j4cbo
100+ Head-Fier
- Joined
- May 13, 2006
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Sweet, someone who knows what they're doing more than me 
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Yep, they're currently open. I put those pads on the board so I could pull the Vcom divider inside the THS4131 just a little bit closer to V+, to bring the output common mode voltage closer to mid-rail. I think I'll just populate it with a 1uF ceramic instead, for now.
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How'd I forget about that? A few fewer resistors to populate; sweet.
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Interesting... I'll take a look at the options. Space is a bit tight, though; I'd like to avoid something that'll need a big cap.
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Good point. Just keep the ratios such that Rdrive * Ciss is the same?
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Absolutely. That one may fit as-is, actually...
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I'll mess around with the opamp power supply a bit and see if I can come up with something better. (Or if it turns out that just omitting it / setting V- = UB- is better.) And it's a 4131, not 4031
Thanks for the feedback - praise is nice, but criticism actually helps make things better, so it's always good to get...
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| Originally Posted by jcx /img/forum/go_quote.gif I hope the R37,38 are open - the Vcom inputs of the diff op amps try to split the supply V and connecting to one rail would a bad idea further the data sheet highly recommends bypass C on the Vcom pin - I would tie the 2 chip's Vcom together and make a AC gnd with a pair of well matched caps, one to each op amp supply rail |
Yep, they're currently open. I put those pads on the board so I could pull the Vcom divider inside the THS4131 just a little bit closer to V+, to bring the output common mode voltage closer to mid-rail. I think I'll just populate it with a 1uF ceramic instead, for now.
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| the fact that the ths4031 servos the output to Vcom does give you an implicit mid supply active gnd the feedback+input resistor strings provide a DC path from the inputs to the Vcom active gnds (common mode) recognizing the feedback R path to the Vcom active gnd then makes R39-44 largely irrelevant until they are reduced to the below the feedback R value |
How'd I forget about that? A few fewer resistors to populate; sweet.
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| I also find the cascode fet ccs more headfi fashion than sound engineering - you loose too much headroom to "light up" the cascode fet ccs and in this unity gain output stage bias application you may not want to give up a few more V on the negative swing Searching the "best" CCS - diyAudio bootstrap R or bootstrap the supply to a single fet ccs and you'd have more headroom and better output impedance over the full Vsupply swing, the 2 bjt feedback ccs has low headroom requirements if you don't want to use more caps for the bootstrap circuits |
Interesting... I'll take a look at the options. Space is a bit tight, though; I'd like to avoid something that'll need a big cap.
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| gate drive R should also probably be different values to give same bandwidth with the different input C of the N vs P mosfets |
Good point. Just keep the ratios such that Rdrive * Ciss is the same?
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| and pcb room and holes for heatsinking the mosfets could be good for those wanting enough bias current for Class A into Grados or K701 and enough V for 600 Ohm Beyers - there are some narrow clip-on heatsinks that may not need mounting holes Standard Board Level Heat Sinks |
Absolutely. That one may fit as-is, actually...
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| just use 1% Resistors – if this circuit’s functionality is needed at all - what's the magnitude of the supply noise or "image" of the load current in the regulated supply and how much more filtering does the ths4031 need with ps supply rejection already >90 dB to way beyond 200KHz? |
I'll mess around with the opamp power supply a bit and see if I can come up with something better. (Or if it turns out that just omitting it / setting V- = UB- is better.) And it's a 4131, not 4031
Thanks for the feedback - praise is nice, but criticism actually helps make things better, so it's always good to get...
