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Old 28th December 2006, 06:36 PM   #11
Bengali is offline Bengali  United States
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Default 6 ohm loads

Hi,

I almost finished building my LM3886.

I'm using a 25V transformer also, however my load is 6 ohms.

I entered 37.3V into the overture design guide. 37.3V is what
I'm measuring without a load.

The overture guides gives me a warning error, Required heat sink is too large with TF package.

Could I be missing some other values in the design guide?

I have the Rf=22.1K, Ri/Ci/Rin/Cin=0, Rb=1K

I'm also curious as to if use the Ri/Ci to reduce the output
dc voltage. So anything under 100mV is safe?

Thanks!
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Old 28th December 2006, 07:25 PM   #12
CarlosT is offline CarlosT  United States
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No...you need to go to liquid cooling
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Old 28th December 2006, 07:26 PM   #13
CarlosT is offline CarlosT  United States
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That warning is purely based on driver impedance (too low) and supply voltage (too high)...tweak away with all other variables...you'll see.
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Old 28th December 2006, 07:34 PM   #14
Bengali is offline Bengali  United States
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Thanks Carlos. I'm using the pcb's from chipamp with the snubber design. So no other values to tweak in.

If I change the impedance from 6 to 8 ohms then the warning is gone.

My only question at this time is if people here omit the Ri/Ci.

I'm waiting for my slab of heat sink today and hopfully will be good to go.
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Old 28th December 2006, 08:01 PM   #15
MikeW is offline MikeW  United States
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Put a 1 to 2 ohm 25 watt resistor at the output of the amp.
It will only cause problems if you have the volume pegged all the time.
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Old 28th December 2006, 11:31 PM   #16
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Hi Mike,

I suggested starting with four ohm and if all is well go lower, if it is really necessary. Think of it 68W on the speakers is the equivalent to 132W on the devices. Stereo operation is 264 watts of heat. Think of it - 13 x 20 Watt soldering irons worth.

Something confuses me a little, that is their SOA does not really explain how they achieve such high power without destroying the device.

Can anyone shed light on this.
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Old 29th December 2006, 02:24 PM   #17
CarlosT is offline CarlosT  United States
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NicoRas you seem very knowleadgeable...can you please address my question here regarding the additional cap across the V+ and V-?


http://www.diyaudio.com/forums/showt...88#post1090688
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Old 29th December 2006, 02:52 PM   #18
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Quote:
Originally posted by CarlosT
NicoRas you seem very knowleadgeable...can you please address my question here regarding the additional cap across the V+ and V-?


http://www.diyaudio.com/forums/showt...88#post1090688
I am sorry Carlos, of course you can drop a cap across +V and -V as long as it is relatively small. so its charge time is an order or so magnitude lower than the time constant of the input circuit, else you will have both high voltage and current across the series transistor and will surely destroy it.

I use this method on several KRELL clones I built (for non technical audio fanatics) and the resevoir capacitors are 4 x 4700 uF each rail, the base capacitor, in fact I use two is 1000 uF and I have 4700 uF on each rail and each amplifier PCB.

This seems to work out pretty cost effective. There is also no plop on the speakers at switch on and the time to get to rail voltage is roughly 700 mS.

For additional safety I place a reversed diode across the series regulating transistor. The reaon for this is that often when working on and tweaking a new amp you will shurely short the input caps and the output cap will also try and source current in the reverse direction through the series transistors and destoy them.

Of course I do not use TIP41/42, they are the drivers in the KRELL CLONE psu.

Kind regards

Nico
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Old 29th December 2006, 04:12 PM   #19
CarlosT is offline CarlosT  United States
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Thanks. I have both 6.8 uF and 2.2 uF film capacitors of appropriate voltage ratings that I could use in this position but I haven't so far. I have built 2 LM3875 IGC mono amps so far without them but just wondered if there was anything to be gained by adding these.

Right now each V+ and V- bank has a 10 uF electrolytic cap at the rectifier board and a 1,000 uF electrolytic cap at the amp board.

I just did not understand the function of a cap across V- and V+.
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Old 29th December 2006, 05:12 PM   #20
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Quote:
Originally posted by CarlosT
Thanks. I have both 6.8 uF and 2.2 uF film capacitors of appropriate voltage ratings that I could use in this position but I haven't so far. I have built 2 LM3875 IGC mono amps so far without them but just wondered if there was anything to be gained by adding these.

Right now each V+ and V- bank has a 10 uF electrolytic cap at the rectifier board and a 1,000 uF electrolytic cap at the amp board.

I just did not understand the function of a cap across V- and V+.
Hi Carlos, I eventually think I understand what you are saying. For putting a mall cap across +V and -V would lower the impedance at high frquancy and that will be beneficial. There is no real need for a high value capacitor on the amp PCB unless it is some distance away from the power supply board.

I also do not know if theis is the case with the GC but in the olden days many times you found motorboating, a sound like a motorboat over your speakers, which was normally solved with a capacitor 10 - 1000 uF very near the IC'a legs. You would also bypass this cap with a low value cap say 100 nF - 470 nF as large capacitors reveals quite a lot of inductance and low values not improving the high frequency noise on the power line.

Does what I say make sense?

Kind regards

Nico
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