Top Drive Preamp

18 April 2009

This is a new preamp design I have called “Top Drive”. Top Drive is a compact tube powered preamp that is quite unique and rocking. The preamp is voiced in interesting way,  the sound is very much like a classic Vox amp. The preamp has got that great top end “jangley” character with that nice almost loose bottom end. It has great clarity and character all the way up the volume control. When the volume control is maxed out there is great singing type of distortion. Top Drive is very much like an old Vox AC30 in the sense that is has a single volume control. With no master volume (gain) control this means you need to crank up the volume for that great tube saturation. There is something about non-master volume amps that make them special and sound different.

Features:

Tube Stage – the tube portion of the preamp features two 12AU7 tubes being run at 50V in cascaded Class A topology. Each of the four triodes are set up for moderate to low gain to allow for maximum tone shaping over a gradual period. The tube preamp portion of Top Drive also has controls for Treble, Middle, Bass and Volume.

FET Boost/Stage – what makes “Top Drive” unique is it has a footswitch-able gain stage in the form of a FET boost. With the high impedance FET boost driving the front end of the pre-amp you get tons of added gain, saturation and sustain. The distortion and break up are incredible, the tone is very much like a pushed over the top Vox. The FET boost stage has a volume control to control the amount of gain and drive going into the preamp. Now this means you now have a volume control at the beginning of the circuit and the normal one at the end of the preamp. So when you switch on the FET boost stage you now have a master volume amp! You can now dial in the perfect amount of gain without having to crank the volume.

As well as the volume control for the FET boost stage there is a voicing control. A three way toggle switch allows you to select bright, flat or fat tones. This is all made possible by putting different RC filters in parallel to the FET’s source resistor. What’s nice about the bright control is it stays bright and clear all the way through the volume control’s sweep, it’s not like an old Fender bright control where the bright fades away as the volume increased.

Footswitch – I have designed a two switch footswitch. The footswitch utilizes transistor driven relays to let the user bypass (true bypass) Top Drive. The footswitch allows one to bypass both the FET boost stage as well as the tube preamp stage. This is really useful as depending on how you set the FET volume and normal volume the footswitch can then act as a clean/dirty channel switch. The switching system even allows one to use just the FET boost by itself. Both switches turn on/off LEDs mounted on the front of the preamp to tell you when which switch is activated.

Schematic:

top-drive-schematic

top-drive-tube-path-vero

Vero layout of tube section

top-drive-relay-bypass

Vero layout of the footswitching relays

top-drive-fet-boost-vero

Vero layout of FET boost

 

 

 

 

 

Build:

Results:

I am very pleased with the tone of this design. Top Drive is very tight sounding overall but with a nice amount of highs and clarity on the top end. With just the tube signal path (no FET boost) the tone can range from clean to great tube distortion. When the volume controls hits 3 o’clock the preamp starts to break up and this great natural distortion is produced. The distortion is not overly heavy, just the right amount of gain. I would describe the max gain as an early 60’s British Vox or Marshall that is cranked. Of course the tone can be tweaked by the simple but the very effect three band EQ.

When the FET stage engaged things let loose. With the FET gain rolled up to about 2 o’clock there is a serious increase in gain. The sound becomes bigger chunkier and there is a lot more gain. Now with the voicing switch set flat the frequency response is well flat, no frequencies are accentuated more than others. This helps retain the character of the tube circuitry. With the voicing control set to fat there is even more gain than the flat setting. The tone also becomes even bigger and the bass is boosted. The sounding is very raw, it provides a great Texas blues type of tone. Finally with the voicing control set to bright well the sound is clear and bright!  The preamp sounds pretty good with the bright on and some gain, similar sound to a 70’s Hiwatt.

The FET booster can also be used all by itself. This means you can bypass the tube circuit with the footswitch and then turn on the FET boost with the footswitch so it’s on all by itself, kind of a neat feature.

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