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★★ Free PCB for JLH 2005 Class-A Amplifier

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#53
Adjustment procedure

Hi there.

I bought a couple of these pcb's. Both the first version you made and 3 sets of the "new" edition. I also bought the power supply you sell on your webpage. 🙂

I'm wondering, what's the "correct" procedure to adjust these boards when first turned on? i feed them with -22/+22V and i get mostly around 20 on the speaker output. To start with, i set VR2 about to mid position and adjust VR1 to try to get 0V on the output but only near the absolute end position of VR1 it changes, and then so fast that it goes from +20V to -12V almost by my finger shaking.

I measured all the components while i soldered, but should VR1 have had another value?

Anyways, when turning it on first time, how should i proceed with adjusting it? and where do i most easily measure to know when the bias is 1.7A?

Sorry for noobish questions, i feel so stupid i probably don't deserve to own a JLH but hey... silly questions are what forums are for, right? :-D

Ben.-
 
#55
siliconray said:
Hi Shamron

Did you installed C3? if not (short 2pins) VR1 should be 200Ohm. If installed, use 2K for VR1.

Measure the voltage on R10 you get the Iq by the equation: Vr=Iq*R10. For 1.7A it should be 0.561V.
Click to expand...


There are no C3 on this first board i got:

An externally hosted image should be here but it was not working when we last tested it.



Or am i wrong? The 3 last pair of board i got, have C3 on them and i soldered it in. shouldn't i have? Still.. what to do with this one?
 
#58
siliconray said:
check the value of VR1. If it's 200 Ohm, remove C3, then short the 2pins of C3.
Click to expand...

Ok, done that. When i check it with the voltages that this page states The Class-A Amplifier Site - DC Voltages

I get -23v on collector on tr1 and +23v on collector on tr2.

When measuring further down the line i notice that the voltage at tr5 collector is also 23v. How?

SOrry for nagging, i am furious on myself for not keeping what i learned at school up to date, but i've forgotten so much 😛

should r13 be 22 or 100 ohm? i got two different schematics saying two different things.

EDIT: Since you have updated this board and i have both versions, i notice on the schematic here http://www.siliconray.com/jlh-2005-class-a-amplifier-kit.html# that r13 is 100 ohm and vr1 is 2k (on the schematic) but the part list says something else. I got a 22 ohm r13, a 200 ohm vr1 and on the new board i removed c3 and shorted the points where is used to be. still, i can't get any of the versions to work. I'm giving it +23/-23V and using mj15003's as outputs.

What could i be doing wrong? 2 boards, one of each revision, exact same problem. Tr8 gets really hot when i turn bias up so that Vr10 is 0,560v. and only one of the outputs gets warm.
 
Last edited:
#60
Well, i've gotten the first one i bought from you to work now. Works nicely actually. I put it in an old case i had for an older amp so it looks a bit reworked, but it sounds ok.

Here's a pic from before i closed it up.

An externally hosted image should be here but it was not working when we last tested it.


Now to see if i can get the new board to work. 🙂
 
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