A-138b

Doepfer 8HP
MIXER UTILITY VCA

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4-channel logarithmic mixer with per-channel attenuators and a master output attenuator. Log taper on pots tracks perceived loudness, favouring audio summing over CV. A-138a is the linear sibling; otherwise panel-identical.

Patch Ideas · 4

3-VCO voice summing
Three VCOs → In 1–3 · blend saw/square/sub · Output → VCF
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 4 connections. Modules: A-138b, VCO 1, VCO 2, Sub Osc, VCF. Signals: 4 audio.A-138bVCO 1VCO 2Sub OscVCFIn 1: 7 · In 2: 5 · In 3: 3 · Out: 8Input 1audioInput 2audioInput 3audioOutputaudioSawaudioSqraudioSineaudioInaudioaudio
Drum bus
Kick/snare/hat/perc VCAs → In 1–4 · master Out → desk
▸ Show diagram
Patch diagramPatch diagram with 6 modules and 5 connections. Modules: A-138b, Kick VCA, Snare VCA, Hat VCA, Perc VCA, Mixer. Signals: 5 audio.A-138bKick VCASnare VCAHat VCAPerc VCAMixerIn 1: 9 · In 2: 7 · In 3: 6 · In 4: 5 · Out: 7Input 1audioInput 2audioInput 3audioInput 4audioOutputaudioOutaudioOutaudioOutaudioOutaudioLine Inaudioaudio
Final stage with master fade-out
Patch output → In 1 · ride the Out knob for crescendo/cut
▸ Show diagram
Patch diagramPatch diagram with 3 modules and 2 connections. Modules: A-138b, Patch, Amp. Signals: 2 audio.A-138bPatchAmpOut: manualInput 1audioOutputaudioAudioaudioInaudio11. Out knob as performance faderaudio
FX blend
Dry to In 1 · reverb return to In 2 · delay return to In 3 · Out to mains
▸ Show diagram
Patch diagramPatch diagram with 5 modules and 4 connections. Modules: A-138b, Dry, Reverb, Delay, Mixer. Signals: 4 audio.A-138bDryReverbDelayMixerIn 1: 8 · In 2: 4 · In 3: 5 · Out: 7Input 1audioInput 2audioInput 3audioOutputaudioAudioaudioOutaudioOutaudioLaudioaudio

Behaviors

Perceptual audio summing audio → any input

Log taper makes channel faders behave like a mixing desk: small rotations near the top produce subtle level changes, bottom of travel fades smoothly to silence. Preferred over A-138a for summing VCOs, drum voices, or VCA outputs.

Master fader / interface to desk Output → external mixer or amp

The master Out attenuator is unusual for A-100 and lets A-138b function as the final stage of a patch. Pull it back to mute the whole patch without touching source modules.

Not ideal for precise CV summing CV → inputs

Log taper distorts linear CV relationships — a pitch offset mixed here will not sum in volts as the knob reads. Use A-138a (linear) for CV-accurate summing.

Unpatched Ch 1 as DC offset JP4 jumper installed · no cable in Input 1

On units from mid-2004, JP4 in right position converts Ch 1 into a ~0 to +5V offset source; left position gives ~0 to −5V. Cable insertion in Input 1 disables the offset and restores audio.

Controls

Channel 1 In 1 Logarithmic attenuator for Input 1.
0 (silent) to 10 (unity) · log taper
Channel 2 In 2 Logarithmic attenuator for Input 2.
0 to 10 · log taper
Channel 3 In 3 Logarithmic attenuator for Input 3.
0 to 10 · log taper
Channel 4 In 4 Logarithmic attenuator for Input 4.
0 to 10 · log taper
Master Out Output level attenuator — also log taper. Lets the mixer sit at the end of the audio chain feeding a desk or amp directly.
0 to 10 · log taper

I/O

IN · 4

  • Input 1 audio / modular-level AUDIO
    Audio input 1. Post-2004 units include a jumper (JP4) that turns Ch 1 into a ±0–5V DC offset generator when Input 1 is unpatched.
  • Input 2 audio / modular-level AUDIO
    Audio input 2.
  • Input 3 audio / modular-level AUDIO
    Audio input 3.
  • Input 4 audio / modular-level AUDIO
    Audio input 4.

OUT · 1

  • Output audio / modular-level AUDIO
    Summed and master-attenuated mix of all four channels.