How to connect monoblocks to a single REL sub with ONE RCA input

Sorry, I haven't quite got it but why do you need to attach the monoblocks to the subwoofer.I would use the y junction.What source are you using?

Hi David (hilde45),

First, congratulations on the purchases!

Second, regarding...

An alternative is that I get a Y cable that will bring two RCA outputs from my Preamp into the single RCA input on the REL?

... Don’t do that! It would result in the main speakers reproducing mono. And although I doubt that it would happen in this particular case, with some preamps doing that might even result in damage, eventually if not sooner, since you would be shorting the preamp’s outputs for the two channels together.

Manuals for a number of REL subs that I’ve seen in the past suggest that to connect their speaker-level inputs to monoblocks, when one sub is being used, what should be done is to connect the red wire from the sub to a + output of one amp; connect the yellow wire from the sub to a + output of the other amp; and connect the black wire from the sub to a ground point or a chassis screw on the **preamp.** I suspect that is likely to work fine, although depending on the particular designs it is possible that a hum will result. But give it a try.

Also, you don’t necessarily have to connect the red and yellow wires to the same output tap (i.e., the 4 or 8 ohm output tap of the amps) that you are connecting to the main speakers.

Good luck. Best regards,
-- Al

@pjr801 — the reason is because the standard advice for this REL is to connect it to the main speaker terminals on a normal (non-monoblock) amp. I am trying to follow that advice from REL but adapting it to monoblocks.

@almarg Thanks! I feel confident about these purchases.

So, before I ordered the preamp, I had Mike wire the "Tape Out" as a *second* main out. Right now, the preamp's MAIN OUT goes to each monoblock and the other (TAPE OUT -- now a second main out) is free. Does that fact change your advice? (I don't know why the main speakers would become mono by doing this, but that's just from lack of general understanding about how these are wired.)

Depending on what you say to the above question, I'll try the monoblock connection approach you suggest — and just listen for a hum. I'll also contact REL. It would be shame if I couldn't use the sub.

Or is there another alternative?

You can connect your preamp  second main out to a 
Stereo to mono signal converter
and connect one of the converter mono output to the REL 328 low level input. This converter utilizes an independent 1:1 ratio audio transformer for each channel, which also function as a ground loop isolator.

https://www.ebay.com/itm/MCM-CUSTOM-AUDIO-50-20795-Stereo-to-Mono-Signal-Converter-/164155969190?_tr...

Thanks imhififan -- I'll definitely keep that in mind if the high level input solution won't work. 

Best video's I've seen that are extremely informative:https://rel.net/how-to-connect-my-rel/

...  the preamp's MAIN OUT goes to each monoblock and the other (TAPE OUT -- now a second main out) is free. Does that fact change your advice?

No, it doesn't affect my comment.  Presumably the change that was made disconnected the two tape out connectors from the internal circuit points to which they are ordinarily connected, and connected them instead to the main out connectors (or to corresponding internal circuit points).  So if you were to connect a y-adapter to the two tape out connectors, to create a mono signal for the sub by shorting the signals on the two tape out connectors together, you would also be shorting the signals on the main out connectors together.  Thus an identical summed-to-mono signal would be provided to both monoblocks.

@imhififan 's suggestions are almost invariably excellent, and from what I can tell from the somewhat limited descriptions shown at the links he provided it looks like the device he suggested should work fine.

Best regards,
-- Al

@lak Thanks for the suggestions. It's because I watched all those videos but did not see an answer to my question that I didn't find them helpful enough. 

@almarg Regarding the primary suggestion you made, connecting the REL's speaker-level inputs to both monoblock amps and then grounding it on the PREamp, do you have a suggestion how to ground it? There is no dedicated ground on the Preamp -- e.g. no phono ground (because no phono stage). Here's a photo: http://quicksilveraudio.com/wp-content/uploads/2017/07/12-1024x293.jpg

Just back off on a chassis screw and put the small wire around it? 

Just back off on a chassis screw and put the small wire around it?

That would be what to try first.

If a hum results, and you are so inclined, you could then purchase an RCA plug, solder the black wire to the plug's ground sleeve connection (while leaving its center pin unconnected), and insert that plug into an unused RCA connector on the preamp (such as either of the "tape out" connectors). Depending on the design of the specific preamp that could very well provide a better ground connection for the sub than a chassis screw.

Best regards,
-- Al

Thanks so much. Sounds like a good plan B.

@almarg FYI, I just found old threads where you were discussing exactly this problem. I wrote to Quicksilver, but don’t have a clear answer (yet) about how my preamp is wired (for the low-level RCA stereo to mono fix) or how best to ground a high level connection (the Speakon cable approach). I got a very brief reply, but it didn’t answer the question. I also wrote to REL, to see what they say. I’ll try to relay what they say.

I am a little concerned that the high-level rig-up you suggest might feed back to my amp and cause a problem. It's a new amp and I'd hate to immediately hurt it!

The adapter mentioned above by @imhififan would be attractive if I knew it was not going to collapse the stereo field on the mains. I suppose that’s the point of that solution, but I’d like to know before I waste $50 or so!

I have a single REL sub connected to balanced mono-blocks as Al suggests and it works fine, I don't even need to connect the ground. While REL recommends against a single sub with mono blocks, due to hum issues, maybe I just got lucky...there is lots of information on the REL help websites about all kinds of connection issues...2 or 4 subs may be ideal but in some systems and rooms you can get great results with a single sub with careful placement and adjustments...I do use the upgrade REL cable

REL's response: "We would recommend connecting low level from your preamp to the R328 as the safest connection for your current setup. As long as it's safe to use a Y splitter out of the QS preamp, this will be fine for going into the single input on the sub."

I will comment that I tried an RCA out from my Streamer and while I could hear bass, it was definitely inferior to the high level connection.

@jl35 When you say "I do use the upgrade REL cable" you mean the "Speakon" cable that clicks into the sub and has 3 wires on the other end? I assume so but these subs can differ. 

Also JL35 (and Almarg)  -- are you at all worried about some kind of damage happening to the amps? @decooney mentioned this happened to him with a Cary amp years ago.

REL makes an upgraded version of the Speakon cable that is included with the sub...I've had this setup about a year with no issues...but I am not technically knowledgeable so can't advise

An alternative is that I get a Y cable that will bring two RCA outputs from my Preamp into the single RCA input on the REL?

Unless there’s a resistor in series to the preamp outputs, using a ’Y’ cable that bring two RCA outputs from your Preamp into the single RCA input on the REL is a big no no!
almarg already explained to you:

connect a y-adapter to the two tape out connectors, to create a mono signal for the sub by shorting the signals on the two tape out connectors together, you would also be shorting the signals on the main out connectors together. Thus an identical summed-to-mono signal would be provided to both monoblocks.

Don’t do that!!!

REL’s response: "We would recommend connecting low level from your preamp to the R328 as the safest connection for your current setup. As long as it’s safe to use a Y splitter out of the QS preamp, this will be fine for going into the single input on the sub."

I can’t understand why REL gave you such advise.

@imhififan I am so very glad to have this forum. I have no idea why they gave me this advice. 

The advice you gave -- the converter box -- would NOT cause the issue of the Y cable though. Is that right? (I know you recommended it but I want to make sure that I gave you a complete enough picture at that point.)

What a headache.

Update from Quicksilver: "I would connect the ground wire to the right channel amp ground just as they [I believe I quoted Almarg] suggest. If there is a hum then you might have to connect the woofer to just one channel that is disconnect the left + wire. A Y connector will mono the outputs on the preamp."

I was in the same position as you, owning one REL sub and utilizing a mono block tube amp. I realized that the value of having mono blocks is to completely separate the signal from the left and the right speaker, with no possible signal corruption as is possible with a stereo amp. As such, it made perfect sense to me to be able to separate the left and right signals that the sub sees into a separate left and right channel for the subwoofer! That is why i think REL suggest this set up ( dual subs with mono blocks) in the first place and it certainly works a lot better this way. I have a stereo amp ( balanced) and with that I use just one of the REL subs, it works fine, but note bass definition is definitely less in this set-up. 

Update from me: I tried the high level. Connected to both mono blocks. No hum. Works fine. (Will it last?)

@hilde45, re your latest update, what is the black wire from the sub connected to?

Also, re the suggestion from QS of connecting the black wire to the ground of the right channel amp, that is a reasonable suggestion. And since I’m pretty certain that your amp does not have outputs which are balanced or bridged, a suitable ground point would most likely be the negative output terminal of the amp. (The black/ground wire of a sub should not be connected to the negative output terminal of a balanced or bridged amp, since in those cases the negative output terminal provides a full amplitude signal rather than ground. Also, there are a few tube amp designs, such as some of those made by Audio Research, in which circuit ground is connected to the 4 ohm terminal, rather than to the negative/"common" terminal). Whether doing that would provide results that are better than, worse than, or about the same as would occur if you connected the black wire to a ground point or chassis of the preamp figures to be equipment dependent and hard to predict. My guess is that more often than not the results would be similar.

Re the mention by @jl35 of the possibility of not connecting the black wire at all, some other members here have reported doing that with fine results. And in fact I recall at least one such member indicating that REL had suggested trying exactly that. The reason that can work in some and perhaps many systems, considering that for an electric current to exist a "complete circuit" must be present from source to load and back, is that the AC safety ground wiring forms the return path in that situation. Again, though, how well that would work is equipment dependent, especially on how or if circuit ground and chassis ground (which in turn is connected to AC safety ground) are interconnected within the specific equipment.

Regarding your recent question about the device suggested by @imhififan , when I looked at the links he provided what particularly gave me a fair amount of confidence was the statement on the packaging that can be seen in one of the photos, that "it provides mono output while retaining L/R input isolation." I interpreted that to mean that the left input and the right input are isolated from each other.

Finally, regarding your question about the possibility of damage, in this particular case I don’t envision any such possibility for any of the approaches that have been referred to.

Best regards,
-- Al

@almarg 
Connections, Subwoofer speakon connectors to monoblocks:

Red to Right +
Black to Right -
Yellow to Left +

Regarding outputs that are "balanced or bridged" I don't know how to tell — but if it's indicative, there are only RCA connectors on the preamp's outs and they run to RCA inputs on the amps. 

Interesting point about the un-connected approach to the black ground wire. If I heard a hum, I suppose I could try disconnecting that wire.

I appreciate your taking a look at the mono/stereo device mentioned by imhififan. In some ways, it still seems like the safest way to go, but I can testify that I tried both high level and low level inputs tonight, and high level is far superior and it's drawing on exactly the signal being sent to the speakers.

I continue to admire the clarity and concision of your replies, and the time you take to research someone else's (my!) question. Thank you.

@hilde45  Is that set up the one suggested by REL? I used that exact set up on my prior ARC stereo tube amp, and it worked fine. I also use that basic set up on my ss balanced stereo amp...although I place the Black to a grounding connection on the amp...as I don't think the minus is a true ground. Works well too, but this is with just one sub connected, not two.

That all sounds good, David (hilde45). And looking at the manual for the amp in addition to its description at the QS website I'm certain that none of the exceptions I cited that would necessitate not connecting the black wire to a negative output terminal apply in this case.

And thanks for the nice words. Enjoy the new purchases!

Best regards,
-- Al
 

Regarding your recent question about the device suggested by @imhififan , when I looked at the links he provided what particularly gave me a fair amount of confidence was the statement on the packaging that can be seen in one of the photos, that "it provides mono output while retaining L/R input isolation." I interpreted that to mean that the left input and the right input are isolated from each other.

Thanks @almarg for clarify, the MCM converter is utilizes an independent 1:1 ratio audio transformer for EACH channel, so the left and right inputs are isolated from each other.

@almarg Great to hear. I certainly want to avoid any issues. I *assume* that if I start to hear a hum that I’ll just disconnect the setup. I asked QS in several emails about grounding to preamp chassis and that suggestion wasn’t even taken up in the (very sparse) replies.

@daveyf

Is that set up the one suggested by REL?

Not for monoblocks. If you look back at some of the REL links people shared above, you’ll see that REL suggests NOT connecting the sub's high level connector to monoblocks. They recommend using the "low-level" input. My problem (as described above) is that my preamp output is stereo and the REL I own only has a single RCA input.

 My sense is that the advice they’re giving here is somewhat inexpert and/or poorly researched. I am not confident that my question got to someone with sufficient knowledge and/or they didn’t really stop and think hard about the question. Their suggestion of a Y cable would (as QS, almarg, told me) collapse the stereo field for the main speakers. While I sympathize with REL’s predicament of having many customers with a wide variety of products, it would be better to say "we’re working on your question" than to give erroneous advice.

I used that exact set up on my prior ARC stereo tube amp, and it worked fine. I also use that basic set up on my ss balanced stereo amp.

I’m assuming by "same set up" you’re saying that you used the high level REL connector. The issues raised by my situation are critically different, insofar as they pertain to grounding the subwoofer’s connection on two separate monoblocks.

Not sure how popular this solution will go over but it will do what you want.

https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4

@djones51 -- Interesting. Just reading the manual. "3.8 MONO/STEREO MODE The 2 Way Advanced 21 plugin supports mixing of two output channels, allowing this plugin to be used in “2.1” applications where a mono subwoofer signal is derived from a stereo signal. To access this selection, click on the System Settings tab."

REL always recommends 2 subs for sonic reasons (not just more          bass). The only reason they state on their website for not using 1 sub with mono blocks is the greater chance of hum   

Yes you place between your pre and monoblocks, you can set the crossover if you want . You send stereo to the amps through channels 3 left and 4 right. The unit mixes stereo to mono output on channel 1 to the subwoofer. In the future if you get another sub you can send the second to channel 2 for stereo. Is also helps on integrating subs.

@djones51 -- Thanks. That may be helpful.
@jl35 Agreed. I got this $1800 sub used for $900 because it just wasn’t in my budget to spend more. It’s now hard to find. I might sell it and get two lesser subs (maybe smaller REL’s). But for my present room, this single REL is definitely powerful enough, though as you and others have pointed out, it’s not optimal.

the point i was trying to make hilde45,  is that one sub can sound great, and if you don't have hum issues, the mono blocks are not an issue...I have mono blocks and one REL and am very happy with the results...

@hilde45  I understood your OP as to the hook up concerns with mono blocks...as I had the same concerns myself with my mono blocks. It always made a lot of sense to me to have two(2) subs in this situation. REL suggest this for probably a couple of reasons...one is that the hook up isn't going to be an issue, secondly the potential for greater extension is also a factor. Two smaller subs will actually produce more accurate bass than one large sub, IME. Lastly, they get to sell another sub..:0)

@jl35 Got you. And that makes me feel good! Since I just got these amps, I will have a chance to live with the setup for a while and see how itis.

@daveyf That's a really interesting point -- two smaller subs. I read somewhere not to go smaller than a 10 inch woofer. I wonder if that's for any and all subs or for just one. And I don't blame REL for their advice -- it does seem optimal for sound and that's what they're known for. It is helpful to have some "escape hatch" for that person who wants/can afford *one* good sub rather than *two* lesser subs.

@hilde45  I have two (2) REL T5i's in my system. They are more than enough for my small room. I believe that two 8" drivers ( in the two T5i) are more surface area than one 10" driver. IME, the room size is the real factor as to how many subs and the size of each...not 10" driver or bigger, etc. 

Thanks, daveyf. I can't remember where I read that it should be at least 10, but my space is only about 500 sq feet so I bet your setup would work well. I'm sure we could start a shooting match over in "Speakers" if we tried...let's not.

@ hilde45
you should NEVER use the high level output. Especially on a tube amp.
Their use is strictly lofi. No professional installation uses anything like them. No matter what anyone says, they cannot produce accurate bass. Especially on a REL328 with no phase control. It's actually a polarity control.

See  http://ielogical.com/assets/SubTerrBlues/TubeAmpSpkrV.png and read  http://www.ielogical.com/Audio/SubTerrBlues.php and https://www.soundoctor.com/whitepapers/subs.htm

What you plan to do will be  H U G E  compromise over what is correct.

@ieales I appreciate your input and will look at the links. There are now a lot of different opinions out there. The amp mfr says it’s fine. REL prefers high level to their own sub if there are two subs. Otherwise, they say low level but have no answer as to how to sum the stereo signal to mono. Others on this thread say both.

Can you clarify what you meant by "Their" when you said:

you should NEVER use the high level output. Especially on a tube amp. Their use is strictly lofi.

Thanks.

@ieales   Cannot use the high level with a tube amp?...go to my system page and take a look at how this can be done...with no issues.

Ian ( @ieales ), thanks for the references.  The OP's amps have a specified damping factor of 20, which at least in theory corresponds to an output impedance of around 0.4 ohms for their 8 ohm tap.  That impedance of course being well below average for a tube amp, and almost approaching solid state territory.  And the Salk speakers he has or will soon have are specified as 8 ohms nominal, and I suspect don't go much below that value at any audible frequency.

Wouldn't such a pairing significantly ameliorate the effects you refer to in the section entitled "Connection" of your paper shown at the second of the three links you provided?  I recognize, of course, that those effects would still be present to some degree.

Best regards,
-- Al

“you should NEVER use the high level output. Especially on a tube amp.”

I am not sure the basis of this statement. I am using a pair of REL subs via its high level input with tube mono amps (8W SET’s). I hear no compromises in bass production or lack there off. 

@daveyf
which system page? please post impulse and phase plots. I had a look. I find the Sonus intolerable. The low end is just awful in every system I’ve heard. [I’ve never owned a ported system]. I didn’t care for Nordost speaker cables in any demo.

@almarg
The QS have low output impedance, but if one looks at response into a real load, it is nowhere near flat.

Additionally, current drives the loudspeaker and voltage drives the sub. For any non purely resistive load I & V differ by an ever changing phase angle. Hence, a sub driven from the LS out can N E V E R be time correct with the mains.

@lalitk

I hear no compromises in bass production or lack there off.

I'm sure there is plenty of level.

I must admit that Time Coherence is my grail and systems without it are uninteresting. IMO, frequency response is, and always has been, overrated.

I have a REL S/812 and two PrimaLuna EVO 400's as mono blocks.

this is what I received from REL

Thank you for contacting us, and for choosing a REL Acoustics subwoofer for your home audio system. While some older REL designs did allow for a single subwoofer to simultaneously be connected High Level to two monoblock power amplifiers at the same time, with all current REL models including the S/812 a single subwoofer should not be connected High Level to two monoblocks at the same time. Doing so will in most cases result in a significant ground loop hum which can be near impossible to eliminate, and in a worst-case-scenario can result in damage to the subwoofer and/or the monoblock amplifiers. 
 
When using a single REL subwoofer in a system that features monoblock power amplifiers, the best option will be to connect it to open outputs on your preamp using the Low Level input. This will allow the subwoofer to safely draw signal from both the left and right output channels, and since the Low Level input feeds through the same filter circuit as the High Level input you will still have full control over the S/812's Crossover dial for tuning it's output to blend with your main loudspeakers. 
 
In this system it is also possible to connect using the High Level input to one of either the right channel or left channel monoblock amplifier. This will of course result in the subwoofer only receiving either left channel or right channel signal, though since sub-bass content in many popular recordings is centered in the stereo mix, meaning that it is common between the right and left channels, it will still be possible to achieve a seamless blend with your main loudspeakers when connecting in this way. When connecting in this way to the Primaluna EVO400, please refer to the following instructions:

• Twist together the red and yellow wires of the subwoofer's High Level cable, and connect to the 8 ohm positive output terminal on one of the two monoblock amplifiers.
• Connect the black wire of the High Level cable to the negative output terminal on the same monoblock amplifier. 

 
I hope that this helps, but feel free to let me know if you have any additional questions or concerns. 

 

 

NEVER use high level outputs to drive a subwoofer. It's a Band-Aid that should be banned.

According to the REL S/812 manual, it has two low level inputs. You can simply Y off the main pre out if there are not two sets of outputs

Ideally, you should roll the lows out the mains with a passive HP filter. Doing so will tighten up the low end and effectively double the apparent power of the PLs

That being said, you should flog the REL and get a pair of subs with real controls. IMO, REL markets bling, not good bass and are not serious about proper system integration due to their dearth of controls making them impossible to integrate properly. e.g. 180° phase inversion is equivalent to moving the sub about 25 feet @ 40Hz. Fixed XO slope will only integrate properly in a tiny subset of rooms and systems. 

Martin-Logan X series and JL Audio are excellent offering full control.

 

 

Sorry but more than one sub in one room seems overkill.Can also lead to standing waves and cancellation not only not doubling bass ability but cancelling each other out in some frequencies.. All seems a bit of a problem really.

@linndec Don't be sorry if that's the way it seems to you. You're mistaken. At least for my room. And you understand that generalizing about this is pointless, since the room's particular dimensions are critical to the number of subs. 

I've googled it for you, just to get you past reflexive opining.

https://mehlau.net/audio/multisub_geddes/

https://www.svsound.com/blogs/subwoofer-setup-and-tuning/75040195-why-go-dual

https://www.audioholics.com/subwoofer-setup/two-subs