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 (http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif)) 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.
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
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 :P
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.
Wait, wait, this does ring a bell...
http://www.diystompboxes.com/smfforum/index.php?topic=74219.0 (http://www.diystompboxes.com/smfforum/index.php?topic=74219.0)
Does this fit the bill?
Quote from: anti-idiot on July 07, 2009, 01:21:04 AM
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!
@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).
Quote from: slideman82 on July 07, 2009, 10:59:13 AM
Quote from: anti-idiot on July 07, 2009, 01:21:04 AM
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?
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.
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 (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..
Quote from: dschwartz on July 07, 2009, 12:04:30 PM
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 (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.
Quote from: dschwartz on July 07, 2009, 12:04:30 PM
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
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).
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.
Quote from: dschwartz on July 07, 2009, 12:47:33 PM
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?
no problem, please show me the link to the schem
Quote from: dschwartz on July 07, 2009, 02:51:16 PM
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)
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 :icon_redface:...
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)
Quote from: dschwartz on July 07, 2009, 03:19:03 PM
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.
Quote from: dschwartz on July 07, 2009, 03:19:03 PM
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.
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...
Wasn't Jack recently commenting on the Minibooster having a kinda high output impedance?
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)
Quote from: dschwartz on July 07, 2009, 05:10:25 PM
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
Hi
Quotei think you wont have any troubles from muamps.. but to be sure, you could use an opamp buffer and be 100% safe
Op-amps make fine buffers, but a JFET buffer would work fine, too, and usually be be simpler.
I seem to recall that many of the traditional (valve amp) tone stacks are designed around high output impedance from the previous stage (up to 100kohms) and high input impedance to the next stage (typically 470k to 1M). These would suit a mu-amp design.
But different stacks have different requirements. For example, the BMP tonestack requires Zout and high Zin, but is often seen where loss of highs will be significant (low Zin). I strongly recommend "playing" with Duncan's Tone Stack Calculator to find out what is what.
have fun!
PS I for one simply don't believe that "tone sucking" is such a great problem. Is it a bad thing? We might also call it "tone mending" if we think of it as getting rid of high-order harmnics and focussing on the tone and the first couple of octaves up. But don't believe me - I'm just some guy on the www. Look at how many great albums were recorded in the 1960s and 1970s through BJT pedals such as the tonebender, fuzz face (around 20k input impedance), a variety of other pedals with Zin of about 50k, or using high capacitance, low quality cables (e.g. Jimi's "tone sucking like a black hole" curly cables).
Gearmanndude will tell you how important that white curly cable is. Owl strap too...
Quote from: brett on July 07, 2009, 10:46:52 PM
Hi
I seem to recall that many of the traditional (valve amp) tone stacks are designed around high output impedance from the previous stage (up to 100kohms) and high input impedance to the next stage (typically 470k to 1M). These would suit a mu-amp design.
But different stacks have different requirements. For example, the BMP tonestack requires Zout and high Zin, but is often seen where loss of highs will be significant (low Zin). I strongly recommend "playing" with Duncan's Tone Stack Calculator to find out what is what.
have fun!
PS I for one simply don't believe that "tone sucking" is such a great problem. Is it a bad thing? We might also call it "tone mending" if we think of it as getting rid of high-order harmnics and focussing on the tone and the first couple of octaves up. But don't believe me - I'm just some guy on the www. Look at how many great albums were recorded in the 1960s and 1970s through BJT pedals such as the tonebender, fuzz face (around 20k input impedance), a variety of other pedals with Zin of about 50k, or using high capacitance, low quality cables (e.g. Jimi's "tone sucking like a black hole" curly cables).
Well, i'm not using different-different tonestacks, just using the Marshall TS, and changing the slope resistor and cap (like the old plexis, the master volume and the Dual Rectifier). Now i don't know how good or bad is it gonna be if i put the 3 tonestacks together and just switch between their outputs. of course there's gonna be lose, but as all of you made me see, i'm gettin' too neurotic. I mean, if Marshall JVM can do it and still sound amazing, why not me? I'll try to finish the schematics ASAP.
Now, for something completely different, what are your experiences with the Presence Control on designs like the Plexizer? Has anybody take a look at the JCM900 4100 Dual Reverb Presence control? it's a Pot-to-cap at the cathode of a tube (post TS, FX-Loop, so i guess it's a recovery stage/buffer for the reverb stage)
Thank you all, you've been great!
A 1/2 vbias could be made with 2 =value small resistors, each stage biased through 1 large resistor from a common Vbias, 1 divider means fewer resistors total and may help reduce noise.
Isn't the upper JFET a weird source follower?
Yupp, it pretty much follows the voltage of it's input, the resistive divider which creates 1/2v, and biases the lower Jfet.
So, what's the deal with this schematic?
http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif
Not sure which deal...
The last stage has 1:1 resistance between the S/D, which would increase the amount of current it can put out, and be about unity gain, for driving whatever is connected to the output it seems....as far as I can calc..
If using the above biasing suggestion [creating a V-bias with say 2 x 10k resistors], a 1 Jfet buffer would be pretty easy put into service [gate biased with a large resistor, see Labs Notebook].
dirty little secrets: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm
That's the one, noiseless biasing.
Thanks RG, of course I've tried it and it works great !
Quote from: anti-idiot on July 07, 2009, 10:13:56 PM
Quote from: dschwartz on July 07, 2009, 05:10:25 PM
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? Kind of, but it's also a load for the lower JFET.
So, what's the deal with this schematic?
http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif The 3 first stages are standard mu-amps with different levels of gain and filtering. The last stage is probably supposed to be a buffer, but the top JFET has no gate bias, and so it won't work right as drawn.
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
You could do that, but if you add a single resistor you'll get basically the same result: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm ...if it was me, I'd replace the stage just before the tonestack with the last circuit in this link, and replace the output buffer stage with a more traditional common drain follower circuit.
Quote from: earthtonesaudio on July 08, 2009, 11:57:46 AM
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
You could do that, but if you add a single resistor you'll get basically the same result: http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm ...if it was me, I'd replace the stage just before the tonestack with the last circuit in this link, and replace the output buffer stage with a more traditional common drain follower circuit.
I'll follow your advice and replace the buffer (i was thinkin' about some stage with a 'presence control')
Ok. I got the 10k/10k, and the 1k between both JFETs. But what's the deal with the 4M7?
So, basically i should use a common 1/2vBias and connect as many 4M7 resistors per stage, right?