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Old 25th October 2004, 01:28 PM   #11
quasi is offline quasi  Australia
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Default Revised circuit

Thanks for your feedback and constructive criticism.

I have made the following changes for the next prototype.

1. Cascode first stage leaving about 20 volts across the diff . pair.

2. Resistor feeding the diff pair upped from 10k to 15k to take some voltage away from Q1.

3. Only one negative rail tying the driver stages to the output FETS.

4. Leaving the higher positive rail to drive the positive output FETS fully and removing the assymetric clipping problem.

5. 12v zeners on the FET gates to provide some protection against overdrive (during excessive currnt draw).

6. Shifting the current sharing resistors to the negative rail FETS source.

7. Input resistor changed from 22k to 33k to match feedback resistor.

The new cct is attached. How does it look now?
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Old 25th October 2004, 01:44 PM   #12
MikeB is offline MikeB  Germany
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Hi quasi !

2 things:

1, For ZD1 use a ~22k or 47k resistor to positive voltage,
2, Paralell to the 33k at input 1nF seems too much to me, try 330pF

I still think you should get rid off VR1, with matched resistors
feedback/input and the 22uf-cap (maybe use 100uF?) DC should
be very low. If not, something else is wrong.
Don't forget, when VR1 is not in middle, the diffamp gets asymetrical,
producing strange distortions.

Mike
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Old 25th October 2004, 01:44 PM   #13
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Default Too tired to read the whole thread, but...

Q12,13,14 should have the resistors in the source leads, not the drains. Did someone already mention this?

Edit -> Oh yeah... duhh...
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Old 25th October 2004, 01:54 PM   #14
quasi is offline quasi  Australia
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Thanks MikeB;
Ok, Ok, Ok. I'll tell you what. I'll use VR1 during the setup phase. Then I'll measure what it's set at and replace it with e metal film resistor network This will be quite easy using PCB stand-offs.

The zener? Doh! Doh! Doh! I just forgot to draw it. I was going to use 22k.

Yeah I'm happy with 330pf.

I would have thought that 22uf in the feedback loop would be enough...oh well a 100uf would fit and I can't see a down side.

Thanks Circlotron;
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Old 25th October 2004, 02:16 PM   #15
AndrewT is offline AndrewT  Scotland
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Hi Quasi,
Add caps to shunt out noise on zd1, try electrolytic//small film.
zd2 & zd3 must come after the gate resistors. Otherwise there is no limit on zd current, then its a fight between mj340 and zd2 to see who survives longer!!! You need a zd across EACH gate to source, not across gate to rs. Check your current through the fets for a 12v Vgs and also check power across the fet in current limit condition. I think 12v might need to be reduced.
Should the feedback time constant (22u*1ko=22ms) be bigger or smaller than input time constant(4u7*33k=150ms)? Someone else suggested NFB cap @ 100uF maybe even bigger?
regards Andrew T.
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Old 25th October 2004, 02:26 PM   #16
quasi is offline quasi  Australia
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Thanks AndrewT;
Youre suggestion about the bypass caps for the cascode zener is noted.

Dunno what I was thinking with these zeners. Put them in there without too much thought. I think I'll have to do without them otherwise my gate resistors will have to be too high to limit the current through the zener (if they ever turned on).

Yeah looks like the 100uf feedback capacitor is a goer.

Thanks & Cheers
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Old 26th October 2004, 07:53 AM   #17
AndrewT is offline AndrewT  Scotland
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Hello quasi,
do not omit the Fet gs zeners. They are the standard method of protecting the Fets.
1. they prevent you exceeding the maximum gate voltage.
2. they give a very simple current limit circuit for both overdrive and short circuit conditions.
design your pcb with provision for fitting them, you could them decide for fit or not. ps. the gate zeners are your last resort to stop the FETs dying.
regards Andrew T.
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Old 26th October 2004, 08:38 AM   #18
dinesh is offline dinesh  India
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Hi Quasi,

Few questions/comments

1) What is the purpose of the 100pF in the input stage between the collectors of Q2 and Q3? That's the cascode mid-point -- very low impedance.

2) 100n pot in the input. Should this not be replaced by fixed Metal film resistors. The large DC NFB should take care of any output DC offsets.

3) Again not sure about the 100pF between the base and emitter of Q7, Q8. I don't see anything in the circuit to ensure stability -- R - C pole zero network placed between the 1K resistors on the collectors of the cascode, or Miller RC between the collector of Q4 and its base.

Dinesh
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Old 26th October 2004, 09:01 AM   #19
quasi is offline quasi  Australia
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Default Those zeners

Thanks AndrewT;
Yes those zeners will provide a degree of overcurrent protection and around 8 volt ones will do it. Trouble is as the cct stands they will blow up either themselves or/and the mje350/mje340 transistors if they turn on.

I could put individual ones on each FET gate but with the gate resistors at only 22 ohms there will be around 2 amps flowing through the zeners. I know this will be momentary but it still is brutish.

I would need to raise the gate resistors to over 100 ohms each (still too low really) but this increases the drive impedance. So I just don't know.

Another way of protecting against over current is to monitor the current through the source resistors on the FETS. A transistor switch here could shut down the driver stage or indeed the whole amp.

I'll ponder all this tonight and hopefully post my decision soon. Any ideas (from anybody) are welcome.

Cheers
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Old 26th October 2004, 01:32 PM   #20
quasi is offline quasi  Australia
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Default Schematic update

This is where I'm at.

Changes made from last schematic are;

1. The BC550's are back in a cascode arrangement with about 35 volts across each one.

2. The constant current source is a BC550 as well with about 30 volts across it.

3. Basic filtering has been applied to ZD1.

4. The gate zeners on the output FETS have been removed. Higher value gate resistors will be tried during testing and if appropriate zeners may be re-introduced and connected directly to each FET gate.

5. The feedback capacitor has been increased to 100uf plus a 33pf capacitor has been added across the 33k resistor.

6. The input bypass capacitor has been reduced to 330pf.

Thanks for everyones input. This amp schematic reflects a lot of the ideas presented.

Now I gotta design and make the new board. Maybe some photos to come later. If anyone is interested I can post a photo of the original prototype.

Thanks again and Cheers
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