Low Frequency Rasping

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Guys I wonder if anyone can help me. I have a set of B&W 685's running off a Harmon and Kardon HK610 amp. Whilst I love the setup in general there is one really annoying issue: when running electronic music with heavy low-end sinusoidal bass lines the speakers give out a subtle, but very annoying, rasping sound. Sounds a bit like someone spitting water out of a trumpet, or a piece of plastic rattling on a windowsill.

It only happens with these types of tunes: Aphex Twin (Syro); Inception OST; Adam F etc. My brother in law thinks it might be that the amp is underpowered to handle the bass at high volume, but I would be interested to know if anyone else had any thoughts?

Cheers. Peace.

 

[tinkywinkydipsylalapo]

Could be the amp clipping as it (literally) runs out of energy, but the symptoms you describe sound more like the mid/bass units in what are after all quite small speakers 'hitting their endstops' - ie the voice-coils are reaching maximum excursion, due to the low frequencies and the high levels at which you're playing them.

If it's the latter, and I suspect it is, then the only solution might be larger speakers with better bass extension.

 

[The_Lhc]

Bad case of port chuffing perhaps?

 

[tinkywinkydipsylalapo]

Bad case of port chuffing perhaps?

Did consider that, but it sounds rather more chuffing serious than that ;-)

 

[hybridauth_Facebook_100000974267673]

So we don't think it's a power mismatch between amp and speakers? That's a pity, as I would much rather buy a new amp than new speakers!

 

[chebby]

It's a 30 Watts per channel amp, I assume you have the bass control turned up and you have the amp turned up loud enough to cause distress at frequencies the 685s are designed to roll-off, (-3dB @ 52Hz and -6dB @ 45Hz).

You first need a more powerful amp (think something like 80 - 100 Watts per channel continuous into 8 Ohms) to avoid clipping and then consider bigger and more efficient speakers with bigger bass/mid drivers.

If you are using the bass control a lot then consider adding an active subwoofer instead.

 

[Vladimir]

Time for a bigger amp. I recommend the HK680 from that series. They are not difficult to find on ebay for under £100.

 

[tinkywinkydipsylalapo]

Christopher Hooper[/size]]So we don't think it's a power mismatch between amp and speakers? That's a pity, as I would much rather buy a new amp than new speakers!

Well, if it's clipping, then a more powerful amp will help; but if these noises have been going on for a while then there's a chance the voice-coils in the speakers may have been damaged.

 

[Vladimir]

So we don't think it's a power mismatch between amp and speakers? That's a pity, as I would much rather buy a new amp than new speakers!

Well, if it's clipping, then a more powerful amp will help; but if these noises have been going on for a while then there's a chance the voice-coils in the speakers may have been damaged.

B&W 685 without a sub is not the ideal speaker for the genre of music such as Aphex Twin, especially on a 30Wpc amp. The whole setup is wrong IMHO.

Chris, I think you should get an amp with a subwoofer out option + an active subwoofer. These don't have to be expensive or new, any would do the job at releiving the B&W 685 from strugling with extreeme bass. Let the sub do the work.

 

[Covenanter]

Turn the volume down?

I'm not being wholly facetious here. Kit works best in the middle of it's specified range and not at the ends. If you have to push the kit hard to get the levels you want then you have the wrong kit. I don't know your music so can't comment on what kit would suit it and others here are better placed to advise. But it does seem that you need more powerful kit.

Chris

 

[hybridauth_Facebook_100000974267673]

Thanks guys. I would tend to agree with the diagnosis of amp clipping. Turning it down is an option - although not preferred!

I may well go for a new amp - 50W per channel, something like that. Any recommendations for manucturers? I like very clear/authentic reproduction (and no, I never touch the EQ controls!).

 

[davedotco]

In my experience a 'rasping sound' does not usually indicate clipping.

It is more likely caused by the voice coils rubbing in the gap due to being driven out of their linear range on the low bass notes. If this is the case, then a new amplifier will not help.

You will know if this is the case when the rubbing wears through the laquer on the coils causing shorting and bass unit failure.

 

[slice]

Hi

Why not borrow more powerful amp from a friend or workmate and see if it still happens. If it does, it will be the speakers at fault, I suppose.

 

[Vladimir]

In my experience a 'rasping sound' does not usually indicate clipping.

It is more likely caused by the voice coils rubbing in the gap due to being driven out of their linear range on the low bass notes. If this is the case, then a new amplifier will not help.

You will know if this is the case when the rubbing wears through the laquer on the coils causing shorting and bass unit failure.

That is easy to test. Just push the membrane at the center area (not pushing the dustcap) back and forward slightly and if you hear scratching sound, the voice coil has come off and needs servicing or a new driver.

 

[Vladimir]

Thanks guys. I would tend to agree with the diagnosis of amp clipping. Turning it down is an option - although not preferred!

I may well go for a new amp - 50W per channel, something like that. Any recommendations for manucturers? I like very clear/authentic reproduction (and no, I never touch the EQ controls!).

Rotel RA-1062 would be a good match IMO.

 

[davedotco]

In my experience a 'rasping sound' does not usually indicate clipping.

It is more likely caused by the voice coils rubbing in the gap due to being driven out of their linear range on the low bass notes. If this is the case, then a new amplifier will not help.

You will know if this is the case when the rubbing wears through the laquer on the coils causing shorting and bass unit failure.

That is easy to test. Just push the membrane at the center area (not pushing the dustcap) back and forward slightly and if you hear scratching sound, the voice coil has come off and needs servicing or a new driver.

Nah, probably won't help.

Any rubbing is clearly only happening at extreme excursions, impossible to check as you suggest, as it is extremely difficult to move the cone and coil that far by hand and in a linear fashion.

Difficult to be sure from a distance, any speaker engineer or even a competent dealer would know in seconds on hearing the actual fault.

To me it simply sounds like the system is being asked to deliver deep bass beyond it's capabilities, the simple answer is to use a subwoofer and filter the main speakers below about 75hz.

 

[Jota180]

Sounds similar to an issue I had with my KEF LS50 on certain tracks basslines when played at volume. The bass seemed to merge and accumulate into a big, unfocussed droning noise that sounded rough.

I had Nexus 6 stands that had a bit of 'play' backwards and forwards and couldn't adjust this out of them. They could rock back and forward easily with not much effort from my hand. Anyway, I bought Custom Design stands and one thing I noticed immediately after putting one together is how solid it is.

There's no play in them at all and no rocking back and forward. The bass on previsouly troublesome tracks are now tight and I don't get anymore droning.

This may not be the same as your problem but check your speaker stands (if you're using them) and see if you can get some easy movement.

 

[Tannoyed]

If like me, you like your music loud you really need a decent power amp. The Pioneer AS500 is on sale at a really good price these days and gives a lot of bang for the money (£200, 70W per channel). I have one and I love it but obviously it is not the most exotic amp around.

If you use an amplifier so that it is clipping and it seems likely that you have, you will only start to hear this when it is severe. Using a 30W amp to drive speakers that require more is more damaging than using a high power amp (sensibly) to drive low power speakers. Clipping starts when the amp is going flat out and there is simply no more voltage available. If the power supply to the output stage is +/- 30 V say, the output transistors can never reach this point and so the maximum useable swing will be about +/- 25V before non-linearity creeps in.

Into an 8 ohm load this gives an rms power of 39W maximum. As you turn up the volume control still further and if you look at the waveform with a scope you would see a solid line forming at the top and bottom as the amp spends more and more of its time at the extremes of its travel (voltage wise). The power output still rises because the form factor of a square wave is larger than a sinusoidal one. Taken to extremes this would become a square wave. Ragged waveforms of this kind contain high energy odd harmonics of the fundamental, extending way up the frequency range. It is these that cause damage, usually to the tweeter units. It starts long before you hear it but eventually it sounds awful!

As Vladimir has said, gently pushing the cones of the bass drivers without touching the dust covers will reveal any distortion of the speech coils which can rub on the magnet pole pieces when this has happened. You will hear a rubbing sound. When this happens the distortion you hear is worse at low levels than at high levels because it is in the mid range of travel (where the cone spends most of its time at low level) that the rubbing is usually (but not always) most noticeable.

Push evenly and gently, applying pressure evenly at several points around the cone. It is possible to bias the cone to one side so that the coil rubs because of this rather than an inherent fault.