Simulation - Pass MF-12 MOSFET Quasi Comp (Original) Modification for HK Citation 12 - Page 3 - diyAudio
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Old 2nd June 2011, 02:16 AM   #21
PB2 is offline PB2  United States
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A matched pair makes a lot of sense for the diff pair but is not critical and it seems that even HK went to 2N5087s in later production - not sure if they matched them. Any similar high beta part with VCE >= 50V and Icmax = 100 mA should do. BC556B or C are also good choices:
Product Folder - BC560 - PNP Epitaxial Silicon Transistor, Fairchild Semiconductor - Global Leader in Power Optimization

There are some low cost duals still out there but most are in tiny modern surface mount packages such as the SC70-6. BC857s:
Product Folder - BC857S - PNP Multi-Chip General Purpose Amplifier, Fairchild Semiconductor - Global Leader in Power Optimization

Or Diodes Inc. IMT4, also in a very tiny package:
http://www.rohm.com/products/databook/tr/pdf/imt4.pdf

2SA1349 is in a normal size package but is getting hard to find:
users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/2SA1349.pdf
A member has them:
FS: Toshiba 2SK389 -GR, 2SA1349 -GR, 2SC3381 -GR

If you buy standard parts like BC556Bs packaged on tape there is a good chance that they came from the same lot and if so will be very well matched.

Then there is always the LM394, and similar MAT02 and SSM2210, that you can read more about here:
Simulation of the JE-990 OP Amp By Deane Jensen
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Last edited by PB2; 2nd June 2011 at 02:35 AM.
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Old 2nd June 2011, 04:02 AM   #22
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Pete,

Thanks so much for all the info. and help you have provided me. It certainly has been a learning experience for me!

The 2N5551's came on the tape reel, so they should be pretty close.

I was able to create my schematic in LTS. I started from scratch instead of modifying yours. I figured I would learn more and how to better use LTS if I created my own schematic.

I believe everything is correct except for the three CCS's; which I used the diode symbol for and the variable BIAS resistor. Could you help me get those symbols correct on the schematic? Thanks again for all your help and time!!!!!!!!!!!
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File Type: zip PASS Citation MF12C.zip (1.6 KB, 57 views)

Last edited by MikeBerry; 2nd June 2011 at 04:04 AM.
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Old 3rd June 2011, 04:38 AM   #23
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That looks good Mike!
Should there be a resistor in series in the top rail?
Check your diode polarities, is there a band on the case
of the diodes, that is the bar in the schematic symbol.
I don't know where to get the correct symbol, you can
search online for LtSpice library add ons, or at the LtSpice
group on Yahoo. You could just make the component
designator start with CR instead of D.
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Old 3rd June 2011, 12:14 PM   #24
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Thanks Pete!

Yes there should be a resistor up there. I will fix that. I will check the diode polarities and rename. I will repost the schematic when I get it fixed. Thanks!
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Old 3rd June 2011, 07:07 PM   #25
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Pete,

I was wondering if you know of any visual learning aids that might be available. For example, I am a visual learner. I need the hands on, visual experience to really grasp things. Let's use my circuit for example. I would love to see a tutorial that depicts the circuit, shows moving, colored lines to illustrate the voltage/current flowing through it. It would show the VAS stage, the differential stage, the power supply circuit. You would be able to see what is happening (using moving colored arrows) when different conditions occur. Such as when the amp is sitting idle and when the amp receives an input signal. Do you know of anything like this? It could also be done to show how components interact together or as a very basic introduction to a single component. Just curious about your thoughts on this.
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Old 3rd June 2011, 09:51 PM   #26
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First download my simulation files for the Quasi version, then also download the comp version. You need the spice library file that I only included in the quasi version. Then load either one into LtSpice and hit the run button. Click on a net to see the voltage plotted as if on a scope, over a component to see the current through it. Hover over a net and look in the lower left corner and it will give you the idle bias point with no signal input. The comp version is close enough to yours to get a detailed understanding of what is going on.
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Old 4th June 2011, 02:20 AM   #27
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Thanks Pete! I will give that a try.
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Old 4th June 2011, 03:10 AM   #28
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Quote:
Originally Posted by MikeBerry View Post
My current power supply filter caps are 17000uf 50v each. The original schematic calls for 6000uf 50v caps. Are 17000uf caps necessary, and if not, what is the lowest uf I could use without hurting the performance of the amp? I was thinking about replacing these as I am sure they must be over twenty years old.
I think it was Nelson who said on here that too much power supply capacitance is like having too much money - well said! The only issue is stressing the bridge rectifier and dimming your lights on power up if the power transformer is very stiff. If it has enough resistance then that will limit the peak current on initial startup.
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Old 5th June 2011, 12:45 AM   #29
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Thanks again Pete! No dimming lights on power up. Just the price of a new 17000uf cap x4 compared to four 6000uf caps.

I have not had a chance to try the simulations yet. My three year old has been a handful the last couple of days! I will hopefully be able to try them soon.
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Old 7th June 2011, 02:45 AM   #30
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Pete,

I was able to get everything loaded as you described. That is pretty cool! I will experiment more with it as soon as I get a chance. Thanks!
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