X amp considerations

[DiMenSioN]

Ok here goes. I've been looking around for quite some time now to find the ultimate amplifier schematic/setup/topology.
Somehow i came to this forum and learnt a lot allready. Especially in the X and alephX threads. The NP aproach with minimal components does sound very good to me.

Below some background info on the amp im building and some general and specific considerations about the X-amp setup.


- I'm a student so i would like to keep the costs down.
- I have two 1.6KVA 2x48V sec torroids wich will supply the juice for this amp (one torroid for each of the two mono-blocks)
- I managed to get 80! 4700uF 80V nippon 105C computer grade caps. I would like to use these for the amp as well.
- For rectifier i have some 35A metal rectifiers and some fast TO220 diodes, not sure if they are slow recovery, will check on that tuesday.
- It gets pretty hot in the listening room during summer so i would like to keep dissipation (well) below 500W idle, maybe some sort of 2 mode operation (winter/summer)This might be optimal solution. The rail voltage is fixed so not sure if this is realistic option.
- Because of the high sec voltage i dont think i can use a symetric supply. So it will be 48V with fantom ground. with symetric amp that shouldnt be that much of a prob
- My speakers are reasonably sensitive and the amps i use at the moment produce to much noise when iddle. So this is a very import criteria for the new amp, no hearable noise, even with ear next to speaker.


I have some considerations about the X amp.
- I do understand why there is no global feedback (to keep speakers from feeding noise into the amp and perhaps oscilation). But because the last stage uses non linear devices this non-linearity doesn't get compensated by feedback. (all active devices are non linear?)
What about using one dummy output stage that delivers into a resistor equal to xr times the load impedance. Were xr is equal to the number of paralell output devices. Then taking feedback from this resistor/stage. This would also solve the dreaded DC offset problems?. Or might this lead to oscilations because of the crosscoupled input stage?
- In the early days nelson recomended slow rectifiers even with 100nf MKP/MKT in paralel with the diodes. In the new XA series he uses fast diodes. Anybody know about this change?
- I see that the irfp240 is a popular device for pass amps. In a document on NP's site about mosfets he concludes that the irf9240 (although it should be) issen't a real good complement to the irfp240 (articles-mos-page9). The irfp9240 is used in many X schematics in the forum. In the commercial X amps are TO-3 hexfets, irfp240 is TO247. Maybe there is a better complementary pair to be used in X-amp?
- Is it possible to use the two 48 secondaries in paralel to have a higher current rating? Should this be done before or after rectifiers? with double or single rectifier? Using one sec winding for the left leg of the amp and the other for the right leg doesn't seem like good idea because there would be less noise cancelation in the supply lines.
- Im not sure about a good way to connect the caps for filtering. I could get them for a very good price. Because i want to use fantom ground they should be even more effective. But how much of them should i use and how to connect them physicaly? A big copper strip/star/plate? Perhaps R or L in between groups of them?
- I got a 2x 9V 4A sec torroid wich i was planning to use for the gain stage. (in serie with the 48V rails) After i read Petters X-amp thread im not sure if this will provide enough headroom.
- i was planning on building a regulated supply for the first stage. but this will cost even more voltage on the first stage.
- I read somewhere that conventional transformer would be more suited for gain stage then torroid. Although i think that torroid has better performance in EM noise radiation. Why would a conventional be better in supplying extra voltage for gain stage?

I know, a lot of information and questions. I would really apreciate it if someone could help me on some of these points.
Hope my english issen't a problem

Jacques

[Zen Mod]

Originally posted by DiMenSioN
.....................
- I do understand why there is no global feedback (to keep speakers from feeding noise into the amp and perhaps oscilation). But because the last stage uses non linear devices this non-linearity doesn't get compensated by feedback. (all active devices are non linear?).......................................... ....

Jacques

you lost me there...........
rethink.......

[BobEllis]

Those transformers are a bit high voltage for an X amp. You'll end up with about 67V rails, meaning you'll be able to swing around 130V as an X amp - a bit over 1KW RMS Suddenly your 1.6KVA transformers don't seem so overkill.

Assuming that you don't need that much power, a more conventional amp would make more sense. The A75 isn't terribly hard to modify to use with higher voltage rails. I've got a pair running with 65V on the front ends and 59V on the outputs. Just watch dissipation and voltage ratings. See Dinu's higher power A75 thread.

[DiMenSioN]

you lost me there...........

What i mean is that all active devices (BJT FET MOST) have a nonlinear curve.
In case of a MOST (Id - Vgs curve).
This means that if you drive a most as a with a clean sinus that the output voltage (in a resistor) won't be a clean sinus. Even in source follower setup. You can compensate for this nonlinearity with feedback. I do agree that this nonlinearity is probably not very big, but still

Those transformers are a bit high voltage for an X amp. You'll end up with about 67V rails, meaning you'll be able to swing around 130V as an X amp - a bit over 1KW RMS Suddenly your 1.6KVA transformers don't seem so overkill.

Thats why i would like to use 48V with fantom ground, so its +24V 0(fantom) -24Volt. The question is can i paralel those 2 secondaries, and how? Is it even needed to paralel them?

[peterpan188]

The reason that linear amplifier is possible with MOSFET, BJT or whatever other active device there are, is that there is a region in the characteristic curve that is, or almost, linear, like a resistor. So nonlinearity is more a imperfection more than a inherit thing.

As for splitting 48V-0 into 24-0-24, I am not sure because I have never tried such.

Regard,
Peter

[jupiterjune]

With those transformers, I think the A-75 would be the way to go as well. One good thing about the A-75 is that you can turn down the bias and it still works, so you have an out if your heatsinks turn out to be undersized.

You can't do that with an Aleph.

You can also use fewer output devices in the A-75 than are used in the Nelson's original article, if you wish to reduce costs and complexity somewhat.

JJ

[DiMenSioN]

Originally posted by jupiterjune
With those transformers, I think the A-75 would be the way to go as well. One good thing about the A-75 is that you can turn down the bias and it still works, so you have an out if your heatsinks turn out to be undersized.

You can't do that with an Aleph.

You can also use fewer output devices in the A-75 than are used in the Nelson's original article, if you wish to reduce costs and complexity somewhat.

JJ

Havven't looked into the A75. I was not planning on building a aleph, aleph-X or XA. I ment the X*00.5 or the X*00
The alephs are indeed to hot. The X should be biasable to different values?

[jupiterjune]

I don't know a thing about the X*00's. Based on Grey's last post about the aleph X followup, maybe it will have a little more in common with them.

I sort of mis-spoke when I said you can't reduce the bias in an Aleph, but doing so reduces the power output before clipping, wheras the A-75 still gives the same voltage out. Right now, I have the bias turned way down in mine and it is sitting inside a cabinet, driving my subwoofer, bridged.

JJ

[BobEllis]

I suppose a virtual ground situation would work with an X amp, with a balanced input.

X amps can probably run at reduced bias, but watch thermal compensation - Probably a good idea to put your Vgs multipliers on the main heat sinks if you go under 100 mA per device pair.

Regulated front end supplies are a good idea. With an extra 9 VAC you'll get 12V more raw DC , which will give you at least 7V above the output rails after regulation - plenty.

You could add the output devices to the feedback loop fairly easily even if Petter didn't. I'd eliminate the 400K resistors surrounding the bias pots (which should be replaced with Vgs multipliers like in the A75) and use a 200K from the output to the junction of the 400K's Petter shows.

If you go for 65 V rails on an A75, don't cut back too far on the number of output devices if you run a low impedance load.

[jupiterjune]

- I see that the irfp240 is a popular device for pass amps. In a document on NP's site about mosfets he concludes that the irf9240 (although it should be) issen't a real good complement to the irfp240 (articles-mos-page9). The irfp9240 is used in many X schematics in the forum. In the commercial X amps are TO-3 hexfets, irfp240 is TO247. Maybe there is a better complementary pair to be used in X-amp?

I think Nelson mentioned somewhere that the Fairchild device makes a good compliment -- I am not sure where that post was, either on the f4 thread or on a thread inquiring where to buy mosfets.

JJ