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Old 16th June 2004, 05:29 PM   #21
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Quote:
Originally posted by fab
hi


The funny thing about this circuit is that it is one of our projetc in EE course back in 1987. I had a weirdy teacher close to retirement and I realised only many years after that he was very knowledgeable.

Fab

Naaah. Couldn't be. Was this Jake?


Francois.
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Old 19th June 2004, 03:35 PM   #22
fab is offline fab  Canada
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Quote:
Originally posted by ronybc
...
(Fab) I tried disconnecting emitter of T1 from output and grounded it through a 100ohms. BUT the result was huge distortions in the output. I don't know what went wrong.. can you help me...

Thank you,
Rony B Chandran
Hi Rony

Sorry, I was away on a business trip.

I supposed that you are referring to your suggested circuit. I am not sure what you are trying to achieve by changing the emitter of T1 from output to ground. Can you explain?

Fab
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Old 19th June 2004, 03:37 PM   #23
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Quote:
Originally posted by DSP_Geek



Naaah. Couldn't be. Was this Jake?


Francois.
no, never heard of Jake. It was Gaston A.

Fab
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Old 24th June 2004, 11:49 PM   #24
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Well, I guess that besides the crossover distortion the point is the mosfet on the upper and the one down the schematic are utterly differently driven.
The 'up' mosfet is driven by an output of the opamp (current less than 150mA I think) so it can't switch the capacities of the mosfet very well. The latter mosfet is driven by a colector of the bipolar trasistor ( and it hasn't difficulties with a Cgs of the mosfet).
But the essence of this schematic is the high-boost filter between the output of the opamp nad the gate of mosfet so eventually problems with mosfet capacity and high-boost filter do do cancel each other out !!!
Nevertheless it looks ugly and i bet it works satisfactory only by accident...
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Old 1st July 2004, 06:29 AM   #25
ronybc is offline ronybc  India
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Hey all...

last week i downloaded a free LTspice circuit simulator from http://www.linear.com/software/ Its so usefull and it really made my life easy, goodbye to those good old 'smokey' days. With the simulator I realise that my first design was an utter failure :) thanks for your comments on it.

The dynamp is now revised, see http://www.ronybc.8k.com

This one is more simplified, but working. Once again I'm requesting everyone's opinions and suggestions about the circuit.

By LTspice simulations.. I've found distortions with low level high frequency signals (0.01v peak to peak, 20kHz) But the output improves with increase in signal level (0.1Vp-p to 1Vp-p), no problems with low frequency region.

All i learn from this is that mosfet's are not that good for (simple)linear applications. Ignorance about the gate charge characteristics lead me to all those failures :) BJT rules..! :)

Have a nice day...
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Old 1st July 2004, 08:03 AM   #26
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Quote:
Originally posted by mandat
At the Polish forum is shown a simple power amp, based on OPA551/2 opamp at the input stage and complementary MOSFET pair IRF540/9540 at the output. The voltage of gates is setting with TL431. The sonic quality is quite impressive.

cześć

could you send a link or address of this site with forum?
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Old 1st July 2004, 02:11 PM   #27
ronybc is offline ronybc  India
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Hey dudes...

sorry for distorting with dynamp...

I've made serious changes to dynamp since my last post. The circuit is entirely changed, now it uses seperate supply rails to the opamp (+/-18V) and MOSFETs (+/-45V). LTspice simulations successfully delivered 32V peak-to-peak clean sine wave into 8ohm load (input: 1.5Vp-p @20kHz).

Yeaaaaahh... and I'm so glad... :) Thanks to everyone...

You can download the LTspice model of Dynamp 2.1 from
http://www.ronybc.8k.com
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Old 2nd July 2004, 04:11 AM   #28
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Hi ronybc

I am glad that you got inspired by my post #3 of "Class AB Amp Project " thread!
The added local feedback in the output stage is a good idea.

With the proper gain at BJT driver stage (like 4 or 5) you should get more than 32V peak-to-peak (16V peak) but close to 40V peak (assuming no drop in power supply voltage and VGS of about 3V).

As the circuit in my post, you could use potentiometers to adjust for mosfet bias current.

I suppose that you could install capacitors at base of BJT to prevent oscillation before applying C3.

I konw it was not in my posted circuit (which was a school project) but you should install serie 220 to 470 ohms resistor at mosfet gates to prevent high frequency oscillation.

Good luck

Fab
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Old 2nd July 2004, 07:38 AM   #29
ronybc is offline ronybc  India
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Hey fab...

that was an error.. the actual output was 64Vp-p. And the simulations went upto 300V peak with +/-320V powersupply... :) that's 10,000Watts..!!! LTspice simulator is cool.. so cool that it never needs heatsinks :)

I'll realise this amp as soon as money arrives.

Thanks for your support

Enjoy DIYaudio...


--offTopic--
Does anyone know why this thread got that black dot with the folder icon placed left side in the thread list page..? I'm new to this forum.
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Old 12th July 2004, 04:54 AM   #30
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A good opamp based mosfet power amp is the project of 300w mosfet power

amplifier by Martin Pipe which was published years ago in Maplin

Magazine/projects book 71, the kit of the same is the velleman kit K4010&K4020,
I wonder if any of you has made it ,I will be greatful if someone could give me the

list of resistor and capacitor values.
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