Hi
I have built this bass guitar preamp (the schematic is pretty large so i'm showing the section where some help would be welcome). It is mainly opamp based but the input section is transistor, with 2x 2N5457, genuine transistors bought from Mouser. The whole thing works overall, but the input discrete section is giving out some noise which is pretty audible. I have decided to use the input transsitor stage as i found it online as recommended as a good guitar preamp with plenty of gain, and it does work like that but with some extra noise in this case.
I'm supplying 15V to the transistor section, same power line as the positive for the opamps. I'm using step up voltage converter that bumps up from normal 9V to symmetrical 15V.
This is what i have discovered so far:
What am i missing here?
I have built this bass guitar preamp (the schematic is pretty large so i'm showing the section where some help would be welcome). It is mainly opamp based but the input section is transistor, with 2x 2N5457, genuine transistors bought from Mouser. The whole thing works overall, but the input discrete section is giving out some noise which is pretty audible. I have decided to use the input transsitor stage as i found it online as recommended as a good guitar preamp with plenty of gain, and it does work like that but with some extra noise in this case.
I'm supplying 15V to the transistor section, same power line as the positive for the opamps. I'm using step up voltage converter that bumps up from normal 9V to symmetrical 15V.
This is what i have discovered so far:
- Volume pot all the down/off or I remove the "A-Signal" which is a jumper to separate the discrete audio line from the opamp section, there is no noise at all, just the very minimal normal noise from the circuit after the pot. Meaning the extra noise comes from the transistor section, the opamp section is dead quiet.
- With the VR2 and VR3 set to about 10k, the noise is pretty audible in the upper mid and high range. When the Trim pot, VR1, is being turned it is creating some light crackling noise and the noise level varies a little as you turn. Have tried different pots with same results.
- Just to experiment, have set the VR3 to about 1.5-1.1k or so, the overall gain is lower than i need but the noise is completely gone, but ONLY when the Trim pot is full clockwise. In this case, turning the Trim pot counter clockwise brings some of the noise back, but crackling is gone when turning it (the noise level is the same at any setting/resistance of the Trim pot though) except when is full clockwise where there is no noise as i mentioned. Only then the circuit sounds like it should sound at any setting. Adjusting VR2 doesn't affect the noise, just the gain of the input transistor.
What am i missing here?
Interesting, I haven't seen this version of this circuit before. I forgot to mention that I have built the same circuit as the one I posted but with 2 band EQ as an onboard bass preamp standalone, instead of the Trim pot and works with no issues (I have not removed the components you have removed), but haven't tried your version.
The question mark caps eliminate radio reception.
The question mark caps eliminate radio reception.
I agree with Patrick. Avoid any DC on pots and pots on DC. They are noisier than common resistors, mainly in the rotor to carbon track contact.
A good stable low noise power supply is very important for these kinds transistor preamp. Please check the step up voltage converter supply is good enough.
The original schematic is a mess, where did you get it from?
NO coupling capacitors where they have some 9Vdc present? WHAT were they thinking?
Please link to the original circuit; the part I already see makes me cast great doubts on the rest.
NO coupling capacitors where they have some 9Vdc present? WHAT were they thinking?
Please link to the original circuit; the part I already see makes me cast great doubts on the rest.
The input impedance at RF frequencies is very low, and that is a classic mistake. Put a series resistor between the gate of the first fet and the input jack, perhaps with C4, and ditch the 100 and 300pF caps. At least 1K, as high as 100K. The 100pF caps may be causing the FETs to oscillate at RF frequencies. Remember caps are actually also inductors and have a series resonance at about 100MHz, especially through hole caps.
Transient output of an electric guitar can be a volt or two, so a preamp that runs on 9V must not have a lot of gain, or it will clip before the gain/volume control. Better to run the preamp at 40V or more (aka 12AX7). Otherwise keep the input gain to less than 10dB (x3.16).
The input is likely to exceed the pinch off voltage of the FET, causing clipping and the pot in the drain is not going to do anything about that. The first circuit added a bit of bias voltage in aid of input clipping, but the second did not. I would separate the gate and source supply voltage by at least 3 Volts.
Finding a balance between gain and noise is not easy. Look for ways to optimize to fit the different use models. One size(gain) does not fit all.
Transient output of an electric guitar can be a volt or two, so a preamp that runs on 9V must not have a lot of gain, or it will clip before the gain/volume control. Better to run the preamp at 40V or more (aka 12AX7). Otherwise keep the input gain to less than 10dB (x3.16).
The input is likely to exceed the pinch off voltage of the FET, causing clipping and the pot in the drain is not going to do anything about that. The first circuit added a bit of bias voltage in aid of input clipping, but the second did not. I would separate the gate and source supply voltage by at least 3 Volts.
Finding a balance between gain and noise is not easy. Look for ways to optimize to fit the different use models. One size(gain) does not fit all.
Thanks all for chipping in, I'm taking notes from all.
I couldn't find the original schematic now of the transistor circuit, i got it a long time ago and did some modifications after with the help of you guys here on the forum. This is where it derived from, pic attached. I have just relpaced the EQ section with one gain/trim pot. I have built this same exact circuit, I drew this schematic I'm attaching actually, and it works like a charm. But with the EQ not the gain single pot, with very low power consumption. So i went for it as i was familiar with it and it sounds great as a boost only EQ, no coupling caps or anything.
But obviously it doesn't work as well with one pot that's meant to be gain.
I don't have issues with distortion here though, it gets just a bit dirty with the gain above 3 o'clock which i like acutally if i want to add some dirt. And that is with active bass, with passive it doesn't distort even on full gain with current settings.
The rest of the schematic is the preamp from the 700RB II bass amp, i haven't modified anything in the opamp section. I have though taken out the 35V transistor input section and replaced it with this as it's way easier to supply 15V rather than 35V from a 9V power adapter. I'm making this a pedal and wanna use normal pedal power adapter. (The full schematic is pretty wide so i'm trying to figure out how to print it in a very wide image to post it. I only have it as a schematic file atm @JMFahey)
The step up converter is the XL6007 which has 400kHz switching freq, which is above the audible spectrum and is advertised to be used in audio among others. Having this mentioned, the current circuit has the same noise when operated on batteries so it doesn't seem like the power supply is the issue atm.
I couldn't find the original schematic now of the transistor circuit, i got it a long time ago and did some modifications after with the help of you guys here on the forum. This is where it derived from, pic attached. I have just relpaced the EQ section with one gain/trim pot. I have built this same exact circuit, I drew this schematic I'm attaching actually, and it works like a charm. But with the EQ not the gain single pot, with very low power consumption. So i went for it as i was familiar with it and it sounds great as a boost only EQ, no coupling caps or anything.
But obviously it doesn't work as well with one pot that's meant to be gain.
I don't have issues with distortion here though, it gets just a bit dirty with the gain above 3 o'clock which i like acutally if i want to add some dirt. And that is with active bass, with passive it doesn't distort even on full gain with current settings.
The rest of the schematic is the preamp from the 700RB II bass amp, i haven't modified anything in the opamp section. I have though taken out the 35V transistor input section and replaced it with this as it's way easier to supply 15V rather than 35V from a 9V power adapter. I'm making this a pedal and wanna use normal pedal power adapter. (The full schematic is pretty wide so i'm trying to figure out how to print it in a very wide image to post it. I only have it as a schematic file atm @JMFahey)
The step up converter is the XL6007 which has 400kHz switching freq, which is above the audible spectrum and is advertised to be used in audio among others. Having this mentioned, the current circuit has the same noise when operated on batteries so it doesn't seem like the power supply is the issue atm.
Ok, i've just tried this, the only thing i didn't change from your edits is the placement of C5. I need to cut traces on the PCB to modify that, so I'd leave that for the very end if it's critical? Not sure if that affects what i'm getting.
With this circuit, VR2 and VR3 set to 10K, there is no change in noise, signal gets through only when the Trim pot is all the way up. Anything below and there is no audio signal, just noise.
Thanks.
This schematic is reasonable, a Fenderish FET based version of their classic Preamps.
If schematic is too wide, you can probably split it in 2 or 3 manageable chunks using some Graphics Editor.
Always leave "a little more" visible so it´s easier to jump from one section to the other.
Show the GK "first transistor" you removed, maybe this FET add-on has way more gain than the original one.
It will not distort by itself, but maybe overdrive later sections, add noise, instability, etc.
To boot, you removed passive Tone Control which introduces significant attenuation ... if present.
Maybe, "just one" Fet was more than enough.
Yes it is Fenderish, or Sadowskiish
The GK one does have a lot of gain too, seems like the original distorts at lower volume than the one i've used. This i think it has just the perfect amount of gain for how i want to use it and it drives the amp's amp very well. My whole idea with this is to skip whichever amp's preamp and use it in the Return input of an amp. Have tested on two amps this way and works i think pretty good other than the noise.
One more thing i've noticed since you mentioned "passive EQ" and "attenuation".
With the Trim pot all the way up the sound is pretty bright, the more counter clockwise you go it attenuates the highs more. I think it's "flattest" with Trim set at about 3 o'clock. Above that boosts treble, below that attenuates.
I've also removed C10, C12, C13 and Hi-freq-att caps and i'm not sure, but maybe the noise is a little lower, maybe. Adding a 1M resistor in paralel with C10 only removed the very high end hiss, but didn't touch the overall noise.
I've split the whole schematic into 4 chunks with overlapping sections. Original is also attached. I'm not sure i can share the full file here of the original so i've inluded mainly the transistor section.
Another link i've found that is similar if not the same to what @patrick101 edited
http://www.riemer.us/two-stage-jfet-preamp-buffer/two-stage-jfet-preamp-buffer
Thanks.
Riemer´s is fine, not sure why you didn´t straight copy it, period.
MAIN problem is you removed coupling caps C2 and C4
Also original GK Fet/bipolar preamp has 4.5X or 33X gain, switch selectable; yours is full blast all the time.
As is, Riemer´s can drive a power amp when fed a passive Bass pickup, now if you pass it though a preamp, gain/noise will be too much.
Riemer´s is fine, not sure why you didn´t straight copy it, period.
MAIN problem is you removed coupling caps C2 and C4
Also original GK Fet/bipolar preamp has 4.5X or 33X gain, switch selectable; yours is full blast all the time.
As is, Riemer´s can drive a power amp when fed a passive Bass pickup, now if you pass it though a preamp, gain/noise will be too much.
Thanks.
Riemer´s is fine, not sure why you didn´t straight copy it, period.
MAIN problem is you removed coupling caps C2 and C4
OK thanks for the input.
I redid the circuit and made it almost as patrick101's suggestion, which is Reimers' basically. I have also taken out all the extra components and now it resembles Reimer's except for the differences below:
On the input instead of an R I have a C, C2 is placed on the wiper instead of where patrick101 and Reimer's is, and there is coupling capacitor on the output, it's just electrolytic 10uF instead of 100nF.
So what you're saying cause of these MAIN differences mine circuit has all this issues with noise, no sound if trim/gain pot isn't on full and boosts and attenuates treble?
As shown, gain pot should NOT affect tone, since it´s a plain resistive attenuator (volume pot) between 2 gain stages.
That's why I included that A-Signal jumper, in case I needed to replace the whole circuit with whatever
I went for the transistors first as they are not perfect thus give this unique sound to instruments. The amp I like a lot, the 700RB II has them in the initial stage, but the original circuit requires 35V, tricky to get from 9V adapter so I substituted that for the current one
I hope I can get that working, too many people have used it with success, I just need to rework the small steps I guess to see what I have missed
Thanks for the suggestion though
I went for the transistors first as they are not perfect thus give this unique sound to instruments. The amp I like a lot, the 700RB II has them in the initial stage, but the original circuit requires 35V, tricky to get from 9V adapter so I substituted that for the current one
I hope I can get that working, too many people have used it with success, I just need to rework the small steps I guess to see what I have missed
Thanks for the suggestion though