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Old 20th August 2013, 02:11 PM   #11
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Make it simple and good but not too simple and less good I think you will conclude that if you want to keep the device small with necessary bells and whistles SMD parts are unavoidable (I know, I hate them too but use them often).

BTW output GND reference resistor is for charging the output cap when no load is connected. You avoid large plops when connecting the device to power amps.
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Old 21st August 2013, 09:41 AM   #12
Calvin is offline Calvin  Germany
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Hi,

the circuit in #1 is singleended class-A.
The maximum output current into the load is limited here by the idle current.
THD-wise the load current must stay well below the idle current.
With 100R current setting resistor value the idle current settles at ~6.5mA.
So the load shouldn´t be lower than app. 10kOhm to keep the load currents reasonably low.
Higher idle current could be something to wish for, but with +-12V power rails the heat power losses in the JFET and the CCS-bipolar reach approximately ~250mW each.
This is manageable with throughhole components but too much for SOT23/SOT363 SMDs.
If output voltages of 2.5Vrms suffice, then the suply rails may be lowered to +-5V.
In that case SMDs may handle even 10mA idle current well.
Suggested parts for the JFET could be BF862, LSK170C and MMBF/SST4393 in SOT23.
The BF862 beeing the lowest noise device, closely followed by the LSK170 and the 4391. As soon as Input filters are added those dominate the noise figure.
THD-wise the following is quite opposite. The 4391 simulating up to 20dB better than the BF862.
Idle current set to ~9.3mA with a 68R resistor, resulting in ~45mW heat power losses in JFET and CCS-bipolar, which the SMDs can handle.
For the CCS I suggest a Dual-NPN like the BC847BS (Diodes/NXP/MCC) or equivalents in SOT363.
Apart from DC-blocking caps and power supply caps, the circuit would be extremely tiny.
Attached is a dimensioned circuit with sim-results.
Just a single word ... cute

jauu
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Old 21st August 2013, 11:20 AM   #13
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Nice !
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Old 21st August 2013, 01:53 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Calvin View Post
..........................
THD-wise the following is quite opposite. The 4391 simulating up to 20dB better than the BF862.....................
How can you eliminate the model accuracies from the reported THD results?
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Old 22nd August 2013, 03:02 AM   #15
Bonsai is offline Bonsai  Taiwan
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Here are some more ideas, incorporating bootstraping etc. I am just grabbing models from the LTspice library - need to try these out with some of the Linear Integrated Systems models as well.

50 Hz PSRR for all of the circuits starts at about -90 dB (middle circuit) with the LHS circuit the best at -110 dB and just gets better from there on. I'll need to do a little investigation as to why the middle circuit PSRR is lower.

I tried the middle circuit using a 2N7002 - it gave not too bad results, although the distortion was a bit higher (10ppm at 2 V out) than the JFET input versions. I would expect though that a mosfet will be noisy in practice.

The middle circuit gives a lot of scope for experimentation with the input JFET in my view, while the performance of the outer two are very good for such simple designs. However, I think you will need to choose the JFETs carefully, making sure in the RHS one that Idss of the botton current source (just shown as a current source) is below the worst case signal current. You can also accomplish that with a resistor in the source of the current source JFET.

Note, the acid test for these circuits (I have not tried it yet - that's the next job) is to test the distortion performance with an input resistance of 5k - this is the worst case situation when you have your volume pot (I am assuming 20k) set to the mid-point resistance position and the effects of the JFET input capacitance make themselves felt. For this test, you can expect low input capacitance JFETS (like the LSK489 for example) and bootstapping the drain to minimize the problem.

I have avoided a JFET+PNP CFP approach - but that is another option to explore.

Finally, I dont think the models will be entirely correct, but they do allow us to get some idea of where to tweak to improve performance - I would not put as much faith in the results as I do with bipolars - but I guess thats half the fun with JFETs.

BTW - there's a good article in the latest EDN by Bob Cordell on the new LSK489 low noise, low input capacitance JFET: http://www.edn.com/design/analog/441...plication-Note



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Old 22nd August 2013, 03:33 AM   #16
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by Calvin View Post
Hi,

the circuit in #1 is singleended class-A.
The maximum output current into the load is limited here by the idle current.
THD-wise the load current must stay well below the idle current.
With 100R current setting resistor value the idle current settles at ~6.5mA.
So the load shouldn´t be lower than app. 10kOhm to keep the load currents reasonably low.
Higher idle current could be something to wish for, but with +-12V power rails the heat power losses in the JFET and the CCS-bipolar reach approximately ~250mW each.
This is manageable with throughhole components but too much for SOT23/SOT363 SMDs.
If output voltages of 2.5Vrms suffice, then the suply rails may be lowered to +-5V.
In that case SMDs may handle even 10mA idle current well.
Suggested parts for the JFET could be BF862, LSK170C and MMBF/SST4393 in SOT23.
The BF862 beeing the lowest noise device, closely followed by the LSK170 and the 4391. As soon as Input filters are added those dominate the noise figure.
THD-wise the following is quite opposite. The 4391 simulating up to 20dB better than the BF862.
Idle current set to ~9.3mA with a 68R resistor, resulting in ~45mW heat power losses in JFET and CCS-bipolar, which the SMDs can handle.
For the CCS I suggest a Dual-NPN like the BC847BS (Diodes/NXP/MCC) or equivalents in SOT363.
Apart from DC-blocking caps and power supply caps, the circuit would be extremely tiny.
Attached is a dimensioned circuit with sim-results.
Just a single word ... cute

jauu
Calvin
Hello Calvin,

I'd put you input filter before the pot - otherwise the -3 dB cutoff is dominated by the pot position.

Since the power amp already has a low pass filter, you probably want to set the -3 dB very high on the buffer - I used 10 Ohm and 47pF so its up in the MHz range. RFI on JFETs is better than undegenerated bips so you should not have a problem in a practical circuit.
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Old 22nd August 2013, 04:37 AM   #17
Bonsai is offline Bonsai  Taiwan
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My post should read '100 Ohm and 47pF'
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Old 22nd August 2013, 07:09 AM   #18
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Quote:
Originally Posted by Bonsai View Post


BTW - there's a good article in the latest EDN by Bob Cordell on the new LSK489 low noise, low input capacitance JFET: Product How-to: LSK489 Application Note | EDN



I just read the article, pretending to understand all of it, and obviously this is great candidate for this circuit. If we want a simple and practical circuit it will surely benefit from the use of such high quality parts. But I do not see LSK389 listed anywhere, let alone LSK489. Would it be possible to find source for this part? Also, since I was unable to find DS for these parts I am in doubt if this is SMD part? This is new part and probably the price is high.

This part is very desirable for our "practical" circuit because there are two j-fets in the same case which allows a small footprint.

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Old 22nd August 2013, 07:33 AM   #19
Bonsai is offline Bonsai  Taiwan
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I have the same concerns. I designed a JFET input opamp (still to build and test) and used the LSK389 (I have 2 in my draw to-72 package!) or LSK489. They seem difficult to get and not at mainstream distributors. Seems to me Linear Systems need to start a web shop and take paypal or debit card - they would do well off the DIY community (Yes, I am a marketing guy . . )

The middle circuit seems for me to be the most flexible, as is Calvins - You can use many differenrt JFETs and can tailor the source CCS per the buffer JFET requirements.

re SMD - I have no problem soldering these down and actually enjoy it - but will the other builders want to use SMD? Maybe 2 layouts - SMD and TH will be needed.
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Old 22nd August 2013, 07:42 AM   #20
Zoran is offline Zoran  Serbia
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nice but maybe too manu condensers on the path of the signal
and "big" one 10uF...
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