Transformers for DAC output stages
Link to spec
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Very high bandwidth type
|1+1:5+5|| 6.8K Ohms
Z voltage out type
The sound quality of almost any commercial CD player or digital music systems can be improved by the use of a transformer to perform the current to voltage (I/V) conversion. There are 3 reasons for this:
1.The usually rather poor operational amp chips which are fed by the I/V resistor are eliminated.
2.The digital and analogue grounds can be isolated which can remove some complex distortion mechanisms.
3.The unique frequency characteristics of the transformer can provide improved filtering of the quantization noise.
The schematic below shows how to connect type 9762 to a CD player. two transformers are required for stereo.
The Iout + and - should be disconnected from the existing circuit and conected to the primary. The secondary windings should be wired in series and connected between the amplifier grid and ground. The internal electrostatic screen should be connected to analog ground. It is important not to conect the amplifier ground to the digital ground of the CD player. The I/V resistor (Rsec) should be connected across the secondary of the transformer as shown.
HOW TO CALCULATE THE I/V RESISTOR: First calcualte the secondary current (Is). This is given by the primary current ( Ip) multiplied by the inverse of the voltage ratio. If the output current pf the DAC is 4 mA p-p, Is = 4/12.8 = 0.33 mA p-p which is 0.33 / 2.83 = 0.117 mA rms. If we want to get 0 dBu (0.775V) at the grid (Vg) we need:
Rsec = Vg/Is = 775/0.117 = 6.6 kOhms.
NOTES: This will work well provided the secondary feeds a high impedance such as a valve grid. It the load is expected to be low the secondary windings should be connected in parallel and a lower secondary resistor used but of course the output voltage will be lower. It should be apparent that it is possible to adjust the I/V resistor and transformer ratio to get a different output voltage but it should be born in mind that most DACs work best with a low I/V resistor. The value of the I/V resistor as seen by the DAC (the reflected secondary load) can be calculated by dividing by the ratio squired. With a Rsec = 6.6 kOhm the reflected laod will be 6600/12.8^2 = 40 Ohms.
PRODUCT OFFERING: We offer a range of transformers suitable for most Digital to Analogue Converters (DAC's). Multi-section winding techniques are used to maximize bandwidth and transient response. All our DAC transformers are constructed on Mumetal (76% Nickel) cores for minimal distortion and contained in heavy gauge Mumetal screening cans. Some models have multiple internal electrostatic shields are provided to minimize noise pick-up.
Type 9762 is designed for most popular applications. Types 9055 and 9545 have twin bifilar primary windings which may be used in parallel driven from one DAC or may be used with twin DAC's carrying out of phase signals. These are subtracted by the transformer which provides the ultimate in cancellation of quantisation noise .
Types 8347 and 9055 are offered for DAC's with up to 0.6 mA dc offset. Most modern DACs do not have significant dc offset. The remaining types in the list above are for use with DAC's which provide an analogue voltage out.