Useful Stuff to Know » article » Electric Guitar Preamp Buffer Circuits » Sep 20, 08:13 PM

Electric Guitar Preamp Buffer Circuits

The tone of passive pickups is very sensitive to what cables, pedals, and amps you plug into. The longer the cable, for example, the more the guitar tone tends toward dull and honky. That is because longer cables have greater capacitance, which together with the inductance of the pickup coil creates a resonant filter that rolls off the highs but boosts the mids or high mids just before the roll off. Some musicians love this tone because it’s like having a wah pedal halfway up, while others loathe it.

The difference between 20, 10, 5, and 1 foot cords is pretty noticeable if you record into an audio interface and listen to the clean signal. Therefore choosing the correct cable length when recording into an audio interface or amp is important; the cable length acts like a tone control. Many recording musicians change strings before every song or two to get the clearest sound, but neglect to use a better or shorter guitar cable.

Active pickups don’t suffer from this problem as much because they have a preamp built in that isolates the pickup from the cable. Passive pickups can be given similar isolation through a buffer circuit. This is a little circuit wired into the guitar that takes the pickup signal without placing a load on the pickup, thereby preserving its tone. It’s like using a 1 inch guitar cable direct into an amp or interface.

The disadvantage to a simple buffer circuit is that you lose the warm highs and boosted mids, thus you get a more hi-fi scooped sound. That is great for cleans and maybe blues, but for hi gain metal it can potentially be too bassy without enough mid-gain grind. While it removes a middy kind of ‘haze’ and ‘dullness’ it also takes away the cut and push of the tone. The more bass goes into a hi gain guitar amp, the more fuzzy and farty the distortion. A pedal like a tube screamer cuts the lows, boosts the mids, and compresses the signal so that a loose mushy amp (like a mesa rectifier on neutral settings) becomes really tight and cutting. So a buffer will have the opposite effect of a tube screamer, as it reduces honk and increases the highs and low lows.

That said, there are three buffers on the net that are easy to build if you have soldering skills:

The Tillman Buffer – This is one of the earliest and most famous. It uses the Fairchild Semiconductors J201 JFET transistor which nowadays which has very low power drain (under 1 mA) and runs on a 9V battery. However that JFET is out of production and hard to find. It can still be bought on eBay. Due to inconsistencies in manufacturing, some J201s will work fine and some won’t therefore buying a half dozen or more and using the one that works and sounds best is necessary. The downside of this circuit is that it doesn’t have much head room, and it also adds gain to your signal so if your pickups are already hot then you may easily overload/clip your audio interface. For single coils it’s fine, but high output hum buckers probably not unless you are using a DI box. Without high headroom, however, the signal will compress and clip in this circuit, which does make for a heavier hi gain sound but overall I wouldn’t recommend it for high gain or high output use.

The Hawes Buffer – This is an improved version of the Tillman Buffer that uses the MPF102 JFET instead of the J201. The MPF102 is also out of production but cheaper/more common than the J201 today on eBay. It draws more power (2 to 5 times more than the J2010) however, but also has more head room. The circuit additionally includes two zener diodes (optional) to prevent the JFET from getting fried by static electricity at the inputs. When the voltage at the input goes above 5.1, it’s shunted through the zeners automatically and therefore acts as a safety mechanism. I recommend this one over the Tillman Buffer. There is still an issue of the output being too hot for an audio interface though.

The Orman Buffer – Fourth diagram at that page. Even simpler, this one uses the MPF102 (or similar JFETs) with a virtual ground formed via two 1M (or 2M) resistors used as a voltage divider. This one has both high head room and gain of about .9 — thus no chance of clipping your interface. Current draw is around 1.75 mA which for a typical 9V battery should last about 300 hours.

I have compared the Orman buffer using the MPF102, to the Tillman circuit using the J201. The latter is definitely louder and has less head room (it starts internally distorting and compressing sooner). However, when volume compensated via interface gain adjustment, and when staying underneath the headroom limit by not strumming too hard, the tone is indistinguishable. Therefore I see no reason to go with the Tillman/Hawes buffer unless you need more gain, like if you had a basic tube amp and wanted to push it into overdrive sooner.

Now, to avoid modifying your guitar, these circuits can be installed in an Altoids tin complete with 9V battery, male 1/4 instrument input plug (yes the plug not the jack) chassis mounted, and 1/4 stereo stereo output jack. The output jack is used as as switch to cut power to the battery when a cable is not plugged in. This is done by connecting the negative of the 9V to the ring, circuit ground to the sleeve, and circuit output to the tip.

This tin can be plugged directly into the guitar (looks stupid but it works) and the guitar cable plugged into the jack. Cable length then won’t matter. If you’re good with EQ, you simulate an idea cable response, which will likely be a resonant high cut filter brought low into the 1k to 4k range. I don’t have a passive bass, but I would imagine that for a bass with passive pickups this buffer should work pretty well because a hi-fi scooped sound is what we typically EQ into bass anyway.

Myself, I will be using the Orman Altoids Buffer for all my cleans because it makes it natural, even, sparkly, and full (though it lacks that vintage 70s cleans vibe that only a crappy cable can bring. It’s said that Jimmy Hendrix used the cheapest longest curly guitar cables he could find because they had the greatest capacitance and gave him that vocal-like wah sound).