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Re: Lundahl Transformer Attenuator??

Posted by gdahl (A) on October 15, 2002 at 23:06:12

In Reply to: Re: Lundahl Transformer Attenuator?? posted by SuperB on October 15, 2002 at 03:11:44:

Hi Super B,

I agree that in many cases adding a 1:1 input transformer will improve results with a resistive attenuator, but you still have to watch the impedance relationships. If you want to use a transformer attenuator, it is important to plan the surrounding stages so that it will perform at its best.

The TX-102 likes to be driven by a fairly low impedance. This might make it difficult to incorporate in certain tube circuits, but in my opinion is worth planning around...most things like being driven by low impedances anyway!

In my system, I use the TX-102 at the input of the Aurora integrated amps, which have their line stage built in. Using an integral line stage allowed a favorable gain structure for the amp...the line stage has a healthy amount of gain, which makes the driver stage's job easier, but the voltage level between the line and driver stage is higher than a typical "line level". With the tranny attenuator at the input, level control takes place at a voltage level that is quite appropriate for its optimum use.

As I see it, one of the main advantages of the tranny attenuator is the fact that as you turn down the volume, you step down the source's impedance. In contrast, turning down the volume with a resistive attenuator increases the source impedance, at first...as you continue to turn it down, source impedance begins to drop as the wiper approaches ground. But wait! Even though the source impedance is dropping, the source has less control over its load. Turning down the volume with a tranny attenuator is like shifting into a lower gear--the engine has greater control over the car's motion. Using a resistive attenuator is like keeping your foot on the accelerator and controlling your speed with the brake!

If you add a 1:1 input transformer to a resistive attenuator, you still have the disadvantages of a resistive attenuator, except its lack of isolation.

Another thing Lynn and I will be doing soon with the Aurora: a fully-balanced version, where the TX-102 is used as a phase splitter at the input. Since phase splitting and level control are accomplished at once, the circuit is elegantly simple.

I have had no problems so far driving the TX-102 with digital sources. As for phono stages, it gets a bit trickier. Phono preamps usually come in two varieties: RIAA only, and RIAA plus line stage. The ones with RIAA only often have a high output impedance, which won't drive the TX-102 well. The ones with an integral line stage end up having too much gain.

Several solutions have come to mind, and I'm planning to try more than one of them. What sort of preamp are you thinking of using? Maybe one of my ideas will be of interest to you.

Best regards,
Gary Dahl

• See my Aurora amps here...   (Open in New Window)

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