Design for my job at station

[Johnny1234]

Hi all any advice is greatly appreciated.

I need to modify a box at work. The original has an amplifer driving a 2 inch 16 ohm speaker.
I need to:
1 Add a line out
2 Have a switch to alternate between speaker and line out
3 Add a volume control for the line out and speaker

Below is a little diagram of my thoughts. All ideas are greatly appreciated, but simpler is better.
Many Thanks

 

[Johnny1234]

Sorry I know the pot has to be before the switch my bad.

 

[ccaudle]

1 Add a line out

How is the signal delivered to the amplifier in the box? Do you already have access to a line level signal coming in, or the speaker is part of something else and you cannot easily access as line level?

What is the level sent to the speaker? (presumably pretty low since it is a 2" speaker, but you will still need to know typical and max to determine how to adapt to line level).

What nominal "line level" do you want? What consumer equipment considers line level and what pro equipment considers line level is quite a bit different.

 

[Johnny1234]

The box is for a college game show that we will host. Signal is derived internally, it just beeps lol. I don't have easy access to the line level input. Speaker level very low I would imagine about 1 volt. Nominal level would be for pro audio input.

 

[Johnny1234]

level to speaker about 17 volts peak.

 

[Brian Roth]

Sorry I know the pot has to be before the switch my bad.

Actually, the pot is in a good location for the mod. However, the way it's drawn means the amplifier's output will be shorted when the pot's setting is turned "off".

Bri

 

[Johnny1234]

Actually, the pot is in a good location for the mod. However, the way it's drawn means the amplifier's output will be shorted when the pot's setting is turned "off".

Bri

Great point, I will put a 100 ohm 5 watt resistor in series with the wiper of the potentiometer.

 

[ccaudle]

I will put a 100 ohm 5 watt resistor in series with the wiper of the potentiometer

Or you could just wire it more typically, where the signal connects to the top of the pot, and the wiper connects to the output.

 

[Johnny1234]

Or you could just wire it more typically, where the signal connects to the top of the pot, and the wiper connects to the output.

Thanks even better. Any ideas for a transformer, it is very low fi just a beep.

 

[Brian Roth]

Thanks even better. Any ideas for a transformer, it is very low fi just a beep.

What are the specs for the input being fed from the beep-o-matic <g>? +4 dBu or -10 dBV line level, low Z mic level, guitar amp instrument level, etc. etc.



Sorry....couldn't resist....

Bri

 

[Johnny1234]

What are the specs for the input being fed from the beep-o-matic <g>? +4 dBu or -10 dBV line level, low Z mic level, guitar amp instrument level, etc. etc.



Sorry....couldn't resist....

Bri

LOL I love it made me smile after a tough week at work. Input will be like a Makie line input 10K ohms.

 

[Brian Roth]

level to speaker about 17 volts peak.

OK....curious how you figured that voltage spec. Oscilloscope? Regardless, seems like a step-down xfmr would be a good idea.

I'll pick this up later after I have some tacos for dinner<g>.

Bri

 

[Johnny1234]

OK....curious how you figured that voltage spec. Oscilloscope? Regardless, seems like a step-down xfmr would be a good idea.

I'll pick this up later after I have some tacos for dinner<g>.

Bri

Thank you Brian, Yes measured on a oscilloscope. Beeps to fast to measure with rms volt meter.

 

[Brian Roth]

Thank you Brian, Yes measured on a oscilloscope. Beeps to fast to measure with rms volt meter.

OK.....peak or peak to peak?

Regardless, maybe something like a xfmr "rated" for 10K:600 Ohms. Nice step down ratio to drop the signal to a reasonable level. I seriously doubt you need the beeps to slam the mixer's VU meter....lol.

Damn....their prices have gone up:

https://edcorusa.com/products/wsm-s...-matching-transformers?variant=41117605298363

What do you have in your junkbox? Like, any xfmrs intended for "70 Volt" loudspeaker distribution systems (ie the 8" speakers attached on frisbies in the ceiling tiles at Applebees)?

Bri

 

[Brian Roth]

What do you have in your junkbox? Like, any xfmrs intended for "70 Volt" loudspeaker distribution systems (ie the 8" speakers attached on frisbies in the ceiling tiles at Applebees)?

Bri

Something like this....much cheaper:

https://www.parts-express.com/70V-10W-Line-Matching-Transformer-300-040?quantity=1

Bri

 

[Brian Roth]

@Johnny1234 ....Just noticed in your first post that you wanted the volume control to vary BOTH the speaker and line output levels. Now I'm second guessing myself on everything! lol

Without "being there", I have a variety of "what if's?" in my poor dumb brain!

I've worked off and on at commercial radio stations over decades and many "one off" projects came up. In YOUR case, is the little internal speaker in Beep-o-matic ONLY used when not on the air (ie, audition in the control room)? Or, is it also needed to hear beeps for anyone sitting in the control room who don't have headphones AND also feed into the mixer?

Damn....I'm over thinking this! <g>

Bri

 

[Brian Roth]

ARRRGH! I looked at the picture you posted...and see a switch for "speaker"/"XLR" and a level pot of some sort, and a male XLR. ???????????????

In post #4 you said "Speaker level very low I would imagine about 1 volt."

Post #5 you said 17V peak into the speaker. Converting to RMS (12V) that is 9 Watts into 16 Ohms which should send a 2" speaker cone ejecting from the box as confetti.

Now exactly what are we doing here? I am just a old, cranky tech! ;-) But, I am 10000% certain an output transformer should be used for galvanic isolation in this science fair project <g>....and I say that with a warm heart.

I worked part time at a struggling FM station when I was a Sr. in High school (1972) and had to figure out how to interface one of these beep-boxes (I built it from a magazine article and mail order kit) into the KWHP production room system.

https://www.ebay.com/itm/156469073284

Bri

 

[ccaudle]

I'm over thinking this!

First rule of a successful engineer is make sure you give the customer what they need, not just what they asked for.

Converting to RMS (12V) that is 9 Watts into 16 Ohms

If it was 17V peak-to-peak instead of peak that would only be about 2W, more reasonable for short term power into a small speaker.

 

[Mihobro]

A lot of this depends on the headphones that you will be using, Some of the low impedence 8 Ohm Sony headphones will get the same power as the speaker, and be horrendously overdriven, High impedance headphones like the 120ohm or so AKG's will be right in their range 1 to 1.5 volts, so the padding of the headphones out will depend on the impedance of the headphones used. To illustrate, we used to use 50 Watt (about 40V @ 8 ohms) power amps to run the headphones in a large session with up to 40 AKG phones sets wired in parallel on the power amp output with no padding, because it did not need it, however it is prudent to have at least an 8 ohm resistor in series to protect the amp from a low impedance situation like a shorted headphone cable. Modern transistor circuits conveniently do this as power output goes up as impedance goes down. So just by using high impedance headphones with this kind of amp, with a safety resistor, you may not need to pad the phones out. Also consider using a jack with switches in case you are alternating, not simultaneously using the speaker and headphones.