Used Signal Processor Buying Guide

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Used Signal Processor Buying Guide

Signal processors are used to alter audio signals in various ways. They can be either analogue or digital. Signal processors alter audio signals, for example, by compression, equalisation (EQ), filtering, noise reduction, or adding effects such as delay and reverb. This has two important purposes. The first is to even out sound, and reduce feedback and noise. The second is to add in effects to create new and creative sounds. The complexity and functionality of signal processors varies, with digital signal processors (DSPs) typically offering more in the way of functionality than analogue processors.

Choosing a Used Signal Processor

Signal processors may be single effects processors or multi-effects processors. Single effect processors, such as equalisers, only alter one component of the sound. Multi-effects processors can alter several components. Look at the key features of the processor to ensure it meets requirements in each area, especially when purchasing a multi-effects processor. Key features include compression and limiting, noise gating, equalisation (EQ) and effects such as delay and reverb.

Compression and Limiting

Compression reduces dynamic range, which is the difference between the weakest, or softest, part of a signal and the strongest, or loudest, part. Dynamic range is measured in decibels (dB). The primary purpose of reducing dynamic range is to achieve a more even and balanced sound. This is especially useful for vocals or instruments with very high and low extremes, such as drums. More extreme compression can be used to create new sounds used for special effect.
The dynamic range is only reduced if the input signal passes a certain threshold, which can be set. Lower thresholds result in more of the signal being compressed and a smaller dynamic range. Compression itself is expressed as a ratio, corresponding to input and output levels. As the compression ratio increases, the dynamic range decreases. A 5:1 compressor with an input signal 5 dB over the threshold will reduce the output to 1 dB over the threshold. The amount the signal is reduced by is known as the gain reduction. In this case it would be 4 dB.
The attack time of a compressor is the speed at which it works on the input signal. The release time is the speed at which the gain reduction returns to 0 dB. Longer release times typically give a more natural sound with less jitter, but release times that are too long can result in the signal sounding squashed. Both attack time and release time are measured in milliseconds. Many compressors allow these settings to be adjusted, but in others they cannot be altered.
Limiting is a type of compression where the ratio is high and the attack time fast. Limiters with higher ratios and faster attack times may be referred to as harder limiters, while softer limiters have lower ratios and slower attack times. Limiting minimises the type of distortion encountered by clipping, which is useful in preventing overload in loudspeakers and similar situations. There is no set ratio at which a compressor is defined as a limiter, but typically it is 10:1 to 20:1 or more.

Noise Gating

Noise gates are used to adjust the volume of audio signals. They allow sound to pass when the signal is above a set threshold, but if the sound is below the threshold, they cut it off. This is useful to prevent background noise, and is frequently used with drums, guitar amps and vocals, amongst other instruments.


Equalisers increase or decrease the amplitude in set frequencies to produce a more even sound. There are various types of equaliser, some with more advanced capabilities than others, though the basic mechanism is the same. All equalisers use filters which do just that; filter the sound in various ways. Two types of filter are high pass and low pass filters, which allow high and low frequencies respectively to pass unmodified, while other frequencies are minimised.
Graphic equalisers have several filters and employ sliders to control them. This is generally regarded as a more intuitive way of using equalisers and allows the sliders (and equalisation) to be displayed as a curve.
Parametric equalisers have several filters with parameters that give precise control over audio signals. This control includes the capacity to adjust amplitude (or gain), bandwidth, and centre frequency. Parametric equalisers typically have rotary knobs.

Delay and Reverb

Signal processors are able to add various effects to sound. Two of the most important are delay and reverb.
Delay can add ethereal tones to music. Delays of around fifty milliseconds with no feedback are frequently used for echoes, while longer delays up to 250 milliseconds are used for a specific type of echo known as slapback. Long delays can create loops. Other delay based effects include flanging, chorus, and reverb.
Reverb is a very commonly used effect, created by the use of multiple delays combined with feedback. There are various types, named after the type of effect they produce, including hall, room, and spring.


Another type of effect can be produced with the use of exciters. Exciters use compression, equalisation, and distortion to add intensity to sounds, making them brighter or richer, especially high frequencies.

Purchasing a Used Signal Processor

If budget is limited, there is a choice between buying a cheaper, and lower quality, signal processor new or buying a higher-quality processor second hand. The latter can be an excellent option in this situation.
Always bear in mind certain guidelines when purchasing an electronic musical device, such as a signal processor, second hand. One of the first considerations is the seller. If they have good feedback and high ratings, this means they have consistently delivered accurately described items and good levels of customer service. Check their returns policy and any extra information.
When it comes to the product description, it helps if there is a thorough description with clear photographs. Look for mention of the key features described above. All items should be in good working condition.
Lastly compare the price and features with other listings. Shopping around can often result in finding some good deals.

Find a Used Signal Processor on eBay

Go to the eBay home page and navigate through the left hand side tabs to the Musical Instruments page. Scroll down and select Pro Audio Equipment followed by Signal Processors & Effects. Scroll down and, under the heading Condition, select Used. Under the heading Type several options can be selected depending on the type of processor required. There are options for multi-effects processors, single effect processors, dynamic range processors or compressors, equalisers and exciters. The types of effects required can be selected under the Effect/Enhancement heading, while further down brand, price and other criteria can also be specified.
Used signal processors can also be searched for using the search function at the top of the page. Type in terms such as “used signal processor” or “used equaliser” to produce appropriate listings.


Signal processors alter audio signals, for example, by compression, equalisation (EQ), noise gating, or adding effects such as delay and reverb. Single effect processors only modify one aspect while multi-effects processors alter several components. Compressors, or dynamic range processors, can adjust the dynamic range to produce a more consistent sound. Compressors with higher ratios can reduce the input signal by a greater amount. Limiters are compressors with especially high ratios and are useful to prevent distortion. Noise gates allow sound to pass when the signal is above a certain threshold, but cut off all sound if it is below, thereby eliminating background noise from a particular source. Equalisers increase or decrease the amplitude in certain frequencies, which results in a more balanced sound. Two types of frequently used equaliser are graphic and parametric equalisers. Signal processors can also add effects such as delay, echo, chorus, loop and reverb to tracks. Exciters use different types of modulation, including compression and equalisation, to add brightness to sounds. Purchasing a used signal processor is an excellent option on a budget. Always remember to check seller feedback and read the product description carefully.

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