High Fidelity Vacuum Tube Audio Amplification Equipment
AP1 FULL PREAMP - PUTTING IT ALL TOGETHER
Building a complete high quality hifi audiophile grade preamp isn't really all that hard. In fact, the hardest part will be figuring out what features you want and how it's all going to fit together and what it's going to look like when it's done! The innards are a "no brainer" with CLASS IX modules.
The diagram at the end of this page shows the signal path connections as I prefer to incorporate on my builds. You might want different features. The features I like to include are listed below. You may decide to use some or all of them, or alternately build in your own features. It's your build, go nuts!
RIAA PHONO PREAMP SECTION
An absolute MUST if you have a vinyl collection . The CLASS IX PP1 RIAA phono preamp module makes it easy to match impedances of the most common moving magnet phono cartridges. My personal choices are the Shure M95ED and the Audio Technica AT140L. The PP1 has a musicality unmatched by solid state phono preamps. Low noise, low distortion, tight adherence to the RIAA equalization curve and employing the use of a low impedance solid state regulated power supply (CLASS IX PS1) impart the characteristics to the signal that are important to audiophiles. Imaging and detail are incredible. Highs shimmer, mids sound natural, bass is tight and punchy.
3 HIGH LEVEL (LINE) INPUTS
Having a few selectable inputs allows me to select between high level inputs like CD, tuner and an auxillary source. The signal is basically routed through the balance and volume pots to the AP1 line amp module and amplified. Gain of this stage is 18.5dB and the output will drive most commercially available solid state and vacuum tube power amplifiers. I use the CLASS IX SW1 microcontroller based switch to select between inputs and for switching external loop and stereo/mono functions.
EXTERNAL PROCESSING LOOP
This used to be call a "Tape loop" in the old days. This is the loop that was used to tap the selected high level signal for recording and the input for listening to the recorded signal. Of course with the introduction of several signal processing devices such as equalizers, compressor limiters/de-limiters, 4 channel matrix decoders and a host of other gadgets, the tape loop was used for anything but a tape machine. So let's call a spade a spade and refer to this versatile switched insert facility as an "external loop". Once again, the SW1 module handles this function. You might chose to eliminate it on your configuration if you're a no nonsense one input CD listener.
I don't listen to a lot of monaural signal sources, but I do find this facility handy for setting L/R balance. In it's simplest form, this switched function mixes left and right channel signals together and sends the resultant mix to the AP1's L and R signal inputs. The SW1 module takes care of all the work, once again.
INDEPENDENT CHANNEL GAIN CONTROLS
I could use a proper balance pot but I really prefer to have 2 individual pots to kick back gain on my strong channel. The radio station I listen to has an inherent weak channel that upsets the L/R channel balance so I find myself having to knock back the signal on my left channel when I listen to this station. Having individual gain pots as opposed to a single balance pot lends to the symmetry of the front panel layout. As well, I don't have to search for a proper balance pot, which is basically 2 audio taper pots, one with reverse log taper resistance. These are hard to find and pretty expensive when you do find a proper one. ALPS makes them, if you're interested. Heck, some folks will eliminate balance control altogether in their design.
HIGH QUALITY GANGED VOLUME POTENTIOMETER
This is the most critical component of the whole preamp, in my opinion. Don't horse around with those cheap ganged pots coming out of China. They're just not worth the aggravation that will ultimately follow. Invest in a good quality "BLUE VELVET" volume pot from ALPS or maybe even a 24 step attenuator that uses individual precision resistors. The internet is full of DIY sites that feature attenuator assembly projects. If you don't have to replace it in 5 years time it's an investment that was well worth the up front cost. Look for units with 100KOhm or higher resistance with "A" taper, or audio taper, resistance tracks. A high quality volume pot will give years of smooth, scratch free operation as well as maintaining tight channel to channel gain balance. Look for a unit with a 1/4" or 6mm solid shaft. This will allow you wide choice in control knobs.
MICROCONTROLLER BASED SWITCHING
OK, it's a bit self indulgent. I came up with a great switching system based around the Microchip PIC16F627 flash microcontroller. Cool thing is that the SW1 microcontroller circuit remembers all the switch settings prior to power down and restores those settings on subsequent power up without the use of batteries or memory storage capacitors. The system controls the switching relays that are placed right at the input jacks. This does away with a ton of shielded cable and gets the signals (essentially) straight from the input jacks to the volume/balance pots and then to the AP1 line amp section. I use illuminated indicating touch sensitive switches on my unit to select between input selection, external loop in/out and mono/stereo selection. The only audio signal wiring is the output pair from the PP1 phono section and the input wiring to the AP1 line amp! The rest of the wiring is data wiring and that's done with ribbon cable terminated with standard low cost IDC connectors. It takes me about 2 hours to wire the selector switches, input jacks and switching unit. Not bad, it would take at least double that if I used a rotary selector with shielded cables. The greatest advantage of using this system is that it keeps signal path connections short and free of hum, buzz, clicks and other annoying extraneous signals. Again, this is not a requirement, but rather, a luxury and a matter of personal preference.
SOLID STATE REGULATED POWER SUPPLY
The PS1 supply provides all the voltages to make the thing work. In fact, it's the main "non-audio" ingredient for the way this preamp sounds and performs. The latest revision provides B+ lines at 285V, 265V and 245V as well as filtered DC 6.3V @ 2.4 amps to run the heaters for 6 tubes. Since the circuit is solid state regulated there is no sag on any of the B+ lines. This low impedance current source is key in providing the rich detail and wide sound stage of the preamp circuitry. The SW1 also employs a small microcontroller to provide 10 second delayed on time for the B+ voltages to reduce cathode stripping in your valuable vacuum tubes. As well, a further 2 second delayed on 5V line can be used to operate a muting relay thus allowing B+ voltages to settle before switching the preamp's output on. This eliminates thumps and snaps to the output signal lines. All the while a third line to the power indicator flashes 10 times slowly to indicate "power on - heaters warming - delayed B+ - muting relay on" and then flashes another 10 times rapidly to indicate "power on - heaters ready - B+ ready - muting relay on". Finally, the unit switches the indicator to steady on and the preamp is ready for action. There is also a voltage tap to operate the SW1 switching circuit. Input voltage may be set for 110VAC North American operation or for 220VAC European operation by way of soldered jumpers on the power supply's printed circuit board.
Figure 1 - CLASS IX complete high fidelity vacuum tube preamplifier signal path
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