Anyone built hotamps noise gate? Any thought of it?

[nguitar12]

Anyone built this hotamps noise gate? If not can someone please comment according to the circuit? It seem that there are no input buffer for this circuit. Does it mean that I am expecting volume drop when turning the pedal on?

[balkanizeyou]

the signal goes into a non-inverting amplifier which has a high input impedance, paralleled with the biasing resistors. The overall input impedance should be high enough not to drag the signal down when turned on.

[Kipper4]

Tried it and it didnt work for me.

[amptramp]

The first transistor seems to have no controlled time constant for the base voltage to drop to ground.  The circuit will give different results at different temperatures.  But even if you get a noise gate working, it is still a gate that switches between on and off suddenly and would be somewhat obtrusive in the music.  Back in the 1930's, RCA came out with a circuit that did what you would probably prefer to combat noise on the shellac records of the day: a lowpass filter that passes all signals when there is sufficient signal but cuts the treble severely when the signal level is low:



The signal enters on the left and exits top right.  There is a 6AV6 triode amplifier and a diode rectifier that drives the voltage negative with high enough signal, cutting off the 6BA6 pentode and letting the signal pass.  If the signal level drops low enough, the 6BA6 acts as a reactance tube and the inverting gain of the tube multiplies the 27 pF capacitance from 6BA6 plate to output, cutting the high frequencies like an auto-wah pedal set for minimum treble at low signal levels.  The 330 pF capacitor in parallel with the 1 megohm resistor at the upper left forms a divider like an oscilloscope divider so with enough input, the low-frequency input and high-frequency input have equal attenuation.  Only with low signal do you get the output to ground capacitance appearing to be so large as to kill the treble.

This is a tube circuit but can easily be recast as a transistor or op amp circuit.  It needs a buffer with high input impedance to be added at the output to maintain control of the gain.  A transistor or op amp stage could substitute for the triode amp and another could be the reactance tube replacing the 6BA6 and another transistor or op amp buffer stage could be added to the output.

The advantage of this circuit is that there is no abrupt switching - just noise reduction at low levels as intended in the original circuit for the RCA VRA 141 high-end radio phonograph.

[PRR]

Magic Monitor recrop: http://oi63.tinypic.com/5ldi0h.jpg

> first transistor seems to have no controlled time constant for the base voltage to drop to ground.

Base will drop to 0.5V (off) at 1Meg & 100nFd (0.1 sec).

> different results at different temperatures.

Yes, say 480mV to 520mV for +/-10 deg C. +/-0.6dB. Well, more because of several diodes in the loop. Still not a large range for any environment you would want to play in. (Yes, the New Years and 4th July gigs might differ; but there's a knob for that.) I might like a better defined transition, but hey, much funkier stuff has been popular.

The transistor clamp (2nd NPN) also has funk as it passes from hi-Z to low-Z, though it was popular as a Mute on cassette decks.

> didnt work

"didn't work" is no use. Didn't pass signal? Didn't gate no-signal? Turned polka riffs into salsa? Went on strike and picketed your factory?

[nguitar12]

It is quite disappointing. I thought it would be a good and easy noise gate to built.