Safety first! Before you connect to the mains

Before you even connect the power plug into the mains, let’s check something first. Assuming your using a plate amplifier, on the rear plate you'll find an AC input and usually fins of a heat sink. Near the AC power cord input, you should find a voltage rating. If you find only an Australian voltage, all is fine. However, many plate amps have been designed to also work in the US. Many of these have a voltage selection switch that needs to be correctly set. Some require the fuse to be changed.

Figure 1 – Voltage selection: be careful to choose the right setting before connecting to the mains!

Connecting to a home theatre setup

In a home theatre system, it’s best to use the subwoofer preamp output. This means you will be able to use the bass management features of your AV receiver or processor as well as adjusting the sub level with your remote. You will need a single RCA interconnect – often it will be called a “subwoofer cable” but the only difference is that it’s a mono cable.

Figure 2 – A Dayton wireless sub sender and receiver avoids long cable runs, providing greater placement flexibility.

An alternative is to replace the cable with a wireless sender and receiver. This eliminates the need for cable and creates some placement flexibility. In some cases, you may want to place a sub around the back of the room, or in a location where cable would run across a walkway. Normally in that situation, you would install concealed cables, running them through wall, ceiling or floor cavities but in a rental this isn’t usually possible, and this device will save the expense and trouble of installation as well as the cost of a long cable run. I used this device in my demonstration room at the 2013 Australian AV show. The sub setup had four subwoofers around midpoints of each wall and this device was used for the rear sub.

Connecting to a preamp

In a two channel system with a preamp, first check if you have a subwoofer output. If not, use the preamp outputs. You will need a splitter on each channel.

Fig 3 – RCA splitter (male to female). This device plugs in to a single One of the outputs goes to your power amp as before, the other goes to the sub amp. Most subwoofers have a left and right input.

Connecting to an integrated amp

If you don’t have any analogue preamp outputs, you can still connect a subwoofer. Simply use high level inputs. This means that you will connect speaker cable from your amplifier to the sub amp.

Your main speakers should not have any bass filtering upstream as it will apply to the sub also. High level inputs have resistors to reduce the signal back to a low level signal. There are two ways to use high level inputs:

1.   If your sub amp has only high level inputs, you will need to connect the subwoofer in parallel with your main speakers. If your sub is near the mains, simply connect another pair of cables from the main speakers to the sub amp. If closer to the amplifier, you may connect another pair from the amp to the sub. The input impedance on high level inputs avoids placing any significant change to the load on the amplifier.

Fig 4 – Connecting to a sub amp via high level inputs (speaker cable with an amplified signal).

Adding a load in parallel in this way will reduce the impedance load seen by the amp. If a 4 ohm load were added in parallel to 4 ohm speaker, the amplifier would see a 2 ohm load. This would be a problem for more amplifiers. However, in this case, the load presented by a plate amplifier’s high level inputs is much higher. As a result, the impedance load added is inconsequential.

2.   If your sub amp has high level inputs and outputs, then you can use a daisy chain arrangement which applies a high pass to the mains. Connect cable from the amplifier to the sub and then from the high level outputs to the main speakers.

Fig 5 – Connecting to a sub amp that has high level inputs and outputs

In this configuration, the speaker outputs will be filtered so that the mains will remove bass content. Amplifier headroom is not improved, but the speakers will be relieved of bass duties. Where this option is available, it may or may not be the best choice for a particular system. The choice depends on the room acoustics challenges faced and user priorities. In some cases, the bass filtering will increase the output of the main speakers, especially in the case of small stand mount speakers. In other cases, an overlap in the bass response of mains and subwoofer will provide a smoother in-room bass response.

Low level vs high level inputs

In most situations, low level inputs should be used where available. There is no advantage to using high level inputs, except where the gain is inadequate. In that situation, high level inputs may be required. Some preamps don’t have adequate gain to drive a subwoofer to its maximum level. The subwoofer volume control may be turned up all the way without realising the full output capability of the sub. In this case, high level inputs should be used.

Bass management settings

AV Receivers vary in their bass management capabilities. Where it’s not possible to independently choose the crossover points for sub and mains, 80 Hz is a good starting point. Where independent control is possible, it will often be advisable to choose the highest possible setting (200 Hz) and then use the crossover dial on the sub amp, which usually offers better control. Unfortunately due to a lack of standards, it’s difficult to suggest an approach to suit all products.

Sub amp settings

There are many subs on the market but most of them have similar controls. One popular example we’ll discuss here is Dayton SPA1000. This is a popular amp that features 1 kW of power when driving a 4 ohm load and parametric EQ, which may be used to dial in the desired bass extension with a sealed box sub or tackle a troublesome room mode.

Fig 6 – Dayton SPA1000.

1 . Mains input

In this amplifier, a different fuse is used for 115 or 250V. By default they are supplied with a fuse suited to US voltage, but when purchased from The Loudspeaker Kit, the correct fuse is already installed.

In the case of Rythmik plate amplifiers, a switch at the back must be selected. Failure to do so prior to connecting to the mains will result in blowing a fuse.

2. Power setting

Most plate amps have the option of auto on, where the sub will switch on when a signal is present. This option is preferred to save power and to enjoy the maximum lifespan of the amplifier. Leaving it on permanently will shorten the life of any amplifier.

3. Low pass (crossover)

The low pass section sets the point where the subwoofer will cross over to the main speakers. This is done with a low pass filter, which “passes” the frequencies below the crossover but filters above a given corner frequency.

Fig 7 – Low pass crossover section. (Dayton SPA1000)

The gain control on the left sets the volume of the sub. The freq knob next to it sets the crossover point and it’s variable from 30 – 200 Hz. The phase control allows for reversal of sub phase.

Gain (volume) control

It should be noted that the gain control does not represent accurately the total output possible from the sub. The sub may reach its limit before the dial reaches 10, or in some cases this position may not realise the full output potential. The input sensitivity of the plate amp along with the signal level are key factors. The driver may reach its limits before the amp or vice versa.

The gain control can be set by ear according to what sounds right or with an SPL meter. Most people prefer to have the sub around 10 dB higher in level than the mains, but this varies based on source material, the quality of the subwoofer, room acoustics and listener preferences. There is no particular SPL level relative to the mains that will suit all situations.

Crossover control (freq)

The crossover (freq) control sets the crossover point. It should be noted that the “actual” crossover point that is realised is the acoustic sum of the driver response and filter response. So this means that setting the control as shown to 30 Hz will not usually result in the actual acoustic crossover point being 30 Hz. This is something that must be measured and there are no effective rules of thumb here.

It should also be noted that when one is using an AV receiver, it will often provide another crossover and the two will be added together. You may wish to set one as high as possible (eg. 200 Hz) so that just one of them is used primarily. Or you may like to sum them together for a steeper crossover slope. Some AV receivers are very limited in their choices, in which case the subwoofer amp can provide fine-tuning of the crossover point.

As a general guide, the knob is likely to be most useful in the 30 – 105 Hz range.

4. Phase control

This is best left in the zero position. Subs typically are out of time alignment with the mains and phase controls simply delay them further. Where phase alignment is required, this is best achieved by compensating with the speaker distance settings. The sub should be entered as further away, causing digital delay to be applied to the mains. This will mean with the correct distance settings, the sub will be time aligned.

Fig 8 – EQ and input sections

5. Parametric EQ section

A single band of parametric EQ is included here with 3 controls – freq, bandwidth and level.

Freq: this sets the frequency of boost or cut

Bandwidth: this sets the width of the filter. A setting around 0.5 might be used for a fairly broad bass boost to augment extension in a sealed sub. A setting of 1 is a narrower filter that one might use for cutting a room mode.

Level (dB): this sets the amount of cut or boost applied. 6 dB of boost requires 4 x the power and this is the reason why a limited amount is available. Much greater cut is available and the maximum cut will often be required when dealing with a room mode.

6. Input terminals

There are 3 RCA input terminals. The LFE direct is for use with an AV receiver and the left and right terminals are for use with a stereo preamp so that both channels can be summed.

Is this as good as it gets?

99% of the time, you won’t experience bass bliss right away. If you’ve added a sub to a music system, you might find the bass becomes boomy and uncontrolled. It might sound overpowering and if you turn the volume down, it then sounds “underwhelming.” Quite often, following basic advice does not lead to great bass. Don’t give up just yet. The answer is most likely related to room acoustics and bass integration. These are problems that can be solved but they are beyond the scope of this article.