Active preamp, no EQ

[rumblinbass]

It's a Behringer 9V pedal supply

Orange tantalum cap soldered from ground to 9V at the jack I added to the cavity cover plate.

Noise is neither hum nor hiss, it's a high frequency signal with repeating chirps and constant whistle. Not loud but loud enough to destroy the experience.

I do have the schematic for the Arion pedal courtesy of the internet but I haven't figured out which part provided the signal conditioning.

don't think you need signal conditioning. if it sounds good with a battery it should sound good w/ the power supply. the problem is w/ the power supply or connection.

I was asking for the schematic for any power supply filtering but the symptoms you describe does not suggest poor ps filtering. of course I could be wrong.

The tantalum should be fine - what value. Also good practice to put a 0.1uF ceramic right at the power pin to gnd.

I'm not trying to insult you, but double/triple check your power connections.

 

[SGD Lutherie]

As for the rotary switch, Dave posted this in another thread, so I stole it:

The rotary pickup selector would not be necessary for my current bass (although it would still be nice), but it's definitely what I want to do for my future build.

Hard to believe that was drawn 25 years ago.

I had installed that in a Hondo II P bass copy, which I converted to a P/J fretless, with a Kahler vibrato! I had a DiMarzio Model P and Schaller copy of a Model J. Wish I still had that bass.

 

[rumblinbass]

Op amp circuits tend to be less susceptible to power supply noise than simple discrete preamps. The power supply on something like a common source amplifier circuit actually has to be quite clean.

I suggest looking up the good old LM317L voltage regulator chip, and in the datasheet there should be a circuit for a low noise or "high ripple rejection" circuit that puts a capacitor across the reference voltage. That's one of the quietest regulators that I know of. You'll have to start with a higher DC voltage on the wall wart, if you want to regulate it down to 9 V.

For a buffer that will be onboard an electric bass, I'd probably build a source follower, because I don't think the application requires any gain. Electric bass output signals are pretty hot to begin with. And the power supply rejection is better. For 9-V use I have come to favor the PN5486 JFET because the output bias voltage comes out somewhere around 4 V on typical parts.

I've got some info under my "bare bones" circuits at my web page, on assembling a simple source follower buffer.

I agree, I don't think that application requires gain.
A source follower is a simple enough circuit.

 

[Rob22315]

Op amp circuits tend to be less susceptible to power supply noise than simple discrete preamps. The power supply on something like a common source amplifier circuit actually has to be quite clean.

For a buffer that will be onboard an electric bass, I'd probably build a source follower, because I don't think the application requires any gain. Electric bass output signals are pretty hot to begin with. And the power supply rejection is better. For 9-V use I have come to favor the PN5486 JFET because the output bias voltage comes out somewhere around 4 V on typical parts.

Thanks for the tip. I'm sure I can find some info on a source follower layout.

 

[fdeck]

Thanks for the tip. I'm sure I can find some info on a source follower layout.

My little article has a pictorial layout, that you could probably adapt to the inside of a bass. If it were me, I'd build it outside the bass first, in its own little box, to get all of the kinks out before digging into my control cavity.

Invalid Link Removed

 

[rumblinbass]

kingrazor,

here's a simple op amp circuit that should work. again, if anyone has any suggestions please feel free.
there isn't any gain but can be added.

 

[KingRazor]

Hard to believe that was drawn 25 years ago.

I had installed that in a Hondo II P bass copy, which I converted to a P/J fretless, with a Kahler vibrato! I had a DiMarzio Model P and Schaller copy of a Model J. Wish I still had that bass.

That drawing is older than I am!

kingrazor,

here's a simple op amp circuit that should work. again, if anyone has any suggestions please feel free.
there isn't any gain but can be added.

Cool.

So, what exactly do I need to buy to make all of this work?

 

[SGD Lutherie]

That drawing is older than I am!

I probably have shoes older than you! I see you are in the Ibanez club, so how about my '87 Ibanez SR-885LE?

I bought it new in '88. It's had all the electronics gutted and has a new Hipshot bridge, and it still plays great.

Cool.

So, what exactly do I need to buy to make all of this work?

If you want to try the JFET route; you can get a circuit board (and schematic) for the Stratoblaster here:

Invalid Link Removed

And an op amp buffer:

Invalid Link Removed

 

[rumblinbass]

That drawing is older than I am!


Cool.

So, what exactly do I need to buy to make all of this work?

you could hit your local radioshack only because it doesn't make any sense for such a small order from Mouser, Digikey, etc.

Radioshack will have all the necessary parts but you won't find any good op amps. Let me see what I can do to find a nice single op amp package to sample. (I love samples!) The Shack will have op amps that will work, but there is no reason not to get something really nice.

Oh, get an 8 pin DIP or IC socket, that way you can change out op amps easily.

C1, C2, C3, & C5 - look for electrolytic caps. for simplicity sake get the same values - something like 47uF to 100uF rated to at least 10V. If there is a possibility that you would ever run at higher voltages, like 18V, get 25V caps. Keep in mind, these caps get bigger as the value and voltage ratings increase.
C3 & C5 filter power supply. Since we are on the subject it might also be a good idea to put a small value resistor (say 4.7 ohms) right at the power input.
C1 & C2 block DC from going out - they also influence low frequency response. The lower the value the more low frequencies you loose.

R1 & R2 set up the bias voltage so that your signal sits halfway between 9V and GND. Make those about 100k.

R3 gets that bias voltage to the op amp and sets the input impedance. Make that something like 1Meg.

C4 should be a 0.1uF ceramic cap. Placement for this cap is critical. Literally solder one pin to +V (pin 7 on the op amp in the schematic) and the other pin to -V (pin 4 on the op amp in the schematic).

The pot - not critical. Most any volume pot should work. If it were me I would prefer one with PC pins so that it can it can be soldered to the board. That way you don't have to worry about where the board gets mounted in the cavity.

I think that about covers everything. The input signal comes from the Balance pot or the switching you want to do. If you go with a pot keep it a high value. Ideally you would put this buffer in-between the pickup an the balance pot but it's not completely necessary. Not to mention - more op amps, less battery time.

 

[rumblinbass]

again, if anybody sees anything that I overlooked please say something. Like I said before, I design pro-audio gear but never for this specific application.

If there is enough interest I could design a PCB and and stuff a bunch of boards...

 

[rumblinbass]

If you want to try the JFET route; you can get a circuit board (and schematic) for the Stratoblaster here:

Invalid Link Removed

And an op amp buffer:

Invalid Link Removed

those prices are really good! this may be the way to go. they also have stuffed boards?

 

[SGD Lutherie]

those prices are really good! this may be the way to go. they also have stuffed boards?

They have kits, but they are for stomp box versions, with the aluminum enclosure, so they are more money.

 

[rumblinbass]

They have kits, but they are for stomp box versions, with the aluminum enclosure, so they are more money.

oh, ok.
$7 for the PCB...I didn't look around too much but I couldn't fine a schematic.

 

[Rob22315]

And an op amp buffer:

Invalid Link Removed

That's a great find and probably what I'll try next. It will probably look better than the proto board I'm using now and hopefully will be less vulnerable to the interference from the 9V power supply. Thanks for posting.

What's nice is how cheap this stuff is if you're willing to do it yourself. Plus, I've relearned a lot of stuff from my BSEE days.

 

[Rob22315]

oh, ok.
$7 for the PCB...I didn't look around too much but I couldn't fine a schematic.

Here's a link to the instructions for the IC based buffer:

Invalid Link Removed

The link on the main page for the PCB is broken. I just placed my order.

 

[rumblinbass]

Here's a link to the instructions for the IC based buffer:

Invalid Link Removed

The link on the main page for the PCB is broken. I just placed my order.

...and I found the schematic

 

[rumblinbass]

the schematic looks good. I would add a few caps but overall it looks ok.

 

[rumblinbass]

ok, a couple things.
The Bill of Materials suggests a TL071 - we can do better than that.

suggested parts to be added:
1. a 10uF (or larger) 16V cap from +9V to GND.
2. a 0.1uF ceramic from pin 7 to pin 4 on the op amp.
3. cap across R4...not sure of the size...possibly 10uF 16V.

 

[Rob22315]

ok, a couple things.
The Bill of Materials suggests a TL071 - we can do better than that.

What do you recommend that's still low power?

 

[rumblinbass]

What do you recommend that's still low power?

I wouldn't really consider a TL071 low power.
Since this is only a single op amp instead of a dual or quad package I would go for audio quality. I'll do some searching but Burr-Brown or Analog Devices should have plenty of good ones to choose from.