Yes it is possible. Whether it sounds better than a active preamp ..depends on the rest of the system and your taste. Here's some info sent to me by a designer of tube equipment and transformers.This unit maybe better than the basic passive attenuator. It's also inexpensive to build.
Passive Preamplifier - No longer an Oxymoron
An introduction into amplification with passive devices.
By Jack Elliano - Electra-Print Audio Co.
INTRODUCTION
Line amplifiers are most commonly used to increase level of a CD player or phono preamp into the amplifiers. Line amps do not have to amplify very much to accomplish this. The average overall gain needed is about four. Normally the job is done with an active device such as a tube with its conventional means to offer a low impedance output to the power amplifiers with standard interconnects.
A not so popular means to increase gain is with the use of a step up transformer plus some sort of a variable gain control. This usually fails due as a result of the output impedance being too high for the interconnect cable capacity and attenuation of high frequencies. However, step up transformers using a low impedance source signal can offer a usable stepped up impedance with very little affect from interconnect cable capacity.
One saving grace that has become more in popular with music source devices is the very low output impedance of about 50 to 100 ohms. New computer based, hard drive music delivery systems use a pro audio sound card that develop an extremely linear sound, using standard 50 ohm balanced and unbalanced outputs.
TRANSFORMER DESIGN
One such step up transformer arrangement that will work with op amp outputs must have certain characteristics that will assure full bandwidth, constant load and low distortion. It's primary winding must have a low DCR (Direct Current Resistance) and enough inductance to react with the low end. This parameter is necessary because the DCR is added to the total impedance and is a loss.
Due to the very low voltage and power at this impedance, a core material must be used that will increase inductance and move fast over zero crossing at low levels. Nickel laminations work very well at these levels and offer very high permeability. This transformer will also need a high coupling, low capacity wind. We tried a few arrangements of step up and settled with a 1 to 8 voltage gain which offered an impedance of 10,000 ohms from 150 ohm source. This worked well with a 10,000 ohm high quality audio taper control as a constant load and a means to vary the gain for the amplifiers. We calculated that 80khz is the -3db point using a 50pf interconnect from 10,000 ohms source. This seemed safe enough. Note that a 400pf interconnect from a 10,000 ohm source at 20khz will give a reactance of 19,000 ohms so a rolloff of about -2db would be measured. When reactance is equal to source impedance, this is -3db.
The transformer we built, and used in this circuit, had an overall bandwidth -1db from 6hz to 55khz. Bandwidth was achieved with 49% nickel core and proper longitudinal balanced windings. The 10K secondary load forced a constant impedance back to the source to operate with. This offered good operational characteristics.
WHAT WORKS AND WON'T WORK
The input impedance of a common tube amplifier is mostly the value of its input tube grid resistor. This impedance does not have to be matched, it is not transferring power. If it is too low, less than 20K, it may lower the total gain of the PVA (Passive Voltage Amplifier, our name for this) output. The input sensitivity of the amplifier is the most important parameter needed. Sensitivities of .25 to 1 volt will work good and the higher input levels may not give full power output.
The output impedance of the PVA is the sum of the resulting transformer secondary and resistance used within the volume control.
Some music sources may not work with the this PVA design due to there higher output impedance’s. One remedy for this is to wind a different ratio step up to offer lower gain levels. The CD players with cathode follower outputs most likely are about 3000 ohms so the voltage level should be sufficient to drive an attenuator only, to amplifier input. It would benefit the waveform to pass it through a 1:1 ratio transformer and its nickel core.
These type transformers can be custom wound to accommodate any ratio needed. Due to the small size, nickel core and levels involved, a wide bandwidth is always a result.
MEASURING OUTPUT IMPEDANCE
One means to measure output impedance of a CD player, with close results, is to have a test CD with a 1Khz signal, play it and measure the open unloaded, output signal on a good AC voltmeter. Then add a variable resistance across it, adjust it until the value is half of the open measurement, remove this resistance and measure its resistance with a standard ohmmeter. That value should be very close to the source impedance, at least at 1Khz.
SIMPLE CIRCUIT DESCRIPTION
As the circuit diagram shows, the volume control wiper then goes to the output, and if
needed to sub woofer outputs. The 1k resistor between these outputs are to isolate any low pass filter effects from the normal output. The circuit arrangement here assures that the bandwidth will not change, no matter what volume level you use. It will not add or subtract inductance and capacity as other tapped ratio devices will. The output or source impedance of this device is within 10K or so. It is best to keep the output to the amplifier interconnect capacity, around 50pf.
If component isolation is necessary due to a ground loop or if a hum develops while interconnecting unit, the input jacks must be insulated from case. Remove the ground from the input winding or jack. Grounds on the output and bass output must be common and cannot be isolated from each other.
LISTENING NOTES
The audible result of this device is very interesting and different. One noticeable difference is the increase of amplitude range from low level to high levels as well as increased detail. It can be said that with a nonactive or passive means to develop amplification has an extremely accurate transfer characteristic result. A tube, as many people experience, will add its signature to the sound. Obviously, with no power supply, this design will have no power supply noise residue despite having gain. Normal listening level ranged from 11:00 o'clock to 2:00 o'clock on the control so it has plenty of gain.
The silver secondary version of the transformer sounded the same as the all copper but a slight but noticeable added softness or silky sound. Silver is more sensitive to flux
variations than copper, it may reveal very high number harmonics.
One added comment is that a nickel core low level audio transformer seems to have an interesting resulting sound. The first thing noticed is a wide swing in dynamic range very different from the original CD player opamp output. The second is a clean, rich delivery of harmonics of strings and brass. It was also noted that it will bring out a poorly recorded CD. We know little at this point as to why these are happening but our theories are piling up.
CONSTRUCTION
The PVA should be built with a steel chassis for best results. Steel will absorb stray magnetic fields from power transformers and chokes in other equipment located nearby. Switching to other inputs can be added to the design in order to accommodate other music sources provided source output impedance’s are between 50 and 150 ohms. Do not exceed 3 volts input or the nickel core will saturate and distortion will occur at low frequencies.
PARTS LIST
2 - PVA-3 Electraprint transformer (nickel laminations with copper wire - $51 each) or
PVA-3S Electraprint transformer (nickel laminations with silver secondary - $250 each)
1 - 10k audio taper volume pot or equivalent
1 - 7"x5"x2" steel chassis with bottom plate or equivalent
6 - RCA jacks
2 - 1k-1/2 watt resistors
