aion Lab L4 BASS PREAMP Manuale - Pagina 13

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BUILD NOTES, CONT.

Ch. 1 midrange, Ch. 1 volume, and Ch. 2 volume potentiometer values

The original L4 amp used 2.5kA potentiometers for both Midrange & Volume of channel 1 and Volume

of channel 2. This is a non-standard value today and not available from any standard parts suppliers.

Tayda Electronics carries 2kA, which is certainly close enough and will work fine in this circuit. But in this

project, to get as close as possible, we included parallel and tapering resistors on the PCB so standard

pot values could be used.

For the Ch. 1 midrange pot, we recommend using 5kA, with a 5k1 resistor in parallel (R7). Since it's wired

as a variable resistor, this drops the resistance value to 2.5k and only steepens the curve slightly.

Both volume pots are wired as true potentiometers with all three terminals used, so it's a little more

complicated. For these, it's recommended to use a 10kB pot with a 2k7 resistor (R13 and R49) between

pins 2 and 3, and a 1k resistor (R14 and R50) between pins 1 and 2. This will give a good approximation

of a 2.5k pot, and the resulting taper will be very close to logarithmic.

LM13700 vs. CA3080

Our original L5 Preamp project used CA3080s as in the original amp. These have been discontinued for

a long time, and while they have been reissued and are not terribly hard to find, they are expensive.

The LM13700 is essentially a dual version of the CA3080/CA3094 and is still readily available. Since the

L4 circuit uses two CA3080s, it seemed like a good opportunity to replace them with a single LM13700.

The only change that this required was to add a second 100k resistor (R77) to cut the compressor LED's

voltage in half, since the LM13700's I

pin sits at twice the voltage of the CA3080's pin. This extra

ABC

resistor prevents the LED from staying on all the time, which was the only side-effect of substituting the

LM13700 in place of the CA3080.

Bypass PCB component orientation

Due to the height of the DC converter and electrolytic capacitors, the components on the switching PCB

are mounted on the bottom, the same side as the footswitch. This is in contrast with most other Aion FX

projects where the switch is mounted on the bottom of the switching sub-board and the components

face up. Components are always mounted to the side with the silkscreen footprint.

L4 BASS PREAMP