Hey all,
I had quite a bit of "room" in my heat sinks, so I decided to remove R19. All biased up from .618 to .650 on the same emitter resistor (.91ohm 3 watt emitter resistors)...all good.
For S&G, I decided to check the other bias voltages on the same OP PCB (Brian's boards). I noticed the middle MOSFET on all OP PCBs had much higher bias than the top/bottom.
655/850mv/650mv
other channel,
649mv/880mv/650mv
Now I used a bar (securing the MOSFETs to heatsink) across all three MOSFETs and can see a slight bow over the center MOSFET...I am using pads too.
Could this be the reason for center bias imbalance across the output PCB?
I plan on replacing the bar with a thicker bar ASAP - I just hate drilling taps into heatsinks as I always break the taps off (I know Peter Daniels has a tried and true method of using a clutched drill to tap into sinks)
This problem most likely was there before I removed R19, but never realized it until I checked it...
Seems an obvious solution, but wanted to ask the pros. I have searched and found nothing much as far as a similar problem with heatsink mounting...
I had quite a bit of "room" in my heat sinks, so I decided to remove R19. All biased up from .618 to .650 on the same emitter resistor (.91ohm 3 watt emitter resistors)...all good.
For S&G, I decided to check the other bias voltages on the same OP PCB (Brian's boards). I noticed the middle MOSFET on all OP PCBs had much higher bias than the top/bottom.
655/850mv/650mv
other channel,
649mv/880mv/650mv
Now I used a bar (securing the MOSFETs to heatsink) across all three MOSFETs and can see a slight bow over the center MOSFET...I am using pads too.
Could this be the reason for center bias imbalance across the output PCB?
I plan on replacing the bar with a thicker bar ASAP - I just hate drilling taps into heatsinks as I always break the taps off (I know Peter Daniels has a tried and true method of using a clutched drill to tap into sinks)
This problem most likely was there before I removed R19, but never realized it until I checked it...
Seems an obvious solution, but wanted to ask the pros. I have searched and found nothing much as far as a similar problem with heatsink mounting...
Oh, and all output devises were matched from Tech-DIY...
And I am using T-Pli 240 Insulators:
http://lairdtech.thomasnet.com/item/gap-filler-pad-tims/t-pli-8482-200-series-gap-filler-material/pn-3893?&plpver=10&assetid=g2
And I am using T-Pli 240 Insulators:
http://lairdtech.thomasnet.com/item/gap-filler-pad-tims/t-pli-8482-200-series-gap-filler-material/pn-3893?&plpver=10&assetid=g2
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Well I changed the bar to a thicker one and all tracks cleanly at 750mv (+/-2mv) across the emitter resistors on one side and 730mv (+/-3mv) across the emitter resistors on the other side...all good.
Heat sinks still nice and toasty and 111F. I will allow the amp to warm up an hour or so.
I had put a 75C switch too close to one MOSFET and was wondering why the amp was shutting down. Sure enough the temp at the center MOSFET was 78C when not fully attached to the sink...
Heat sinks still nice and toasty and 111F. I will allow the amp to warm up an hour or so.
I had put a 75C switch too close to one MOSFET and was wondering why the amp was shutting down. Sure enough the temp at the center MOSFET was 78C when not fully attached to the sink...
Thanks for your reply Nelson...
When the middle MOSFET is a bit warmer (in my case - carrying more of the current load), is the sound quality compromised?
When the middle MOSFET is a bit warmer (in my case - carrying more of the current load), is the sound quality compromised?
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