replacement for the 2SK117 JFETS in a Boss BD-2 circuit

[mordechai]

I've been looking at a few of the schematics for this circuit, most of which seem to substitute the original 2SK117s.  I built up a version and it sounds good, but the 2N5457s I used for the discreet op-amps are a little chunky sounding.  I have a few of the 2SK117s have built a stock version of this circuit, but I have a few questions about possible substitutions:

1) Since MPF102s are much lower in gain than the 2N5457s, can they be used in this circuit, or is there something about them that makes them unsuitable?  (I am not sure that gain is really the issue regarding the chunky sound I don't like in the 2N5457s).

2) Is there a more readily available equivalent to the 117s that people like in this circuit?  Does anybody use J113's?

3) This is unrelated to the JFET question but...f I wanted to tame the volume output a bit, I assume I would need to add a fixed resistor somewhere toward the tail end of the circuit to hold back some of the output.  If I'm right about that, this should be done BEFORE the output buffer stage, correct (I'm using a schematic where the IC is a TLO72)?  Where could I insert this additional resistance to bring down the output without offsetting the frequency response?  Is there a different way to limit the output volume without interfering with impedance?

I'd appreciate any insights here.  It's a great circuit and the version I already built sounds very nice, but I'd like to explore the options without departing drastically from its characteristic tone.

[Mark Hammer]

Hi,

I'm seeing 2SK184 FEts in the schematics I have.  Are your questions about the K117 related to e-switching, or to gain sections?

[antonis]

...f I wanted to tame the volume output a bit, I assume I would need to add a fixed resistor somewhere toward the tail end of the circuit to hold back some of the output.

Not necesserily..

You may add a resistor between Tone pot wiper & Volume pot upper lug..

[mac]

2SK117 are hotter and have lower Vgs than Mpf102 and 5457.

mac

[mordechai]

Hi,

I'm seeing 2SK184 FEts in the schematics I have.  Are your questions about the K117 related to e-switching, or to gain sections?

I'm mostly curious about gain, but Mac, below, noted that the 117's are higher gain than the other types I mentioned, but have lower Vgs.  So I suspect the interplay between one factor and the other is significant here.

[Eb7+9]

No such thing as jFET gain, hotter, etc ... something else entirely

The BD-2 circuit incorporates a pair of discrete (3 transistor) op-amps in its signal path ... if you know how op-amps are designed in general you'll know what Boss went thru to make their circuit work well enough and reliably snuff from pedal to pedal (duplication) ... once you figure that out you'll want to do the same to achieve comparable results

[bushidov]

I know on my BD-2 Clone, I used 2N5457's and it sounded close enough for government.

[imJonWain]

I've used 2n5485's in the discreet opamp section of other boss pedals for what that's worth (I just have a big bag i got for free).  The FZ-2 and OD-2/3 have a similar section.

[Eb7+9]

Apologies for being vague - you could "measure" the Vgs(off) voltage of your jFETs see if it makes a performance difference when using front-end op-amp pairs with matched turn-off voltages // try swapping their relative positions in the unmatched cases ... all you need is a 9v battery and a DMM to get Vgs(off)