A 1987 Westerly-made GF-40 is my first Guild acquisition

welshtoast

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I just had a thought... no need to use a 9V battery. The circuitry will work with whatever I throw at it... 9V, 12V, 18V, etc. So how about using a 12V MN21/23 battery in there instead? That's a whole 3V of extra headroom!

One downside is longevity: the average rechargeable 9V rectangular PP3 battery has 600mAh capacity, which would last for 600mAh / 4mA = 150 hours of use (although not really because the voltage would dip too much before being exhausted).

On the other hand, a 12V MN21/23 battery has only 50mAh capacity, which would last 50/4 = 12.5 hours (again, not really). It's long enough for a short set. Good to have options!

Edit: Whatever I do, I should measure the output impedance of the 12V batteries to make sure they're suitable for a dynamic load, such as an audio op-amp. Also, 12V batteries are small and easily paralleled for increased capacity and reduced output impedance (both are desirable).
 
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welshtoast

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What's the power source, two 9Vs in series? I'm cautious about that because it'll double the internal resistance / output impedance of the power supply, which is bad in audio applications.
 

welshtoast

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"Uses two "AA" batteries"

That's only 3V. I bet he's using something like a LT1054 charge pump to increase the voltage. I considered it, but those things work by using a switching oscillator at around 40kHz (if I recall correctly) and (a) they take more current than I want to use, and (b) the extra circuitry to cut down the switching noise is... less than ideal.

Like I say, I considered that option but ultimately decided to go a more purist route without the charge pumps, etc. Thanks for the link!

Edit: This picture gives a big clue:

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See that gold can? It's a shield. I imagine there's DC-DC converter or a buck converter or somesuch under there. I'm not against them per-se, but they require very careful finessing to get them quiet!

Edit edit: I'm a little old-fashioned in my electronics preferences and even using an op-amp was a compromise for me! I seriously considered using a discrete transistor-based amp design, but in the end got lazy and used what I had in the bits box! It's cost me nothing to build so far :)

Edit edit edit: I'm not saying these voltage-boosted systems are bad. They're not (necessarily). For example, the well-known Klon Centaur pedal uses a charge pump to boost internal voltage to 18V. The fantastic Shin-Ei B1G1 boost pedal designed by Lee Jackson uses one. There are Echoplex clones around that use them, etc etc etc. It's just... I'm a luddite and a curmudgeon who doesn't want to!
 
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welshtoast

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For sh!ts and giggles I put together a voltage doubling circuit made from an LT1054 charge pump I had lying around:

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It’s using 10uF film caps for absolutely the best ESR I could find. The diodes are old germanium NOS ones that have super low forward voltage of 190mV!! Modern silicon ones are typically 700mV and I want to avoid that much voltage drop.

Output voltage is 17.89V (17.23V under load), which is really great. Power consumption went from 4mA to 13mA while powering the preamp, which is fine in this use case. A 9V battery will last almost 2 days. I could probably squeeze an LM7815 voltage regulator in there, which would help mop up some of the noise and help prevent the preamp power rails sagging under the dynamic load of the op-amp.

Best of all is that I can put 4V pk-pk into the preamp and get 13V pk-pk output without clipping!! At this point I’d be clipping the house audio system before my preamp.

Noise performance is fine in the audio band, but there’s a bunch of crap at 40kHz. It won’t hurt anything so I’m tempted to see if I can shoehorn the voltage doubling circuit into my preamp enclosure!
 
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JohnW63

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When I was in electronics in school and I had spent a year of classes learning about analog stuff and then, they showed us an op-amp...Oh man. " All we have to do is use one of those to replace all of this? " SOLD!
 

chazmo

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Recent information that I read about Turner's multi-source system (I think?) was that higher voltage helps to prevent piezo quack. I'm not sure how much of that actually holds true in practice, but I remember one of our members pointed me at that. Very interesting stuff. If true, then increasing the voltage should help you build a great system, welshtoast!
 

fronobulax

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Recent information that I read about Turner's multi-source system (I think?) was that higher voltage helps to prevent piezo quack. I'm not sure how much of that actually holds true in practice, but I remember one of our members pointed me at that. Very interesting stuff. If true, then increasing the voltage should help you build a great system, welshtoast!

I followed the thread and swallowed the Kool Aid. The piezo quack comes from the initial transient. That transient seriously overdrives the typical preamp and thus produces The Quack. The solution is to give the preamp more headroom so that it can handle more of the transient before distorting. So basically a higher voltage preamp reduces The Quack. My source for all of this is ultimately Rick Turner.
 

welshtoast

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I followed the thread and swallowed the Kool Aid. The piezo quack comes from the initial transient. That transient seriously overdrives the typical preamp and thus produces The Quack. The solution is to give the preamp more headroom so that it can handle more of the transient before distorting. So basically a higher voltage preamp reduces The Quack. My source for all of this is ultimately Rick Turner.
This is what I concluded through observation. The initial transient is - like I said earlier - disproportionately massive compared to the rest of the signal. When it clips (and it will clip/overdrive most, if not all, low-voltage / low-headroom preamps) I'll bet it sounds like sh!t. Quack quack.

Based on these lessons learned I propose to tackle the issue in three ways:
  • Provide a LOT of headroom using 18V power rails (as per JohnW63's suggestion) with the voltage doubling circuit I showed above
  • Adjust the gain of my preamp to either 2x or even unity gain to further reduce the chances of clipping the initial transient
  • Put a high-headroom compressor on the floor as my first (and maybe only) pedal to squish the transient before it hits any other audio gear.
When I first embarked on this project I assumed that a lot of gain would be needed, but I'm coming round to the idea that I was wrong.

Based on what I've seen in my own experiments and heard from you folks here, I think that applying gain, especially to piezos, should be firmly in the realm of the sound engineer. Let him/her control input levels to mitigate clipping and boost output levels to make it sound good in-house. The more gain I apply at my side, the more likely it is I'll drive a piece of audio gear into clipping before it gets to the mixing desk, and at that point it's too late because even the best engineers in the world can't un-clip an overdriven signal.

Given all of that, my preamp's job should be to simply provide input impedance matching for the piezos, volume attenuation, and low-impedance buffered output to quietly drive long cables in a venue.

Boy, has this has been a fun project so far!
 

JohnW63

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I've read somewhere that high impedance input on the pedals or the channel used on the amp also calms the piezo tone. The theory was that piezos were designed for stage use in the 70s and what amps were used then were big Mashalls with high impedance inputs. I did some testing at my house and found the Randall 110 watt amp I have has almost no quack. The other amps have varying amounts based on their intended use.
 

welshtoast

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JohnW63 said:
I've read somewhere that high impedance input on the pedals or the channel used on the amp also calms the piezo tone. The theory was that piezos were designed for stage use in the 70s and what amps were used then were big Mashalls with high impedance inputs. I did some testing at my house and found the Randall 110 watt amp I have has almost no quack. The other amps have varying amounts based on their intended use.

K&K specify an input impedance of 1M Ohm due to high input impedances causing the bass response to become overblown and boomy:

https://www.kksound.com/help-troubleshooting said:
In addition to this there is most likely an impedance match issue going on as well. Acoustic amps and most competitors' preamps are designed to work best with undersaddle pickups or other very high ohmic piezo pickups. They feature extremely high input impedance (5-10 meg), which boosts the bass response. Most competitors' pickups need this high input impedance to boost the bass response of their pickups, because most of them sound tinny in passive mode.

The Pure has a healthy bass response to begin with. It is overkill to boost it with extremely high input impedance. The Pure pickup sounds best with lower input impedance like 500 k to 1 meg, even just in a line input of a mixing board.

Based on this I built my preamp with a 1meg input impedance.
 

welshtoast

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Having decided to go with an 18V supply rail for the preamp, I built a small 9V - 18V boost circuit based on my prototype. I again used only parts from my collection (I don't want to buy anything else, I've hoarded enough!!) and it turned out really well. It consists of:

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After confirming it worked properly I added hot-melt glue to keep all the components from vibrating against each other; it's also an important cable stress relief technique. I glued a piece of cardboard under it for a little protection against accidental shorts, too.

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JohnW63

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Nice. At one point, I was thinking I was going to be an electronics engineer and design audio gear. I ended up in computer science. This sort of stuff makes me want to get even another hobby. I just don't have the time. My wife says I always take my hobbies to 110%.
 

welshtoast

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Nice. At one point, I was thinking I was going to be an electronics engineer and design audio gear. I ended up in computer science. This sort of stuff makes me want to get even another hobby. I just don't have the time. My wife says I always take my hobbies to 110%.
My day job is in cybersecurity! Hacking and reverse-engineering both hardware and software are my daily bread; guitar stuff is my catharsis.
 

JohnW63

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My day job is now the network guy at a school district, so I manage web filters and the firewalls and the network gear. Maybe we'll end up having private message conversations!
 

welshtoast

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And back to the scheduled programming! Bob Colosi’s bridge pins, nut and saddle arrived and I found a few hours to put into shaping things and fitting them. They’re still rough cut just now - the saddle needs lowered slightly and the nut slots still need work, but it’s getting there.

The saddle and nut come oversized for better shaping and fitting:

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Shaping the nut is pretty easy, but takes patience and attention to detail. To fit it to size I usually just tape some 220 grand sandpaper to my work surface and slowly start removing material after marking the shape on the nut in pencil:

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After getting it to thickness and properly shaping sides I copied the positions of the string slots from the old nut and used a .008” precision saw to start each slot in the direction I wanted.

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After that I took the nut files and made the slots just big enough to grip the strings snugly:

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The slots are clearly waaaaay too high, but the finessing is done while strung up.

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The bridge pins are the only things that need no work - they just drop in:


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The parts looks great together and are almost identical to original Guild parts from what I can tell.

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Lovely!

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I’ll see about making some time this Sunday to work on it some more.
 

welshtoast

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I finished up the preamp today! In order to squeeze everything into the tiny enclosure I ended up using an externally-mounted Switchcraft 227 plug:

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It's mounted to the plate of the enclosure using some small bolts/nuts held fast with Loctite Threadlocker Red:

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To power the preamp I opted for a rechargeable 9V battery made by EBL. This one is fantastic because it can be charged with a USB cable, which means I never have to open the enclosure:

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I used my dremel with a tungsten carbide cutter to make a hole suitable for a USB charging cable:

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The output jack and volume put are squished closely together!

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It really is cramped in there:

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You can see where the USB cable goes in:

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I made sure to add an on/off switch to disconnect the battery from the preamp circuit so that I can charge it without powering the amp at the same time.

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I tested the preamp with my Telecaster into a Tweed Deluxe and it works very well! It is noise-free, sounds great, and the volume control has a very nice sweep of range. I just need to get the pickups fitted to the Guild and I'll be in business.
 
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