Bass guitar FET preamp

[Rick PA Stadel]

Which first stage FET voltage drops by 50mV?

You are welcome to fiddle with the Source resistors if you must(!), but I wouldn't. We're already substantially impaired trying to do this kind of work with no oscilloscope, and you already have the gain that you want (remember, Drain-res/Source-res approx sets gain). Playing with the Source resistances is a little too much like throwing darts blindfolded -- it'll trigger a petulance streak that fogs my thinking (worse than usual). Before we do that, let's nudge up the bias slightly. 6k or 6k1 in series with the diode would be a good starting point, raising the voltage by about 100mV. That original bypass electrolytic on the bias is much larger than it needs to be -- try 0,1 to 0,47uF; that'll help the turn-on thump, too.

Then let's see what the Drain voltages are, before doing the listening tests.

If you're still troubled by the 5k Source resistors in the original design, remember your FETs are nothing like the FETs they selected, you have double the supply voltage, and your pickup is almost certainly much higher output.

Cheers

 

[Schotky]

Which first stage FET voltage drops by 50mV?

The 15V drain

You are welcome to fiddle with the Source resistors if you must(!), but I wouldn't. We're already substantially impaired trying to do this kind of work with no oscilloscope, and you already have the gain that you want (remember, Drain-res/Source-res approx sets gain). Playing with the Source resistances is a little too much like throwing darts blindfolded -- it'll trigger a petulance streak that fogs my thinking (worse than usual). Before we do that, let's nudge up the bias slightly. 6k or 6k1 in series with the diode would be a good starting point, raising the voltage by about 100mV. That original bypass electrolytic on the bias is much larger than it needs to be -- try 0,1 to 0,47uF; that'll help the turn-on thump, too.

OK I'm not touching the 1k8 then. For now the 100uF bias cap is still in until I get lower value, probably I can get 47uF that will physically fit in the spot. Let me know if we need to do further testing with the new value bias cap so I can speed up the replacement.

Then let's see what the Drain voltages are, before doing the listening tests.

I've placed 5k1 in series with the diode, the closest I had on hand. With it:

(all measurements taken before listening tests)
Drain voltages start at about 16V and gradually drop to around 11.5V in about a minute and a half, and then continue dropping slower to around 10V in another 5-10mins.
(Note: with the resistor in place, it doesn't drop with hard playing, but yeah significantly drops gradually by itself, load or no load)

As the voltages drop, the total current draw goes up from about 0.5mA at the very start, to around 1.4-1.5mA by the time drain V's get to around 10V.

Could this voltage /current weird thing be caused by the 100uF bias cap charging that slow or what when a second resistor is added in the bias? Without it this doesn't seem to happen...

Sound vise, no change at all, same as without the resistor.

If you're still troubled by the 5k Source resistors in the original design, remember your FETs are nothing like the FETs they selected, you have double the supply voltage, and your pickup is almost certainly much higher output.

Not troubled, probably I just gave irrelevant suggestion. Will leave them be 1k8.

 

[Rick PA Stadel]

Sorry for the delay. This time it was Windows 8.1 that took a dump on me, and not just Chrome. Had a decent little bit composed, too ..

Give me another 1/2 day or so to recover my wits.

One key point I do remember was, "Lift one leg of that 100uF bias filter for now. Even if we accept the reasoning of the original designer, we won't need more than 1/10 of that value." In the mean time, the only damage is a very slight increase in the noise floor.

Still almost can't believe it -- silly thing had been rock solid for over 300 days! Best it has ever done!

Regards

 

[Schotky]

Not a problem.
Ok i'll disconnect one leg on the bias cap and see what that will do. I might have missed this if it was mentioned before.

For the Windows thing, I strongly suggest you get the Win 10 if you can, much more stable compared to Win 8, significant difference and saves a lot of headaches.

 

[Rick PA Stadel]

Thanks for the tip -- don't know if I'll have the courage. Some of the last few generations of Windows *upgrades* have brought me to my knees for weeks. Having to discard and reinvent work habits reduces my already limited productivity more than I can readily tolerate at times ..

Going to have to give it a good think, though. Thanks.

Cheers

 

[Schotky]

Sure, Win 10 it's basically Win 8 just vastly improved and way more stable. Fresh install instead of an upgrade through Win 8 is always better

 

[Rick PA Stadel]

The R-C time constant of 1 megohm / 100uF is 100 seconds, so the Drain voltages dropping over the course of a few minutes is reasonable. But a stable, arithmetically-sound operating point is a must. Because we originally had trouble getting your '5457s to turn on hard enough, we set up a higher voltage at the divider.

Now it's looking more and more like either a threshold shift has occurred, or our original measurements were compromised by the charging of the bias filter capacitor. Whichever the cause (not immediately important), we need to know what it is now.

Now that one leg of the 100uF electrolytic is lifted, are the Drain voltages stable immediately (within a few seconds) of applying power? What are they? They should drop approximately with the supply voltage (down to a B+ of ~14V that would be a reasonable minimum). Do they?

Do you still have the slight distortion with hard striking noted earlier? We may need to fit a signal pick-off at the tone stack. There should be about 30 dBV of gain being produced using these Source / Drain resistor values. We can certainly change them if needed. But it would be good to know where that excess is going in the meantime; a systematic approach needs a known starting point.

No need to measure current draw -- its easily calculated from the Drain resistor values and the drop across them. The bias divider will now only use 13 to 17uA.

Thanks for being patient. Sorry I took so long getting this posted.

Regards

 

[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.

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

 

[Rick PA Stadel]

Thanks for your contribution, WhiteDragon -- and many well-reasoned assertions. Unfortunately, you're about 6 months behind that point on this project. The PCBs have been designed, ordered, arrived, and stuffed.

All that remains for this design is a little tweaking to find the best/right balance between power consumption and performance.

I have personally worked with more than one bassist who dared to plug such contrivances directly into a hi-fi amp and custom, hand-made cabinet(s). The potential might surprise you.

Cheers

 

[Schotky]

The R-C time constant of 1 megohm / 100uF is 100 seconds, so the Drain voltages dropping over the course of a few minutes is reasonable. But a stable, arithmetically-sound operating point is a must. Because we originally had trouble getting your '5457s to turn on hard enough, we set up a higher voltage at the divider.

Ok, one important update on the "unstable" drain V. Just have accidentally noticed that there was tiny cold joint on the divider on Vref, the two 1M gate resistors were disconnected from Vref. This fixed the SO unstable voltage drop. Sorry if this has taken some unnecessary thinking time. More info below.

Now it's looking more and more like either a threshold shift has occurred, or our original measurements were compromised by the charging of the bias filter capacitor. Whichever the cause (not immediately important), we need to know what it is now.

Now that one leg of the 100uF electrolytic is lifted, are the Drain voltages stable immediately (within a few seconds) of applying power? What are they? They should drop approximately with the supply voltage (down to a B+ of ~14V that would be a reasonable minimum). Do they?

So, now that i fixed the cold joint, with the 100uF cap in place drain V get stable in several seconds at about 14.4V. With one cap leg lifted the drain V is stable immediately at 14.36V.

Do you still have the slight distortion with hard striking noted earlier? We may need to fit a signal pick-off at the tone stack. There should be about 30 dBV of gain being produced using these Source / Drain resistor values. We can certainly change them if needed. But it would be good to know where that excess is going in the meantime; a systematic approach needs a known starting point.

No change in the distortion, still present - gets more pronounced with hard playing.

No need to measure current draw -- its easily calculated from the Drain resistor values and the drop across them. The bias divider will now only use 13 to 17uA.

Good to know, thanks!

Thanks for being patient. Sorry I took so long getting this posted.

Thanks for investing your time in this!

 

[Schotky]

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]

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!