Cloning the Luxman IC known as DML-02 - diyAudio
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Old 4th January 2012, 03:54 PM   #1
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Default Cloning the Luxman IC known as DML-02

Luxman used an IC in some of their otherwise excellent preamps that was unreliable. There were two actually, the DML-01 and the DML-02. This is about the latter, the DML-02. When these quit you are left with a boat anchor.

I have more anchors than boats so I'd like to clone this IC, but I need some help understanding the circuit so I can select some non-obsolete components. I'm semi-clever around tubes but not the hottest iron on the bench around solid state circuits, so this may involve a box of these: Just a warning, type slowly.

This site has a sketch of a circuit that might work but It uses lots of obsolete parts and I suspect that the pin 5 connection may have been inadvertently omitted (not sure). My C-12 amp has a replacement board that was probably a Luxman workaround once they figured out there was a reliability issue with the potted IC so I removed it and drew it. That drawing is attached. It's similar but uses only six active devices rather than eight.

I have intentionally left the bottom pair of devices ambiguous as I am not sure what they are; no marks on the devices (all TO-93). Looking at the other drawing, they are probably a 2SK60 which is some sort of FET that I am not familiar with - and since I don't really get the circuit, I'm leaving it that way until I am a bit smarter about it.

I'm hoping to get some input on how the circuit works and some thoughts on substituting modern devices. Perhaps dual devices would make sense.

One more note on that drawing. The first two pairs are symmetrical but the third (mystery pair) are not. By that I mean that the drain will be at the top on one and the bottom on the other.
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File Type: jpg Luxman DML_02.jpg (62.3 KB, 722 views)

Last edited by Captn Dave; 4th January 2012 at 03:57 PM.
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Old 4th January 2012, 05:00 PM   #2
gk7 is offline gk7
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Are you sure that drawing is correct ? As it is I canīt see what it is supposed to do.
If the bottom pair is unknown how do you know that they are not both connected with
their sources to the 22k resistor ?

If I change some connections I could imagine a working cascoded differential amplifier:

The two 2SA836 form a current mirror as they are connected.
The two 2SC1435 could be for cascoding. In this case their bases must be connected to the point where the 56k and 47k resistors meet (pin4)
The two FETs would be connected with their sources to the 22k resistor which then provides the current through them.

Then the gates of the FETs would be the inputs and pin 5 and 9 the outputs of a differential amplifier.
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Old 4th January 2012, 05:47 PM   #3
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Quote:
Originally Posted by gk7 View Post
Are you sure that drawing is correct ? As it is I canīt see what it is supposed to do.
If the bottom pair is unknown how do you know that they are not both connected with
their sources to the 22k resistor ?

Oh Yeah!! That drawing is WRONG! There should be a node in the center of that drawing. The junction of the 56K and 47K resistors happens at pin 4.

As for your other points, all I can say with any authority is that these devices are in TO-92 packaging and that the pin numbers on the drawing are correct. I only think that they are FETs because of the similarity to the other drawing. I can't know with any certainty how the FETs are connected within the TO-92 package. I'll think about what you have said about the FETs

I used a scope and a signal generator to study both the one existing (still working) encapsulated IC and this workaround part. They seem to function the same and the notes about the pin 4 input and pin 11 output are correct.

The link I posted in the IP has a schematic of the entire circuit so that one can see how the IC works within the larger circuit.

Last edited by Captn Dave; 4th January 2012 at 06:05 PM.
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Old 4th January 2012, 06:16 PM   #4
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Here is a repost of the corrected drawing.

Also a couple of shots of the circuit card and devices.
Attached Images
File Type: jpg Luxman DML_02_Rev1.jpg (310.7 KB, 657 views)
File Type: jpg photo (12).JPG (36.5 KB, 586 views)
File Type: jpg photo (13).JPG (25.4 KB, 574 views)
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Old 4th January 2012, 06:25 PM   #5
gk7 is offline gk7
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But something must be wrong, the gate of a FET canīt be an output.

http://www.hilberink.nl/codehans/luxdml2.jpg (The picture "Below the schematic diagram of the DML-02") shows what I have just described, except that instead the 22k resistor there is an active current source.

Ah now I see (in the complete schematic) pin 11 is the negative _input_. You can measure the output signal on pin 11 because the feedback loop is connected there.

Ok I think we make progress :-) Iīll look into the details tomorrow (itīs already evening here in europe) and post schematic.
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Old 4th January 2012, 06:32 PM   #6
gk7 is offline gk7
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Should not be too hard to build. Can you tell me the supply voltages ?
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Old 4th January 2012, 07:22 PM   #7
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OK, I'm beginning to understand the circuit a little better now. But if this is a cascode circuit, as it appears, then looking at the right side of the drawing, the input would be to the base of the lower BJT (pin 4) as it is, but the signal out should come from pin 8. I'm thinking here of an analogy to the tube cascode circuit where the input is from the grid of the lower triode and the output is from the plate (anode) of the top triode. But I can't see than being correct within the context of the larger circuit.

I suppose the purpose of the bottom two FETs is to hold a constant DC voltage on the emitters of the lower BJTs in the cascode. Have I got that much right?

The top rail runs at 29 volts. Everything else is at a lower voltage.
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Old 4th January 2012, 09:16 PM   #8
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The top rail runs at +29.5 and the bottom rail runs at -29.5.

So pin 7 runs at the top rail voltage.
6 and 8 at 29.3
5 at 28.8
4 at 13.7
3 and 11 at zero
2 at 0.5

I put a small signal into the phono stage and set the volume contol for 200mv at the output. Then I measured signals on the 2,3,4 and 11. 4 showed 16.6 mv and 2, 3 and 11 showed 30 mv.

I can now say with a great deal of confidence that I definitely don't understand this circuit
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Old 4th January 2012, 10:09 PM   #9
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Quote:
Originally Posted by Captn Dave View Post
the notes about the pin 4 input and pin 11 output are correct.
No, that should be pin 3 input and 11 output. Note that pins 2 and 4 are only a .001uf cap that bypasses the 47K resistor. Nothing else going on there.

Last edited by Captn Dave; 4th January 2012 at 10:13 PM.
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Old 5th January 2012, 02:14 AM   #10
sregor is offline sregor  United States
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Throw in my 2 cents - since this is part of a circuit, not the whole circuit, the closest to an output would be pin 9, which should go to the next stage of the circuit. AC wise most of the pins should have close to same signal as the input. The top 2 transistors form a current mirror, the middle pair are cascoded to the input pair of FETs. Someone on the previous thread mention 2sk68 which are probably what the FETs were, not 2sk60. Pin 9 is the only pin with a lot of ac gain, but still don't expect to see large signals.
Pin 3 and 11 would be non inverting and inverting inputs - feedback would be fed into 11 if 3 is input. Pin 9 should have highly amplified Difference between pins 3 and 11.
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Last edited by sregor; 5th January 2012 at 02:21 AM.
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