Has this been done b4?

[night0wl]

Hi all,

I have an idea (which may or may not be original) to put a small booster circuit into a 1 humbucker guitar and use a toggle (SPDT?) to switch between normal signal and the boosted signal (for solos).

I recently bought a small preamp kit from D!ck Smiths Electronics, just to start learning about circuits and I was wondering if it could be used in the guitar boost idea? There is a link to the kit here
If it is possible to use this circuit, I would like to add a volume pot to it.

If the site details aren't enough I can scan the schematic that came with it.

Shane

[UnklMickey]

Shane,

yes, it's been done before, but don't let that stop you.

there are 5 things i can think of right away that you should consider:

1 will it fit physically?

2 is the input impedance high enough? (over 500k)

3 how long will a battery last in this circuit?

4 get a switching type jack for the guitar, so the power will be shut off when your cord is unplugged.

5 in addition to a volume control, you'll also want a trimpot to set the maximum output level.
40dB gain = 16 times as loud. WAY too much!



unk

[night0wl]

Thanks unk,

1. The circuit would fit in a matchbox so I reckon I could squeeze it in a guitar

2. Total noob response here.... how do I tell if the input impedence is high enough?

3. The guide that came with the kit says the current drain of the preamp is 2mA @ 9V. Does this help?

4. I will have to look into switching type jacks....never seen one! lol

5. I thought 40dB gain would be too much.

The guide says "The input signal is supplied via a 0.1μF capacitor to the base of Q1. The bias voltage for this transistor is set up by the 2.2kΩ, 100kΩ and 150kΩ resistors.........the overall gain is set by the ratio of the 2.2kΩ resistor and the 100Ω resistor also connected to the emitter of Q1. The full gain equation is: Gain = 1 + (2200/1200) = 23 which is equivalent to 27dB. The 22μF electrolytic capacitor in series with the 100Ω resistor sets the lower frequency response to 72Hz. The output is taken from across the 2.2kΩ collector load resistor of Q2 via a 10μF Electrolytic capacitor."

Almost all of that is Chinese to me but maybe you could help me make some sense of it? I am hanging to start soldering something! lol

Shane

[UnklMickey]

night0wl said:

Thanks unk,

1. The circuit would fit in a matchbox so I reckon I could squeeze it in a guitar

2. Total noob response here.... how do I tell if the input impedence is high enough?

3. The guide that came with the kit says the current drain of the preamp is 2mA @ 9V. Does this help?

4. I will have to look into switching type jacks....never seen one! lol

5. I thought 40dB gain would be too much.

The guide says "The input signal is supplied via a 0.1μF capacitor to the base of Q1. The bias voltage for this transistor is set up by the 2.2kΩ, 100kΩ and 150kΩ resistors.........the overall gain is set by the ratio of the 2.2kΩ resistor and the 100Ω resistor also connected to the emitter of Q1. The full gain equation is: Gain = 1 + (2200/1200) = 23 which is equivalent to 27dB. The 22μF electrolytic capacitor in series with the 100Ω resistor sets the lower frequency response to 72Hz. The output is taken from across the 2.2kΩ collector load resistor of Q2 via a 10μF Electrolytic capacitor."

Almost all of that is Chinese to me but maybe you could help me make some sense of it? I am hanging to start soldering something! lol

Shane

1 - cool.

2 - newbies welcome here. if the first transistor (at the input) is a FET, it's pretty much a given that the input impedance will be nice and high. FETs are essenntially voltage driven, current sinks. the gate impedance is high, and the gate biasing circuit will have high resistances. the input resistor will also high.

i'm guessing your's is bipolar (nothing personal, i'm talking about the transistor.) that's essentially a current driven, current sink. bias resistors in the 10k~100k range. input resistor around 100k. you can approximate the input impedance to be equal to the value of the input resistor.

it sounds like this circuit is going to load down the pickups enought to change the tone. you might compensate for this in the circuit itself, but if you leave the input connected (SPDT)when you bypass, that loading will dull your non-solo sound.

you might be able to give away some gain, and use an input resistor that is considerably larger like 500k or 1meg.

you might be well off to connect things up outside the guitar.

test it there before you install it. make whatever changes you need first.

3 -- since an alkaline 9v has a capacity of about 600 mAH, @ 2 mA, you should get 300 hrs of use. 5 hrs of use per day, every day, would give you 60 days. i don't know how often, or for how long you play, but those numbers sound like a lot. so it looks like you won't need to change batteries for 2 months or more.

4 -- go here to see some of the possibilities:

www.switchcraft.com/products/pdf_files/jack-65_schematic.pdf

and

www.switchcraft.com/products/pdf_files/jack-65b_schematic.pdf

using a jack to switch off the power when unplugged will prevent accidental dead battery. using a switching jack is a bit classier than using a stereo jack.



5 -- yeah.

[JohnH]

I agree with what Unkl says. Wire it up outside the guitar to test how it works and to tweak it. If you build it in, think about how to change it later because there are other booster circuits which you can build using FETs that you may find are better.

With all that gain, you can probably afford to loose some gain , as a way of raising the input impedance. On current consumption, 600mAH will take a 9V battery down to about half voltage, so you may find that you change batterys a bit sooner than that

Although there are other more purpose designed circuits, this looks like a great option for a first active project however, so all power to you!

John

[4real]

Yes...

That link wasn't direct to the preamp in question but I know it well. This preamp, with some mods is what I currently use as part of my sustainer system so I have made a few.

Sure...it's fine with guitar pickups but you may want to change to a 10k pot for the volume control after the preamp.

A question came up (again) about the "black ice" overdrive over at PG and although not the best thing with average pickups, you could add this two diode circuit to add clipping to your preamp...maybe replace the the tone pot cap with this to have a variable overdrive as well as the clean boost.

Finding room for the battery is going to be harder than fitting this little critter in there for sure.

[night0wl]

4real wrote: That link wasn't direct to the preamp in question but I know it well.

I didn't realise the link wasn't right, I just copied and pasted it here. The product number to search for on the D!ck Smith site is k5608.

Could you please enlighten me about "black ice"? I am new to all this and this is the only forum (besides DIYstompboxes) that I visit.

Also, would I place the volume pot after the preamp (separated from the circuit) or replace the last 100k resistor on the circuit board with the pot?

JohnH wrote: Although there are other more purpose designed circuits, this looks like a great option for a first active project however, so all power to you!

You wouldn't have any schematics to a more purpose designed circuit that I could check out? It would be good to see some different diagrams so I can get a good idea of how all this works.

Thanks
Shane

[JohnH]

Shane - I have been developing an on board overdrive, using JFET transistors, and Ill post my latest circuit soon. My starting point was this, the Tillman preamp:

www.till.com/articles/GuitarPreamp/

It is very simple. It gives just a small gain, but if you put a 10MFD capacitor in parallel with R2, you can boost it to a significant gain. I also multiply R2 and R3 times a factor of 3 to reduce current consumption. My circuit is based on two of these in series.

At www.runoffgroove.com there are several clever circuits using a few JFETS to mimic tube amps, including some classics from Marshal and Vox.

So good luck with your first step - there is lots to explore with guitar design, once you give yourself the benefit of a little battery juice!.

John

[night0wl]

great stuff! Thanks heaps for this!