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Help! High level subwoofer input on a class D amp introduces ground loop.

I know my opinion diverges from the seeming majority of swarm sub guys on this forum, but I believe that (1) subs are very important for stereo music, and (2) connecting subs via their high level inputs is preferable for stereo music.

In my limited experience, the difference between high and low level inputs isn’t subtle. Through the high level input, the sub disappears into the mains. Full disclosure, my data set consists of two different, low end subs. So, yes, I am extrapolating from my limited experience to conclude that high level connections will also be best for other subs as well. To speak to the swarm guys, I do understand that multiple subs produce smooth sounding bass; I just want my bass to be well integrated too.

The wrinkle with regards to the high level input on a class D amp is the introduction of humming from the sub. I notice that when I set the sub to auto (where the sub turns itself off after X minutes of non-use), the sub doesn’t turn itself off anymore. It remains on with a slight hum now. In fact, the sub was on all night and seems to have knocked out my internet, which was on the same outlet as this sub. The internet promptly came back when I turned the sub off manually.

My sub is not a REL, but I can see that REL has instructions on how to use their high level connections with class D amps (https://rel.net/connect-rel-class-d-gear/). If I understand their article, REL recommends achieving ground either by running RCA from the preamp to the sub or attaching the (-) speaker cable to the turntable ground like the ones found on the back of NAD and Peachtree integrateds. I currently run a Peachtree, but it is an older model without the TT ground.

Correct me if I’m wrong, but here’s my understanding for connecting high level to a sub with 5-way binding posts on an integrated that doesn’t have a TT ground: run speaker cable between the amp and sub for the (+) of the R/L and don’t run anything for the (-) of the R/L, then run an RCA connecting to the (-) on the pre and sub. How’d I do?

128x128classdstreamer
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almarg's avatar
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almarg
9,670 posts
 
I currently run a Peachtree, but it is an older model ....

Certain older class D amplifiers utilized class D modules which were designed such that both the + and - speaker-level outputs are offset from ground by a large DC voltage. Something like 24 VDC if I recall correctly. And I’m fairly certain that some older Peachtree designs utilized those modules.

While a large DC offset on both the + and - outputs won’t bother a passive speaker, powered subs should not be connected to the outputs of such amplifiers without a special adapter which inserts capacitors in series to block the DC. And it may be hard to find such adapters, as class D designs produced in recent years no longer have those offsets.

Depending on the designs of your particular subs it seems conceivable to me that the symptoms you have described could be the result of connecting them to such outputs without a suitable adapter.

My suggestion is that you contact Peachtree and ask them if what I have described applies to the specific amplifier model you are using. If so, the alternatives are to connect at line-level, or to try to find the kind of adapter I described, or to make such an adapter yourself.

Good luck. Regards,
-- Al



classdstreamer's avatar
classdstreamer OP
169 posts
 
Thanks, almarg!
erik_squires
16,056 posts
 
In my limited experience, the difference between high and low level inputs isn’t subtle. Through the high level input, the sub disappears into the mains.


This in my mind has a lot more to do with the crossovers than the connections. That is my goal, and I always achieve it using strictly signal level connections.

To speak to the swarm guys, I do understand that multiple subs produce smooth sounding bass; I just want my bass to be well integrated too.


I have tried for months to co-exist with swarm people. It's not possible, it is a cult and you either believe everything or you don't know anything. I suggest you lock your doors when you see them coming up your driveway.
classdstreamer's avatar
classdstreamer OP
169 posts
 
@erik_squires haha. The swarm guys do proselytize here.

To be fair to the swarm guys, I believe a swarm setup is the right compromise in a home theater space. In a HT space, bass needs to be similar across a larger seating area, and stress can be relieved from amplifiers that are running multiple channels by passing the low frequencies only to the subs. I’ve tried two subs in my HT room. Two was a big improvement over one. Really smoothed out the low end. 

But for my stereo system, I believe I can still get some of the swarm benefits for music by running stereo subs with high level connections.

ES and Al, what do you guys run? No sub(s), sub(s) via high level, sub(s) via low level?
gdnrbob
4,444 posts
 
@OP,
I am not ES or Al, but I have two systems running with 2 subs, one with a high pass filter (Vandersteen), the other running full output from preamp, and using the sub's crossover.
In my limited opinion, the high pass system is far more integrated, and it relieves the amp from all the demands of low frequencies.
Bob
millercarbon's avatar
millercarbon
12,531 posts
 
Stay away from those swarm guys. Physics is a cult, and if you don't believe the science they will be all over you! I tried for months to convince some of these zealots but they refuse to even look at a REL ad or even GIK! GIK! DSP! I'm telling you stay away they are nuts!
Some are so fanatical they actually research circuit diagrams and say things like your high level input has a resistor in series that lowers it to line level! Like anyone would believe such nonsense! Lock your doors! If you see one, run! Just run!
almarg's avatar
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almarg
9,670 posts
 
I’ve never used a sub or subs in my system.

The speakers I’ve used over the years have all been spec’d down into the mid to upper 20s of Hz, with my present speakers (Daedalus Ulysses) being spec’d down to 28 Hz +/- 2 db, and with the rolloff below that frequency being gradual. And I use the digital signal processing functionality provided by the DEQX HDP-5 I use as my preamp to introduce some small and narrow equalizations which mostly compensate for what I’ve measured to be fairly minor room-related peaks and valleys in frequency response.

Some users of Daedalus Ulysses speakers use subs with them, but my impression is that most do not.

Regards,
-- Al

classdstreamer's avatar
classdstreamer OP
169 posts
 
@ghnrbob Interesting. My had a high pass filter, where the speakers connected to the sub and only received the higher frequencies. The sub I now use (an older Jamo) connects like a REL where the speaker receives the full range of frequencies and the sub supplements the lower frequencies. Both subs integrated better via speaker cable than when connecting via RCA/Coax. 
classdstreamer's avatar
classdstreamer OP
169 posts
 
@millercarbon I love the pictures you have for your system. I imagine it sounds fantastic. If you could only run two subs, would you run them high or low level? Also, how many subs do you run? 4?
classdstreamer's avatar
classdstreamer OP
169 posts
 
@almarg I'l have to read up on your speakers. I hear equalizing software provides real benefits for music. 
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