Subwoofer amp blowing fuses

[JimD]

I built a subwoofer in a ported box to go beneith the floor in my family room and shoot out a fake heat vent. I houses a 12 or 15 inch sub (I don't remember, it's been several years) and is powered by it's own 100W amp. After getting my new Samsung 6300 LCD (LED) TV and LG 5700 blu-ray (WiFi) going, the bass didn't sound right so I checked the fuse on the amp (requires taking a panel out of the shop ceiling to see the back of the amp). It used to reset itself if I turned it off and then back on so I thought that might be the issue. It didn't help so I checked the fuse. It has a 2A which was blown. I got a new fuse which also blew instantly after installation.

Is there something I should check before replacing the amp? PartExpress has a replacement for about $100 which doesn't seem bad to me. Other ideas? It's a little pain to work on because of the location but I can take the amp out to look at it better if I need to. Probably a good step before placing the amp order.

Jim

[LCHIEN]

before assuming the amp is bad, consider the coincidence or not that the amp had problems right after installing the new A/V receiver. Possibly it could be something like the new receiver has a turn-on transient when powered up that is large enough to blow the fuse. A turn on transient can be a click or a thump or it can even be sub-audible DC that you can't hear. Some receivers mute their speakers (but it may still get out the LFE line) for about a few seconds after turn-on to prevent speaker damage.

A 2Amp fuse might be adequate to smallish for a 100W Amp, I dont know how good your amp is, their peak draw can vary a lot depending on design reserves and internal capacitance etc. Being DIY, how was the original fuse size selected? Whatever worked at the time?

The first thing I would check is to unplug the LFE (bass signal) signal to the sub and replace the fuse, Turn the sub on and off to see if that blows the fuse, if it still blows then you probably have a bad sub amp, if not then try a few power on and off cycles with the receiver and see if somehow the fuse blows immediately. If not then replug the LFE to the sub amp - try on-off. If it blows then a turn-on transient signal from the A/V RCVR might well be the problem, not a bad sub amp.

If it is the turn-on transient, then, try temporary 5Amp fuse in the sub amp, and repeat the power up down cycles to see if that blows the sub. If not, then consider leaving the 5A fuse in. Or use some kind of power sequencing, like manually turning the sub on after the receiver, or even using a Sears accessory switch like we do for shop vacs, which has an on delay for the slave device (sub amp) when your receiver is plugged into the master outlet..

Good luck.

[Knottscott]

Loring's advice is excellent. I can't add much, but it entered my mind that it's remotely possible that the new equipment you've added has caused enough voltage loss and amperage increase in the line to overload a fuse that was already borderline. Can't rule out the possibility of the replacement fuse being defective from the get go.

[BobSch]

I'll bet Loring is right, but is the fuse for the power or is it on the output to protect the speaker itself? You might have a bad speaker. Just something else to check.

[JimD]

Loring,

I will probably try another fuse with the input disconnected. I did not change the receiver sending the signal to the sub, however. It is the same Kenwood as I have used for more than 5 years. I added a blue ray but the signal from the blue ray goes to the Kenwood receiver (using a digital coaxial input) and it plays out the other channels of the reciever (left, right, center, and left and right rear). It is just the sub that is giving me trouble.

But disconnecting the input seems like a reasonable thing to try before taking the amp out and looking at it. I think I will also try unplugging it, disconnecting the input, then plug it back in with a new fuse. I do not see how unplugging is different from the fuse blowing but maybe something is still getting power. It's also easy (everything plugs into a power strip with protection). It also has an on/off switch which I will switch off. I will switch it on while I am looking at it and can see if the light comes on (it has a tiny led).

Waterworld looks nice in HD from Netflix, by the way.

I do not have a bigger fuse, I bought an assortment that tops out at 2 amp. I probably should have purchased the assortment that goes down to 3 amps. They are an odd small glass body fuse. I may try a bigger fuse but I will probably pull the amp out of the speaker box first.

Jim

[JimD]

I tried disconnecting the input, the power supply to the amp, then reconnecting the power (but leaving the input disconnected). The buse still blew. I tried it more than once with the Min-Max adjustment in different positions. When none of that worked, I pulled the amp from the box. It looks OK but still blows fuses.

I also measured the resistance of the driver. It is about 6 ohms. And it makes an audible thump when the resistance tester is hooked to it.

So the good news is the driver seems fine. The bad news is I need an amp. For tonight, my son hooked up his powered sub. It is not as big but should be far better than nothing.

Thanks for your thoughts.

Jim

[wbsettle]

I built a subwoofer in a ported box to go beneith the floor in my family room and shoot out a fake heat vent. I houses a 12 or 15 inch sub (I don't remember, it's been several years) and is powered by it's own 100W amp. After getting my new Samsung 6300 LCD (LED) TV and LG 5700 blu-ray (WiFi) going, the bass didn't sound right so I checked the fuse on the amp (requires taking a panel out of the shop ceiling to see the back of the amp). It used to reset itself if I turned it off and then back on so I thought that might be the issue. It didn't help so I checked the fuse. It has a 2A which was blown. I got a new fuse which also blew instantly after installation.

Is there something I should check before replacing the amp? PartExpress has a replacement for about $100 which doesn't seem bad to me. Other ideas? It's a little pain to work on because of the location but I can take the amp out to look at it better if I need to. Probably a good step before placing the amp order.

Jim

The fact that you seem to have experienced multiple "self protection resets" suggests you've been pushing that poor little amp pretty hard. You've likely been killing it slowly (heat is the main enemy)...last rites confirmed by the testing in your last post.

Do you have any more info about the driver and alignment you used? If you'd been reaching the driver limits with the 100 watt amp, you should have noticed a "clanking" as the driver bottomed out or other sounds of mechanical distress. The lack of such suggests you're amp limited, which is how I prefer to design subs. However, you could be *too* limited if the driver/alignment is capable of meeting your needs, but the amp can't get near the driver's limits. With the driver and alignment info, I'd be happy to run some models to suggest a path forward (different amp and/or alignment changes possible).

-Brent

[JimD]

Brent,

Thanks for the offer. It's been too long since I designed this, however. I think it's a Pioneer driver but I don't even remember the size for sure, let alone the driver parameters. It is a ported box about 4 ft3 in size but I do not remember the 3db down point. It's around 30hz, maybe 25hz.

I did a little google searching for a new amp. Best deal right now seems to be a Dalton 240W (into 4 ohms) for $99. I don't think the driver will take that much power but I wouldn't turn it up that high anyway. It has an adjustable crossover. It also has a 6db bump at 30 hz that I would rather not have. The same source has a 100W for only $10 more without the bass boost and a 240W for a bit more that doesn't have the bump.

The main thing I want this to do is add some impact during movies. I don't like big bass bumps, I think they sound silly in music reproduction, but this low maybe it wouldn't be so bad for movies.

Jim

[wbsettle]

Brent,

Thanks for the offer. It's been too long since I designed this, however. I think it's a Pioneer driver but I don't even remember the size for sure, let alone the driver parameters. It is a ported box about 4 ft3 in size but I do not remember the 3db down point. It's around 30hz, maybe 25hz.

I did a little google searching for a new amp. Best deal right now seems to be a Dalton 240W (into 4 ohms) for $99. I don't think the driver will take that much power but I wouldn't turn it up that high anyway. It has an adjustable crossover. It also has a 6db bump at 30 hz that I would rather not have. The same source has a 100W for only $10 more without the bass boost and a 240W for a bit more that doesn't have the bump.

The main thing I want this to do is add some impact during movies. I don't like big bass bumps, I think they sound silly in music reproduction, but this low maybe it wouldn't be so bad for movies.

Jim

Agreed. With a ported box, you're not likely to want the SA240 with boost...the SA240B is intended to work with the natural rolloff of a sealed box. Your 6 ohm meter reading suggests you have an 8 ohm nominal driver so the SA240 will only produce about 170 watts into that load; shouldn't be a dangerous power jump over your current amp (depending on how it was rated). Especially since you likely killed the current one by running it at its operational limits fairly often, which is even more likely if the 100 watt rating is for 4 ohm loads, making it's actual delivery to your 8 ohm driver in the 50-70 watt range. Get the flat SA240 or the BASH300, both available from Parts-Express (and through Amazon). Set the SA240's lowpass to its highest frequency or use the BASH300's LFE input and let your receiver do the bass management.

Jack Hidley, formerly of NHT, has some surplus gear, including Foster plate amps for $35 + shipping. Suitability really depends on what your current amps real power delivery is. Jack's Foster amps are an honest 115@4/80@8...he also provides instructions for bridging a pair for 220@8.

If you're using the receiver's bass management, you don't necessarily have to go with a "subwoofer plate amp". A tour of your local pawn shops/Craig's List might turn up an inexpensive receiver with suitable power. You'll likely lose the auto on feature that most plate amps have now, but you could end up with a really good "amp" for $20 based on some of the finds people have made over the years.

-Brent

[sailor55330]

Very good advice from all.

You mentioned that the amp is older. Another possibility, although slight, is that with today's newer soundtracks being more and more demanding, the amp may have worked itself to death, with heat being a primary culprit as was mentioned. I'm curious, do you know if the 100w rating was continues or peak? If it was peak, then there is a good chance that the amp may be begining to fail as that is a fairly small amp for a driver that size. Also, you mentioned that the amp was rated at 4 ohms. It's been a long time, but I seem to remember that most home theater equipment run with 8ohms resistance, not the four that you mention (unless a dual voice coil sub is used). This could generate further heat and stress to the amp. The exception to that is if you use a monoblock amp designed for bass applications, then a 2 ohm load is fairly common, although the heat sinks are designed with that in mind. If you do purchase a new amp, I would consider one that has a minimum of 250-350 watts continuous and peak as high as you can get (as an example, my Velodyne 12 has 300w continuous and peaks to around 700 I think and it has never clipped regardless of how hard I push it). Also, try to find one with a THD of .01% or less at rated power for a cleaner signal. I would definitely try to avoid an amp that has a "bass boost" as that is only trying to compensate for a driver's shortcomings in my opinion. Lastly, since you have a ported enclosure, your efficiency has been increased, with the potential sacrifice of sound quality. Ported usually leads to "boomier" base while sealed is tighter and more controlled. I'm sure the manufacturer gave the port sizes so hopefully that is not a problem.

Then again, let's hope it's a fuse.

[JimD]

Brent,

Thanks for the tip on the $40 plate amps. I just ordered one (I got version 2 without the 18db rolloff starting at 29 hz). The price is certainly reasonable and I think it is enough power for me.

I don't know if I overworked the old amp or not. It was a really cheap amp. I bought some cheap speakers that I use in bedrooms where we are not listening critically, we just want to be able to play the radio. They came with a subwoofer. It was an 8 inch in a sealed box. Really cheap driver. So I pulled the amp and put it on my much bigger driver and box. The amp looks like a typical plate amp but it does not have adjustable crossover. It may have been designed assuming the impedence of the 8 inch driver in its sealed box. In that case I may have overworked it but not by playing things real loud. You can always talk over top of my listening level unless it is one of the really loud parts of the music or movie (sometimes you might have to raise your voice some but not really shout). I might have been OK with a 25-50W amp but it's nice to have some headroom to avoid clipping. My front speakers have about 90db sensitivity. My rears are less efficient so they are set to get 3db more power from the amp. The sub was plenty powerfull, I had it set to -11dB power. The pot on the amp was turned to max, however. We had to turn the receiver balance up with my sons sub to about the same as the front channel speakers. I may have been fighting some big bass boost in the circuitry of the old amp.

I'm probably fortunate the old amp lasted as long as it did (~5 years).

Jim