SSL Duende Tutorial 3 – Basic EQ

Tutorial 3: Basic EQ

EQ in its most basic form is a tone control, used to change the frequency balance of a signal. We have all encountered this in the home hi-fi featuring either hi, mid and low tone controls or as a graphic equalizer. As you move the respective controls the sound is changed in such a way as to give more treble, less bass etc. By doing this you can get the music sounding to your taste, and with a well featured EQ can solve problems caused by a difficult listening environment or badly recorded material.

The Theory

The Duende Channel EQ is predictably rather more complex and powerful than those found on a hi-fi, yet works on the same principal. It is a four band EQ, meaning four sets of different frequencies can be attenuated or boosted at the same time and the Q (or bandwidth) is variable on the middle two bands. The high and low bands are switchable from bell to shelf independently, and to top it off hi and low pass filters are also simultaneously available. In addition to the four band EQ, two filters are simultaneously available.

The diagrams below attempt to clarify the difference between shelf, bell and filter shapes.


As can be seen a filter cuts out the frequencies above or below its cut-off point. A lowpass filter will let through only the low frequencies therefore cutting out the highs, where a high-pass filter will cut the low frequencies. These are useful when unwanted frequencies are present at either the high or low end of the spectrum. An example of this would be cutting out the stage rumble picked up on microphones in a live situation.


With the BELL button up on the LF band of the Duende Channel a shelf EQ will be selected which will boost frequencies from the cut-off point downwards, or upwards in the case of the HF band. The shelf shape gives a gentle roll off, as opposed to the more heavy handed cut action of a filter.


If the BELL button is down then instead of the shelf shape, a curve is selected which will only boost frequencies around the cut-off point with a gradually reducing gradient towards the zero gain point. The HMF and LMF bands have one more control alongside the gain and frequency. The Q, also referred to as bandwidth, is the width of the filter and governs how much of the frequencies either side of the center frequency are boosted or attenuated by the EQ. This type of EQ is referred to as fully parametric. A smaller Q value will give a wider curve, therefore not only affecting the cut-off frequency but many others either side. This is often said to be a more musical way to EQ, and can be good for general sound shaping when the tonal balance of a signal needs to be changed.

With a large Q value, the curve is narrowed allowing you to zoom in on frequencies more precisely. This can be useful for attenuating problem frequencies within a signal, but large gain changes can sound unmusical.

The Practice

Now we are going to use the channel EQ over various drum tracks to clean them up and increase the separation between them.

Load the appropriate DAW project and let’s have a look first at the individual channel EQs.

Solo the kick track and open the corresponding Duende channel. The kick drum is not sounding so good with the channel bypassed. There are some frequencies in the middle which tend to mask the lower and higher end of the kick drum. Instead of adding low and high EQ, it can be preferable to pinpoint the nasty middle frequencies and tone them down.

Now press the BYPASS ALL button to bring the processing into place.

  • Using the LMF band an 8db cut has been made and by experimentation the offending frequencies have been found to be around 600Hz. The Q has been decreased so a more musical result is achieved.
  • A small boost at 3.6kHz has been used to give the kick slightly more click, emphasizing the range of frequencies caused by the beater hitting the skin.
  • Also a small amount of bottom end at 64Hz is added to give the kick slightly more weight.
  • The filter section has not been used as we need all the low frequencies of the kick drum to be present and the high end is important to retain the attack information.

Now solo and open the Duende channel over the snare drum. Listen to the unprocessed signal and then press the BYPASS ALL button to enable the channel processing.

  • The high pass filter has been placed at about 90Hz to remove the unwanted frequencies, as there is little of interest happening down in this area in the snare drum.
  • To compensate for this low end cut the LF band is used in shelf mode to boost some of the frequencies above this cut so the snare still has some weight.
  • Again there is some nasty middle present, this time by sweeping the LMF with an 8db cut and high Q value it has been found to about 285Hz. The high Q has been used so that the frequencies either side of 285Hz are not attenuated too much as there is a significant amount of wanted information around this area.
  • To give the snare a bit more snap the LMF band boots about 2kHz with a low Q value.

Solo the hi-hat and again listen to the processed and unprocessed signals.

  • Here you can see the high-pass filter has been wound up to 500Hz to cut out the kick and some snare drum in the hi-hat track as this is not wanted. Because the hihat is nearly all high frequency information this can be done.
  • Pulling 6db out of the signal using the LMF band at a frequency of 540Hz significantly reduces the presence of the snare in the hi-hat track.
  • Some of the harsher frequencies have been attenuated using the HMF band with a slight cut at 7kHz.
  • Then to compensate for this a slight amount of air has been added using the HF band in shelf mode at 17kHz.

By toggling the BYPASS ALL button in and out the difference in clarity is quite noticeable in the EQed signal.

Finally solo the toms-oh track and open the appropriate Duende channel. Again the middle frequencies are quite muddy, and some work should be done to clear these up.

  • The high-pass filter has been used at about 60Hz to clean up the bottom end and a slight cut at 180Hz using the LF band in BELL mode reduces some of the boomyness in this area.
  • Using the LMF band a small cut has been made at 430Hz to reduce the boxyness of the snare drum.

The Proof

Now solo the processed track and A/B with the unprocessed track. By carefully using filters, EQ and a small high end boost in places the overall clarity and openness of the drums has been improved.

Please bear in mind that all this EQing has been done with the channels in solo. This is often not the recommended way to mix, as to help a signal to sit in the mix you need to hear everything else that is going on around it. For these purposes however, the solo method has been used to help clarify what is happening with the EQ as otherwise it can be difficult to hear.