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Old 25th July 2012, 02:56 PM   #1
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Default Amplifier 4/8 ohms Selector

Hello everybody,

I have a yamaha RX-V457 avr and recently acquired a jbl 5.1 speaker system SCS 140 BK. The yamaha avr has a 4 and 8 ohm selection switch which basically switches the transformer secondary tapping providing a higher voltage to the amp power supply rails when 8 ohms in selected

But in the operation manual and on the rear panel o the amp where the speaker connectors are it is mentioned that minimum speaker impedence should be

Front L & R - 4 ohms, Centre - 6 ohms, Surround L & R - 6 ohms & Surround back - 6 ohms. my questions are

1. why only the front channels are speciied for 4 ohms, i studied the schematic and all 6 channels of the amplifier appear to be identical in all respects.

2. i read an article somewhere ( i will post the link when i find it again) that even if i am using 4 ohm speakers it is advisable to select 8 ohms setting on the amp,

3. all my jbl sats are rated at 4 ohms. you think i could safely operate the amp at a reduced voulume level without overloading the output trans and power supply

please advice on above

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Old 25th July 2012, 03:41 PM   #2
Join Date: Jan 2005
Set the switch to 4ohm mode and leave it there.
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Old 25th July 2012, 07:10 PM   #3
Mooly is offline Mooly  United Kingdom
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1. It could be because of thermal constraints and the distribution of heat produced via the output stages. The heatsinking and PSU will only be able to cope with so much. Maybe separate smaller heatsinks are used for the other channels or just that the distribution of "power" has to be dished out some way and assuming the fronts do most of the work less of the available "power" and "heatsinking" is left for the other channels.

2. Someone somewhere will have there reasons...

3. Probably... but I would be tempted to use the 4 ohm and lower the supply. Heat produced goes up rapidly as supply increases. Also remember that impedance ratings are "nominal" and not constant throughout the frequency range. My B&W's are nominally 8 ohm but have a 3 ohm minimum. Your JBL's at 4 ohm are probably more "honestly" specified.

If you have a circuit you can see what the different supply voltages are re 4 and 8 ohm and do a quick calculation on the available power output.
Installing and using LTspice. From beginner to advanced.
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Old 28th July 2012, 02:18 AM   #4
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Hi Mooly

You have a point there, maybe the heat sink dissipation or the power supply current rating is designed based on a maximum load of 2 x 4 ohms + 4 x 6 ohms and since i will be using only five channels of the amp all with 4 ohms i guess i would remain within the limits. also as you rightly mentioned how the manuacturer labels his impedence values.

I think it makes sense in selecting the switch in 4 ohms position as stated by tekko also

(can i post links on this forum to articles on external websites, i located the article which explains why the 8 ohms position shd be used even or 4 ohm loads)

The heatsink is a single piece common to all channels with six pairs of 2SA1695 and 2SC4468 mounted on it and the schematic shows two power supplies from the transformer (AC 79V & AC54V) either selected thru a relay. The amp DC dual supply corresponding would be around +/- 55V & +/- 30V


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Old 28th July 2012, 03:44 AM   #5
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Originally Posted by viki1967 View Post
The amp DC dual supply corresponding would be around +/- 55V & +/- 30V


mmmh...too much spread . Probably it switch between 35 and 40 VAC .
I think the thing is for adapting the amplifier to the load , and it's useful near
clipping ,because you can hear it .

Edit : I read now the period before the quote : AC 79V & AC54V , so it's a little beast !

Last edited by picowallspeaker; 28th July 2012 at 03:46 AM.
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