# Using an external preamp with the SB12-Plus



## Christopher (Apr 17, 2009)

I recently purchased an SVS SB12-Plus as a replacement for two Yamaha YST-SW100 subs. The Yamaha subs have gain and crossover controls on the front of the sub making adjustments on-the-go rather easy. The SB12-Plus has a much broader menu of controls, but they are all located on the back of the sub. I’m tempted to use a clean and neutral pre-amp located in the equipment rack to make gain settings more accessible and easier. Is this a bad idea?

Background:
After setting-up the SVS…listening…adjusting…and listening some more (with more adjustments), I have not been able to find a universally perfect gain setting on the sub. What is perfect for Vivaldi is not ideal for Bach…this disparity is even more pronounced when moving from one genre, like Jazz, to another, like R&B.

A possible solution:
I have an Adcom GFP 565 pre-amp that I’m not using. In addition to being clean and neutral sounding, the GFP 565 can also be used in bypass mode which routes the signal away from processors. I’m tempted to introduce the GFP 565 between the SB12-Plus and the main pre-amp. The master pre-amp for the system is a Benchmark DAC1 PRE.

Has anyone tried something similar? In general, does this sound like a bad idea since I will be effectively triple pre-amping the SVS sub (Benchmark>>Adcom>>SVS gain)?

Your thoughts and advice are very much appreciated.

Thank you,
Chris 
(Manhattan, NY)


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## JimP (May 18, 2006)

Chris,

I've never heard of anyone trying what you are preposing.

Your problem with varying subwoofer levels may be either going from a digital input to an analog input that bypasses bass management and/or a problematic frequency response. Have you tested your frequency response and have intergrated some form of subwoofer equalizer?


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## Christopher (Apr 17, 2009)

Hi Jim,

Thanks for the response and your questions. I have not tested the sub’s frequency responses in the room and I’m not using a sub equalizer. 

Here’s a quick summary of the setup:
CD transport connects to a Benchmark DAC1-PRE with an AudioQuest OptiLink 3 cable. The Benchmark is both the digital to analog converter and pre-amp (there is a POT for volume control). There are balanced outputs from the Benchmark that feed a Rotel RB-1080 (200 watt) amp. The amp feeds a pair of B&W CM-7 speakers with Type 4 AudioQuest speaker wire.

In addition to the balanced outs on the Benchmark, there are also unbalanced RCA outs. I’m using the unbalanced outs to feed the SB12-Plus directly.

Sub settings:
I’m not using the Parametric EQ controls on the sub. Room conditioner is set to large The main room is 23’ X 12’ and it is completely open to an adjacent room which is 15’ X 12’ (total cubic feet is about 3,650). The Crossover is enabled and set at 90Hz (the frequency response of the CM-7 speakers are 62Hz - 22kHz ±3dB on reference axis). Phase is set at 1 o’clock. Room Comp is set at Large. Gain is set to the lowest indicator mark at 8 o’clock.

Listening experience:
With the settings listed above, the sub sounds musical and detailed. I’m pleased for the most part. However, the sub is a little too ‘present’ with some tracks and a little understated with other tracks. For the most part, it’s just right. I’ve been listening to the same CD test tracks for many years, in some cases for over 15 years, so I have good historical experiences to compare to.

What I’m hearing and experiencing is not boomy or muddy bass. It’s tight, punchy and accurate. I would prefer, however, to bring the gain up a notch in some cases, and lower it just a bit in other cases. I’m not looking for significant changes in gain.

The idea…
Since the sub’s gain control is currently set very low at 8 o’clock, I’m considering the following steps. Have the unbalanced low level output from the Benchmark run to the Adcom GFP 565 pre-amp in bypass mode (this will bypass processors like treble, bass and balance). Go out from the Adcom pre-amp to the sub. Increase the sub’s gain setting to 12 o’clock. Set the Adcom Pot to a relatively low level that will come close to the sub’s current gain setting of 8 o’clock. With this as a reference point, I can tweek the gain on the sub up and down with the Pot on the Adcom.

Since the B&W CM-7 speakers are running independently of the sub with balanced outputs from the Benchmark, the main speakers will not be affected at all. The goal is to have an easily accessible, accurate and neutral volume control for the sub. 

I realize this idea is unconventional. But, it may work. I’m wondering if anyone has tried this approach and if there are any unforeseen downsides.

I’m glad to be here as a new member of Home Theater Shack. The reviews and feedback available here are largely responsible for me purchasing the SB12-Plus. I’m a first-time owner of an SVS sub and so far I’m very impressed. I trust the opinions here at the Shack and thank you all in advance for sharing your thoughts.

Chris


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## JimP (May 18, 2006)

Is the unbalanced output from the benchmark going to the sub from a subwoofer output or just regular front main speaker outputs normally intended for the amp?

Does the benchmark have a seperate output labeled sub or subwoofer?

Does the benchmark have its own bass management where you select speaker size and crossover?


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## eaglerider94 (Mar 19, 2008)

Chris,
It looks like you're just going to have to set it up the way in which you just described and report back to everyone here. I'm interested in anything having to do with SVS subs particularly SVS's New PB12-Plus as it is one of two I have narrowed down as my next sub upgrade. 
Good luck and let us know what your results are. Hey, why not post a photo as well!


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## Christopher (Apr 17, 2009)

Hi Jim,
Thanks for the questions. The unbalanced outputs from the Benchmark are not specifically designed for a sub – they are full range. There are no outputs labeled sub or subwoofer – only full range balanced and unbalanced outs. The Benchmark is a lean piece of equipment and, other than being a DAC, does not have any processors – there is no bass management, crossover or other controls like treble, base and balance.

While lean and simple, the Benchmark does a very good job producing a clean and accurate signal. I’ve checked with the tech team at Benchmark and they see nothing wrong with using both the balanced and unbalanced outs at the same time. People commonly use both to bi-amp speakers.

Hi Rick,
Thank you for suggesting that I just go for it! I took your advice and added the Adcom GFP 565 to the mix. It works and, so far, I’m happy with the results. I’ve added a few pictures of the system and its environment in ‘My photos.’ Since you’re in the market for an SVS sub, I’ve added some more detailed thoughts below.

General thoughts and observations:
This approach is not for everyone. First of all, I would not recommend that anyone run out and buy a pre-amp to tweek gain settings on a sub, especially an SVS sub. In my case, I already had the pre-amp and my needs are very specific.

When set-up properly (especially in a home theater environment), there should be no need to constantly adjust gain settings on a sub. The best advice is to get it right once – set it and forget it. However, my situation is different than most. 

In my case, I’m running a two channel system with significant differences in the quality of source material. In addition to playing CDs with a transport (16-bit/44.1kHz), I’m also playing high resolution 24-bit/96kHz downloads with an iMac (it connects to the Benchmark DAC1-PRE via USB). Bass response varies greatly depending upon the quality of the recording and the source material. If only playing 16/44.1 CDs and DVDs, I probably wouldn’t need to, or want to, adjust the gain settings on the sub.

This leads me to an important point. The SB12-Plus is powerful sub. I’m tempted to say that it’s an engineering marvel. How SVS was able to produce a sub with this much power, accuracy and depth in such a small box is simply astonishing. If I was using a less robust sub, the differences in bass output would probably not be so pronounced and I wouldn’t be motivated to tweek things.

In conclusion, 99% of the readers here at Home Theater Shack will probably never need to consider an approach like this. If used unwisely, it can also be a possible source of distortion in the signal. Minor tweeks in gain is one thing – maxing out the volume on the pre-amp for a lot more bass is another thing completely. Finding a healthy balance with the sub’s gain and pre-amp’s POT is essential. I’m not sure that anyone else will have my exact set of circumstances. If you do, then take comfort knowing that adding a pre-amp works. But, for most people, don’t try this at home.

Thanks again to Jim and Rick for responding. If you’re reading this and have additional thoughts, questions, or suggestions, please weigh-in.

Thank you,
Chris


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## eaglerider94 (Mar 19, 2008)

Thanks for the feedback Jim,
I tried viewing your photos but they appear too small and I couldn't make anything out very well.

As for your comments on varying bass responses, my understanding is that "If you try to use the sub to adjust for recordings that lack bass you'll go nuts, adjust the sub to well recorded music and leave it alone. You'll never get it perfect for all music because the music isn't recorded consistently!"
This is a quote from Mike's website at ACI in the subwoofer setup section. 
Although this is referring to a sub and in your case your using the Adcom, wouldn't you still be constantly adjusting it for each type of music which lacks bass? Seems to me it would become a hassle after a while. 
I find myself adjusting too much every time I listen to music and sometimes I simply want to put something on relax and enjoy the music. 
Have you considered using presets? I don't know if there is such a unit but that would be awesome.


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## JimP (May 18, 2006)

Chris,

If I understand what you are saying, it seems that your speakers are getting a full range signal and are good to around 60 hz.

You've got your sub crossed over at 90 hz. So you are getting double bass between 60 and 90 hz which may be part of the problem. Content heavy in mid bass is too loud.

Is it possible to route your speaker wires out of your amp (by the way, I've got Rotel amps too) go through the subwoofer (using its interal crossover) then back out to your speakers?


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