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International equivilents

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(@greywolf)
Posts: 1
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Hey Mate,

        Thanks for your incredible projects. I look forward to each one that comes out. I am currently gathering parts for your Microphone project. Going to use it on a parabolic dish for long distance recordings. Just thought I would let you know in you don't already. Some of your components are really difficult if not impossible to source in some countries and the markup and postage for import is huge. Also the wait times are awful,

The 2n4416 for example, you can get it OK from China, for $4 or $5 but the wait times are horrendous. I went to order a couple and estimated delivery was up to 31st Jan 2023. UK sites are 14 pounds and 20 pounds delivery. The US, one site was going to cost $55 US and still Jan 2023 delivery. I had the same problem with the THAT1512, ended up being about $70 by the time it got here which took 9 weeks.

Most of this stuff is not even available in Australia. I am still trying to find a 2n4416 or equivalent I can get in Australia without waiting over 2 months for an international delivery.

It would be great if in some of these projects you could find some equivalent components available more readily to folks not in the UK.

Some of these parts are so rare people are even posting on Reddit looking for equivalents.

Your projects are incredible, as is your presentation and explanations.

But the difficulty in sourcing stuff is getting prohibitive,

Anyway, thanks heaps, Look forward to you next project.

Ray

 
Posted : 15/11/2022 12:51 am
marcdraco
(@marcdraco)
Posts: 487
Member
 

I hope you got sorted, but here's some stuff that comes to mind.

It's an odd choice of FET but probably inspired by the original design which was as I recall for a project called Alice. The designer names his builds like that, it doesn't mean anything.

The 2n4416 has a screened can which is the lead Matt trims off. Actually, it could go to the screen if you had one but as the rest of the assembly is screened that's really overkill

A couple of things matter in finding an equivalent.

This is a 30v *self-biasing* circuit which avoids calibration of the Vgs (pinch-off) so the FET sits in its saturation region which is where it's supposed to operate. For this FET it's 

FETs are weird as they don't have easily defined properties so we need to either bias them manually with a potentiometer OR use a self-bias which many designers say is a cop-out. It's not, it's a choice.

But it's also not an audio FET - that's not to say it's not suitable for audio, just that it was designed for RF right up to 400 Mhz. Words like overkill come to mind.

A 2SK170 is what I've used in a similar circuit which is superior to this design but as is doesn't produce the differential output. For that, you'd need a bipolar stage to split the signal. Not difficult but not ideal

The 2SK170 has a very tight pinch-off of 0.2 to 1.5 v compared to the 2N4416 which is -2.5 to -6 volts. The circuit I designed runs down to 6V and will probably go a lot lower, perhaps 2V which is great if you're stuck with batteries.

My usual jellybean solution is the 2N3819 which is pretty easy to source but it won't handle the 30-volt supply so that's out of the running as is the J109 for the same reasons.

Given the design requirements, I would guess the 2SK170 (and its equivalents) should be OK as the self-bias will look after the biasing and it can work at those voltages/currents. The 2SK545 is a closer match and can handle the higher voltage but I don't know what supplies are like down under. The 2N4856 looks like it might work too but I have to stress, I have NOT tried any of these.

It depends on how much help you need with this circuit - you'll see the design I cribbed from Doug Ford (formerly of Rode Australia) under the official topic for this mic and the conversations over the necessity for such complications over a few CM of wire. The complete "Schopes" circuit does provide the balanced output for something like a THAT1512 (also serious overkill, sorry Matt) but it needs a lot more components to do it. I haven't provided that schematic but there are loads of examples on the web. 

Hopefully, Matt and I will be able to improve this over the coming months for V2 which uses more easily replaceable or sourceable parts. Matt's already found a cracking, modern JFET but as far as I know, it's only available in SOT23 which is surface mount.

The nasty little NEEWER I'm using for a donor mic uses a 2SK596 but again, it can't tolerate the 30V. But the point of the K596 and the K170 is that they are designed specifically for magnetic cartridge heads and (drum roll) microphone impedance matching.

If you care to jump back in here, I'd be happy to help design an improved version of this which should be more cost-effective and still provide excellent low-noise performance. I think we can do away with the THAT1521 altogether and use a lower-cost (ultra-low voltage) op-amp to drive the ADC that runs from just 5V. Or we can use a jellybean bipolar emitter follower to do the same job. It may still need a voltage gain stage, however.

This post was modified 1 year ago by marcdraco


Take everything I say with a pinch of salt, I might be wrong!

 
Posted : 16/02/2023 3:48 pm