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Old 2nd July 2002, 05:33 PM   #1
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Default FINALLY I modified it

Hi all,

Some of you must remember me of one of my first threads about a year ago(can anyone help me with this amp?), it's about my first try to make an amplifier but unfornately it blew up several times, so I asked you to help me, and you did. But I stopped the project then because I was too impatient. I decided to start reading about how amplifiers work and how to design them. And I learned a lot of this site since then.
Yesterday, I took back my first (toasted) amp-pcb and I decided to redesign the schematic to delete the mistakes and to make it stable.
I just finished my first version, it ran smoothly in simulation, and I would like your comments on it.

NOTE:
*) I'll use in the future an extra pair of transistors (MJ15003/15004) after the TIP142/147
*)the diodes must be mounted on the heatsink and not in the excisting holes in the pcb
*)the same pcb can be used
*) maybe I'll change the value of the emitter resistors to adjust the quiescent current
*)I'll recommend a small heatsink for Q5
*)I'll use a potmeter of 2.2k for R7
*)all resistors 1/4W, exceptions:
R1 & R2 => 5W
R6 & R8 => 1W
R9 => 2W

HB.
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Old 2nd July 2002, 05:38 PM   #2
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here's the original one
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Old 2nd July 2002, 05:49 PM   #3
Jean is offline Jean  United States
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Sorry, I won't be of any help but could you tell me what Circuit simulation software you use ? I am really interested in getting something like that and start simulating circuits, but MicroCap is way too expensive for me.

thanks
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Old 2nd July 2002, 05:55 PM   #4
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Quote:
Sorry
he, no excuse we're here to help everyone

I use circuitmaker, student version
you can download it freely at www.circuitmaker.com

also check this: http://www.terrypin.dial.pipex.com/ECADList.html

best regards,
HB.
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Old 2nd July 2002, 06:21 PM   #5
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I've changed all the 300pF capacitors into 100 pF caps, otherwise there will be distortion at higher audio frequencies.
And I tested what C6 in the original circuit does to the signal - I've never seen that big distortion, so I left it away.

best regards,
HB.
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Old 2nd July 2002, 08:06 PM   #6
e96mlo is offline e96mlo  Sweden
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I'm curious about D5-D8 in the original circuit. What are they doing? Wouldn't they cause non linearities when they start to conduct?

In the new circuit I'm wondering about C6 and C7. Are they improving anything? Also you could bypass the bias diodes with a 100 uF cap and see if that helps up the higher frequencies.

/Marcus
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Old 2nd July 2002, 08:26 PM   #7
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D5-8 is a rectifier bridge wired to bypass the 1 ohm emitter
resistors. Its purpose is to provide a low impedance path to
the load when the voltage across the 1 ohm resistor exceeds
about .6 volts. In this way, you can ballpark a bias voltage
across the resistor at less than .6 volts and not have the
bias current wander too much because 1 ohm is quite a
bit of degeneration.

Done properly, it's a viable technique, but it can be done
improperly as well.

I used it on the Threshold 800A, 400, and 4000, and later
I saw it on the "Barney Oliver Amplifier" which was an
n-house employee project at Hewlett Packard. You may
recall that Barney Oliver was a head of development at HP,
and was later deeply involved in SETI.
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Old 2nd July 2002, 08:40 PM   #8
e96mlo is offline e96mlo  Sweden
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But doesn't it require the diodes to be very linear directly after the threshold voltage (and further on in the graph of course)?

/Marcus
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Old 2nd July 2002, 10:39 PM   #9
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If you bias them right up short of conduction, it works
better, and if you minimize the resistance, it works better.

After that: feedback.
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Old 3rd July 2002, 11:05 AM   #10
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Yes, I was thinking about placing a cap over the bias diodes but a lot higher value than in the original circuit (470nF was too small), I think I'm going to try some different values and pick out the best.
I've made some changes to the circuit, there the original ran into oscillation of higher frequencies when no input was applied:
1) I've placed some 100nF capacitors over the transistors, and one at the input
2)I've lowered the input-impedance, and also the gain-factor by making the values of R5 and R1 equal to eachother (they must!!), and changed them to 10kohm.
3)I've placed 10 ohm resistors in the emitters of the diff.amp transistors, to handle differences of the two transistors.
4)I've changed the zobel network to the mostly used values
5)the diodes must be placed in better thermal contact with the output transistors, they are placed too far away on the PCB
6)I'm also going to use MJ15003/15004 and a bigger heatsink with a low noise fan on it.
7)I left the D5-D8 diodes away, the drills in the pcb I'll use later on to detect the voltage over the emitter resistor (for protectioncircuit, clipping-level detector, ...). I had to change the value of these resistors, and they must be 5W instead of 1W.
8)I'll use a trimpot for R6 (or R7 in the modification) there I remember there was once a DC-offset when I had changed that transistor

so what do you think about it?

HB.
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