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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old 28th December 2016, 07:20 AM   #3441
Wierd is offline Wierd  Netherlands
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Thanks for the sugestions,

I wil try the different opties en reported to this forum.
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Old 28th December 2016, 02:59 PM   #3442
strawa is offline strawa  Bosnia and Herzegovina
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Default brand new old jlh69

Hi to all!
Inpired by this thread I made JLH 1969 version just to see what is that sound everyone here is talking about
I made one channel while I was waiting for some resistor/caps etc. to start working on Apex fh9 xrk mod and I was just blown away with that litle amp in mono version! So I decided to make second channel and try stereo version on my main technics/bose system.
I was thrilled again, sound is awsome, details are a lot better than any of mine previously used amps (ab and d class), bass is strong and tight! I made it entierly of scrap parts I had laying over the room. Outputs are 2 different pairs of old original 2n3055s, bd139 and ei nis 2n2905 transistors. It was so good that I have decided to give it a nice housing and it is my main amplifier now. Higly recommend it to anyone!
My heatsinks are small and with 1.5A per channell it needs active cooling, but it is not a problem beacuse my comp is next to it so I cant hear that 12cm fan and I can keep my fingers comfortably on heat sinks
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Old 28th December 2016, 04:58 PM   #3443
Mooly is offline Mooly  United Kingdom
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JLH 10 Watt class A amplifier
That looks great
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Old 28th December 2016, 05:17 PM   #3444
Wierd is offline Wierd  Netherlands
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Looks nice,

Thanks for sharing
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Old 28th December 2016, 06:15 PM   #3445
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Wierd View Post
Bigun,

The jlh69 is a fine amp and i use it almost every day.




I think you are right it can by oscillation.
I will try compensation.
Also i ordered a new set 3264 they have also higher gain.
It seem on low volume there is more detail.
Try Hood's compensation a capacitor from collector 2nd transistor to emitter 1st transistor.
330 pF should do the job. Without this lag component your system is analagous to reset an ignition system so it fires dangerously close to to dead centre. There will be no interuption to the feedback at hf which is possible with Miller compensation - an approach deplored by Hood
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Old 28th December 2016, 07:13 PM   #3446
Jonathan Bright is offline Jonathan Bright  Australia
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Hi Weird. Might be an idea to read gannaji's post (#3439) first as JLH's letter there speaks of the need to use all THREE elements of compensation and NOT just one......
Cheers Jonathan
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Old 28th December 2016, 10:02 PM   #3447
Wierd is offline Wierd  Netherlands
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I will try the different options
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Old 29th December 2016, 12:14 AM   #3448
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Jonathan Bright View Post
Hi Weird. Might be an idea to read gannaji's post (#3439) first as JLH's letter there speaks of the need to use all THREE elements of compensation and NOT just one......
Cheers Jonathan
Been there done that 40 years ago. The amplifier was stable without these components if there was separation of the output wiring and the input.

The ClassAB amplifier that followed this used the same voltage amplifier layout, the driver running in Class A leaving the complentary outputs MJ481 and MJ491 as the only switching stage. This circuit is in "The Class A Amplifier Site" hosted by Rod Elliott,

Hood subjected this to listening trials with test subjects against the Class A amplifier.

The audience could not tell them apart. However for builders who preferred Class A operation for intellectual reasons it could be built to run in Class AB or Class A.

The Class AB used 4MHz outputs and the compensation capacitor value was 390 pF. There is a zobel but that is it.

If 2SA and 2SC devices are your choice, you could put them to good use there.

Of course in a non switching stage i.e. there is no switching to slow the ouput stage down - 330 pF should be enough either the Class A or Class AB circuits.

What one is looking for in selecting a capacitor deployed as per Hood is choose you roll off frequency and select a capacitor value equal in impedance to the feedback resistor value.
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Old 29th December 2016, 12:41 AM   #3449
Jonathan Bright is offline Jonathan Bright  Australia
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Hi Wierd that is just the point. JLH said that they are not options....its all or nothing.
refer to the page that is posted #3439.
Cheers JB
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Old 29th December 2016, 05:36 AM   #3450
mjona is offline mjona  New Zealand
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[QUOTE=Jonathan Bright;4931253]Hi Wierd that is just the point. JLH said that they are not options....its all or nothing.
refer to the page that is posted #3439.

The cut off frequency of the mod you refer to is around 50 kHz whereas the output transistors are capable of 4MHz.

The stipulation applies to those limitations to deal with a problem of circuit layout. Hood did not incorporate this mod into his own unit. Having tried it myself I can appreciate the reason he didn't.

If 2SC devices are used changes in phase will arise at higher frequency so these are further up the frequency scale and less critical - certainly the phase correction components are not to be seen in other Hood amplifiers using the same voltage gain structure as the Class A amplifier.
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