Inexpensive JFET Preamp

[HKC]

most multimeters are less accurate when set to read amps.
Some multimeters are not sensitive enough to read mA or even tens of uA.
The voltage drop across the multimeter is unknown and so the applied voltage across the DUT is also unknown.

Use a resistor and voltmeter.

Hi Andrew

Thank you for your explaination. I understand now.

 

[jonusgrumby]

HKC,

There are some good suggestions here and feel free to use what works best for you.

I think you will be OK using the milliamp setting on your DMM. It is true that the voltage drop is unknown, but it will be consistent and negligible to the measurement.

I agree that the milliamp reading can also be less accurate than the voltage reading, but in my experience with many different meters, they are consistent with the measurement. I think the DMM will work fine provided you measure all the devices in the same timeframe. You really just need repeatability here so you get closely matched devices. The absolute idss is not as important.

I allow 30 seconds for the idss reading to stabilize before I record the value. It will change slightly as the device warms up (slightly at 9 or 10 volts) due to the current flow.

 

[HKC]

HKC,

There are some good suggestions here and feel free to use what works best for you.

I think you will be OK using the milliamp setting on your DMM. It is true that the voltage drop is unknown, but it will be consistent and negligible to the measurement.

I agree that the milliamp reading can also be less accurate than the voltage reading, but in my experience with many different meters, they are consistent with the measurement. I think the DMM will work fine provided you measure all the devices in the same timeframe. You really just need repeatability here so you get closely matched devices. The absolute idss is not as important.

I allow 30 seconds for the idss reading to stabilize before I record the value. It will change slightly as the device warms up (slightly at 9 or 10 volts) due to the current flow.

Hi Tom

Yes the suggestions and advices are all valuable to me. It is a good chance to me to learn from practice. Thanks to all again.
I will keep posted on progress of this impressive pre-amp.

Best Regards

 

[mlloyd1]

for these small jfet measurements, i simultaneously run a bunch of parts on a protoboard with some precision resistors (you could also just hand-measure some resistors, too). i lucked out on some precision 100 ohm and 1K ohm resistors. i let the bunch of circuits "cook" while i do something else for some time. then measure and pick my matches. works well, no finger problems.

mlloyd1

 

[HKC]

for these small jfet measurements, i simultaneously run a bunch of parts on a protoboard with some precision resistors (you could also just hand-measure some resistors, too). i lucked out on some precision 100 ohm and 1K ohm resistors. i let the bunch of circuits "cook" while i do something else for some time. then measure and pick my matches. works well, no finger problems.

mlloyd1

Hi Mlloyd

Your method sounds very good for measuring JFets. Thank you for sharing.

Best Regards

 

[HKC]

The other option of JFet Jig use resistor instead of ammeter. Please help to check the drawing is correct or not.

Thanks

 

[AndrewT]

you need to add a method of measuring the current through each Jfet.

 

[HKC]

you need to add a method of measuring the current through each Jfet.

Hi Andrew

Yes I will use the DCV of the multi tester to measure the current through each JFet. Am I right?

 

[AndrewT]

no,
you need a drain resistor in in FET path.

 

[HKC]

Just exactly what some of us are looking for. Thank you. I've thought about the FORSSELL discrete etc and when I've done the sums I find the better quality FETs are too expensive and v.hard to get so this is good. Now, I am not up to speed on the maths of biasing etc but I was wondering is it possible to run a couple (or even 3 or 4) Fets in parallel in the input stages and gain some benefit in noise reduction in the same way that they have achieved with bipolars in the LM394 for instance? I recall them speaking of a "root2" decrease in noise for every doubling of the junctions I think.
If it is possible with Fets please can you put up an appropriately modification to your current offering for the theoretically/mathermatically challenged?
Thanks again, Jonathan.

I have built the FORSSELL discrete. I am still using it. I like the sonic quality it reproduces.
I am preparing to build Tom's Jfet pre-amp. I am in progress of measuring the idss of the 2N5459/2N5462. I ordered 100 pcs. of each from Mouser. I will keep you guys posted.

 

[KLe]

thanks HKC

 

[HKC]

Tom

I measured half quantities of the 2N5459 & 2N5462. The idss between the two JFets are very various.
2N5459 from 8 ~ 10ma whereas 2N5462 from 3.5 ~ 5.5ma. It seems I have matching problem. Any suggestions?

Best Regards

2N5459:

 

[HKC]

2N5462:

 

[HKC]

Hi Tom

I sorted some 2N5458/2N5462 JFets with idss from 5 ~ 5.5ma. I am not sure if this current range is good for both the input and output stages. The JFets are matched within your suggested tolerance. Please advice then I can go forward to finish the amp.

Best Regards

 

[lineup]

First
you write 2N5458 ... shouldnt be 2N5459 .. no?

See Attachment of amplifier scheme.
If looking at schematic of this jonasgrumsby inexpensive JFET amp,
I say, my consideration would be slightly different for Input vs. Output jfets.

==============================================

For input -
Get as high current devices as possible.
I may go for using 75-90 % of IDSS in both N and P transistor.
That is of the one with lowest, NJFET/PJFET
==============================================

For output -
Get as high current devices as possible.
That is, get both PJFET and NJFET have as high IDSS as you can do Complementary with.
For output I may run a bit lower than 75-90% of IDSS.
Say use 50-65% of IDDS.
==============================================


Hope this makes sense.
According to John Curl, our real JFET expert,
we should almost never try to STARVE our JFET.
That is use and run them at lower currents than we can.
And we should buy those with HIGHEST possible IDSS
Like 2SK170V/2SJ70V .. or at least 2SK170BL/2SJ74BL


Regards Lineup

 

[HKC]

Hi lineup

Thank you again for your prompt reply. To answer your question #1 1st. I am not going to use 2N5459 even though I have 100 pieces in hand. For the reason why not use it please refer to my post #92. Under this circumstance I have to use 2N5458 which I have 50 pieces and measured their idss can match with my 2N5462 within it's current range.

Best Regards

 

[lineup]

Yeah.
Using 2N5458 does not change much anything.
They are practically identical to 2N5459.

 

[jonusgrumby]

I was very surprised at the low idss range of the 2N5462. I have ordered many batches of those parts and never had a set come in with that low of idss, but it is within the part specification. I have many of these parts, I'll look at what I have and find ones that are good matches for your N channel devices. If you PM me I'll make arrangements to send them to you.

These devices are not starved. The operating points for the input and output stages were carefully selected for linear operation. Other devices could be used and they will need different bias points. I designed this many years ago and at that time the 2SK, 2SJ parts weren't available in small quantities and were quite expensive.

 

[lineup]

I have many of these parts,
I'll look at what I have and find ones that are good matches for your N channel devices.
If you PM me I'll make arrangements to send them to you.

These devices are not starved.

HKC will be pleased, I think.
Most kind of you, jonasgrumby.
He even might send you one of his PCB boards, in return

In schematic:
These devices are not starved

I know. I was more talking general knowledge here.
For all other beginners/readers considering JFET amplifiers.

Lineup

 

[HKC]

I was very surprised at the low idss range of the 2N5462. I have ordered many batches of those parts and never had a set come in with that low of idss, but it is within the part specification. I have many of these parts, I'll look at what I have and find ones that are good matches for your N channel devices. If you PM me I'll make arrangements to send them to you.

These devices are not starved. The operating points for the input and output stages were carefully selected for linear operation. Other devices could be used and they will need different bias points. I designed this many years ago and at that time the 2SK, 2SJ parts weren't available in small quantities and were quite expensive.

Hi Tom

Yes please send me the parts which you have extra and you think the idss is good for input stage. I will pm you my mailing address.
I will be pleased to send you two boards if you like.
lineup, this is a very good suggestion. Thanks.

Best Regards