SOLVED: Spirit Box - Unity Gain Issue

[jdduffield]

I built the spirit box Pcb and noticed a drastic volume drop when the pedal is on. In other words, I need more volume to achieve unity gain. I’ve been reading that the best approach is to increase the resistance value of R4. I started out with 10k. I tried 18k, 20k, 47k, and 68k. The higher I go the closer to unity gain I get. Most people seem to only increase to 18k or 20k. I still need more volume but not sure how high I can go before breaking something. Can I try 100k or is that a bad idea for some reason?

 

[jdduffield]

The 470n caps I’m using are large but I’ve shaped the legs to fit the board. I’m still new to schematics so wondering if these can affect volume/unity gain.

 

[Robert]

R6 should be 470R (470 Ω), it looks like you have 470K installed there.

That could definitely be the cause of the problem.

 

[jdduffield]

R6 should be 470R (470 Ω), it looks like you have 470K installed there.

That could definitely be the cause of the problem.

View attachment 86292

 

[jdduffield]

Oh wow. Good eye! I’ll swap that out. Thanks!

 

[Feral Feline]

I did the same exact thing, @jdduffield, but was working from a hand-drawn (by me) copy of the original schematic of the circuit I was breadboarding — so when I checked my circ against the schematic (copy), it was “correct”.

After many times going through checking it, gave up and sent a pic to a Guru-mentor friend — he spotted my error immediately, “the transistor will never turn on with 390k, use a 390r”. I’d inadvertently added a “k” after that resistor’s value when copying it.

I’ve told the story many times on the forum, a cautionary tale, that
— thinking it’s right doesn’t mean it’s right.

So glad Robert spotted your build’s 470 Ohm-error so quickly.
After first reading of your plight elsewhere last night, I had to catch some sleep. First thing I did when I woke up was search for this thread, wishing to maybe help, so quite a relief a pro already had it sorted. I’m not the best trouble-shooter, obviously.

I hope there are no other hiccups for your build, and if not this thread can be added to the marked “SOLVED” pile.

 

[jdduffield]

I did the same exact thing, @jdduffield, but was working from a hand-drawn (by me) copy of the original schematic of the circuit I was breadboarding — so when I checked my circ against the schematic (copy), it was “correct”.

After many times going through checking it, gave up and sent a pic to a Guru-mentor friend — he spotted my error immediately, “the transistor will never turn on with 390k, use a 390r”. I’d inadvertently added a “k” after that resistor’s value when copying it.

I’ve told the story many times on the forum, a cautionary tale, that
— thinking it’s right doesn’t mean it’s right.

So glad Robert spotted your build’s 470 Ohm-error so quickly.
After first reading of your plight elsewhere last night, I had to catch some sleep. First thing I did when I woke up was search for this thread, wishing to maybe help, so quite a relief a pro already had it sorted. I’m not the best trouble-shooter, obviously.

I hope there are no other hiccups for your build, and if not this thread can be added to the marked “SOLVED” pile.

I’m amazed at how people are able to scan an entire build and spot things so quickly. I’ve been doing this under a year and have a long way to go, but hope to get good at this stuff with more practice and time.

 

[jdduffield]

Just a follow-up. All is well. Finished the build. This is a video showing how it turned out.

 

[ftumch]

@jdduffield did you end up with the stock R4 value or leave it at 18k? Just putting one together now

 

[comradehoser]

Edit your thread and change the prefix to SOLVED

 

[jdduffield]

@jdduffield did you end up with the stock R4 value or leave it at 18k? Just putting one together now

I ended up with R4 at 20k which turned out to be just right for my ears in terms of unity gain. The video is an exact representation of the volume change from pedal OFF to pedal ON. Even if technically there is a tiny boost happening, that’s ok in my mind because reverb sets the signal back in terms of depth anyway. If there is technically a slight change in volume, I doubt it would be enough to cause any amp feedback or anything like that.
R8 was changed as well, to a 12K. That was a preference thing because I felt that the dwell knob (or in my case, the “vibe” knob) wasn’t doing enough at the suggested default value.

 

[ftumch]

@jdduffield cool thanks I'm going in now

 

[jwin615]

I’m amazed at how people are able to scan an entire build and spot things so quickly. I’ve been doing this under a year and have a long way to go, but hope to get good at this stuff with more practice and time.

A good quick check is to pull up someone's build report with a clear gutshot and doing a quick comparison, paying close attention the that next to last band.

 

[ftumch]

I went with 12k for R4 in the end and perceive a slight boost when engaged

My use case is probably a little different to the norm, I like the trashy Belton brick sound into a dirty amp, complex reverbs can turn to mush used this way

The pre-delay keeps the reverb away your playing which keeps it clear when things are noisy

I went for the short brick so I can crank the mix up more

Such a cool idea to combine a PT2399 and reverb brick in this way