Solid State Tube Sound
I always liked the sound of tube amps, and even if I already own one, sometimes I would like to be able to have the sound without having to use one. Like when I want to record something fast on my computer. Or when I plug straigh into the PA at a jam. Etc...
So when I saw Gabe's article about his work on tubes and tubes sound with transistors and mosfets, I decided to try it.
Also, don't forget to read his other articles about the tubes.
It had a nice clean sound, but the overdriven sound wasn't as good as I though it would be.
Note: This first schematic can still be used for a clean preamp if you add a trim-pot in series with R16. It will permit you to adjust the gain of the second stage.
But while I was at it, I decided to build another prototype, but this time, build entirely on his design. I just moddified it to work on +9V, and added a few parts here and there to be able to adjust it for a clean sound, even with a strong input signal.
I used NP 1uF caps in this circuit, so there's no (+) sign to identify how to connect polarized capacitors. If you use polarized caps, connect the (+) on the side that has a more positive voltage. For example for C2, the (+) goes on the +5.6V.
The result is a preamp that can handle a signal as strong as 1V peak2peak and stays clean, when the trim pot (R23) is adjusted at its maximum position.
Now, does't it really sound like a tube preamp?
I can't say for sure. One thing I can tell you is that it seems to make the sound smoother. Just as tubes do. And it's very quiet, even when laying on the floor without a box and connected to my amp..
Here are 2 simple sound clips.
This one was recorded both with my guitar connected straight into the sound card, and with the preamp added between the guitar and the sound card. 7 seconds straight, then with the preamp, then straight, and again with the preamp.
It shows the unloading effect of a preamp to record a guitar with a soundcard.
And in this one, the first 20 seconds are with my guitar into a Fishman GII preamp connected into the soundcard. The next 18 seconds are with the SSTS preamp added between the guitar and the Fishman preamp.
The last part is with my guitar connected into the Fishman preamp, and then into the SSTS preamp, and then into the soundcard.
In that part of the recording, the Fishman preamp was at maximum volume. And the SSTS preamp had its trim pot (R23) adjusted for maximum resistance. It was to check the sound with a stronger signal going into the SSTS preamp adjusted for minimum gain.
Can you hear a difference?
I will let your ear be the judge.
You want distortion?
Ok! Put the trim-pot (R23) at its minimum position, and feed the circuit a strong signal. Use a guitar with hot pick-ups for example.
You will get distortion. Not enough?
Put a resistor in the R20 position and a capacitor in the C9 location. You will be able to overload the second stage easily. Or add a pot in the location marked "Drive Pot" and cut the trace as indicated on the pcb layout at R20. You will be able to change the amount of signal you send to the second stage of the circuit. Choose a value for R20 that will determine the minimum drive you want from the first stage.
Here is how the mod will look when wired as in the pcb layout.
But I can't promise you a good sounding distortion.The circuit just wasn't designed for that. But if you are willing to work on it, you could find how to make it sound good when overdriven. I wasn't able to do it, and don't have enough time right now to work on that.
You can also experiment with your favorite Tone Control. The output signal is strong enough for that.
And don't forget to use a Volume Control... You will need one. I use a 350K.
More mods? You can use smaller capacitor values in the circuit if you want less bass.
Also, Gabe suggested to use higher values for R3 and R4 for a fatter sound. Something like 330K and 220K, or 470K and 330K. But we both agree that it could result in more hiss generated. In my really first tests with a single SSTS stage, I used higher values like 1-2Meg and had a hiss problem.
Another way to get a higher input impedance would be to use 2 small resistors and a 1-10Meg resistor for the bias.
I will add a picture of the finished preamp with a tone control as soon as I put it in a painted box...
See you later with more ideas.
Again, don't forget that this is based on Gabe's work, and that he only kindly agreed to let me use his design in my guitar preamp project. It does'nt mean that he agrees with what I did with it...
And take a look at his site if you want to know more about his work with tubes.
(Thanks again Gabe)