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Old 13th February 2011, 04:51 PM   #11
NYCOne is offline NYCOne  United States
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I'm happy to be wrong.

Here's what I have -

Each channel of the circuit for the Balanced - Cascoded F5 has four outputs, two IRFP240s and two IRFP9240s. I measured the voltage across the 0R47 resistor at each output device. I measured 0.470V at each of four outputs to yield the following:

Amps = 0.470/.47 = 1 per output. I have four outputs, each with 0.470V across the 0R47 resistor, 4A right?

In the stock F5, you bias to 0.6V on each output and get 2.6A total. I assumed each output is additive.

As I've said before, I'm relatively new to this hobby, perhaps I don't fully understand the implications of the circuit. That's why I like to experiment. I learn.

Quote:
Originally Posted by AndrewT View Post
this cannot be right.
The bias for a standard F5 is 1.3A, not 2.6A

I suspect you have not read that other thread yet!
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Old 13th February 2011, 05:04 PM   #12
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Quote:
I assumed each output is additive.
here is the mistake.
take the stock F5.
At quiescent,
the two outputs can be considered as serial connected ( no current into the load)
So, it's 1.3A total bias.
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Old 13th February 2011, 06:27 PM   #13
a.wayne is offline a.wayne  United States
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NYCOne,
Did you ever finish your Cascoded F5 ? if so what were your final impressions ?


regards,
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Old 13th February 2011, 07:35 PM   #14
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Quote:
Originally Posted by NYCOne View Post
I assumed each output is additive.
Each output PAIR is additive but NOT each output.

This is good news.
If you have 22V rails you can safely increase the bias to around 100W dissipation.
P=VI

I=100/(2x22) = 2.27A.
You have two output pairs. That is 1.135A through each source resistor.

Don't go higher than 130W. That is don't go higher than 2.95A total bias.
So if you are happy with the heat, you can safely take it to 2.6A (ie 1.3A per output pair).

Last edited by Melon Head; 13th February 2011 at 07:46 PM.
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Old 13th February 2011, 07:48 PM   #15
NYCOne is offline NYCOne  United States
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I finished one channel, then converted the board to this project. If (or when) I un-"X" this board, it will be two cascoded channels for an F5. I'll get to all of this the first chance I have.

The plan is to try the Balanced Cascoded F5 channel sonically tonight (hopefully), then un-"X" it and try the Cascoded F5, then try the Stock F5 (different build). In the background, I have a Balanced (not cascoded) channel set up for the Toshiba outputs that I will also try sonically - but one must have the time for all these experiments. I suspect by this time next week, I'll have tried them all at different settings.

Depending on the first listening session, I'll likely order a few more toroids - I'm considering 12V for the Balanced and ~28-32V for the Cascoded. I haven't thought too much about the ideal for the Balanced Cascoded, but I'm sure the answer will come.

BTW, if anyone is in the NYC area and wants to come by, PM me. If you have different toroids, bring them along!
Quote:
Originally Posted by a.wayne View Post
NYCOne,
Did you ever finish your Cascoded F5 ? if so what were your final impressions ?


regards,
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Old 13th February 2011, 08:03 PM   #16
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Quote:
Originally Posted by NYCOne View Post
I'm considering 12V for the Balanced and ~28-32V for the Cascoded.
I assume that is 12V AC not DC. Why so low?
That is around 15V DC. Probably lower once you add the crc supply
I think a torroid with 15V secondaries would be a better match.
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Old 13th February 2011, 08:35 PM   #17
tinitus is offline tinitus  Europe
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input Jfet might not be too happy with low voltage
its been debated in with B1

and as Nelson said, outputs are happier with higher voltage
but high bias still needed

seem to kind of bite each other
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Old 13th February 2011, 11:33 PM   #18
NYCOne is offline NYCOne  United States
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I was trying to get the most Class A power for the given outputs. Lower voltage yields more "space" for the current when trying to limit dissipation per device to 30W.

That said, I'm not married to 12V - I could see 15V. That's what I'd planned initially.

Quote:
Originally Posted by Melon Head View Post
I assume that is 12V AC not DC. Why so low?
That is around 15V DC. Probably lower once you add the crc supply
I think a torroid with 15V secondaries would be a better match.
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Old 14th February 2011, 06:54 AM   #19
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Keep in mind, anywhere between 30W to 35W per mosfet is fine.
As discussed earlier there is a trade off, the lower you go in Vds the higher Ciss becomes.
I wouldn't want to go much below 15V dc, but if you already have the transformers then it does not hurt to try.
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Old 14th February 2011, 08:38 AM   #20
NYCOne is offline NYCOne  United States
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Default All Smiles...

I hooked up my solo channel of my new Balanced Cascoded F5. The bias settings from last night were dead on tonight - a great start.

I've built a working Stock F5, and played around with various permutations of the F5, but I NEVER get tired of that first song though a new amp - pure unadulterated bliss. I chose Jet "Are You Gonna Be My Girl", followed up by a little Professor Longhair - "Tipitina"

Needless to say, the amp is wonderful. It seems similar to the Stock F5 in many ways: very detailed, excellent highs. I say this but must tell you, this is not the way to compare amps - I have only one channel, hooked to a makeshift FF85K speaker (in a Logitech enclosure). To say the least, I need to finish my OBs (all parts here, just need time).

So far, what I can say is the Balanced - Cascoded F5 is viable and worthwhile.

Tests to do when time permits:
  1. Stock F5 vs. Balanced Cascoded F5 (both with IRFP outputs, and already built)
  2. Stock F5 vs. Balanced Cascoded F5 vs. "Stock" Balanced F5 with Toshiba outputs (just need to finish the "Stock" Balanced Toshiba - one night or so)
  3. Balanced Cascoded F5 vs. "Stock" Balanced F5 (both with Toshiba outputs - need to convert Balanced Cascoded F5 to Toshibas and finish Balanced F5 - two or three nights)
  4. Cascoded F5 vs. "Stock" Balanced F5 (have all the bits, just need to build - a week or so)
  5. Balanced Cascoded F5 vs. Cascoded F5 (this is a long way off - lots of shared bits at this point)
I'm having a lot of fun. I am not well set up for technical analysis, I'll leave that to folks with the right equipment. I just like to build things and listen to music - pretty simple really.

I'd like to take a moment to point out none of this would be possible without Nelson Pass. I would like to thank him for his kindness and generosity - they are both valued deeply in NYC!
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