[Bakke]

Hi. I'm going to make the PIMETA amp, it is my first DIY headphone amp project. I've got some electronic experience. So my questions is most about the parts that i cant find on www.elfa.se/en because im going to order all the parts from elfa. And i'm using the guide on www.tangentsoft.net

So, the parts that i've not found on elfa is as follows:

Op-amp OPA132PA, single. Can i swich this with the OPA134PA?

JFET transistor for current source. Dont know which JFET i shall use. What exactly does the JFETs in the circuit?

Current-regulating diode, 1.2mA, DO-35. Can i use this diode? 70-095-41

And, is this the "Blue Velvet"?

Edit: Does the PIMETA work well with high impedance headphones? Like my Beyer DT880 (250ohm).

 

[flecom]

Quote:

Originally Posted by Bakke Hi. I'm going to make the PIMETA amp, it is my first DIY headphone amp project. I've got some electronic experience. So my questions is most about the parts that i cant find on www.elfa.se/en because im going to order all the parts from elfa. And i'm using the guide on www.tangentsoft.net So, the parts that i've not found on elfa is as follows: Op-amp OPA132PA, single. Can i swich this with the OPA134PA? JFET transistor for current source. Dont know which JFET i shall use. What exactly does the JFETs in the circuit? Current-regulating diode, 1.2mA, DO-35. Can i use this diode? 70-095-41 And, is this the "Blue Velvet"? Edit: Does the PIMETA work well with high impedance headphones? Like my Beyer DT880 (250ohm).

if you are going to use the JFET's for the cascode then you dont need the CRD...

also the PIMETA works just fine with high-z headphones, works great with my BD 990PRO's (250ohm also)

 

[jnewman]

Read this to find out about which transistors to use (if you want to use them). The JFETs are to make a constant current source to pull enough current through the opamps to make them run in class A. That's not a blue velvet - the blue velvet is the RK27 series. It looks like it'd work fine, althogh I haven't heard anything about it - if you're going to use it, get the "D" curve.

 

[tangent]

Quote:

Can i swich this with the OPA134PA?

Yes.

Quote:

Dont know which JFET i shall use.

They're not mandatory, but there are European JFETs that will work. Just match the Idss rating and the pinout, and they should work fine.

Quote:

Can i use this diode? 70-095-41

Yes.

Quote:

if you are going to use the JFET's for the cascode then you dont need the CRD...

I don't think I talk about CRDs used in place of the cascode in the PIMETA docs. Almost certainly, the original poster is talking about using it for the LED cut-off circuit.

 

[mjw]

Quote:

They're not mandatory, but there are European JFETs that will work. Just match the Idss rating and the pinout, and they should work fine.

I'm thinking about using BF245A (for 2N5484) and BF245C (for 2N5486) here. The only problem I see is that pins 1 and 3 need to be swapped. But this can probably be solved by rotating the device 180 degrees and bending pin 2 to the other side. I will see how that works out mechanically.

 

[Daroid]

Quote:

Originally Posted by mjw I'm thinking about using BF245A (for 2N5484) and BF245C (for 2N5486) here. The only problem I see is that pins 1 and 3 need to be swapped. But this can probably be solved by rotating the device 180 degrees and bending pin 2 to the other side. I will see how that works out mechanically.

I can confirm that atleast BF245A works in a lone jfet configuration. Yes, just turn it 180 degrees and it will work fine. I knew the difference between the 2N548X and BF245A/B/C, printed out both datasheets and tested for Idss on the BF245A while looking at the 2N548X pinout on the datasheet by mistake

. Nothing has happened though, it still works. Both BF245As had an Idss of ~4.6 mA so that would be ideal for class a biasing without current limiting resistors.

Does anyone know if measuring Idss "the wrong way" COULD have harmed anything if done wrong ? (i.e. gate and drain is shorted instead of gate and source, connect the negative side of a 9V source to them, putting the amp-meter positive lead to the plus pole on the battery / 9V source and the negative lead from the amp-meter to source)

EDIT:
Bakke, the alps potentiometer you linked to is an RK18. The "Blue velvet" has the model name RK27. The RK18 would work fine, tohugh - if it is pin-compatible

 

[jonesy]

Hi Bakke

I got all my Pimeta parts from elfa. The 2134/134 opamps worked fine! I used a Bopla alubos enclosure with a bit of filing needed. One bit of advice: try to get the printed catalogue (a monster!) as its easier to compare specs and pin layouts etc. I'm not sure about the LED current regulating RFET - there must be one that matches, though.

If in doubt, give them a call.....(You're Swedish, right?)

Hope this helps!

Jonesy

 

[jonesy]

That elfa pot (d curve) works fine -it's much smaller than the Blue Velvet and fits the same pcb holes. It might give you fewer problems finding an enclosure!

PS I had no electronics experienc when I built my first Pimeta, now on my second. Just respect the numbers and read everything Tangent says carefully....

 

[mjw]

Quote:

Does anyone know if measuring Idss "the wrong way" COULD have harmed anything if done wrong ? (i.e. gate and drain is shorted instead of gate and source, connect the negative side of a 9V source to them, putting the amp-meter positive lead to the plus pole on the battery / 9V source and the negative lead from the amp-meter to source)

According to the datasheet I have for the BF245 (Philips) this device is a symmetrical jFET, i.e. Drain and Source are interchangeable. This is explicitly mentioned in the datasheet.