breaker between the component and the wall plug, so if there is ever a problem inside the unit it will trip the breaker instead of blowing the fuse.  I may have to give this a try, at least on the amp for now.  If this really works to improve the sound I may eventually add one to each component.  Thanks for the info and the clarification.

@vandy357 That all sounds right, but I can't be totally sure about your amp fuses. You can call Mark at the phone number on the website URL I gave at the bottom of the review and he'll get you sorted out.



Welcome to phone me on my mobile phone - 303.594.7586


Happy to help anyone that needs any questions answered.

Thanks - Mark 

@gladmo Thanks again, big help.  I am starting to wrap my brain around the concept of these devices, I honestly just didn't understand the application at first.  I worked on cars all my life and have witnessed people replacing fuses in automobiles with everything from aluminum foil to screw drivers, with a few that caught things on fire by doing so.

@verafiaudio thanks Mark for the phone conversation and the help with the product.

My pleasure to help, guys.

For anyone that is using SDFBs, I ended up switching back to the polished copper Sluggos in my amp after a brief time with the unpolished ones. It became apparent to me that, actually there is some small loss of fidelity due to the oxidation-based corrosion on the unpolished Sluggos. My recommendation is to go for polished Sluggos with any SDFB you have installed.


Has anyone tried applying a light coat of Graphene Contact Enhancer to the ends of the Sluggos?

OK…. I caved. And bought one Swiss Fuse Box. Maybe because Mark is a pleasure to work with. Or maybe easy picking for my Innuos ZENith MK3, for which I was already using a SR Purple fuse (and before that the Orange). It’s pretty simple to deal with fuses on Innuos. So there is that.

All I can say is …. OMG! I know I sound like a teenage girl 👧 now 🤷‍♂️🤦‍♂️. More later when I really have a chance to listen to for an extended period of time this weekend, but first impressions are fantastic. And I am a big believer in first impressions.


Oh… and it was extremely easy to set it up. Very simple. It literally takes minutes to put to work, if you have connections ready. And I am happy the adapter I bought from Amazon (link below) works great in putting it directly to my power distributor (existing power cord on the other end to Innuos)





Today I received (finally) the first of the next gen of Sluggos. I love the name Slug Club

Trying coatings, polishing and exotic materials.


Thanks – Mark



So glad to read this. I’ve had one of those amazing days. Just incredibly glad for you and the others that have stepped up

Very nice fellow in the 901 that tried a SDFB for this BHK 250. His results make me so happy for him – and my guys in Switzerland that made this all possible for us. The note was incredible. More of this daily now.



Register that name. 😊

I love it

Best to all – Mark


Yep, I caved and bought one of these thing too.  I am so glad I did and want to give a huge thanks to @gladmo for his initial write up on this great product.  Everything about the sound of my system is now better, even with my meager setup abilities I have never heard my music sound this good.  Thanks Mark, it is a pleasure to do business with you sir!  Next will be one for my PS Audio MK2 DAC, I can't wait to find out what that will do for my system.


Got slugs? *inserts a photoshopped photo of an attractive celebrity with a half-eaten slimy slug on her upper lip*

I appreciate all the raves, but..

my main issue (only because I have no personal experience with what it/they does/do top to bottom) is a slightly hot top end that I only noticed recently. Now I can't discount that for over a week now I have been using a product called Sonofit (drops in the ear canals) in hopes/expectations that it will resolve decades of tinnitus, which is bad enough that I sleep with the TV on to cover the constant ringing

@tweak1 I have also suffered with tinnitus for 30 plus years which seems to get worse with age, but still doesn't sound as bad as what you are describing with your case.  Tell me more about the Sonofit drops, do the seem to be working?  Now on to what I can tell with the Swiss Digital Fuse box installed.  The first thing I noticed is how voices sounded, I was listening to a Van Morrison tune, when the music began his voice seemed to come out of nowhere, then his female backup singer sounded like she took a step or 2 back and a couple steps to her right, (she is to the left in the sound stage).  When I listened to this same tune before the fuse box the backup singer almost sounded like she was singing into the same mic that Van was using.  Then I noticed how much smoother my system sounded, this is hard to explain but to me the music sound a lot more coherent than it did before from top to bottom.  Honestly, I am flabbergasted at the difference the SDFB made in my system.  

I certainly will be eager to hear more about how the drops are working for your tinnitus.  Good luck with that.



Thanks for posting your impressions. I must admit that your unique solution, the adapter on one end of SDFB definitely peaked my interest. I wasn’t too keen on adding a garden variety pigtail in the signal chain. You know the PC’s in my system :-) 

@lalitk They are basically the same adapters that I mentioned briefly in the review.  I'm currently using one on all four of my SDFBs.