Power amp under development

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#3,342
yes i was previously using a solid state preamp. the preamp was just a simple opamp circuit with basic tone controls.

It worked great (except for some distortion if the bass tone was turned down too far, but that was a preamp issue). This is my first tube preamp I have tried with this amp.
 
#3,343
I dont have much experience with tube amps, but the impedance matching may be an issue here which is causing the NMOS to oscillate. The power ground and the signal ground being at different potentials in the NMOS may also be a contributing factor (with the tube pre-amp connected).
Hari
 
#3,344
You do not have to buy another. If you hav another 39pf just put this in parallel with the allready fitted. In this way you will have 78pf as miller cap. Try this first. In this way you can add as many as you like and as a result you will increase the values step by step.
 
#3,345
MAKING QUSAI'S 500WATTS AMPLIFIER

I AM MAKING QUASI'S 500WATTS AMPLIFIER.NEED HELP.
I WAS MAKING THE AMPLIFIER I WASNT ABLE TO FOUND MJE340 &MJE350 TRANSISTERS SO CAN I USE MJE3055T AND MJE2599 TRANSISTER I AM USING A SUPPLY OF 45-0-45 TRANSFORMER GIVING 63.4-0-63.4 VOLTS DC AT 6 AMPEARS.
 
#3,346
no,
these are not equivalents.

Your 45+45Vac 500VA transformer will not give anywhere near 500W into 4r0.
It should just about give 200 to 240W into 8r0 if you build it appropriately.
The PSU will sag terribly trying to drive a 4ohm speaker.

You will be lucky to get an extra 50% of power into 4r0 compared to 8r0.
 
#3,347
Re: MAKING QUSAI'S 500WATTS AMPLIFIER

ASHKARMALIK said:
I AM MAKING QUASI'S 500WATTS AMPLIFIER.NEED HELP.
I WAS MAKING THE AMPLIFIER I WASNT ABLE TO FOUND MJE340 &MJE350 TRANSISTERS SO CAN I USE MJE3055T AND MJE2599 TRANSISTER I AM USING A SUPPLY OF 45-0-45 TRANSFORMER GIVING 63.4-0-63.4 VOLTS DC AT 6 AMPEARS.
Click to expand...

MJE340/350 are freely available, at least here in Mumbai. Where are you located?

Hari
 
#3,348
Re: MAKING QUSAI'S 500WATTS AMPLIFIER

ASHKARMALIK said:
I AM MAKING QUASI'S 500WATTS AMPLIFIER.NEED HELP.
I WAS MAKING THE AMPLIFIER I WASNT ABLE TO FOUND MJE340 &MJE350 TRANSISTERS SO CAN I USE MJE3055T AND MJE2599 TRANSISTER I AM USING A SUPPLY OF 45-0-45 TRANSFORMER GIVING 63.4-0-63.4 VOLTS DC AT 6 AMPEARS.
Click to expand...



hi there

no problem in finding them in delhi as well.beware of fakes of on semi
better prefer cdil devices.for voltages around +-50 vdc you can use bd139/140.



ravs



be a vegeterian.
 
#3,349
ok, i tried upping the miller capacitance to 50pf... nothing is different. is VR1 ment to do something?

I have checked the output impedance of the preamp and its 1.5k. the preamp circuit then feeds into a volume control 10k pot, which then goes into the Nmos 500 amp.

any more ideas on this mysterious 1.5v dc?

thanks
 
#3,350
if VR1 does nothing then something has been assembled incorrectly.

BTW,
an unbuffered 10k volume pot with a 1k5 source impedance will present a variable source impedance to the power amplifier that varies from near Zero ohms to ~2900ohms.
That will have a profound effect on what the power amp is able to do with the signal.
 
#3,352
ok i will try an opamp buffer between the valve and the volume control. I already have a buffer in there going to a line out, so it shouldnt be hard for me to quickly hook it up for a test.

as for the 1k pot doing nothing, im at a loss. I will try a new one in it soon and see if that helps.

thanks for the help 😀
 
#3,354
ok, just an update for anyone interested. My issue was grounding. the nmos 500 has a 10 ohm resistor to separate the signal ground from the amp ground (its R2 on the schematic I have). once it was rewired (the preamp ground is now seprate from the output ground) all my problems faded away.

Just a Q thou, can i remove the 10 ohm resistor and replace it with a wire link? will that cause me more harm than good?

cheers....
 
#3,358
Thanks Hugh. Although I have a stereo amp of Quasi's amp playing for nearly 2 years now, I have not been able to replicate it with a changed PCB layout - separate input and output stage boards. Even the diode + resistor drop in the rail voltage to the LTP is not working.

The current source at the bottom of the LTP does not behave consistently when the resistor to ground is split and their mid point decoupled to negative rail.

What I fail to understand is that the current draw alternates between tens of milliamps and zero when the trimmer in the Vbe is rotated both in practice and in simulation. I wonder if there's something wrong with the whole front end of this amp.
 
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