Fulltone - Secret Freq  [traced]

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Re: Fulltone - Secret Freq

Postby mictester » 26 Apr 2015, 10:59

MoonWatcher wrote:Sorry for the necro-post, but it dawned on me what this thing is most likely lifted from.

I bet it's based on the Pearl OD-05.


You're exactly right! There are a few component changes (most notably around the filter circuit), but it just acts as a cocked wah in front of a Tubescreamer circuit. I have my own version of the OD5, which I'll put up on here shortly.
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Re: Fulltone - Secret Freq

Postby snofla1900 » 26 Apr 2015, 12:51

I have my own version of the OD5, which I'll put up on here shortly.



Please , do !

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Re: Fulltone - Secret Freq

Postby zoetrop » 24 Jun 2016, 13:16

Today I got my hands on a Secret Freq. Out guitar player bought this and I decided to check out the circuit. Unfortunately, Mr. Fulltone seems very afraid of rip-offs... He sanded off the OP, and the pcb is multilayer with no visible traces - the only traces I saw were from and to the LED - kinda. Actually, one trace goes to the drain of T1, which is a BS250, even though the solder lug is labeled with LED2.
Top and bottom layer is ground plane. The highs pot has a 2k resistor in series to ground, maybe it cut off too much... the frequency pot is connected to an additional small pcb, which is glued to the case. Its complpetely covered with grey epoxy, it seems like there is a capacitor, but I couldn't find out more. Maybe that's the gyrator?
Funny thing: there are a few old-style carbon-compound resistors, R4 (9k1), R6 (10k), R7 (1k) and R9 (9k1).
As this isn't my pedal, I didn't rip off any components to measure connections between the vias. So, no schematic, sorry.

But with this informations I think it's safe to say that it differs from the Pearl OD-05 - this one has only one IC (which I think is a double Opamp, Pin 4 goes to ground...)

Cheers
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Re: Fulltone - Secret Freq

Postby MoonWatcher » 29 Jun 2016, 20:20

zoetrop wrote:... the frequency pot is connected to an additional small pcb, which is glued to the case. Its complpetely covered with grey epoxy, it seems like there is a capacitor, but I couldn't find out more. Maybe that's the gyrator?

I couldn't figure out how the frequency control would work without a dual-pot, but I looked at the pictures a little more closely. It looks like 3 pins go through the pcb, but I think I see another set of 3 pins on top of them, which would indicate why there's some off-board wire connections.

So chances are it's a dual 100k reverse-log pot, and under the epoxy are two resistors (probably 2k2) and two capacitors (4n7 and maybe 68n, although the second one could possibly just be 100n).

There's probably also a transistor under the epoxy for the gyrator, as it will work the same as an op amp.

The main drive can probably be accomplished with just two op amp stages, and it looks like there are two small-value caps connected at pins 1&2, and at pins 6&7. The OD-05 has a 470pf parallel to a 10k resistor for the pre-clipping stage, and then a 47pf parallel to a 100k-lin gain pot. Since the resistors are mounted vertically, and since there are primarily just overhead pics or they are off to one side, I can't make out the values of those key components.

...There will also probably be a 100nf cap off of either pin 1 or pin 7, and that would then probably connect to pin 5 or pin 3. Also, either pin 3 or pin 5 will probably connect right to the V-bias.

If you have a multimeter, you don't need to tear apart anything to determine voltages and potentiometer resistances. And even if the pcb traces are hidden, you can usually determine at least some connections with a meter, too. That is one of the benefits of the vertical-mount resistors - you have one lead that is very easy to get a probe tip on! :thumbsup

Unless the frequency control doesn't indeed use a dual-pot, then I'm still leaning towards considering this to be more OD-05 than not. The essence is the pre-clipping frequency control that allows for a cut or boost from low (~100hz) to high (~4khz) frequencies.

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Re: Fulltone - Secret Freq

Postby zoetrop » 04 Aug 2016, 15:39

So, I got my hands on the secret freq again and "traced" the schematic, i.e. I continuity-checked from each solder point to every other and derived the schematic from that. Quite tedious, but here is the schematic.
- Some thoughts: there is a BS250 P-FET and a Zener diode, both of them have something to do with the LED (and not with the sound). I forgot to write that down, but I'm sure it's nothing too special. Just hook up the LED to your 3PDT switch.
- no input buffer
- no output buffer
- the gyrator schematic is covered in gray goo, therefore I couldn't read the values. I build a farily simple OP-Amp schematic, hooked up the gyrator and measured where the frequency response had it's maximum, it was around 1.1 kHz. With that, I calculated and simulated said OP-Amp schematic in LTspice and chose the component values iteratively to fit the frequency response. So, Fulltone may have used different values, but I believe it's pretty close.


Have fun!
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Re: Fulltone - Secret Freq

Postby zoetrop » 05 Aug 2016, 14:25

Hi all,

I want to clarify some aspects, since I think it might be a bit unclear if one does not read the whole thread.
The OP-amps part marking has been sanded off, therefore I couldn't find out the part number. In the schematic v1.0 I labeled it as AD822, which is my ECAD-softwares standard op-amp. I changed it to the much more common 4558. I'm not sure whether the 4558 is correct, best is to use a socket and try different types.

R13 and R14 are both 10k and in serial. This may seem odd, but one of the two resistors in soldered where in the original pcb the frequency-pot had to be soldered (please see pictures above). You may replace them with one single 20k (or 22k) resistor.

The gyrator's transistor is a wild guess but not too off, I think. It's covered in gray stuff as well, but since the 2SC1815 is a nice low-noise audio transistor, I chose it. You may use an american type instead (e.g. 2N3904, 2N2222) or BC550, whatever comes in handy.

Please find an updated version of the schematic with my remarks attached.

zoetrop
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Re: Fulltone - Secret Freq  [traced]

Postby PlayerofPlexis » 16 Dec 2016, 18:38

Wow, that is so not even close that I have to correct you, enjoy.
Satan
SecretFreq.png
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Re: Fulltone - Secret Freq

Postby zoetrop » 19 Dec 2016, 12:14

PlayerofPlexis wrote:Wow, that is so not even close that I have to correct you, enjoy.

Hey, thanks a ton! Yeah, the gyrator was a wild guess, since I wasn't allowed to remove the goop. Good to know that theres an additional capacitor from the Freq-pot to ground

Still I'm a bit unsure, since your schematic says R16 = 100k, but on the board (see photos above) it's definitely not 100k but 82k (in my schematic it's R11). And also you see the serial circuit of R23 and C5, but R23 here is 10k and not 1k (3rd band is orange, not red). Are there different versions of the Secret Freq out there?
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Re: Fulltone - Secret Freq

Postby PlayerofPlexis » 20 Dec 2016, 18:51

You can always bet on changes having been made, I'll check into it.

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Re: Fulltone - Secret Freq

Postby audiohub » 07 Feb 2017, 05:46

So, is there any further info on the gooped board for this?

I've bought an inexpensive Joyo pedal, the sweet honey overdrive, that looks to be a good candidate to mod as a Secret Freq, and i'm trying to collect as much info as I can about it.
Player of Plexis, did you actually degoog the gyrator board and get cap values?
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Re: Fulltone - Secret Freq

Postby modman » 08 Feb 2017, 23:14

audiohub wrote:Player of Plexis, did you actually degoog the gyrator board and get cap values?

Why would he do that if he went through the effort of gooping it first?
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Re: Fulltone - Secret Freq

Postby okgb » 05 Sep 2018, 02:14

It seems he hasn't [ obviously anyway ] got on the Dumble or Klone bandwagon has he ?, not that he's humble But I'm sure he thinks he should have the same mythical status to prove
he's a great designer and he's trying [ booster with germanium diode limiter ? ] lots of people rag on the klone as not that special but jump on Bill's coat tails to make money off his work
maybe mike is really a re-doer or updater , is any of his music original ? I never bothered to listen.

I had a couple of emails with him where I wanted to mod [ change the Q of the gt 500 mid inductor ] and he simply said there were no mods [ change one resister like any wah ]
when I said I'd check here for a solution, he referred to freestomp as a bunch of pirates , more like Robin hood I'd say !

Where else could you come up with a term like yats!
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Re: Fulltone - Secret Freq

Postby Jone the Bone » 15 Sep 2018, 07:21

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Re: Fulltone - Secret Freq

Postby Ben N » 17 Sep 2018, 22:06

okgb wrote:Where else could you come up with a term like yats!


Even that is derivative (I think) -- from Jack Orman's YAFF. So no need to unload on old Mike, there really is nothing new under the sun.
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