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Old 27th October 2012, 05:09 PM   #61
dadod is online now dadod  Croatia
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Quote:
Originally Posted by AndrewT View Post
why is D8 added to TT version?
To lift a bit Vbe multiplier voltage, otherwise thermal compensation with TT diodes is undercompensated. I simulatid it with normal diode and schottky diode was better suited with its lower voltage drop.
dado
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Old 28th October 2012, 08:04 AM   #62
AndrewT is offline AndrewT  Scotland
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Can I rephrase that reply?

Without the added didoe the Vbe output is at the required bias voltage. That also multiplies the diode compensation by the same ratio.

With the added diode D8, the Vbe output voltage is the same but the multiplier is at a slightly lower ratio. This results in the diode compensation being lower due to the lower multiplier ratio.

The added diode reduces the temperature compensation. Without the added diode, the bias voltage would be over compensated.

That last conclusion is opposite to what you said.
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Old 28th October 2012, 08:08 AM   #63
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I think you will find that you can "tune" the temperature compensation, without incurring the "step change" of using discrete added diode/s.

Simply change the current in the TT diodes and the compensation changes. This is easily done by adding a VR parallel to the diode string. This VR bypasses some of the current around the diodes.
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Old 28th October 2012, 09:29 AM   #64
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No is the answer on both!
Thermal compensation has two part, TT diodes to compensate thermal drift of the output transistors and Vbe multiplier to compensate thermal drift of the drivers(that is why Vbe multiplier is on the same separated small heat sink). To be effective the Vbe multiplier needs more voltage and this is why D8 was added. I this case we have four transistors Vbes plus D8 diode voltage drop, and that is close to 3.2 V(not 2.8 without diode).
Without that diode drivers are undercompensated, with diode drivers are a bit overcompesated(better then temperature run away).
I have a resitor parallel to the TT diodes for fine tuning.
I simulated this thermal compensation using Cordell models for output, tt diodes, drivers and Vbe transistors, but I need to test it in real amp, not sure when as I am occupied with other project. Maybe somone can do that?? I tried in my simulation Cordell's Vbe multiplier(figure 14.19a page 307) but it a was way overcompensated according to the simulation. Could be that the simulation(models) is not acurate enough, so practical test will be neaded.
Cordell, in his book describes Vbe multiplier for output transistors only, drivers not being mounted on the main heatsink, neither Vbe transistor. Hi mentioned that if predrivers are mounted on the main heatsink then Vbe transistor should be there too.
I want predrivers on separate heatsink and compesated with Vbe transistor on the same heatsink.
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Old 28th October 2012, 09:38 AM   #65
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I don't follow your explanation. I seem to be on another planet where everything is opposed.

Have you only simulated the compensation?
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Old 28th October 2012, 10:13 AM   #66
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Quote:
Originally Posted by AndrewT View Post
I don't follow your explanation. I seem to be on another planet where everything is opposed.

Have you only simulated the compensation?
For this amp only simulation, but something similar I used in my TT TRIPLE amp and that amp was built.

As English is not my native language I am not good in explanation. Do you have Bob Cordell book? Its good reading.
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Old 28th October 2012, 10:22 AM   #67
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Yes, I have started my re-read. Some bits of gems have only started to become apparent on the re-read.
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Old 29th October 2012, 05:35 PM   #68
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Default BOM for Little-GEM-N

Here is bill of materials for Little-GEM-N with not TT output transistors.
Small heatsink Fisher SK 548-50.
dado
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File Type: txt BOM-Little-GEM-N.txt (1.0 KB, 65 views)
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Old 23rd May 2013, 04:36 AM   #69
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Default A bit late

Hi Damir
Sorry this is a bit late but I have had a few ideas lately that may be relevant.
Have not simulated to see if the theoretical improvements make any practical difference but may do so in a few days.
.
If you increased the Current Mirror emitter resistors then you would have less noise and also it may then be possible to balance the mirror better.

I don't think you need TO220 drivers. TO126 devices like 2SC3503 have adequate power capacity (SOA) and are faster so a little more feedback could be safely applied.

Best wishes
David
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Old 23rd May 2013, 05:17 AM   #70
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Quote:
Originally Posted by Dave Zan View Post
Hi Damir
Sorry this is a bit late but I have had a few ideas lately that may be relevant.
Have not simulated to see if the theoretical improvements make any practical difference but may do so in a few days.
.
If you increased the Current Mirror emitter resistors then you would have less noise and also it may then be possible to balance the mirror better.

I don't think you need TO220 drivers. TO126 devices like 2SC3503 have adequate power capacity (SOA) and are faster so a little more feedback could be safely applied.

Best wishes
David
Hi David,
Good ideas, I will try it, at least in simulation first. I know about CM emitter resistors, but I did not think about it when simulated Little GEM:
BR Damir
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