ME850 mods designed by Art Vandelay

[Art Vandelay]

OK, other channel now fixed.@georgehifi

 

The old transistors exhibit slightly more overshoot on the -ve cycle, but otherwise similar and excellent into 0.1uF. 

 

I'll test the alt comp schemes tomorrow and will upgrade the transistors in the non-upgraded channel too when I've determined the most optimum scheme. It mightn't be until Wednesday that I'm able to reassemble and audition. 

 

10kHz, 1.5uF.

 

Edited June 25, 2018 by Art Vandelay

[georgehifi]

9 hours ago, Art Vandelay said:

OK, other channel now fixed.@georgehifi

 

It's interesting your getting more overshoot in the negative phase than in the positive? Maybe one of your neg comp cap is out of whack. Don't think I got it, with the same transistors, and you still get up to 6 cycles of ringing before total settling.

This was .1uf, what happens with 1.5uF or 2uf

 

Cheers George 

Edited June 25, 2018 by georgehifi

[Art Vandelay]

2 hours ago, georgehifi said:

 

It's interesting your getting more overshoot in 

This was .1uf, what happens with 1.5uF or 2uf

 

Cheers George 

No, that was with 1.5uF, so it's a good result. 

 

FTR, in both modules the rail caps are brand new. 

 

 

[georgehifi]

14 minutes ago, Art Vandelay said:

No, that was with 1.5uF, so it's a good result. 

That says to me, not worth the expense/labour and extra trouble changing the output transistors, as mine seemed even more stable with only same initial amplitude overshoot then 2 cycles to settle.

Now we just have to see what the reconfigured compensation caps yield on the scope.

 

Cheers George     

[Art Vandelay]

28 minutes ago, georgehifi said:

That says to me, not worth the expense/labour and extra trouble changing the output transistors, as mine seemed even more stable with only same initial amplitude overshoot then 2 cycles to settle.

Now we just have to see what the reconfigured compensation caps yield on the scope.

 

Cheers George     

@georgehifi

The difference with the small ripples is most likely from a difference in equipment and test conditions.  

 

The pic below is 10kHz, 1.5uF with the new transistors and new compensation scheme. It's slightly improved from the original scheme, and no signs of instability with any loads. 

 

Edited June 26, 2018 by Art Vandelay

[georgehifi]

23 minutes ago, Art Vandelay said:

The pic below is 10kHz, 1.5uF with the new transistors and new compensation scheme. It's slightly improved from the original scheme, and no signs of instability with any loads. 

 

 

Seams to me what I did to mine in the first post, is the most "cost effective" way of improving all the ME models, still with a ? over what the 240 can take with all the steps. 

 

Rack'em and stack'em Zaph and Arty, I'm sure there's going to be a few for you guys to do these mods to, maybe someone should get to the Asia forums, as I hear there's are many more ME's over there just waiting for these mods.

 

Cheers George  

 

[Art Vandelay]

@georgehifi

 

And finally, 10kHz, 1.5uF, old transistors with new compensation scheme.

 

Slightly less overshoot with the new scheme. 

 

No stability issues at all either. 

 

[Art Vandelay]

3 minutes ago, georgehifi said:

 

Seams to me what I did to mine in the first post, is the most "cost effective" way of improving all the ME models, still with a ? over what the 240 can take with all the steps. 

 

 

For the 850, probably yes. 

 

However, I'll put mine back together with new transistors and the new comp scheme on both channels and will lend it to you for subjective evaluation. 

 

 

[zenelectro]

Guys, I've been successful building BJT OP power amps that showed almost perfect 10kHz square wave

into 1.5uF // 8R, (actually I got it up to 50kHz then cooked the OP stage due to too much level =  bang )  but just a few things to think about:

 

The edge rate of 10kHz can vary quite a bit depending on your generator, the amp front end BW and any low pass filtering

in the path (cables / amp). This makes a significant difference to how the OP stage handles the leading edge and the massive curent demand

if it's very fast.  If it is very fast the OP stage layout (ground plane etc) starts to really come into play.

It's also worth experimenting with the Base input resistors.

 

WRT servo's... We did a huge amount of work on DC servos and found to make one completely dissapear sonically was a real challenge.

Did find a solution but it was not easy by any stretch. People tend think servos are like a black transparent box and this is not the case at all.

 

I think using higher speed more modern BJT's will generally sound better, even with similar OP stability into cap loads. 

 

WRT Emitter resistors, IME the smaller the better - if you can keep the whole shebang thermally stable.

I have used NONE but that is a veeery tricky business...... and usually end up with a small value.

From memory, Technical Brain (love that name) Amps had none but did have a penchant for blowing up for a while.

 

Good luck in your quest for ME pefection!

 

T

 

 

 

 

 

 

 

 

 

 

 

 

[Art Vandelay]

8 hours ago, zenelectro said:

Guys, I've been successful building BJT OP power amps that showed almost perfect 10kHz square wave

into 1.5uF // 8R, (actually I got it up to 50kHz then cooked the OP stage due to too much level =  bang )  but just a few things to think about:

 

The edge rate of 10kHz can vary quite a bit depending on your generator, the amp front end BW and any low pass filtering

in the path (cables / amp). This makes a significant difference to how the OP stage handles the leading edge and the massive curent demand

if it's very fast.  If it is very fast the OP stage layout (ground plane etc) starts to really come into play.

It's also worth experimenting with the Base input resistors.

 

Agree with all. ME amps in question were built way back in the late 80's, well before the modern era of low cost cad software and low cost of multi-layer pcb's. If the amps were designed and built today the layouts could / would employ inductive cancelling techniques as well as multi layers with RF and AF ground planes and earth returns etc.   Having said that, the ME amps are a low / nested feedback topology, which makes all of the above much less critical. The output stage is configured with 8dB of gain and hf compensation is achieved with lead and lag caps, and no output inductor is required for unconditional stability.  

 

Re testing and methods, yes, my sig gen (Rigol DG1022) is no doubt different to George's, so results will look different.  

 

Quote

 

WRT servo's... We did a huge amount of work on DC servos and found to make one completely dissapear sonically was a real challenge.

Did find a solution but it was not easy by any stretch. People tend think servos are like a black transparent box and this is not the case at all.

 

I found the same too, and my own SS  design is a 2 pole design with filtering after the op-amp. Probably not perfect but it's pretty good. 

 

The ME amp mods replace electro's with a couple of box MKT Wima's which sound better and provides longevity.  

 

Quote

 

I think using higher speed more modern BJT's will generally sound better, even with similar OP stability into cap loads. 

 

That's my experience too. The amp in question uses MJ15003/4's in the old TO3 package , so replacement options are limited.

The MJ21193/4's are fortunately manufactured in TO3 and they exhibit less beta droop and being more complementary there's less distortion too. Also, the beta / hfe  varies less with op conditions - a good thing. 

 

 

Quote

WRT Emitter resistors, IME the smaller the better - if you can keep the whole shebang thermally stable.

I have used NONE but that is a veeery tricky business...... and usually end up with a small value.

From memory, Technical Brain (love that name) Amps had none but did have a penchant for blowing up for a while.

 

Good luck in your quest for ME pefection!

 

T

 

Edited June 26, 2018 by Art Vandelay

[Art Vandelay]

 

FTR, My refurbished ME850 will be available for purchase on SNA in the near future, so feel free to PM if you have any questions in advance. 

[Art Vandelay]

All back together and working very nicely. 

 

I'll give a more detailed appraisal after a few days but what I'm hearing thus far is really quite phenomenal.

 

This might be the best amplifier I've ever had in the system.   

 

 

[MrRogers]

 
FTR, My refurbished ME850 will be available for purchase on SNA in the near future, so feel free to PM if you have any questions in advance. 

I can offer a free quality control test.

[andyr]

18 hours ago, Art Vandelay said:

All back together and working very nicely. 

 

I'll give a more detailed appraisal after a few days but what I'm hearing thus far is really quite phenomenal.

 

This might be the best amplifier I've ever had in the system.  

 

So why sell it? 

 

Andy

[georgehifi]

1 hour ago, andyr said:

So why sell it? 

He's got more, and also 1400 and I think a 75 and 150

 

Cheers George

Edited June 27, 2018 by georgehifi

[13000km]

Någon som kan förklara hur 100W klass A kan ha så lite som levererar bort värmen, för det är väl förstärkarbilden i början av tråden det gäller?

En lösning som jag kom på nu är att det kanske är en D förstärkare som går i A (finns det sådana?), i nutid finns det väl?

Som ni förstår så är jag inte helt på det klara men ni kan väl förklara?

[muzzagruzz]

13 minutes ago, 13000km said:

Någon som kan förklara hur 100W klass A kan ha så lite som levererar bort värmen, för det är väl förstärkarbilden i början av tråden det gäller?

En lösning som jag kom på nu är att det kanske är en D förstärkare som går i A (finns det sådana?), i nutid finns det väl?

Som ni förstår så är jag inte helt på det klara men ni kan väl förklara?

I couldn’t agree more 

[MrRogers]

 
So why sell it?  [emoji37]
 
Andy

Andy, are you interested in buying it?

[Art Vandelay]

2 hours ago, andyr said:

 

So why sell it? 

 

Andy

 

Too many amps.

 

But if it doesn't sell I won't be unhappy. 

[Art Vandelay]

50 minutes ago, 13000km said:

Någon som kan förklara hur 100W klass A kan ha så lite som levererar bort värmen, för det är väl förstärkarbilden i början av tråden det gäller?

En lösning som jag kom på nu är att det kanske är en D förstärkare som går i A (finns det sådana?), i nutid finns det väl?

Som ni förstår så är jag inte helt på det klara men ni kan väl förklara?

 

The ME850 is a class AB design but outputs approx 0.5A rms before transitioning out of class A, so it's regarded as a high bias class AB design. The high standing current generates about 120 watts continuous at idle - in the form of heat.   

 

I hope I've answered the questions, particularly given that I can't read or write in Swedish, so I have no idea what you've asked. 

[muzzagruzz]

6 minutes ago, Art Vandelay said:

 

The ME850 is a class AB design but outputs approx 0.5A rms before transitioning out of class A, so it's regarded as a high bias class AB design. The high standing current generates about 120 watts continuous at idle - in the form of heat.   

 

I hope I've answered the questions, particularly given that I can't read or write in Swedish, so I have no idea what you've asked. 

Eloquently asked question with an equally eloquent answer. 

 

Couldnt have answered it better myself. 

[andyr]

19 minutes ago, Art Vandelay said:

Too many amps.

 

But if it doesn't sell I won't be unhappy. 

 

Aah, understood.    Thanks,

 

45 minutes ago, MrRogers said:

Andy, are you interested in buying it?

 

 

Unfortunately, Mendo doesn't (understand it). 

 

Andy

 

[13000km]

Totally understood here with the answer to my question. I'm a gossip with fantasy in cubic which makes it easier for you to understand!



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[georgehifi]

17 minutes ago, vinilink said:

Maybe drop in to listen to your ME amp one day, if ok with you.

Yeah give me a ahoy after the school holidays and we'll tee it up, have to a be weekday though.

 

And if you keep your eye/ear here Scott (Audionutz) listened to it a couple of days ago and was quite floored by it and is soon doing impartial small review on what he heard.

 

Cheers George

 

 

Edited July 4, 2018 by georgehifi

[vinilink]

18 minutes ago, georgehifi said:

17 minutes ago, vinilink said:

Maybe drop in to listen to your ME amp one day, if ok with you.

Yeah give me a ahoy after the school holidays and we'll tee it up, have to a be weekday though.

 

And if you keep your eye/ear here Scott (Audionutz) listened to it a couple of days ago and was quite floored by it and is soon doing impartial small review on what he heard.

 

Cheers George

 

 

Thanks, i may bring my 550II HC along to compare.