Hafler SE 120 Schmatic Explanation

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I have a working Hafler Se 120. I have attached a schematic and I am trying to figure out how the two transistors circled work together. Also the circled 5.1 ohm resistor does get very warm and I am trying to figure the voltage drop and current through it. This 5.1 ohm resistor is 5 watts not 2 watts as listed. This is more curiosity than anything. Thanks to anyone that can help. Mark
 

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That resistor should be 5.1K Ohms NOT 5.1 Ohms. If you measure the voltage across that resistor then divide the voltage by the Ohms you'll have the current thru it.

Q6 and Q7 make a cascoded VAS, Voltage Amplifier Stage, the final voltage amplification stage of the amplifier, the following stages are current amplifier stages. The signal from the collector of Q1 drives the base of Q6, the collector of Q6 then drives the emitter of Q7 and the final amplified voltage for the driver stage is taken from the collector of Q7. The base of Q7 is held constant by the circuit connected to it. Clear as mud now right!

Craig
 
Thanks for the explanation. I am always trying to learn what happens behind the scenes. I am a pure hobbyist, no real formal training. I know the basics and try and learn from the DIY family. This amp sounds great. I know what dual differential inputs and currents mirrors are, but never saw this configuration. with that said I have never scene this input either. Mark
 
llwhtt. Okay I measured the voltage and they both measure 103 volts. That gives about .02 amps or exactly 2 watts through the 5 watt, 5.1 ohm resistor. Of course this now brings another question. The amp only as 54 positive and 54 negative volt rails. The diodes drop .7 volts but how can plus 54V and minus 54V equal 103 volts. The voltage drop be only one rail minus the diode drops...right???
 
The 10 Ohm resistor and the string of four diodes will drop approx. 2.6 - 3 volts plus the 103 so there's 106 volts. On the -rail you have a string of three diodes dropping another 1.8 - 2.1 volts, not sure what Q27 will drop. Current will flow from the negative rail thru the diodes, transistor, and resistors to the + rail. Another thing is that the rails will only be +/- 54VDC as long as the voltage from the wall is 120VAC. Did you measure the rails when you measured the 5.1K resistor?

Craig
 
Craig, sorry to say I did not measure the rails. Our power in Michigan fluctuates between 114 and 122 VAC depending on the time of year. I ran close to the same calculation, but doubted my numbers as I keep thinking this is a perfect system. I am trying to learn (tolerate) that these calculations are not always perfect or exactly what the schematic voltages say, like the perfect 54 volts that is unregulated. With all this said I also measured the the left right current and it measures a rock steady .098 amps against a specification of .100 amps. Lastly, thanks for all your help. I have only a few basic books and am trying to educate myself as I move to retirement. And is all for fun. Mark
 

PRR

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little off topic ...How it is possible to lounch tex SaturnV to the moon 50years ago and still have electric network like from begining of electricity era .. :mad:

The utilities give *POWER*.

Not accuracy.

An electric motor gives a variable torque and RPM and power depending some on power voltage and frequency. If you need exactly 1,000,000.00 pounds of thrust, *you* supply a precision controller which will trim my 108V-125V to *exactly* however much the job needs.

Accuracy in terms of thrust traditionally comes from weighing scales. You set the rocket motor down horizontally, turn a weight-scale up to measure horizontal force, and fire the rocket into the scale. (Tie a bottle-rocket to a fish-scale, you can read the grams of thrust.) Train a tiny electronic demon to watch the scale display and turn the raw power up/down to get the right power.
 
Hafler Se120

were and PRE, I never know what the "line voltage will be day to day. Today was a steady 121.7 VAC. Yesterday was the same. This past weekend it was 120 VAC.

All, one more question about the schematic I attached in the original post. What exactly is the 2n2222 doing in the circuit. I know the bias pot is setting the bias current...I just cannot figure out how it works in the circuit. Thanks. Mark
 
The bias pot, P1, controls the conduction of Q8. The more Q8 conducts the colder the bias, lower idle current and vice versa. In most BJT output amplifiers the bias xsistor is mounted on the heatsink so as to track the heat of the heatsink. The hotter the heatsink the more Q8 conducts and tries to lower the bias to avoid thermal runaway. Certain MOSFET output stages are immune to thermal runaway, not sure about this Hafler.

Craig
 

PRR

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Joined 2003
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...What exactly is the 2n2222 doing...

2N2222(A) was EXTREMELY popular at one point in e-history. We used it for "everything". It was an early Silicon device with much lower leakage than Germanium devices. It had an EIA number and second-sources, unlike the TI house-number Silicon jellybeans. Its family (includes 2N2219) was heavily promoted; my father had a gift-pack of '2219 from a sales-rep.
2N2222 – The Most Successful and Widely Used Transistor Ever Developed
It had a name so memorable that a small MOSFET was named after it (VL2222).

While other later parts had "better" specs, there were still many jobs the '2222 was totally fine for, and we all had half a box of '2222s left over from when they were new.


...I never know what the "line voltage will be day to day. Today was a steady 121.7 VAC. .... This past weekend it was 120 VAC.

So? 1.4% shift of line voltage is "nothing". 5% shift leads to about 10% shift of brightness on incandescent lamps, which is barely noticeable (much like the 1dB threshold on the ear for change of loudness). Your 1.4% shift is 5% on lamp brightness which is hard to see in A/B test, impossible to detect from one day to the next. That's plenty good for almost anything.
 

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llwht, i forgot to say this 2n2222 is not mounted on the heat sink. It is in the middle of the circuit board. This amplifier has on circuit board with both right and left channels on it. The users manual lists all parts, voltages and schematic. The voltages on the transistor pages are mixed up. I had to go item by item to correct the page. This amp sounds great. I am using it and have a Hafler DH200 also. Mark
 
I have a new question about the original post. I know p1 sets the bias of 100MA. what does p2 do and what is the measurement in volts or amps to make sure it is adjusted properly. I have a line on another one of these amps for $40 with one channel working an done channel out. I figure for the money it is worth tinkering with. My current se120 is a great amp. runs cool and sounds great.
 
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