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Old 31st January 2009, 01:21 PM   #2591
AndrewT is offline AndrewT  Scotland
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18Vac is likely to give >27Vdc in worst case conditions, i.e. 6% overvoltage, 5% regulation on little or no load.

I have 15+15Vac running loaded at the moment and it gives +-22.1Vdc feeding 25Vdc caps and rises to 22.4Vdc off load.

I wouldn't dream of using 18Vac on 25Vdc caps.

You can use upto 22Vac on 35Vdc caps.
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Old 31st January 2009, 01:22 PM   #2592
rhysh is offline rhysh  United Kingdom
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Quote:
Originally posted by AndrewT
18Vac is likely to give >27Vdc in worst case conditions, i.e. 6% overvoltage, 5% regulation on little or no load.

I have 15+15Vac running loaded at the moment and it gives +-22.1Vdc feeding 25Vdc caps and rises to 22.4Vdc off load.

I wouldn't dream of using 18Vac on 25Vdc caps.

You can use upto 22Vac on 35Vdc caps.

Glad i bought the 35v model in this case
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Old 31st January 2009, 01:31 PM   #2593
EUVL is offline EUVL  Europe
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> you say you should change other things..can you show us redesigned f5 amp for 2sk1530/j201 ?

I can tell you off what I am using for my balanced version, but no guarantee for single ended :

Rail +/- 16V, 2A bias per MosFET
Source resistor 0R22
Gate resistor 100R
No thermistors, no current limiter (build at own risk !!!!)

Choose JFETs with Idss around 6.5mA.
For the 2SJ74 ONLY, change source resistor from 10R to 13R, and feedback resistor from 50R to 65R.


Patrick


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Build at your own risk !!
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Old 31st January 2009, 01:32 PM   #2594
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Default Re: Bridging Power

Quote:
Originally posted by dcbingaman
about 2.5A
Each side of a bridged F5 pair biased at 2.5A means an idle dissipation of 57W for each output device (24V rails minus the voltage drop across 0.47 Ohm, times 2.5A)
You'll slow-fry both the output devices AND the 0.47 Ohm/3W resistors (0.47*2.5*2.5= ~3W )
Only way to safely up the bias on a bridged F5 is more output devices in parallel.
Easiest is just placing two F5's in parallel, the joy of the steep input impedance of JFETs and the ease with which they allow to be put in parallel. (in particular with degeneration)

200W in 4 Ohm is +7 Amps output current, just the drop of 3.3V across a 0.47 Ohm resistor will make the spec impossible to reach by bridging just a single pair of F5's. (same story with the 120W/8 talk)
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Old 31st January 2009, 01:34 PM   #2595
EUVL is offline EUVL  Europe
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> Only way to safely up the bias on a bridged F5 is more output devices in parallel.

Or lower voltage and drive in balanced mode (see my post above), with additional even harmonics cancellation.

Unless you like even harmonics of course (like MSG in food).
I don't !!




Patrick
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Old 31st January 2009, 01:43 PM   #2596
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These are the ones I use -- I bought 8 for $20 on EBay a few years back.

Click the image to open in full size.

I think that the world is gonna have to learn to live with different P-Channel JFETs at some point.
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Old 31st January 2009, 01:55 PM   #2597
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by AndrewT
18Vac is likely to give >27Vdc in worst case conditions, i.e. 6% overvoltage, 5% regulation on little or no load.

I have 15+15Vac running loaded at the moment and it gives +-22.1Vdc feeding 25Vdc caps and rises to 22.4Vdc off load.

I wouldn't dream of using 18Vac on 25Vdc caps.

You can use upto 22Vac on 35Vdc caps.

I'm used to use anything between 20x10mF/35V up to 10x68mF/63V for amps with similar current consumption .
but - 'm also used to see 25V caps in much worse conditions than in Papamp .

350VA xformer , with filter in CRC will not give you more than 22V5dc ;

that's - including 10% overvoltage in mains , still bellow safe margin for these caps .

for me - that's not issue , but - if someone is on tight budget - its better to spend few greens on quality and capacity , than on voltage

Click the image to open in full size.


that off course not means that Papa is on tight budget .......... he simply have enough miles under the .... pants ........ so he knows what's practical
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Old 31st January 2009, 02:36 PM   #2598
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Zen Mod
I'm used to use anything between 20x10mF/35V up to 10x68mF/63V for amps with similar current consumption .
but - 'm also used to see 25V caps in much worse conditions than in Papamp .

350VA xformer , with filter in CRC will not give you more than 22V5dc ;

that's - including 10% overvoltage in mains , still bellow safe margin for these caps .

for me - that's not issue , but - if someone is on tight budget - its better to spend few greens on quality and capacity , than on voltage
long post to tell us nothing.
What transformer Vac are you recommending for 25Vdc, 35Vdc or 63Vdc smoothing caps?
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Old 31st January 2009, 02:49 PM   #2599
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by AndrewT
long post to tell us nothing.
.....

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Old 31st January 2009, 03:21 PM   #2600
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Quote:
Originally posted by EUVL
Choose JFETs with Idss around 6.5mA.
For the 2SJ74 ONLY, change source resistor from 10R to 13R, and feedback resistor from 50R to 65R.
[/B]
The following paragraphs are a question, rather than a statement. :-)

When using the Fairchild parts, the transconductance of the the P-channel parts is higher - so in the upper half of the circuit, you've got a lower S jfet and a higher S mosfet, and the lower half of the circuit you get a higher S jfet feeding a lower S mosfet, and these two combinations (partially?) cancel each other out.

Now when you're using the Toshiba mosfets, based on the fact that they're so well matched for transconductance, and that the jfets still aren't, you need to reduce the current output of the P-channel jfet (2sj74) and this is why you've increased source and feedback resistors.

Is this in the right ball park?

Thanks for the tips Patrick - I may end up going Fairchild unless I manage a bit better understanding here.

-j
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