[lordearl]

The other option is of course to simply cut the traces on the PCB which are leading to where I've incorrectly soldered the BD137, then solder a jumper on the underside of the board to where the BD137 should have been soldered in the first instance.
 
That is probably the easiest solution as I won't be using any other transistors besides the BD137/BD138, the desoldering is simply too perilous!

 

[tomb]

  Argh!
And I was attempting to be so meticulous - two fundamental errors, putting the BD137 in the wrong spot, TWICE!
 
How important is the matching?  I can try to source two BD137 from a local electronics store.
 
Oddly enough, I have two pairs of BD139/BD140 already mounted to heatsinks here with me, will they work at all?

The BD139/BD140 offer sub-standard performance in the MiniMAX circuit.  It has to do with the fact that there's no feedback.  Feedbacked transistors sound pretty much the same, but with no feedback, you get a lot of sonic differences.

 

[tomb]

  The other option is of course to simply cut the traces on the PCB which are leading to where I've incorrectly soldered the BD137, then solder a jumper on the underside of the board to where the BD137 should have been soldered in the first instance.
 
That is probably the easiest solution as I won't be using any other transistors besides the BD137/BD138, the desoldering is simply too perilous!

 
I would strongly recommend against this:
 
1. Traces are not all that easy to cut - you'd have eight of them to do.  The board will end up pretty messed up by the time you get through with it.  It might not even be usable.
2. To make matters worse, you have to engineer a crossover.  You can't simply solder some spent leads in a different direction - they have to cross over one another.
 
The simplest answer is to remove the transistors.  You only need to make a blob of melted solder big enough to cover all three pins.  A lot of the time, the transistor will simply drop out of the board.  If not, you can tie a string or something similar through the mounting tab hole.  Pull on the string when you have the solder blob melted and it will come right out.
 
You don't absolutely need the heat sink mounting pads in a case like this.  Yes, it's nice to start with them, but you could simply smear some Vaseline on the backs of the transistors and probably get 90% of the heat transfer effect.  For that matter, since the transistors are all plastic-bodied, you could mount them dry and get probably 75% of the heat transfer effect.  The differences may be even closer than that - I'm just guessing.
 
Anyway - cutting traces on a PCB is the absolute last resort.  Never do it if you can help it.

 
 
Edit: Changed a word that might be interpreted as vulgar by our international folk.

 

[lordearl]

OK, I may as well attempt the de-soldering of the BD137 initially, then buy some more if it doesn't work out.
I'll turn the board upside down, tie a weight to the transistor, then de-solder using the blob technique - fingers x'd it will fall out.
 
Is the transistor at risk of being damaged if I apply too much solder heat?

 

[tomb]

  OK, I may as well attempt the de-soldering of the BD137 initially, then buy some more if it doesn't work out.
I'll turn the board upside down, tie a weight to the transistor, then de-solder using the blob technique - fingers x'd it will fall out.
 
Is the transistor at risk of being damaged if I apply too much solder heat?

 
They can withstand a lot of heat, but remember that all you're trying to do is melt the solder blob.  Once the transistor comes out, clean up the pads with de-soldering braid.  A dental pick can sometimes help clear out the holes.

 

[lordearl]

To quote an earlier post - "Another Minimax lives!!"
 
Switched the transistors to their correct spots very easily - there's two holes on each heatsink, so I just clipped them out and then re-attached them to the lower hole in each heatsink, which meant no desoldering was even necessary!
 
The sound is phenomenal, even without any burn-in time.  I'm using jumpers at the output, with 12AE6 and DT880 headphones....B E A U T I F U L !!!
 
Will post more impressions as the burn-in continues, but for now - a massive thanks to Tom B for all his help, an absolute legend.

 

[tomb]

  To quote an earlier post - "Another Minimax lives!!"
 
Switched the transistors to their correct spots very easily - there's two holes on each heatsink, so I just clipped them out and then re-attached them to the lower hole in each heatsink, which meant no desoldering was even necessary!
 
The sound is phenomenal, even without any burn-in time.  I'm using jumpers at the output, with 12AE6 and DT880 headphones....B E A U T I F U L !!!
 
Will post more impressions as the burn-in continues, but for now - a massive thanks to Tom B for all his help, an absolute legend.

 
Thanks for the kind comments!

 
I'm glad you tried just using the jumpers.  I think the amp sounds best that way, but not many people are willing to try it for some reason.  The "noise" I reference in the parts kit might be a little hiss with very low-impedance, high-efficiency earbuds or IEM's, but almost every solid state amp has the same issue in those cases.  There are very few SS amps - especially opamp-based ones - that do not have some resistance on the output.
 
For the great majority of headphones, though, no output resistance is ever needed on the MiniMAX.

 

[lordearl]

Agree - I've tried a number of SS, hybrid and full tube commercial offerings, and all my low impedance headphones (eg KRK KNS8400 @ 32 ohms) have a slight amount of hiss.  The minimax actually has VERY LITTLE detectable hiss, and on the 600 ohms DT880, silent as a grave!!

 

[lordearl]

Tom - a couple of quick questions if I may:
 

• I'm about to build a small non-oversampling DAC which uses passive current-to-voltage converters in the form of a single resister per channel.  There is no filtering at the output, so I'm not sure on whether I need to implement some ultra-sonic frequency filtering in the chain before the Minimax.  Does the Minimax have any filtering of this nature?  If so, obviously I won't need to solder the capacitors into my DAC circuit.
• What is the input impedance of the Minimax?

 

[tomb]

  Tom - a couple of quick questions if I may:
 

• I'm about to build a small non-oversampling DAC which uses passive current-to-voltage converters in the form of a single resister per channel.  There is no filtering at the output, so I'm not sure on whether I need to implement some ultra-sonic frequency filtering in the chain before the Minimax.  Does the Minimax have any filtering of this nature?  If so, obviously I won't need to solder the capacitors into my DAC circuit.
• What is the input impedance of the Minimax?

1. It doesn't really matter - the entire output of the MIniMAX is filtered with capacitors.  Whatever offset may exist, it's stopped at the output (and the relay-delay is there to ensure the caps are charged prior to a load switched in).  I also doubt that the tubes are going to do anything in the ultra-sonic regime.
 
2. The input impedance is the impedance of the volume pot - 50K.

 

[dentaku81]

Has anyone tried a miniMax with Mad Dogs? Does it output enough power to drive them? I seem to remember reading that the amp output is about 0.8 watts at ~50 ohms which should be enough in theory. I would just like to hear from someone who has tried the combo.

 

[tomb]

  Has anyone tried a miniMax with Mad Dogs? Does it output enough power to drive them? I seem to remember reading that the amp output is about 0.8 watts at ~50 ohms which should be enough in theory. I would just like to hear from someone who has tried the combo.

I'm not sure it's that powerful, if that's what you think is really needed.  From my observations, ortho power requirements are wildly over-stated.
 
In any event, the Class A current capability can probably be dialed up to 75ma with the 1" heat sinks and custom case.  That would equate to about 0.28W.  A MOSFET-MAX can be dialed up to 125ma bias, which should result in 0.78W with a 50 ohm load.

 

[dentaku81]

  I'm not sure it's that powerful, if that's what you think is really needed.  From my observations, ortho power requirements are wildly over-stated.
 
In any event, the Class A current capability can probably be dialed up to 75ma with the 1" heat sinks and custom case.  That would equate to about 0.28W.  A MOSFET-MAX can be dialed up to 125ma bias, which should result in 0.78W with a 50 ohm load.

I asked Dan from Mr speakers what the power requirements were and he replied "at least 200mW at 50ohms", so it looks like the minimax with the custom case might work. Is the custom case the one that comes in the beezar kit?

 

[tomb]

  I asked Dan from Mr speakers what the power requirements were and he replied "at least 200mW at 50ohms", so it looks like the minimax with the custom case might work. Is the custom case the one that comes in the beezar kit?

That seems more reasonable for an ortho.  Yes - the custom case is the one that comes in the beezar kit.

 

[dentaku81]

  That seems more reasonable for an ortho.  Yes - the custom case is the one that comes in the beezar kit.

Awesome, im currently building a cmoy to re-learn how to solder, but i will start on the minimax after that.