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Old 29th March 2010, 03:56 PM   #101
klewis is offline klewis  United States
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I'm measuring the bias of the an amp similar in configuration to Panson's.
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Old 30th March 2010, 01:57 PM   #102
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Quote:
Originally Posted by robertvalk View Post
Panson, do these developments mean that there will be e new PCB in the near future? I am still awaiting my main components and am about to order a pair of pcb's. Should I just go ahead and buy or would you suggest to wait for a while?
Hi Robert,

Sorry, I was in somewhere without internet access yesterday.

A newer PCB is not needed since we can use the current one (Rev 2.1) to do the fine tune.

BTW, thank you for your order.

Panson
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Old 30th March 2010, 02:06 PM   #103
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Originally Posted by klewis View Post
I'm measuring the bias of the an amp similar in configuration to Panson's.
I measured one Re=0.22R volt drop.

Ken, can you check for higher heatsink temperature.
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Old 31st March 2010, 12:44 AM   #104
tomchr is offline tomchr  United States
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Quote:
Originally Posted by panson_hk View Post
Out of curiosity, I put the two 15030 diodes on the main heatsink, similar to your suggestion. Since the main heatsink is hotter than that of driver, I can effectively increase the overall bias tempco. It results in a very stable system. The Re drop varies from 5.x to 6.x mV.
Nice...! Just remember (and I know you will) to track the bias voltage over a longer period of time (60-120 minutes at least). Thermal systems have a nasty habit of having very, very low bandwidth and may oscillate at sub-Hz frequencies.

~Tom
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Old 31st March 2010, 03:29 PM   #105
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Originally Posted by tomchr View Post
Nice...! Just remember (and I know you will) to track the bias voltage over a longer period of time (60-120 minutes at least). Thermal systems have a nasty habit of having very, very low bandwidth and may oscillate at sub-Hz frequencies.

~Tom
Hi Tom,

Sure I will do extensive test. Thanks!

Panson
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Old 31st March 2010, 03:42 PM   #106
klewis is offline klewis  United States
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Quote:
Originally Posted by panson_hk View Post
I measured one Re=0.22R volt drop.

Ken, can you check for higher heatsink temperature.
Panson,

Is it conventional to measure across only 1 Re?

I was thinking that the target 22mv was from the Audioman54 findings - where he found that a bias of about 22mv was better slightly than the "normal" 44mv for the 49811.

I've been looking at my temp simulations and found that the 3k/2k R3/R2 combination give the lowest bias rise with temperature, of about 2x with the Cordell configuration. Using the aforementioned Audioman54 findings, setting the initial bias at 22mV and knowing that it would rise to 44mV under heavy loads the amp should preform pretty well.

I'll try to get the heat sink hotter. I'm only running 33v rails into 8 ohms and it just doesn't want to get much past 40c - unless I bias it into class A - but that goofs up the whole test.

Ken
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Old 31st March 2010, 04:01 PM   #107
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Quote:
Originally Posted by klewis View Post
Panson,

Is it conventional to measure across only 1 Re?

I was thinking that the target 22mv was from the Audioman54 findings - where he found that a bias of about 22mv was better slightly than the "normal" 44mv for the 49811.

I've been looking at my temp simulations and found that the 3k/2k R3/R2 combination give the lowest bias rise with temperature, of about 2x with the Cordell configuration. Using the aforementioned Audioman54 findings, setting the initial bias at 22mV and knowing that it would rise to 44mV under heavy loads the amp should preform pretty well.

I'll try to get the heat sink hotter. I'm only running 33v rails into 8 ohms and it just doesn't want to get much past 40c - unless I bias it into class A - but that goofs up the whole test.

Ken
Ken,

Taking one Re volt drop is fine unless you have a very unbalanced upper and lower half.

Yes, Mark has recommended 22 mA for 49811. My measurements agree with him. You can get the heatsink hotter by using a 4 ohm load.

My target is to have <= 10 % bias variation.

Panson
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Old 31st March 2010, 05:14 PM   #108
klewis is offline klewis  United States
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Quote:
Originally Posted by panson_hk View Post
Ken,

Taking one Re volt drop is fine unless you have a very unbalanced upper and lower half.

Yes, Mark has recommended 22 mA for 49811. My measurements agree with him. You can get the heatsink hotter by using a 4 ohm load.

My target is to have <= 10 % bias variation.

Panson
Panson,

Let me ask another my question another way is Mark's 22mA bias recommendation measured across both Re's - from emitter to emitter?

I have to buy more power resistors...

Good luck on the 10%, I await your solution!

Ken
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Old 1st April 2010, 03:29 PM   #109
AndrewT is offline AndrewT  Scotland
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with the conventional temp comp systems I have used, I prefer to see a small fall in bias current with increasing temperature.
Expect some bias changes from cold to warm. then after it is warmed up, try artificailly heating the heatsink, hairdryer maybe. Or what is the bias immediately after a few minutes at say 30% maximum power?

The overcomp allows for higher bias when the amp is cool and in cool ambient. As the ambient rises the bias falls very slightly. This is the opposite of thermal runaway.
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Old 1st April 2010, 05:12 PM   #110
superR is offline superR  Netherlands
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Any idea if this amplifier will function properly on a +/- 24V PSU? I have a high current toroid but it has low voltage output.
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