HEX-FET 60W/4ohm Amplifier - diyAudio
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Old 7th February 2002, 04:10 PM   #1
djdan is offline djdan  Romania
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Default HEX-FET 60W/4ohm Amplifier

What do you think about this amplifier schematics ?
( AN-948 aplication from International Rectifier )

Q5 and Q6 (Hex- Fet power devices) are IRF532 and IRF9532.

1. It can be replaced with IRFP240/IRFP9240 and supply this amplifier with more voltage ( +/- 40v ) ?

2. Did anybody build this amp and can tell me something about the sound quality ?

3. Did it worth to build this amplifier ?

Thank you !
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Old 7th February 2002, 05:54 PM   #2
subwo1 is offline subwo1  United States
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I like the simplicity. Has anyone else observed a possible increase in distortion from connecting a capacitor directly to the base of a transistor, possibly a rectification-storage effect? Maybe consider adding in a small resistor right before the base of the input transistor.
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Old 7th February 2002, 07:37 PM   #3
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How about a Gate resistor on Q6. Also, you can dump
R14 and the output coil without fear.
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Old 8th February 2002, 12:06 PM   #4
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Suggestions:
-> Eliminate R14 (short it)
-> Eliminate R8,R9,C4. Connect a constant current source (5mA to 10mA) between Q3 collector and +Vdd.
-> Connect a 50uF capacitor between Q3 emitter and collector
-> A la Nelson, add a gate series resistor to Q6, a similar value to R10 (both should be a couple of hundred ohms).
-> Optional depending upon the open and closed loop gains: add a capacitor between Q4 base and collector of between 50pF and 150pF (depends on the stability of the circuit).
-> For short-circuit protection of the FETs or output current limiting you can put a zener and diode in series between the gate and source of each FET. This will limit Vgs max.
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Old 8th February 2002, 12:15 PM   #5
Geoff is offline Geoff  United Kingdom
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Personally, I would keep R8/R9/C4. There are many examples where a bootstrap current source in this position is audibly superior to a 2-transistor or diode-transistor ccs, provided C4 is adequately sized to minimise low frequency distortion.
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Old 8th February 2002, 01:00 PM   #6
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Geoff,
Would you elaborate on the reasons the bootstrap can sometime improve the sound quality? I would like to understand this.

I could see that if R8 were a CCS then R9 would effectively be connected between the gate and source of the FETs. Perhaps this would provide some phase improvements at high frequencies or help to reduce the effect of Zgs mismatch.
BAM
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Old 8th February 2002, 01:50 PM   #7
Geoff is offline Geoff  United Kingdom
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BAM

I wish I knew the reasons. Perhaps a reduction in the number of active devices? Or the fact that a resistor is more linear than a transistor? Or a reduction in high frequency spuriae?

Nelson has indicated on many occasions his preference for a resistor, as opposed to a css, on the input differential of his amps because of the sonic benefits. Over the years, many highly regarded amps have used the bootstrap arrangement for the voltage amplification stage, including designs by John Linsley Hood and, more recently, the AKSA.

I will admit that my preference is to keep a circuit as simple as possible, with as few active devices as one can get away with whilst maintaining an acceptable level of measured and, more importantly, audible performance.

Perhaps if Hugh drops by he will be able to throw some more light on this subject.

Geoff
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Old 8th February 2002, 11:35 PM   #8
AKSA is offline AKSA  Australia
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Default Bootstraps

Bootstraps - relic of the past, or glimpse of the future?

The most vexing problem of SS amplifier design, in my view, is stability. Because any amp with a global feedback loop has the potential for its NFB to turn positive and nasty at HF, we must find ways of pulling back the gain at HF so that at the upper pole point of the amp, the gain is below unity and it can never oscillate.

The big challenge is to do this without affecting the sonics of the amplifier. In short, pulling back gain at HF almost invariably affects the quality of the sound.

There are many ways of doing this, and usually a combination is used. Often there is a cap across the output feedback resistor, and always there is a small cap between collector and base of the voltage amplifier. There may even be a small cap between the collector of the voltage amplifier and the feedback node; this approach is suggested by Linsley Hood and is useful. These caps all conspire to pull back gain to just below unity at the point in the spectrum where global negative feedback turns positive and can send the amp into destructive paroxysms. This requirement is called the Bode/Nyquist criteria, after the mathematicians who originally postulated and researched it.

The voltage amplifier is crucial to the sonics of an amplifier, and must be thrown in manacles to meet the Bode/Nyquist criteria. These manacles, usually capacitive but sometimes inductive, can wreck the sonics, so they must be minimised whilst holding good stability into reactive - read difficult - loads.

A constant current source has DC to light frequency response. That is, it will hold very high output impedance at its collector well into the MHz range. This means that any voltage amplifier supported by a CCS needs a substantial manacle to keep it stable at HF, and this in turn mandates use of a large compensation cap between its collector and base.

A bootstrap using a conventional electrolytic capacitor starts to rapidly loose effectiveness at higher frequencies. This means that as the frequency rises above 50Khz - well into the ultrasonic range - the collector of the voltage amplifier sees a steadily increasing load - reducing impedance - and this pulls back its gain very nicely without recourse to large lag compensation.

Everything old is new again.........

Cheers,

Hugh R. Dean
Research/Technical Director
www.printedelectronics.com
Melbourne AUSTRALIA
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Old 9th February 2002, 12:39 AM   #9
Geoff is offline Geoff  United Kingdom
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Thanks, Hugh. In view of your AKSA design I thought you might have a constructive comment. Now if only you would remove the input LTP :-)

Geoff
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Old 9th February 2002, 02:06 AM   #10
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AKSA,
Thanks for the detailed explanation. I think I understand it - now that C4 is assumed lossy at high f.

Consider the transimpedance of Q4 Ic to Vo. Ignore Q5 and Q6 input capacitance. At frequencies where C4 can be approximated by a short circuit the small signal transimpedance will be about R9xGmxRo (Gm is the combined transconductance of the two FETs and Ro is the load, say 8-ohms). When C4 can be approximated by an open-circuit the transimpedance is about R8+R9. Therefore R8+R9 << R9xGmxRo to cause a reduction in open loop gain at high frequencies.

But I'm not sure about the choice of circuit. I am concerned that C4 isn't a good short-circuit at low frequencies, that it may add distortion being an low performance electrolytic and that +Vdd noise is coupled through R8. Would it be better to have a CCS from Q3 collector to +Vdd and a series cap/res from the same point to ground? You could then use a low value, high f cap and a resistor of value R8+R9. Loop gain at low frequencies would be very high thus minimizing output offset at dc.

BAM
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