High gain jfet/mosfet preamps - Tone stack issues.

[J0K3RX]

Ok, I am having some problems... No surprise huh. Anyway, I have built both the Uberschall and Engl using the Russian GTLabs schematics with the Engl using a combination of J201's and 2N7000's and the Uberschall using all J201's and both are totally awesome except for the tone controls! Neither one have enough low end and most of the range is undesirable, like the treble goes into the super high ranges where only dogs can hear and really has a negative affect so if I keep it all the way off it sounds good. The low end seems to be missing well, the lows and the mids are ehh, ok. The mids don't really seem to control the mids just kinda muffle them... They both use the standard Marshall tone stack and I have tried everything from using the Dr.Boogie values and other variations but all sound like sh!t, to me anyway. Anybody ever try the Cobra or Blackstar ISF controls or even the SansAmp GT-2 active tone controls for something like this?

Any ideas, thoughts etc..? I am open for about anything as long as it works! Both of these designs are just killer and the Engl is smokin bad ass and really really nails the Engl tone!! A shame not to have a really good tone control on these!

So, I have built the the Russian Uberschall, Engl 3, and I have the Krank layout made just not built yet.. I have to say that the use of the 2N7000 mosfets blew my mind when I first built the Engl WOW!!! Plus they are easy to get and cheap! You can use the BS170's also I have read but I haven't tried them yet... The only thing I am struggling with is the tone controls.. I mean they are ok but not a whole lot of options. I can dial in a really good tone but any variations or deviations from where I have it set sounds horrible Not really into "set it and forget it" tone stacks...!



Here is a video of the Engl 3 with modded tone stack I believe, not my vid..? Mine pretty much sounds just like this but this guy seems to have a much better working TS...

Here are the schematics, you can find them in the Russian forums also if you dig for them.. I have amassed quite a large collection of them myself
I wouldn't trust the board layout on this site tho
http://www.guitarristas.info/foros/muestras-sonido-messa-soldano-krank-engl-vht/88517/pagina2

Here is a really old post here in the diysb forum from 2004 with xbananov and Medved talking about the D90x2 which looks like a real beast I will be attempting it real soon! Most of the links are dead but lots of good info about different jfets and what to sub for the Russian stuff etc..
http://www.diystompboxes.com/smfforum/index.php?topic=27925.40

[brett]

Hi
although I've only skimmed the material, I have an idea that might be wroth thinking about.
Some tone sections are strongly affected by the output impedance of the prior stages and the input impedance of following stages. In the case of prior JFET stages, the output impedance can be quite high (Rd + some). A simple check would be to compare the Rd of the prior JFET stage with the R values in the tone section filters. The tone section filters should be much greater. (and the input impedance of the following section should be higher again - easy to achieve with JFETs).
cheers

[J0K3RX]

Brett, good advice... That's probably what is happening here. I think I got it, very usable tone control now! I pretty much stuck exactly to the schematic except I temporarily installed trim pots for  R22, R24 and R25. I also tried different caps for C5, C10, C17 and C18 in an effort to clean up the power but I went back to the original values. Keep in mind this thing normally will have only one trim pot for the first J201 and all the rest of the gain stages minus the buffers are the 2N7000 mosfets with 56k fixed resistors on the drain.. Nice to not have to bias!!

Here is a real quick snip-it of the sound with the gain maxed and the tone knobs @ 12 o'clock except for the treble was @ about 9 o'clock.. I probably could have dialed the gain back a bit but I wanted you to be able to hear how much gain this thing gets.. Thoughts? Like, no like, don't care etc..?

http://dl.dropbox.com/u/11178619/E3_test.mp3

[Ronan]

Sounds good. Plenty of bottom end (at my end of the internet). I typed the following before you posted, it might not be applicable now, but here goes:

Just some thoughts, I think that some solid-state "distortion engines" need a specific fixed eq after (or before) them to get them to "sound right", then after that, you can apply the standard tone stacks derived from tube amps, or active ones, to give some tonal variety. See what they did with the GT-2, it sounds terrible without those fixed filters after the distortion IC's. However the majority of designs seem to be tweaked just right to overcome this.

If you are building a lot of distortion circuits, it might be worth trying to pick up one of those old dual graphic equalizers from old hi-fi component systems, they are usually 10-band, very cheap, and use one of the eq sections to run your new build through and get an idea of which frequencies need boosting or cutting to get the sound you like. Or something like that, maybe use a PC graphic and sound card.

[J0K3RX]

That's a good idea Ian! I really need to pick one of those up... I can use a VST but I would rather have a real EQ! I have a hell of a time with all of these solid state preamp tone controls! Almost makes me want to install a one knob big muff type control at times... Frustrating!  

I really need to try the Framus Cobra TS or the Blackstar ISF controls! I have heard they are very good but I haven't really heard anybody using them for these preamps? I was hoping that somebody might be familiar with either of them and comment...

This sounds good to me now but, then I will wake up tomorrow and hate it all over again

[RonaldB]

I had the same issue with my Dr Boogie build.
I didn't like the tone stack add all. I was going for a John Petrucci sound with this one but the treble was to harsh and lack of low end.
Also I have to mention I wanted to use the Dr Boogie as a pedal not as a Preamp. So I'm running it true my 15watt Tube amp on the clean channel with bandaxall tonestack.

First thing I did was removing the TMB tonestack.
What I did was adding this BMP tone control with pressence pot after Q4. I removed Q5.
http://www.muzique.com/lab/tone3.htm(It's version 2)

And added 2 LPF after this tone controle (10K/2n2) and the to the Level potentiometer.
This gave me the sound I wanted, Very nice and plesant lead tones and of course rythem tones.

After this I wasn't completly satisfied so I hooked up a 3band EQ after the Boogie and now it Rocks. Really nice tones, deep low end, strong mids and nice highs.

I will be building a new Dr Boogie with this concept but then with J201's and no BMP tonecontrol to see how this will turn out.
The ENGL looks promishing to,

RonaldB

[deadastronaut]

^ interesting,  i have the boogie on breadboard at the mo, because i fancied having a go at 'nailing the poxy tonestack too...

ive knockep up a 1590b layout for it, but still not happy with the tone controls..

i swapped out  R14 4.7K for a 5.6k

c11 for a 470nf instead of 220nf....a bit thicker, but the interaction between the controls are just plain weird..

like jim says, it'd be nice to get a 'good responsive' tonestack for theses high gain monsters

[JebemMajke]

I had a similar issue with my Alembic f2-b, fet version. Because you see I use it both with bass and guitar. It had a lot of mids and treble, and not enough lows. So I've decided to modify it's tonestack

Here's what i came up with

It's obvious that this is not my design, but some things borrowed from different versions of fender tonestack. But it works. Deep switch gives and instant "modern" bass sound ( with bass guitar ), really deep but with preserved highs and mids. Fat gives somewhat 3D sound ( with guitar ).

I'm not sure if this is going to help you but it could be a starting point.

PS sorry for bad paint drawings

Pss i've made a Cobra for a friend of mine, and it's tonestack sucked too. Only with mids scooped all the way down it had nice, round lows.

[J0K3RX]

Ok, starting at the top

RonaldB

Really interesting.. You can probably do just fine with a big muff control if you are going into the front of an amp, plus you also have all of the amps eq where you can tweak the tone! I think the 3 band eq is a good idea also!

Rob

My Boogies sound real good as far as the tone controls are concerned. One thing I noticed about the Dr.Boogie is that they divided most of the values of the tone stack by 10 including the pot values. So, for example on the Dr.Boogie schema R14 is 4.7k but on the Mesa Boogie Dual Rec schema it is 47k. If you look at a real Boogie you will see what I mean. They scaled back the tone stack but the rest of the values of the preamp remain stock. You may want to try the original TS values? Another thing I notice is that if you remove the presence completely the whole tone stack reacts better, all the controls work better!


JebemMajke

That is interesting as well! I will have to experiment with this..! Very nice!

Here is the Russian schematic for the ENGL I am working with now and in red are my edits/notes. The mosfets are just out of this world in the gain department! The tone stack works really good now but if I roll the volume knob back on my guitar I get noise at lower volumes? No noise with the vol on the guitar all the way up just about half way down I get a lot of noise?? Pissin me right off! Even if I dail the gain on the pedal back to half way 12 o clock, as soon as I turn the volume on my guitar down I get noise, wtf? Ok, now if I stick a tube screamer in front of the ENGL then my volume works perfect..? Double wtf? Get all that sorted and I will box this beotch up!

[Quackzed]

if its an impedence issue, which i think it is, could you just insert a lowish value resistor to ground after the stage driving the tonestack?
that would bring the impedence down and the stage driving it should be able to handle driving it as well as the tonestack. just a thought.
i guess it depends on the driver stage wheather it would knock out the bias etc... but on the engl for instance if you made r22 4.7k instead of 47k that would
lower the impedence driving the tonestack and should get rid of the treble heavyness if it is indeed impedence by making it harder for the driving stage to push all that treble through.
i dont think the driver would be very negatively effected , i believe its just an emitter follower and r22 is the load, i'd try lowering r22 (start at maybee 4.7k or 10k and see) may need some adjusting, maybee try and figure out the out impedence of that stage in the real amp and match it?

[J0K3RX]

if its an impedence issue, which i think it is, could you just insert a lowish value resistor to ground after the stage driving the tonestack?
that would bring the impedence down and the stage driving it should be able to handle driving it as well as the tonestack. just a thought.
i guess it depends on the driver stage wheather it would knock out the bias etc... but on the engl for instance if you made r22 4.7k instead of 47k that would
lower the impedence driving the tonestack and should get rid of the treble heavyness if it is indeed impedence by making it harder for the driving stage to push all that treble through.
i dont think the driver would be very negatively effected , i believe its just an emitter follower and r22 is the load, i'd try lowering r22 (start at maybee 4.7k or 10k and see) may need some adjusting, maybee try and figure out the out impedence of that stage in the real amp and match it?

Yeah, I tried everything from 10k up to 47k with varying results. I think I got the tone stack worked out now, sounds really good but I still have the problem when I roll the volume back on my guitar. No noise with the volume pot all the way up but if I roll it back 3/4 of the way on the guitar it surges progressively until reaches a peak then it stops with the volume all the way down. The lower it goes the noisier it gets until the volume pot is all the way down and then no noise. Then if I put a tube screamer in front of the ENGL the volume on the guitar works perfect? Something stupid I just know but can't figure it out??  

[Quackzed]

  try removing c1? its so small its pretty much out of the circuit without any series resistance if front of it like with the vol up, maybee as you roll off the guitar its progressively more in or on, maybee just a bad cap? or also maybee a bad trimmer?

[deadastronaut]

gotcha, i'll try it without the pres...

might try your engl stack too...good work man.  cheers.

[JebemMajke]

How about this



I saw schematic of a modified shredmaster with this tonestack. The guy claimed that it gives enough bass for chung chung riffs. If it helps that guy was ( and still is ) Russian. xD

[jymaze]

Try shielding the case of your pots, of course the gain pot in priority. I use a bridge made from the leg of a resistor: it goes from the pin of the pot that goes to ground, soldered to the side of the pot case. You may have to sandpaper the area on the casing before to get the solder to flow right.

It helps when the circuit is not in a box yet, and it help even after since the connection to ground is way better than 2 oxidized surfaces pressed together by nuts.

[J0K3RX]

JebemMajke - Damn, you're killin me smalls  

Seriously, can you get it any smaller? Anyway, I tried all sort of combinations, I am sure that was probably one of them...

jymaze

Damn, yer killin me too! I have those pots that come with the plastic dust covers, you mean I gotta take'em all off just kiddin, I will try that but I think the problem is elsewhere? BTW-You said pressed together by nuts?   Sounds gross and I ain't like that  

[JebemMajke]

How about enlarging c4 and c7 to 47n?

Ps the smaller the better :p

[jymaze]

I think your tone pot is acting like an antenna and picks random radio white noise. When at the mid setting it has the maximum impedance so the picked signal is louder. When it is at the minimum, it does not pick any signal because it is grounded, and on the other side it has 0 impedance. Overall, boxing it should resolve the problem, but hard-grounding the pots has always been good to me anyway.

[Groovenut]

One thing to keep in mind with both of these real amps is that the pre amps filter out most all of the low end. The negative feedback loop around the power amp is used to restore the low frequencies filtered out before the distortion in the preamp. If you are only building the pre amp, there is no way you are going to achieve the real amps sound. There are ways to emulate this negative feedback filtering though. Quite possibly using opamps or the like. Have a look at the resonance circuits in the real amps for a clue as to where to start.

[Ronan]

The negative feedback loop around the power amp is used to restore the low frequencies filtered out before the distortion in the preamp.

I don't understand what you are saying, how can low frequencies going into the preamp be "restored" if the feedback loop is only around the power amp? Is the feedback loop somehow sampling low frequency levels before the distortion stage? Anyone have a link to the real amps?