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JLH Headphone Amp
JLH Headphone Amp
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Old 26th November 2017, 06:24 PM   #721
mod_sonic is offline mod_sonic  United Kingdom
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Default JHL Zener Diodes

Kroto,

Good idea.

I have 3 x 1N4148 in the right channel and 4 x 1N4148 in the left channel and I can't hear any difference, so maybe it really doesn't matter !

It would be interesting to know how the 'JHL' pcb designers got from John Linsley Hood's 2 unspecified diodes D1 and D2 in series to a 2.7V Zener diode.

See attachment :

jlh hpa schema.gif

Mark
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Old 27th November 2017, 07:49 AM   #722
AndrewT is offline AndrewT  Scotland
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I'm not sure that the voltage value at the bottom of the resistor is that important.
What is important is that the voltage varies less than any variation that will happen on the -ve supply rail.

The two diodes D1 & D2 act like a shunt regulator to give a near constant -ve voltage at the bottom of R1.
The current flows from Q4 emitter > Q4 base > through R2 (4k7) > through R1 (47k) > to regulated voltage ~-1.3Volts to -2.7Volts.

After Q4 warms up to operating temperature, we have a near constant quiescent current flow and you choose resistor values to minimise the input offset voltage. This current flow will vary if the output offset varies. The output offset should be pretty low for this dual polarity supply. The voltage at Q4 base will be -ve and sit >0.6V below the output voltage.
This means the voltage across R1+R2 = regulated Voltage = Base voltage.
With only two diodes, the V(r1+r2) is <0.6V
increasing to three diodes increase this to <1.2V and substituting a 2.7V Zener increases this still further to <2.1V
You can see that the current variation for the 0.6V diff will be much larger than for a 1.2Vdiff or 2.1V diff.
It could be that output offset variation improves as the regulated voltage is increased.
Diodes, or LEDs, or Zeners could all be used.
Worth experimenting to see if any combination is better or worse than others.
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Last edited by AndrewT; 27th November 2017 at 08:05 AM.
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Old 30th November 2017, 12:03 PM   #723
mod_sonic is offline mod_sonic  United Kingdom
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Default JHL Diodes

Andrew,

Thanks for the detailed response. If I understand you correctly the diodes are there to reduce the variation in the voltage to Q4, and that the voltage variation will reduce as you increase the voltage drop across the diodes i.e. more diodes = more voltage drop = less current variation to Q4 and potentially less (DC ?) output offset. Have I understood you correctly ?

If this is correct then I can presumably measure the effect of adding more/different diodes on the output offset by checking the DC voltage between the output and the star earth. However, how/where would I measure the effect of adding more/different diodes on the variation in voltage using a multimeter ?

Regards

Mark
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Old 1st December 2017, 05:09 PM   #724
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by mod_sonic View Post
Andrew,

Thanks for the detailed response. If I understand you correctly the diodes are there to reduce the variation in the voltage to Q4, and that the voltage variation will reduce as you increase the voltage drop across the diodes i.e. more diodes = more voltage drop = less current variation to Q4 and potentially less (DC ?) output offset. Have I understood you correctly ?
Yes, I am fairly sure you will find this to be true.
Quote:

If this is correct then I can presumably measure the effect of adding more/different diodes on the output offset by checking the DC voltage between the output and the star earth. However, how/where would I measure the effect of adding more/different diodes on the variation in voltage using a multimeter ?

Regards

Mark
You can measure output offset at the output when no speaker is attached and when the input is shorted, or fitted with a dummy load to mimic your source.
You can measure the variation in output offset as the amp warms up. You can also warm up the heatsink artificially using a hair drier or similar.
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Old 11th December 2017, 08:16 PM   #725
Buzzy is offline Buzzy  Singapore
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I tried simulating the JLH circuit 1 in LTspice. This is what I got for the input and output signal.

JLH Headphone Amp


Why does it look so weird ? The output signal is not centered at 0V line but is centered at 0.8V. I am still learning to use LTspice which means I must have done something wrong. Anyone knows what I did wrong making the signal output look weird ?

This is what I simulated.
JLH Headphone Amp
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Old 12th December 2017, 12:00 AM   #726
roclite is offline roclite  United Kingdom
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JLH Headphone Amp
The LTspice circuit looks OK and the plots look pretty good, apart from the output not being centered around 0V. I am not familiar with the JLH circuit 1 but did build the "JHL" clone amp based on the same design, which was further modified as per my posts much earlier in this thread. The problem is the output DC bias operating point is not set properly. For a quick and easy resolution, just try 2 simple things in simulation to see if things can be improved:

1) Remove or reduce R3 (prevents Q1/Q3/Q7/Q9 from biasing properly)
2) Add extra diodes to D3,D4 to see the effect on the output DC operating point

Finally, simulate a potentiometer voltage divider (two resistors) fed from the diodes reference to R6 to see if you can get close to 0V DC output. You will need to implement this component if you want to adjust the output DC operating point of a real circuit.
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Old 12th December 2017, 07:34 AM   #727
AndrewT is offline AndrewT  Scotland
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I think your simulator is telling you that the AC output has a DC offset added on.
Try reducing your input signal in steps of -20dB (1/10th) and see what happens to the prediction as you approach an output of 0.1mVac.
Roclite has given some clues to where an offset adjust could be fitted.
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Old 12th December 2017, 11:07 AM   #728
EUVL is offline EUVL  Europe
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How long do you think it will take C6 to reach steady state, in 10ms ?


Patrick
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Old 14th December 2017, 05:11 AM   #729
Buzzy is offline Buzzy  Singapore
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Does using other output transistor other than BD139 change the sound ? Will transistors like MJE15034 or MJE243 from ON Semi make it sound worse ? Anyone tried other transistors other than the BD139 ? Can share your experience pls ?

Last edited by Buzzy; 14th December 2017 at 05:33 AM.
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Old 14th December 2017, 05:47 AM   #730
Buzzy is offline Buzzy  Singapore
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I found an old post searching this thread that says that the MJE15030 sounds better than BD139. So I think that answers my own question on whether diff output transistor changes the sound.
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