How to address unwanted distortions (english)

Some users may experience clipping and noise artefacts after updating. While some distortions are inherent to the simulation (and also to be found on a real Hammond organ, like AO28 triode distortions and power amp overdrive), sometimes an old parameter settings or a too aggressive tapering may lead to unwanted distortions. Here is a short manual to locate and defeat these, valid for HX3.5 as well as HX3.6 boards.

Distinguish Clipping and Modulation Artefacts

While clipping will sound like a very hard overdive, modulation artefacts are "swirling", grainy noises and non-harmonic "alias" frequencies. Modulation artefacts are independant from swell pedal setting; they may occur when modulation depth of vibrato, phasing rotor or rotor/horn delay is set too high. Mostly modulation artefacts are caused by rotary horn modulation, especially with higher notes on rotary FAST.

Locate Rotary Modulation Artefacts

While HX3.6 uses a more sophisticated oversampling technique to obtain an authentic spread modulation of horn and rotor sound beams, HX3.5 does not allow oversampling here due to FPGA space restrictions.

To locate the source of modulation noise, temporarily reduce LFO Mod Horn parameters #2116 ff. in Editor's Rotary Models section while rotary is on FAST. Modulation artefacts will dissappear with lower modulation factors; however, effect "spread" will suffer with lower values.

HX3.5 Sound Engine Issue

Late 2023 versions of HX3.5 sound engine do have a nice spread modulation, but suffer from modulation artefacts, especially on rotary FAST due to a too high "diffusor spread" modulation.

Diffusor spread modulation is derived from LFO Mod factors, it has no separate parameter to tweak. We reduced spread modulation in 2024 FPGA sound engine versions to avoid these.

See HX3.5 Firmware Update Changelog resp. HX3.6 Firmware Update Changelog and update to newest FPGA sound engine from BETA directories.

Locate Clipping Source

Locating the distortion source is crucial. There are several places where clipping may occur in the simulation, as internal audio processing is limited to 16 bit due to FPGA constraints:

• Busbar accumulators
• Matching transformer
• AO28 preamp
• Tube power amp
• Speaker crossover
• Speaker simulation

Finding the distortion source is handled from top to bottom. Play a handfull of notes known to be critical at full swell. Repeat your tests always with the same notes played. Set Rotary and Tube Amp (Tabs #1136/#1137) to Bypass and Volume Pots AO28 Tube Age #1089 to 0, so they won't affect your sound. If reducing a parameter mentioned does not reduce clipping, leave it where it was. Do not change parameters randomly, as this will limit the organ's dynamic range as well as (intended) tube amp overdrive.

Clipping still occurs when Rotary and Tube Amp set to Bypass

This means that clipping occurs in the organ itself (first three points).

• Reduce AO28 Trim Cap Swell #1088.
• Reduce manual/pedal/percussion levels, parameters #1082 to #1085.

If reducing these does not help, a too aggressive tapering may lead to busbar accumulator overflow. Update the taperings to latest version or create new taperings with a lower scaling value (Defaults tab in HX3 Tapering Designer).

Clipping only occurs when Rotary and Tube Amp are active

The purpose of a tube amp overdrive is to produce distortions, so we will miss out this point here. However, unwanted clipping may occur in the speaker crossover and rotary simulation. In this case, try the following:

• Reduce Rotary Inits, Input Level #2104.
• Reduce Rotary Inits, Horn Level #2105 and Rotor Level #2106, especially when clipping depends on speaker's horn/rotor position.

Please note: These settings are saved to the particular speaker model.

Ground Loop Noise

Ground loops are a common source for a humming and buzzing noise on audio outputs. While analog and digital grounds are strictly separated on the HX3 mainboards, improper wiring inside the instrument can result in an unintentional ground loop. If you experience a buzzing noise that changes slightly when pressing a panel button (no manual keys pressed), a ground loop may be the cause. Chassis or metallic parts should have a one-point connection to ground only, preferably to mainboard's power supply connector. Be sure to separate analog and digital grounds in your instrument.