Bass guitar FET preamp

[Schotky]

Originally Posted by WhiteDragon

most amplifiers have preamps
dont see the point for basic fender tonestack EQ in the bass.

Fender tonestack works rather well for amplifiers.
But running a tone curve into another tone curve isnt always that great.

Preamp in a bass guitar is a must for me, it's completely different story onboard and in-amp EQ. Access on hand, you get the EQ curve you want on different amps, and good onboard preamps usually have 18-20dB of boost/cut defined at specific frequencies. The in-amp EQ's usually have less headroom and usually not the peak frequencies I want though. With this, i barely use the EQ in the amp and can get same or similar curves i'm looking for on different amps. There are many advantages of onboard bass preamps if you're seeking a specific sound as i do.

Since we dont know if the fets used are even real
assume this is just a very average preamp or a very poor but working radio receiver

not a single drop of tube sound in these fets
be easier to stick the same tonestack in between a opamp and call it a day.

its really all your getting is a EQ curve. trying to get more overhead with 18 volts is a good idea in theory. Since your not getting distortion and looking for a clean signal.
no point in using fets or transistors thinking its more tube like.

if a little asymmetrical distortion was wanted, be easier to just use BJT transistors.
you get the same waveform with bjt or fet.

or if you wanted more tubey more appealing distortion, be better to emulate the bassman 6G6 method of driving the tonestack. with transistors.

be alot easier to design and model this pre in spice if we knew we were working with actual real devices not fakes. And using normal 2n5088 or some other common low noise transistor. Or if need be a Fet use a common available device.

there shouldnt be so many issues going from 9 volts to 18.
and if your running a passive volume control into the pre input. then id assume loading of the circuit would be different than what has been modeled.

alot of work for a EQ curve. just put a fender tonestack in between 2 opamps.
if worried about battery life, there is tones of incredible low current rail to rail opamps.
and if you need some sort of assurance their is something tubey in there. use a fet input opamp

All this sounds good and would have went with that. Just as Rick said i have already designed the PCB and had it made and populated. If i knew this before i started i would have probably went with the option of putting two different tone stacks in one opamp-based circuit and save a lot of time and fiddling around. Just all this in the first place wasn't the plan, but at this point i'm about 90% there into finishing this one. Still those 10% are killing me, hah xD

[Rick PA Stadel]

Now that the bias is stable, what is the bias divider? .. both circuit and resulting voltage.

Disappointing that the distortion is still present -- would'o thunked/hoped that fixed itself with the bias fix.

You do still have room for a couple hundred more uA's of battery draw, right? (currently ~750uA if my arithmetic is correct)

Regards

[Schotky]

Originally Posted by Rick PA Stadel

Now that the bias is stable, what is the bias divider? .. both circuit and resulting voltage.

The circuit is 1M in series with one diode, no cap. I'll get the voltage when I get back home.

Disappointing that the distortion is still present -- would'o thunked/hoped that fixed itself with the bias fix.

I thought so too... Do you think there could be something with the transistors after all?

You do still have room for a couple hundred more uA's of battery draw, right? (currently ~750uA if my arithmetic is correct)

Yup, your calculation is on point. We can if needed, if there isn't other option to fix the distortion

[Schotky]

Ok ok ok, now I had some more time to fiddle around with the preamp, trying every combination possible, even running the bass passive which actually made me figure something is off and that led to new discovery. I found the root of the distortion. It's not the preamp, it's the load from the other preamp, but while it's only unpowered and inputs are connected together on both preamps. You had a really good point about it Rick, I just wanted to deal with it at the end but there it is! But when both preamps are powered at all times, and only outputs are being switched, both preamps sound flawless!

I don't know why for the hell of me I didn't power the other preamp at least once while working on this one. Probably cause I needed a second cable plugged in in the second jack on the bass used for testing and I guess that's why it didn't occur to me....

Both preamps together draw about 1.84mA, which actually seems great considering it's two high performing preamps. I can definitely live with powering them both all the time at that current feed and switching the output only.. haha. This also simplifies switching between the two drastically.

However, will need to connect everything together to have my final thoughts if absolutely everything is working as it should when everything is in the cavity. This is still wires everywhere, some components still hanging on wires etc.

So, we have the diode and 1M resistor bias now with no cap. Are we putting in a cap of some value? 10-47uF? I can easy go with 47 as I know I should have those on hand in the correct size, will need to recheck for 10uF.

Rick, I can't explain how thankful I am to you!!!

[Schotky]

I'll post some photos of the finished products when I get there

[Rick PA Stadel]

You're welcome! It's been a fun run! Plus, that ax is absolutely GORGEOUS!
Take your time and work carefully, but I'm anxious to see the pics.

If you have something still smaller for the bias divider filter, would be fine. Anything bigger than about 0,1 to 1uF won't give any additional filtering.

Good catch on the unpowered preamp input loading .. I had kinda forgotten about that!

Again, mondo congrats!

Cheers

[Schotky]

Congrats to you for being able to provide extraordinary help remotely! Thanks again and

OK, so I can go for the cap as low as 0.1-1uF. I think I have 1uF, will need to check the voltage rating on it, it may fit. And to put the rest of components on the pcb instead of hanging on wires.

I'll report how it goes, now I'm eager to finish it, put it inside the cavity with the other preamp and wire everything as a final complete product. Since they'll be both powered simultaneously now, I'll have one line empty on the DPDT switch, I'll use it to break the connection on the input as well.

[Rick PA Stadel]

With the bias divider using a diode, the voltage can't exceed about 0,6V -- no need to check its voltage rating.

You may have a worse pop problem by switching the inputs, too. As long as both preamps are powered, the loading caused by the input of the switched-off preamp should be pretty minimal.

You COULD wire both sides in parallel. Won't make a lick of difference today, tomorrow, next week, or next month, etc. But someday, one side may get noisy before the other does.

Regards

[Schotky]

Thanks for the input. I used 4.7uF, it's what I had as a spare capacity. When I was getting the components I got the very long life hybrid polymer caps with ultra low ESR to extend the life span of the preamp as much as possible. Works perfectly even though it has plenty of excess capacity.

Wired everything, switching both input and output. The only common thing the preamps share is the power supply at the same time. Current draw for both is 1.76mA, slightly better then my rough estimation. There is literally zero clicking or popping noise when switching between the two. This solution works great and doesn't require any additional components.

I couldn't be more happy with this solution and how it performs for both. Give me a couple of days I'll post pics of both. Would post sound samples but don't have any recording gear unless I figure something out in spare time.

[Rick PA Stadel]

Excellent!