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Old 30th May 2013, 10:12 PM   #131
diyAudio Member
Join Date: May 2010
Hi folks!
Building this amp now, and I have some questions. This has probably been answered, but I still don't get it. Could someone please explain to me what VR1, VR2 and VR3 does, and what/where to measure when adjusting?
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Old 5th June 2013, 04:05 PM   #132
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VR1 - Used to set 0v on output, measure on output terminal.
VR2 - Used to set BIAS, measure voltage over R10. Vr=Iq*R10

VR3 - ???
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Old 29th June 2013, 03:44 PM   #133
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Join Date: May 2001
Location: Canada

I just ordered a stereo kit, for your answers please see Fig. 2 from my link below, it's the schematic Ray used to create his PCB ;-)

I hope the small transistors are matched.

The Class-A Amplifier Site - JLH Class-A Update


Last edited by e_fortier; 29th June 2013 at 03:46 PM.
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Old 3rd July 2013, 08:15 PM   #134
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Join Date: Feb 2006
Location: Viborg / Denmark
This is very interesting...!!

-I just found this thread - because i was to repair a mis-handled JLH made just before 2000 by me, for a friend... - Shortly why im in this thread today...

In 96' the "revised" JLH 15W was also presented in a Danish magazine ( High Fidelity ), and off course i immediately built it...
Quite soon we started argumenting about the usage of a poor voltage stabilizer to obtain af constant current ofr the input / feedback stage - And a CCS by two tranny's was mounted instead - Spitting Image almost down to every detail exact the schematic used for this one...!! - Just my five cent's .
One minor difference, Though. We found the mounting of a 4.7 Ohms resistor in series with basis of the "negative" output transistor - And utilizin' Match'd Sanken 2SC3264 with superb result's. ...

Nice PCB and service from the Author...!! - I think i'll redraw my own and make a new amp, maybe witt a new input stage - extra transistor to countermeassure the 0.6V mismatch in feedback offset...

/ Lars
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Old 3rd July 2013, 08:28 PM   #135
diyAudio Member
Join Date: Feb 2006
Location: Viborg / Denmark
Originally Posted by zajin View Post
Hi folks!
Building this amp now, and I have some questions. This has probably been answered, but I still don't get it. Could someone please explain to me what VR1, VR2 and VR3 does, and what/where to measure when adjusting?
VR2 = Bias... - Set to app. 5-700mv Across R10 330mOhm ( Equals 1.5-2Amps in Bias Current... )

Power the amp to Full Heat ( app. one hour of operation ), and udjust VR1 to 0.00 Volt DC Across the output terminals ( With input shorted / no input / No Speaker )

Turn off the Amp, and allow to cooooool of.
Re-adjust in cold state DC Offset to almost 0.00 with VR3.

Wait for full heat, and repeat adjustment of VR1 + 3 until you're satisfied with the difference in cold and hot state. It should be possible to obtain app. +/- 30 mV DC offset - Which is quite OK. ...

Be aware that End adjustment should be carried out connected to the power plug where the amp is to be used - as the DC Offset is sensitive to variations in Rail Supple - EG dont make the adjustmen on the Lab Vario Transformer, as you dunno exactly that you hit the supply Spot-On. ...!!

Lars / Viborg
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Old 26th October 2013, 10:08 PM   #136
Shamron is offline Shamron  Norway
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Join Date: Sep 2010
Originally Posted by Kermit100 View Post
Thankyou for your quick reply
so would this transformer be acceptable?
Buy Toroidal Transformers Transformer,toroidal,120VA,2x18V,3.333A RS 81548RS online from RS for next day delivery.
If so, do i connect the +18v and -18V to the psu you linked me?
The amplifier draws 1,7-2A continuous current on each channel. If you invested in a good capacitor bank with some capacity, this will also require a good transformer to recharge the capacitors between bass / drum strokes which pull a lot of juice.

I'm not even close to remembering the formulas to calculate this, but i wouldn't go with a transformer that barely keep up when the amplifier is idle.

This is the transformer i use for this amplifier and even this one becomes nice and warm. Not hot, just warm.
Antek - AS-4218

The price is nice as well. The shipping costs the same as a transformer, so i would order 2 at the same time. Same shipping cost.

I Have 3 of this amplifier now, with capacitor bank from 2x30.000uF to 2x100.000uF. I can't really say i hear a lot of difference, maybe the bigger one is "feels" a bit stricter with sharp drums, not sure.
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Old 2nd January 2014, 10:45 AM   #137
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Join Date: Jul 2011
I've soldered two of these cards now and are using the Hi-End Power supply board with speaker protecting.
Been trying to use the sc5200 chip.
Anyhow, I burst the fuse on the powersupply card or add a much larger fuse and I heat up R10 on the amplifier PCB. I cannot find the error and cannot find a faulty spot on the soldering. Where do I start to search?
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Old 3rd January 2014, 11:19 AM   #138
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Join Date: Mar 2009
can I run this amplifier on +/- 9 volt power supplies ?
how much output power it will give with 4 amp bias on 3.5 ohm speakers?
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Old 4th January 2014, 05:56 PM   #139
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speedless's Avatar
Join Date: Jun 2013
Location: just short of paradise
Default JLH 2005 help

A friend and i just build a set .
His a faster builder than i am and he send me a sms;"DO NOT CONNECT TO POWER YET
We checked his boards against mine and the diagram,all just as the doctor ordered.
Components;right values,right placed and orientated,No shorts,no Discontinuity.
Newer the less,my boards acted exatly the same as his.ARRRGG!!!
The bd139 blows after a few seconds,taking the mj15003 with it !???
Whats wrong?
What should i check ?
Is there a startup position of the potmeters,BEFORE power is applied?

Getting tired and annoyed.

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Old 5th January 2014, 12:39 PM   #140
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speedless's Avatar
Join Date: Jun 2013
Location: just short of paradise
Just some more info.
Powersupply: +25/0/-25V
With the Cap.mutifier board.
No C3 (shorted) and VR1:200R
No speakers connected,input shorted.
18R/10W in serial with power rail.

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