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Build your own onboard preamp

#61
MglMatador said:
R1 should be changed to 200k if it goes immediately after the pickup leads....wasn't sure if I was clear about that. 3M is too high to load down the pickups properly, and it will sound really bright if placed right after the pickup. 200k is reasonably close to a volume pot and tone control in parallel (from a loading perspective).

If you place this preamp after the volume and tone controls you should leave R1 high (1M+).
Click to expand...

Ah, gotcha.
Yep, if you are going straight after the pickups with no volume pot to load them down, you need a lower resistance there to simulate that volume pot.

Even then, 200K is a bit low for my taste. I would have tried maybe a 400K.
 
#62
line6man said:
200K for the gate-ground resistor? Since the impedance is close to being purely real, it's accurate enough to say that the impedance at the input is equal to the value of R1.
You want a high input impedance, and 200K is relatively low.
I would say that an acceptable input impedance is above 1M ohms.

Adding a capacitor parallel to R2 will increase the gain.

I'm not sure how you jumpered across C2?
Since it's negative is grounded, and it's positive is connected to the battery's positive, shunting it would short the battery out.

Whatever you do, don't short out C1. That's a coupling capacitor used to block the 9V DC power from reaching the output. Without that cap, there is a risk of frying whatever the bass is plugged into.

Also, even though the voltage and current is low, it's never a good idea to short out electrolytics. It decreases their performance, and can even cause them to fail.

I accidentally shorted out an electrolytic capacitor that was charged with 526V by trying to drain it with a resistor whose value was too low, a few months ago, and that spark scared the hell out of me! The capacitor was still ok afterwards though.:hyper:
Click to expand...

oops I meant I took the cap out the board. I saw that +9V going directly to ground was not good. the 100uf cap did increase the gain. I noticed with this preamp harmonics ring out really loud. with a pickup out of phase I can even hear the harmonics an octave higher in the same spot. going through this Varitone I'm hearing tones I never heard before :bassist:
 
#64
Have a Dean EUB with Piezo pups, an eBay special sold without the blown pre-amp.

Have been using a Digitech pedal with amp simulation and EQ as a pre-amp on my pedal board (an ugly solution as I have to change the output cable from one chain to the digitech pedal when I switch to EUB).

Have been following the thread, and thanks to all for sharing your knowledge, but I am not quite certain whether to:

1. get the piezo kit from Fishman or similar
http://cgi.ebay.com/Fishman-Powerch...tem&pt=Guitar_Accessories&hash=item563b13f302

2. go with a generic like the $39 wonder discussed here

3. go with a Aguilar Tone Hammer pedal and pay relatively big bucks but not sink anymore into a relatively cheap instrument (Dean EUB) and have a transferable pedal.

Any advice?

BTW: I think I could manage the soldering and hole cutting to mount a small board in the large cavity in the Dean EUB; there are two 9 volt battery holders already installed.

Thanks - richard
 
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#65
line6man said:
Ah, gotcha.
Yep, if you are going straight after the pickups with no volume pot to load them down, you need a lower resistance there to simulate that volume pot.

Even then, 200K is a bit low for my taste. I would have tried maybe a 400K.
Click to expand...
Tillman.gif

WOW! popped in a 510K resistor at R1 and it went to another level :hyper: :bassist: the mids are really accentuated. now I see why guitarists and bassists love these preamps so much. amazing how one transistor can totally change the sound of a bass. The Alembic Stratoblaster is next! C2 will have to bumped up on this one :hyper:

main.php?g2_view=core.gif
 
#66
MglMatador said:
The cool thing about adding a cap across R2 is that you can introduce a gain step wherever you want it to emphasize higher frequencies....that's the secret to the tube screamer kind of gain controls. If the RC time constant is below the lowest note (e.g. the capacitance is large enough) then gain at all audible frequencies is increased. This is a good way to increase the gain of this circuit as the only other ways are: decrease R2 (moves the quiescent operating point), increase R3 (which increases output impedance), or use a higher gm JFET (more noise).

If you size the cap just right you can emphasize the resonant bump of the pickup for a very bright and punchy sound (in the 2-4kHz range).
Click to expand...

I took out the 100uf cap across R2 because it was way too much gain. I dont want to add a pot because I want everything straight to jack. how would lower cap values (.1, .047, .022uf) affect the sound if put across R2? my preference is a low middy sound :bassist:
 
#68
BassLife77 said:
I took out the 100uf cap across R2 because it was way too much gain. I dont want to add a pot because I want everything straight to jack. how would lower cap values (.1, .047, .022uf) affect the sound if put across R2? my preference is a low middy sound :bassist:
Click to expand...

The basic equation is:

f3_gain = 1 / (2 * pi * Rs * Cs)

or re-arranged:

Cs = 1 / (2 * pi * Rs * f3_gain)

Where Rs = source resistor (R2), and Cs is the cap across the source resistor.

So let's say you wanted a gain boost that was up 3db at say...500Hz. That would be:

Cs = 1 / (2 * pi * 2200 * 500) = 0.145 uF

So 0.15 uF is a standard value that would get you pretty close. Putting in a 0.22uF will move this down from about 500Hz to about 330Hz.

You can also see that the one you had previously (100uF), if you plug it in to that equation, you'll see the f3_gain is 0.17 Hz, which means gain boost for essentially everything above what is for all intents and purposes DC.

Plug in different values and experiment: it all depends on the sound you want, there are no right or wrong answers. Bigger cap values will move this gain step downward, and smaller ones will move it upward. Keep in mind that what you are doing is introducing a gain step, where certain frequencies (higher in this case) will get more gain than the lower ones which can give you a brighter sound depending on the cap value.
 
#70
MglMatador said:
The basic equation is:

f3_gain = 1 / (2 * pi * Rs * Cs)

or re-arranged:

Cs = 1 / (2 * pi * Rs * f3_gain)

Where Rs = source resistor (R2), and Cs is the cap across the source resistor.

So let's say you wanted a gain boost that was up 3db at say...500Hz. That would be:

Cs = 1 / (2 * pi * 2200 * 500) = 0.145 uF

So 0.15 uF is a standard value that would get you pretty close. Putting in a 0.22uF will move this down from about 500Hz to about 330Hz.

You can also see that the one you had previously (100uF), if you plug it in to that equation, you'll see the f3_gain is 0.17 Hz, which means gain boost for essentially everything above what is for all intents and purposes DC.

Plug in different values and experiment: it all depends on the sound you want, there are no right or wrong answers. Bigger cap values will move this gain step downward, and smaller ones will move it upward. Keep in mind that what you are doing is introducing a gain step, where certain frequencies (higher in this case) will get more gain than the lower ones which can give you a brighter sound depending on the cap value.
Click to expand...

thanks for the info! now I have an equation to work with. after trying a bunch of different caps I realized I need a volume pot for this bass. I can still control the tone with the varitone. with 72 different pickup selections including pickups being out of phase there is too many different volume levels going straight to jack. I read in another forum the 51K resistor can be swapped with a 50K pot to control the volume. so it will be similar to the schem below.

powerpot_sch.gif
 
#71
StyleOverShow said:
Have been following the thread, and thanks to all for sharing your knowledge, but I am not quite certain whether to:

1. get the piezo kit from Fishman or similar
http://cgi.ebay.com/Fishman-Powerch...tem&pt=Guitar_Accessories&hash=item563b13f302

2. go with a generic like the $39 wonder discussed here

3. go with a Aguilar Tone Hammer pedal and pay relatively big bucks but not sink anymore into a relatively cheap instrument (Dean EUB) and have a transferable pedal.
Click to expand...

I'm gonna recommend the Fishman. Definitely don't go with the pedal: the more wire between the piezo and the preamp, the worse it is going to sound. Get a preamp inside your bass for sure.
 
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#72
just got a bunch of 2SK117GR's for $2. I just figured out how to adjust the voltage drain. not much info on the web about these JFETs. is the ideal voltage drain 5 volts for maximum headroom? I'm still researching and learning everyday :hyper:
 
#73
BassLife77 said:
WOW! popped in a 510K resistor at R1 and it went to another level :hyper: :bassist: the mids are really accentuated.
Click to expand...

What you did was lower the input impedance to 510K. So now you are loading the pickup more, and have lowered the resonant peak to the mids. You will likely notice less high end as well, more like a passive tone. This is the idea behind the Z switch on the Audere preamp.

The original impedance was 3.3M. You can also try other resistors like 1M, etc.
 
#74
SGD Lutherie said:
What you did was lower the input impedance to 510K. So now you are loading the pickup more, and have lowered the resonant peak to the mids. You will likely notice less high end as well, more like a passive tone. This is the idea behind the Z switch on the Audere preamp.

The original impedance was 3.3M. You can also try other resistors like 1M, etc.
Click to expand...

thanks for the info. maybe I'll pop in a switch to select different input resistors after trying them out. I got the 2SK117 tuned to 4.7v at the drain. sounds great :hyper:
 
#77
MglMatador said:
The output impedance of the Tillman preamp is 6.8k (the drain resistor). I would trade that 50k pot for a 100k pot: you will get more signal that way.
Click to expand...

when I biased the Vd correctly at 4.7v for some reason the gain increased a lot as well as better tone and headroom. when I had it biased wrong at 8.5v it had very little gain. I have the bass plugged into the active -10db input on the amp with the volume on the bass at 75% and it is a very hot signal. I think it's the nature of these 2SK117's, once it's biased right it takes off! :hyper: :bassist: The reason I used a 50K (actually 64K) pot was to match the resistor it replaced. But I'll keep in mind about a 100K pot that can be used instead.
 
#78
line6man said:
Yeah, I was thinking of how useless that cap was.:hyper:
It's not like we're filtering ripple from a rectified power source or anything, it's just a battery powered circuit with a single active component. You can add it or skip it, it really doesn't matter, but I kept it in there for good measure.
Click to expand...

looks like the C2 cap has a use in this circuit I'm tweaking. when the cap is not there the voltage spikes 2v through the output when the bass is plugged in. when I put the cap back in the voltage spike stops. still learning :hyper:
 

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