Simple Mosfet Compressor MODs

Another designer tried my mosfet compressor using a darlington pair as the gain stage  device Q1. He described some improvements in performance – so I tried this as well. I love it! Using a darlington such as a MPSA14 (or a homemade one) is a drop in replacement for the MOSFET Q1. It gives a little more gain and reduces noise in the control loop slightly (allowing more freedom in changing and or eliminating C7, c8, C9). I definitely can reccomend it.

New Schematic:

https://circuitsaladdotcom.files.wordpress.com/2012/11/mosfet-compressor1.gif

New compressor design – updated 10/28/12

Here is the schematic for a new compressor design, which has very small parts count but flexibility in threshold, attack and decay settings.

The design uses generic NMOS FETS such as the BS170 or 2N7000 and the only critical part is the one dual op amp which needs to tolerate voltage swing at or  below ground. I am using a LMC6482 but others will work. Another designer built my original compressor using the LM358 and after looking at the data sheet believe it will work here as well.

I am using a 2N7000 as a voltage controlled resistor and it works well but cannot tolerate a drain to source voltage of even 100mV. I solve this by using shunt feedback from the drain to gate in the amplifier stage. This creates a cancelling signal at the input node, proportional to the gain, which reduces the net voltage seen across the 2N7000 down to tens of mV(with a 2 volt input).

The 2N7000 has turn on voltage starting at about .8 volts. The circuit has a 2N7000 configured as a supply independent voltage reference which provides the bias for the voltage controlled resistor. This reference is adjustable and can be used to set a variable threshold of compression or fixed at whatever threshold desired.

This design also uses a simple op amp peak detector which would be normally used in a sample and hold circuit. A bleeder resistor is added to create a decay response as desired. Because a peak detector such as this tracks instantaneous level changes – it needs to have it’s very abrupt shifting of output level smoothed out to eliminate sharp noisy spikes as the amplitude changes rapidly. This is achieved with a simple low pass integrator on the input of the 2N7000 voltage controlled resistor. This smoothing filter sets the attack. The peak detector can use just about any diode. I am using an LED which lights up and varies in intensity with respect to the amount of compression. This provides a visual indicator of how much compression is occurring. No adjustments are required to use different diodes.

In all, you can adjust the attack, decay, threshold, compression level (from 1:1 to greater than 3:1), and output level with this design, and it only uses three mosfets and one op amp.

Check out the schematic here: Consider this obsolete….. look below for improved design

https://circuitsaladdotcom.files.wordpress.com/2012/10/simple-compressor.gif

Update: I am changing the design of this compressor somewhat to eliminate some noise caused by the closed loop peak detector. The updated circuit is below. It uses an open loop compensated  peak detector instead of the closed loop type. Also I separated out the compression level indicator – which really works well. It has the benefit of giving visual indication of threshold adjustments which can be set by R6 and R10.

I don’t need the final buffer either because the threshold settings and control loop gain allow the output match input level at max compression.  Many of the component values can be adjusted so expect to play around with some values. Currently, I am using : R6(100k), R9(1Meg), R10(10k), C10(.1uF). Changing R9 and C10(smaller cap and larger resisitor) lets the op amp slew a little faster – no big deal. If I have any big revelations that something is better – I will post it!

New Schematic:

https://circuitsaladdotcom.files.wordpress.com/2012/11/mosfet-compressor1.gif

 

OPTO FET issue solved with Negative Feedback!

By applying shunt shunt feedback from the Drain to the Gate of the first JFET stage of my compressor, I reduce VDS across the optofet  by a factor of 5! So with a input of 1 volt, the the VDS of the optofet is about 200mV worst case. Now it performs beautifully. The feedback consists of a 470k ohm resistor in series with a .1uF cap from drain to gate. Now the source must be bypassed with a 10uF cap – where before it was optional. The gain of the stage with this feedback is now about 4. What happens is that the feedback subtracts from the input at the optofet drain node greatly reducing the VDS across the voltage controlled resistor – while still providing gain.

Further Simplification of the Enhanced Orange Squeezer Compressor

It occurred to me that if one is willing to sacrifice some flexibility in the decay length of my compressor design; that it can be simplified by removing the source follower and the Zetex current sensor. What this means is that the control voltage is only half wave rectified instead of full wave, but if you just increase the filter cap – the circuit should still work fine. The trade-off is that you get limited to longer decay times only, but for most guitar applications this is fine. I have not verified this circuit but I will do this soon. I am confident it will work well. Removing these parts may make it more attractive to the DIY builder.

New Schematic:

https://circuitsaladdotcom.files.wordpress.com/2012/09/compressor3.gif

Pedal and Amp demo videos to be posted

I have been building a lot of guitar electronics these days … and more to come I hope! Having said that, I think it is important to show how they can be used in real music and how well DIY designs can perform as compared to commercial products. My first demo is of my portable busking amp and my FET discrete signal chain compressor. This demo  represents a classic jazz sound using the compressor to understate all of elaborate grips on the guitar and make the lines very smooth sounding even with heavy down strokes.

check it out at:

more demos to come shortly!

Enhanced Orange Squeezer type Compressor

This is my twist on the Orange Squeezer compressor. I use an h11M1F optofet instead of a JFET as a current controlled resistor (not voltage controlled). The signal chain is discrete FET and the control signal is generated from a ZETEX  ZXCT1041 current sensor – used for battery chargers. It is configured to act as a full wave precision rectifier. Unlike the Orange Squeezer you can adjust level and compression – it has all sorts of potential for mods. The  ZXCT1041 can be replace by an opamp such as the one I use for the peak detector, configured as a precision full wave diode(see op amp data sheet) – not much difference either way.

Schematic: https://circuitsaladdotcom.files.wordpress.com/2012/09/fet-compressor.gif

Picture: https://circuitsaladdotcom.files.wordpress.com/2012/09/img_4862.jpg

Sound Sample: https://circuitsaladdotcom.files.wordpress.com/2012/09/compressor-blues.mp3

recording starts with no compressor -> compressor on-> off->on->off-> ends with compressor on

UPDATE: 10/14/2012

I have noticed that at very low compression settings with high guitar input levels(>.5Volts) there can be some distortion. The Opto FET  I am using appears to have distortion that looks like crossover distortion with signals greater than 200mV. When compressing the signal across the OPTO FET decreases so that normally eliminates this issue but at low settings – this may be a problem. I am going to explore just using a linearized JFET in its place and or other solutions.

UPDATE:10/17/2012

By applying shunt shunt feedback from the Drain to the Gate of the first JFET stage of my compressor, I reduce VDS across the optofet  by a factor of 5! So with a input of 1 volt, the the VDS of the optofet is about 200mV worst case. Now it performs beautifully. The feedback consists of a 470k ohm resistor in series with a .1uF cap from drain to gate. Now the source must be bypassed with a 10uF cap – where before it was optional. The gain of the stage with this feedback is now about 4. What happens is that the feedback subtracts from the input at the optofet drain node greatly reducing the VDS across the voltage controlled resistor – while still providing gain.