Roland Ap-7 Jet Phaser Fuzz section question

[pete_g]

Hi all. I've built around 6 pedals and am still new to this so I was wondering if I can get some help on building the fuzz section of this pedal.
Can someone please highlight and help me isolate the circuit of just the fuzz section of this schematic so i can build a pedal from this circuit?

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[Keppy]

It looks to me like everything from the input to the "Jet Level" pot will do the trick. If you jumper R82 the volume control will go all the way to zero. Note that the circuit uses +/-9v, so you'll have to use two batteries or a charge pump if you want it to sound the same.

[pete_g]

I have a pedal power 2 plus and a Y cable that can use 2 9v output jacks from it. So this uses a chip LM324 as well. I have a spare 741 IC. Will that work instead? I'll try building it sometime soon and hopefully all will turn out well.
I'll see if I run into any trouble with it.

[duck_arse]

knock it up on the breadboard. the lm324 loves to run single supply, but you're only using a single of the quad. you could shove in a tl071, with a bias string maybe, or use the dual version of the 324. looking a bit harder, there is some junk around IC3B, and ic3b itself, you'd want to include. leave off R17 and all following it, and the down-going connections to R22.

[Keppy]

I have a pedal power 2 plus and a Y cable that can use 2 9v output jacks from it.

I don't know how your power supply is wired, but you need 9v above ground and 9v below ground for the schematic to work as pictured.

So this uses a chip LM324 as well. I have a spare 741 IC. Will that work instead?

A 741 IC will work, but it will sound different in this circuit as it's being used to distort the signal. Of course, anything you do will sound different than the original if you're leaving out the phaser. Just watch the pinout.

looking a bit harder, there is some junk around IC3B, and ic3b itself, you'd want to include.

IC3B is driving the mix of dry fuzz signal with the phase shifted signal, as well as a feedback path to intensify the phaser. I still say leave it out if you just want the fuzz, unless I missed something (which I probably did because I have no idea why R21 is there, unless it's just providing a load for the opamp).

leave off R17 and all following it

R15, R17, & R18 create a voltage divider in front of IC3B. The gain of IC3B is set at about 3, so the divider knocks it down to about .5. Removing R17 alters the gain structure of the original, though the extra boost might be nice after the clippers and tone stack.


Here's a thought: Instead of a bass/treble switch, just make R13 a B100k pot. The extreme settings would be exactly the same as the settings on the switch, but you'd have all sorts of in-between settings too. In fact, you'd have a Big Muff tone control. No extra work, no extra parts, just extra sounds.

[pete_g]

So I don't need to include the IC if I want just the fuzz? Can you highlight in red on the schematic where the circuit gets cut off from the IN or which end of R8 or R9?
Thanks.

[PRR]

Use the '741. It'll fuzz.

The '324 is not a "good" audio amp unless you treat it right. The "un-good" *may* be part of the fuzz tone. But I bet 99% of the tone is in the diodes after the opamp, and the tone-shaping before and after the diodes.

They used the '324 because the phaser section needs a LOT of opamps and at one time the '324 was significantly cheaper than any alternative. Since you are not building the phaser nor counting your tenth-cent profit losses, this does not apply.

[Keppy]

So I don't need to include the IC if I want just the fuzz? Can you highlight in red on the schematic where the circuit gets cut off from the IN or which end of R8 or R9?
Thanks.

You still need one opamp. IC3A is mandatory, though you can substitute a 741 for the 324. Cut off everything after VR1.

[pete_g]

Ok that cleared it up. I'll have a go at it some time this week and post whether it was a success. Thanks for that.

[duck_arse]

^ yer, wot they said.

[pete_g]

A couple of questions.
Do the pin numbers on the LM324 match with the 741 IC numbers?
Can a Bass / Treble pot be put in place of the JET TONE SW3 switch and do the lug numbers match?
Are the V+ symbols I've put a Red question mark next to the positive power supply connection?
And would you place a DRIVE control pot in place of the R9 or R10 resistor or somewhere else?
Thanks.

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[nocentelli]

If you use a 741 single opamp, use pin 2 and 3 as per the schematic: Pin 4 of the 741 should be grounded, pin 6 is used for output instead of pin 1, and +9v goes to pin 7. Pin 1 and 8 should not be connected to anything.

A 100kB pot could be used instead of the switch, and would give a decent impression of a BMP tone pot, but you'll need to reverse lug 1+3 compared to the schematic numbers if you want the standard "clockwise = more treble".

The only thing that I am not so confident about is the fact the original used a bipolar supply: Since you will have grounded the opamp -ve (pin 4), you should probably rig a 10k/10k voltage divider and hook the vref created to those 10k resistors off the opamp inputs instead of connecting them to ground. You might want to wait for further advise on this.

[moosapotamus]

Do the pin numbers on the LM324 match with the 741 IC numbers?

Data sheets are very easy to find on the net. Just type the IC number into Google.

Presto...
http://www.ti.com/lit/ds/symlink/lm124-n.pdf
http://www.ti.com/lit/ds/symlink/lm741.pdf

Comparing pinouts is pretty easy once you get used to looking up the datasheets.

I say, better to learn for yourself than to keep asking the same kinds of questions over and over again.

RE: +/-9V supply
Myself, I would probably add an LT1054 charge pump and three caps to get -9V for the opamp.

Cheers
~ Charlie

[Mark Hammer]

A 100kB pot could be used instead of the switch, and would give a decent impression of a BMP tone pot, but you'll need to reverse lug 1+3 compared to the schematic numbers if you want the standard "clockwise = more treble".

Exactly.  The old Little Big Muff used the same sort of arrangement.  Underneath was a standard BMP tone circuit, but the 100k pot was replaced by a fixed resistor, and a switch that either picked from the full treble side or the full bass side.  That way, by using a fixed "Sustain" setting, EHX could stuff a cheaper BMP into the same 1-switch/1-pot box they used for a bunch of other pedals.  Small wonder that thing didn't sell like hotcakes; its best tones are not found at 5:00 and 7:00.

And I suspect that the 470pf cap should probably be 47pf, if you want to have any discernible sizzle.  470pf and 2M2 gives a treble rolloff starting around 153hz.  47pf will move that up by a factor of 10 to 1.5khz, which will give you some treble to remove, at least.

[duck_arse]

and being a big muff type tone control, you might want to follow it with some buffering/gain make-up, as in the big muff. a single transistor stage, like in the big muff, or an opamp stage, probably like in the roland jet phase, would do the trick.

[Keppy]

and being a big muff type tone control, you might want to follow it with some buffering/gain make-up, as in the big muff. a single transistor stage, like in the big muff, or an opamp stage, probably like in the roland jet phase, would do the trick.

This seems like a good idea. Oddly, the Roland doesn't appear to have any gain make-up, since the last gain stage (2x) is preceded by a voltage divider (.17x, really less due to the tonestack). I thought that was weird.

[onthetundra]

How would I swap out the switch for a tone pot and add a gain control?

[nocentelli]

A 100kB pot could be used instead of the switch, and would give a decent impression of a BMP tone pot, but you'll need to reverse lug 1+3 compared to the schematic numbers if you want the standard "clockwise = more treble".

Remove the 100k resisistor. Replace with a pot. Connect the lugs to the matching numbers on the schematic BUT reverse lug 1 and 3.

For a gain control, you could do several things: Replace the 10k/10k in front of the opamp with a 20k pot wired as a big muff style voltage divider "sustain" pot; Replace the 2M resistor in the feedback loop with a 2M pot (not common); Replace the 10k to ground from the inverting input with a 100k or 250kA pot.

[Mark Hammer]

The 2M2 feedback resistor and 10k grund-leg resistor give a gain, for that stage, of 221x.  I'm assuming the transistor stage before it has some gain as well.  So whatever gain it provides is multiplied by 221x, which could easily go into the thousands (although R7/R8 divide the output of the first stage by 2).

Given all of that, I'll suggest that a suitable gain control might be replacing R9 with a 3k3 resistor in series with a 10k pot, preferably C-taper, but I suppose linear will do.  That will give the op-amp stage a range of gains between 166x and 667x.  So you'll be able to go from  noticeably less saturated than stock, to much more saturated than stock.

If you wanted something moving in the direction of "grind" but not fuzzy, then consider using 6k8 and a 20k or 25k pot.  At max pot resistance, the stage gain will be 83x, using a 20k pot and 6k8 resistor, with a max gain of 324x.

[onthetundra]

Awesome suggestions. Thanks.