Wednesday, April 28, 2010

Mini³ finished at last

Today I received some parts I ordered. I bought two 9 volt batteries (one 200mAh and one 250mAh) hoping one of them would fit. They both do. After that I connected both battery terminals to the 250mAh battery and clamped the battery to the PCB. Next I finally soldered the terminals to the PCB. The last thing to check is the battery charging circuit. 

Also in my parcel was a DC-plug & cable assembly to charge the Mini³ battery with. A check with my Velleman P1832 PSU @ 15 volts output shows that the battery charging circuit checks out OK. Charging current is ~18.5mA. For the 250mAh battery to be fully charged it would take about 13½ -15 hours. Modern speed chargers laugh at this but slow charging is better for the battery.

The battery on the left is an old 200mAh that doesn't fit inside the case. The right one is the new 200mAh that does fit. The left one is a 9.6volt (8x1.2v), the right one is 8.4volt (7x1.2v).

Sunday, April 25, 2010

Butchered my iPod LOD

I bought a nice line-out cable for my iPod Nano 4G. This way I got to bypass the mediocre amplifier inside the Nano. The difference is indeed remarkable. Teamed up with an external headphone amp my Nano sounds much better using the line-out than using the headphone output jack. Problem solved right? Well no. The nice line-out cable introduced annoying pops & clicks in the audio. Using the headphone output jack there's no annoying pops & clicks. I found the pinouts of the iPod connector and discovered that only three pins are used for audio line-out. But what are the other two pins for in my line-out cable? As it turned out both were connected internally. Most likely pins 11 and 15 if I didn't lose count. I cut away the connection.

I now have a line-out cable with a broken iPod connector. On the upside, now there's silence. No pops, no clicks. Oh, and Loud Jazz. Of course. 

Saturday, April 24, 2010

Mini³ - it's alive

I now know my Mini³ works. I still haven't got the proper battery for it, but I manged to power it up. The blue LED lights up nicely, not too bright nor too dim. The amplifier's total quiescent current draw is 24.2mA, which nicely matches the "about 25mA" that the AMB Laboratories site specifies. DC-offset is 1.5mV and 1.4mV for the right and left channel. Both are good low values, and are indeed lower than the specified 5mV. Still to check is the battery charging circuit, which can only be done when the DC-plug & cable assembly I ordered gets here.

That's all very well and groovy, but how does it sound? Clarity, depth, separation, detailed and punchy bass. All at ear splitting SPL...

Friday, April 23, 2010

Mini³ nearly finished

I started working on my (hi-performance) Mini³ today. It's now almost finished. I still have to find a 9v NiMH battery that fits the case. Soldering the SMD opamps was a bit tricky. With a fine soldering tip and thin solder wire (0.5 mm) I managed to do it. The end result looks OK.

Here's how I soldered the opamps: apply a tiny amount of solder to one of the corner pads and allow to cool. Align opamp on all its pads. Re-heat the pre-soldered pad. Keep opamp aligned in the process. Allow solder to cool. The opamp is now soldered to the pad. Check alignment of opamp. If all's OK solder the opposite corner first then all the other pins. If alignment is off re-heat the pad and re-align opamp. For best results use thin solder wire and a fine tip on the iron. I've got an Ersa 832SDLF 0.8mm pencil style tip.

Sunday, April 18, 2010

mini Aleph working again

My mini Aleph is working again, I put it together yesterday afternoon. I completely rewired the amp and implemented a starground. I also mounted the IRFP250's instead of the original IRFP240's I had in it. After powering it up I started tweaking the bias and DC offset. The trimpots really make adjusting the bias and DC offset very easy. Turn the bias pot up and the voltage drop over the source resistor goes up too. Cool. The offset pot works likewise, for a negative DC offset (gently!) turn clockwise. Over the course of the evening I adjusted the left and right channel bias to 1.1 amps (heatsinks at ~37°C). DC offset in both channels is now below 10mV, one channel was 70mV before the tweak.

The amps bias and rail voltages makes an amp of 2x 15Watts @ 6 Ω.

How does it sound? Well just like before I pulled it apart, only a touch better. A bit more clarity and depth. I feel that the MKP1837's are responsible for that.

Tuesday, April 13, 2010

mini Aleph mods done

I bought four blue Bourns trimpots for the mini Aleph today, two 500Ω and two 200KΩ. The 500Ω trimpots will be used to adjust DC offset. The 200KΩ trimpots will enable easy bias adjustment. I soldered pins 2 and 3 together, this way rotating the set screw clockwise will increase resistance. I preset the trimpots to the default resistor values for R8 and R13 before I mounted them onto the PCB.

I also bought four IRFP250 MOSFETs for 1 euro a piece, pretty cheap IMO.

Monday, April 12, 2010

mini Aleph mods

My lovely sounding mini Aleph got partly disassembled today...

After I built it a few things struck me:

  • bias current wasn't at 1 Amp but lower (0.86 Amps)
  • left and right channel bias wasn't equal
  • left and right channel DC offset a bit too high to my liking

All these things can be traced back to tolerances of all the parts that make up the amplifier. I upped the bias to ~1.2 Amps by installing additional 1Ω resistors in parallel to the 0.47Ω resistors but it didn't make left and right channel bias equal.

So I want to modify it slightly. I want to:
  • install DC offset trimmers
  • install bias current trimmers
  • install 0.01µF bypass caps

The bypass caps are mounted on the reverse side of the amplifier PCBs, every 220µF Elna cap now has its own 0.01µF MKP1837 bypass cap. For the trimmers I'm gonna use those square cermet types. With the trimmers it's probably not needed to have the 0.47Ω source resistors paralleled with 1Ω resistors to increase the bias current so I removed them.

A local store has IRFP250's on offer, at 1 euro a piece, I think I'm gonna pick up four of those puppies to replace the IRFP240's I now have. According to Nelson Pass, the IRFP250's "will give a better bottom end"

Wednesday, April 7, 2010

My Mini³ parts are here

My Mini³ parts arrived today, in a sturdy package. Everything I ordered @ AMB Laboratories is here now. I did some test fitting just now and came to the conclusion that the battery snap holders needed filing down, I knew that this would be the case. The pins of the 3.5mm stereo mini phone jacks needed some filing down too. They were both slightly rotated clockwise with respect to the PCB silkscreen. I didn't know that they would need some work too... Doesn't really matter since they now fit like they should.

Still on order are the PSU caps and the Hammond case. I can't do anything yet, I've got to fit the battery contacts and battery first and for that the case is needed. Space in the Hammond case is very tight, also you just can't order any 9v volt battery and expect it to fit. The 9 volt battery must be a slim type so the PCB and battery can slide in and out of the case. 

Sunday, April 4, 2010

Mini³ parts in transit

I placed an order for the Mini³ PCB and all the additional stuff on the AMB website, including those lovely front and rear panels. I ordered the Hammond 1455C802BK enclosure and caps locally. I have the rest of the parts needed to make the Mini³ (resistors, LEDs, diodes etc.) already in house.