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High Rollers
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Posts posted by MASantos

  1. On 11/27/2020 at 1:49 PM, rumina said:

    it's a BRZHIFI BZ2607  case from aliex..., size is  260*311*70mm.  i have to many silver boxes so i like compact sizes even if the layout isn't perfekt.

    the trafo is a 100va /24volt from triad magnetic VPT24-4170 (mouser), with a bias of 200ma the amp needs only around 40va so a lot of reserve here, next time i would chose one that is a bit smaler.

    Don't forget that a transformer shouldn't work near max output current. Most recommendations for Class A amplifier are to use something with at least double the output current capability. 

  2. I replaced the opamp and now it reads as the other 3. The offset still drifts around below 10mV. All other amplifiers I built (cfa, ckk3) with dc servos were very steady once the servos were installed. 

    Is this drifting normal in the dynalo mini? I can't seem to find any solder bridges, it is as if the servos are not doing their job at all. 

    Sound is now centered and similar in both SE and BAL ouputs, it is just the drifting that is puzzling me. 

  3. So I found a dead resistor in the R+ channel and replaced it. The L+ channel is the one acting up.

    I removed the servo jumpers and dialed the offset to less than 5mV on each channel. All fine. When I reinstalled the jumpers the L+ dc offset jumped to around 350mV while all the other channels remain below 2-3mV (but still wondering between 0 and 2-3mV, not ultra stable). 

    The servo opamp for the L+ channel is getting power as it should. Might the IC be damaged and causing this?

    On further inspection, the 100k resistor next to the opamp measures 28K when soldered (I removed it and it measures 100k as it should) 

    The opamp is measuring 1.1ohm between pins 2 and 3 (input + and input-) this is different from all the other opamps so I'm ordering a new one. 

  4. 12 hours ago, Pars said:

    I assume that the values shown are mV? Also, when you say bias, I assume you mean offset?

    The first place I would start would be to measure the bias voltage across all of the 20R resistors, noting any variances. I would then do a comparison of the biasing circuits (consult the schematic) and go from there. It can be difficult to spot solder bridges on these, so checking resistance across all resistors (power off) might show you something. I know I had a resistor shorted that looked fine.

    Those are dc offset mV indeed. long day yesterday...

    I'll take readings on the resistors and check the biasing circuits to start!

  5. My dynalo mini started acting up. Balanced output works fine, but SE output had low volume and distortion.

    I did another thorough clean up with alcool, double checked everywhere for solder bridges and it started working again, probably some bridge of flux I didn't spot before.

    It now has almost even level between channels, but just a bit lower level on the left channel. It could almost be due to a potentiometer inbalance but it is perfect in balanced output.

     Dc is 0.1-0.2 on the right channel and 2.4-2.6 on the left. When starting up the bias goes down quickly on the right channel but take a bit longer on the left one. this is with servos in the circuit path. I took VDC measurements around the board on most transistors and servo opamps and I couldn't spot any differences between the 4 channels. 

    Any ideas of what might be causing this? Thank in advance for your help!

  6. 8 hours ago, thuytn said:

    How hot does the heatsink get? Seems to me it's like 220-250mm long?

    That power hole seems to be meant for the 12mm Bulgin-like switch...



    I have about 160mA bias atm and the heatsink is about 34 Celsius. Transistors are about 10 C higher. I'll push it a bit closer to 180-200 once it is all done to see if it makes any difference.


  7. 24 minutes ago, Pars said:

    The case looks decent... something from ebay/aliexpress?

    It looks like the switch was drilled for a Bulgin type vandal switch...

    The case is alright. Not top notch but also not a bad price considering it has most of the drilling done. 

    The whole for the switch is 13mm wide with a little bevel on the front. The cap on the pushbutton didn't fit right so I made that cosmetic upgrade. 

    I am considering painting the front panel mate black. 


  8. It's alive! Finished wiring today, only thing missing is drilling the front panel for the 4 pin XLR, waiting on a drill bit. 

    Probably the best amplifier I had for the HD650. Absolutely dead silent, great sound! I'll need to make a better comparison with the dynalo mini, but at first glance it sure seems like a winner.

    Thanks for all the help I got during the build!



    • Like 6
  9. So I cleaned the boards once again and checked every transistor for orientation and location. Both boards match. 4 LEDs appear to be ok, similar voltage drop.

    I powered both boards and took some random VDC readings to ground. This is where things show that something is wrong. Most of the small critters have different readings. They all came from the same 100 unit strip bought from mouser. I guess I'll go ahead and start replacing them all.

  10. So I wired the amplifier board to the GRLV.

    One channel fires up ok. Both leds giving light, about 150mV across the output resistors 90mA offset without servo and input open.

    On the other channel I have one of the LED not lighting. No reading on output offset.

    all the LEDs test ok with a multimeter I diode check and emit a small light. 

    On the mjf15030, the output resistor reads 30mv across. On the 15031 nothing across the output resistor. 

    I don't see any solder bridges. Should I start replacing stuff?





  11. Apparently mouser forgot to send me the opa134 needed so I plugged a pair of opa445 for the CFA servo. 

    No smoke. Voltages are +25.02 and -25.11 I didn't match any of the LEDs or resistors so this seems pretty good to me and probably won't make a difference in real life. 


  12. On 6/1/2020 at 10:26 PM, RudeWolf said:

    Hunting for power switches is a nightmare, unless you're willing to go with a bulgin vandal-proof type of thing. I'm mounting my power switch on the bottom plate near the faceplate, so you can reach under the amp and easily push the toggler switch.

    This is also pretty okay - https://eu.mouser.com/ProductDetail/Amphenol-Audio/ACJM-MHOP?qs=t8VhaDIDl4uYvnn1Z02tXA%3D%3D

    But generally the choice isn't too big.

    I had the under switch in my power amp, but this one wont have feet high enough to reach under.

    I'm ordering a latching anti vandal switch and will be using a 4 pin xlr even though the amp is single ended. 

    I already have my hd650 with a balanced connector so I wont have a need for adapter cables.

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