Persistent Rattling Noise in Neumann KH750 DSP Subwoofer – Need Advice

[Mimeophon]

Hi everyone,

I’m reaching out to see if anyone else has encountered similar issues with the Neumann KH750 DSP subwoofer and if there are any recommendations on how to address them.

About a year ago, we set up our studio with a Neumann monitoring system, including the KH750 DSP subwoofer, two KH120 II monitors, and the Neumann Monitor Alignment Kit. The setup initially performed flawlessly, but after a few months, we started noticing a rattling sound from the subwoofer when playing low frequencies, especially around 30Hz.

I should also mention that the sub is not in protection mode when this happens; the LED on the back remains green.

Here’s a link to a Dropbox folder with videos showing the issue:
Dropbox Link

The first time we encountered the issue, we contacted the seller, and the subwoofer was sent to Sennheiser for warranty repair. The report indicated that both the subwoofer and the mainboard were replaced. However, a few months later, the rattling returned under the same conditions. We sent the subwoofer back again, and this time the woofer itself was replaced. After several weeks, we received the unit back, but it seems that there is still some rattling noise when playing a low frequency signal trough it loud enough.

We’ve tested the subwoofer using a 30Hz sine wave from Ableton, where the rattling is most noticeable. Our KH120 II monitors don’t show any similar issues, so the problem seems isolated to the KH750 DSP subwoofer. The sub is placed on a stable surface in a well-treated room, so we don’t think external vibration is the cause.

Has anyone else experienced this issue with the KH750 DSP? If so, how did you resolve it?

Any advice or insights would be greatly appreciated! We’re at a bit of a loss after two repairs with no real change in the situation.

Thanks in advance for any help!

 

[Audio Science Pal]

The first time we encountered the issue, we contacted the seller, and the subwoofer was sent to Sennheiser for warranty repair. The report indicated that both the subwoofer and the mainboard were replaced. However, a few months later, the rattling returned under the same conditions.

So, it was okay for a few months after the first repair?

We sent the subwoofer back again, and this time the woofer itself was replaced. After several weeks, we received the unit back, but it seems that there is still some rattling noise when playing a low frequency signal through it loud enough.

From an outside perspective with my limited information and listening to your Dropbox video on my laptop's tiny speakers, I sort of wonder if the subwoofer has possibly been over-driven, causing the voice coil to bottom out? But, it could be something else. Can you send it back again?

 

[amirm]

Sounds like a bad woofer to me. Pretty nasty stuff.

Just to isolate something else, and I know it is not easy, but are you able to shake the sub and see if anything makes noise in it when it is off?

 

[restorer-john]

It sounds like a damaged voicecoil/former to me. Possible delamination of windings.

 

[Doodski]

I'm with @restorer-john on a damaged voice coil. If you use a non-stepped frequency sweep you can get right to the exact frequency where the noise is the worst.

Online Tone Generator - generate pure tones of any frequency

szynalski.com

 

[Wolf11Man]

You are not allowed to open it without loosing your customer rights.
Return it to the seller and ask for a refund if they are unable to get a permanent fix. They don't take your complaint serious.
Probably they didn't change any parts with the second repair and just want to push you over the warantee time, as the budget for a repair is exhausted. Most service companys don't get any money from the manufacturer if the same fault returns. They don't repair in such a case, but store it in a corner for a month or two, then send it back as it was. This is common practice.
Maybe have it play near clipping level for a while, if it is the voice coil, it will get worse until it stops working and they can't return it with "no fault found, customer is stupid".
On your side, the next step after the 3. repair attempt is refunf or you have to talk to a lawyer. This is time sensitive!

If it is not the voice coil, which equals a defective driver, it sounds exactly like the wires to the voicecoil tapping the cone. In this case it will play full volume for hours without any problem, just the anoying noise.
The service guy may just have bend them back a little, giving a temporary fix, until the wire returned to it's favorite position. I repeatedly had such problems with large drivers.
If you know what you do, simple to fix without any cost, but that would mean you are a speaker specialist with years of experience. A hint: Usually a drop of glue will do. The cause is the wires are just a little bit too long, maybe a few Milimeter. Good subwoofer driver have these wires fixed on the spider. The 750 is a budget product by Neumann's.
Anyway, you are not allowed to open it by now. If the warantee is expired, give it to someone who knows such stuff, like a speaker builder.

 

[Doodski]

Probably they didn't change any parts with the second repair and just want to push you over the warantee time, as the budget for a repair is exhausted. Most service companys don't get any money from the manufacturer if the same fault returns. They don't repair in such a case, but store it in a corner for a month or two, then send it back as it was. This is common practice.

This not common practice. Stop making up rubbish.

 

[Wolf11Man]

Obviously you don't know how things are handled in the industry today. Maybe get out of your chair, and have a look at the real world outside, in 2024...

Neumann/ Senheiser have no own repair facility in any country. They have independend service centers, that do not get individualy paid for any case, but have general agreements for each product. Usually they have a stock of free service parts that is calculated on the average fault rate of anything sold. The simpler the repair, the better for them. A second repair of the same item is not paid for, they have to fix it for free. If the stock of replacement parts is exhausted, problems start. Also, after warantee ends, leftover spare parts are used for customer paid repairs. That makes them something they don't like to waste on a returning case. They have fixed maximum return times for the first repair, a second repair can take longer, as the industry now partly blames the customer for the defect.
This is very complicated and not very customer friendly practice, the large brands don't want you to know about. Samsung, LG or whoever doesn't fix their own products any more, but it is done by people that are paid less and promise to do it cheaper.

If you had something fixed by a repair man and it breaks down again with the same fault, a week later, you would not like to pay for it a second time, but insist on a free repair. Neumann does just the same and the repair guy is not happy about that.
These independend repair centers are a "we can do cheaper" industry, with the lowest offer getting the contract. They have a constant fluctuation of personel and no skilled worker will stay there for long.
Something from the chapter "what we don't want to know about". Like sausage making...
It explains why you are unable to talk to a real technician today and only have some idiot on the phone, reading from a computer program possible, but mostly useless fixes to you. "Take out the mains plug and wait for two minutes..."

 

[Doodski]

Obviously you don't know how things are handled in the industry today. Maybe get out of your chair, and have a look at the real world outside, in 2024...

You are very biased and obviously are making up facts.

after warantee ends, leftover spare parts are used for customer paid repairs. That makes them something they don't like to waste on a returning case.

More rubbish.

If you had something fixed by a repair man and it breaks down again with the same fault, a week later, you would not like to pay for it a second time, but insist on a free repair. Neumann does just the same and the repair guy is not happy about that.

90 days repair warranty has been a high standard for several decades or more. This is nothing new.

You seem to have an ax to grind and are bent on making this situation worse than it already is.

 

[Old_School_Brad]

While it’s true that third-party repair services encounter challenges like those mentioned, some aspects may be somewhat exaggerated.
I've earlier had both good and bad experiences with this. So YMMV.

 

[Doodski]

While it’s true that third-party repair services encounter challenges like those mentioned, some aspects may be somewhat exaggerated.
I've earlier had both good and bad experiences with this. So YMMV.

With a 90 day warranty period on all previous service work the rate of parts use actually goes up and not down as the commenter alludes to. It's all grinding the ax.

 

[kyuu]

You are not allowed to open it without loosing your customer rights.

This is incorrect, in the U.S. at least. Manufacturers cannot legally refuse to honor a warranty just because you broke a seal or otherwise opened up the warrantied item. They certainly do like to scare you into thinking that's true, though.

But of course, if the item is under warranty generally the best thing to do is have the manufacturer make it right rather than trying to fix it yourself.

 

[Mimeophon]

Thank you for your responses. I spoke with the importer for Neumann and consulted a friend who has more expertise with speakers. They suggested that I might be pushing the subwoofer too hard by sending a very low and loud sine wave into it, and that perhaps one KH750 simply doesn’t have enough power for my needs. Adding a second subwoofer would give me an extra 6 dB in output. Given the issues I’m having with this one, I’m hesitant to buy another KH750.

My question is about the built-in limiter on the sub. I don’t understand how it could have been damaged, as I assumed the limiter would protect it from receiving an excessively loud signal. Shouldn’t the limiter prevent this kind of problem?

 

[574stereo]

A 10" driver isn't large enough to reproduce sub-bass frequencies at elevated levels.

If you add in a second sub and have them placed next to one-another to get proper coupling you should gain between 5.5 and 6dB of additional level. That'll make a huge difference to how hard the driver is working (6dB less level is dramatic)...

 

[Old_School_Brad]

A 10" driver isn't large enough to reproduce sub-bass frequencies at elevated levels.

If you add in a second sub and have them placed next to one-another to get proper coupling you should gain between 5.5 and 6dB of additional level. That'll make a huge difference to how hard the driver is working (6dB less level is dramatic)...

Getting an additional sub will give +3 dB. Not 6.

 

[Wolf11Man]

You should get 3dB for double input power and 3dB for double cone area I have heard.

 

[dshreter]

Are you encountering this issue in ordinary use or only when playing pure tones? And how loudly are you playing this tone - is it near the limits of the subwoofer’s capabilities?

I’m curious if what you’re hearing is the excursion limiter kicking in to protect the driver from damage. That’s different from thermal protection and soft clipping of the amplifier.

 

[Old_School_Brad]

You should get 3dB for double input power and 3dB for double cone area I have heard.

Yes, a 6 dB boost is theoretically possible if both subwoofers are positioned identically and in-phase, which maximizes constructive interference. However, in practical terms, it’s often more effective to place the second subwoofer in a different location within the room. This placement strategy is aimed at smoothing out the bass response rather than relying on constructive interference, which can vary significantly with room acoustics and positioning.

Constructive interference -where the sound waves from both subs reinforce each other -is not always desirable. It can lead to excessive emphasis on certain frequencies due to room modes, which might actually worsen the overall sound quality. In my experience, adding a second subwoofer typically results in a reliable 3 dB boost due to the doubling of power output alone. This gain is more consistent and manageable than trying to achieve the ideal 6 dB boost, which is highly dependent on exact positioning and room-specific factors and the risk of missing the opportunity to fill in dips in the response.

As a result, if you're not positioning your subwoofers together (which is typically not recommended for a living room setup), you can expect a consistent 3 dB boost. This is also what most power and SPL calculators for multiple speakers are based on.

 

[574stereo]

The ~6dB gain only happens if they’re placed right next to each other. Spacing the subwoofers apart might allow you to even out the frequency response at the listening position but you’ll gain a lot less level. 3dB is an approximation but it’s usually less than that.