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Old 11th October 2012, 09:23 AM   #61
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Hi again andrew .
Can i not just use the existing bias transistor wich is a to92 package and glue that to the output device????
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Old 11th October 2012, 09:45 AM   #62
AndrewT is offline AndrewT  Scotland
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A To92 is massive compared to a sot23.
The high mass of the To92 will reduce the response speed and increase the temperature error from the Tc you are trying to measure.
We really need Tj, but even the NJL and Sanken can only get us closer to Tc. I know of no device that allows us to monitor Tj.

Adding a current mirror changes the open loop gain (OLG) of the amplifier. That increase in OLG may make the amplifier oscillate and destroy it's self.
You can reduce the OLG by adding in or increasing the LTP degeneration, but you would still need to adjust the compensation to make the amp stable into all likely loads.
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Old 11th October 2012, 10:05 AM   #63
AndrewT is offline AndrewT  Scotland
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Default Moving the Multiplier to the heatsink, or To3, or Collector lead

The plastic package transistors have a metal backplate that is directly connected to the Collector of the BJT transistor (Drain in a vertFET and Source in a latFET).
The contact face of the backplate is at Tc.
The inner face of the backplate is at a slightly higher temperature than Tc (Tc + a bit). This is due to thermal resistance to heat flow across the thickness of the backplate.
The plastic package in contact with the front surface of the backplate will be at the same slightly elevated temperature of Tc + a bit.
The outer surface of the plastic package will be at a very considerably lower temperature than Tc + a bit. Again this is due to the thermal resistance of the plastic to the heat flow from the backplate through to Ta on the inside face of the amplifier. It is likely that the plastic is >10C lower than Tc and possibly >20C lower. The heatsink Ts adjacent to the backplate will be Tc - a bit, again due to thermal resistances and heat flows.
The collector lead is directly attached to the backplate with a fairly thick copper lead. Some are welded on, but many (collectors) are part of the backplate copper. The temperature of the lead next to the plastic package is very close to Tc in temperature. This is because there is little heat flow out through the collector lead.
Nowhere, external to the transistor package, is there any temperature that monitors Tj.
All locations are a compromise. That compromise determines what ratio of temp change the Vbe multiplier has to apply to approximate the tempco required to maintain near optimum ClassAB bias conditions for minimising
I reckon that the collector lead is the closest in Tc for a monitoring temperature outside of the package.
We could use D.Self's or Cordell's simulation methods to arrive at a model for this collector lead monitoring.
Any volunteers?

I am going to place a copy of this post in a new Thread. Any related comments should be in that new Thread.
Moving the Multiplier to the heatsink, or To3, or Collector lead
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Last edited by AndrewT; 11th October 2012 at 10:15 AM.
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Old 11th October 2012, 11:19 AM   #64
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Quote:
Originally Posted by AndrewT View Post

Adding a current mirror changes the open loop gain (OLG) of the amplifier. That increase in OLG may make the amplifier oscillate and destroy it's self.
You can reduce the OLG by adding in or increasing the LTP degeneration, but you would still need to adjust the compensation to make the amp stable into all likely .
This is one of my concerns oscilations . This is taking me into the relms of needing a scope. Unfortunatley for me my old scope is no longer working.. I will look up software to use My PC as a scope and get to grips with that before i make any modifications .. I understand the to92 case is large surley afixing it to the output device is an improvement to just being close to the main heatsink. Not the best way i agree but a step in the right direction . Also my concerns for the pre drivers not having adequate thermal disipation will need to swap them out to. Just a thought maybe the amplifier is set up as it is because of PCB layout issues and increasing the gain may bring instability problems . This is a bit of a guess . based on the layout of the pcb the power tracks are a mess the input signals are way to long also there is a problem with the neg feedback take of point so im assuming there are other problems hence my guessing..

Kind Regards Mark Ps Would it make ocilations less prone if i over compensate then start to reduce compensation in steps untill a good compromise ids found ...

Last edited by madtecchy; 11th October 2012 at 11:21 AM. Reason: PS
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Old 11th October 2012, 11:27 AM   #65
AndrewT is offline AndrewT  Scotland
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a PC used as a digital scope cannot see the fast changes needed to identify oscillations that can kill an amplifier.

The problem oscillations are likely to be in the range 1MHz to 200MHz.
A very fast digital sampling scope of the order of 1Gs/s will see these fast changes.
A reasonable analogue scope of 50MHz should be just about good enough, 100MHz would be better.
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Old 11th October 2012, 11:55 AM   #66
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Nico Ras View Post
A lot of people (on DIY) stick this transistor on top of one of the output devices. I have seen this a lot in the threads here.
My "school of thought" is to make the VAS (including temperature compensation) free of stray capacitance. I like to put both drivers/VAS transistors very close with each other. As the consequence of this, the compensation transistor will be put in the middle of NPN and PNP output transistors, and consequently, on the heatsink.

Quote:
Originally Posted by Nico Ras View Post
I don't subscribe to this and believe it is best to tie this transistor onto the heat sink so it responds slowly to the temperature variation a form of hysteresis.
The question is, is the compensation transistor used to linearize gain and to lower distortion or SIMPLY as protection against thermal runaway.

Quote:
Originally Posted by Nico Ras View Post
Tying onto one of the output transistors can introduce thermally generated distortion in that the thermal compensation reacts quickly to one transistor but slowly to the other thereby distorting or compressing the output signal.
Logically, for protection purposes we need to control only one device: the most disturbing one. For most power BJT, it should/usually be the NPN.

Quote:
Originally Posted by Nico Ras View Post
This is measurable, but maybe not audible at lower temperatures.
This "fact" is more important for practical less theoretical minds. Theoretically, to get the perfect tracking is very possible, but there are too many variables to be considered (properties of transistors used, heatsink size, bias used related to TR properties, circuit used, etc etc), leading to question: is it necessary? May be not for most circuits.

Easier is to change the level of temperature tracking/coupling physically or mechanically without "changing" the circuit (just an idea). I guess using TL431 in combination with diodes it is possible (by shorting some of the diodes based on measurement). I'm not sure tho...

But primitively changing the contact level between the TC transistor and tracked transistor, physically, is possible (no detailed idea how the mechanics should work, yet).
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Old 11th October 2012, 11:58 AM   #67
AndrewT is offline AndrewT  Scotland
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Jay,
I have started a new Thread so that off topic discussion does not encroach too deeply into this Thread.
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Old 11th October 2012, 12:03 PM   #68
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by madtecchy View Post
This is one of my concerns oscilations ..
My concern is sound quality. Do you really need extra low THD, or extra OLG?

Imo, adding the mirror has advantage and disadvantage. To justify the use of it, you have to get every possible advantage, so that it would be justifiable to add the mirror. In your case, I cannot backup my intuition with theories, but imo it is unjustifiable.
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Old 11th October 2012, 12:11 PM   #69
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by madtecchy View Post
..................
Line Outputs
max output level +2dBV (monitor off), +8dBV (monitor on)......
If I am reading this correctly, then 0dBV = 1Vac
+2dBV = 1260mVac
+8dBV = 2520mVac

But I don't understand why the higher output when monitor is ON.
Is that showing that there is a +6dB (2times) gain stage being added into the signal route?
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Old 11th October 2012, 03:17 PM   #70
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Hi Jay
Yes i agree sound quality is the formost quality that is my wish. however the ociilations are a danger and a concern.. I dont know if i like extra low thd or the effect of higher open loop gain on the sound signature . anything im prepared to try on thia amplifier will be easily reversable so if i dont like what i hear i can switch back to what i like and would also in the process have learnd usefull knolage.. you may be right about what im proposing being unjustifiable. there are other areas i should improve that dont have such knock on effects . More realistic! a seprate psu for each channel then maybe another isolated psu for the front end of each amplifier or maybe just improve psu decoupling between the diffrent stages.. i can deal with the bad psu layout by hard wiring and the same with the feedback take of point.

Kind Regards Mark
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