Alcapulco Gold LTSpice [project help]

[KingGeedorah95]

Hi everyone,

I'm trying to characterize an Acalpulco Gold circuit in LTSpice. I've been using LTSpice for the past year during my electrical engineering degree so I'm familiar with the software but this is my first time trying to simulate a guitar pedal.

Here is what I go so far...



The pot and op-amps might be hard to read but they are 100k at 50% and LM386s

Now, I took the peak-to-peak of both Vin and Vout and obtained the gain. I multiplied Vin by the gain for verification and as you can see I've got a gain of ~5500. That's waaaaaaay too much isn't it? Wouldn't that be an upwards of +70 dB? Shouldn't it be something like +20? Idk. I can't find specs like this anywhere so please let me know if I'm wrong.

Am I using the incorrect input for my guitar signal? Right now it's set to 0.132 [mV] @ 1kHz. The rest of the schematic is in good shape, yeah?

[FiveseveN]

Hello and welcome!

I've got a gain of ~5500. That's waaaaaaay too much isn't it?

Should be 40000-ish, it's a high-gain distortion, you will have a hard time getting a clean sine out of it. Which brings us to...

Right now it's set to 0.132 [mV]

That's more like the noise floor of a guitar signal. Here's an article by Jack Orman that goes into some detail: http://www.muzique.com/lab/pick.htm

[anotherjim]

The LM386 chip's datasheet does say the cap between pins  1 & 8 raises gain to 200. Total voltage gain with more than one stage multiplies by the gain of each consecutive stage. x3 followed by x3 is x9, not x6.
x5500 is too much, but I suppose there could be factors in the sim (noise & distortion?) and measurement (pk-pk vs RMS?) that can give such a large discrepancy because of the multiplying nature of the 2nd gain stage.

[duck_arse]

does your sim program recognise R1 as 1Meg or 1milli? [I know 0 about sims.] and welcome.

[KingGeedorah95]

Hello and welcome!

I've got a gain of ~5500. That's waaaaaaay too much isn't it?

Should be 40000-ish, it's a high-gain distortion, you will have a hard time getting a clean sine out of it. Which brings us to...

Right now it's set to 0.132 [mV]

That's more like the noise floor of a guitar signal. Here's an article by Jack Orman that goes into some detail: http://www.muzique.com/lab/pick.htm

Glad to be here and thank you for the article!

[KingGeedorah95]

The LM386 chip's datasheet does say the cap between pins  1 & 8 raises gain to 200. Total voltage gain with more than one stage multiplies by the gain of each consecutive stage. x3 followed by x3 is x9, not x6.
x5500 is too much, but I suppose there could be factors in the sim (noise & distortion?) and measurement (pk-pk vs RMS?) that can give such a large discrepancy because of the multiplying nature of the 2nd gain stage.

I managed to find this information from the datasheet that you might be referencing...

"9.2.1.2.1 Gain Control

To make the LM386 a more versatile amplifier, two pins (1 and are provided for gain control. With pins 1 and 8 open the 1.35-kΩ resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 1 to 8, bypassing the 1.35-kΩ resistor, the gain will go up to 200 (46 dB). If a resistor is placed in series with the capacitor, the gain can be set to any value from 20 to 200. Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground."

So I am going to try removing the second gain capacitor and maybe placing a pot there for gain control. Even using Vrms instead of Vpp there is less than a 1% difference in gain which leads me to believe there is a different problem at hand.

Thank you for your response!

[KingGeedorah95]

does your sim program recognise R1 as 1Meg or 1milli? [I know 0 about sims.] and welcome.

Hahaha nope! I must've replaced 1Meg by mistake when I was doing some remodeling. I don't use Megs often so I am always forgetting that "M" isn't recognized by the syntax in LTSpice. Now that simulation looks like this...



There is some hard clipping on the output signal but that's desirable in a high gain/distortion pedal, isn't it?

I also changed my input voltage to 150 [mV] @ 640 Hz.

[Vivek]

Could you email me your LTSPICE file ? Thanks

[KingGeedorah95]

Could you email me your LTSPICE file ? Thanks

The link below should be shareable with anyone who uses it. All of the files are there including the LM386 .asy and .sub and the 1N4001 SPICE model. Let me know if you have any trouble accessing it.

https://drive.google.com/drive/folders/1dUwpvGqqzTOBoi_ezlWijx5tabg0q2hx?usp=sharing

[Vivek]

Will you actually build this circuit after SPICING it up ?

[ElectricDruid]

does your sim program recognise R1 as 1Meg or 1milli? [I know 0 about sims.] and welcome.

Hahaha nope! I must've replaced 1Meg by mistake when I was doing some remodeling. I don't use Megs often so I am always forgetting that "M" isn't recognized by the syntax in LTSpice.

Me too. I *think* I might now have got into the habit of using my new favourite 999K resistor value!

[KingGeedorah95]

Will you actually build this circuit after SPICING it up ?

Yeah! So I made a breadboard build and it sounds decent. Have you ever built this one? Any mods I should be aware of? I want to add gain control to the second op-amp.


I found another schematic that is slightly different so I'm going to SPICE it up and breadboard that one to see which I like better. Then I'm moving over to perfboard it and fit it into a tiny little 1590A. Just being introduced to KiCad so I'm not going to do a full out PCB layout just yet.

[KingGeedorah95]

does your sim program recognise R1 as 1Meg or 1milli? [I know 0 about sims.] and welcome.

Hahaha nope! I must've replaced 1Meg by mistake when I was doing some remodeling. I don't use Megs often so I am always forgetting that "M" isn't recognized by the syntax in LTSpice.

Me too. I *think* I might now have got into the habit of using my new favourite 999K resistor value!

Hell yeah! Haha that's great. -1k ain't going to make a difference at that level anyway.

[PRR]

1000k is a legal value.

[composition4]

So is 0.001G

[jdoughty]

I just built this effect myself and it's awesome, so very excited to see if you can add a usable gain control! The variety of tones you can get with the volume knob is excellent, so I'm sure there's plenty of ways.

[Vivek]

Will you actually build this circuit after SPICING it up ?

Yeah!  I'm going to SPICE it up and breadboard

It would be great if you decided to calculate expected signal level and distortion percentage/ wave shape at each stage

Off the cuff, I would assume that gain of first stage should be around 80 for the gain control of the second stage to be meaningful

( I cannot calculate more precisely since it depends upon

output of guitar signal

and impedance of guitar pickup

and also your tastes : Do you want from almost clean boost to mayhem  ? or mid distortion to high distortion ?

And loss in the inter-stage Low Pass filter)

[ElectricDruid]

( I cannot calculate more precisely since it depends upon

output of guitar signal

and impedance of guitar pickup

..both of which are widely variable, unfortunately.

A "guitar signal" is not a well-defined standard. In fact, the only way it *can* be defined is as "anything a guitar produces"!

Hence the effect of pedals that depend on gain and/or signal level for their effect is likely to be similarly variable. There's no way around this. The simplest solution is player-operated knobs that compensate for different signal levels, but then of course every player has a difference experience, which isn't ideal when people want to share "sweet spots" on the controls.