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xrk971 Pocket Class A Headamp GB

List looks good. A few questions: you selected 270k vs 270R resistor. Do you really want to buy 100 of the MOSFETs? Thats $55 worth. It appears you have selected a dozen of the 1000uF Oscon output caps. That’s enough for 6 pocket amps. You only need a pair. Maybe double them up for 2000uF if wanting to drive lower impedance headphones.


2017-07-11 4:14 am
Thanks for catching my error. I have ordered two boards and I was considering a third.

The hundred mosfets is to get multiple matched sets. I also put a hundred down for things as a place holder until I figure out how many of each I'm going to get.

Also I've lost my compontent tester, do you have any suggestions? Thank you for your time.
Wima are film caps - not orientation specific like electrolytic caps.

Higher bias current allows use with lower impedance headphones.

This headphone amp can work on lower impedance headphones if they are high sensitivity. For example, they work with my 8ohm IEMs but they are 116dB/mW. My 96dB headphones at 55ohm works too. However, 32ohm headphones that are under 96dB are a difficult load. 250ohm headphones at 92dB are fine though. The max you really can set is 75mA within the cooling capacity of this amp baord.
I made a Pocket Class Amp,

C3A & C3B= 2200uf 16V Nichicon VK(M)

C2A1 & C2B1= 1000uF OSCON 16V

C2A2 & C2B2=560uF OSCON 20V
C2A2 &C2B2 bypassed with Wima 1uF 63V caps.

Input Power supply: Old linear Lab power supply set to 2A and 18V

the problem is that my whole board is hot along the edges (dissipating heat there), looking for the source of this heat, it seems it is the OSCON caps that are hot to touch. So much so that I can touch them for a maximum of 3 seconds. Is this normal operation? if not, where should I look?

Sounds real good, this gem of an Amp. Thanks XRKAudio. I will try another configuration for better treble or I will just get used to the lovely sound this produces.
I did not know that I wont be able to edit my previous post in the future to add relevant info.

So here i am in the next post to my last one, with more info.

Here are some pictures of my setup having been running for about half an hour, crusing at 92-93 degrees celcius. Input voltage is 20 Volts. Current demans from amp is a 253-254mA

Is this a ventilation problem? I am netting 5 watts.
Am I doing something wrong? Driving Sennhieser HD 25 Limited edition (70ohm)

Can somebody show me how to do the maths? I am searching parallel to this thread and will post if i reach an answer.


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Try dropping the lab supply to 16.5v. That’s about what it runs at with rechargeable 9v batteries. If you want to run it at 20v, you need the Desktop Class A (DCA) with TO-220 MOSFETs and big resistors.

This amp should not run more 75mA bias current at 18v. Or 1.35w dissipation per channel. The PCB was not designed to cool more than that passively.

What JFETs are you using and what resistor values are you using? This looks like 125mA bias current per channel used on DCA.

Adjust R3 and R11 (drop from 51R to 33R). That should help

Last edited:
Thanks for your input, it helped me confirm the fault.
You see, I placed an order for the parts using the updated BOM with the new BJT and JFET.

However upon assembly, I followed the default BOM on page 1 of this thread and wondered why I had bought 2.2k 1% resistors and no 1K 1% ones and same for R4 and R5. And did not realize the values I ordered were for the correct JFETS and BJT that are available at this time. So i pulled 1 K from another board I had lying around, and went camping. Now i realize that i made the rookie mistake of thinking i made a rookie mistake.

Looking through the thread I realized my mistake, replaced the resistors with the correct values and my PCA is back to Hand Warmer mode again. I have to say though, i really like the overclocked performance with 20V haha

Can I ask, at this moment I feel that when I am using the PCA, i feel the bass can be a bit lacking. I gain much better control over the LCD 2 bass when driving the amp at 20 Volts.

My question is, what caps should i upgrade, and which ones in this design will cause the most difference when driving the somewhat hungry LCD 2?
Now I know what you will say, that the amp is able to power the LCD 2 amazingly, however I want the deep bass extension, and was checking out this amp with my LCD 2 listening to tracks like Bass I love You Bass Boosted, Hideho Tropkillaz, Bass boosted. I dee that every time the bass needs to rumble, my current usage on the amp goes up which is to be expected. But at this point i'd like more capacitance to rely less on the power supply for those transients.

Please feel free to correct me if I am wrong, as I imagine I can very easily be, in my expectations or understanding
You should use the DCA, desktop version and that lets you run 20v and 125mA bias current as before. Make sure you have at least 2000uF of output capacitance.

xrk971 Desktop Class A (DCA) Headphone Amp

Use a 24v wall supply with at least 500mA output. This goes through the built in cap multiplier for 20v rail that is very quiet. It has 125mA bias current and can use even 4700uF caps for output.