Is a battery powered preamp feasible? Is it practical? Will it sound good? In this series of articles we will examine these questions.
Some groundrules: in order to be practical, the batteries must last at least 100 hours before needing changed. The preamp must cover all normally needed functions: this includes RIAA phono preamp, line stage, tape monitoring with isolation. It will also include tone and loudness compensation.
Tube Complement / Power Budget
4 6088s (2 per channel). The first section is operated pentode mode, the second section is operated triode mode. This provides a gain of about 60 in the first stage and about 10 in the second stage. Since RIAA requires a 20 dB loss, and since the non infinite plate resistance of the first stage requires a little "compensation of the LF poles" (see pole/zero technical report on these pages), the overall gain will be about 50. For low Z low output cartridges, a stepup transformer can be used.
For the pentode mode, I'll "tap" the B+ batteries at 27 volts for the screen supply. Overall current consumption on paper came very close to the actual phono preamp built. The filaments were run slightly starved, as that provided a reduction in distortion, and improved the sound quality.
Filament requirements: 72mA (instead of 80 mA). One Alkaline D Cell will provide about 15A-H at this draw, so the battery should last about 200 hours! A single Alkaline "C Cell "will last about 100 hours.
B+ Requirements: Total current consumed (anode and screen) is about 2mA. Since the B+ is all run from one set of batteries, we will calculate the overall life later.
This is a nicety more than necessary. Some tape recording machinery load the "tape output" when not used, so it is usually a good idea to buffer the tape output. Normally, a cathode follower could be used, but that's not convenient with DH "battery" tubes. So, I decided to use 2 6418 in a unity gain buffer. (This is sometimes called an "anode follower" just to confuse people.) Filament was also run slightly starved at 9mA (instead of 10mA) so total current is 18mA. B+ requirements was about 400 microamps (triode mode) for a total consumption of 800 microamps.
I decided to use 2 6088 per channel, each operated triode mode. This provides a total available gain of about 75., which is sufficient for an overall "line" stage gain of 10 (20dB) and provide "room" for balancing and tone controls. Current consumption on the filaments was 74 mA and B+ consumption was 2.9mA for all 4 stages. For the filament battery, 74+18=92mA, so a Alkaline D cell powering these will last about 160 hours.
9V battery consumption:
Total B+ usage is 2mA + .8mA + 2.9mA= 5.7mA. Alkaline 9 volt batteries provide about 575mA-Hr, so the 9 volt batteries will last slightly over 100 hours. (This assumes the phono section is ALWAYS run. When switched to CD, tape or tuner, the phono is unpowered and does not consume current. Under that condition, the resulting 3.7mA consumption allows the batteries to last about 150 hours!
I built each phono section on a separate circuit board. It looks like this:
The "yellow stuff" wrapped around the tubes is that nice acoustically dead foam stuff. (See microphonics section on these web pages), although, these tubes are surprisingly free from microphonics. The light orange caps are the RIAA components and are specially built poly units designed for an old telecom application. They are 1% tolerance and sound quite nice in audio. The 2 boards were then placed in a separate case for further shielding:
A 12AX7 is shown for size comparison.
The line stages (6 tubes) were then built on a separate board. This looks like:
The "pins" sticking up will be where the low C coax cable will hook the "active electronics" into the switching "stuff". The 2 6418s are the "barely visible" tubes on the left, sticking straight up.
Mounted into the overall preamp "box" the electronics looks like this:
The funny little copper segmented "box" will hold the 8 9V batteries. All the active electronics is shown here, prior to hooking up the inputs/outputs and controls. The top/front is shown swung up in this picture.
What this will look like is:
Notice that I make no claims to be good mechanically :-)
Aannddd, my sisters inherited al the artistic talent in my family.
There are two (almost invisible in this picture) LEDs that provide an indication of the state of the batteries and power indication.
The rear connections look like this:
Next time, we will cover some of the circuitry in more detail.