Amplifier question - output to 600 ohm headphones

[Mick]

I have built a small amplifier using an LM 386 which is connected to a pair
of 600 ohm headphones which recieves an output from a gold detector.
the problem im having is that there is only a small amount of improvement
over the output without the amplifier.
Any help would be appreciated.

thanks in advance
Mick

 

[Rheilly Phoull]

The LM386 output is for a load from 4 to 16 ohms, probably the most
efficient at 16 ohms.
You need a transformer to put into the output that has a primary impedance
of 16 ohms (on the output of the chip) and a secondary impedance of 600 ohms
(for the phones). I imagine anything that has close to those values would
give you extra performance as desired.

 

[Phil Allison]

I have built a small amplifier using an LM 386 which is connected to a pair
of 600 ohm headphones which recieves an output from a gold detector.
the problem im having is that there is only a small amount of improvement
over the output without the amplifier.
Any help would be appreciated.

** So you have a "gold detector" ????????????????????

Care to share the design with the rest of the world ???????

Have you got a patent ?????????????



.............. Phil

 

[Trevor]

600ohms would eat up the spare power in an instant, the LM is use to driving
4 / 8 ohm speakers, most headphones are 32ohms, 600ohm is a MASSIVE drop in
output power

Big deal, headphones don't require much power. High impedance headphones
require more *voltage* than low impedance types, and that's where the
problem lays.

Trevor.

 

[Rheilly Phoull]

I can see that 600 ohms would definitely be a massive drop in power which is
exactly what is being complained about. I would like an explanation of how
600 ohms would "eat up the power" though ???

 

[Mick]

Ha ha, my mistake. To have a gold detector which only found gold now that
would be an wonderous device.
In future i shall stick to calling it a metal detector which you find more
often than gold.....

But seriously if someone can direct me to a suitable coupling transformer
for this application then it will help me a lot.

thanks
mick

 

[amstereo-matt2]

ok, easiest way to look at it. an 8ohm 100watt rms amp, when connected to a
4ohm speaker will deliver near enough to 200watts rms. BUT going to 16 ohm
will deliver 50 watts rms.

4 ohm = 200
8 ohm = 100
16 ohm = 50

double the resistance, half the power.

the CHAMP (lm386 based jaycar amp kit) delivers near to 1 watt in 4 ohms.
you are talking about driving this in to 600 ohms thats a stepping of 7 1/2
times, so the reverse is 0.007 at best. , my sony 'style' headphones
sensitivity rating is 96dB .01 watt 1mm so you can see the problem here.
your headphones must have been designed to take a good output from a home
amp.

 

[Rheilly Phoull]

Sure, I have that on board
My problem is how increasing the load impedance "Gobbles" the power. To my
limited knowlege I would think that the power used would be much less with
the higher impedance.

 

[amstereo-matt2]

the amp has only so much voltage / current it can distribute, when the ohms
goes up, the resistance goes up, and when the resistance goes up, less power
is getting through. think about it this way, resistance, the higher it is
the smaller the water pipe, your tryna gravity feet water through the tip of
a surynge when its only got enough pressure to feed through garden hose, Bad
analogy but first thing to mind.

 

[Eugene Rosenzweig]

P = V^2 / R, hence from 4R to 600R we have 150x decrease assuming V stays
constant.
Better leave the waterpipe analogies for highschool, they may be good for
getting the feel of things but don't really wash in a quantitative argument.

 

[Mainlander]

I have built a small amplifier using an LM 386 which is connected to a pair
of 600 ohm headphones which recieves an output from a gold detector.
the problem im having is that there is only a small amount of improvement
over the output without the amplifier.
Any help would be appreciated.

600 ohms is a fairly high load for one of these which is designed to
drive a 4 ohm or 8 ohm speaker. Try putting a 600 ohm resistor in series
with a speaker and then without. I'm sure the difference in volume will
be huge.

I'm familiar with musical instrument speakers that have a wirewound pot
of only 50 ohms in series with the speaker - that makes a BIG difference
to the volume level.

 

[Mainlander]

The LM386 output is for a load from 4 to 16 ohms, probably the most
efficient at 16 ohms.
You need a transformer to put into the output that has a primary impedance
of 16 ohms (on the output of the chip) and a secondary impedance of 600 ohms
(for the phones). I imagine anything that has close to those values would
give you extra performance as desired.

Just get a chip with a higher power output. It will still not use a lot
of power to drive the high impedance load.

 

[Mainlander]

I can see that 600 ohms would definitely be a massive drop in power which is
exactly what is being complained about. I would like an explanation of how
600 ohms would "eat up the power" though ???

Say you are running it on a 6 volt battery, Maximum voltage at the output
6 volts. Using ohms law, the current through a 4 ohm speaker i = v/r is
1.5 amps. The power in the load, v x i, 9 watts. Actually an LM386
produces about half a watt so the voltage must be lower?

Anyway if the load impedance is 600 ohms the current will be only 0.01 A
and the power will be 0.06 watts. Pretty small.

 

[Mainlander]

Sure, I have that on board
My problem is how increasing the load impedance "Gobbles" the power. To my
limited knowlege I would think that the power used would be much less with
the higher impedance.

The higher resistance reduces the current flow, because power is voltage
times current the power will be dropped.

It is correct, the power is dropped. You can't afford that.

 

[Phil Allison]

Just get a chip with a higher power output. It will still not use a lot
of power to drive the high impedance load.

** An ordinary op-amp ( eg TL072 or NE5532) will drive a 600 ohm
load - so a dual will drive a pair of headphones. Much gain as you like
then.





................ Phil

 

[Mainlander]

** An ordinary op-amp ( eg TL072 or NE5532) will drive a 600 ohm
load - so a dual will drive a pair of headphones. Much gain as you like
then.

Interesting. Is that because they are designed to drive balanced audio
lines (600 ohm)? What is the output impedance of one of these, or does
that depend on the circuit they are used in?

 

[Phil Allison]

Interesting. Is that because they are designed to drive balanced audio
lines (600 ohm)?

** They can deliver about +/- 15 mA - limited by by heat
dissipation mainly.



What is the output impedance of one of these, or does

that depend on the circuit they are used in?

** Depends on voltage gain and frequency - the more of either the
higher.

With a gain of say 10 it is less than 1 ohm up to 10 kHz or so.



............... Phil