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LJM MX50 kit amp
LJM MX50 kit amp
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Old 10th July 2018, 06:59 PM   #251
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Drdorkmeister View Post
I keep burning out a 47 ohm resistor on one of my channels. I'm running 27v-0-27v ac to 35vDC rails. I replaced the output transistors that are associated with the the resistor but the problems continues.

Any thoughts?
Your accompanying diagram gives the value of C12 as 100uF instead of 100n. Is this error simply confined to the diagram?
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Old 4th September 2018, 10:43 AM   #252
zdr is offline zdr  Belgium
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I just assembled my MX50X2. As someone said before for MX50, it's so cold I thought it was broken. Then DC offset at the output... made me think my Fluke was broken, 0,0mV?? No power on thumps either... Looks good so far!
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Old 23rd October 2018, 10:14 AM   #253
dagfaune is offline dagfaune  Norway
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LJM MX50 kit amp
About darlington VAS.
At the beginning of this thread # 21 and # 43 there are some comment about the absence of darlington VAS. In my set, labeled "MX50 SE 2004-2018", is (still) not a darlington VAS such as on the schematic MX50 SE V6.
How important is this in relation to distortion?
Someone who has comments about this, and is it worth worrying - making a modify on the pcb?
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Old 23rd October 2018, 11:19 AM   #254
ivanlukic is offline ivanlukic  Serbia
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No need at all for Darlington VAS. Do not get me wrong, it will not spoil anything but it's just not necessary. The only very small difference would be in the specs. Doug Self use it in his Blameless amp but it's purpose is just to get somewhat better specs. Amp will function perfectly without it.
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Old 23rd October 2018, 11:46 AM   #255
dagfaune is offline dagfaune  Norway
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Quote:
Originally Posted by ivanlukic View Post
No need at all for Darlington VAS. Do not get me wrong, it will not spoil anything but it's just not necessary. The only very small difference would be in the specs. Doug Self use it in his Blameless amp but it's purpose is just to get somewhat better specs. Amp will function perfectly without it.
So you think that darlington VAS does not make any audible difference, only measurable?
How about using a foil Miller capacitor instead of ceramic?
Doug Self also writes about 2-pole compensation. ...has no purpose in terms of audibility?
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Old 23rd October 2018, 01:20 PM   #256
ivanlukic is offline ivanlukic  Serbia
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Many people recommend using foil Miller cap but I had very good experiences with ordinary high voltage disc ceramic caps (as supplied in the kit). I have not noticed real advantage in using foil or silver mica as far as sound quality is concerened.

MX50SE uses CFP output stage and if you read Doug Self's book you will see that with this kind of output stage 2-pole compensation does not have any advantages and is not necessary.

Just build the kit with supplied parts. That's all you need.
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Old 23rd October 2018, 09:47 PM   #257
ZoSoNCSU is offline ZoSoNCSU  United States
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I have four of these LJM MX50 SE kits and I have built two of them. However, I never actually used them for anything until recently and I have seen two fires already, lol.

It's always one of the 47 ohm resistors near the (included) heat sink that goes up in flames. It only occurs if I set my Variac to supply +- 30V from my rectifier + filter board (also by LJM). This is usually around 80V (AC) on the Variac dial.

I am trying to calibrate a Potomac AA-51 Audio Analyzer and it requires an 80Vrms 1000 Hz signal during one point of them calibration instructions. I can only supply a 20Vp-p from my signal generator, so I'm trying to boost it with the MX50 SE.

From my reading of this thread, I believe the issue for me is the load I'm using. I am running 3x Dale HL-35-08Z 25ohm (5%) 35W in parallel, which comes out to 8.45 ohm (on my Tenma 72-1020) and 105W. I'm using a 500VA toroidal transformer with 34V x2 and 20V x1 secondaries.

Please advise what load I should use to accomplish my goals without setting resistors on fire. I have plenty of spare speakers I could use, but I didn't think/know using a dummy load would do this. In my 15+ years of building/soldering, I've never seen a fire until now.


Quote:
Originally Posted by Drdorkmeister View Post
I keep burning out a 47 ohm resistor on one of my channels. I'm running 27v-0-27v ac to 35vDC rails. I replaced the output transistors that are associated with the the resistor but the problems continues.

Any thoughts?
Sounds like we are having the same problem, what load are you using?
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Last edited by ZoSoNCSU; 23rd October 2018 at 09:58 PM.
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Old 24th October 2018, 05:52 AM   #258
ivanlukic is offline ivanlukic  Serbia
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80V AC from Variac can not give only 30V DC from PSU.
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Old 25th October 2018, 12:26 AM   #259
slimecity is offline slimecity  New Zealand
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Multiply 35 VAC by 1.4 to get the expected DC output voltage once rectified - that gives you 49 DCV
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Old 12th November 2018, 03:57 PM   #260
audio_tony is offline audio_tony  United Kingdom
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Default Which is the current board?

Can anyone tell me which one is the current model?

I have two kits (one bought last week, the other about 5 years ago) and the board I received last week is different to the older boards I have.

These are the boards I have (generic images from the internet - both my actual boards are blue - which I believe marks then out as genuine LJM boards).

The blue one is the same as the older boards I have - the green one is like the newer board I've just bought.

So which is the updated version?

Thanks!
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