Smiley fuzz face bias indicator LED

[markusw]

Have a look @ http://myweb.absa.co.za/g.landry/Smileyface.htm

Looks like a good idea. LED is green when bias voltage is below 4.5 V and red when bias V is above 4.5 V (or vice versa). Exactly what I was searching for. Any comments from the FF gurus?

Markus

[Alex C]

While we're waiting for a guru, I thought I'd pipe up and say thanks, that's a neat article, and a real neat idea with the bias indicator LED.  Very interesting read.  Thanks!

Alex

[Gus]

Looking the archives I descibed a circuit like this to Joe G for his petals years ago

[Paul Perry (Frostwave)]

What a small world! I came across that guy's site  when I was looking into ultrasonic air leak detectors, but never got to the music page!!
(note some air leak detectors and bat detectors are based on ring modulators to shift the ultrasonic hiss down to audio).

[cd]

How long do you think it's going to take for boutiquers to integrate this as the latest and greatest feature in their pedals, even though the PDF is dated 2002?

[markusw]

Looking the archives I descibed a circuit like this to Joe G for his petals years ago

Could not find it in the archives. Any hints?

Markus

[markusw]

May I use a TL072 instead of a LM358?

:?:

[Hal]

whats the current draw on the relay?  I've thought of using this type of bypass for a while, but I figured it would either eat batteries, or I would need a double latching type - with a cost at that of a stomp switch, already.  O-well  :roll:

[markusw]

OK. I´ve breadboarded the indicator part of the above circuit and connected it to my FF.

It´s a PNP/neg gnd FF with an input and an output op amp buffer. Although it´s not yet boxed it´s working and actually hum-free. Just at higher fuzz settings when both the pregain pot and the volume control (on the guitar) are turned close/to zero there is a hum. It gets higher in frequency when you slowly turn the pregain pot up until it disappears. Above about 3k for the pregain you don´t get anymore oscillations regardless how the other pots are set. So I´m pretty happy with it. By the way Q1 is 2N2904 with ~70 hfe and Q2 is a AC128 with ~100 hfe (and a leakage current of 0.4 mA).

When I connect the indicator circuit to the FF board with crocodile cables I get oscillations even if the volume pot is fully up. Just when I turn the fuzz pot completely down and the pregain pot up to at least 10k it stops. Then it is really nice to adjust the bias using the indicator LEDs. When the cable between the collector of Q2 and the indicator circuit is not connected the hum is gone. However, I would like to keep it connected all the time.

I have to admit there are a lot of crocodile cables around for connecting the FF PCB with the breadboard and also some pots to the PCB. Any ideas whats happening? Is it just a breadboard phenomenon or.....no idea...
  :?:

[markusw]

Bump  

[stm]

John Hollis made a nice temperature stabilizing circuit for a fuzz face based on a single opamp a LED and a photoresistor.  This approach is fully automagic, so you don't need to manually set the bias point every time you wanna play.  Of course you may miss the nice dual coloured LED...

Check the RockFace schematic at:

http://www.hollis.co.uk/john/circuits.html

[The Tone God]

If you interested in self-biasing transistors do a search for "servo" bias circuits. There was thread about this awhile back that resulted in a couple of auto-biasing circuits.

Andrew

[markusw]

Thanks for your answers!!  

John Hollis made a nice temperature stabilizing circuit for a fuzz face based on a single opamp a LED and a photoresistor.

I really like the idea with the LED/LDR combo. Since I already populated the board it is unfortunately impossible to implement it. Therfore I have to use it for my next project, for sure.

If you interested in self-biasing transistors do a search for "servo" bias circuits. There was thread about this awhile back that resulted in a couple of auto-biasing circuits.

Great stuff to read through. Did´nt know Q1 needs bias adjustment too. Maybe in my next project I will try to implement auto-biasing for Q1 and either auto-biasing for Q2 too or an external bias pot for Q2, if I can manage it.

But for my current FF project I first have to get rid of the oscillations caused by the single bias indicator circuit. Any suggestions  :?:  :?:

[markusw]

Got rid of the oscillations by sticking a low-pass filter consisting of a 1 M resistor and 0.01-0.1 µF cap between Q2 collector and the smiley indicator circuit. Works like a charm now.

Markus