Alembic F2B FET Version with Balanced Output

[gtudoran]

Had a little time for a friend request (he is a bass player and wanted a little preamp for small / medium gigs). I've used a FET version of the Alembic F2B bass preamp and attached an balanced output - for maximum headroom the pedal will use AC power supply (9V AC) - the OA is using +/- power supply.

Schematic:http://dl.dropbox.com/u/15275178/F2B-SS/f2bss-sch.pdf
PCB: http://dl.dropbox.com/u/15275178/F2B-SS/f2bss_top_silk.pdf
PCB Transfer: http://dl.dropbox.com/u/15275178/F2B-SS/f2bss_brd.svg

Hope you'll enjoy this (not so) new project.

Best regards,
Gabriel Tudoran
Analog Sound

[Morocotopo]

I don´t quite get the power supply. Are you using 18V AC to get +/- 9V?

[PRR]

9VAC is 12V peak DC.

9VAC through this voltage-doubler gives +12V and -12V DC.

The bias-trim is not needed if you have large voltage, especially if you have bipolar rails. You can "nail" the desired plate/drain impedance, current, and idle voltage by running a large source resistor to the far rail:



The 470u-100-470u may work fine for the opamp (I'd lean to 220 ohms). The FET stages have much less supply rejection, especially at the drain-resistor side. Since you will only have 0.4mA to both FETs, add a 3rd R-C dropper/filter with 2K resistors to 100uFd+ caps, this will be very clean.

[Gus]

I would clean up the power like PPR posted and to add to PRRs post about using the - supply
http://www.diystompboxes.com/smfforum/index.php?topic=93817.0

What is R10 value?   

[gtudoran]

I would clean up the power like PPR posted and to add to PRRs post about using the - supply
http://www.diystompboxes.com/smfforum/index.php?topic=93817.0

What is R10 value?   

Hey Gus,

R10 = 10k.

@PRR - thank you for intervention - i allready modifyed the schematic according to your advice - i will post an updated version ASAP (schematic and PCB).

Best regards,
Gabriel Tudoran
Analog Sound

[gtudoran]

Took a little time to make it, but now seems ok, hope i get your explanation rigt PRR (Thank you again).

Latest version!

Schematic: http://dl.dropbox.com/u/15275178/F2B-SS-V2/f2bss-schematic.pdf

PCB: http://dl.dropbox.com/u/15275178/F2B-SS-V2/f2bss-pcb-assm.pdf

PCB transfer: http://dl.dropbox.com/u/15275178/F2B-SS-V2/f2bss_brd_toprint.svg


Best regards,
Gabriel Tudoran
Analog Sound

[PRR]

Gus> What is R10 value?

Gus too is curious about that output stage. I had squinted and also assumed R10=10K to get the same gain as the other side. BUT... Why a gain of 2* 1.2? Why not unity-gain or real-gain?

Sorry, I just noticed the 1Meg pot (and 39K drain-load) driving the output stage. The input impedance of the output stage is essentially 10K. This is a real heavy load on 1Meg pot, even on 39K amp.

Hmmmmmmmmm

There is a ton of signal at Q2. The output stage does not need gain. Even if it did, wire IC1A for gain as needed. Then wire IC1B for unity-gain inverter and fed from IC2A _output_.

I'd take R11 R12 higher, 200-300 ohms. R13 R14 are not needed.

[gtudoran]

Hey guys, after some time i've decided to digup this project, so i've modified the schematic and pcb (still a work in progress). I would like to hear your opinion and if you see any big mistakes around it.

Best regards,
Gabriel Tudoran
Analog Sound

Schematic: https://dl.dropboxusercontent.com/u/15275178/Alembic%20F2B%20FET%20ver/f2b_fet.pdf

[teemuk]

Sorry to say, but aside tonestack response I can't see how this circuit could sound or behave even remotedly like an Alembic F2B.

- The headroom and response of the FET gain stages are totally different from the original tube circuitry. For example, the tube circuit tolerates about 5Vpp input signal before the stage overdrives, the FET circuit powered from 9VDC overdrives already at 160mVpp input. This is a HUGE difference headroom-wise.

- With operational drain bias value the FET starts to overdrive by soft clipping the negative halfwave. It takes about a 800mVpp signal before positive halfwave begins to clip as well. The ratio of input amplitude from one halfwave clipping to both halfwaves clipping is about 1:4. This means extremely asymmetrically clipped output. The tube on the other hand starts to overdrive by hard clipping the positive halfwave. It takes about 10Vpp input signal before negative halfwave begins to overdrive. The ratio is about 1:2, half of that of the FET-based circuit. The overdrive characteristics and the resulting harmonics and tone in this regard are once again entirely different.

- The aforementioned headroom and frequency response of the FET gain stages also change drastically each time you touch those stupid trimmers. The operation of the stage is in fact highly dependent on drain resistance. If it changes, so does the circuit's performance and pretty much ALL characteristics (headroom, symmetry of clipping, response, output impedance, input impedance, etc.). This also means those characteristics are highly interacting with the widely varying tolerances of the FETs used in the circuit.

IMO, this is an amateurish design, which cannot be built repeatedly even with modest consistency on mind unless FET characteristics are always closely matched to allow the exact same drain bias for all of them. Always. In every build. Despite that it won't behave even remotedly like the circuit it tries to mimic.

[gtudoran]

I think you didn't look closely the circuit is power by a bipolar power  supply that is at least +/- 9V so we are talking about at least 18Vpp power supply, so a 5Vpp input signal i don't think it would be a problem. And the opamp is also powered by a bipolar psu.

Best regards,
Gabriel Tudoran
Analog Sound

[teemuk]

Ok then, allow me to correct.

The values for asymmetric clipping threshold are (about) 340mVpp (vs. 5Vpp of the tube circuit). 800mVpp to clip second halfwave (vs. 10Vpp of the tube circuit).

Needless to mention, still as interactive with the drain resistors as ever.

No. Despite higher power supply voltages still not even remotedly similar peformance to Alembic circuit.

[PRR]

The cathode caps should go to ground, not negative rail. (Polarity is OK.)

Bias stability IS excellent. Overload levels probably will be less than a 12AX7 job (and certainly less than 5Vpp input!), but probably acceptable.

[gtudoran]

Thank you PRR for your answer, i've modified the schematic. Also i've made a little experiment that i've done it with tubes: a resistor from decoupling electrolytic caps to ground, it seems that helps a lot regarding the symmetrical clipping. Signal is 1Vpp, first is without a resistor and the second picture is with resistor:

https://dl.dropboxusercontent.com/u/15275178/Alembic%20F2B%20FET%20ver/withoutR.jpg
https://dl.dropboxusercontent.com/u/15275178/Alembic%20F2B%20FET%20ver/simulation.jpg


Best regards,
Gabriel Tudoran
Analog Sound

[PRR]

> a resistor from decoupling electrolytic caps to ground

Where?

[gtudoran]

Between C4 and GND.

Best regards,
Gabriel Tudoran
Analog Sound

[gtudoran]

This is the last version of the schematic and the first version of board. Build it ... put it on oscilloscope, everything looks good (i don't have a bass guitar right now for some tests but  already spoked to a friend of mine to come and give it a try).

SCH: https://dl.dropboxusercontent.com/u/15275178/Alembic%20F2B%20FET%20ver/f2b_fet.sch
BRD: https://dl.dropboxusercontent.com/u/15275178/Alembic%20F2B%20FET%20ver/f2b_fet.brd

If you would like to build it you are more then welcomed

Best regards,
Gabriel Tudoran
Analog Sound

PS: It works wonderful with guitar also.

[gtudoran]

Hmmm i have a little bit of a strange behavior... do you see any reason why the input of this effect would be microfonic? Without any cable in the input jack... if i tap in the box i hear it in the speakers, do you see any ... major design flows in the schematic?! Is the first time that happend to me and is strange as hell...

An input cap would be advisable here?

Best regards,
Gabriel Tudoran
Analog Sound

[gtudoran]

A sound sample with various settings directly into sound card.

http://soundcloud.com/analog-sound/little-bad-as-bass-preamp

Please don't throw with tomatoes .... i'm just a guitar player

Best regards,
Gabriel Tudoran
Analog Sound

[gtudoran]

Hey guys, some recordings made by a friend of mine who is a bass player (not like me ). The EQ was at 12 o'clock and in the name of the clips you will find the Gain and Volume settings (ex G9 V8 is Gain at 9 o'clock and Volume at 8 o'clock).


https://soundcloud.com/analog-sound/g5-v8
https://soundcloud.com/analog-sound/g9-v1
https://soundcloud.com/analog-sound/g3-v8
https://soundcloud.com/analog-sound/g12-v10

There are also 3 clips recorded with the bass directly into sound card and one with Sansamp BDDI and one with M80 from Mxr. The clips ware recorded for comparison.

https://soundcloud.com/analog-sound/bass-direct-placa
https://soundcloud.com/analog-sound/bass-sansamp-bddi
https://soundcloud.com/analog-sound/bass-mxr-m80-di


Bass: Warwick Streamer Stage 1 5 string Active
Strings: Warwick Black Label (Steel) 040-130
Sound card: Lexicon Lambda


Enjoy and any comments are greatly appreciated.

Best regards,
Gabriel Tudoran
Analog Sound

[tubegeek]

Some caps can be pretty microphonic with a high gain circuit.