Help ZEN's cousin to come to life!

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Hi guys,
I am currently building PLH and mini Aleph and lack of the final fets that my supplier will send me soon, so I jumped on something else to have my time occupied by something cool...so I jumped on Nelsons schematics for the ZV8 but had no J fets here, so I took a N channel mosfet I had lying here and a IRFP 240 and built this(picture) yes it works ...sound is not bat at all, but on sinewaves seems that the upper part of the sinewawe is cut straight, would anyone be able to have a look at this schematic ant tell me what I did wrong? i'd really like to fix it as the amp sounds really good til 5watts then it starts ti clip on the upper side of the sinewave..:bawling:
Vince
 

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Hi Guys!
thanks for comments, I actually tried to trim the 22k (that is varianle resistor) and managed to get 9V at the gate of the IRFP 240 ...
but this adgusment does not help to solve tho whole problem...
any other idea? why i can't get the right rail value?
Thanks mates!;)
Vince
 
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bobsinclar said:
Hi Guys!
thanks for comments, I actually tried to trim the 22k (that is varianle resistor) and managed to get 9V at the gate of the IRFP 240 ...
but this adgusment does not help to solve tho whole problem...
any other idea? why i can't get the right rail value?
Thanks mates!;)
Vince

you need to readjust biasing (consequently-Iq,too) and Rload (in this case 14E) to achieve symmetrical clipping;

when you achieve that ,with output node at ~ Ub/2 ,that's it .
in other words- tweak divider for cascode biasing,Rload and even 0R47 to ground
I can't tell you more without looking at precise graphs.......my toob brain works just in that way ;)
 
HI, I love the looks of that thing because it's simple, and I have seen that such circuits can work pretty well.

:2c:

The bias on the lower fet : looks like a 300K resistor from drain to gate, and no resistance from gate to ground - meaning, I think, that the driving source has a big fat gate-to-source capacitance to deal with - which is not so good.

I believe that if you put a voltage divider pot on the lower fet, of a value between 10K and 25K, you can tweak it to maximum output.

You have a 14 ohm resistor dropping 13 volts, so the Idle current
would be 926 mA. For 5 watts into 8 ohms(?), we need 18 volts pk-pk, 9 volts pk, so 9/8 = 1.12 Amps Idle current minimum is needed. So there must be some modulation of the current source going on there, I think. I am not familiar with the operation of that current source you are using. I have a very simple one I like, but it can't be modulated. It uses the gate-to-source voltage to regulate itself. In my experiments, I seem to recall it provided better regulation than the Aleph current source, using dummy loads (although whether that is so important, I don't know).

:soapbox: If you use resistors in the value around or below 10K on the lower mosfet, and make it a standard voltage-divider bias, I think you will have more success. :wrench:
 
Hey thanks guys!
here is the improvment:
carpenter and zen higlighted the fact that the supply value should be divided by 2 at the drain of the 240 by trimming it's gate resistor, actually I just had to use another value of resistor and we got it:D the amp clips symetrically, but only when trimming gives me 16v at the 240 drain when I have 18v at the supply (I don't know why, but when I put half the supply at his drain clipping isn't symmetric...)
Now the amp sounds wonderful very punchy bass and sweet meds...
peak to peak I get about 12v that is to say (if i am correct 6Vrms) on a dummy 8ohm load
I'll try to work on the cascode small fet now just like JCM proposed to...
but this seem a very good start , maybe i'll had to apply some feedback? any comment is welcome!
Vince
 
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bobsinclar said:
...................
but this seem a very good start , maybe i'll had to apply some feedback? any comment is welcome!
Vince

agree with everything ,especially with glorifying Carpenter and me ;) ........

feedback-you already have that :devilr: ,but if you mean on regular loop job,than is time for standard resistor in line with lower transistor gate and one from ldsp node to gate........in this case you have cap induced mumbojumbo included in feedback ;)
 
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bobsinclar said:
Oh just forgot,
what could I do to get more power out of it without replacing the supply? I'd like to get 10wattsrms is that possible?actually I didn't choose to clip at about 4 watts:D it's just the way it ended but i'd prefer 10!;)
Vince
use inductor instead R or use CCS .........whatever
in both case you need to readjust biasing divider(s)
or just make F2..........if you are lazy
 
"agree with everything ,especially with glorifying Carpenter and me ........"

hehe

:D you and Carpenter made my amp alive;) he'll never forget:cool:

thanks for the tips, I'll go on working on it following your advises on the global feedback loop...
Best regards!and many thanks Zen , and also JCM and Carpenter,
really nice place here!
Vince
 
:magnify: ~ I just noticed I totally missed that that was a cascode design. Somehow I looked at that 14 ohm resistor as a filter, not a load resistor, but I see there's no dc cap. there; and that the output is taken from there:Ohno:

The only thing I can think to get more power is to increase the idle current, maybe increase the supply voltage. Might replace the load resistor with a current source - that might be the single biggest improvement. As it is, with the 14 ohm load resistor and an 8 ohm load, the amp is driving a 5 ohm load.
 
bobsinclar said:

the amp clips symetrically,
but only when trimming gives me 16v at the 240 drain
when I have 18v at the supply
(I don't know why, but when I put half the supply at his drain clipping isn't symmetric...)

Now the amp sounds wonderful very punchy bass and sweet mids...
.

To have output at half the supply is many times NOT the best solution. Sometimes is, but far from every case.
We have several good examples of this fact.
In Nelson Pass amplifiers and others.


To adjust for symmetrical clipping, if you want high and good QUALITY output
is more important, than then V/2 output.
Especially when we like in this case
deals with a non-symmetrical amplifier topology.

For so called, dual supply and totally symmetrical amplifiers
using NPN + PNP transistors in mirrored configuration,
is another case.
These uses 99.9% of the time output at half the supply.


lineup
Lineup Audio Lab
http://lineup.awardspace.com/
 
JCM said:
The load resistor being 14 ohms, and the load being 8(?) ohms, in parallel, the load is 5 ohms, normally, anyways, I mean for less exotic kind of amps.

The theoretical most effective output stage and maybe also best quality higher power output,
is reached when the amplifier load resistor (DRAIN resistor)
is equal to load average impedance (LoudSpeaker).


But lower the resistor from 14 Ohm to 4/6/8 Ohm
in this case will probably increase the total power too much.
A lot of heat to dissipate :att'n:

But if you have a another good transformer with a bit lower voltage
then it might be worth a new experiment.
Some other component values would need to be adjusted, for sure,
and you may even have to use some other MOSFET.
.... it would be another amplifier .... with another performance ....

lineup
 
The theoretical most effective output stage is reached when the amplifier load resistor (DRAIN resistor)
is equal to load average impedance (LoudSpeaker).
True - theoretically.

But lowering the resistor... will [result in] ...a lot of heat to dissipate.

One could always take a cue from N.P. and use light bulb filaments for the load resistor. Without turning up the idle current, and maybe supply voltage, a ccs is probably the fastest, maybe only way, to crank up the power output.
 
JCM said:

.... a ccs is probably the fastest, maybe only way,
to crank up the power output.

I agree fully, JCM.
One P-Channel IRFP9240 on top as costant current source.
Not only for cranking power, but for lower circuit distortion at equal power output.


But I don't know ...
... there is something special about the sound of a Power resistor loaded Single end output!
And resistors are more linear than any semiconductor.
Even wire wound power resistors are.

It is much up to what the designer himself fancy to do. :)


There are always so many choices ... so many ways and combinations.
This is a bit of what makes audio amplifiers so interesting.
There is always a new idea to test ... in theory, simulation
.. and :clown: even in plain reality, ........ sometimes.

lineup
 
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