Ok..need help

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
ok somebody told me that instead of one high wattage resistor witch could end up being wirewound and therefor inductive i could use sevreal smaller ones,My question is you've all seen my project and i want to know if for regular music signals a 1 watt resistor would be enough for my output network resistor or i need more? Also how do i use more than one for higher rating? Do i connect them in series or paralell?

Edit: My expected output per network of like 80Watts max.




Those are the three main ones that give the idea,Sorry for my impatince i'm not in a good moood tongiht. :( i would have thought series would have been better since in paralell the power would go to one resistor instead of mutiple ones together if one was slightly lower but in series it's forced to go through both??
Joined 2002
Paid Member
I use 1/2 watt on my output networks, in a 100 watt STK 4191 bridge amp. I suspect I could get by with 1/4 watt, the 1/2 watt seems like overkill, doesn't cross room temp, when amp sinks are at 80C.

The idea of the Zobel is to reduce oscillations at high frequency. The resistor will pass power only at extremely high frequencies, maybe 40/50 KHz. A heavy-duty or precision resistor is not needed in terms of design intent.

That said, overkill is great. If you use 1% metal film, the difference in current between the resistors will be about 1%. Not a significant variation. Also series resistors will not help in terms of current draw. To illustrate, if you have two 1/2 watt 22 ohm resistors in parallel, the resulting resistor is a 1 watt, 11 ohm resistor. Two 1/2 watt, 4.7 ohm resistors in series will form a 1/2 watt, 9.4 ohm resistor.

BTW I read you were using computer heatsinks with fans. Those are loud mothers!! I prefer large passives, or if you must have fans, I would suggest 60-80 mm adapters to reduce noise. The PIII heatsinks aren't the best coolers around anyway because the PIIIs weren't very hot CPUs to begin with. Look at some of the high end Athlon coolers, specially some of the new Thermaltake coolers with 70 mm adjustable-speed and thermally-controlled fans.
Joined 2002
Paid Member

The Slot 1 CPUs put out around 40 watts when at full load. A pittance in CPU terms nowadays, and without a look at the datasheets I don't know how much heat the STK is gonna be putting out at 70% power (which is typically where most of the heat generation happens, I dunno why but that's about right) so can't say. You need to look at the datasheets to figure that one out.

More like something like this, from the same store:


Or should it be



I dunno why they've got two prices on the same item. Your money would probably be better spent elsewhere. All the guys I know in the States buy Computer hardware from www.newegg.com or www.tigerdirect.com I know www.directron.com would also be a good place to look.

Some other good (I mean capable of dealing with 70+ watts of heat energy such as put out by the exteremely hot P4 and Athlon XP processors) HSFs - Heatsink/Fans - are:


These are nutty overclocker territory fans, which means Thermal resistance of about 0.5 to 0.7 C/W. Which is partly because of copper or whatever, and partly because of the loud and fast fans. That is one aspect that is bad about this kind of solution: The noise.

The second is: How do you plan to mount the devices to the HSF? I would imagine most of these come with clamps that go onto sockets of CPUs, you'll have to design a custom bracket that the devices will mount on, and then thermal epoxy it to the HSF. Which will create higher thermal resistance, which is bad.

I would imagine passive sinks (large ones) with low-rpm fans should be good enough for this critter. In computers, there is a limit in terms of heatsink size. If you face the same limit in your amp, then well, you'll just have to use the HSF route. If you don't, I would look at a large HSF with a thermally controlled fan or two, which is made very simply with a 555 and a few BD 139s attached to a heatsink.

The BD 139s act as thermal diodes, and though the solution is not perfect, it works great. You'll need one more BD139 to act as a current amplifier to switch on a fan. Throw in a RC (1K/220 micro) network for hysteresis control, and you're set. If you have access to thermistors, then even better. I have implemented the above circuit with great success in an amp I built 6 years back for myself (My last ever DIY project, unfortunately), and it keeps the amp nice and cool.
Power Resistors

It does not matter if you put your resistors in series OR parallel. In series each resistor takes a share of the voltage drop, thus sharing the total power dissipated. In parallel you use larger values of resistors together to form a lower value resistor.

Two 1/2 watt, 4.7 ohm resistors in series will form a 1/2 watt, 9.4 ohm resistor.

Actually they would form a 1W 9.4 ohm resistor because although the current draw is not being effected the resistors are 1/2 the total value and therefore dissipate half the power P=I^2*R
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.