This particular Sparkle Boost has got me stumped

ayayay! · October 28, 2009, 09:55:57 PM

Here's the same old Sparkle Boost I always build:

It's a great circuit, and this is my 3rd one. No mods other than doubling the input cap to .047uF, which I've done on the previous 2.

However on this one there is almost NO dirt whatsoever when I max the gain knob. I've tweaked the trimpot several times, and no matter what I do, the only gain I get is super farty/unuseable. No happy medium. It works great as just a boost, so an audio probe doesn't really help here.

I've used 3 J201s, an MPF102, new 10uF caps, new 2.7k resistors (including 2.2k,) Ohm'd out all resistors (all OK,) trimpot is fine... I just can't figure out why on earth it's doing it.

The simple thing would be to just build a new one, but I want to know why it's doing this.

Ripthorn · October 28, 2009, 11:50:03 PM

I had an issue sort of like this once and turns out I had one tiny strand of my stranded wire touching something it shouldn't have.

ayayay! · October 29, 2009, 07:59:12 AM

Yeah I'm kinda thinking new build at this point. Luckily it's quick and easy and I have all the spare parts, but I just find it so odd.

All I can think of is that I have a goofy trimpot somehow. Everything "works," just not nearly as well as it should. So weird.

oliphaunt · October 29, 2009, 11:04:34 AM

Quote from: ayayay! on October 29, 2009, 07:59:12 AM
All I can think of is that I have a goofy trimpot somehow.

I ran into a batch of bad trimpots recently. I test them all now before I use them.

I've been meaning to put together a Sparkle Boost. i'm gonna go breadboard that thing right now...

rustypinto · October 29, 2009, 11:10:05 AM

Can you compare the voltages around different nodes to a "known-good" unit? Who knows, 3 different J201's may not be enough. Are the J201's from the same manufacturer? trimpots can also be unreliable depending on construction, resistance material, tolerance spec, etc.

ayayay! · October 29, 2009, 11:32:16 AM

Quote from: oliphaunt on October 29, 2009, 11:04:34 AM
Quote from: ayayay! on October 29, 2009, 07:59:12 AM
All I can think of is that I have a goofy trimpot somehow.

I ran into a batch of bad trimpots recently. I test them all now before I use them.

I've been meaning to put together a Sparkle Boost. i'm gonna go breadboard that thing right now...

May I ask where from? I used a new trimpot from SB (Gray. Bourns?) that I've never used before.

Quote from: rustypinto on October 29, 2009, 11:10:05 AM
Can you compare the voltages around different nodes to a "known-good" unit? Who knows, 3 different J201's may not be enough. Are the J201's from the same manufacturer? trimpots can also be unreliable depending on construction, resistance material, tolerance spec, etc.

I did compare the voltages last night on my good one and this one. For measurings sake I set the drains to 4.5V. I can't recall what the voltages were but the Source & Gate readings were close between JFETs. Argh.

The J201's were Fairchilds. You think maybe try a different brand/batch? I guess I could...

Like I said before, it works, just not as intended. Those are often harder to t-shoot.

At first my money was on the 10uF or the 2.7k, but both tested out great, and were the same after replacing. The 10uF on my good Sparkle Boost came out about 9.5uF. My "bad" pedal is about 10.1uF

oliphaunt · October 29, 2009, 11:53:17 AM

Quote from: ayayay! on October 29, 2009, 11:32:16 AM
May I ask where from? I used a new trimpot from SB (Gray. Bourns?) that I've never used before.

They were some blue square ones from Mouser. Not sure of the brand. If your trimmer is adjusting the bias of the JFET then you should be ok as far as that goes. The bad ones I got simply do nothing.

kurtlives · October 29, 2009, 12:09:58 PM

Id check out your gain pot. Might try substituting it with a non working one.

oliphaunt · October 29, 2009, 01:35:18 PM

Interesting, I just built the Sparkle Boost with a J201 on my breadboard. I get absolutely no distortion from it of any kind even with the gain wide open. It's a great sounding boost, extremely natural on the bottom end with a little bit of extra crispy stuff on the high end. I found that removing the 1M resistor will make the top end behave more naturally. I replaced the 1M with a 100K, assuming it has something to do with input impedance. That still sounds basically neutral.

If I bais the JFET to about 1V I get some overdrive, but it's not really that good sounding.

For kicks, I built a second SD and drove one into the other. That makes for a nice low gain overdrive, with the output volume of the first SD controlling the gain. It has tons of output at the end though, way too much really, the volume pot is too sensitive. I think Maybe I'll add a third stage and see what happens!

Der Groovemeister · October 29, 2009, 01:59:55 PM

You are a sick man!

LOL

oliphaunt · October 29, 2009, 03:19:48 PM

I stil have no useful information for the OP except I have built three stages of the SB and none of them have much overdrive by themsleves. Now, stack all three together and then you 'll hear some drive!

I'm really liking this triple SB. The JFETs overdrive smoothly and there is a lot of gain and a LOT of output available. I have been tweaking the in and out cap values on each stage and the 1M resistor is a different value on each stage trying to find the right EQ. I think I have it pretty close, but I need to get my giging amp out of the band trailer to do a final check as my old Bassman rig that I am testing on now is sometimes a bit forgiving of harsh high end. I permantly wired the gain control each stage, tweaking that for each stage also. I will have to post a schematic and sound clip sometime soon once it is fully dialed in.

jayp5150 · October 29, 2009, 03:30:37 PM

I built a version with a 5457 that I loved, so I threw one together for my dad as well, but all I had at the time was a j201. It was completely clean...

The 5457 had a good amount of grit to it, though.

I will agree that you can get some cool tones out of a couple cascaded SB's though

ayayay! · October 29, 2009, 04:22:40 PM

Quote from: kurtlives on October 29, 2009, 12:09:58 PM
Id check out your gain pot. Might try substituting it with a non working one.

That's a very good point, but the gain pot on the stock Sparkle Boost has a behavior that let's me know it's working: Volume boost. The Sparkle Boost in the schem above does boost the volume some in the last 25% of the pot travel. This is well documented in several threads and I believe Andrew even offered up a fix for it in one thread IIRC. So that's a good point, but that's not my culprit.

Quote from: jayp5150 on October 29, 2009, 03:30:37 PM
I built a version with a 5457 that I loved, so I threw one together for my dad as well, but all I had at the time was a j201. It was completely clean...

The 5457 had a good amount of grit to it, though.

I will agree that you can get some cool tones out of a couple cascaded SB's though

Yap! Tried one of those too. Same thing.

...Helluva clean boost right now though. Ha!

ayayay! · October 31, 2009, 12:14:12 PM

Well I built a new board and all is well. Oh well, guess I'm putting this one to bed.

oliphaunt · October 31, 2009, 05:52:28 PM

Did you use the same components? Are you using a J201? I'm still curious as to how to get grit out of this curcuit, I can't get any on my breadboard.

The "Triple Sparkle Boost/Drive" thing is coming along very nicely! I'm tempted to take my breadboarded version to the gig tonight to test it in the mix, but it would likley be destroyed by some deranged, spirited partier, it being Halloween tonight and all... still I might do it anyway!

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This particular Sparkle Boost has got me stumped