Hsu Research bass optimizer. (Equipment).

Manufacturer: Hsu Research, 3160 E. La Palma Ave #D, Anaheim, CA 92806; 800/554-0150 Price: $100, factory-direct Source: Manufacturer loan

This is a short review of a nifty, low-priced product that some of you will want to rush right out and purchase and others simply will not feel they need.

Normally, an unequalized Hsu TN-series subwoofer is flat down to 40-50 Hz, with a gradual rolloff below that point. The $500 TN1220HO unit that I reviewed in Issue 67, for instance, is down about 6 dB at 20 Hz if run unequalized with a conventional amplifier. Any kind of amp would do the trick, including that old stereo integrated unit or receiver that you retired after upgrading to surround sound, although you would only be able to use one channel if it was not possible to bridge the outputs.

This natural rolloff is actually rather modest (precious few full-range speaker systems, and even the majority of subwoofers, are down "only" 6 dB at 20 Hz), and with typical room gain an unequalized TN1220HO sub will still have exemplary low-bass performance. Indeed, the low-end rolloff is not as extreme as what we find with THX-certified subwoofers, which take into account room gain in their designs.

So, the Hsu approach allows a subwoofer freak to get killer low bass on a shoestring, for sure, particularly if they have their own amp. On the other hand, in the past if one wanted to get all-out performance with a Hsu subwoofer and equalize the response genuinely flat down really low, they would also have had to purchase a Hsu amp. Those Hsu amps, in addition to being quite powerful, have built-in equalization that contours a TN1220HO for flat response to below 20 Hz and contours the smaller TN1225HO model to go flat to about 22 Hz. The amps are independently configured to work with either sub.

Hsu amps do cost money, however (the 500 watt job is $700 if ordered along with a sub, the new 250-watt job is $350 with a sub, and the 150-watt unit is $200 with a sub), and so Hsu has now offered those with amps of their own to continue have ultra-grade Hsu subwoofer bass on a serious shoestring budget.

The Hsu Optimizer is basically the equalizer section found in all Hsu amps. However, in this case it is installed inside of a tiny, almost cigarette-box-sized, enclosure that allows you to stash it away on a very small shelf, or even behind a component that is occupying a shelf of its own. The actual dimensions are 4 x 2 x 4 inches (h/w/ d).

It is designed to do a few basic things, with the most important being to equalize the Hsu subwoofers mentioned above for flat response when hooked up to a non-Hsu amp. The Optimizer should also be able to equalize an unpowered, older-model Hsu sub that is not used with a Hsu amp, such as the HRSW12Va that Robert Thompson reviewed in Issue 67.

The Optimizer is DC-powered, coming with a "wall-wart" power supply and a long skinny wire that you plug into the unit's backside. The backside also has a pair of input jacks and a pair of output jacks (left and right subwoofer channels). Normally, you would only need to use one input jack and one output jack, and in that case you simply use the right-channel hookups. If you have stereo subwoofers, you would use both.

The front panel has a switch for stereo/mono. It basically is a blend switch and if you have stereo outputs to a pair of subwoofers you can use the switch to determine which sounds best. In the mono mode, the outputs are internally coupled to mono and so you should get, as Hsu says in the brief instruction booklet that comes with the Optimizer, "lower distortion and higher headroom." This is because mono bass from two subwoofer units will add coherently, whereas with the stereo-mode setting they may not do this, depending upon the program material. Why offer the option at all? Well, some enthusiasts may feel that stereo low bass has an ambiance edge over mono bass. This is debatable, but the Optimizer lets you try out your theories.

The front panel also has an equalization switch, marked EQ1 and EQ2. You would use EQ1 for the TN1220HO (or the HRSW12Va), and use EQ2 for the TN1225HO. Hsu warns that using EQ2 with the TN1220HO will result in a degree of unevenness in the overall bass response, with a serious loss in output below 25 Hz. This is because the EQ2 position electrically rolls off the signal below 25 Hz, and also boosts the signal between 50 and 25 Hz a bit more than what we get with the EQ1 position.

If you use the EQ1 position with the TN1225HO, you risk damaging the woofer driver, since that setting applies boost clear down to 18 Hz, which is below the reflex enclosure's resonance point where the speaker system has control of the driver.

As evidenced by the amount of rolloff I described previously, the boost level is actually rather small. The EQ2 position applies it between 60 and 22 Hz, with the maximum reaching about 4-5 dB at the lower frequency. The EQ1 position applies boost between 50 and 18 Hz, with the maximum reaching about 6 dB at the lower frequency. Below each peak point, the response is electrically rolled off rapidly, which helps to protect the ported Hsu subwoofer systems from driver overload at ultra-low frequencies.

The front panel also has a level control. Normally, your existing processor, or the amp you might use, would have its own level control. However, some power amps some do not. The level control allows for level balancing and fine-tuning over a wide range, and has unity gain at about the 1 o'clock position.

The specifications for the unit are rather straightforward. The volume control allows for up to 14 dB of gain above unity (not counting the bass EQ applied), the input impedance is 10k ohms, the output impedance is 100k ohms, the shipping weight is one pound, and the power draw is 5 watts. The Optimizer comes with a one-year warranty.

One anomaly with the device that some users will need to pay attention to involves its behavior with DC coupled amplifiers. The Hsu amps are not DC-coupled, and so they do not have problems with the equalizer as it is internally installed. However, the power supply in the Optimizer was designed as a single-rail device, and as a result, depending upon the input impedance of the power amp it is used with, there may be some DC offset as the capacitors charge up.

Most amps have DC blocking capacitors at the inputs, which protects them from DC offset problems. However, if you have a true DC-coupled amp there might be some offset applied to the woofer driver if you turn on the amp before or immediately after turning on the Optimizer.

Actually, the Optimizer has no on/off switch at all, and since it is a DC powered device that draws only 5 watts of power, it is best to plug it into the wall outlet and leave it on all the time. If the power is cut while it and the amp are on, the worst that will happen is that you will get a moderate thump sound from the sub. And if the amp is not DC-coupled, a simultaneous turn-on will cause no problems.

I tested the unit with my own TN1220HO, using it in conjunction with an AudioSource Amp One that I have had on hand as a test amp for some time. It can be bridged to about 200 watts, and when used with the Hsu Optimizer the sub's performance was on par with what I got with my 250-watt Hsu amp. Very clean and very impressive. Not much more to say.

Well, there is one thing more to say. I have had an SV 16-46PC subwoofer in my middle system for some time now. (I reviewed it in issue 88.) It is a great sub, but even the curves that SVS supplied to me showed that it rolls off the bass slightly below 35 Hz and is about 5 Hz down at 20 Hz. With 99% of the program material out there (music and home theater) this sub will hold its own with just about anything. However, below 30-35 Hz it rolls off faster than an equalized Hsu TN1220HO or subs like the Velodyne F1800RII (reviewed in Issue 67) or the Paradigm Servo 15 (Issue 71).

Putting the Optimizer (set at EQ1) between the sub-out connection on my processor and the input to the SV sub's on-board amplifier brought up the response at 20 Hz, and I recommend using the Hsu equalizer with the SV sub if you already own the system and want to augment its bass performance below 30-35 Hz.

In conclusion, if you want super bass on the cheap and have a 150+ watt amp of your own, obtaining the Optimizer and a Hsu subwoofer may be your most efficient ticket to superior subwoofer performance. Or, if you already have a Hsu sub and your own amp in operation (or have one of the SVS units), and you want to hear ultra-deep pipe-organ sounds better than ever, the Optimizer will be the best way to up the ante.

Actually, you could purchase two TN1220HO models and the Optimizer, and after hooking them up to your spare, reasonably powerful stereo amp, you would have an absolutely killer, dual-module subwoofer system for $1100. Folks, deals like that do not grow on trees.