Ampeg Based Tube Preamp

[blown240]

This is my first post, so please bear with me.

I play an upright bass that has a dual piezo pickup. *It came with a preamp that sounded so bad I literally threw it away. *What I want to build is an Ampeg b15n based dual channel preamp that I can use to plug into house sound, my regular bass amp, or use to record direct. *

My bass has a stereo jack on it, so the preamp will have a single stereo input. *It will have separate volume, treble, and bass controls for each channel, and I will use push/pull tone pots for the bass and treble boost.

I want to make the preamp circuits as close as possible to the original B15n but obviously setup for a piezo. *Also, when playing at louder volumes, or when the acoustics of a room are terrible, my bass feedsback at about 100hz. *So I want to do a notch filter on each channel that drops 100hz, but leaves everything above about 120hz and below 80hz. I will have a separate notch for each channel that is switched with a small micro switch.

Here is a link to the pickup I am using:
Bass Master RB - K&K Sound

Here are some questions I have:

1. *Input Resistor – the pickups input resistance is 1m per channel. *Should I match that, or should I got with something like a 4.7m input resistor like a lot of the peizo preamps I see online.

2. *Rectifier – I want to use a tube rectifier, but I am not sure if the tranny I am using will let me, or what rectifier tube to use. Here is a pic of the small trans that I will be using (tranny on the left):



3. *Notch Filter – No idea how to do this. *Any ideas?

4. *Is there anything else I need to do to the basic b15n preamp circuit to better work with a peizo?

5. *I want to make sure I have loads of clean headroom, so anything that will help that...

Here is a schematic I did to start with. *I still need to make sure the input impedance is correct for a piezo. *And I need to figure out where to put the notch filter(s) and exactly how to do them.



Thanks for any input!

[MarkusBass]

I play an upright bass that has a dual piezo pickup. *It came with a preamp that sounded so bad I literally threw it away.

I think it was a bad idea to throw away the preamp. Preamps with piezo pickups usually sound bad due to impedance mismatch (between output impedance of the pickup and the input impedance of the preamp). Simple solution to this problem is to build a JFET buffer (2 buffers in your case), which will cost you just few dollars. And now you want to build advanced preamp which will cost you several hundreds $. And I'm not sure whether it will help you. The problem is that without a buffer, the sound of the pickup can be greatly influenced by the lehgth of the cable between the pickup and the preamp. So my suggestion is to check the pickup with any valve amp. If it sounds good, go for it. If not, check why.

*What I want to build is an Ampeg b15n based dual channel preamp that I can use to plug into house sound, my regular bass amp, or use to record direct. *

The lauout of B15N was published on this forum few days ago (in Schematic request section). This may be expensive experience but it's up to you if you want to do it.

1. *Input Resistor – the pickups input resistance is 1m per channel. *Should I match that, or should I got with something like a 4.7m input resistor like a lot of the peizo preamps I see online.

Most probably you are talking about preamp input resistance (because a pickup does not have input resistance). It should be at least 3.3 MOhm.

4. *Is there anything else I need to do to the basic b15n preamp circuit to better work with a peizo?

It seems to me that you need some kind of a mixer to mix both channels. At the moment you have just 2 resistors. This may be not enogh.


Mark

[J M Fahey]

Nothing special to add about Marku´s post.
What I find weird is the pickup system itself.
Why only 1 bridge pickup, and unsymmetrical to boost?
I don´t very much buy the idea it picks up "all 4 or 5 strings evenly"
I would much prefer having one at each side, and even more the classic one which looks like a long thick brass screw, which goes side to side within the bridge arch itself.
Another idea that scratches me the wrong way, even more, is the second "Fingerboard" pickup.
*Where* do you apply it?
If anything, I see it more as a novelty and not picking "musical" sounds.
All "extra" sounds, be it finger noise, string slight buzz against the fingerboard, even a coat button scratching the instrument body somewhere *will* be picked by the main pickup, like it or not, it´s a characteristic of Piezos.
What worries me is that the fretboard pickup will also pick string sound, but being placed elsewhere there will be a phase difference , which will cause cancellations at unpredictable frequencies.
That comb filter effect can´t be solved by any EQ or preamp.
Maybe you dropped your original preamp off, personally I think the Electronics can´t be *that* bad, I much suspect the pickup system itself.
The idea *looks* cool, but from idea to successful blemish free product there is a long hard way.
jm2c

[MarkusBass]

I think that the name of the pickup: Bass Master Rockabilly explains why there are two pickups. In specific types of music (e.g. rockabilly) you may need to be able to get additional sounds : slaping, poping and all kinds of percussive sounds from the fretboard. So in jazz you could live with a single pickup but it rockabilly you need more. Just to test the pickup you may use the bridge part only.

Mark

[blown240]

So my suggestion is to check the pickup with any valve amp. If it sounds good, go for it. If not, check why.

My normal bass amp is a Fender 300ps. Its sounds amazing, but the head is 75lbs and its just too big to lug around to practice, record, etc. Thats why I want to build this preamp. I can simply plug it into the PA for practice and direct to the board when we record, if I want to bring the big Fender.

It seems to me that you need some kind of a mixer to mix both channels. At the moment you have just 2 resistors. This may be not enogh.

Mark

I was planning in mixing it using the volume controls on each seperate channel. Most of the time the fingerboard piezo will be turned all the way down. I really only use it for rockabilly or blues/bluegrass. Basically only for slapping.

What I find weird is the pickup system itself.
Why only 1 bridge pickup, and unsymmetrical to boost?
I don´t very much buy the idea it picks up "all 4 or 5 strings evenly"
I would much prefer having one at each side, and even more the classic one which looks like a long thick brass screw, which goes side to side within the bridge arch itself.

It seems to work pretty well. I have had it for years and I know alot of other pickups use a similar one sided setup. It is in the side of the bridge that is above the sound post, so maybe that has something to so with it.

Another idea that scratches me the wrong way, even more, is the second "Fingerboard" pickup.
*Where* do you apply it?
If anything, I see it more as a novelty and not picking "musical" sounds.

The idea *looks* cool, but from idea to successful blemish free product there is a long hard way.

The Fingerboard pickup is only there to get the sound of the strings hitting the fingerboard when playing rockabilly or bluegrass. Sure the bridge will pick that sound up, but when playing without a drummer its nice to emphasize the slap of the strings.

I realize its a long project, but thats the idea. I love doing stuff like this.

[blown240]

I think that the name of the pickup: Bass Master Rockabilly explains why there are two pickups. In specific types of music (e.g. rockabilly) you may need to be able to get additional sounds : slaping, poping and all kinds of percussive sounds from the fretboard. So in jazz you could live with a single pickup but it rockabilly you need more. Just to test the pickup you may use the bridge part only.

Mark

EXACTLY!!

[blown240]

Here is an updated schematic:

I have added a cathode follower to boost my output in case there are long runs to house sound, also I have changed to a SS rectifier since there really is no reason for a tube rectifier in this. I still need to figure out the notch filter and I may add a trans and XLR out for a DI.

[km6xz]

A T-Notch filter is not hard to design, but to leave the adjacent frequencies intact, the Q will have to be very high which presents two problems...component matching tolerances and ringing. A 3rd problem involves the acoustic environment, that is that the room plus cavity resonance will vary over a range that will require careful tuning of the notch filter. A manual way of doing that is creating a variable high order high pass filter and a high order low pass variable filter and tune them to be a band-stop filter where the feedback actually is. Not easy for you to while playing. That is why PA digital feedback suppressors have become so popular, they work, they are cheap and they are auto-tuning. Setting up a line level feedback suppressor at the output of your preamp, its line driver will be low enough Z to drive long balanced lines...two problems solved.

[MarkusBass]

The heater power supply on your schematic is floating. You need to add at least two resistors (connected to the ground) or supply the heater with DC voltage. Nice schematics of balanced output can be found on Gyraf Audio web site.

Mark

[blown240]

Here is where I am at now.

I have added:

1. Ground Switch to isolate the grounds after the standby switch in case there is a ground loop issue. (shown in green)
2. Added a DI out
3. Added a variable notch filter with a DPDT on/off
4. A couple other minor tweeks