Aleph 5 bias diferential

john65b · 2nd January 2010, 02:02 PM

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...

john65b · 03:00 PM

Oh, and all output devises were matched from Tech-DIY...

And I am using T-Pli 240 Insulators:

http://lairdtech.thomasnet.com/item/...=10&assetid=g2

john65b · 2nd January 2010, 06:34 PM

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...

Nelson Pass · 3rd January 2010, 12:44 AM

Even when the pressure is the same, the one in the middle
tends to be warmer.

john65b · 3rd January 2010, 01:23 AM

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?

Nelson Pass · 3rd January 2010, 03:26 PM

I don't think so.

2nd January 2010, 02:02 PM	#1
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	Aleph 5 bias diferential 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 speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

2nd January 2010, 02:47 PM	#2
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	Oh, and all output devises were matched from Tech-DIY... And I am using T-Pli 240 Insulators: http://lairdtech.thomasnet.com/item/...=10&assetid=g2 __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V Last edited by john65b; 2nd January 2010 at 03:00 PM.

2nd January 2010, 06:34 PM	#3
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	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... __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

3rd January 2010, 01:23 AM	#5
john65b diyAudio Member Join Date: Jan 2005 Location: Chicago	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? __________________ "I speak Spanish to God, Italian to Women, French to Men, and German to my Horse" - Charles V

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3rd January 2010, 12:44 AM	#4
Nelson Pass The one and only Join Date: Mar 2001	Even when the pressure is the same, the one in the middle tends to be warmer.

3rd January 2010, 03:26 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	I don't think so.

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