amplifier/receiver question

Guest
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&langId=-1&productId=8198552921665061593&XID=A:1225267:10396361:sduidp0t744321&ref=http://forums.slickdeals.net/showthread.php?t=744321

Manual says 8-16 ohm impedance, and I'm wondering how bad will it be
if I connect my 4-ohm subwoofers to it?

Are we talking Blue Smoke here?

Michael
 
On Apr 14, 3:11 pm, mrdarr...@gmail.com wrote:
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Manual says 8-16 ohm impedance, and I'm wondering how bad will it be
if I connect my 4-ohm subwoofers to it?

Are we talking Blue Smoke here?

Michael
Click to expand...

No blue smoke, but you wont be sending the maximum power to the
speakers (and thus hurting yourself in the long run).

Check out "maximum power transfer", it basically says that impedance
in must equal impedance out for power to be transferred effectively.

http://en.wikipedia.org/wiki/Impedance_matching
 
<mrdarrett@gmail.com> wrote in message
news:14f24ee1-9240-4f82-b4c0-a0ef993d7a63@w1g2000prd.googlegroups.com...
On Apr 15, 6:04 am, NoS...@daqarta.com (Bob Masta) wrote:
On Mon, 14 Apr 2008 22:10:49 -0700 (PDT), Delsol



Aaronofk...@gmail.com> wrote:
On Apr 14, 3:11 pm, mrdarr...@gmail.com wrote:
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Manual says 8-16 ohm impedance, and I'm wondering how bad will it be
if I connect my 4-ohm subwoofers to it?

Are we talking Blue Smoke here?

Michael

No blue smoke, but you wont be sending the maximum power to the
speakers (and thus hurting yourself in the long run).

Check out "maximum power transfer", it basically says that impedance
in must equal impedance out for power to be transferred effectively.

http://en.wikipedia.org/wiki/Impedance_matching

The old "impedance matching" thing almost certainly doesn't apply here
because the amp specs are not based on output impedance. (Unless this
amp has an output transformer... which pretty much went out with
vacuum tubes.) Modern solid state amps have output impedances that
are near zero at low frequencies.

The "8-16 ohms" rating is more likely based on power handling
capability of the output stage. If you use a lower impedance load, it
takes more current at lower voltage to get the same power, which
means the output stage has to dissipate more power. Like a pair of
voltage regulators with changing setpoints, the positive and negative
output devices have to pass the output current while dropping the
voltage difference between the supply voltage and the output voltage.

Let's take a simple example: Say the supply is 40 VDC and at some
instant the positive output device of the amp has to deliver 50W into
its load. (No RMS here, just instantaneous power.) So with an 8 ohm
load the output is 20V to give 20^2/8 = 50W, and the device itself has
40-20 = 20V across it, while the current through it is the same as the
current through the load = 20/8 = 2.5A. So the device has to
dissipate 2.5 * 20 = 50W.

Now consider a 4 ohm load. First of all, you need to turn the volume
control down to get the same 50W into the load, since now the output
voltage must be 14.14 since 14.14^2/4 = 50W. So the device now has
40-14.14 = 25.86V across it. But its current is 14.14/4 = 3.535 and
the power it dissipates is 25.86*3.535 = 91.4W.

So if the power rating of the amp is based on the power that the
output devices can handle, you'd need to derate it significantly to
use a 4 ohm load. Some amps list separate power ratings for 4 ohms
(and sometimes 2 ohms). If the 4 ohms rating is less than the 8
ohm, the output devices are the limiting factor. If the amp is more
sturdily built (a pro amp, or better grade home amp) the 4 ohm rating
will be higher, indicating that the limit is more likely just the
supply voltage. (+/-40V gives 28VRMS, which into 8 ohms gives 100W,
or into 4 ohms gives 200W.)

But this amp wasn't rated for 4 ohms, so you may have to derate it.
And you'd be taking a chance that someday somebody didn't crank up
the volume too high.

Best regards,

Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!



Ok, thanks for the detailed explanation.

If I place my two 4-ohm subs in series, that would give an 8-ohm
impedance, right?

Would sound quality suffer if I put two speakers in series?

Michael
Click to expand...

Get some big 4ohm resistors, and put them in series. That will protect the
output drivers from excess dissipation.

The resistors need to be rated at 100W, of course...

kind of pricy...

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=C300K4R0E-ND

so use 2 50W 2 ohm resistors for each instead:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=HLB-2.0-ND

Regards,
Bob Monsen
 
mrdarrett@gmail.com wrote:

So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&langId=-1&productId=8198552921665061593&XID=A:1225267:10396361:sduidp0t744321&ref=http://forums.slickdeals.net/showthread.php?t=744321
Click to expand...
Yes very likely.

And please don't post in HTML.

Graham
 
Delsol wrote:

On Apr 14, 3:11 pm, mrdarr...@gmail.com wrote:
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Manual says 8-16 ohm impedance, and I'm wondering how bad will it be
if I connect my 4-ohm subwoofers to it?

Are we talking Blue Smoke here?

Michael

No blue smoke
Click to expand...
Whjy do you think not ?

Graham
 
Bob Monsen wrote:

Get some big 4ohm resistors, and put them in series. That will protect the
output drivers from excess dissipation.
Click to expand...
That's plain silly.

The right answer is to buy an amp suitable for 4 ohm loads.

Graham
 
On Apr 15, 6:04 am, NoS...@daqarta.com (Bob Masta) wrote:
On Mon, 14 Apr 2008 22:10:49 -0700 (PDT), Delsol



Aaronofk...@gmail.com> wrote:
On Apr 14, 3:11 pm, mrdarr...@gmail.com wrote:
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Manual says 8-16 ohm impedance, and I'm wondering how bad will it be
if I connect my 4-ohm subwoofers to it?

Are we talking Blue Smoke here?

Michael

No blue smoke, but you wont be sending the maximum power to the
speakers (and thus hurting yourself in the long run).

Check out "maximum power transfer", it basically says that impedance
in must equal impedance out for power to be transferred effectively.

http://en.wikipedia.org/wiki/Impedance_matching

The old "impedance matching" thing almost certainly doesn't apply here
because the amp specs are not based on output impedance. (Unless this
amp has an output transformer... which pretty much went out with
vacuum tubes.) Modern solid state amps have output impedances that
are near zero at low frequencies.

The "8-16 ohms" rating is more likely based on power handling
capability of the output stage. If you use a lower impedance load, it
takes more current at lower voltage to get the same power, which
means the output stage has to dissipate more power. Like a pair of
voltage regulators with changing setpoints, the positive and negative
output devices have to pass the output current while dropping the
voltage difference between the supply voltage and the output voltage.

Let's take a simple example: Say the supply is 40 VDC and at some
instant the positive output device of the amp has to deliver 50W into
its load. (No RMS here, just instantaneous power.) So with an 8 ohm
load the output is 20V to give 20^2/8 = 50W, and the device itself has
40-20 = 20V across it, while the current through it is the same as the
current through the load = 20/8 = 2.5A. So the device has to
dissipate 2.5 * 20 = 50W.

Now consider a 4 ohm load. First of all, you need to turn the volume
control down to get the same 50W into the load, since now the output
voltage must be 14.14 since 14.14^2/4 = 50W. So the device now has
40-14.14 = 25.86V across it. But its current is 14.14/4 = 3.535 and
the power it dissipates is 25.86*3.535 = 91.4W.

So if the power rating of the amp is based on the power that the
output devices can handle, you'd need to derate it significantly to
use a 4 ohm load. Some amps list separate power ratings for 4 ohms
(and sometimes 2 ohms). If the 4 ohms rating is less than the 8
ohm, the output devices are the limiting factor. If the amp is more
sturdily built (a pro amp, or better grade home amp) the 4 ohm rating
will be higher, indicating that the limit is more likely just the
supply voltage. (+/-40V gives 28VRMS, which into 8 ohms gives 100W,
or into 4 ohms gives 200W.)

But this amp wasn't rated for 4 ohms, so you may have to derate it.
And you'd be taking a chance that someday somebody didn't crank up
the volume too high.

Best regards,

Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
Click to expand...


Ok, thanks for the detailed explanation.

If I place my two 4-ohm subs in series, that would give an 8-ohm
impedance, right?

Would sound quality suffer if I put two speakers in series?

Michael
 
mrdarrett@gmail.com wrote:

On Apr 15, 9:58 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com
mrdarr...@gmail.com wrote:

So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Yes very likely.

And please don't post in HTML.


I didn't. I pasted a link.

What did it look like on your end?
Click to expand...
There was an HTML tag which screws up the editing for the reply. That'll often
happen when posting links. With Netscape it warns me about that and I can turn
off the HTML (which I do when posting links).

Graham
 
mrdarrett@gmail.com wrote:

Eeyore <rabbitsfriendsandrelati...@hotmail.com
Bob Monsen wrote:

Get some big 4ohm resistors, and put them in series. That will protect the
output drivers from excess dissipation.

That's plain silly.

The right answer is to buy an amp suitable for 4 ohm loads.

Or just buy new speakers...
Click to expand...
That too. I'd buy a more comfortably rated amp myself though.

Graham
 
On Apr 15, 9:58 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
mrdarr...@gmail.com wrote:
So I'm looking at this ($149.99 at Sears)

http://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?cat...

Yes very likely.

And please don't post in HTML.

Graham
Click to expand...

I didn't. I pasted a link.

What did it look like on your end?

M
 
On Apr 15, 10:00 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
Bob Monsen wrote:
Get some big 4ohm resistors, and put them in series. That will protect the
output drivers from excess dissipation.

That's plain silly.

The right answer is to buy an amp suitable for 4 ohm loads.

Graham
Click to expand...

Or just buy new speakers...

M
 
<mrdarrett@gmail.com> wrote in message
news:298fc06e-7b7b-4ebb-9d29-f6bb60b67f82@i36g2000prf.googlegroups.com...
On Apr 15, 10:00 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com
wrote:
Bob Monsen wrote:
Get some big 4ohm resistors, and put them in series. That will protect
the
output drivers from excess dissipation.

That's plain silly.

The right answer is to buy an amp suitable for 4 ohm loads.

Graham


Or just buy new speakers...

M
Click to expand...

If you can buy new speakers, that is the way to go. If you can't buy new
speakers, or just love those old 6 foot tall woofers, you can use the
resistors to protect the amp.

BTW, isn't Eeyore a stuffed donkey?

Regards,
Bob Monsen
 
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