KISS ... Keep It Short & Simple

Hi!

Some people like it simple and only listen with one source. In case of systems with my transformer coupled D3a phono stages this can simplify things considerably. Said phono stage is well capable to drive the inductive load of an AVC. Hence a passive volume controller is all that is needed if the system gain is sufficient.

Just a single knob for volume control.

One set of inputs and two parallel wired outputs (just in case).

I usually prefer a proper active line stage which decouples the sources from any difficult load and in turn provides low output impedance to drive long cables to power amps. But in controlled environments like those this unit is intended for, a passive one will do.

Best regards

Thomas

Passive Transformer Volume Control Unit

Hi!

Here some photos of a passive transformer volume control which I just finished.

Independent volume control switches for left and right channel. The two jacks on the left side are for headphones (both wired in parallel).

The unit uses Intactaudio AVCs (auto former volume control). Here a shot of the inside before it got mounted into the wooden enclosure:

All the wiring is done with teflon insulated solid core silver.

The backplate :

Just single sets of RCAs for  inputs and outputs.

The unit is similar to the passive line stage which I showed last year, except that it has no input selection switch but just a single stereo input.

Passive AVC units like this are meant for use with low impedance sources which are known to be able to cope well with the inductive load. Otherwise active line stages are my preferred solution.

Best regards

Thomas

The Modular Preamplifier, Part 3: passive line stage

Hi!

The next module is finished, the passive unit with transformer volume control and input selector for line level signals.

A bit more work to wire this one up compared to the MC transformer shown in the last post, but still a rather quick job.

Slage autoformers are used for the volume control, ELMA switches for input select and attenuator. All wired up with solid core silver.

For best sound the autoformer needs to be driven from low output impedance sources like the phonostage modules which will be built to complement this modular system. Separate buffer stages will be used for sources which do not have the drive capability. The resulting sound can be tuned be using bufferstages with a specific sonic charactder to complement the source. For example a UX201A buffer stage for CD sources which have a tendency towards the harsh side.

The line control unit has the same size as the MC transformer:

This unit is not to be confused with the usual passive resistive controls which tend to have a high output impedance and thus are influenced by cable parameters and input impedance of power amps. It will be used with low impedance sources of 300 Ohms and less only. So the worst output impedance will be 300 Ohms. Much less at lower volume settings.

The only reson I put it into a separate box is to adhere to the modular approach and to be able to experiment with different gain stages without the need to put a volume control in each of them. This will allow to mix and match different gain stages according to taste and amplification needs.

This approach will be helpful to experiment with all directly heated preamp stages and to see how far upstream DHTs will be practical. Starting with the output section of the phono preamp, mixing it with an indirectly heated input stage. Later also a DHT phono input stage will be added.

I also plan to build a linestage in this chassis style with input selctor and volume control built in. This will use one of the longer chassis shown in the previous post.


Best regards

Thomas