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Old 10th December 2002, 12:46 PM   #211
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Just some observations:-

Having blown one channel of my Elliott Death of Zen (a JLH lookalike) from what appeared to be thermal creep and an unoticed too-small TO3 mica insulator, I decided to make some tweaks when repairing. Even though I loved the sound, I thought I could improve on it, so I replaced the existing MJ15003's (Hfe=70) with higher gain and better matched MJ15003's (Hfe=125) on the output. (I'd made that switch from 2N3055's to MJ's shortly after completing the amp a while back and it definitely makes a difference). I also replaced the existing NTE373 (2SC3421 substitute) driver & current source which were Hfe=100 with JE180's (BD139 substitutes) with an Hfe=250.

The net result is the amp just sounds hard, dry and seems to have lost the magic. As I know the MJ's aren't the issue, I'll be reverting to the NTE373's or will try to find some 2SC3421's.

The only other change I made, because I'm now using 4 ohm speakers, is I've dropped the voltage and raised the current (18V rails @ 3A quiescent instead of 40V@1.7A. Once I get a new toroidal xformer I'll raise the rails to 30V and drop the current a little). I have noticed that the JE180's run cold with this lower voltage configuration whereas the NTE373's ran warm previously and wonder if there's any sound coloration going on due to that? So I'll probably fix the power supply first before I do the transistor switchout as I'm concerned that the lower Ic max of the NTE373's may be an issue in a 4 ohm configuration. If the sounds still off once the power supplies where I want it (30V@2.5A) I'll revert to the 373's/2SC3421's. I'll update later if anyone's interested
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Old 10th December 2002, 01:08 PM   #212
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Please do so report on your findings, I'll be interested to know the differences..
Chris
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Old 10th December 2002, 01:49 PM   #213
argo is offline argo  Estonia
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Sidney

And while you are there (I mean transformator change, power supply redesign and everything) maybe you can try to modify the whole thing even a little bit further and try out one of the JLH versions. I have always been interested to make some comparison between those two but newer managed to build DOZ myself.

Argo
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Old 11th December 2002, 03:23 PM   #214
Joel is offline Joel  United States
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Default Tranny to use?

Hi,

I just finished wiring up the PCB of my 1969 version. I have a power transformer I lifted from my old crap Pioneer SX2000 receiver, that I was thinking of using for my JLH.
Will it be up to the task?
I don't have any specs on it, but the Pioneer was 60W per channel.
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Old 11th December 2002, 03:48 PM   #215
halojoy is offline halojoy  Sweden
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Default Class A

Quote:
Originally posted by Drafance007
Sorry, but i am a little confused right now.. Which one of all this schematics works good. I am intrested in building some low power class A amplifier and i am asking someone who allready built some amp to give me the link or schematic. Thanks! Dragan.
hello
Drafance007 YESTER-ME. YESTER-YOU, YESTERDAY /Stevie Wonder (paul mcCartney "Michelle/Yesterday"
i am no sherlock holmes
but you should keep asking these "Solidarity poland" guys
anything
and the lord will help you to have a little nice
Classy amp, after Christmas, January/February

I have built
my own mono class a design
that can deliver 4 Watt pure into 8 ohms
i love it!
will make another one for stereo at summertime!

/halojoy home of music and for real chess brains
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Old 11th December 2002, 10:54 PM   #216
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Exclamation Re: Class A

I been on the link you posted but there isn't any schematic of that 4 w amplifier ?
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Old 18th December 2002, 08:58 PM   #217
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Thumbs down Class A

There...something that i get to work...schematic form yugoslavijan magazine "radio amater" from 1984.
Attached Images
File Type: jpg classa.jpg (60.4 KB, 2617 views)
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Old 11th January 2003, 01:05 AM   #218
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Default Re: Jlh poasted by Les is not Class A methinks

In Oct 2002 a cct was posted by Les as the JLH 1996 Class A amp. It looks Class B to me. Shouldn't the speaker be connected to the low Voltage rail by a capacitor and not to Ground as shown!? In the 1996 JLH article the speaker was connected to -V through a cap. I am just tryin' to learn this stuff again, but I did do 3 years of EE 20 years ago before dropping out so pardon me if I am wrong. Thanks.
Greg
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Old 12th January 2003, 03:35 PM   #219
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Default Temp Measurements

Thanks to one of our member posted the thermometer model from Radio Shack in another thread that I went and bought one and now do not have to use my hand to test the heat.

The measurements are for my JLH for ESL between both mono blocks, if anyone can foresee a protential problem please let me know and warn me about it, thanks.

8 Output transistors case temp MJ15003 best to worst, 63C-67C
8 Heat sink temp for MJ15003, 48C-52C
4 Bridge Rectifiers case temp, 63C-67C
2 Transformer(22-0-22 300VA) top plate temp, 52C
4 Regulators LM338K no worries, did not take measurement.
Supply rail after regulation is 20V and Iq bias at 4A. All measurements were taken after 2 hours at room temperature of 21C.

Thanks in advance,
Chris
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Old 12th January 2003, 05:51 PM   #220
rmgvs is online now rmgvs  Netherlands
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A heat sink temperature of around 50 degrees is very fine for a JLH design. In my case, I have smaller heatsinks and about 1,5 A idling and do measure around 70-75 degrees on the heatsinks, which is very hot indeed (not recommended). But it works fine for more than 3 years now.
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