New Amp!

The "New" Vacuum Tube Integrated Amplifier Nov/Dec 2003 Return to Eldocountry

It started innocently enough...looking to one-up my previous amp project I decided to combine some popular amp designs on the net, but add a twist..microprocessor control!

I decided I'd like to build Bruce Rozenblit's grounded-grid preamp, and a pair of his 150w monoblocks. Now, for comparison's sake, my previous amp was rated at 30wpc, and this is tube equipment, so 150w is more than a person would ever need, I guess I just like to think big! Besides bringing the pre and power amps onto the chassis, there was a unique "Headwize" active equalizer, using tubes of course, that followed op-amp topology. Why not I thought? Wireless remote would be nice, so I located a PIC based 12 channel infrared receiver and transmitter kit and used that as the foundation of the remote system. I liked the idea of having relay switched inputs, and the possibilities of incorporating control over the system's B+ sections, so a BasicX processor made sense. Adding an LCD display and rotary encoder opened up a lot of possibilities. For clean sound, I wanted a robust power section, and this time I wasn't going to tolerate ANY hum! Although it upped the tube count (hey, that's a good thing!), the EQ and preamp are regulated with 4 OC3 gas-regulators, supplied by 4 6X4s.

Appearance- The overall process of putting this amp together took approximately a year off-and-on to come to a conclusion. The chassis is my regular aluminum sheet with riveted lip bottom allowing air circulation from the front, and a soft neon glow from below. The paint is a three step process of Cadillac pearl coat, with the large volume knob having been masked in sections and painted with blue metallic to match the transformer end bells. A huge toroid supplies the 700v B+ for the poweramps and is concealed under the upper chassis that contains the motorized volume pot, microprocessor, LM3914 LED control board and the LCD display and inverter. The output trannies are Hammonds rated for 150w, although they're not working that hard. There's a transformer on the left of the chassis that supplies the filament voltage for all the tubes, 15 amps worth. Under the chassis is the preamp/EQ transformer with a modified enclosure and miniature silent muffin fan to keep the winding temp within reason. Plus there's the transformer that runs continuously for the remote control board and the microprocessor. Finally, a 7th transformer supplies the negative bias for the output tubes. Guess you could say it's kind of cramped under there.

Controls- The design for this amp was originally going to be a pyramid with tubes mounted along the sides, but I eventually reverted to what you see below due to soldering considerations. To acheive audio balance, I used two seperate motorized pots in the signal path that are also controllable from the remote. From left to right, the controls are as follows: Shadow-pushbutton to bypass the EQ, Bass, Midrange and Treble adjustment, Enter button, Encoder to select menu items, sleep timer/dimmer button, left channel trim, eye-tube for audio level, right channel trim, and power button. The small circles under each control illuminates a symbol or letter representing the function (except the far right hole, that's the remote eye). On the back are gold plate binding posts for both channels, 8 RCA jacks for 4 inputs. A 'hard' power switch, bananna jacks to set tube bias, the main fuse, a trimmer knob and toggle for the eye tube, the 3 prong power jack and a couple receptacles. On the rear of the upper chassis is a special 8 pin jack that allows a laptop to be connected serially for programming, and future accessories to be connected that are wired to the remote control board. There's also a mini-toggle to select between 'dot' and 'bar' mode of the volume control leds.

Performance- This amp has a very distinctive sound. No hum, whatsoever, and it just bleeds power. It does sound more solid-state than I initially cared for, but that's because the sound isn't colored or padded by conventional topologies. Either the tubes have burnt-in nicely, or I'm just becoming accustomed to the sound, but I really do prefer it to my other amp.

Now for those who have the eagle eye, you're right, those aren't the magnoval based EL-509s the circuit calls for. I had nothing but trouble with those tubes...cracking bases, shorting plates, dud filaments. Finally I gave up after my amp was damaged twice before it even left the bench. I reconfigured the supply with help from the DIY Audio Forum and converted to run with good 'ol octal EL34's. They don't have the power of the EL509s but they're dead reliable and I happen to have a decent supply on hand.

Features- Now for the goodies...Every I/O on the BX-24 is full, leaving me little choice for any other option but I believe I made the best compromises I could. The power button locks in a relay that only the processor can disconnect by pulsing a relay designed to open the low power circuit. So once the processor gets power (as well as a parallel relay controlling all the filaments), it enters its program that powers up the preamp HV after a set amount of time. The poweramp HV follows. While this is going on, the processor checks to see what the last selected audio input was and selects the respective reed relay. It also goes ahead with its housekeeping tasks, including adjusting the volume control if auto-volume was previously set.

Auto-volume? Yes, this option allows the amp to adjust the volume knob physically! either to a preset position of your choice, or to the position it was last left at. During shut down it takes note of the location, stores it in memory and turns the control down to zero.

There's also an hours-accumulated setting so you can see how long your tubes have been burning. One of the processor inputs monitors under chassis temps- You can view the temp on the LCD or shut that feature off. Either way, it will warn you of an over-temp situation and go into shut-down mode to protect the amp. I've found the indicator LEDs to be very, very bright. So to take the edge off, pressing the SleepTimer button dims the LEDs or brightens them depending on which mode they're in. Press and hold that button for several seconds and you enter SleepTimer mode. The amp will then automatically power down in 30, 60, 90 or 120 minutes, and to make selecting this choice easier from across the room by remote, an LED flashes once per 30min block while setting so you know where you stand.Once set, the led remains lit.

Via remote you can toggle through the four audio inputs, individually adjust the left and right channel volumes, adjust the main volume, dim the LEDs and set the sleep timer, as well as 3 more undeclared functions that are accessible on a jack at the rear of the amp, designed for future audio components.

Oh, and one last really cool feature- There's no physical indicator on the volume knob to let you know where you are. At first you'd think this would be difficult to get the hang of, but the trick is that there's 20 LEDs arranged in a circle under the perimeter edge of the knob. Regardless of position, a cool white glow spills from underneath the knob. As you increase the volume, a red glow appears up to the point of selection. Flip a toggle at the rear and it becomes a single red "dot" under the knob. The knob and leds have the relation that half a circle indicates quarter volume, and a full circle indicates half volume. This is due to the fact that the knob will spend most of its life being less than quarter turned so you have greater accuracy in this range. I have rarely ever run the amp past a full red circle (half volume) as it's extraordinarily loud, but I love to push it some mornings if I'm in some other part of the house.

The real beauty of this amp lies under the chassis, all I have left is the wooden side panels for the upper and lower chassis and some software for the LCD to tidy up. Then I'll snap some under-hood pics!

A peaceful glow...	Serial Interface
The rear layout	Color-changing microprocessor
Gas Regulators takin' care of business	EQ/Preamp Section