My_Ref Fremen Edition - Build thread and tutorial

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P

Panelhead

Member
Joined 2002
giacintopapa said:
Hi!
A week ago I finished the construction of the My_Evo Rev. A.
After over 40 hours of listening with a passive TVC preamplifier, I am very impressed with the sound quality. Sound control is remarkable.
For less than 450,00 Euros, I think it's almost a miracle.

I would like to remember and thank Mauro Penasa's brilliant intuition on how to use in an original way a LM3886 and the great work of implementation and fine tuning done by Dario Inserra. What have they done!? My god, they did!

For budget reasons I used the Mouser industrial BOM with the exceptions of C9 (Cerafine on Ebay less than 6,00 Euros) and C13 (a friend of mine gave a couple of Mundorf Supreme to me). Transformers are Triad Magnetics, the cheapest ones found by Mouser. Nothing less than other more expensive and famous, they work perfectly.

Always for budget reason I choose as enclosure a Galaxy 31x28x8cm (less than 40,00 Euros). According to Dario, this is the minimum solution to contain the amplifier. The only recommendations are to use 20mm spacers to bring the LM3886 to a height that does not encounter grooves on the heatsinks and use high quality thermal compound (I used Arctic MX-4) to achieve the best thermal coupling.

The last week in Rome, where I live, we have had very high temperatures, over 35 degrees Celsius. Under these conditions, the heatsinks were warm, but not very hot.
Now the temperature is near or below 30 degrees Celsius and heatsinks are tepid-warm. My speakers are three-way closed box, 91 dB, minimum impedance 4 ohm. Not a very difficult load.

After first power up DC offset were about -4,0mV and -6,5mV. After 30 hours of listening were about -4,5 and -8,0 mV (this is the only value I don't like too much). I hope DC offset will stabilize (possibly within + or - 10mV)...
The amplifier has never made noises, electric discharges, it is totally silent and has not presented any anomalous overheating.

My little problem is the fuse: I've bought Schurter ceramic ones, slow blow (from datasheet: time-lag T, high breaking capacity - ceramic tube, 1,25A 250V).
In a week, three times, turning on the amplifier, I found the fuses broken. What do you think is the cause? Can I use a 1.6 or 2 amp fuse in the hope that it will last? Or is it dangerous?
The only thing I added to the project are two X2 capacitors (0,01uF 275VAC) mounted on the bipolar switch to avoid burning the contacts. Are they the possible cause? Or I was wrong to choose the type of fuse?

Thank you all for your attention
Giacinto
Click to expand...

Try a larger value fuse. Also a smaller gauge power cord.
 
J

Joseph K

Member
Joined 2004
Generally I use 3,5A fuse. If you use timed, (T) it could be a bit smaller.

1,25 is a safe bet for blown fuses..

Edit: ok, i was thinking a stereo module (an Evo, 80k uF)
For the case of monoblocks, 2-2,5 could be enough..

And I think George hinted at quality, not functionality, by the power cord. He is saying try a large diameter power cord.

I'm with him. But it's just an another marshy shallow
 
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P

Panelhead

Member
Joined 2002
What Imeant about power cables is that smaller ga power cable will allow a smaller amperage fuse to work.
When I first built a apir of TP My_Ref mono's I was using two 18 power cords while testing.Once put in the system I connected two 14 ga power cords. Blew both fuses on initial power up. Repeated this. Raised the amperage to 2 amp slo blo. The 18 ga cords were limiting inrush current enough to protect the initial fuses.
I think 18 ga is fine for mono My_Ref. Power demand is small once operating.
 
P

Panelhead

Member
Joined 2002
Gathering parts for Fremen

Pulled the TP boards out of the chassis. These Fremen are larger, will fit chassis, just will require changing orientation.
I built the EVO amps differently than the TP Mono's. But did pull the S102 resistors and 220 ufd BG NX caps. I used a 4.7 ufd BG NX for C21. Not sure what this was.
 
S

Schlumpfpeter

Member
Joined 2015
Hi guys!


My Fremen edition amp is now sleeping long enough in my drawer, so I want to finish it. Can you help me with the transformator? I am having a hard time on figuring out which one to buy. I generally do not want to spend too much money on them, but also do not want to buy the wrong/undersized transformator.


I would like to order here: 30 VA - Shop Toroidy.pl


Can somebody have a quick look and recommend me the right tranformators? I guess I will need to buy two. Actually three, as I want to buy one for a chipamp.com kit which is utilizing the same chip and is sleeping next to myref fremen kit in the drawer :D


Thanks,


SP
 
S

Schlumpfpeter

Member
Joined 2015
0rphu said:
Hi, 30VA is way too low. If you want to use dual transformers, 24V or 25V 160VA per channel is the recommended starting value. You could also use a single transformer, some builders, including myself do. In which case look for 300VA. I'm using a single 24V / 250VA
Click to expand...


Thanks mate!


I was not intentionally linking to a specific transformer, I actually just wanted to show the list from the shop I wanna buy from.


What is the downside of using only one transformer? Or, what is the benefit of using two?
 
0

0rphu

Member
Joined 2017
Hi, I'm no expert by any means, it was stated, maybe in this thread or in the build manual that using a single transformer for both boards can lead to hum issues, but some builders reportedly went this way and didn't experience issues. I did too and don't have problems. I'm sure there are more experienced people in the forum who can shed more light on the topic.
 
S

Schlumpfpeter

Member
Joined 2015
0rphu said:
Hi, I'm no expert by any means, it was stated, maybe in this thread or in the build manual that using a single transformer for both boards can lead to hum issues, but some builders reportedly went this way and didn't experience issues. I did too and don't have problems. I'm sure there are more experienced people in the forum who can shed more light on the topic.
Click to expand...


Thanks again.


I will check the prices, but I am leaning towards using a single one as well, just for price reasons. I am building in addition to this one here a second chip amp, and I do not want to spent hundreds just on the transformers.
 
O

olivosunlight

Member
Joined 2013
i replaced D4 and receive a good result as:
-33.2V/ +33.2V
- 14.57/ + 14.56 V

- 13.99/ + 13.95 V (zener)
- 1.252/+1.249 V
0.583V
DC offset: 0.37 mV
Led is solid light so far with 1.95V


Q: I use tortoise transformer 230V/300VA/2 x 25V, single board, but my power grid is only 220V/50Hz so

- Is that a reason transformer output is lower than its specs? and can not reach -35V/+35V
or any other reason?


Best regards
/NMN
 
ClaveFremen

ClaveFremen

Member
Joined 2005
inserra.it
Paid Member
olivosunlight said:
i replaced D4 and receive a good result as:
-33.2V/ +33.2V
- 14.57/ + 14.56 V

- 13.99/ + 13.95 V (zener)
- 1.252/+1.249 V
0.583V
DC offset: 0.37 mV
Led is solid light so far with 1.95V
Click to expand...


Good to hear :)


You can try connecting a spare speaker to see if you get sound.


olivosunlight said:
Q: I use tortoise transformer 230V/300VA/2 x 25V, single board, but my power grid is only 220V/50Hz so

- Is that a reason transformer output is lower than its specs? and can not reach -35V/+35V
or any other reason?
Click to expand...


With 220V at the power grid I would say it's normal.


Your transformer is rated for 25V output with 230V input so at 220V it will output less.
 
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