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Old 3rd August 2005, 09:52 PM   #1
CBS240 is offline CBS240  United States
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Default New homade job

Hi.

Joy to me for discovering this forum. Lots of interesting information here. God bless modern communication and free will. Anyway, I made a new amp circuit needs picked apart. Figured some of the SS experts here can do some damage. I will start by saying it does work and sound good, and the DC offset is very stable, like plus or minus 5mV or so. This was achieved when I put a “heater” on top of the VAS because I noticed room temp. seemed to affect the DC offset, in the form of a 150K 1/8W resistor from +60 to -60, be it small but affective. Think Roughly that the difference of 90 degrees F to 60 deg. F made like a 20mV difference. I noticed that when I turned my AC on after not being home for a while the offset changed. I verified this with a soldering iron.
This circuit is designed toward driving lower impendence with frequencies less than about 7KHz with mixed reactance. This is where most of the power in the full range audio spectrum is anyway and I will use smaller different amp for the highs above this range obviously with different speakers.
The Vbe of the outputs is lower than the drop across a 1A diode, so bias is limited but is controllable with the darlington servo. Vbe output bias seems very stable after cranking it for a while.
TIP141/146 are cheap and I have a lot of them, in case anyone wonders why I used them…they are cheap and generally seem to perform that way.

1845/992 ----- FJV1845E/FJV992E SOT 23
5550/5401 ----- MMBT5550/MMBT5401 SOT 23
BC846/856C ----- SOT 23

NTE 54/55 is the fasted driver I happen to have so as to help counter the affects of the slight nonlinear but larger Hfe of these outputs even up passed 10A! Obviously triple darlington circuit isn’t the fasted with GNF but I didn’t design for speedy frequencies.

What would be the obvious pro’s and con’s compared to your experiences?

apologize for the hand drawn circuit but I don’t have the luxury of all the fancy simulation software.
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Old 4th August 2005, 04:30 AM   #2
jaycee is offline jaycee  United Kingdom
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LTSpice is a free simulation package you can download from: http://www.linear.com/company/software.jsp
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Old 4th August 2005, 07:46 AM   #3
ilimzn is offline ilimzn  Croatia
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what is the point of having an input transformer if both sides of it are strapped to ground?
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Old 4th August 2005, 06:52 PM   #4
CBS240 is offline CBS240  United States
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Good point!

I originally used it for DC stability when I was building the circuit, and the input can be completely isolated but I think I will try a capacitor input and see if I get as good of stability and no drift.

Isolation don't seem necessary, but a transformer can cause phase differences at lower frequencies.
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Old 4th August 2005, 06:57 PM   #5
CBS240 is offline CBS240  United States
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BTW

I am in the process of buying a function generator and analog O-scope. After shoping around I am thinking about a 40MHz from BK instruments. It's about $800. Is there any better deals out there in your opinions? (not having to pay duty is preferred)
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Old 4th August 2005, 07:03 PM   #6
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I would look around for a good used or rebuilt O scope. They would be a much better value for your money. Try to go with a major name brand..Tektronics or HP or such. While you might find the 40 MHz one good for starters(I've never used BK's scopes), If you can find a good 100 MHz one used, it'd be allot more useful IMHO, but YMMV.

Tall Shadow
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Old 9th August 2005, 06:55 AM   #7
CBS240 is offline CBS240  United States
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I have made some changes that greatly increased the quality and the gain!

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