140v single rail Nmos amp..

[Junm]

Good day gentlemen....

I already have this things, 220vac trafo with 100v single winding secondary (isolated) at 1000VA used as isolation trafo, 30pcs 1200/250v caps as filters in parallel, 35A br rect., 2-10,000uf at output coupling and several N-Ch power mosfets as outputs. Now all i need is a good single rail amp working at around 140vdc.

To the designers and members out there can you help me out and give me a working schematics please!....I just want to turn this things to usable amp rather than sitting on storage as junks..

[AndrewT]

you need to add DC blocking at both the input and the output to allow the quiescent voltage in the amplifier to sit @~70V above Zero Volts.

The input cap is easily solved a 100V 1uF feeding Rin~100k will let through all the audio signal you want/need.

The output cap is different. It must be able to pass speaker currents and block voltages <=70V (+mains tolerance)
A 10mF 100V cap will allow >=20Hz signals to flow to 8ohm speaker with virtually no attenuation, and progressive roll of from 20Hz down to 2Hz.

If this 10mF is connected to a 4ohms speaker then the filtering effect of that DC blocking cap moves up an octave to passing >=40Hz and F-3dB~4Hz.

[CBS240]

you could build a balanced amp, full bridge, and use no cap at the outputs.

[AKSA]

BUT, these amps still can be made to sound very, very good. And they have intrinsic speaker protection, no small consideration.

Hugh

[Junm]

Hi andrew thanks for the info, at what capacitance do i need to compensate even at 4 ohms load.

to CBS 240.

Yes it can be by having it at full bridge it double the rails at +-140v and 1 thing more is the filter caps must be very large.

Hi aksa

...thanks for dropping in.. you're an expert here can you recommend a good design schematics? I am not after a low THD as long as it will work and stable, anyway this will be use for a disco or videoke purposes.

[AndrewT]

each time you halve the load impedance, you double the coupling/blocking capacitor value to maintain the same RC.

[AKSA]

Jun,

Any stable, good quality two rail design, such as Quasi's design on this forum, can very simply be adapted to single rail operation by adding an output coupling cap and creating a bias voltage around 70V by using a simple voltage divider. You can even use the same pcb with minimal additional components. Take a look at the Quasi thread and ask around; lots of people there have built this amp with outstanding results and could help you.

AndrewT's recommendation for 10,000uF coupler rated to aboutg 100VW is fine, don't worry too much about low Z loads, that will be fine.

Hope this helps,

Hugh

[Junm]

Hi andrew and AKSA... Thanks bro for your kind advise....

Actually i had in mind the Quasi design suitable since it is quasi complementary using Nmos. I had already ask Quasi on how to modify the schematics to work on single rail.

In my knowledge it is not posible to just put a coupling cap on the input and output since there is a differential input. Am I correct?

[AKSA]

No, Jun, wrong.

You can easily drive such an amp through coupling caps at input and at output.

No problem at all, just have to be careful of switch on spikes, nothing more.

Hugh

[stocktrader200]

The power on/off thump will be huge, Hundreds of watts.
You will need at least a 47ohm 20w resistor and a relay to switch the
speakers off / bleed resistor on.
5- 10 second on delay instant off.
Leaving the resistor on will just waste output power and raise the low cutoff frequency.