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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?

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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



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classdstreamer OP
160 posts
 
Thanks, almarg!
erik_squires
15,657 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.
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