aion Heliosphere Manuel - Page 5

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BUILD NOTES

C2 capacitor

In the stock circuit, at higher gain settings, the tone control must be at absolute maximum otherwise the

treble is overly muffled. This is entirely caused by the 10n feedback capacitor which cuts a significant

amount of treble in the first gain stage, with a corner frequency of 318 Hz (!) at max gain. Compare this

to the RAT whose feedback capacitor is 100pF in this position, filtering above 31.8kHz at max gain.

We strongly recommend changing the C2 capacitor to 100pF, which solves this problem entirely and

turns the circuit into a really interesting lower-gain RAT.

RAT modifications

The Drive-O-Matic is essentially a modded RAT, so if you want your build to be a little closer to the

source circuit, you can make any or all of the following changes. Even if you revert back to a standard

RAT, it's still well worth building due to the footswitchable drive channels.

• C2: 10n → 100pF (restores the treble frequencies)

• C10: 10n → 1uF (increases bass at output)

• R1: 2M2 → 1M (reduces input impedance for lower op-amp noise)

• R3: 680R → 560R (increases gain of both channels)

• R4: 100R → 47R (increases gain of channel 1)

• R5: 100R → 47R (increases gain of channel 2)

• DRIVE 1: 50kA → 100kA (increases gain of channel 1)

• DRIVE 2: 50kA → 100kA (increases gain of channel 2)

LM308 and OP07

Like the original RAT, the Stamps Drive-O-Matic used the LM308, an early op-amp with a very low slew

rate compared to modern alternatives. The slew rate is critical to the unique sound of the RAT.

In the mid-1990s, the LM308 supply began to dry up, so after some research & experimentation, Pro Co

changed over to the OP07 for their production RATs. Despite insistence from gear enthusiasts that the

LM308 sounds better and the OP07 is inferior, it's been shown on more than one occasion (both by

DIY community

and more thoroughly by JHS) that the OP07 performs and sounds identical when used

in the same circuit with the same settings.

Because of this, we have specified the OP07 for this build and recommend it over the LM308.

The LM308 has been obsolete for a long time, and most of what's available today on eBay and

component distributors in Asia are relabeled fakes, so it's not recommended to risk using one. Let the

article from JHS convince you the LM308 is not special, and use the OP07 with a clear conscience!

C3 compensation capacitor

The OP07 op-amp has an internal slew-rate compensation capacitor while the LM308 does not. C3

(27pF) should be omitted when using the OP07, but it's required for the LM308 to work properly.

HELIOSPHERE DUAL DISTORTION

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