homebrew variable load buffer

[Chris Reesor]

After a long period of procrastination, with the parts sitting around, I finally built my own take on Brad Sarno's Freeloader. One JFET, two caps, four resistors, a 500K audio taper pot. Add a diecast box, a couple of 1/4 " jacks, a battery clip and terminal, a knob and a leg clip and a piece of circuit board and the total parts cost was maybe 40-45 bucks CDN.
Well, now I really know just why the Freeloader is such a great box, and vastly superior to the passive tone control on some steels for taming harsh top end.
The passive tone circuit kills the harshness, but also all your top end. The Freeloader type buffer smooths the resonant peak without killing all the sparkle.
If you are reading this Brad, I'm also liking the pickup load in the 150K area. Pickup is a 12 string Tonealigner.
If anyone is interested, I'll post the circuit I used here. It is about time I learned how to post images anyway, after eight years on this forum.

[Robert Parent]

Hi Chris,

Please post your schematic as I for one would be interested.... I built the current buffer that I am using. It is based on a LF353 op amp, it works great, but always like to try different ideas.

Thanks,
Robert

[Chris Reesor]

Here is the updated schematic for the buffer circuit
https://photos.app.goo.gl/B5UIL7mgngN2Vxov1

[Robert Parent]

Thanks Chris, I'll give it a try.

[Jon Light]

I'm trusting that you did not just reverse engineer the Freeloader, right?

[Chris Reesor]

No, Jon, I just used a basic JFET buffer circuit I found on the web, and added a variable resistive load right across the pickup. The basic idea is Brad's, I believe, at least that is where I got the idea. How closely this corresponds to the innards of a Sarno Freeloader, I don't know. But it does work well, and whether it would be better with another transistor in terms of hiss, it will take ears with better high frequency response than mine to say. Forty years of driving heavy trucks does take a toll.
Peace.

[Jon Light]

Cool. I hoped not. Thanks for the answer.

[ajm]

I looked at the schematic.

At the maximum pot setting, the voltage input to the FET circuit will be divided by around 10:1.
I would think that you would notice a large volume drop.
In addition, because of the hearing characteristics of our ears, you may also notice a perceived drop in high frequencies.

At the minimum pot setting, the input impedance to the circuit is around 47K. Assuming that you are feeding the circuit directly from the guitar, that will probably load the pickup enough to get rid of some highs.

Discuss.

[Scott Duckworth]

I had posted a schematic here based on the author hand drawn first version, but since so many changes were made, I decided it best to delete it. Sorry.

[Chris Reesor]

Yes, ajm, there is an error in my quickie schematic. Input jack is actually connected to the .1 mfd. cap with the pot and 47K resistor directly across the input jack terminals.
Good spot. Thanks. I'll post a corrected schematic when I find my camera (!) and figure out how to post images here.

Edit: I see Scott was posting a schematic while I was typing. Could you please correct the circuit as I have described?
Much obliged.

[ajm]

Here's what I think you want corrected on Scott's schematic.

1) That resistor should be 47K, NOT 4.7K.

2) The pot and the 47K are connected in series, not parallel.
That would vary the resistance from 47K at minimum to 547K at maximum.

3) The input jack is connected to the junction of the 47K/500K series and the 0.1 uF cap.

If any of this is not correct please advise.

If all of this is correct, what DC voltage do you measure on the source of the FET?
Ideally I think you want it to be half of the supply voltage.

[Chris Reesor]

ajm, the corrections you suggest sound right.
I didn't take any voltage readings on this circuit before putting it together for testing, however if my theory is correct,the gate should be at roughly 50% of the battery voltage (10 % tolerance resistors in the bias divider), and the source at that voltage minus .6 V since the source-gate junction is reverse biased.
I am going to build another one and install it in the guitar, since there is already a tone control with a switch to take it out of the circuit when fully CCW. Lots of room under there for a little shielding box and a battery bag.
OK, back to making supper.

[Brad Sarno]

After a long period of procrastination, with the parts sitting around, I finally built my own take on Brad Sarno's Freeloader. One JFET, two caps, four resistors, a 500K audio taper pot. Add a diecast box, a couple of 1/4 " jacks, a battery clip and terminal, a knob and a leg clip and a piece of circuit board and the total parts cost was maybe 40-45 bucks CDN.
Well, now I really know just why the Freeloader is such a great box, and vastly superior to the passive tone control on some steels for taming harsh top end.
The passive tone circuit kills the harshness, but also all your top end. The Freeloader type buffer smooths the resonant peak without killing all the sparkle.
If you are reading this Brad, I'm also liking the pickup load in the 150K area. Pickup is a 12 string Tonealigner.
If anyone is interested, I'll post the circuit I used here. It is about time I learned how to post images anyway, after eight years on this forum.

Nice work! Yeah, 150kOhm load is totally usable and often a sweet spot. I definitely find myself dialing the load down that low at times and with certain pickups too. I remember years ago checking out one of Tom Brumley's ZB volume pedals, and it had a military spec Clarostat potentiometer in it that was 100k audio taper. So he was working with a loading of 100k, FAR lower than most anyone else uses. And he made it work. He had such a fat, warm tone yet with plenty of clarity. I think I personally tend to operate in the 180k to 300k range mostly when it comes to load impedance values. I can't remember where I was with the Tonealigner when I had it in my guitar.

Brad

[Chris Reesor]

Thanks for the comment ,Brad. Too bad I didn't pay attention to where Greg Leisz had his Freeloader set with his 12 string Tonealigner when I saw him play a small club with Bill Frisell a few years ago. He was using a Fender DeVille , and the result was warm and full but certainly didn't lack clarity.
The observation about Brumley and his 100k volume pot just emphasizes the point about the overall effect of pickup load on your sound, I think.
I wonder about how an active pickup similar to an EMG would work for PSG. With low inductance and distributed capacitance, the pickup resonance could be raised well above the frequency band we are working with, I think.

[ajm]

A couple more comments (open to correction/criticism/etc).....

1) I'd add a resistor to ground on the output.
If you don't have one, you might get a large pop when you connect it up due to DC voltage charging on the capacitor.
Value?
Maybe 10K or 100K or 1 Meg or ??????

2) This is a unity gain, or less, buffer.
There is no amplification here.
Then again, amplification is not what we intend on.

3) The biasing will result in less than symmetrical clipping at the output if you ever drive it hard enough.
This may or may not matter.

4) I'd add a reverse biased diode from V+ to ground in the event that you ever connect a battery up backwards (even momentarily).

[Dave Hepworth]

Hi,
Could you please post an amended circuit diagram of the suggested mods that work on this.Also if I wanted to use a J201 in this circuit ,what would the pin out be on the circuit please ,with annotation/ alignment .
Regards Dave

[Chris Reesor]

I have posted a new link to the corrected schematic in the third post of this thread.
Dave, the J201 has the same pinout as MPF102, which I used simply because my nephew gave me several. I added a little pinout drawing to the schematic.
I'm wondering whether protection for reversed battery polarity is necessary here; the 3.3k resistor would limit the channel current to around 3mA, well within the device limits. The forward biased G-S junction current would be limited by the 2.2 M bias resistor to a few microamps.
I'll have to whip up another one, hook up the battery backwards and see if the transistor survives.

About the suggested resistor across the output circuit: So far, I have only run this thing into my passive Goodrich VP, so not an issue. However, we will see what happens when I try some dirt boxes before the VP.
I'm not hearing any clipping with my hardest picking.
Yes, it might clip asymmetrically, which is not necessarily a bad thing (watch a Champs' output waveform on a scope as you drive it into distortion some time) but I doubt you will get it there with even the hottest passive steel pickup.

[Dave Hepworth]

Hi Chris,
Thanks for the ammended circuit diagram .I didn't see it on 3rd reply !!
I have quite a few J201 s and have built a buffer and guitar treble booster with them.This project looks interesting .......a buffer with tone adjustment ,controlled by altering the impedance ......am I correct?
Regards Dave

[Scott Duckworth]

OK, I redrew the schematic for you in my computer, hopefully it is right...

(Edited 11-24-2017 @ 5:40PM ET)

[Chris Reesor]

That's got it, Scott.Thank you.
I did find my camera, just a few minutes ago. I seem to be getting better and better at hiding things from myself lately; must be a sign of "maturity" -
I should note that if you hook the pot up correctly, fully ccw will show the pickup a 47k load, mid rotation about 100k, and fully cw about 550k.
It probably wouldn't be a bad idea to add a 1 meg resistor across the output jack as ajm and Georg suggest; I'll add it to the next iteration of this circuit, to be built into my Excel.
Excelsior!

[ajm]

Diode protection: I like Georg's idea of putting a diode in series with the V+ line. Probably easier and fewer parts than what I originally suggested.

Regarding Scott's redrawn schematic on 23 Nov: IMO it's about 99% perfect.

The "problem" is with the battery connections on the left to the input jack as they are drawn schematically.
If you take away the verbage for the Ring/Tip, and the verbage down below about the battery, whomever builds this will build it wrong. (The same applies if someone just looks at the "picture" and doesn't read the notes.)

"Picture wise" you show the tip of a plug being connected to the -9 VDC, and the ground sleeve of the plug connecting to the input of the circuit.

If you pull up virtually any effect schematic on line you'll see what I mean.

True, as long as whomever builds it reads every note they'll probably be fine. If they follow the picture and don't read it carefully they'll have a problem.
From my experience in electrical engineering, the less that someone is required to think about what they're doing it's usually for the better.

[Dave Hepworth]

Hi all,
How would this circuit look on Veroboard please.
Regards Dave

[Chris Reesor]

Try this site, Dave. Should get you going.

http://www.zen22142.zen.co.uk/Prac/vero_circ/vero.htm

I had to look up Veroboard, since I hadn't heard the term in decades. I actually built mine on a small piece of some knockoff Asian version that I had around.