introduction

hey there,

I.ve just registered so i want to introduce myself i little before posting anything serious. I'm mainly concerned with power amp construction and at that till now mostly concerning the hiraga designs, i am however considering the linsley hood design as well now. I've built both the hiraga le monstre and the le classe with great results and i'm planning to go ahead. going ahead means i'm looking in to combinations of cascode amp design on the basis of the lavardin patents and a pass article applied to the hiraga designs. Basically the idea of a constant power amp.
For now my main concern is still to get rid of the hum resulting from using a dual mono psu for the le classe and as i'm out of options i'm planning on turning it in to a monstre amp.
first experiments on both the switchover and the cascode seem promising but i still want to lay low on that, if anyone has an answer concerning my hum problem ( I've been at it for three years) please do so. got a nice idea to get rid of the offset swing tough.
As for the hood design i looked at geoff's website (great by the way) and one thing that struck me was the fact that it was said that it isn't good to use more modern transsie's since it would cause instability. Now i think at first sight of course that this has to do with the feedback factor. Using better transistors should improve linearity and hence reduce the need for feedback, reducing the amount of feedback in turn reduces the loop stability problem i'd say. But then again it's just a hunch?

Greetings Jazz

Oh by the way it was an article by rudy v. Stratum that got me started on this gig and cost me so much money, no I don't want a refund!
 
Welcome to the forum, Jazz. It's good to have a fellow low-power, BJT, Class-A enthusiast here. Particularly one who appreciates some of the older designs.

I included the word of warning about high ft devices in the output of the JLH Class-A as some people have had problems with oscillation when trying to use them. However, I know of others who have used the 2SC3281 or similar without any apparent problems.

I suspect that the physical layout has a significant effect on the likelyhood of oscillation and that the 1996 version is more prone to instability than the original 1969 circuit. I hope to do some practical testing in the New Year to determine what additional compensation is required to ensure stability with the likes of the MJL3281A. The existing circuits rely on device characteristics to ensure that the open-loop gain drops to below untiy before the phase shift reaches 180deg. With high ft output transistors I think there is a definite need for a more controlled high frequency roll-off.

Finally, yes, Rudy does have a lot to answer for :)

Geoff
 
A little late this reply, but I see it only now.

Concerning the hum in the Hiraga class-A (if I understand it correctly):

The Hiraga has a strange thing: the hum can only be rid off by putting in a small power resistor in the +/- power supply lines, a value of around 0,22/0,33 or 0,47 Ohm/5-25 Watts is okay.

Even when building double mono this tric is needed. And to be sure: the original Hiraga amplifier as manufactured in France shows this same phenomenon.

Other Hiraga amps such as the Monstre do not have this characteristic.

Rudy
 
Hi Rudy,

Well as a matter of fact i build both hiraga's on the basis of you're article in audio&techniek a few years ago (i think it was you're article at least?). As for the sound quality / peculiarities of both hiraga's i can agree with you're comments in general but ultimately like the monstre more, maybe that has something to do with that hum as well. The le classe is more warm & friendly in character and ultimately easier to listen to. The monster more refined and detailed in it's imaging etc. (you know the usual vagueness). If you still have one of the monsters around try bypassing the lower half of the resistor divider on the input cascode. This brings the sound a bit closer to the le classe, try it. Furthermore iv'e build both with and without bandwith limiting and i think they both sound better without. I did use the mentioned resistors to as high up as one ohm.
The hood design has never gotten any further then a makeshift prototype and only one channel at that so I can't really comment on that one (yet).

greetings joris

btw. if you did write that article you got some answering to do for getting me in to this thing. ;)
 
I did write the article, that is true. So tell me what questions I should answer for. I never told it would be easy and/or cheap. For most normal people I advice to just buy a decent amp in the shop (oh maybe the editor did leave that conclusion out of the article).

The hum issue wonders me. I never had any hum with the Hiraga's. And also did I build without bandlimiting filters, no problems and better sound, I agree.

It is a pity I do not have any Monstres around anymore. Maybe it is an idea that you let hear me the modified Monstre, and I let hear you my JLH. We both live in Holland. Just for the sake of this soft science.

Rudy
 
rudy,

I made the remark as a joke more or less and certainly not in a negative way. The article got me interested in trying something like you described and i got there, in the end. And you did warn about the unexpected results one might get. The amp did sound very good from it's first humble beginnings though. Anyway it got me into something i've enjoyed very much so please see it as a compliment. one of the pta's you described in an earlier article is still on my things to do list btw.

As for that hum it still surprises me as well I've spend hundreds of hours with my scope hunting for possible ground loops, build a psu of some 190.000 uf which was very good, used shielding, starwiring to the max, in short I tried everything.

As for the possible comparison That will have to wait since two months of bad luck cost me my preamp, speakers and cdp. The only things surviving are the monster and turntable. and i'm very busy at the moment as well. I can try to post some pics of this first monster though.

Greetings Joris
 
pics

hi There,

here are some pics of the monster amp. remember this was pretty much a sunday afternoon project with parts i had lying around.
 

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