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Old 10th November 2011, 01:27 PM   #2071
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Tim,

Thanks for the link to CPC, tried a search for MJ15003 on the net but CPC didn't come up. There's a price break on 10+ so since I need 8 for four channels, it was cheaper to order 10, which I did this morning.

I'll have a few listening sessions before I change the transistors, I just hope my cloth ears can tell the difference.

Regards

Ned
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Old 14th November 2011, 06:06 AM   #2072
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Default my jlh-2005

Repost it here by Johnathan's suggestion...

Just finish my first JLH-2005. The circuit is from this link:
The Class-A Amplifier Site - JLH Class-A Update

schematic and pcb attached.

A few thought about the design
1. The original 1969 design use 2N3055 as Q1 and Q2. Then changed to MJ15003. I'm thinking of some other subsititutes such as 2N3773, MJ21194 or 2SC5200

2. Can I use 2SA1358 as replacement of BD140? it's complementary of 2SC3421.

3. How to set the value of VR1, VR2 and VR3? in other words, what current should those 3 current sources be?

4. Is the large polygon pour on PCB good for audio amplifier?
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File Type: jpg jlh2005.jpg (137.6 KB, 407 views)
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Old 14th November 2011, 06:14 AM   #2073
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Quote:
Originally Posted by NED1955 View Post
Tim,

Thanks for the link to CPC, tried a search for MJ15003 on the net but CPC didn't come up. There's a price break on 10+ so since I need 8 for four channels, it was cheaper to order 10, which I did this morning.

I'll have a few listening sessions before I change the transistors, I just hope my cloth ears can tell the difference.

Regards

Ned
You should order more than you need and measure the hfe on each. These parts MJ15003 I found had huge variants on hfe. I also ordered 10 of which 2 were not usable at all as the hfe readings were below the specified value. I was quite disappointed with tolerance actually.
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Old 14th November 2011, 06:18 AM   #2074
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Quote:
Originally Posted by siliconray View Post
Repost it here by Johnathan's suggestion...

2. Can I use 2SA1358 as replacement of BD140? it's complementary of 2SC3421.
Yes you can, but no sonic improvement has been reported with that part. In other words, it will not matter which part you use there.
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Old 14th November 2011, 07:22 AM   #2075
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it's better to order pair matched parts. hfe doens't mean all. The curves should be matched.
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Old 14th November 2011, 08:54 AM   #2076
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I think I'll have to set up a test circuit to verify the dc gains of each of the transistors. The first thing however will be to check the markings on the transisitor case to see if they correspond with the manufacturers data. I've hear that the MJ15003 is one of the most faked transistors.
Assuming everything is ok with the transistors I'll select the best matches for each output stage.

Cheers

Ned
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Old 14th November 2011, 10:07 AM   #2077
AndrewT is offline AndrewT  Scotland
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Sil,
I suggest you add a series resistor to each of the current setting Vr1 Vr2.
Try 100r for Vr1 and 10r for Vr2.
This allows you to check and set the target currents for these two stages.
Initially you set the Vr to maximum resistance. This passes minimum current through each stage and that way you don't blow up the amplifier at first power up.
Start with Vr3 set to maximum. This will leave Q5 becoming current starved as you increase IcQ4. The current through R11 must be >>IcQ4/100 and could rise as high as IcQ4/2
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Old 14th November 2011, 01:33 PM   #2078
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Hi Andrew,

Thanks for the suggestion. If I use 100r for Vr2, in series with a 1.9k, does it have enough range for adjustment? the same with vr2
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Old 14th November 2011, 02:07 PM   #2079
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Quote:
Originally Posted by siliconray View Post
it's better to order pair matched parts. hfe doens't mean all. The curves should be matched.
As for power transistors: Beta droop at ~1.5x quiescent
current should be matched. If they don't match at lower
current, its no big deal, the global loop and offset CCS
can handle it. But if droops are wildy different, you are
screwed! At that point you might look to SRJLH if you
still determined to make use of mismatched devices...

The npn driver and pnp error detector are doing completely
different things. Not optimal that they should compliment.
PNP should be optimally sized for gain at only a few mA.
And the NPN driver sized for best gain at around 250mA.
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Old 14th November 2011, 02:12 PM   #2080
AndrewT is offline AndrewT  Scotland
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Keep the Vr values the same.
2k +100r gives a range of 100r to 2100r, i.e. a current range of 0.3mA to 6mA
50r +10r gives a current range of 10mA to 60mA
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