FET/RACK Operational FAQ

Posted by Hairball Admin on Mar 13 filed in DIY Resources

When I power the FET/RACK in GR metering mode, the needle sits one-half to one full dB short of zero. Is this normal?

Yes. The FET/RACK, like the original, has a fully discrete meter circuit. The components in this section are sensitive to heat and when powering a cold unit, your meter will rest a little sort of zero. It will take at least 1 hour for the meter to slowly climb back to zero and it may still fall short or overshoot zero. After hours of usage, it may fall a little again. The variance in this part of the circuit is why the designers placed a zero adjust accessible through the front panel. If you’re keen to have it at exactly zero, you’ll be using this adjustment hole a lot.

Should Q6 be getting so hot?!

Yes. It’s the Class A output stage transistor and it’ll run very hot. It should always have the supplied heat sink attached to it to avoided thermal damage to the component.

The large resistor on the VU meter lamp PCB is getting hot!

That’s normal. That resistor is used to drop the positive supply rail from +30V DC to about +12V DC and illuminate the meter safely. It needs to dissipate 18V DC at around 100mA.  Most of this power (voltage x current) is dissipated in the form of heat, which is why the resistor gets so hot. The resistor is rated at a higher wattage than its handling and though it will get hot, there is no need for a heat sink.

I think I smell something burning is this normal?

If it’s subtle, yes. Q6 and the stud diode can emit a slight burning smell.  Smoke and a horrible burning smell that fills the room is not normal.  Should this happen, turn the unit off and consult the help documents and support threads.

When I calibrated the unit I could hear the sine wave emitting from the output transformer. Is this normal?

Yes at high-gain this is completely normal. The transformer windings are moving at your test frequency and their vibration is audible.

When selecting “All Buttons In” mode, my meter needle slams to the right.

That’s what it should do. ABI mode was not intended to be a proper operating mode. It’s a hack, so to speak, and the meter will slam hard right with no signal. As you apply gain reduction it will move back left chaotically.

4:1 seems to show more compression on the meter than 20:1. Are my ratios reversed?

No, your ratios are fine, what you’re seeing on the meter is GR at one given point. This is not the same as ratio. The FET/RACK has a fixed threshold that is generally lower at lower ratio settings (4:1) and higher at higher ratio settings (20:1). Generally, you will see more GR at 4:1.

Ratio describes the rate of change between the input signal vs the compressed output signal. If your input is 2dB and your compressed output is 1dB then suddenly your input increases to 4dB and the resulting compressed output increases to 1.5 dB, your ratio is 4:1. The input has increased by 2dB and compressed output has only increased by 0.5dB.

The original manual provides a nice graphic showing both threshold and level vs ratio.


My attack control doesn’t seem to do anything.

The attack time for the FET/RACK is 20 microseconds to 800 microseconds. To put it more plainly, that is 0.00002 seconds to 0.0008 seconds. You’re not going to clearly hear the attack, it really just effects how much transient is compressed.

The input seems really hot. I’m getting a lot of compression at the “36” setting.

Here is the one difference between your FET/RACK and a vintage unit. The input pot (t-pad attenuator) on the original units had a very odd taper. It’s not a taper you commonly see in potentiometers today. Your input attenuator has a more common taper, which leads it to be a little “hotter” earlier in the clockwise pot turn. If you’re finding it too hot, decrease the level of the signal source.

In active link mode the units do not seem to be tracking in stereo.

You may notice that your GR is a little less in one unit due to side chain tolerance differences. To get the two units tracking the same, you'll need to adjust the attack on one unit slightly. Typically adjusting the attack on one unit will make tracking worse, and the other will make tracking better. Now that the units are tracking properly, they may have slightly different output levels. Using your DAW metering or +4/+8 on the FET/RACK units, adjust the output level using the output controls. Note that using a pure sine wave is not a good method for testing the active link.

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