Jfet gain stages: Supercharge your old mpf102!

[Gringo]

Well, after some crazy cloning period, followed by a "modding the clones" period, i finally got to the "experimenting" one.

I (finally) bought some stripboard, and God send me a personal smile in the flesh of some j201 fet samples. No more excuses left. I could finally taste some of the "forbidden" projects!!! (bsiab, DougH jfet series,ROG latests), those said to not sound good when a j201 was subbed.

Talking about this with Prive, he told me that i should build something and make a mpf201 sound like the j201 (we both are from argentina, where you can find the greatest meat and women, but j201 are as rare as a dino egg).

After some websearching/reading, i chose the peppermill jfet gainstage out of many, just because of it's simplicity. Trimpot at the drain, 1k res and 1uF cap at source.

Results(after all this yadda yadda, it better be good ones, right?)

Yes, i could make a common, easily available (at least here) mpf102 sound *very much* like a j201. Not *exactly*, but close enough. With a 2.2k~2.7k resistor instead of the 1k, and a 2.2uf~4,7uf cap, i got the same volume(even a bit more) and clipping (almost).

Side by side, in the same stage, both sound similar, but the j201 has more volume and cruch (gee, thats a revelation!).

Increasing the resistor at the source yielded more output, adding to the cap increased the gain.

After matching volume and gain, the mpf102 sounds more round (a little more bass or a little less treble, it's a subtle yet present difference) than the j201. This can be solved reducing a bit the output cap from the stage, i think.

At the stripboard, there was slightly more noise when using the hotrodded mpf102. As in life, you can't have it all, i guess.

I'd like to hear the comments of people that can explain the theory of what i accomplished empirically, to have some criteria that allows a "quick and dirty" table of mods to gainstages from other circuits.

I searched for a 'tweak-the-gain-of-your-fet-gainstage for dummies', but no cigar (I'm sure it's out there under my nose).

I hope this helps.

[brett]

Hi.

I agree that MPF102s often get bagged because they have different characteristics to J201s and other JFETs.  The graph below show how different the Rdrain requirements are for MPF102s, compared to JFETs with much higher Vgs (ie less negative).  While there's no J201s on this graph, they would mostly be up around the -1V level.
[Edit]  These optimal Rdrains are for Rsource = 2.4k ohms.  It is also possible to get MPF102s into their "zone" by tuning their Rsource while leaving Rdrain constant.



Also, you've done a neat job of greatly increasing the effective transconductance of the MPF102 by adding the source capacitor.  As you say, it's probably gone from 2 to 3 times lower than the J201 to higher than the 201.

good work.

PS I'm in Australia and have to get my J201s from the states, too.

[Prive]

When i need some answer i call GRINGO!!!!!!!!!!!!!!!

Hahahaha, ok boy, when you have it working you can go to sleep again.

Hahahaha!!!!!!!!

:wink:

Mentira, pero despues pasame la data jajaja

Suerte!!!!!!!!

[Gringo]

Che Marcelo, now you don't have excuses either

I guess i'll try different gainstages and different fets, to come with a "rule of thumb" a little more generic, for people like us that can't get j201's and are unable for some reason to shop at steve's (or just for the "heavy socketers").

Any suggestions about what gainstage/fet should i take to a j201 sound? I chose the mpf102 because it's the easiest to get, cheapest fet around this place. I have 2n5457, and a couple of nte subs (nte457, and the one for bf245, nte 133, thanks to Arn  : i can compare and tweak.

What do you think?

[Gringo]

After the avalanche of requests and suggestions ( :twisted: ), i picked the BSIAB as the next project to tweak for mpf102 (ab)use.

It turns out this one is based around Jack Orman miniboost, not the ordinary fet gainstage. Using a "divide and conquer" approach, i'll mod the miniboost first, then apply the results to the BSIAB.

Now the question: How can i increase the gain of this block?

I read the SRPP article at geofex which suggested adding a 1k resistor between source and drain of the fets to achieve higher gain. What else can i do?

I hope to *gain* (couldn't let this one pass ) a better understanding of this piece of circuitry by messing with it, as the theory didn't quite stick on me yet.

[Brian Marshall]

I have found that mpf102's with higher gain there is a little secret that helps make them sound a little better.  

If you set up an mpf102 at high gain with large resistors follow it with a bood sized resistor before the next gain stage.  50k to 100k or so.

The reason (my geuss is anyways) that because the ouput impedance is flying arround all over the placw with every cycle, the resistor helps stablize it a little bit, and makes and pasive filtering more effective.

that gets rid of some of the noise.

MPF102s seem to be more suseptible to radio waves for some reason.

[Gringo]

If you set up an mpf102 at high gain with large resistors follow it with a bood sized resistor before the next gain stage.  50k to 100k or so.

What do you consider as "large resistors"? The 50/100k resistor you mention is supposed to go in series with the output of the stage?

[swt]

Hey Prive and gringo!. I'm also form argentina, neuquen, specifically. If you want a bsiab, or any other j201 fet design to work like a charm, i never use mpf 102, as the voltage swing is too low. Just grab 10 or 20 bf245A, and only the suffix A will work. Use a fet matching technique to know what the voltage swing actually is, for example, if you've got a 9 volt power supply, and supposing you are using the common gate and source to ground, drain to 1k resistor to b+, the higher the voltage you read at the drain the better. Use the ones with a voltage of 5 or 6 for the upper fet in minibooster tyoes of circuit, or for buffer stages, and get the ones with 6.5 or up, for distortions stages. I've been using these ones for phasers, distortions and booster without changing any values in the original circuit, and in my opinion sound a little less harsh than ths j201 i could get. The higher voltage i could get was about 7.18volts. Try them, you won't regret. Any qs, just pm me or let me know if i can help you. Good luck!.

[Brian Marshall]

What do you consider as "large resistors"? The 50/100k resistor you mention is supposed to go in series with the output of the stage?

yes in series... sorry i wasnt clear.

50 to 100k isnt really large, but not relatively small either... it's "good sized"

[Gringo]

Use a fet matching technique to know what the voltage swing actually is, for example, if you've got a 9 volt power supply, and supposing you are using the common gate and source to ground, drain to 1k resistor to b+, the higher the voltage you read at the drain the better. Use the ones with a voltage of 5 or 6 for the upper fet in minibooster tyoes of circuit, or for buffer stages, and get the ones with 6.5 or up, for distortions stages.

What fet measuring circuit do you use? I use the one in geofex, is that the one? I'll save for a batch big enough of those 245's and compare

[swt]

i use the first that was publish at geofex. It's fairly simple and it works for this purpose. Drain to a 1k resistor, to 9 volts, gate and source tied together, to ground. Then measure from drain to ground, and that's sort of the voltage swing. Let me know your experiences and results!