Hi ZeroZero, this is very strange, particularly if you're not using the built in amp. If you're bypassing the internal amp, are you able to do it by passing signal directly to the driver from the external amp? If so, then I can't think of why it would be cutting out, unless the external amp was the problem. If you can connect the driver directly to the external amp, then you can unplug the power to the subwoofer completely.

The fuse (in the internal amp?) blowing when the correct one in will be because it's drawing too much current. If it was a complete short, then I'd think a 5 amp fuse would blow too (but I could definitely be wrong there). Maybe it's a problem with a voltage regulator? But again, all moot if you are bypassing the internal amp.

What happens when you use the internal amp? Do you get the same issues with the fuses when only using the internal amp? (a good basic test would be plug the sub into the wall, and plug an mp3 player to the inputs on the built in amp - this way you're guaranteeing a very simple singnal chain and only one AC power source etc).

If you decide to take the sub apart, my instinct would be to try disconnecting the driver and then running a speaker wire direct from the driver to the speaker out on your external amp and see if you get consistent performance.

I'm not sure if this is much help :)! This is definitely a strange problem. Best of luck!

food for thought - thanks

rmsaudio has given you some things to try, but what you haven't told us is the make and model of the sub.

I don't understand why the sub needs to be plugged into the mains at all if you are driving the speaker from an external amp and not the internal one. Are you sure that the socket you are using for connecting to the speaker is not meant to be an output from the internal amp to drive a second passive speaker? If it is, you would be back-driving the internal amp output with your external one, and it would not be surprising that it kept cutting out and also taking out the fuse.

Are you the sax player in the summerhouse in St Albans?

the make is Tannoy its a fifteen inch all it says is 5.1 sub 15 on the back no model number. on the back are six XLR baLANCED inputs only one of which I am driving -from my Sony str-DB780 amp ( ddicated phono type out for the sub. Ther are level controls for each speaker fiter switches and a phase switch, plus terminals for the other speaker outputs to be hooked to if required. There is a seperate power section with a kettle lead type female a power switch and a light,

and yes I am

OK, I understand now. You are driving at domestic line level into the amplifier input of your sub, so you are in fact using the sub's internal amplifier. On a quick look on the Tannoy website, I can't find any active 15" subs that have a row of 6 XLR inputs on the rear, either in their current range or in their archive, so I'm no closer to identifying the model.

If it's similar to other subs such as the KRK range, the sub houses the crossovers for both L and R channels of mid-top speakers, and itself takes the lows from both L and R inputs and combines them as a mono LF channel. Tannoy used to have a range of active subs that contained class D 100W amplifiers, and yours could well have one of these inside it. Class D amps can be prone to cut-out (see separate thread on problems with the Yamaha Stagepas 300 system), so it could be simply that you have a faulty unit.

One of the controls on most active speakers is a time-out, controlling the switch-off of power after a period of what it thinks is inactivity. Usually this can be disabled. It's worth seeing if your Tannoy has this enabled, because it could be that driving the line-level XLR inputs with -10dBV domestic levels is not triggering the keep-alive circuit, although I would think you would get little LF output from the sub if this were the case.

If I am using the amp then I must be using both amps for the sub - seems unhealthy then. Maybe I should hook up the sub directly to the sound card (edirol FA101).

thanks for the input

You would have to describe exactly how the sub's input and output connectors are labelled to be certain, but it's likely that it's wired internally the way I described for the KRK: L + R full-range inputs, L + R high-pass outputs and the single mono sub driven from a sum of the L + R inputs low-pass filtered.

I have managed to locate the specs for your Sony AV receiver/amp, and I see it has a pre-processed mono sub output at a 2V level. This is fine for taking into one of the XLR inputs, but how have you got it wired? It should be signal to pin 2 and ground to pins 1 and 3. If you are getting enough subwoofer level it is probably OK as it is, but if not, you could wire a cable to the other XLR input (L or R as appropriate) in parallel with the first, and it should give you an extra 6dB of gain.

It's sounding more like there is a fault with the internal amplifier in the sub.

As a matter of interest, what was the "correct" mains fuse rating?

thanks for the input Boswell. Yes, when working the sub gives very good repsonse. There is a low pass filter control on the circuit too. I may have been misleading about the fuse rating. The back says 2.5 amp but this I think might refer to an internal DC fuse rather than the mains fuse. I am behinning to suspect an issue with the power supply unit. This is a seperate circuit which has the light, mains switch and kettle lead input - maybe this is dicky?

As for the wiring, all I can say is that it is a standard phono (from amp) to XLR lead of good quality which has worked before and does function when the speaker works. I dont know how its wired, both ends are sealed units.

It's worth buzzing out the XLR - phono lead using a multimeter to check its wiring, although it sounds as though that's not your problem.

I would certainly think that a 2.5A fuse has to be a 20mm internal one, as there is no such UK mains plug fuse rating, and in any case, Tannoy products are sold worldwide, including into former colonial territories that are not sufficiently enlightened to have fuses in their mains plugs.

Your suspicion of a power supply problem may well be correct, but, whether the trouble is in the internal amplifier or the power supply, if you want to get it fixed, unless you can find someone locally who is prepared to take it on, it's looking like a trip to the Tannoy service centre in Lanarkshire.