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The Multi Amp aka Dynalo Mk2


spritzer
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Interesting problem, at least to me, and might help someone in the future with these. Fired up the 3rd board yesterday, and all looked well (LEDs, etc.). However, there were problems:

Problem 1: Went to check DC offset, and the R channel (both +/-) was showing 4V offset. Not good. After looking the board over, I suspected one of the small transistors (MMBTA06/56), as there were a couple that were a bit crooked and I couldn't tell if all pins were soldered or not. Q3_2 looked suspect, so removed with hot air, which also took the 3.01K resistor right by it. Cleaned and fluxed the pads and got it put back together. I also touched up the solder connections on the others on both channels in this section of the board. Fired it back up, same problem. I then looked at Q1_2 and Q2_2. Straightened these and touched up the solder joints. Success! But I wasn't quite done...

Problem 2: Adjusted DC offset (no pot in yet, so will recheck once that is in). I then started going down the 20ohm resistors checking the biasing. All looked good until I got to the L+ channel. These  alternated from 290/220mV (they should be balanced between the NPN and PNP sides). The 20 ohm for the closest to the center NPN showed no drop across it. I retouched the solder joints for this transistor, but same problem. Removed and replaced the transistor. Success. Now bias was the same as the other channels and consistent across all resistors.

Ohming that transistor out, it appears to be good. I couldn't see any evidence of solder on the bottom of the tab (collector). I know when I checked voltage on it, it showed V+, but I was probably pushing it down with the probe. The collectors aren't connected on the middle small pin on these from what I can tell in the gerbers. I had used hot air to install these originally, but had hit the tabs with an iron with a wider chisel tip.

These are considerably more difficult to troubleshoot/rework than the full size dynalos are, so it pays to take your time building them. Before I checked the end transistors in #1, I wasn't relishing having to do more extensive troubleshooting/rework on this. The other two boards came right up with no problems. And I'm still not fond of SMT pots :)

Also, since I don't think this is documented anywhere, with the board facing you (pot, outputs, etc.), looking at the channels as quadrants, they are laid out as follows:

L-           L+

R-          R+

Edited by Pars
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  • 2 weeks later...

I seem to be the only one participating in this thread... hmmm :)

Just got the heatsinks today (45x35mm).  After reading what some have said regarding the heatsinks tunneling heat down the transistor rows, I'm considering drilling 2-3 holes in the heatsinks along the rows to allow them to vent. I'll run the thermal heatsink tape in rows just on top of the transistors. Anyone think this is a bad idea?

 

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Well, if you can get the heat sinks to transfer some heat to the chassis (shimming extra metals on top the sink so it can get touch the chassis top), doubt that the heat in the channels would be a problem.  My Mini's temp is about 32C with help from the chassis to dissipate the heat; without the chassis help, it was well above 50C.  I didn't do any hole into my sinks.

I see you didn't attempt QFN solderings with your new re-flow gun?    

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Agreed. I know mypasswordis and cspirou were two who managed it. I was trying to use the advice I got from mypasswordis in my last attempt.

I would think the lighter copper weight on the GB boards would make this more doable. Many of the pins on the regs go to ground, and on the 3oz boards, difficult to heat. Kerry's prototype board that he did was 1oz copper, and he told me via pm that he doesn't think the board needs to be any heavier than that. I would agree.

Edited by Pars
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I think they are still talking 2oz for the group buy. I don't know what the basis for the current moar is better philosophy on copper weight is. All of the older GBs (dynalo, dynahi, etc.) were 1oz boards, many of which I have reworked several times with no problems. Some boards need the heavier copper (Stax stuff, the GRLV/GRHV). Most don't.

Edited by Pars
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I managed to get it soldered with reflow oven and smd rework station. It wasn't really so bad as long as the right amount of solder is used (less is better). Though it could be easier.

I agree that if the board was designed for 1oz that it should stay 1 oz. Considering how hot the transistors get, I do think the extra copper helps with heat dissipation. but there's probably much better ways to implement that.

Why do STAX boards need higher copper weight considering that electrostatic headphones are low current?

 

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I don't think the Stax amps need more than 1 Oz copper because of the low current, but firstly everything here is overkill so if 2 Oz copper is good then 3 Oz is surely better :P 

Secondly, the 2 Oz boards will generally take more (re)heating so they stand a better chance of surviving rework which does come in handy for every DIY'er at some point.

Last but not least, most of the amp designs have originally been offered to the community through early large GBs and when you are ordering 30/50/100 PCBs the additional cost per board becomes almost negligible.

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I just looked (way) back. Amb apparently uses 2oz copper on the beta22, sigma22 and CK2III boards. The Dynafet boards were also 2oz copper. AFAIK, all of the prior GB boards (imagineering) were 1oz boards (the old Dynalo rev B and C boards, the original Dynahi, etc.)

I'm not in for any dynalo mini boards, but this is from the experience of building 3 of them, plus talking to Kerry. I may fuck around with one of the spare boards I have and some new parts (which I still have). Reflow oven seems to be the best bet and I may commandeer the toaster oven briefly again. This worked pretty well for putting the tantalum caps on the underside of the last board I did. Even with my Hakko 936 and a 3.2mm chisel bit, cranked up to 750F +, I had to use a hot air rework station to get some of the connections to ground to flow well.

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Try Metcal PS-900. Clearer better than Hakko 888d (I have both). Hakko 888d is a great station, but sorry to say, Metcal is better. Reason I still keep Hakko is that I already invested in lots of Hakko tips, including so called 'microwave' tips which I use for soldering SOIC packages and the like (when heat transfer speed is not critical). But most of soldering is done by Metcal, including joints to heavy ground planes.

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Sure, I'd love a Metcal and I'm sure they are better than the Hakko I have. They also seem rather pricey, even on ebay. Also difficult to tell if you are getting the full soldering station or not as most sellers seem to sell everything separately.

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