Getting more volume from germanium clipping

[russpurdy]

So I put a clipping/diode switch in one of my big muffs and im digging the germanium diodes for clipping but there is a monster volume drop and its a tad bass heavy. What would be the best way to solve this? I was thinking...

1) replace one germanium diode with a red led or possible silicon diode

2) try out asymmetrical clipping. Am I correct that in this case I would need a third germanium diode and have two going one way and the third going the opposite way? Is it possible to use silicon for the third diode to get more volume and bite?

Thanks!

 

[amz-fx]

Use four Ge diodes - two in series to replace each silicon diode. The volume will be similar to the silicon but you will have the Ge tone.

regards, Jack

 

[aaronmcoleman]

asymmetrical (with 1 silicon/led and 1 ge) will probably not sound good because it will take so much to clip one side and nothing to clip the other.

Three options i've tried and like in other circuits (who knows how you'll like them in a BM):
1) quad ge like jack said.
2) asymetric (1 silicon going one way, 1 silicon and 1 ge going the other way). this way the asymmetry isn't as massive as with an LED or silicon opposite a ge.
3) BAT41/46 somewhere in between ge and silicon.

theory lesson for the day: the most important characteristic when choosing a clipping diode is forward voltage. silicon is around 0.7v, germanium around 0.3v, LED are typically around 1.2-2v. the lower the forward voltage, the more clipping you'll get (for the most part) and the less volume you'll get...higher forward voltage will create less clipping early on and more volume.

when you combine diodes just add up the forward voltages (i.e. 2 silicon one way, and 2 the other way would add up to 1.4v each way). so two ge each direction will add up to 0.6v). when you're working with asymetric clipping you don't want a huge difference between the two sides most of the time it'll sound weird (1 silicon is 0.7v and 1 silicon+1 ge is about 1v).

Does this make sense? hope this helps in picking the right clipping diodes.

 

[midwayfair]

A couple things going on here.

1) Bass heavy: The Big Muff's diode clipping is frequency specific. A capacitor (usually 390pF) in the feedback loop of the second and third transistors cuts a bunch of treble -- that's neither clipped nor amplified. The other half of this is a capacitor in series with the diodes forms a high pass filter (everything about a certain frequency passes), which means only those frequencies are passed. In modern big muffs, it's 1uF, which is ... a lot, but really doesn't get all the bass frequencies. In some earlier big muffs, it was smaller. The green Russian, for instance, was 1/10 the size of modern big muffs, which is why those have so much more bass.

2) Volume drop: The diodes act as a kind of ceiling on the size of the signal. Germanium will clip the signal at .3V. Silicon (stock diodes) will clip it .6V -- twice as much signal!

What's going on when these two happen at the same time? You are putting a much lower ceiling on everything above the bass frequencies. Much lower volume, and a lot of what's left is bass.

Fixing this is complicated.

First, you have to decide whether there's actually something about germanium clipping itself that you (a) like and (b) can even hear.

It could simply be that you like that there's more clipping. In that case, using four germanium diodes will end up being disappointing, because there will be less clipping again.

Then there's a question of whether you are hearing any "germaniumness" at all. I'm not trying to insult your ears, but the big muff has so much filtering and multiple distortion sources that I sincerely doubt a human can hear a difference between two germanium diodes on each side an two silicon diodes that have exactly the same forward voltage. Plus, the diode clipping is technically soft clipping, so one of the benefits of germanium diodes (softer knee) is already taken care of by the circuit itself. You could test this theory by getting a few schottky diodes (for example BAT41 will clip at .35V, like a 1N34A, and BAT43 will clip at .25V, like some other germanium diodes) and trying them out in place of the germanium diodes. Most of the time, they will sound almost identical.

You should also put two diodes in series with each other on each side, so that their forward voltages are equal to the silicon diodes -- and switch between the diodes. Try to pick pairs of diodes that match each other pretty close, otherwise it's not scientific. Can you hear a difference? If you can, and you like what you hear, you've found your solution.

If it's just not giving you "that" feeling, then you need to look into a solution that addresses three things at once:
1) Corrects the tonal balance of bass frequencies.
2) Corrects the overall volume drop.
3) Switches out the diodes.

Let's tackle #1.

There are a couple ways to go about this. The easiest is to include in our switching scheme a way to increase the size of that capacitor that's in series with the diodes. If we make the capacitor big enough (10uF should do it), it'll pass all the guitar frequencies, and the bass will be clipped along with the treble. This will keep it so that the bass isn't overwhelmingly loud in the rest of the signal. This isn't a perfect solution: You will end up with a little less bass than you started with on the silicon diodes, but it's certainly more satisfactory than having way too much bass.

Next we have to correct the overall volume drop.

Fortunately, the Big Muff has a make-up gain stage at the end of the pedal. You've dropped the total drive of the pedal, and now that last transistor isn't getting hit as hard. We need more volume, and Q4 can give it to us. The easiest thing to do here is just to ground its emitter. This will push the last stage harder, and should give you quite a bit more volume.

Finally, we need to switch out the diodes. That's easy. Except you've probably used more than one pole on a switch, and you are like, "Well, my switch doesn't have enough connections". Fortunately, we can do all of this with just a double pole toggle:

These are the six lugs of your switch:
1 4
2 5
3 6

Create a small daughter board with the following on one side:

-<always connected to the collector of Q3 ->
positive side of the 10uF capacitor (it will be polarized) ->
negative side of the capacitor connects to one side of the germanium diode pair ->
Other side of the germanium diode pair connects to lug 1

On another part of the little daughter board, put this:
-<always connected to the collector of Q3 ->
one side of the original 1uF capacitor that's in series with the diode pair (if it's polarized, put the positive side toward the collector of Q3) ->
other side of the capacitor connects to one side of the silicon diode pair ->
Other side of the silicon diode pair connects to lug 3 of the switch

Lug 2 of the switch connects to the base of q3.

Now you can switch out the whole assembly at once.

For the other row on the toggle, connect lug 5 to the emitter of Q4, and lug 4 (same side as the germanium diodes) to ground.

Now when you flip the switch to the germanium side, you'll get a volume boost and change the clipping diodes. When you switch to the silicon side, it's the stock circuit.

Other things you can try:

1) Use an on-off-on switch: Now your center position will disconnect the diodes completely.
2) Change the diodes in Q2 instead of in Q3. This will change the gain structure of the pedal in a different way -- in fact, the pedal may sound less dirty overall when the germanium is switched in than it will if you're doing this on Q3. And you may end up will less of difference in bass. It could be an overall improvement.