BLACK FOREST High Gain Preamp - Based on the Bogner Uberschall

[fretzburner]

Liked the mercury box too high gain OD that will sound like this ENGL but lesser gain than ENGL but can do light OD tones on lower gain settings.About this Engl did changed the 4.5v cap to 10uf tant and the 2.2uf tant on V1,add 100ohm resistor at 9v line.i can hear improvements(tolerable for me) and the whistling sound too.Will change later other caps to 47uf,22uf,22uf as per krankenstien schematic posted by Brymus.Will still add 22pf caps as jmaze suggested.

[meffcio]

Build update as of today


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How do you do traces like that? Do you tin each and every pad used?

[fretzburner]

Yes i solder all components then do the routing by soldering.I'm used to this because i'm lazy doing pcb drawing then etching just for one build or two.It's like hand wired point to point amp style.

[deadastronaut]

@fretz:  looks cool, i'd get lost doing that..

[fretzburner]

@fretz:  looks cool, i'd get lost doing that..

Just like a puzzle or scrabble deadastronaut but i have to just to build my own pedals.And it's rewarding in the end just like this engl very contented with the result.

[deadastronaut]

cool, so is the hum/pot noise issue sorted then?...i'm just finishing up my reverb so i'll have a clear breadboard to try this as well soon...

[J0K3RX]

I built the long board Engl with the on-board pots last night and it seems a little less noisy than my last build which was kinda just thrown together and all the pots were attached by wire... I also used smd tantalum caps mounted/soldered directly to the solder side of the board for C17 and C18. I may change C10 and C5 also. Still getting noise with the volume roll back on the guitar and a low hum but not nearly as bad as before.

Caps like this but the ones I used are 50v - 10uF for C18 and 22uF for C17

[J0K3RX]

Ok, you can run this @ 18v, my caps are all rate @ 50v so no problems there... I tried it tonight but still the same issue with the noise. 18v seems to open this up a bit and give it a more tube like sound but it already sounded tube like before so... Gotta get rid of the hum and then this will be GOLD!

[JebemMajke]

I've noticed in my tube to fet builds that bf245c kinda kills a lot of hum and hiss. It's low gain fet, but it's worth a try.

[fretzburner]

Yes JOK3RX giving it higher voltage supply sounds huge and tube-like but increase noise too.My build now almost tolerable noise but still stronger tha normal.Used shielded wires on input and output.Will change gain wires too later and make it shorter.Gain up to 60 percent is useable now.Still experimenting.Changed back to 100uf on C10 and C17.C5 is still 22uf tant while C18 is 10uf tant.

[J0K3RX]

Yes JOK3RX giving it higher voltage supply sounds huge and tube-like but increase noise too.My build now almost tolerable noise but still stronger tha normal.Used shielded wires on input and output.Will change gain wires too later and make it shorter.Gain up to 60 percent is useable now.Still experimenting.Changed back to 100uf on C10 and C17.C5 is still 22uf tant while C18 is 10uf tant.

I didn't get an increase in noise with increased voltage, just the same noise... I can tame it down to almost nothing by adjusting R24/the noise get and a buffered pedal in front of it but that ain't a fix, just a band-aid....

[J0K3RX]

Yes JOK3RX giving it higher voltage supply sounds huge and tube-like but increase noise too.My build now almost tolerable noise but still stronger tha normal.Used shielded wires on input and output.Will change gain wires too later and make it shorter.Gain up to 60 percent is useable now.Still experimenting.Changed back to 100uf on C10 and C17.C5 is still 22uf tant while C18 is 10uf tant.

Hey fretz, I think I am on to something here! Try this - Remove C3 all together and use a 2N5457 for the 1st stage and tell me what you think!? Almost fixed, I think

[fretzburner]

Just did a quick fix because time now is limited just removed the 22uf cap and the rest still stock yes it did lowers the noise level.But seems the gain is lowered too without the cap.Don't checked the bias if still 4v without the cap.Maybe later if have enough time.
Thanks JOK3RX for the tips.

[J0K3RX]

Just did a quick fix because time now is limited just removed the 22uf cap and the rest still stock yes it did lowers the noise level.But seems the gain is lowered too without the cap.Don't checked the bias if still 4v without the cap.Maybe later if have enough time.
Thanks JOK3RX for the tips.

Ok, try a 220nF cap for C3 and did you notice that C2 is a 220nF cap? Is this a mistake, maybe? Shouldn't it be more like 22nF or 47nF??? Anyway I lowered C2 to 47nF like on a regular Engl tube amp schematic and even better!! It's getting there!!

[fretzburner]

That 220nf C2 is something to think about hahaha. Now lots of things to do on my free time JOK3RX.Will investigate that later.We're getting close.

[J0K3RX]

That 220nf C2 is something to think about hahaha. Now lots of things to do on my free time JOK3RX.Will investigate that later.We're getting close.

I really think that C2 - 220nF was a mistake on the schematic! I stuck a 47nF in there like on almost all Engls... I looked at all of the other similar preamps and non of them are that far off, actually they are the same as the actual tube preamps they are emulating, except for this one!?

I am going to try to copy/convert the first stage values to the AMT E1 I traced a long time ago. That one had no issues other than it didn't have as much gain (probably due to it not using the 2N7000's) but there was no hum or noise with it at all.

[fretzburner]

Already tried changing C2 to 22nf problem still the same and almost same sound.placed back 220uf in the circuit.Larger amount of noise starts at the 3rd mosfet or V4.Signal is stronger at the base of V4.Maybe 33k is too small from V3 to V4 line.

[J0K3RX]

Already tried changing C2 to 22nf problem still the same and almost same sound.placed back 220uf in the circuit.Larger amount of noise starts at the 3rd mosfet or V4.Signal is stronger at the base of V4.Maybe 33k is too small from V3 to V4 line.

yup... try 100k for R19? Some values are scaled down by x10 and others are not... Some sort of mathematical mind f__k theory that I am not getting?

The real tube preamp schematic goes something like C14 - 47n / R18 - 470k / R19 - 330k

Some of the schematic follows this divide by 10 formula and other parts don't? Got me? 

[jymaze]

I did some quick calculation and changing C2 from 22n to 47n should not change the bandwidth for a 7-string, you can even reduce to 22n for a 6-string. It must be a mistake because it is way over sized. Having 22 or 47 can only be better than 220n because you reduce frequencies of motor-boating type of noise. I would choose 22n, way enough.

On the other hand changing C3 to anything lower than 4.7u makes a bass cut for the stage. 22u is way too big though. I would say 4.7u is adequately sized.

There are some circuit optimization you can try, it optimizes the gain structure, and provides a better biasing for the MOS which means less noise:

A. Optimization of gain structure (less gain wasted, less degradation of the noise floor), should be the same voicing or very close:

1) jumper instead of R5
2) change R6 to 100k
3) jumper for R12
4) R13 to 47k


B. Better biasing of the devices to get better noise figure:

1) change R10 to 22k and C8 to 220n
2) change R17 to 10k
3) change R21 to 10k
4) change R22 to 10k

C. Get rid of some gain (just a little, nothing horrible, I know it makes you nervous): Change R20 to 10k

I think it should make it quieter and keep the voicing. Let me know if it works, it should get things better, trust me.

I still think that, even after all that, there will be noise because there is just a tad too much gain in this circuit and it may have to be tweaked too.

[J0K3RX]

I did some quick calculation and changing C2 from 22n to 47n should not change the bandwidth for a 7-string, you can even reduce to 22n for a 6-string. It must be a mistake because it is way over sized. Having 22 or 47 can only be better than 220n because you reduce frequencies of motor-boating type of noise. I would choose 22n, way enough.

On the other hand changing C3 to anything lower than 4.7u makes a bass cut for the stage. 22u is way too big though. I would say 4.7u is adequately sized.

There are some circuit optimization you can try, it optimizes the gain structure, and provides a better biasing for the MOS which means less noise:

A. Optimization of gain structure (less gain wasted, less degradation of the noise floor), should be the same voicing or very close:

1) jumper instead of R5
2) change R6 to 100k
3) jumper for R12
4) R13 to 47k


B. Better biasing of the devices to get better noise figure:

1) change R10 to 22k and C8 to 220n
2) change R17 to 10k
3) change R21 to 10k
4) change R22 to 10k

C. Get rid of some gain (just a little, nothing horrible, I know it makes you nervous): Change R20 to 10k

I think it should make it quieter and keep the voicing. Let me know if it works, it should get things better, trust me.

I still think that, even after all that, there will be noise because there is just a tad too much gain in this circuit and it may have to be tweaked too.

jymaze - you are right about C2, I tried 22n and 47n and not much if any noticeable difference between them.. So it's a go on that and it does sound better than 220n. Same goes for C3 - 4.7u and reduces the noise considerably... I actually tried a 470n and then a 1u there but it made it a little too tight for me (never thought I would hear myself say that!) and Lord knows it's already tight anyway... I think I settled on a 2.2u but I will try a 4.7u also.

Ok, on the R10/R17/R21/R22 are you sure that isn't going to throw off the bias of those? Remember there are no trim pots on these! As for R20 I am fine with that and I can do with a little less gain, not a problem because it has plenty of gain to burn like you say..! The only thing that makes me a bit nervous is the drastic changes to the source resistance of the mosfets, hoping it doesn't require changing the drain resistors? But, I am gonna do all of this as you say anyway  

Over all I had this thing almost noiseless last night but there was still a slight increase in hum when lowering the vol pot on the guitar and that my friend BUGS THE LIVING HELL OUT OF ME!!!  

Ok, off to solder land I go... wish me luck

Edit: Note to self: Since yo dumb @ss don't like to bread board these preamps, next time socket every damn component!