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Old 24th January 2009, 11:11 PM   #1
rtill is offline rtill  United States
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Default cap polarity.... another mosfet amp

I'm working on another mosfet power amp...I have 1 question in regards to it. C1 is a 2.2 uf cap, however, all I have is a 2.2uf electrolytic. Can I use this and if I can, which way does + face? towards the circuit or towards the input?
http://www.elecfree.com/electronic/b...2sk15302sj201/
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Old 24th January 2009, 11:26 PM   #2
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In reality I dont think it really matters...But , since your CCS
is on the positive rail the bases of your LTP will be + 20-40mv.
so + on the EL would go toward the base of the tranny.

All of my DIY amps have CCS negative, and I have used
bipolar and polarized EL with + facing input.

Worst case the reverse voltage would only be 20-40mv
unless the CCS or differential failed.
OS
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Old 24th January 2009, 11:42 PM   #3
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Quote:
http://www.elecfree.com/electronic/...-2sk15302sj201/
I looked at the amp, and they list both vertical (IRFp's) and lateral
mosfets (2sj162/sk1058).
With the verticals they fail to mention that you need
thermal compensation and you would most likely encounter
problems if you used the IRFP's with that circuit.

A simple mje340/bd139 Vbias circuit replacing P2 would
be the preferred setup for the IRFP's.

Os
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Old 25th January 2009, 01:30 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
with R3 from 330r to 470r, the VAS stage runs ~ 12mA to 15mA in each half.

I would drop R3 (150r to 200r) to increase the current in the LTP, add 200r Emitter resistors to the LTP and then drastically reduce the 3k9 collector loads to get the VAS stage down <=10mA per half.

But, whoever specified those output FETs does not know what he/she is doing.

I would not rely on anything he has posted if he makes fundamental mistakes like that.

The bases of the LTP sit positive relative to signal ground when idle.
They swing +- slightly when signal is present.
The positive voltage determines the direction of both the input cap and the NFB cap.

The value of this positive voltage is determined by the input offset current times {R1 + R2} resistor values.
Input offset current ~ LTP half current / hFE of the input transistors at that current.
R27 should match 47k + 2k2 i.e 49k2 (not 27k) for minimum output offset voltage, if the hFE of both LTP transistors are matched at the same Vbe and stage currents.
Slight alterations of R1 will precisely set the output offset to near Zero Volts. But unequal currents or unmatched transistors in the LTP will cause drift in output offset. The more accurate the matching of both devices and balancing of currents the less will the output offset drift with operational conditions. It will also sound better.

Find a better schematic.
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Old 25th January 2009, 05:02 PM   #5
rtill is offline rtill  United States
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so...after reading your replies... this won't work? Is there a mosfet amp using the irfp240/irfp9240 pair someone could post?
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Old 25th January 2009, 06:06 PM   #6
CBS240 is offline CBS240  United States
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Quote:
Originally posted by rtill
so...after reading your replies... this won't work?

Not likely for very long. If you look at FIG 7 in the datasheet for IRF240, you will see the transfer characteristics for 3 different temperatures. The zero temperature coefficient bias point is at 14A or so. Unless you set your bias to 14A, you will have bias drift. So if your bias is 200mA, you can see that as temperature increases, the required Vgs vs conductance decreases. If your bias doesn't have thermal compensation, you will have run-away.

However, if you look at the same transfer figure for a lateral fet like a 2SK1058 you will see the zero temp co point is 100mA, which means they turn off as they get hotter and don't need compensation. Your circuit would require lateral fets for this reason.
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Old 25th January 2009, 07:35 PM   #7
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Quote:
this won't work? Is there a mosfet amp using the irfp240/irfp9240 pair someone could post?
there are the quasi amps..
Power amp under development

http://www.adam.com.au/cgpap/QuasiWeb/index.htm

These amps use all IRFP 240's ,N-channel,

Then there is my collection - 30+ amps.. assorted mosfet..
http://71.203.210.93/pdf1/Electronic...Mosfet_25-1kw/

And the one I have working with BJT's now (schema is set for
IRFP 240/9240's , my next sub amp.. (see attachment)
OS
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Old 25th January 2009, 07:52 PM   #8
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ostripper, isn't 2SA1837 the complement of 2SC4793, or have you deliberately got 1847?
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Old 25th January 2009, 09:47 PM   #9
rtill is offline rtill  United States
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ok...would this work?
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Old 25th January 2009, 10:13 PM   #10
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rtill,
as CBS240 said, not likely for very long.
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