But this BJT circuit is a common emitter design so I guess it's there for additional amplification.
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Active preamp, no EQ
- Thread starter KingRazor
- Start date
But this BJT circuit is a common emitter design so I guess it's there for additional amplification.
One more question - I'm going to relocate one of my opamp-based buffers into my looper. The looper will have 18v for powering the onboard preamp in my main bass through a TRS cable as you've recommended. It will also have 9V for powering the LEDs in the looper. Is it better to power the buffer off 18V or 9V? I'm guessing I can only do this with the one opamp-only buffer (unity gain) since the JFET and opamp/BJT based buffers are tuned for 9V. I know when I set up the JFET buffer, I tuned the self-biasing circuit for 9v.
I can power the buffer off the 9v available in my pedal board but do I limit headroom or is that largely BS as the roughly 4v swing with 9v and the opamp-based or opamp/bjt-based buffers is enough for almost any setup? Alternatively, should I go with 18v to the opamp-only buffer?
I can power the buffer off the 9v available in my pedal board but do I limit headroom or is that largely BS as the roughly 4v swing with 9v and the opamp-based or opamp/bjt-based buffers is enough for almost any setup? Alternatively, should I go with 18v to the opamp-only buffer?
for the op amp ckts, as long as your caps are rated for the appropriate voltages you are good to go.
18V will give you more headroom, but the signals aren't very big so 9V is enough.
The JFET ckt is a bit trickier but adjusting R values should work. I would have to look at the ckt to be certain, but the main consideration is current thru the device. Keeping the resistor ratio equivalent will maintain bias voltages but since the rail voltage is going up the resistor values will also have to go up in order to maintain the same current.
If you would like to run your JFET ckt off 18V send me the ckt (agian?) and I might be able to help.
If it's worth anything...
I am designing a complete system in which my rack preamp is going to supply the rail voltage to the discrete active buffer on my bass. I have a few different topologies in mind for the main preamp and I plan on building both. The voltage rails for these preamps will have different rail voltages - one +/-18V and the other may be a single rail 36V!
Most JFET op amps have max rail voltages of +36V or +/-18V. So as long as you do not exceed those voltages and all other parts (espectially caps) have the appropriate ratings there shouldn't be any problems.
here is a goodLink Removed that walks you thru designing single stage JFET amplifiers.
If I remember correctly, it is similar to the ckt you are using (?).
If I remember correctly, it is similar to the ckt you are using (?).
engineers
skipping over the 6 pages of jfet and opamp discussions for an OP who says he doesn't even know how to use a soldering iron, an EMG PA-2 will make a perfectly good buffer. stuff it inside the control cavity with the boost switch set to "off" and you have unity gain, or switch it to "on" and tweak the level trimpot up just a pinch and you'll have a clean volume increase. (the distortion comes when you dime that level trimpot and blast the front end of your amp.)
run your regular volume and that rotary switch first.
One reason I've considered things like this is because the bass I've considered modding with a buffer is an Ibanez with 4 knobs. If I went passive (unless I wanted two tone controls) I'd have at least one empty whole where a pot once was. If I got this little EMG unit I could use it to plug up the last hole.
Only thing I'm wondering is if the PA2 or afterburner are capable of getting me what I want which, to remind everyone, is:
- Low output impedence
- No additional noise (if my bass was silent before I installed this, it should still be)
- Lack of (or reduced) signal loss, allowing longer cable runs
- Gain boost (more than likely, I won't need more than 3-6db of boost)
I've considered things like active/passive switches and series/parallel switches. But really this little switch is all I'd need if I already had a volume, blend and tone knob.
That would have been helpful when I was designing mine - especially the part about isolation from the power supply. The one thing I didn't see there for our application is how to self-bias the circuit for low current draw (ensure long battery life). I'll see if I can find my notes and post or at least post a link to the website that I used. Also, for us this is more of a buffer than an amplifier so getting maximum gain isn't a design objective.
IIRC, since there can be large variance in the JFET performance, I had to put my transitor in a test jig and measure the cutoff voltage and quiescent current then use those to calculate the various resistors. The source resistor sets the self-biasing to control the zero input current draw and the drain resistor sets the zero input drain voltage so there's enough head room below the supply voltage to avoid clipping. The J201 did really well in this application, better than the one or two others that I checked out. I don't about all JFETs or MOSFETs so there may be others better suited.
If you have an extra hole, put in a switch to go from passive to active or series/parallel one of your pups. Alternatively, you could put an LED in there to indicate when your preamp was getting power.
I tried to series/parallel one of my Dimarzios, I think it was a model P. All it did was get softer in the parallel config - no change in tone. I decided to use the switch to go between active and passive.
it would indeed do exactly all of that. the PA2 might be the better choice, since you could pre-set the slight volume boost you want and it won't change. with the afterburner, pulling up on the knob puts the boost level control in your hand, where it would be really easy to go "too far" and risk blowing something up.
i'm pretty sure you can run these at 18 volts, which would be good for increased headroom (not increased output).
I have been working on a discrete design for some time now but since this thread I have changed direction.
The only design criteria I had was to use all 4 holes on my Stinray control plate. This means more features than I would normally want...
There are 2 amplifier blocks. Both are high input impedance and low output impedance.
The first buffer has 6dB of gain and drives the volume pot.
The 2nd stage is after the volume pot - this insures that the output stays low and constant. Typically the output from the onboard preamp is taken off of the wiper of the pot. This means that as the volume is turned down there is a series resistance that also causes a high frequency roll-off - may be subtle, but it's there.
To take up the 4 holes there will be a 250K or 500K volume pot, a switch to bypass the output buffer and another switch to bypass the first buffer. The switches allow me to take advantage of the high frequency roll-off and go completely passive.
The last hole will be for gain control as soon as I can find something that will work. The best solution is a rotary pot but I have been having trouble finding a single-pole 3, 5, or 7 position rotary with the right shaft dimensions. I could use a pot but the nature of the circuit doesn't quite work with any taper. Using a rotary switch let me choose resistor values to get gain needed gain levels.
The only design criteria I had was to use all 4 holes on my Stinray control plate. This means more features than I would normally want...
There are 2 amplifier blocks. Both are high input impedance and low output impedance.
The first buffer has 6dB of gain and drives the volume pot.
The 2nd stage is after the volume pot - this insures that the output stays low and constant. Typically the output from the onboard preamp is taken off of the wiper of the pot. This means that as the volume is turned down there is a series resistance that also causes a high frequency roll-off - may be subtle, but it's there.
To take up the 4 holes there will be a 250K or 500K volume pot, a switch to bypass the output buffer and another switch to bypass the first buffer. The switches allow me to take advantage of the high frequency roll-off and go completely passive.
The last hole will be for gain control as soon as I can find something that will work. The best solution is a rotary pot but I have been having trouble finding a single-pole 3, 5, or 7 position rotary with the right shaft dimensions. I could use a pot but the nature of the circuit doesn't quite work with any taper. Using a rotary switch let me choose resistor values to get gain needed gain levels.
Neat!
I'd have 4 holes to fill as well on my current (or just about any future) Ibanez.
Although one bass I've really been GASing for has 3 holes and one really tiny one for a little toggle switch.
If you want some additional ideas, someone has reverse engineered the NTMB circuit. It's a two stage opamp design with the first stage being a blend/buffer/volume control and the second for tone control. The tone controls are in the feedback loop of the second opamp. The mids portion uses a couple of inductors which I thought was pretty cool.
I'm not keen on the freq setting of the NTMB - the treble corner frequency is a bit high for my tastes (~6.5khz) and the bass is a little low (~30hz). The mid is tuneable although I haven't seen a true sweepable implementation for it.
I figure it would cost me about $20 for parts, not including pots.
I'm not keen on the freq setting of the NTMB - the treble corner frequency is a bit high for my tastes (~6.5khz) and the bass is a little low (~30hz). The mid is tuneable although I haven't seen a true sweepable implementation for it.
I figure it would cost me about $20 for parts, not including pots.
The Baxandall topology can be done with either caps or inductors...typically caps are used. Inductors do have a bit of a different tonal characteristic.
What you have described is a common signal path. The Tone Control op amp driving a volume pot.
The idea for what I am doing came from KingRazor's criteria for low output impedance. Again, in most onboard preamps (all that I know of) the output is taken from the wiper of the volume pot. When the volume is all the way up, the signal is taken directly from the amplifier - which of course is a low output impedance. However, when the volume is turned down a series resistance is introduced. That coupled with cable capacitance will create a low pass filter. As the volume gets turned down, the series resistance increases thereby lowering the corner frequency in the low pass filter.
What I did was add a buffer that will drive the output eliminating that low pass filter and providing a consistent (low) output impedance. I did include a switch to bypass this buffer so that if the high frequency roll-off is desired, it is there. With the gain on the first stage (6dB) I can compensate for at least half of the volume loss. This will give me unity output with a little high frequency cut...theoretically.
Also, because of the circuit topology when gain is added there is an increase in second order harmonic. Especially when operated on 9V power rail. It can be run on higher voltage rails (up to 36V) but as the voltage rail goes up the distortion goes down. The added benefit of this distortion as gain goes up is a fatter, fuller, richer tone. Almost sounds like the lows were boosted without any muddiness. This coupled with the high-frequency roll-off could sound very nice
.I have listened to the gain circuit and am happy with it. Have not tried the whole circuit yet. I am confident that it will work just not sure how subtle/drastic the high frequency roll-off will be. Need to get it built and put in a project box to test with a passive bass.
I am not the type of bass player that wants/needs onboard tone control. I believe that is what the preamp in my amplifier is for; not to mention technique. I am not discounting what other bass players want or need - just stating my approach. That said, I do think that the tone shaping of this preamp will be subtle but noticeable.
- Aug 21, 2008
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If you are powering that from batteries, than be aware that LEDs will be drawing current the whole time, which will help drain your batteries.
The first time I put a JFET buffer into a bass (my '73 Rick with Hi-A pickups), I installed a bypass switch. I never used it in passive because it sounded so lifeless! The buffer extended the top end and tightened up the bottom.
Here's a dual JFET buffer I just put together for a set of low Z pickups.
Parallel is brighter. You might not have heard it because of the tone you use.
Here's a Neo Jazz bridge pickup:
Link Removed
- Aug 21, 2008
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The other way you see it is on something like the Aguilar preamps where in some wiring variations they have the passive volume control before the preamp, and the output of the preamp directly drives the cable.
That's not an ideal situation either because of the loading on the pickups. I like your idea of the buffered volume control.
On a preamp like the NTMB with the tone control op amp, would you use an additional stage after that?
If I homebrew an NTMB, I'll probably add this for the reasons rumblinbass stated. I haven't figured out if I can adapt the unity gain buffer I used before to be 18v or 9v yet. IIRC, something in the GGG unity gain buffer had to be retuned for 18V. I don't want to re-layout the vero board by buying a quad opamp either.
Yes but I did it knowing that I'm converting my setup to power the onboard preamp through a TRS cable and a pedal power supply. This assumes I can work out any noise issues from the power supply.
I also set up my LEDs with 1K current limiters so I'm only drawing 9 milliamps or less. I didn't want the LEDs to be too bright.
The NTMB design has a volume trim pot in the feedback loop of the initial opamp so that shouldn't load down the pickups. The only passive loading is for the blend knob - it uses an MN dual pot. The blend knob output feeds the first opamp.
I suppose the ultimate in minimal loading would be an active blend circuit with JFET based opamps then an active volume circuit. Of course, each of these stages drives up the current draw.
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If you build the NTMB, I'd give it a little more gain. Those preamps are on the quiet side.
Be aware that some JFET buffer circuits; such as the Stratoblaster type; actually work better on 9v. You can change the components to make it work with 18v however, but they work great on 9v.
