Connecting An Audio Output to 20 Headphones...

[Shahriar]

Hi

I want to connect a VCR OR VCD audio output to
twenty headphones, this device is to be used in
an English Institute Laboratory, as you can see in the picture
i have name it Black box, The second problem is connecting
twenty Microphone to the Black box, What kind of Amplifier is
suitable?
The swiches are to be controlled by the teacher.
The quality of Sound is important...

ThanX a million

 

[Shahriar]

Sorry, here is the Picture

View attachment 35606

 

[audioguru2]

Shahriar,
Your "black box" headphones distribution amplifier will be a 10 to 20W (for 8 ohms load) PA amplifier with 2 or 3 line-level inputs.
Each headphone connects to its output through a 120 ohm/1W resistor in series (use 2 for stereo).
Your microphones "Amp" will be a 20-input microphone mixer that connects to one of the inputs of the headphones amplifier.
Contact a local PA equipment dealer that sells worldwide TOA brand.

 

[Shahriar]

Dear Sir
Thanks very much for your reply, But some questions

View attachment 35615

 

[audioguru2]

Shahriar,
Your FET Audio Mixer will work fine if you select a FET that has a correct amount of self-bias current.
But your Microphone Mixer looks like a line-level mixer, that needs a separate mic preamp for each mic. If you design it with enough gain for direct mic connection, and you use standard 600 ohm dynamic mics, then its gain will change for all mics if only one input control is varied.
Maybe you don't need 20 mic preamps and 20 controls. Instead, use one mic preamp like your circuit, and 20 input switches. Also use a mic transformer to increase the mic's level and impedance.

 

[Shahriar]

Dear Sir
Thank you so much for your valuable recommendation

View attachment 35598

 

[hotwaterwizard2]

CHECK THIS OUT!
http://www.paia.com/hda.htm
http://www.paia.com/hdasch.gif


NE5532 Datasheet Click Here

NE5532 Application Note Click Here

View attachment 35628

 

[audioguru2]

Shahrier,
1) Check the spec's for your VCR to see whether it can drive the parallel combination of the TV and amplifier.
2) You have 2 posts about this subject. Please see my reply in your other post about using gold-plated relay or switch contacts for audio.

 

[Shahriar]

Hi,

I hope I can find such relays.

Can anybody tell me what is the exact meaning of RIAA (Recording Industry Association of America)?
Is Normal VCD or TV audio ourput a kind of RIAA?

ThanX...

 

[audioguru2]

Shahrier,
Relays and switches with gold-plated contacts will mention their use in "dry" (very low voltage) circuits. They will say "AU clad or AU flashed", and are usually limited to 24 or 30VA.
A silver-plated high-current contact relies on high voltage or high current to burn off the corrosion. They might pass a microphone signal when new, but will fail after a short time.

Old amplifiers have an preamp input for magnetic phonograph pickups. They use a standard RIAA frequency response equalization which boosts the bass and cuts the treble, the opposite to the response that is used during recording.
When they cut the bass during recording, the modulated grooves don't take too much space on the vinyl record, then the RIAA preamp boosts it back to normal.
When they boost the treble during recording, the RIAA preamp cuts it back to normal, then the hiss is also cut.

 

[hotwaterwizard2]

I guess My circuit was not what you needed even though your topic asked for such a circuit. I found it and posted it and Datasheets. It took me about 2 hours. You guys just Ignored it. What a waste of my time. Maybe I should just forget the whole Helping thing all together. What a let-Down.

 

[MP1]

Hotwaterwizard, I see nothing wrong with your post. It looks like they have changed the subject without posting a new topic. Believe me, your efforts are not a waste. Others will appreciate it.

others:
Gold plated relay contacts? give me a break. Better for arcing? Go ahead and waste your money!

MP

 

[hotwaterwizard2]

For what it is worth here is a mic Mixer Circuit to go with the other.

 

[audioguru2]

Hotwaterwizard,
Thanks for posting your headphones amp circuit and datasheet. It is really appreciated to see how things can be done differently. I am sorry that we didn't acknowledge your post, it is because the topic's author decided to use an amp that he already had, a few days before your post, and we moved on to discuss mic mixers.

MP and others,
Many relays and switches with gold-plated contacts are manufactured and sold for the specific purpose of reliably switching low-level (dry) signals such as from microphones. Their gold plating doesn't cost much or any more than higher-current silver contacts. Silver corrodes and a spark burns it off. Gold doesn't corrode and a microphone doesn't make a spark.
A typical relay with silver contacts has a minimum current rating for the contact, so a spark can burn off corrosion. A gold contact doesn't have a minimum current rating, therefore is spec'd for "dry circuit". See contact ratings in Tyco's relay datasheet here:
http://relays.tycoelectronics.com/datasheets/R10_DS.pdf

 

[audioguru2]

Hotwaterwizard,
Thanks for the good mic mixer circuit.
I like the way that it uses an opamp to provide a "VR" supply voltage for the electret mics.

 

[MP1]

MP and others,
Many relays and switches with gold-plated contacts are manufactured and sold for the specific purpose of reliably switching low-level (dry) signals such as from microphones. Their gold plating doesn't cost much or any more than higher-current silver contacts. Silver corrodes and a spark burns it off. Gold doesn't corrode and a microphone doesn't make a spark.
A typical relay with silver contacts has a minimum current rating for the contact, so a spark can burn off corrosion. A gold contact doesn't have a minimum current rating, therefore is spec'd for "dry circuit". See contact ratings in Tyco's relay datasheet here:
http://relays.tycoelectronics.com/datasheets/R10_DS.pdf

I am not convinced. But then I am basing what I say on actual applications with both and not someone's glorified data sheet. Besides, a mechanical relay has a life of X number of ON/OFF states. All materials used for contacts far exceeds this spec. That is, unless you abuse the specs.

MP

 

[audioguru2]

I always honour datasheet spec's and ratings so that my designs are reliable.
I guess that Yamaha didn't know about gold contacts on my intermittent stereo receiver. I must clean off the silver corrosion by clicking the selector switch back and forth a few times.
And GM ignored the current rating of the ignition switch and left out the starter motor's relay on hundreds of thousands of cars, and had to recall them all to install the relay. They simply caught fire!

 

[MP1]

Sure, the data sheet must be followed. But somewhere in there you need to distinguish between advertisement and technical notes. I have heard you get on your soap box more than once about Watts ratings of power amps. Do you honor these manufacturer's specs? I think we both know that we are not talking about data sheet technical specs but instead, we are talking about glorified hype.

BTW... the GM recall was not because they forgot to add a relay to the ignition. They had to add a time-out relay for idiots whose cars would not start and held the key in the start position for a long period of time even though the starter is not cranking. Try this with an older car and you will also get smoke.
Hundreds of thousands did not catch on fire. There were 159 incidents. I have one of these "recalled" cars and I would not hesitate to buy one again...I have not taken it in for the fix.

MP

 

[audioguru2]

Smoke?
Many years ago I used the starter motor to propel my car to the nearest gas station. A fair distance. No smoke, no problem. The relay at the starter motor conducted the high current very well.

The ignition switch and its wiring were catching fire on those GM cars. Mine had a heavy duty relay added near the battery, without a timer. The starter motor ran faster after my car's recall. Doesn't that relay reduce the current flowing through the ignition switch and its wiring, and allow most of the battery voltage to reach the starter?
It's no wonder that those cars wouldn't start during winter.
Get your relay added soon. Your ignition switch is strained each time you use it. When it finally melts, you'll have a nasty fire between your legs, unless it's a Saab.