Bridged Center Loudspeaker, Dope from Hope

[WMcD]

There have been requests for the DfH on the mini-box mixer used by PWK to generate center channnel. I will post it in the next window.

Gil

[WMcD]

There you go. I was getting errors when the file name had a comma in it. You learn something every day.

Center Loudspeaker.pdf

[Daddy Dee]

Thanks Gil.

[WMcD]

D.D.,

Yes, I set this up in the interst of your mental health.

Maybe now your mental health is improved. Or maybe not. Smile.

It is a frequently requested subject. I believe some things get lost at the bottom of strings or are difficult find in a search.

Gil

[Daddy Dee]

Gil,

I am indeed feeling better. Thanks.

[Deang]

I take it then that I wasn't the only one to bombard Dee's mailbox?

Gil -- have you done this?

[dbflash]

I wish Mark would offer this box along with his Peach and Blueberry. It could be made to look like a mini preamp. Put in a nice wooden box,stained to match his preamps.

Danny

[Daddy Dee]

Danny,

That is a neat idea. That polycarbonate (or whatever it is) face on Mark's preamp would look cool on a minibox. Match the knobs on the BB and Peach,... sweet!

[Seadog]

I built a minibox that I am not using at the moment (I'm currently using the derived center out built into my Scott 299D). Although a minibox is easy enough to build, if someone wants to borrow mine to play with, let me know. It is based on the DFH Figure 2 (line level), but it has a fixed resistor (12K ohms) instead of the variable, so the center speaker volume must be adjusted using the center amp volume control. If you don't have a mono amp, one side of an integrated stereo amp will work for the center amp.

[WMcD]

I've not set up a system with the minibox and separate amp.

Instead, I use three RS 70 volt line transformers to generate a combined signal at the speaker level. No extra amp in needed.

Gil

[Deang]

O.K. Gil, this sounds interesting as well. Are you saying you are achieving this with a two channel amplifier?

[Daddy Dee]

I'd be interested in hearing more about that solution.

[WMcD]

Yes, just a two channel amp.

I'll have to draw it up and run some tests for the consideration of critics.

Essentially you power the speaker side of two transformers from L and R. Then the 70 volt outputs are wired in series to power the 70 volt side of the third transformer. This third transformer is getting L plus R.

Then the center channel speaker is hooked to the speaker side of the third transformer.

You can play with the taps to get the correct level.

This was inspired by the John Eargle article (work done at Klipsch) from the Klipsch papers. This will be next weekend's reading if no one has objections.

PWK had a scheme early on. He used a tube amp having output transformers. He'd wire the center speaker directly across the two 8 ohm (or whatever) output taps. That is what I infer. So you were getting R - L. That is to say, the voltage difference.

Remember, when you consider the voltage across a device, it is the difference of voltage in two leads.

PWK wrote an article arguing that two channel stereo recordings have random phase, at least live ones. There was a picture of a 'scope with L and R to the respective inputs to the scope. It showed a circular hash. A mono component would be a line.

I believe this was to say, the center with L-R is not different than L+R because of the randomness of the information in his recordings.

This certainly is not true today with the use of mixing boards where a vocalist is mixed hard center with L and R in phase. There were some ads for devices to remove a vocalist from a recording . . . I suppose to allow a Karioki (sp) thing. It must take the difference between the channels and thus a center mono vocalist goes away.

In the Eargle article the center is still wired across the output "hot" leads. But the grounding of one channel (say Right) is reversed, and the speaker too. Therefore the polarity of the speakers is preserved despite the fact that the output of the Right is inverted Right . . . or minus Right.

But now the voltage delivered across the taps is Left minus a minus Right. Voltages do work like math so minus a minus is a plus, and you get Left plus Right.

There is I believe a little assumption here. It is that the number of windings between the 16 ohm tap (now ground) and the 8 ohm tap (on one side) is the same as between the ground tap and the 8 ohm (on the other). Something there give me pause.

= = = =

I haven't written it up earlier because I wanted to run some impedance and freq curves.

One natural concern is that Eargle's and my system imposes a load on the amp which it doesn't like. Per the Eargle article and PWK, the center channel should be 6 dB down. That means that, in my set up, your imposing a load between the amps of 4 times the impedance of the center speaker. So, nominally, we're talking 32 ohms.

There may be some issues if you're using a less sensitive center speaker, like a Heresy with K-Horns. I've used the inputs to the transformers as auto transformers. I.e. feed the 16 ohm taps and wire the Left and Right speakers across the 8 or 4 ohm taps.

I do it on the thought the amp might do better with less of a load overall, which that wiring accomplishes. However, it does knock down the L and R power, and hence a Heresy could probably be used.

Best,

Gil

[Seadog]

"This was inspired by the John Eargle article (work done at Klipsch) from the Klipsch papers. This will be next weekend's reading if no one has objections."

No objections!

[Daddy Dee]

Next weekend works for me!

[Deang]

I'm glad Erik brought this subject front and center.

Seadog, can I borrow that box for about a week or so?

Thanks Gil, I'm looking forward to the reading.

[Seadog]

Dean,

PM me your shipping address and I'll drop it in the mail.

David

[artto]

Ive been using a center channel (Belle between to Khorns) since around 1979. My center channel box is the simple one with only one potentiometer on the center channel for individually controlling its volume level (using separate mono power amps for each channel. They have stepped level controls, not the continuously variable type as usually found)

I can assure you that over the years Ive found you need more precise control over the volume level of the center speaker. There are many variations in the way recordings are produced that affect the way the center speaker reacts to the program material. For instance, if the source material was recorded as a true stereo recording, such as was usually done in many early classical recordings (such as Mercury Living Presence) , it may be perfectly acceptable to have the normal 3dB gain at the center channel. If the recording being played back is a monaural recording, youll probably find that the bass is too excessive from the center channel, distinctly bringing attention to the center speaker as a sound source, especially in the bass range, thereby necessitating the need to reduce the center channel to as much as 6dB or more. Then there are the differences in analogue verses digital recordings. Analogue playback sources tend to have much more left and right channel crosstalk than their digital counterparts. This too, produces much more bass output from the center channel. On many fully digital recordings you may find that you can have the center channel turned up to the same volume (-0dB) as the flanking speakers. And then there are the deviations in right to left stereo balance of certain recordings (theyre not all balanced correctly to say the least). To get the imaging precisely correct you really need to use a derived center channel circuit that allows you to precisely adjust the level of all three speakers as the balance control in most preamps is usually not enough. Ill be building the 3 potentiometer version, hopefully in the not too distance future. It will employ the same military grade high precision potentiometer currently used in my existing box (10 turns full on to full off). If all three of your power amps (channels) have a volume control you can circumvent this to some extent. But I still find I have to calibrate the right and left channel balance with an SPL meter at close range at each speaker using a test tone such as a 1Hz sine wave. Then you can dial-in the center speaker to your liking from there.

[Daddy Dee]

Hope there are a few who still might be following this thread. I think I want to tackle the minibox project.

There are a few questions I could use some help with, which will reveal my ignorance even more than usual.

Of the two miniboxes, fig.1. and fig.2, is there any advantage of one over the other? In figure 2, does the figure two method of deriving the center from the speaker level leads take any appreciable power away from the power amps?I'm thinking probably not, but with flea powered SET, I wonder.

Figure 1. is my preference to build. In reading this schematic, are there seven resistors?

Another thing I'm not clear on: RCA jacks have two poles don't they? center and outside? What I'm not clear about is how the schematic in Fig 1 and Fig 2 are different. The fig. 2 has a single lead connection for each channel plus one ground. Is the outside connection on the RCA just go to the alum box?

One thing I don't know how to read is how the RCA output and pot are connected. I see the black arrow from the output RCA plug that looks like it goes to the center of a resistor. I know that the resistor has a lead on each end, so don't know how to interpret this.

Any help is appreciated. When explaining, please talk real slow. Thanks.

I don't know much, but am interested in learning.

[Daddy Dee]

Mark, I could just imagine that sitting on top of the Peach. Sweet.