JFET Stages (in an Amp Emulator)

[anti-idiot]

Hi everyone.

I'm goin' to build an amp emulator using JFETs and I wanted to know which method was better, using the typical one-transistor (Thunderchief, Plexizer, etc) or using a Mu-Amp as a stage.

What are the advantages of using the Mu-Amp vs. the F4T. BTW, I'll use > +9v (maybe 15v o 24v, but I'm makin a 4-stager).

A fellow member of this forum (Mensur) did this JFET-amp using a Dr. Boogie@24v plus a TDA7294 (and it sounds pretty cool). He suggested me using the Mu-Amp setting, and even gave me this schematic (JCM800) and i thought it was a cool idea. Do you guys have any experience with that config.?

http://i20.photobucket.com/albums/b228/Anti-Idiot/Stages.png

So, what are the pro's and the con's.

Thanks for your time.

[brett]

Hi
these modes are quite different.
I'm no expert, but here's my 2c....
The mu-amp is a high gain configuration (for a FET), because the "top" JFET is an active load.  It is reasonably unlike the triode stages in a pre-amp.
The standard common-source stage has lower gain than the mu-amp (around 20 to 30 with a J201, IIRC) and lower gain than a valve amp's triode gain stage (40 to 100), but has a number of other similarities to a typical triode stage.
cheers

[dschwartz]

well..i think the best you can do here is breadboard both options and compare them..
I prefer Mu amps because it´s auto biased, and have very high gain.. perfect for jcm800 emulations..But, for thicker lead tones, single fet stages seems to work better..

for an amp, i´d go with mu-amps, and forget about trimpots, , but i´m quite sure that is not just changing the triode for a mu amp..again, i recommend you to breadboard a 2 or 3 stage mu-amp preamp and experiment with cap and resistor values for best results...my texas brownie was born like that

i think a well designed 3 stage mu-amp preamp at 24v should sound awesome....you can use 1 stage for clean, 2 stage for crunch , and 3 stages for hi gain mayhem.

[Ben N]

Wait, wait, this does ring a bell...
http://www.diystompboxes.com/smfforum/index.php?topic=74219.0
Does this fit the bill?

[slideman82]

A fellow member of this forum (Mensur) did this JFET-amp using a Dr. Boogie@24v plus a TDA7294 (and it sounds pretty cool).

I've done this too but using a mosfet power amp, sounds amazing, but sure you'll have to try several JFETs, some mid, some low gain ones.

I also going to try the Mu-amp configuration using probably 24V because I don't like the way it sounds with 9V, but sure a 3 stage will be enough gain for such preamp!

[anti-idiot]

@Daniel: My plan was doin' some JVM-style amp -bypassing stages-. This was supposed to be a pedal, but now i want it to be an amp (full-SS). The Mu-Amp has a lot of gain, but no so much as a complemetary push-pull (there's an article at Geofex) and running it at 24v will do the trick (i don't want each stage to have massive gain, i want 'em to be overdriven little by little, like the JCM800).

A fellow member of this forum (Mensur) did this JFET-amp using a Dr. Boogie@24v plus a TDA7294 (and it sounds pretty cool).

I've done this too but using a mosfet power amp, sounds amazing, but sure you'll have to try several JFETs, some mid, some low gain ones.

I also going to try the Mu-amp configuration using probably 24V because I don't like the way it sounds with 9V, but sure a 3 stage will be enough gain for such preamp!

What did you do? The Mu-Amp thingy or the Fet4Tube turn into an amp? Please let me know how it goes...

MY amp is supposed to have 4 stages (1959SLP has 2, JCM800 has 3, JCM900SL-X has 4; only pre-PI stages without Cathode/Source-Follwer)

Now, I have this silly question. Which one is the better way to bypass a stage? (in a 4-Stage Amp like JCM900SL-X). The JVM method (http://forum.metroamp.com/viewtopic.php?f=25&t=21932) or using a "true bypass" method:

http://i20.photobucket.com/albums/b228/Anti-Idiot/BypassingStages.png

Marshall's JVM uses the first one, but i don't know about tonesucking. BTW, On JVM's tonestacks, both OD1 and OD2 tonestacks are together. How much tone is sucking that configuration? 

[slideman82]

I've done it with single JFET stage. I always recommend try different ones, and avoid most gainny ones like J201 or 2SK117. And if you build it with the tonestack before power amp, place a buffer after it, sounds better! Solid state power amps don't have the 2M input impedance as the long tail pair phase splitter type you find in well known guitar amps.

[dschwartz]

for bypassing stages you can use relays controlled by a flip-flop and a simple actuator switch..or something like this :
http://www.diystompboxes.com/smfforum/index.php?topic=77630.0
you can use 2 switches, one on the amp, and the other on a remote footswitch.
If you are going for the cmos switching, you should use 15Vdc to get more headroom..(but better use the relay option)

i´d be happy to help you design this pre..if you need any help, count on me..

about single fet v.s mu amp:
Mu amps can sound very, very good i you configure them to lower gains..i use a 8.2K source resistor with no bypass cap..low gain, but very tubey..you want to use the soft clipping zone, not full clipping..

You could also use hybrid design intercalating muamps with single fet stages...a mu amp would drive the hell out of a single fet..

IME, the source of the distortion is, by far, not as important as good filter and gain choices between stages..Fets, CD4049s, mosfets, diodes, and opamps, can achieve amazing tones by just being careful about which frequencies you are letting pass though and how much gain between stages gives you the best touch sensation and overal gain..

[anti-idiot]

for bypassing stages you can use relays controlled by a flip-flop and a simple actuator switch..or something like this :
http://www.diystompboxes.com/smfforum/index.php?topic=77630.0
you can use 2 switches, one on the amp, and the other on a remote footswitch.
If you are going for the cmos switching, you should use 15Vdc to get more headroom..(but better use the relay option)

Well, In fact i have the switching ckt (Groove Tubes Trio, 'cos I use 3 channels). I just need to know wich method (http://i20.photobucket.com/albums/b228/Anti-Idiot/BypassingStages.png) produces less tonesucking.

i´d be happy to help you design this pre..if you need any help, count on me..

about single fet v.s mu amp:
Mu amps can sound very, very good i you configure them to lower gains..i use a 8.2K source resistor with no bypass cap..low gain, but very tubey..you want to use the soft clipping zone, not full clipping..

You could also use hybrid design intercalating muamps with single fet stages...a mu amp would drive the hell out of a single fet..

IME, the source of the distortion is, by far, not as important as good filter and gain choices between stages..Fets, CD4049s, mosfets, diodes, and opamps, can achieve amazing tones by just being careful about which frequencies you are letting pass though and how much gain between stages gives you the best touch sensation and overal gain..

I'll mail you as the project goes by (which will be easier for me, 'cos we can talk in spanish). I really need all the help i can have. Thanx

[anti-idiot]

BTW, this is the famous Groove Tubes Trio. It's a 3 channel preamp, and the switching allows you to go from any channel to any channel (unlike most 3-channels amp). Most 3-ch has this config. Clean-OD1/OD2.

http://i20.photobucket.com/albums/b228/Anti-Idiot/gtrio.gif

I'll replace the opto-switches with relays (6 relays: 2 for switching gain pots, 2 for bypassing stages and 2 for switching tonestacks, pretty much like the JVM).

[dschwartz]

a menos que pienses usar BJT´s no deberias preocuparte por el fantasma del tonesucking...ese problema es de fuzzes y wahs..cualquier cosa con entrada de fet u opamps no sufre de tonesucking (a menos que le bajes la impedancia de entrada a propósito)..el resto es MITO.

[anti-idiot]

a menos que pienses usar BJT´s no deberias preocuparte por el fantasma del tonesucking...ese problema es de fuzzes y wahs..cualquier cosa con entrada de fet u opamps no sufre de tonesucking (a menos que le bajes la impedancia de entrada a propósito)..el resto es MITO.

Yep, I'll use either a MPF102 or 2N5457. I guess if I use the JVM style, I'll have to use pull-down resistors; right? Can you take a look at the JVM schematic?

[dschwartz]

no problem, please show me the link to the schem

[anti-idiot]

no problem, please show me the link to the schem

http://forum.metroamp.com/viewtopic.php?f=25&t=21932

The JVM bypasses stages to have so many modes. It also switches gain and tonestack pots. If you look carefully, each stage has a 4M7 or 10M pulldown resistors (and uses the voltage divider of each stage as a Pull-Down too). On the Tonestack/Gain sheet, You can see how they're grouped (Clean Vol-Crunch tonestack / OD1 TS - OD2 TS). Both OD's Tonestacks are conected together, do you think it will suck tone? (I dread tonesucking, as you can see)

[dschwartz]

hey..the switching part of that amp is fairly complex!..
I suggest not worrying about tone sucking if you´re building an amp this complex..it makes you look as a total newbie  ...
Leave tone sucking worries for tone snobs that don´t know of what they are talking about..

as i said before..the signal is not passing through any low input impedance device..(fets have hundreds of mega ohms, some opamps have even more)

[anti-idiot]

hey..the switching part of that amp is fairly complex!..

I know, right? I like the whole bypassing stages thing, that's why i took the idea and "simplify" it a little. only 3 modes (versus the 12 modes of the JVM) based on 1959SLP, JCM800 and JCM900SL-X.

as i said before..the signal is not passing through any low input impedance device..(fets have hundreds of mega ohms, some opamps have even more)

About the stages, ok, the JFETs has lots of M of impedance. But at the JVM OD's Tonestacks, as you can see, both are connected. Now, i don't know if the Mu-Amp (which is kinda "Self-Buffered") will solve the tonesucking thing at the tonestacks, 'cuz I'll use 3 tonestacks (56k/250p, 33k/470p, 47k/680p; one for each respective channel)

I'll post the schematic ASAP.

I'm Obssessive-Compulsive when it comes to sound.

[dschwartz]

ohh. you are talking about tonestack loss..
i think you wont have any troubles from muamps.. but to be sure, you could use an opamp buffer and be 100% safe..remember that all passive tonestacks have losses (well except some based on coils)

is hard to  visualize your idea of switching.. maybe you can draw a block diagram showing the switching scheme...

[petemoore]

  Wasn't Jack recently commenting on the Minibooster having a kinda high output impedance?

[dschwartz]

yes, muamps have rather high output impedance..compared to a buffer, of course.. but putting a buffer before the tonestack should be a must independently of what you are using for gain stages (well not for opamps, maybe)

[anti-idiot]

yes, muamps have rather high output impedance..compared to a buffer, of course.. but putting a buffer before the tonestack should be a must independently of what you are using for gain stages (well not for opamps, maybe)

Isn't the upper JFET a weird source follower?

So, what's the deal with this schematic?

http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif

What if i use an additional source-follower? (pretty much like this one http://www.runoffgroove.com/tc-tone.png)

http://www.geofex.com/Article_Folders/foolwfets/foolwfets.htm