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Old 2nd March 2012, 01:49 PM   #81
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Quote:
Originally Posted by chalkandtalk View Post
So which boards to use
Whichever pleases you ?

Cascode devices can be bypassed with a wire link, common cathode diodes can be parallel connected to source resistors with wires.

Ahhh, not the PS275, but the PS257. Why didn't you say so.
The thermal resistance number for fan blown mentioned there is at 5m/s air velocity, don't think you'd want that.

That heatsink model in 150mm height does about 0.25 C/W, say 0.35 C/W derated for 50C operating temperature.
With a fan blowing at 2m/s, one heatsink would do half that number, ~0.175C/W.
Two of the fan/heatsink setups would handle the 260W dissipation, at under 25C above ambient.
(quicky wet finger estimate)

Means 4 heatsinks and 4 fans for stereo.
Scary, no ?

Addendum : Option would be to mount two heatsinks facing eachother, forming a duct, with one 120mm fan mounted on them. Would require a DIY plenum between fan and heatsinks to make the crossover from 120mm to 178mm.

Balanced/bridged with 25V (to 35V) rails is the most sensible choice for an F5T build, btw (imo)
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Old 2nd March 2012, 02:38 PM   #82
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> Ahhh, not the PS275, but the PS257.



That's the one. The table gives this as 300mm long, which will be halved and put back to back and made into a tunnel with a fan at one end. (Comme ihquam)

> Cascode devices can be bypassed with a wire link, common cathode diodes can be parallel connected to source resistors with wires.

Yes Yoecutter's balanced input boards with individual boards for each output device is favourite.

Last edited by chalkandtalk; 2nd March 2012 at 02:41 PM.
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Old 2nd March 2012, 11:58 PM   #83
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by jacco vermeulen View Post
Think output level of 35V in 2 ohm, with 75V rails.
40V drop between drain and source, 17.5A current going through the mosfets : 700W dissipation.

Worst load for an output stage occurs when the product of output current times voltage drop is highest.
A sine curve shows that this happens below 50 percent of max output.

Pushing out full power is relatively harmless for an output stage : high output current, but low Vds.
OK ... get yah ....!!!!
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Old 3rd March 2012, 07:16 AM   #84
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Just one thing that may be worth a mention

Your CL60 or CL 30

Bigh Traffos with Loads of C and loads of current at Idle

The Surge supressors are going to work overtime and going to get hot

You lot beter start to think about By passing them with a relay maybe
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Old 5th March 2012, 03:22 PM   #85
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Quote:
Originally Posted by jacco vermeulen View Post
2 ohm, with 75V rails.
Jacco, just for curiosity's sake, how many input pairs would I need for 8 output pairs, 75V rails and 2 ohm load?

Many thanks
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Old 5th March 2012, 04:35 PM   #86
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For a MHz, i'd think 28 pairs.
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Old 5th March 2012, 05:04 PM   #87
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Quote:
Originally Posted by jacco vermeulen View Post
For a MHz, i'd think 28 pairs.
It's a shame you don't like sharing your knowledge.
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Old 5th March 2012, 05:12 PM   #88
AndrewT is online now AndrewT  Scotland
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Ask the right question.

State the bias current and the operating Tc for your proposed output stage and just maybe someone could help.

But since it's
Quote:
just for curiosity's sake
why should any Member waste their time?
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Old 5th March 2012, 05:39 PM   #89
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Quote:
Originally Posted by AndrewT View Post
Ask the right question.

State the bias current and the operating Tc for your proposed output stage and just maybe someone could help.

But since it's why should any Member waste their time?
Don't know why you wasted yours with me then. And sorry if I'm a beginner and don't know exactly what I should state to have my question answered. Stupid me that thought everyone would be welcome here.
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Old 5th March 2012, 06:03 PM   #90
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8 times as many output devices.
70V out instead of 20V, 3.5 times the voltage stretch.
8 times 3.5 is 28 JFET pairs per channel.

Use 1 pair of JFETs, 8 MOSFETs in parallel, 75V rails : ~35KHz (1 MHz divided by 28) ~57Khz with the revised feedback.
Nice subwoofer amp
Maybe 12 pairs if you take the 32V of the V2 as a reference.
Mr Popa managed to parallel as many as 4.

Easier to make a small high quality power amp than a biggy. (the prime reason for FW's existence)
Maybe you should read the F5T article thoroughly.
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