How to add a subwoofer to a Spectral system?

[acousticsguru]

Dear all!

Need help with what would seem like a simple question unless the amplification in question weren't Spectral. I'm thinking of adding stereo subs to my system, in which I use a DMC-15SS, which has one set of unbalanced inputs. Spectral advises against using a Y-splitter RCA cable at the output of the preamp, as the parallel RCA cable might act like an antenna, which in turn might drive a Spectral power amp into oscillation (I'm a "burnt child" in this respect, happened to me once before using non-MIT speaker cables with a Spectral power amp).

Curious to hear how you guys solve this problems?

One presumably "safe" way of doing it would be to tap off the speaker outputs (or loudspeaker inputs) using Jensen Transformers Iso-Max SS-2NR Stereo Speaker to Line Audio Converter:

https://www.cs1.net/products/jensen_transformers/SC-2NR.htm

Any suggestions? Thanks in advance!

Greetings from Switzerland, David.

 

[ddk]

You should try a splitter, there are multiple input and output interconnects in close proximity anyway why don’t they become antennae? You can try one of these;
https://www.crutchfield.com/S-SRbOv...jXxP3_Ql3OCbwIPegVOfAjzET-PV0dtcaAkvWEALw_wcB



david

 

[acousticsguru]

You should try a splitter, there are multiple input and output interconnects in close proximity anyway why don’t they become antennae? You can try one of these;
https://www.crutchfield.com/S-SRbOv...jXxP3_Ql3OCbwIPegVOfAjzET-PV0dtcaAkvWEALw_wcB

View attachment 73865

david

If I interpret Spectral tech support correctly, the difference is none of those other cables are directly connected to the input of the power amp. The preamp doesn’t appear to be the problem in this scenario.

Greetings from Switzerland, David.

 

[ddk]

If I interpret Spectral tech support correctly, the difference is none of those other cables are directly connected to the input of the power amp. The preamp doesn’t appear to be the problem in this scenario.

Greetings from Switzerland, David.

I can’t speak to your preamp’s design the one I use has 6 outputs. I use 4 of them without issues this is a cheap enough solution to try and see for yourself.

david

 

[acousticsguru]

I can’t speak to your preamp’s design the one I use has 6 outputs. I use 4 of them without issues this is a cheap enough solution to try and see for yourself.

david

You have a Spectral preamp with six outputs? What model is that? And no, I’ve driven a Spectral power amp into oscillation once, was costly and a huge hassle as there’s no importer in this country, not going to take any risks “trying” - want to be certain.

Greetings from Switzerland, David.

 

[ddk]

You have a Spectral preamp with six outputs? What model is that? And no, I’ve driven a Spectral power amp into oscillation once, was costly and a huge hassle as there’s no importer in this country, not going to take any risks “trying” - want to be certain.

Greetings from Switzerland, David.

No it’s not Spectral, my point was about the antenna effect.
david

 

[acousticsguru]

No it’s not Spectral, my point was about the antenna effect.
david

Again, it’s not about the preamp but the power amp input. Spectral power amps have a ca. 1.8 MHz bandwidth. Different category of problem.

Greetings from Switzerland, David.

 

[sbnx]

If noise & feedback into the preamp is the concern then the MSB sub isolator would be the solution.

 

[acousticsguru]

If noise & feedback into the preamp is the concern then the MSB sub isolator would be the solution.

Guessing that’s a Y-splitter with an isolation transformer on one leg? There is a balanced and unbalanced version by Jensen that could be used with after the Y-splitter David suggested above, presumably in its combination the exact same thing:

https://www.jensen-transformers.com/product/sub-2rr/

Greetings from Switzerland, David.

 

[Diasoft5]

I also wanted to try subwoofers and eventually add a stack to my system. Vitus has the same problem as Spectral. The amp goes up to 1.000.000 Hz. Never use a high capacitance speaker cable, the amp may oscillate. My integrated only has one XLR output. It goes into the power amp for biamping. Here is what Vitus told me about connecting a subwoofer and an power amp to the XLR output:

The XLR splitter would in principal also work, however the problem is that without knowing the inputstage of the subwoofer, i cannot advise you about this.
What you do if you use the splitter is, is that you load the preout more, not that this would nessecarily be a problem.
However the inputstage of the subwoofer could have some kind of feedback to the preamp and poweramp, that could potentially cause trouble.

The only way to do this "correctly", would be to get a XLR unity buffer box, with 1 input and 2 seperate outputs (run by seperate unity gain amplifiers).
But unfortunately i dont have any recommendations on any specific box. This would also change the sound of the system of course, since we dont know how these boxes are built.

 

[acousticsguru]

UPDATE: I made more inquiries and have come to the conclusion a Jensen Transformers Iso-Max SS-2NR Stereo Speaker to Line Audio Converter could be used to step down the loudspeaker output of a Spectral power amp to line level provided the loudspeaker cable is non-inductive and, preferably, short:

https://www.cs1.net/products/jensen_transformers/SC-2NR.htm

I have been informed the information presented in Post #1 is wrong, in other words, adding isolation transformers in parallel does not present a significant additional load:

"The speaker inputs represent a trivial additional load for the driving power amplifiers - maximum input power is 0.08 W,0.4 W, 1.5 W, and 5.4 W respectively in the four input ranges."

The SS-2NR version is said to be optimized for subwoofer applications, being flat to 5Hz. Normally, these units/devices are placed close to a subwoofer, using them with a jumper cable in parallel to the speakers (either at the power amp output or loudspeaker inputs in a dual mono version: Iso-Max SS-1NR) one will "lose the advantage of lower EM ingress by putting the device near the amp instead of near the sub, but it would serve to isolate the amp from stray RF. Assuming that RF doesn’t happen to get in on the speaker cable between the amp and the ISO-MAX."

It's my understanding that the RC and/or LCR type circuits built into MIT speaker cables act as an amplifier choke that in designs other than Spectral would be placed at the end of a power amp output stage, so adding an isolation transformer in parallel should in no way affect its efficiency. An isolation transformer would seem to be much preferable in this scenario to the (cheaper) resistor circuitries normally built into commercial subwoofer high-level inputs stages, as it'll eliminate any ground and hum issues on top of keeping EM and RF ingression to either side of the isolation transformer.

My speakers have external crossovers placed next to them on the floor, using low-inductance cables (Kimber) running from the crossovers to the drivers. The different models of MIT/Spectral speaker cables I've used running from the power amp to the crossovers have invariably kept the Spectral power amp(s) from harm, so what I'm going to do is tap off the signal there (although I have a hard time believing the circuit that protects the power amps from oscillation is serial, but as I'm not going to open the boxes of my Spectral/MIT cables to check and make sure, I'll just just continue to follow Spectral's philosophy here).

Thanks everyone for their contributions!

Greetings from Switzerland, David.

 

[acousticsguru]

I also wanted to try subwoofers and eventually add a stack to my system. Vitus has the same problem as Spectral. The amp goes up to 1.000.000 Hz. Never use a high capacitance speaker cable, the amp may oscillate. My integrated only has one XLR output. It goes into the power amp for biamping. Here is what Vitus told me about connecting a subwoofer and an power amp to the XLR output:

The XLR splitter would in principal also work, however the problem is that without knowing the inputstage of the subwoofer, i cannot advise you about this.
What you do if you use the splitter is, is that you load the preout more, not that this would nessecarily be a problem.
However the inputstage of the subwoofer could have some kind of feedback to the preamp and poweramp, that could potentially cause trouble.

The only way to do this "correctly", would be to get a XLR unity buffer box, with 1 input and 2 seperate outputs (run by seperate unity gain amplifiers).
But unfortunately i dont have any recommendations on any specific box. This would also change the sound of the system of course, since we dont know how these boxes are built.

There's also a symmetrical version of the Jensen isolation transformer unit:

https://www.cs1.net/products/jensen_transformers/SP-2SX.htm

I specifically inquired about this, and was informed that it will make a difference only IF the subwoofer uses TRUE balanced input circuitry. This has got me wondering how many subwoofers offer true balanced inputs? Certainly not the majority (if none) of the ones I've been looking at.

Greetings from Switzerland, David.

 

[LL21]

Hi David,

I am definitely interested in this thread, as we have used a sub out of one of the preamp outs for years. I am now thinking about what to do to create more isolation/feedback buffer to protect the preamp and amp as this thread has suggested.

Have you looked at this? https://www.cs1.net/products/jensen_transformers/SUB-2RR.htm

Specifically for subs to go directly just before you go into the sub. I am thinking about this myself.

Additionally for the first time, we have a bit of a ground loop hum issue, and if we disconnect the output from the preamp to the sub...the buzz instantly stops and the system is utterly silent. I am hoping this Jensen piece will put a stop to that buzz.

FROM THE WEBSITE
_______________________
The Jensen Transformers SUB?2RR ISO?MAX 2-Channel Low Frequency Audio Input Isolator is designed specifically for subwoofer amplifier isolation in home theater and automotive applications. The specially designed transformers in this unit provides unparalleled low frequency response while providing over 100dB of common mode isolation, to solve even the most difficult subwoofer installation problems. The zero dB insertion loss also prevents carefully calibrated system levels from being disturbed.

The SUB-2RR is housed is a sturdy metal enclosure, measuring 4.32" W x 2.5" D x 1.45" H., including the flanges. The flanges have mounting holes, which are handy for permanent installations.

If you're not sure which model to choose, please check out our Jensen Transformers Application Guide.

Please do not confuse the SUB-2RR with similar (but inferior) products. Jensen Transformers' ISO-MAX Studio-Quality Audio Isolators provide the ultimate signal interface solutions. They solve the fundamental problems associated with ground loops, without degrading signal quality.

With inferior products coming from all over the world, we think it is a genuine treasure to find a company like Jensen Transformers, actually designing and manufacturing some of the finest products in the world, right here in the USA, and we are proud to be able to offer their products. The SUB-2RR is made in the USA, is backed by the manufacturer's comprehensive one-year warranty, and has a remarkable 20-year warranty on the transformer device itself.

Ground Loops?​

(or why the best planned systems sometimes sound bad)​

It is a subject that is seldom discussed in manufacturers publications, or even in magazines of the trade, but ground loops can happen even in the most carefully planned A/V system. When ground loops happen, the result is unwanted hum. This hum is most often heard in powered sub-woofers, but may also be present in the main channels as well. Most powered sub-woofers use a signal-sensing circuit to turn the amplifier off when no signal is present, but ground loops can induce enough hum on the input to keep the amplifier running continuously.

Ground loops are often the result of the fact that sub-woofers tend to be powered by a different electrical branch circuit than the rest of the A/V system. It's easy to understand why, because the sub-woofer may be across the room, and/or it may require so much power that a single branch circuit just isn't enough to handle the load.

The result is that the neutral or ground side of the two circuits can be at a slightly different potential. It can be because one of the circuits is loaded more heavily than the other, or because one run is longer than the other, or any number of other reasons that are beyond the scope of this article.

Ideally, ground loops would be prevented at the design level, but if you are working with an existing room, it is not always possible or practical. Ground loops can occur in a mobile installations as well, because all of the amplifiers in the system have their own internal power supplies, and voltage drop percentages in a 12 volt system can be even more challenging than in a home system.

The Solution​

The Jensen Transformers SUB-2RR is elegant in its simplicity. Just insert it anywhere in the audio lines between the receiver or preamp LFE output and the sub-woofer input and it immediately provides these benefits:.

• Eliminates the inherent ground noise coupling mechanism
• Common-mode ground noise rejection of 105 dB at 60 Hz
• Extended low frequency response: -1dB at 0.4Hz, -3dB at 0.15Hz
• Extremely low distortion: 0.01% at 20Hz and -10dBV
• 0dB insertion loss with 600 Ohm source and 47k Ohm load

Immediately, the effects of the ground loop become a non-issue. Problem solved. There are not too many things that one can install in an audio system that can do so much with so little effort. It is a small device, 4.32" long, 2.60" wide, and 1.45" tall, so it fits just about anywhere.

Don't Forget the Cables!​

Good cables are very important in the fight against hum and noise. You will need at least two short patch cables in order to insert the SUB-2RR into your signal path, and you may want to consider upgrading your existing cable(s) as well. The SUB-2RR does not include any cables, but if you need them, we have some very high-performance subwoofer cables available. Please click here to see them!

 

[hogen]

Dear all!

Need help with what would seem like a simple question unless the amplification in question weren't Spectral. I'm thinking of adding stereo subs to my system, in which I use a DMC-15SS, which has one set of unbalanced inputs. Spectral advises against using a Y-splitter RCA cable at the output of the preamp, as the parallel RCA cable might act like an antenna, which in turn might drive a Spectral power amp into oscillation (I'm a "burnt child" in this respect, happened to me once before using non-MIT speaker cables with a Spectral power amp).

Curious to hear how you guys solve this problems?

One presumably "safe" way of doing it would be to tap off the speaker outputs (or loudspeaker inputs) using Jensen Transformers Iso-Max SS-2NR Stereo Speaker to Line Audio Converter:

https://www.cs1.net/products/jensen_transformers/SC-2NR.htm

The problem there is that a step-down isolation transformer represents an 8? load in parallel with the main speakers, which in my case are 6? nominal (forget what the minimum impedance is). [EDIT: the crossed-out bit information is WRONG, see Post #11 below!]

Any suggestions? Thanks in advance!

Greetings from Switzerland, David.

Can't you drive the subs from you amplifier with speaker cables? I do this in my system.

 

[acousticsguru]

Hi David,

I am definitely interested in this thread, as we have used a sub out of one of the preamp outs for years. I am now thinking about what to do to create more isolation/feedback buffer to protect the preamp and amp as this thread has suggested.

Have you looked at this? https://www.cs1.net/products/jensen_transformers/SUB-2RR.htm

Specifically for subs to go directly just before you go into the sub. I am thinking about this myself.

Additionally for the first time, we have a bit of a ground loop hum issue, and if we disconnect the output from the preamp to the sub...the buzz instantly stops and the system is utterly silent. I am hoping this Jensen piece will put a stop to that buzz.

Yes, I have looked into this model as well. It would need to be placed after an RCA Y-splitter at the preamp output, between preamp and subwoofer, as one can absolutely NOT put an isolation transformer between a Spectral preamp and power amp. In doing so, you're effectively replicating the MSB device mentioned in Post #8 above.

One will need to observe the maximum cable length on either side of the unit as per the manual for the SUB-2RR:



The cable recommendation is the same I got (cannot comment other than in my experience, Canare know what they're doing, looks well-shielded and reasonably priced).

Whether or not tapping off at the preamp output is as safe and/or sounds better, I'm unable to tell. I've had so-so experience with Y-splitters sonically, hence my preference for tapping off where multiple terminations are being tapped off simultaneously anyhow: the speaker terminal (or in my case, the crossover box that's sitting on the floor next to each main speaker).

Greetings from Switzerland, David.

 

[acousticsguru]

Can't you drive the subs from you amplifier with speaker cables? I do this in my system.

That's what I'm proposing in Post #11, only that instead of using the in-built (cheap, non-isolated, non-protective from the perspective of a megahertz bandwidth power amplifier) input resistor circuit normally used in commercial subwoofers, I plan to use isolation transformers to step down the signal. Or are you referring to passive subs? Guess not judging from your components list.

Greetings from Switzerland, David.

 

[hogen]

That's what I'm proposing in Post #11, only that instead of using the in-built (cheap, non-isolated, non-protective from the perspective of a megahertz bandwidth power amplifier) input resistor circuit normally used in commercial subwoofers, I plan to use isolation transformers to step down the signal. Or are you referring to passive subs? Guess not judging from your components list.

Greetings from Switzerland, David.

I see. I am referring to active subs I do not have any knowledge about the input resistor circuits however. The quality of these may vary on different subs maybe.

 

[acousticsguru]

I see. I am referring to active subs I do not have any knowledge about the input resistor circuits however. The quality of these may vary on different subs maybe.

No doubt. There are commercial subwoofers with isolated inputs, too, although I only know of one or two models, I'm guessing there may be more.

Greetings from Switzerland, David.

 

[microstrip]

IMHO, unless your sub amplifiers are poorly designed/manufactured you will not need insulating transformers - they are very useful, Jensen has excellent audio transformers, but only if needed.

Considering the MHz band, just add a 1000 pF film capacitor in parallel with the pull down resistor of the resistive dividing network.

I have tried the top Jensen JT10 KB - I have a pair with the different RCA /XLR options in my tool box - with the cj GAT2 and the JL Audio's F112 and preferred the sound without transformers. But did not try them using the loudspeaker terminals.

 

[LL21]

IMHO, unless your sub amplifiers are poorly designed/manufactured you will not need insulating transformers - they are very useful, Jensen has excellent audio transformers, but only if needed.

Considering the MHz band, just add a 1000 pF film capacitor in parallel with the pull down resistor of the resistive dividing network.

I have tried the top Jensen JT10 KB - I have a pair with the different RCA /XLR options in my tool box - with the cj GAT2 and the JL Audio's F112 and preferred the sound without transformers. But did not try them using the loudspeaker terminals.

Interesting Micro...I have a soft, steady ground loop hum...cannot figure out why. (BTW, it is a loud ground loop hum unless I select the 'Float' option on the Robert Koda K15EX preamp.

What I do know is that disconnecting the IC from the preamp to the Sub solves it immediately. I was hoping that this Jensen Sub-2RR would therefore solve it. Any thoughts on whether that is a fair assumption?