Quote:
Originally Posted by jdkJake /img/forum/go_quote.gif
TomB,
Realizing there are some fairly major part changes, are you going to be releasing an updated BOM for the entire project or just the delta changes to the existing BOM?
Thanks!
jk
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As BMF said, I've revised a BOM for him so that he could order parts. However, your question makes me think that perhaps I should just make a BOM that highlights the changes. That way, both the existing MAX/MiniMAX and MOSFET-MAX BOM's would still apply.
Essentially, there are three major changes that distinguish the MAX V1.2a board from old:
1. Power Supply - exactly the same as the MiniMAX. Although, you have the option to use 1-1/2" sinks, which means you can use two 2000uf 50V electrolytics.
2. Headphone Relay - cetoole has gone to 24VDC relays throughout. This is combined with a 78L24 voltage regulator that replaces the resistor/zener diode combination used in competing relay circuits. The result is that parts count is reduced and interestingly - relay transients at the headphone jack are reduced to being immeasureable. Many relay-delay circuits have a few full volts of transient at the point of shutoff, depending on the headphone load. This transient is not enough to cause any harm, but still - it can be measured even with a $9.99 DMM. The 24V relay snaps so fast that I can't measure anything. I've used this arrangement on a modified MiniMAX for the past year and can attest to its reliability.
3. Signal Input Section - Lastly, this is the most significant change with the MAX V1.2a board. As many of you have seen before, the PCB allows for a BantamDAC to be mounted directly to the board, with relay switching between the Bantam as a source or normal RCA input jacks.
The previous prototype I built performed flawlessly with this switching arrangement. However, it used only one relay and so depended on the dB quieting that existed between the contacts in the relay itself to control cross-talk between the two competing sources. Unfortunately, if both sources were "live", there was audible bleed-through of the signal. (Absolutely nothing was wrong with it if the other source was switched off, however.)
For this proto, cetoole has designed a couple of mini-relays that will independently switch the two sources. Further, he has designed in some resistors that will "short" the un-used signal source when not in use, to further remove any possibility of crosstalk above the SN ratio of the amp itself.
One last thing - we've been playing around with the idea of using the IRF510 (used on the Starving Student) and its complement, the IRF9510, as the MOSFETs to make up the MOSFET diamond buffer. Further, cetoole has identified some Xetex FETs that will replace the SK-type Toshibas that were always in short supply. Both the MOSFETs and input FETs have lower gate capacitances than have been previously used, which should allow better performance at lower Class A biases. I have been running the prototype almost every day for the last several months at 125ma bias - even had it up to 150ma bias for short periods, with no ill effects. However, lower biases noticeably affected the sound quality, IMHO. Still, I haven't cased it up yet, so who knows if that bias can be maintained? The new MOSFETs and FETs should alleviate some of this and again - looking forward to trying the combination out.
