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Old 3rd February 2010, 06:25 PM   #11
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JLH steering output transistor base current, ignoring output voltage and current.
(except in the GNF loop, after the paralleling). Seems a fairly compatible topology
for parallel abuse. Certainly with the 74LS04, as you got 120 ohms extra insurance
built into each output totem.

Could piggyback a dozen or more if you could figure a way to get the heat out...
PNP Collector pulldown must be proportionally smaller the more gates you drive.

Sim a dumbed down inverter logic gate in LTSpice is not the same, and it does
not work in this application. You need the full internal schematic to understand
why the input has to be pulled down.
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Old 4th February 2010, 07:04 PM   #12
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I got the board done only to find that I'd forgotten to reverse the image !
Luckily it can still be used as the TO92 leads can be made to fit the reversed holes. The board is now fully populated. Hope I get some time tomorrow to try it out. Not a very nice layout but functional.

Will accept 12-0-12 transformer or 0-12 transformer or +12 Vdc from a wall wart.
Time to crash out today.It's 12.30 am!

Will post details after the listening test.....to determine if it's worth posting the details at all.
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Old 4th February 2010, 09:58 PM   #13
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Ashok, are you trying to make a .pdf from Eagle? If so, PM me and I can assist if you are still interested,
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Old 5th February 2010, 08:09 AM   #14
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Default Initial test result.

This was just a hasty test. I don't want to make any sweeping statements that I might have to retract again.
Initial impression is that it sounds very good. Without a level matched comparison with another headphone amp using an opamp front end with TO92+TO126 output stage running in classA mode, I think it sounds better. But I should make a level matched test to be really sure..........maybe , but the sonic difference is quite large right now. Wonder if level matching will negate that. It sounds much cleaner I think.

Now to put it under the scanner to see what it is really doing. This might take some time.
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Old 5th February 2010, 09:01 AM   #15
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Default Circuit diagram and some notes.

The circuit diagram is attached.
I used a 12-0-12 Volt power transformer feeding two FR107 diodes to a 2200uF cap. This is connected via a 10 ohm resistor to another 2200uF cap. This is the supply to a capacitance multiplier made up of a BD139 with 470 ohm resistor from collector to base. 470uF cap from base to ground and 100uF cap from emitter to ground.
I don't hear any hum or hiss from the headphone. Must try this at night when the ambient noise is very low. The LED on board is a red 5mm diffused type and isn't very bright. The heat sink I've used is about 30x30x1.6 mm. Both output transistors are mounted on it using mica insulators.
The heat sink after one hour is about 39 deg C and the ambient temperature is 25 deg C.
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File Type: gif JLH- tested board.gif (9.2 KB, 2477 views)
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Old 5th February 2010, 12:25 PM   #16
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Default The board

Here is a picture of the board.
Not a clear shot.

When it's very quiet I can hear faint hum in the can's ! So a better supply regulator is required. I will of course rip up the grounding scheme of the board to deterimine if that is the cause. So more work ahead . A second listening session shows up the other headphone amp as sounding muffled in comparison. Is this one boosting HF ? Will have to run the tests now.
Note the cheapo yellow box 1uF input cap and standard Samwha elco's. Still sounds very good. No sloppiness in the bass. Nice and tight / deep bass and very good HF . On phones it has shimmer on the cymbals ( on some tracks)! Wonder how it would sound as a pre amp. That's for later!
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Last edited by ashok; 5th February 2010 at 12:36 PM.
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Old 8th February 2010, 06:02 PM   #17
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Default On the test bench.

Nothing much to say about frequency response. It's flat out to 20 Khz. About -0.04dB at 20 Khz . At 20 Hz it's down about -1.3 dB with a 33 ohm load and less than -0.2 dB with a load higher than 100 ohms.

Distortion spectrum is great with the 2nd harmonic prominent and rapidly decreasing higher harmonics. Tube like I'd say !
The max output is just about 2.8 V rms with a 12 V supply. Enough for most listening as it's quite loud.

Hum as expected is very high . Only about -65dB down and getting poorer as the load impedance decreases. Needs more work on the power supply. A simple capacitance multiplier is not good enough.
However the noise floor looks quite low , especially at HF and overall looks good. The low quiesent current isn't enough for a 32 ohm load for full output ( about 4 V peak ). That's expected. With a 0.135mA quiescent current the amp clips at just about +4V peak and the heat sink is running at about 45 degC when the ambient is 24 deg C . Distortion with a 33 ohm load also drops dramatically over the operating range with a higher quiescent current.

Will work on the power supply and see what I can do. I think the hum component ( 100Hz and higher harmonics ) should be brought down by 30dB or at least 20dB. Bigger caps yes, but then it will not bring it down fast enough. Maybe a TO220 regulator. I wanted to avoid using one ! Will try something tomorrow. I should also test it at higher supply voltages.
Will post some graphs later.

It's turning out to be a great headphone amp.
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Old 8th February 2010, 07:03 PM   #18
Mooly is offline Mooly  United Kingdom
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Interesting reading your progress
I thing using a 7812 is the way to go, and make sure the grounding is correct... proper star point taken as a spur from the negative end of the main smoothing cap. Even connecting grounds tightly together isn't good enough... it has to be on a spur to eliminate the AC ripple that flows in the PSU.
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Old 9th February 2010, 03:30 AM   #19
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Hi Mooly,
Yes the unit is star grounded. The signal paths and supply ground approach the ground node from opposite sides. But then again there could be other mistakes and I will be examining them today. The simulated capacitance multiplier has far lower ripple in it's output than the practical measured values. Must figure out what's wrong. It's wrong by about 60dB ! Maybe the simulation is wrong !
Cheers.
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Old 9th February 2010, 07:42 AM   #20
Mooly is offline Mooly  United Kingdom
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And is the star point on a spur off from the main reservoir caps ?
If it not it doesn't work as you can never get down to a single point free of AC ripple. It's so important
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