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Old 31st January 2013, 12:08 AM   #2221
Eric is offline Eric  United States
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Hi Graeme, Thanks for the link. I remember reading through that before - perhaps it makes a little more sense to me after letting some time go by... I have 3 amps that I want to convert to JFET inputs and I have three matched quads of 2SK170's from Spencer.

So now I have a bunch of detail oriented questions:

1) You used four 2SJ109's in conjunction with four ZTX550's. This is a little confusing to me, since it looks like others have just used four matched 2SK170's (directly replacing the 109's in your diagram). Are the 550's necessary if I'm using the 170's?

2) I can't seem to find a definitive diagram for N-channel JFETs. In the schematic that you linked, is it the Source or the Drain that is opposite of the Gate? Some sources I've found indicates that at low signal levels, the Source and Drain are interchangeable... I guess more directly, is it the Drain or the Source of the 109's that is connected to the Drain of the 9610 above?

3) You seem to have made a number of other changes such as the input network as well as various resistor values (R16, R46), R22 was omitted, and you've added a diode to ground at the 550's. I don't understand the significant and purpose of these changes. Are they necessary if I use 170's?

4) I presume the 109's on the "left" side of the diagram need to be thermally coupled with one another, as do the pair on the "right" side of the circuit. Is this correct? Is the same true for the 550's on each side? I have some of the dual JFET heat sinks that were sold here at DIYAudio some time ago...

I'm sure I'll have other questions shortly.

Thanks,
Eric
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Last edited by Eric; 31st January 2013 at 12:30 AM.
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Old 31st January 2013, 02:44 AM   #2222
gl is offline gl  United States
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Quote:
Originally Posted by Eric View Post
Hi Graeme, Thanks for the link. I remember reading through that before - perhaps it makes a little more sense to me after letting some time go by... I have 3 amps that I want to convert to JFET inputs and I have three matched quads of 2SK170's from Spencer.

So now I have a bunch of detail oriented questions:

1) You used four 2SJ109's in conjunction with four ZTX550's. This is a little confusing to me, since it looks like others have just used four matched 2SK170's (directly replacing the 109's in your diagram). Are the 550's necessary if I'm using the 170's?

2) I can't seem to find a definitive diagram for N-channel JFETs. In the schematic that you linked, is it the Source or the Drain that is opposite of the Gate? Some sources I've found indicates that at low signal levels, the Source and Drain are interchangeable... I guess more directly, is it the Drain or the Source of the 109's that is connected to the Drain of the 9610 above?

3) You seem to have made a number of other changes such as the input network as well as various resistor values (R16, R46), R22 was omitted, and you've added a diode to ground at the 550's. I don't understand the significant and purpose of these changes. Are they necessary if I use 170's?

4) I presume the 109's on the "left" side of the diagram need to be thermally coupled with one another, as do the pair on the "right" side of the circuit. Is this correct? Is the same true for the 550's on each side? I have some of the dual JFET heat sinks that were sold here at DIYAudio some time ago...

I'm sure I'll have other questions shortly.

Thanks,
Eric
OK, I'll do my best and answer your questions in order. I will be talking about one channel here when I make circuit references.

1) The 2SJ109BL is dual transistor part. It's the equivalent of two 2SJ74BL's. I created the diff pair (I'm talking one channel) by putting one transistor from each 2SJ109 on each side of the diff pair. I hope that makes it clear. Please note that I didn't match the two 2SJ109BL's (4 transistors) to each other. I just took the next 2SJ109BL from the bag as I was assembling the PCB's.

Please note here too that the 2SJ109 and 2SJ74 are P channel parts, just as the 9610 is a P channel MOSFET. That is what you must use. The 2SK170 is an N channel part and won't work here.

The cascode transistors aren't absolutely necessary. I used them for several reasons: 1) I believe that 8V-10V is a sweet spot for the J74/K170 parts, 2) thermal dissipation on the JFET's is kept down to almost nothing so possible drift is reduced/eliminated and thermal coupling is not necessary, 3) I wanted to avoid JFET source degeneration resistors in order to keep the gain up but I needed a way to avoid current hogging, and 4) the maximum Vds for the 2SJ74 is 25V on the data sheet and the AX100J uses 27V rails. I have since found out from NP that these parts are actually OK to higher voltages. But I didn't know that at the time. And even then these were rare and valuable parts.

2) The P channel JFET symbol I used shows the gate close to the source. The sources of the JFET's are connected to the drain of the 9610. The source and drain of the 2SJ109/2SJ74 can be interchanged but I always connect them as shown on the data sheet.

3) I will need to get back to you about the specific resistor designators you reference. I need to check a couple of things on that. The resistor and zener that are connected to the ZTX550's form the voltage reference for these cascode transistors.

4) I didn't thermally couple anything. The cascodes keep the dissipation of each JFET down to 40 milliwatts (P = 8V * 5ma), so IMO all the JFET's are staying at room temp. So no need to thermally couple. I am seeing no drift so I think I'm good. A lot of people here are big believers in thermal coupling. If you want to do it then go ahead. Bottom line it won't hurt and it may help. If it does improve things then definitely let me know.

If I've been unclear let me know.

Graeme

Last edited by gl; 31st January 2013 at 03:04 AM.
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Old 31st January 2013, 02:50 AM   #2223
Eric is offline Eric  United States
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Another question: if I use a quad of sk170's to replace the 9610 differential, what Ids should I aim for with matching? Does this matter, given that I can adjust the bias with R23/R25?

The matched sets that I have are matched at about 8.5mA. Will these work, or should I get matches as a different Ids?
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Old 31st January 2013, 03:13 AM   #2224
gl is offline gl  United States
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Hi Eric,

You can't use 2SK170's. They are N channel parts and are the wrong polarity. You must use P channel parts. That means 2SJ74BL's or 2SJ109BL's.

I substituted two 2SJ74BL parts (2SJ109BL - same specs) for each 9610. So if the 9610 is running 10ma of bias then each JFET will have 5ma of bias. So you need to pick JFET with an Idss higher than that. I'd pick an Idss of 7ma or more. Almost any BL part would work here. The Idss match should be within 10%.

I did what I did to avoid having to change the bias in the diff pair.

Graeme
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Old 31st January 2013, 03:18 AM   #2225
Eric is offline Eric  United States
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Hi Graeme,

Thank you for your detailed responses! I didn't see your reply before I made my second post, so please don't think I'm not appreciative of your help!

Thanks for pointing out my error on the 170's vs the 74's - that could have caused a problem...
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Old 31st January 2013, 04:17 AM   #2226
gl is offline gl  United States
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Hi Eric,

OK I've reviewed the schematics. So here are some comments about those resistor value changes.

R22 is easy. JFET's don't need the 220 ohm gate stopper resistors - at least not when there's just one (or a couple in parallel). Note too that the input protection diodes were deleted as well - not needed with JFET's.

R16 and R46 (and a few others in the AX100J) are values that came from schematics of the pass Labs X5 and X2 circuits that NP posted briefly here 'once-upon-a-time'. These are/were JFET input supersymmetric circuits. Some of the other value changes were calculated by me as a result of updating R16 and R46 etc.

Graeme
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Old 31st January 2013, 02:21 PM   #2227
Eric is offline Eric  United States
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Thank you for the continuing details, Graeme! This is very helpful! I'm sure I'll have more questions as I think about this more carefully. Time to start drawing things out to see how best to implement these changes. My first thought is to add a small secondary perf-board to hold the new transistors...
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Old 31st January 2013, 03:44 PM   #2228
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Hi Eric,

you could also have a look here:

AlephJ-X

Depending on the amount of input fets (2 or 4) you will need to change the drain resistors from 392R to something like 910R to account for the reduced bias.

You will also need to change the current source resistors to be able to set a lower bias if you go from 20mA to 10mA

And last but not least you need to change the McMillan resistors (start with the double value you have now)

I don't think I changed anything else. Coupled the input JFets to each other and used a 10R pot as a replacement for source resistors and to set the relative dc offset.

William
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Old 31st January 2013, 05:47 PM   #2229
Eric is offline Eric  United States
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Thanks, William!

I think I'm starting to have a better understanding now. Some time ago I purchased a bunch of 2sk170's and not I see I got the wrong part. Time to try sourcing the right stuff and drawing out how to "graft" this onto my PCB...
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Old 5th February 2013, 07:13 PM   #2230
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Default Temperature compensation

Hi,

today I tried to put some PTC's in the current source for the input diff pair.

I've used KTY81-110 as suggested by Salomon (thanks again!).

Until now the current source was build around Q6a (IRF9610) with 3x2.9V zeners and the following values for the resistors:
R24 2k7
R26 680R
VR2 200R

I've replaced R24 with a KTY81 and a 1k5 or 1k2 resistor, the rest remained unchanged.
The PTC is coupled to the IRF9610 which is coupled to the top of the case.(see pics)

Well, it seems to work fine. The calculated 1k5 seems a bit high (the absolute DC get's more negative during warmup) so I've put 1k2 in the other channel which is better. I'll watch some real cold starts and do some long runs to decide which one is better or if I have to go even lower.

This setup will probably only work for my amp but with a different combination of resistors it should also be possible to make it work for IRF9610 or quod JFet inputs.

William
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