Yet another N-Channel Amplifier

[MikeB]

hi pro !

Maybe you made something wrong with the DC-servo, so that it
puts the -input of diffamp to virtual ground ?

Mike

[pro]

MikeB:
I have followed the schematic. the only thing I have changed is the value of R14 from 100K to 50K, because with 100K the servo didn't work. With 50K I see 100uV of offset.

[MikeB]

maybe you could test to remove the dc-servo, and place a dc-blocking-cap instead. between r10 and gnd, ~100uF
Then gain should be as intended.

Mike

[pro]

Hi Mike.
I have already tried it. Nothing changes.
Ciao.

[Pelle]

pro: I don't no what the problem might be. 80dB, that's the openloop gain of this amp, so it sounds like all feedback is gone in your simulation.

MikeB: I've also changed the value of R14 to 22k.

I now have a complete amplifier on my desk and I will hopefully try it tomorrow. An initial test with +-12V showed that no dc was present at the output and that it did amplify the noise made by my finger touching the input. This was without DC-servo and with a blockingcap at the feedback.

I'll get back to you when I know some more.

-Pelle

[pro]

Hi Pelle.
Did you resolve the cross talk problems and how did you discovered you have them?
Let us know how daes it work and how does it sounds.
What bias are you using?
Ciao.

[Workhorse]

Originally posted by Lars Clausen
Pelle: Nice and straightforward design! However the big problem is (like the Citation 12 clone) that there is really high rail loss. At each rail this design starts clipping at some 10 Volts under the rail.
In practical life this means that to build a 100W amp you will have to use not 44V rail voltage but 54V, to satisfy the amplifier's rail loss. It wastes power like a 180 Watt amplifier, so you power supply and heat sinks need to be size: 180 Watt. Still the amp put out only 100 Watt.
Not a very efficient design.

R36 why not make it 50 Ohms, and then place Q13 on the heat sink as well. R17 could be 10 Ohms. Then it will work without cross conduction.

All the best from

Lars

I agree with the Lars Statement regarding the High Rail Loss.
Perhaps in our design we used push pull drivers and the VAS stage has voltage of about +-65VDC and Driver+Mosfets are operated at 58VDC to get maximum Efficiency.
Secondly Gate resistors must be of values ranging from 15 To 25 ohms to eliminate the cross-conduction at high Frequencies of order@20KHZ.
Thirdly, increase the driver stage current also.
With Regards,
WorkHorse Audio Group

[Pelle]

Hi!

I have increased the current through the phase-inverting driver to the same current as in the VAS (approx. 8mA). In simulation I get no crossconduction, even at 500kHz. But I have to measure the actually amplifier to confirm this.

To raise the voltage at the vas is not an option for me as it will increase the cost of the amp, so I have to live with the somewhat bad efficiency.

-Pelle

[Workhorse]

Originally posted by Pelle
Hi!

I have increased the current through the phase-inverting driver to the same current as in the VAS (approx. 8mA). In simulation I get no crossconduction, even at 500kHz. But I have to measure the actually amplifier to confirm this.

To raise the voltage at the vas is not an option for me as it will increase the cost of the amp, so I have to live with the somewhat bad efficiency.

-Pelle

Carry on pelle With ur Design and have Fun

Cheers
W.A.G

[Pelle]

Now I have done the first real test with +-30V. But the result was not what I expected. I got oscillation at 10MHz, about 100mV peak-peak. Didn't have time to test any modifications today, so it has to wait untill next week.
Here is a picture of the schematic I used for testing:

And here is the amplifier:



-Pelle