My_Ref Fremen Edition - Build thread and tutorial

Hi!
I own a pair of PCB since 2021 but never could buy chips and never gave time to it.
Centurion53 posted recently that Mouser had LM3886 in stock so I bought 4 of them! ✌️
Did anyone use SMSP to power this amp ? Or compare this amp with ?
I think about Audiophonics Connex SMSP300REh, but it's only +/- 30V output.
 
Hi Nico,

I assume by 'SMSP' that you meant switch mode power supply, smps. Most are using linear supplies on this amp, in particular because it has the rectifiers and power supply caps on board. This is a topic that can turn out heated discussion. I personally try to avoid smps because it puts out high impedance noise. That said, there is no doubt that it is easier to get low ripple from smps. I also recognize that some claim that they can filter out the high frequency noise, but I haven't experienced that. In the end, this is a hobby, so do what you would like and share your results.
 
I haven't really given this much thought, so I am probably missing something.

Since the SMPS puts out DC, you will have to bypass the onboard diodes, but the speaker protection circuit needs +24VDC and it has it's own diodes so you will have to figure how to manage that. If the power to the speaker protection circuit isn't +24VDC, more than the relay would need to change as the whole circuit that protects against DCV on the output is balanced against the 24V.

Assuming the SMPS is above 16V or so, the ompamp power supply should be OK. Reducing the voltage to LM3886 is really hard to predict because there are compensation circuits that are there to manage stability. Lower voltage and different output impedance of the SMPS will have unknown, at least to me, interaction with the LM3886 compensation. Of course, lower supply voltage will decrease the amp power.

The more I think about this, the riskier it seems. The whole design of the circuit as a current pump involves compensation and feedback used in unusual ways. It isn't as easy as a simple LM3886 amp where lower volts in would just mean lower power out. I actually think you are asking for trouble. If you do try it, make sure to watch for smoke and heat when powering it up for the first time. If it powers up OK, look at the output on an oscilloscope if you can, just to make sure its not oscillating.

As always, there are more knowledgeable folks on this forum. I hope one of them will respond with their thoughts.
 
Quick question, how do you connect toroidal transformer when you only have 3 wires 21v - 0 - 21v ? can I split the white wire and connect it to NAC1 and NAC2, and take the 2 black wires and connect them to AC1 and AC2? My PCB is Rev. 1.7.

Toroid.jpg
 
I fifnished my build of this amp in the middle of 2021 and have tried it with a few combinations of speakers and knew that it was a great sounding amp. However I re-homed a pair pf Lampizator Saba Green Cone/ Eminence speakers in a new baffle that I thought would be better for the size of my room. Currently runnng them with the RefC and the combination is just right. Perhaps it took a more revealing speaker to see the quality of this little amp, Really good sound!
 
  • Like
Reactions: 1 user
I struggle with finding bad component in my build. When I first did the power up I missed the transistor T101 was soldered rotated 180 degrees.
I also used wrong transformer causing short between rectifier diodes... After few years I decided now to finally fix the circuit but with no success.

After fixing the power section and checking all supply voltages (I could read -14.7, +14.7, -35, +35) I soldered new lm318m (lm3886 also, just in case) . It seems the op-amp failed straight away as I could no longer read 14.7V on one of the supply lines (voltage dropped to -0.5V, other line was ok).

I removed lm318m and rechecked voltages - all ok.
What is strange after power on (without lm318m) the relay is activated but the LED dims slightly (fluctuates) after a while. Sometimes it goes off and relay clicks.
I measured 35v relay supply voltage instead of 24V as shown on the schematic. Is this correct? The relay is 24VDC. Or is it half wave rectifier for the relay?
Many questions and a lengthy post. I hope someone will come up with some suggestions on how to fix the amp and what to check.
 
Did you check R11? when You insert the lm318, you close the global feedack loop both AC an DC.. if the input ground reference (R11) is not in place (burned previously) - i could imagine the mentioned effects produced..

Then: If the relay is not CLOSED at the moment when you try to measure the coil voltage - then the auxiliary diode from the main secondary will just charge the cap C14 accross R14 to the peak voltage of the sinusoidal.. 35V. When You close the relay coil circuit, then current flows, the voltage on R14 drop, (a voltage divider formed by R14 (470ohm) and the coil impedance) - so the coil will see ~24V.
 
Last edited:
I suggest a unity gain preamp, given the MyRef high gain.
I use a TVC passive preamp.
I listened to a pre with ladder resistors attenuator and unity gain buffer based on fet-opamp. Possibly the closest thing to a "wire with gain" I've ever heard.
Maybe Joseph K can explain it better, he knows something about it!...

Ciao
G