Shaka 5

[aron]

> Some where less than unity.

What do you mean by this?

[Kipper4]

I set up a srpp from the RG page linked above
used
560k from Vbias to gate of top fet
1k from top fets source to bottom fets drain.rvrvrv
100nf from top fets gate to bottom fets drain
output from the top fets source

i tried 2n5485 mpf102 j113 2n3819 j201 2n5457
the j201s and 2n5485 where the only ones that actually boosted the signal all the others had less than unity gain. I hope this clarifys things Aron

[Kipper4]

What are you thoughts on the mini booster in this circuit. Does it make a significant contribution to the sound.
What if i was to use a common or garden booster instead say an lpb1.
why the lpb1 because its low parts count as opposed to an op amp booster.
The circuit is fine as is the reason I ask is because im looking to reduce the amount of j201s in use

[Cozybuilder]

This circuit is on my breadboard now. Overall very nice, super responsive to controls, the tone circuit needs work though. The Shaka Syrup?
And its got some gain to it!!

[bloxstompboxes]

Sure to end up in a 1590a too, cozy?!!

[Cozybuilder]

Maybe an LB

[Kipper4]

try swapping that c5 (220pf) for a higher value, Im using 10nf you might like it better.

What value resistor are you using between the source and drain on the srpp?

Have you tried the srpp with other fets?
I'm trying to preserve my stash of j201s.

Why add the op amp gain stage after the tonestack? i would have thought the mini booster (srpp) should have plenty to push the stack without the need for the extra stage.

Is the tone stack better with a 10k than  100k?
As it is in the original schematic the highs are needlessly high and harsh for my rig. Fender pj.

Cosy you'd be lucky to get that in a 1590a even with 2n7000s rather than the lrf520's.

[Cozybuilder]

Hi Rich-
The resistor is 1K (Q2-S to Q1-D). The Op-Amp is to invert the signal to be in phase with the input, the volume there works real well.

The 10K pot works a lot better in the tone circuit- larger values give a dead spot in the middle. Its ice picky at max treble, and wooly at bass- definitely needs work. I forgot to draw in the coupling cap- I stuck a 100n between Tone and the op-amp, it smoothed the response a bit- maybe my imagination?

I just breaded it this morning, haven't had a chance to try other JFETs or the other component values other than the vol and tone pots. I'm sure this can be improved, you and Sam Hay had some good ideas to try, so I did, and was surprised at how good it sounded from the start. Thats not usually the case. There are certainly a lot of things that can be tweaked, I'm looking forward to it.

Oooh, a challenge for a small box build!

[samhay]

Looks good Russ, but you should probably put a cap between the tone stack and second op-amp or connect the 22k to Vref rather than ground. You can loose the 470k resistor to the (+) input too.

[Cozybuilder]

Thank you Sam- there is a 100n cap there now, I neglected to draw it in. Another mistake, the cap from Q2-G to Q1-D is 33n, not 100n.

I'll remove that 470K on pin 5.

This is definitely a work in progress, nowhere near ready to box.

[duck_arse]

Thanks Duck Arse
10 on the way. I tried every type of fet i have and j201s work best in the srpp on my breadboard.
Some where less than unity.
Now I'm wondering if the srpp can be done with BJT. I searched and googled but found nothing useful.

rich - you should be getting bucketts of boost outta the srpp, esp w/ j201, maybe summink 'off'? I think joe davisson has done srpp bjt, maybe. mosfet too.

[Kipper4]

I do get loads of gain with j201 but all the rest bar one or two where not functioning in this circuit as I have it set up. Maybe I'll try again asap with a 4m7 resistor to Vb

There are bjt srpp they are even simpler in terms of parts basically an npn and pnp emitter to collector arrangement.
some include the srpp in the feedback loop of an op amp IIRC for stability.

I havent found a suitable mosfet alternative but I'm not done yet.
Just finishing up my directo-matic with an xlr conector then I'm gonna work on the corner freqauncies of the bmp tonestack with the duncan calculator.

Cosy I didnt bother with the op amp at the end of the circuit yet.
I've got lots of spare room on the perf for upgrades.
I did try bs170s for clippers but the lrf520s are better IMO
How do the 2n7000s sound?
I will try the 10k tone pot too.

Smoke me one, I'm on my way.

[Kipper4]

Bugger me  look what I just found thanks DA

http://www.diystompboxes.com/analogalchemy/old/ccsdrive.gif

[Cozybuilder]

Rich- Nice find! Always great to have the option to use 2cent transistors instead, will have to try these out.

Mine was gated, so I spent the morning on a quest to sort out the right bias combination. Its simple, just add the 1M resistor back in, to the ground side of the 100K trim pot to gate of Q1. Now on to reducing 60 cycle hum, adjusting the tone stack, and trying some component value changes.

[Kipper4]

I think I know the one.
Similar to Rod Elliot's fet srpp here note the power supply voltage.


http://sound.westhost.com/valves/preamps.html


Thanks Cozy I'm glad you like it enough to experiment with it.
The wife's not home till late tomorrow so I'll crank my amp a BIT and see how I like it then.

"It's not over until the drive sings"

[Cozybuilder]

I've been busy trying different things with the tone stack, and wound up with a modified SWTC-3. I think with these values it gives a good even response across its range.

This thing is sounding good, but still needs some work, it swells with higher gain settings. I tried down to 100K drive pot, which helped quite a bit, but there must be a better way.

This is the current circuit on my breadboard:

[Cozybuilder]

Its easy. Add a cap at the output over a resistor to ground- mine has 100nF over 10K. Louder, takes care of the swell, it sounds good as is, lots of components you can tweak to get your own sound if you don't like it as drawn above.

[Kipper4]

Godam Cozy.
I never heard the hum on the test amp. It's there on a bigger amp though.
This could be a big headache.
How can it be that were getting this 60hz hum kinda sound.
I tried a B250k for gain and liked it.

[mac]

Bugger me  look what I just found thanks DA

http://www.diystompboxes.com/analogalchemy/old/ccsdrive.gif

thanks DA

mac

[Cozybuilder]

I like the 250K drive pot better too. I find the hum really distracting at higher gain settings with the 500K, less so with the 250K, the 100K just doesn't have quite enough grit on tap. Now, how are we gonna reduce the 60 cycle and still have gobs of gain?