Speaker Line Out

[tubenit]

About 8 yrs ago, I built an amp for a "zero volume" stage (something I'd never heard of prior).  The "line out" arrangement that I used looks to be a simple paralleling speaker jacks.  It worked well for yrs.


I'm wanting to do this again on another zero volume amp head.  This time the amp will use 6BM8 tubes if that makes any difference?



Is this a reasonable approach?  OR ........... is there a "better practice" approach that would be wiser to consider?


with respect, Tubenit

[sluckey]

Why not build it into the amp and eliminate that red box and external dummy load? I recently worked with this circuit from a new Magnatone Twilighter amp. I added the 8Ω/25W just for you. Simply unplug your speaker plug to engage the dummy load for zero volume. The three line out components can be mounted between the speaker and line out jacks.

[tubenit]

Perhaps unfortunately, I didn't know one could be built in.   


And I've already purchased the humongous 100w/8R dummy load resistor and the Hughes and Kettner Red Box.


The Red Box does have some "cabinet emulator" features that did seem useful on the previous older model one that I put on the other amp head.


So, in light of that, I'll probably stick with the Red Box approach.  Note the schematic doesn't match the photo of the Red Box exactly as they are different versions.


If I am understanding you correctly, your version would still have allowed using a speaker plugged in ........... OR disengaging the speaker which would automatically have engaged the dummy load resistor?  Is that correct?   


Does the use of the 6MB8 tubes change the paralleled speaker jack approach ........... OR is there something else I need to add to the line out?


With respect, Tubenit

[sluckey]

If I am understanding you correctly, your version would still have allowed using a speaker plugged in ........... OR disengaging the speaker which would automatically have engaged the dummy load resistor?  Is that correct?

That's correct.

Does the use of the 6MB8 tubes change the paralleled speaker jack approach ........... OR is there something else I need to add to the line out?

The type tube is not a factor in this.

I'm curious as to why you want a 100 watt resistor for a 7 watt amp?

Does your sound system "require" a balanced XLR input? Most will accept an unbalanced phone plug also.

[tubenit]

Sluckey,    THANKS for the replies and the direction !   


The reason I used a 100w dummyload last time is because I wasn't confident in what I was doing and went the waaaayyy overkill approach. And because the one I bought seemed easy to hook up to wires into a male speaker jack.  And I knew I could mount it on the cabinet head ceiling as the dummy load had holes to put screws thru to mount it to a piece of wood. 


When I looked at the other types of dummy load resistors, I couldn't figure out if they needed to be attached to the chassis to cool off enough?  I figured at that previous time that the 12w amp wouldn't get the 100w dummy load resistor too hot to be a concern on a piece of wood?


Simple answer is I didn't know any better. 


I don't know for sure about the balanced XLR being needed/required this time.  My understanding is the sound guys wanted that the last time around that I did this. Whether it was actually "needed" or not ............. I don't know?   However, it did work well and I heard it literally 100's of times in a band setting and thought it sounded great.


So, I was trying to recreate what I knew worked last time.


Now, I noticed the Red Box 5 is different than the Red Box 3 that I previously used.  The Red Box 3 had a "line in" plug, so I'm guessing my thinking was I needed a "line out" from the amp head going into the "line in" plug/jack?


However, I am seeing this is very simplified with the Red Box 5.   I go from the amp into the "IN" and then from the "THRU" to the dummy load.  On the other end of the Red Box is the XLR direct jack.  So, I'm gonna try it that way and I think it should work just fine. 


Once again, I was over complicating and over thinking my approach. 


THANKS for the help!  Always appreciated. 


with respect, Tubenit

[CascoSieg]

Clarification about Sluckey's option... it looks like the 1uF cap and 470R would form a low pass filter. I assume this is to tame the sometimes tinniness of line out signals?

Thanks!

[thetragichero]

i have built a single-ended el84 amp with a shunt tip jack and attenuated line out (added a pot to get some level control). if i were to do it again i'd certainly add a low pass filter as some of the high frequencies that normally would not come out of the pretty poor speakers (in terms of frequency response) we love for guitar sound pretty terrible direct. so a cab sim and balanced xlr are certainly beneficial (along with a ground lift) for sending to FOH

i played at a church that was zero stage volume and while at first it was an adjustment from super loud stages i came to prefer it. in ears with an adjustable mix (bury the singers in the mix since playing bass i don't really need to hear them) and a click track helps folks play super tight. the only negative is that a competent sound tech is an absolute MUST (just because the pa has a ton big subs doesn't mean the bass should only come out of there!)

[AmberB]

One nice thing about a way oversized load resistor is that it won't get hot, even when you crank the amp...

[pdf64]

... it looks like the 1uF cap and 470R would form a low pass filter…

I think that the 47ohm resistor will dominate the ‘R’ term of the LPF corner frequency calculation.
So more like 3k4Hz, rather than 340Hz.
I think of it as the source resistance of the voltage source feeding the cap.

[PRR]

> 47ohm resistor will dominate  .....   like 3k4Hz

Both enter into it, making roughly 47.7Ω 42.7Ω and about 3,764Hz. Which makes perfect sense for a tenor string.

EDIT: 2 and 7 look a lot alike....

[pdf64]

> 47ohm resistor will dominate  .....   like 3k4Hz

Both enter into it, making roughly 47.7Ω and about 3,764Hz. Which makes perfect sense for a tenor string.

I suspect a typo, should read 42.7Ω 
Calculated by the 47 and 470 being effectively in parallel, and assuming that the amp's output impedance is 0Ω.
If there was no NFB, then the amp's output impedance would be non zero, but dunno what the large signal level output impedance might be for a 6K6 AB1 amp; maybe somewhere in the range 50 to 100Ω? Whatever, that would add to the 470Ω term.