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xrk971 Pocket Class A Headamp GB

Founder of XSA-Labs
Joined 2012
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The desktop version with 125mA bias current can drive headphones as low as 32 ohms just fine. If you really have demanding headphones and want more SE Class A power, the LuFo Lite is the way to go.

https://www.diyaudio.com/community/threads/lufo-lite-a-1-transistor-se-class-a-headphone-amp.374760/


1664123268092.jpeg
 
Thanks. Perhaps the desktop version of the PCA? BTW what do you think of those headphones? I've been using studio headphones for awhile (Sennheiser HD560s, Beyerdynamic DT-770) and would like something with a little more detail and excitement.
I can't comment on the desktop version, but I think the headphones sound really nice and open with or without EQ. They are an upgrade over my past HD 600s and 660S. I drive them with a Topping A90. The only flaw I really can take with them is the bass definitely has that planar-magnetic signature to it, and can be kinda dry. But I would definitely say it's an upgrade over the DT770 and 560s.

Massdrop has them on sale for less than $400 occasionally.
 
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Yes, both of them are my own layouts. I've attached a zip with the Gerber/drill files, schematics, and BOM to this post. Note that on the Cap Mx, you're going to want to use a 47uF 25V cap instead of the 100uF 16V... until they make 100uF 25V in 1210 size. ;) (It was a slight oversight in my thought process that 16V would be good enough, in fact there's about 18.5V coming into the cap Mx for 16V out... I didn't feel like risking popping the relatively expensive little guy to find out if it could go that far.) Also, note that OSH Park doesn't do slot drills, so you'll have to trim the micro-USB jack to fit if you use them. A nice sturdy pair of wire cutters worked great, the one on the BOM is steel so don't use your good ones

I'd like to build the capacitance multiplier but the 1210 format capacitors with 25V or higher voltage rating are only available in 22uF or below (as far as I've been able to find). Any suggestions?
 
Use multiple ones in parallel or larger 2512 format. Can get 100uF. Or use electrolytic.

How can multiple 1210s fit in the space? Stacked? All I can find in stock at Mouser/Digi-Key with 25V or higher rating are 22uF. I suppose two of those might be enough since raptorlightning reported that 47uF worked.

I can't find any good candidates in the 2512 format and at twice the size I'm wondering how it would fit.

What about a radial lead 100uF cap horizontal off the edge of the board? I see some that are 8.2mm diameter x 11mm long.
 
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I have sort of neglected to update the BOM since the BF862 and ZVN4396 have been discontinued.
[....]
Here is what I think is the BOM that goes with the 2SK209GR and ZXMN10:
C1 Input cap 5mm pitch up to 5.5mm wide (Wima 2.2uF MKS2B042201F00JSSD)
C2_1 Output cap 3.5mm pitch, 8 dia (390uF 20v Panasonic OSCON 20SEPF390M) or Nichicon AK 470uF if you can fit it Edit: I highly recommend a 1000uF 16v OSCON here for low impedance phones.
C2_2 Output cap 5mm pitch, up to 9mm wide (Wima 1uF MKS2C041001F00KSSD) Edit: I highly recommend a second 1000uF 16v OSCON here, if you have 30ohm or lower phones, and place Wima on SMT side.
C3 Rail cap 100uF 3.5mm pitch, diameter whatever you can fit in the tin (100uF 20v Panasonic OSCON 20SEP100MX) - or highly recommended Nichicon 2200uF 16v - lay on its side
C4 Rail cap 100nF 1206 package (50v X7R)
C5 Additional 100nF rail cap 1206 package (50v X7R)

Could you clarify what I should be doing for the input and output caps? I have:

C1: Elna Silmic II 2.2uF
C2_1: 1000uF 16v OSCON
C2_2: 1000uF 16v OSCON
C3: Nichicon 2200uF 16v

For C2_2 do I need to combine the 1000uF 16v OSCON with a 1uF on the SMD side? If yes, I have an Elna Silmic II I can use.

My headphones are 48R (Beyerdynamic DT 700 Pro X) but I thought I'd build the PCA to be capable of driving low impedance headphones, just in case. Hopefully that won't compromise performance with my current cans.

TIA.
 
I am very excited to follow these pages xrk971 Pocket Class A Headamp GB. I have a Sony wm1z. I replaced the capacitors in it, removed the relays and wires. My headphones are Gjallarhorn GH50 JM. 32 ohms. I ordered a circuit board and a set of transistors from ETSY. Now I'm going to look for other parts at Mouser or another store. I am looking forward to the sound of class A. I also ordered the BF862 some time ago
From Aliexpress. I did not believe that they would send me solid transistors, but out of curiosity and the low price, I ordered it. It arrived today from Aliexpress. The cases are marked w28u. What is it? I tried to test it and you can see the result in the photo. Thank you all for your work on pocket Class A.
20221005_215421.jpg
20221005_215356.jpg
 
Founder of XSA-Labs
Joined 2012
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That’s not a BF862 which is N JFET. W28U code is an N channel MOSFET. That’s what your tester says. File a complaint with Aliexpress and they will refund you. Provide proof of photo of the code stamped on package.

https://m.yoycart.com/Product/562584991156/

I sent you a matched pair of genuine BF862 in your order. They are not made anymore so kind of hard to find. You can also use 2SK209GR or BL. There are different resistor setpoints provided in the thread.
 
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Could you clarify what I should be doing for the input and output caps? I have:

C1: Elna Silmic II 2.2uF
C2_1: 1000uF 16v OSCON
C2_2: 1000uF 16v OSCON
C3: Nichicon 2200uF 16v

For C2_2 do I need to combine the 1000uF 16v OSCON with a 1uF on the SMD side? If yes, I have an Elna Silmic II I can use.

My headphones are 48R (Beyerdynamic DT 700 Pro X) but I thought I'd build the PCA to be capable of driving low impedance headphones, just in case. Hopefully that won't compromise performance with my current cans.

TIA.

My question is: does combining in parallel a 1uF and a 1000uF capacitor do something more than simply their combined 1001uF? Something to do with impedance at different frequencies? I'm asking not only so I can complete my PCA build but also hopefully learn some electronics theory.
 
If the 1 uF is a film cap, generally lower equivalent series resistance (ESR) and this will provide some improved high frequency response and better transients. It is a subtle effect though and you may not hear it. On a power supply rail cap, it is called a bypass and helps to suppress higher frequency RF noise.

Thanks. Is it only C2_2 that can benefit from the small+large cap combo? What about the other output cap C2_1?
 
Founder of XSA-Labs
Joined 2012
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Both output sides left and right can benefit.

This is one way to do it.

1665169257147.jpeg


Later I generally put the big 2200uF rail caps aligned axially and pointed towards the connector.

Like this and I put the filmcap bypass on the other side:
iss_fullxfull.10685270_2hh2zbsh.jpg

Like this:
1665169475719.jpeg

The main places to have a bypass cap are the outputs circled here:

1665169638562.jpeg
 
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Thanks for the tips. I've got my board assembled except for the smaller film caps on the SMD side that'll combine with C2_1 and C2_2. My understanding is they are good to have but the amp should work without them. I also haven't connected the power indicator LED.

Without anything plugged into the input or output jacks I connected a couple 9v batteries and turned the switch on. No smoke but after 10 seconds C3 (2200uF) on the end of the board with the output jack starting getting warm. I turned off the power before it could get too hot.

M1 is ZVN4306GTA. Q1 is 2SK209-GR with the recommended resistor values:
R3 1k (1%) ** use 2k2 for 2SK209GR **
R4 33R (1%) ** use 67R for 2SK209GR **
R5 270R (1%) ** use 470R for 2 SK209GR **

I know the soldering isn't great (first time with SMD components this small). Does anything look amiss?

IMG_4946.jpg

IMG_4945.jpg IMG_4944.jpg
 
Founder of XSA-Labs
Joined 2012
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The board serves as a heatsink for the mosfet and resistor bank. It is class A and dissipates about 1/2W of power so getting warm is normal. Maybe 35C to 40C. Not hot though. When inside a tin, makes a nice hand warmer in the winter. You want to use rechargeable batteries because it goes through regular 9v in about 2-3 hrs. The board may be conducting heat to the cap via the leads or radiation. Is your cap polarity correct?
 
The board serves as a heatsink for the mosfet and resistor bank. It is class A and dissipates about 1/2W of power so getting warm is normal. Maybe 35C to 40C. Not hot though. When inside a tin, makes a nice hand warmer in the winter. You want to use rechargeable batteries because it goes through regular 9v in about 2-3 hrs. The board may be conducting heat to the cap via the leads or radiation. Is your cap polarity correct?

You're right, it was just the normal heat dissipation. I plugged my iPhone into the input and some easy to drive headphones (48R, 100+ dB) into the output and it works! Well, sort of. The amp isn't as loud as driving the headphones directly from the iPhone. If I turn up the PCA volume or the level of the input (iPhone volume) I start to hear distortion before the listening level gets just a little beyond normal levels. Clearly something isn't quite right.

My current caps:
C1: Elna Silmic II 10uF
C2_1: 1000uF 16v OSCON
C2_2: 1000uF 16v OSCON
C3: Nichicon 2200uF 16v

Update:
Checked my old rechargeable 9v batteries and they are at 8.1v. As an experiment I switched to alkaline and it seemed louder for a second and then the output cut off (both channels). Switched back to the rechargeables but still no output. Evidently something fried although I didn't turn it up very much.
 
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With the rechargeable batteries they sometimes go into shutdown mode because of the high inrush current startup. There is a self protect circuit in the battery to prevent it from burning up due to a short. You may have to click it on/off a few times to charge up the rail caps to overcome the auto off.

What is voltage of the source (pin 3) at the mosfet? This will tell you if bias current is correct. It should be about a little less than 1/2Vcc. Across 137.5ohms will give you bias current.

So for 8.1v battery x 2 is 16.2v. 8v across 137ohms is 58mA bias current. That should be ok for 60 to 100 ohm headphones.

If your amp distorts with 40ohm Headphones, try increasing bias current a bit to 70mA. You can change the 4 parallel resistors to a lower value.