[net.audio] question

ron@trsvax.UUCP (10/31/84)

Does anyone have any experience with the Heathkit Sub-woofer system they
have?

	{microsoft,ctvax}!trsvax!ron

jf4@bonnie.UUCP (John Fourney) (03/29/85)

*** REPLACE THIS LINE WITH YOUR MESSAGE ***
Here's the situation:
I have two headphone signals (walkman + stereo keyboard) that I
want to mix together to hear on one headphone.
Can you 'Y' this stuff togther directly or do you need a mixer.
I tried mixing both left channels and both right channels but
the mixer attenuated the signals a lot.
Is is possible to mix speaker level signals?
Thanks
John

fish@ihlpg.UUCP (Bob Fishell) (04/02/85)

> *** REPLACE THIS LINE WITH YOUR MESSAGE ***
> Here's the situation:
> I have two headphone signals (walkman + stereo keyboard) that I
> want to mix together to hear on one headphone.
> Can you 'Y' this stuff togther directly or do you need a mixer.


> Is is possible to mix speaker level signals?


***     AC  T              YOUR     AGE ***

It depends on the output impedance and levels of the devices.  Generally,
it's not a good idea to "y" outputs together, since you'll frequently
wind up with two C/S collector circuits glaring at each other -- bad
for both of them, especially if they share a ground connection.

For higher impedance headphones, e.g. Sennheisers, all you need to
do is put a couple of resistors in series with each output device,
i.e.
               R1
device 1 o-/\/\/\/\-:----headphone
               R2   |
device 2 o-/\/\/\/\-'

R1=R2=150 ohm.

This will provide sufficient isolation, in most cases, to eliminate
any problems that might be caused by the amps loading one another
down.  If you have a floating ground output, you'll have to use
two sets of resistors, one on each lead.  This scheme will work
(I've done it) for headphone jacks as well as speaker drive
terminals, as long as you're driving a relatively high impedance.

If you have speakers or other low-impedance loads, you'll have to use
a mixer.

/_\_

				Bob Fishell
				ihnp4!ihlpg!fish

sjc@angband.UUCP (Steve Correll) (04/02/85)

> I have two headphone signals (walkman + stereo keyboard) that I
> want to mix together to hear on one headphone.
> Can you 'Y' this stuff togther directly or do you need a mixer.
> I tried mixing both left channels and both right channels but
> the mixer attenuated the signals a lot.
> Is is possible to mix speaker level signals?

Generally, no. A good audio amplifier strives to present an output (or
"source") impedance much lower than that of the speaker, so when you
connect two amplifier outputs together, each amp looks almost like a
short-circuit to the other.

Because high-powered hi-fi amps put out much higher voltage than
headphones require, many amps have a fairly large resistor between the
last amplifier stage and the headphone output. In such cases, you can
mix signals by connecting the headphone outputs together.  I suspect
that the Walkman lacks such a resistor, however, since its amplifier is
low-powered enough to begin with that it matches the characteristics of
headphones directly.

I don't understand why your mixer attenuates the signals too much. Does
the mixer have an output intended for headphones? A mixer output meant to
drive an amplifier probably won't produce enough power to drive headphones.
-- 
                                                           --Steve Correll
sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc

don@umd5.UUCP (04/03/85)

> Is is possible to mix speaker level signals?
> -John

[]
Yes, it is quite possible to mix speaker level signals, but one should
do so with a resistive network to maintain proper impedances in the
system. Use three 2.7 ohm (8 divided by 3 and standardized) 5 watt
resistors (5 watts audio is LOUD). Hook them together like this:

               Input 1 >-------------< Input 2
                        \           /
                         \         /
                          \       /
                           \     /
                            \   /
                             \ /
                              |
                              ^
                             Load

The resistors form the sides of the triangle. Cheap and effective.

-- 
-----------------------------------------------------------------------------
"Space, the final frontier .." Final, hell! It's the frontier of frontiers !!
-----------------------------------------------------------------------------
-==- IDIC -==-                                           (Thanks Bob!)

SPOKEN: Chris Sylvain
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aopal@wateng.UUCP (Ajoy Opal) (04/05/85)

> *** REPLACE THIS LINE WITH YOUR MESSAGE ***
> Here's the situation:
> I have two headphone signals (walkman + stereo keyboard) that I
> want to mix together to hear on one headphone.
> Can you 'Y' this stuff togther directly or do you need a mixer.
> I tried mixing both left channels and both right channels but
> the mixer attenuated the signals a lot.
> Is is possible to mix speaker level signals?
> Thanks
> John


Yes , it is possible, but if you attempt to mix signals at the output
directly one amplifier will basically be attempting to drive the output
of one amplifier against another...ie they would blow up, if it were'nt
for the short circuit protection provided by the makers of both
the stereo keyboard and walkman.

A crude way of mixing would be to use three resistors in a "Y" 
configuration, but this will result in some (maybe a lot ) attenuation
in the signal.


	   walman out  ---/\/\/\/\----|
			  20 ohms     |
				      |
				      ----   mixed output
				      |
				      |
				      |
	keyboard       ----/\/\/\/-----
      output	          20 ohms

the values are not exact. Larger values will attenuate the signal
but will provide better mixing. Larger relative vaules of the walkman
resistance shall decrease the amount of signal from the walkman, and
similarly for the keyboard output.

The best place to do the mixing is at some stage prior the output
section...and then feed the whole signal to a an output stage.

ray

klein@ucbcad.UUCP (04/05/85)

>                Input 1 >-------------< Input 2
>                         \           /
>                          \         /
>                           \       /
>                            \     /
>                             \   /
>                              \ /
>                               |
>                               ^
>                              Load
> 

Oops, doing a quick analysis of this network shows that the load presented
to each amp channel is 7R/11, or in this case, R = 2.7ohms, or 1.72 ohms.
The amp will have a very hard time here, and almost none of that power
makes it to the 8 ohm load.  Also, the load is driven through an R/2
output impedance, so damping factor suffers.

Using a resistive network will always mean losing power and damping factor.
A big transformer is certainly the best way to do this, ideally, but
expensive for good quality.  You could also buy speakers with two voice
coils, if you have that luxury and are only interested in mixing bass.

-- 

		-Mike Klein
		...!ucbvax!ucbmerlin:klein	(UUCP)
		klein%ucbmerlin@berkeley	(ARPA)

don@umd5.UUCP (04/08/85)

>>                Input 1 >-------------< Input 2
>>                         \           /
>>                          \         /
>>                           \       /
>>                            \     /
>>                             \   /
>>                              \ /
>>                               |
>>                               ^
>>                              Load
>> 
> Using a resistive network will always mean losing power and damping factor.
> 		-Mike Klein
[]
Ackk ! Attack of The Memory Bug! (Thanks Mike!)
It didn't seem to me that damping factor would be of any consequence for
the asker's application, and since he was using low power anyway (headphones),
and what's a half a watt among friends, anyway?
The revised schematic is:
              Input 1 >---/\/\/\---o---/\/\/\---< Input 2
                                   |
                                   |
                                   |
                                   |
                                   ^
                                  Load
The resistors form either half of the top of the "T" and the "|" portion.
[Look at the referece book before you post something off the top of your
head !!! ... Let that be a lesson to you all !! :^) ]


-- 
-----------------------------------------------------------------------------
"Space, the final frontier .." Final, hell! It's the frontier of frontiers !!
-----------------------------------------------------------------------------
                                -==- IDIC -==-

SPOKEN: Chris Sylvain (transient user of Don Preuss' account)
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