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Vintage threads from the first ten years

De-Hiss

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6/21/2004 4:47 PM

Hey Guys,
I've been building FX for a while now, but I still keep coming back to my FuzzFace. I think the tone is great. The only drawback is that the circuit is quite noisy, especially with the 5k Battery drain maxxed out. Is there any way to cut down the amount of hiss and static this thing produces when the guitar is not being played? I don't want a noise gate or anything, just a quiet version of my beloved Box!
I added a 22pF ceramic cap from the collector of Q2 to ground which has pretty much killed all the radio interference, but the hiss is still there.
Any ideas guys??

*b

6/21/2004 6:10 PM

The classic solution is either a downward expander or noise-filter or both.

Downward expanders are the complete inverse of limiters. Instead of blurring any differences in signal level ABOVE some threshold, they accentuate small differences in signal level BELOW a designated threshold. So, you won't notice any gating or unusual effects on your playing dynamics, but once you stop playing, the hiss will seem much lower in amplitude than before.

Noise filters work more or less like Mu-Trons. That is, you get full bandwidth when the signal is of reasonable strength, but the treble (and hiss) is rolled off when you stop playing or play softly. The basic principle is that hiss is not noticed under some conditions but becomes more noticeable under other conditions. The filter kicks in during those other conditions.

The Rocktron HUSH system implements both of these in a single device. The National Semiconductor LM1894 (and its successors) are commercially available single-chip noise filter devices. Read the appnotes for the 1894 and you'll find out more.

7/2/2004 3:27 AM
RedHouse	I add a 100pF ceramic-monolithic across the collectors of Q1 and Q2, seems to help the hiss and any aberant oscillation issues.

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