Eros Tape Head Preamplifier Kit
What’s different?
Oh boy, we were hoping you would ask! Here’s the rather interesting development story –
When we were working up the Seduction phono preamp design in 2000 we tried several different circuits that Paul Joppa had cooked up. One of those was a direct coupled EF86-to-6DJ8 circuit, a very simple design with a unique passive RIAA eq and a very basic power supply. Up against the equally simple Seduction circuit that used a 6DJ8-to-6DJ8 design, the circuit was deemed too tubey and dull sounding, and so the simple all 6DJ8 circuit was the design we developed into the Seduction kit.
Somehow that breadboard EF86/6DJ8 phono preamp prototype managed to avoid being scavenged for parts for other projects, and over the years it was given a shunt regulated power supply and eventually C4S active loads were made to work in it. The result was a dramatic improvement in performance with just the right balance of open highs, smooth midrange and bass punch. Once upgraded like this the redesigned circuit seemed to stay in our reference system in place of our best Seduction. From there the circuit was further refined and used in our $4000 Tube Repro amp for tape playback, and also put into a soon to be released phono version of the Tube Repro. The Eros is a direct descendant of that top of the line phono/tape preamp, streamlined to be cost effective and easy to assemble without sacrificing sonics.
There are a number of changes from the Seduction design. The input stage is now an EF86 pentode rather than a 6DJ8. The EF86 is perhaps the finest sounding low noise pentode made, being used in some exceptional microphones. At low signal levels like those of phono cartridges and tape heads the high gain of the tube can be taken advantage of without the sonic penalties often associated with the use of pentodes in higher power amplifier circuits.
Our unique use of the C4S constant source on this pentode offers a high level of isolation from power supply noise that might otherwise be amplified by the high gain tube, resulting in an all tube phono preamp with great resolution that can work with medium output cartridges without adding excessive noise. A servo circuit on the cathode of the EF86 accommodates production tolerance variations of various brands of EF86s and 6DJ8s, keeping the operating point at its optimum.
The power transformer is our own Bottlehead PT-5, a new low noise design made specifically for the Eros. The hybrid shunt regulator provides separate high voltage regulation for each channel and gives the Eros an added sense of speed and a sense of frequency extension that exceeds that of Seduction. The heaters are also regulated.
Gain is up from the Seduction’s 40dB to 50dB. This extra gain allows easy use of pretty much any tape playback head – Ampex ATR, Studer, Nagra T, Technics, Otari, even lower output tape heads such as those found on Revox tape recorders. And it works exceptionally well with the Flux Magnetics extended response heads we offer as an option for Technics and Otari decks.
The enlarged 8 x 10″ chassis – laser cut .080″ grained aluminum – now matches our other kit chassis footprint and is big enough to accommodate the two extra tubes, a bigger, more quiet power transformer and easy to stuff PC boards for the circuitry that doesn’t carry audio signals. We even managed to retain the option of adding a Hammond aluminum box from the Seduction design, to shield the circuitry for those who live in high RFI/EMI environments. The wood base supplied is the same beautiful Pacific Northwest alder as our other kits, which you assemble and apply your favorite finish to. The kit is supplied with two EF86 tubes, a 6DJ8 tube and a 12BH7 tube for the regulator.
Components
Signal wiring is all point to point with Teflon insulated wire and solid core shielded twisted pair cable. The power supply is easily assembled on PC boards. It is an efficient Schottky rectifier supply feeding dual active loaded hybrid (12BH7/LM431) shunt regulators in parallel with each channel of the preamplifier circuit. The DC heater supply is a full wave bridge of super quiet Schottky rectifiers feeding a Linear Technology regulator. Each channel of the preamplifier circuit is a C4S isolated EF86 voltage amplifier direct coupled to 1/2 of a C4S isolated 6DJ8 voltage amplifier, with the passive, switchable NEB/IEC equalization network positioned between the two gain stages. Only one output coupling capacitor is in the signal path of each channel – a metallized polypropylene type. Resistors are a combination of high quality metal film, carbon film and carbon composition resistors, each chosen for optimal performance in the intended application. Filter capacitors are overrated for long life. Tube sockets are high quality ceramic with aluminum shields. A ground post is provided for phono cable and turntable grounding.
Performance
Gain is 50dB
Input impedance is 47K ohms
Output impedance is 4K ohms (a preamp of at least 15K input impedance and interconnect cables of 1 meter or less are recommended, naturally we recommend the Extended Foreplay III as a great match for the abilities of the Eros)
Phase is not inverted
Because of the varying output levels of tape heads, a S/N ratio figure cannot necessarily tell you how the preamp will behave in your system. The Eros was designed to have 10dB more gain than our Seduction kit, and it may be more suitable for lower output tape heads. In the lab we typically measure a noise floor of 0.7Vrms and is dominated by “1/F” noise from the tubes themselves. What this means is you can usually hear a little tube hiss only if you turn your preamp volume up way beyond tolerable listening levels.
We consider this kit to be Skill Level 3. It has a lot of parts, requires stuffing several PC boards and will take some time to complete, probably at least a couple or three evenings. A beginner who wants a challenge and is willing to take their time can complete the kit too.
A very thorough assembly manual is included in printable Adobe PDF format on CD ROM. It contains step by step instructions you check off as you go, detailed photographs of the assembly that can be blown up for great detail, and a detailed checkout procedure, along with tips on soldering and tool selection. All the builder needs to supply is solder. You will need a copy of the free Adobe Reader software to use the manual.