Center Channel Mixer with Transformer

[WMcD]

Skip, today, nudged me again about a schematic for a center channel adder.

I have been remiss in mentioning it on the forum and then not posting it. I wanted to do some serious testing and write up all the findings. However, that will take some time.

The medium-short and disorganized story is as follows:

The virtue of a center channel goes back to the '30's as shown in publications by Bell Labs. Essentially, two speakers for stereo are good, but a center channel speaker running Left plus Right in the middle allows better placement of the sonic image and allows for a better sound stage overall. It fills the hole. All the experiments point in the same direction.

This is now available in HT amps. But not everyone may want to go that way. Years ago, it was not so easy to achieve at home.

Early on, PWK was experimenting with two channel recording and using a "phantom" center channel for playback. (Stereo was high tech in those days. He was a pioneer of reporting "hands on" results, not hype.)

He opined, and gave good evidence, then, that the phase of the derived center was random. Essentially, L+R and L-R were indistinguishable, with a few comments that perhaps phase should be honored.

That issue did not hold sway for too long. And certainly doesn't today. L+R is the right way to go.

PWK implemented a passive low level mixer to allow the use of an amplified center channel. Of course this requires a third amp. This resistive mixer is the subject of a Dope from Hope.

In the collection of "Klipsch Papers" was an article by John Eargle. Smart guy. This may well have been with the assistance of Klipsch. The schematic there shows a set up for L+R. Essentially, there were two Klipschorns at the flanks and a Model H at the center. The grounding of the tube output transformers was altered to allow a bridged center channel to receive L+R. An L pad was added to allow adjustment of the level of the center channel Model H.

I don't have that here to give you.

Let me add that the Model H in those days was specifically designed to be a center channel. The bass driver was about 6 dB down and the mid and tweeter were up at the sensitivity of the K-Horns. That changed later when the Heresy became a speaker for "flat" response.

I built a set of speakers which are basically K-Horns and a "bigger Belle" for the center. All the same midrange and tweeter.

I drive the things with a hand me down Sony transistor amp. The issue became, how to drive the center channel.

I struck upon a setup using Radio Shack 70 volt line transformers. Essentially, this follows the Eargle circuit. But it all outboard of the amp. No modifications to the amp.

Center channel is wonderful. I must comment that my personal experience follows all people who experimented and published before. Quite uncanny. These results were from typical pop and classical records, not DVD audio. Nothing specifically mixed for center channel.

You'll see in the attached my concern that this set up may present a difficult load to the amp. I've had no problems.

My experience is with a center channel speaker which is has equal sensitivity to the flanking units.

An important issue is how loud should the center channel play. All evidence is that it should be about 6 dB down. This is not the same issue of the Heresy bass driver issue.

The 6 dB down issue is linked to whether the circuit will yield good results without imposing a bad load.

If you're of a mind to experiment, please do consider this. I'd be pleased to have your comments. I may be able to present some more info in a week.

I'd be pleased to answer questions.

Regards,

Gil