Diode Switching / Volume Level Issue

[fuzzy645]

So I have a rotary switch hooked up to switch between various diode & LED combos (both symmetric and asymmetric) in the Easy Drive.  The sound of the different combos is great, however there is a significant volume difference between some of the settings which I am speculating is due to the variation in voltage properties.  Continuing with the speculation (ie grasping at straws) I am guessing that a resistor(s) might  help here.

Any ideas on how to balance out the levels?

Thank you

[teemuk]

Resistor dividers or alternating gain. Whichever you choose to use, you naturally need to switch it while you switch diodes. So, be prepared to replace that diode selector switch with one that has more poles and to add some extra stuff to the circuit.

There's also two quite different methods to approach the entire thing...

1) Equalize the clipping thresholds. Say, a germanium diode maybe clips at about 0.350mV (I say maybe because they actually have a great variation), a silicon diode at about 500mV and a LED maybe at about 1.8V (again there's a great variation depending on the type of LED). So, you attenuate the signal proportionally before it hits the clipping stage. As a result though, you may find out that all those switched diodes actually will sound pretty much the same. Yep, a germanium diode does have a softer "knee" than LED but all in all, in practice, you won't really be hearing that in any great extent.

which brings us to...

2) Equalise the signal amplitude post clipping. As said, the forward voltages of different diode types are different, LED-clipped signal will have much higher magnitude than germanium-diode-clipped signal and in essence you must attenuate the signals "somewhat" proportionally to that. (Or alternatively tweak the gain level introduced by following stages to achieve the same effect). The "somewhat" in quotes comes from the fact that when you clip with LED in comparison to germanium diode and equalize the levels the germanium diode, due to its much lower clipping threshold, compresses the signal more resulting into higher average amplitude, which you will in turn perceive as louder signal (and naturally as a greater amount of distortion too). So the correct ratio at which to equivalize the signal levels - so that they all end up sounding about as loud - is a matter of a bit of experimentation.


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Of course, some people just add a simple volume control potentiometer and adjust the level accordingly after flipping the diode selector switch. It's an effective and easy addition but of course not marvellously versatile if you want to change the clipping diodes while playing.

[nexekho]

Could use trimmer pots to find the right level - they're dirt cheap if you use the caseless ones (don't know how good they are)

[digi2t]

Here you go, diode switching, with a trimmer (VR6) for the volume offset. You could even mount a pot outside, and use is as a boost level;



Idea courtesy of Mark Hammer.