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Old 14th March 2009, 12:17 AM   #201
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it would probably be easier to get the jfets than rework the circuit.

if you write down all the voltage drops of the individual componenets in the front end of the cricuit perhaps we can help

-Dan
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Old 14th March 2009, 12:56 AM   #202
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well it will be an very interesting experiment and very instructive to see how it sounds after the rework.
what if no difference is noticeable?
then we will have a bipolar monster, a crazy one that can be built by lots of passionate ...a great legacy!!

i will keep on trying any help is apreciated.
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Old 14th March 2009, 01:18 AM   #203
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i understand your goal,

it will be difficult to match the jfet inputs performance but it may be possible

-Dan
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Old 14th March 2009, 01:30 AM   #204
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Quote:
Originally posted by ericfe1975
well jfets easily picks up rf , bipolars dont do it as easily.
In my experience the opposite is true. I have never had a FET amplifier
give me trouble with RF.

Le Monstre is a classic design, and there are several contemporary
pieces which are homages to it whether the designer knows it
or not.

I do think, however, that device technology has improved enough in
the last 26 years that it can be made better.

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Old 14th March 2009, 01:49 AM   #205
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Thanks for your information Mr Pass ; as jfets has an extremely low input impedance comparated to bipolars I was sure that it would means more sensitivity to RF .
I can remember days in my youth when i was playing with jfet devices picking up AM rf noise an thus having a very simple radio .

I also have heard extremely good things about Mr Pass amps that are built around FET devices.

I think that good job can be done with the monster, usually good job starts with some experimentation.
Maybe it is not such a good idea to replace the jfet with a compound bipolar device, maybe it is...trying it is the only way to know.

In your personal opinion, Mr Pass how could the "le monstre" took some advantages of the new hardware and philosophy developped nowadays?

Maybe it is time for the "le Monstre " to have a son or a bride, just like the Zen did .
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Old 14th March 2009, 12:15 PM   #206
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Try to think RF interference as bad shielding and not as component thing. To minimize RF interference in audio:

1. Use thick aluminum chassis all the way around the box of the amplifier (the best way to minimize RF).
2. At the input, Use a ferite ring and wind 1-2 windings of the same coaxial cable that comes from your RCA to the input of the amplifier.
3. Use coaxial cables and connectors. Coaxial cables for low level signal. I use coaxial cables in my new monster for the speakers too. Have you thought that unshielded speaker cables can pick RF? They can. If possible get rid of the "must" philosophy of many companies. Use RF coaxials and plugs. Low capacitance RF cabling is much better and has much stronger shields than many high quality audio cables. And since impedance in audio cables is not of too much importance, one should give priority to shielding and low capacitance of the cables.
4. If you can, try to think a way to power the monstre from batteries. This is a more extreme approach that I am thinking, but in such low voltage amplifiers it is feasible. Power supplies may pick RF interference from the high voltage network.

Finally think that you will get much more RF interference from your phono cartridge or your passive preamplifier if you have one, so shielding is of much importance there.
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Old 14th March 2009, 12:44 PM   #207
juma is offline juma  Germany
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Quote:
Originally posted by ericfe1975
... as jfets has an extremely low input impedance comparated to bipolars...
Please can you explain how did you come to this conclusion ?
One of the main reasons to use jfet in some audio circuits is its almost infinite input impedance.
Friendly advice: read the k170/j74 datasheets, read the jfet articles on Borbely's site, it seems that you lack a basic knowledge on this matter.
Best regards!
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Old 14th March 2009, 01:09 PM   #208
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Excuse me!!! you are totally right, i said exactly the opposite i had in mind: fets are voltage controled devices , so almost no curent is needed wich means an extremely HIGH impedance and thus an extremely low current level signal can have a noticeable effects.
i mixed those ideas in a very bad way.

In fact RF interference depends also of the components : a bc560 with an Ft or more than 200 Mhz, and a high gain is more likely going to pick up RF than a 2n3055 with an Ft of 0,8Mhz and much lower gain.

Jfet are comonly fast devices with an extremely HIGH input impedance , exactly what you need to pick up RF .
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Old 14th March 2009, 01:14 PM   #209
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Too bad this forum does not allow one to edit comments after the posting :P.
Thanks Juma for pointing out my BIG typo mistake!
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Old 14th March 2009, 01:16 PM   #210
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Yes, the human minds have the tendency to wonder off sporadically
I see that you had in mind to replace input JFETs in LeMonstre with BJTs. One way to do it would be:
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