FETZILLA - The Builders Place

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Greg,Just do what you think is best.What do you mean with a long time?

Meanman,

By long time, I mean perhaps 15 minutes, but this is in no way damaging to the speaker. It just means the amp takes some time to reach optimal operating conditions. Actually the extra H2 distortion created by having some DC offset probably will sound nice!

I have no personal need for a 150-200W Fetzilla and I now prefer to make designs which use cheap and readily available parts (no lateral fets, depletion mode mosfets or JFETs). If you want a 150W fetzilla I am happy to advise you along the way and help you with all the details, but this time I will not be building the amp myself.

Let me know if you would like to proceed and I will post a schematic to try.

In the meantime, the 72W quasi amp I am designing should sound pretty good and provide reasonable power. Should be done in a few weeks.

Cheers,

Greg.
 

Wow, "Quasi" really seems to be a buzz word on these forums. The interest and number of personal messages this has generated is quite amazing. I hope the amp lives up to expectations.

I think the reason chip amps sound as good as they do is the quasi output. Couple that output to a simple single ended input stage and vas and you should have a good sound IMO. Anyway, this is getting off the feztilla topic so I will shut up now.
 
A quasi output stage mandates different loop gain for each half cycle. This promotes H2 and H4 output. THD overall is not as good as a complementary design, but many say that quasi amps sound better. This is a close paradigm that higher THD increase sound quality, but really is highly controversal.

Go for it, Greg!

Hugh
 
I think the reason chip amps sound as good as they do is the quasi output. Couple that output to a simple single ended input stage and vas and you should have a good sound IMO.

I think the opposite. The reason chip amps sound as mediocre as they do is the quasi output...

I don't think that a quasi amp can be "the best". We have LM overture series, we have Naim clones, etc. but I think "we" rely more on your capability than the dreadful quasi topology (yes, we have a LOT more NPN than PNP).

But please keep the original "spirit" of Lineup's circuit: current feedback. Visually I want to see no higher than 2k2 of feedback resistance, and I prefer the feedback applied to emitter/source.

BTW, since the beginning around almost 2 years ago, Hugh always wrote LINEOUT. Something I am missing?
 
Hugh: If you are chasing asymmetry in the output stage, does this mean that you would prefer a quasi output without the baxandall diode?

Jay: I think the thing that lets down the sound of the chip amps is the very complex circuit before the output, not the quasi itself. Be rest assured, my quasi amp will stick to the fetzilla topology with its single ended input and vas and current feedback.
 
Meanman,

By long time, I mean perhaps 15 minutes, but this is in no way damaging to the speaker. It just means the amp takes some time to reach optimal operating conditions. Actually the extra H2 distortion created by having some DC offset probably will sound nice!

I have no personal need for a 150-200W Fetzilla and I now prefer to make designs which use cheap and readily available parts (no lateral fets, depletion mode mosfets or JFETs). If you want a 150W fetzilla I am happy to advise you along the way and help you with all the details, but this time I will not be building the amp myself.

Let me know if you would like to proceed and I will post a schematic to try.

In the meantime, the 72W quasi amp I am designing should sound pretty good and provide reasonable power. Should be done in a few weeks.

Cheers,

Greg.

Greg,
I want to give it a try.As I said earlier it may be a real bass amp going to 200Hz top but if you can design a high power FetZilla be my quest.The FetZilla produces such a bass it's amazing strong ,tight and rocksteady fast that I want.
 
Sorry, been very busy with work and getting home late.

It's taking shape, haven't had a chance to simulate it or finalise the design yet. Will probably be something like this. Should get it finished on the weekend...
 

Attachments

  • Clipboard01.jpg
    Clipboard01.jpg
    237 KB · Views: 464
BTW, R5 is a fixed value resistor and will need to be determined empirically. I once tested four devices and made a chart which should provide a guide (see attachment).

The Vgs multiplier is used for biasing because 40mA is probably too much for a trimmer, though you could use one if you like. Probably worth a go actually, just be sure to put a zener across it in case it goes open circuit. If you go with the Vgs multiplier it needs no heatsink.

The input also needs zener protection and a DC offset voltage reference. I would make this with a zener and a trimmer - like fetzilla but with the LED replaced by a zener.

I will try to get a final circuit posted this weekend.
 

Attachments

  • DN2540 curves.jpg
    DN2540 curves.jpg
    103.9 KB · Views: 559
Hi GregH2,

please excuse my lack of electronics Knowledge, is the feedback via R2/R1 and via C1? how does this effect phase?

Please excuse me for my lack of knowledge in the 1st place. Im also Looking at something that goes down stable at 1.8ohms and I am hoping that this will do the trick. I currently have an old 250w Hitachi base jfet/mosfet amp that is deployed but looking at different approach that can give me better microdymanics and better separation of instruments
 
Greg,

Sorry for late reply; a bit slow off the update.....

Yes, use a Baxandall diode, along with a 100R and a 10nF cap, all in parallel, supporting the emitter of the phase transistor. This works as well as anything and some amps that sound wonderful use this approach, as in deed JLH used then for years too. You cannot expect that clip is identical however, so don't expect perfection at the highest levels.

Cheers,

Hugh
 
Great thread - I'll be following this with interest. I really appreciate the knowledge here, I've been lucky enough to have recently built three amps, an LM3875 chip amp with regulated 28V rails and a 6DJ8 based tube buffer, a Hypex NC400 Ncore set of monos, and the highlight of course was the Fetzilla. I would be hard pressed to sell the Fetzilla, it is such a good amp. The thing I like about it most is the lack of listening fatigue, it's got all the attributes I like, punchy bass, detail etc, but the gem is in the relaxed presentation. When comparing it against the Ncore for example, the Ncores can sound more dynamic, more detail, however ultimately the Fetzilla always snuck back and replaced them after a while. I really thought 50w (I used 120va) would not be enough to drive my 87db 8ohm speakers, especially as the trafo's were a little smaller than I would have liked and always intended to replace them with 160's at some point, but the specs don't always tell the whole story, the Fetzilla seemed to punch above its weight, that's how I would characterize it. I never tried driving it to party levels though.
I'm not smart enough to contribute along with others here on achieving higher power output, but all the same I will be interested in how it comes together and how the sound will compare with the original.
AJ
 
Great thread - I'll be following this with interest. I really appreciate the knowledge here, I've been lucky enough to have recently built three amps, an LM3875 chip amp with regulated 28V rails and a 6DJ8 based tube buffer, a Hypex NC400 Ncore set of monos, and the highlight of course was the Fetzilla. I would be hard pressed to sell the Fetzilla, it is such a good amp. The thing I like about it most is the lack of listening fatigue, it's got all the attributes I like, punchy bass, detail etc, but the gem is in the relaxed presentation. When comparing it against the Ncore for example, the Ncores can sound more dynamic, more detail, however ultimately the Fetzilla always snuck back and replaced them after a while. I really thought 50w (I used 120va) would not be enough to drive my 87db 8ohm speakers, especially as the trafo's were a little smaller than I would have liked and always intended to replace them with 160's at some point, but the specs don't always tell the whole story, the Fetzilla seemed to punch above its weight, that's how I would characterize it. I never tried driving it to party levels though.
I'm not smart enough to contribute along with others here on achieving higher power output, but all the same I will be interested in how it comes together and how the sound will compare with the original.
AJ

AJ,
Nice review,anyone that's heard the FetZilla says the same.This is realy a good sounding amp people must build it they will not be disappointed .
 
Oops,

I didn't see that there had been a reply.

Music lover, yes, you have the feedback concept correct. As to your questions about phase I will leave that for someone who knows more than me to answer.

Hugh, thanks for your response. Can I ask what the 10nF cap is for? I haven't seen it used anywhere.

Meanman, try this:
 

Attachments

  • Clipboard01.jpg
    Clipboard01.jpg
    302.5 KB · Views: 456
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.