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xrk971 Pocket Class A Headamp GB

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#2,162
xrk971 said:
Here is the latest BOM that should be able to work with your low impedance headphones. It uses the 2200uF Nichicon KA output caps. Install the 1uF Wima bypass on the SMT side. On inputs, use just the 10uF Silmic II. Tuning for low impedance cans involves adjusting R4 to get a slightly higher bias (circa 65mA to 70mA as measured through R7). Usually a 47R or 51R value gives higher bias. However, it is at the expense of higher 3rd order HD relative to 2nd order. The other way to do it is to choose 'hot' (higher Idss JFETs). The ones I sent you should work well with a 47R value for R4. If it is too high, add a high value like 220R in parallel to lower it slightly.
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I have been looking for this attachment for a long time. Well thank you xrk971
 

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#2,165
Thank you all for this project. I had no problems getting it up and running. I also want to thank Hugh R. Dean, who deserves credit for this.
 

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#2,167
I recently stumbled across this amp and it interests me because I find that My I-pod touch (not my first choice but it was a present) sounds thin and lacking in dynamics. I have tried it with different IEMs but it never sounds involving, despite the good reviews it had when it came out. Strangely the I-pod classic that it replaced sounded nicer! (although still lacking a certain something). Even when I use my over ear Sennheiser momentum 3's it still sounds uninspiring.
I can't help feeling that maybe an amp like this could help with this issue and make it a more enjoyable listening experience?
If so would it be ok with IEM's as the two pairs I currently have are quite low in impedance (18 & 30 ohms)?
I should perhaps also mention that this will be for portable use mainly so I am not looking for a desktop amp at the moment.

Paul
 
#2,168
Hi Paul,

Sorry you are having issues.

Well you are the first user to ever report this experience. There are literally over 200 happy builds out there. I have listened with IEMs before and it sounds great. I suspect there may be an issue or something is not right. Can you please take a few high resolution closeup photos of the amp at oblique angles (not straight from above). Top side and bottom side.

Also, with the amp powered up, use a DMM and measure the DC voltage at the source pin of the MOSFETs (pin 3). Measure pin 2 to get the battery rail voltage.all with respect to GND (-ve of battery).

Finally, measure the voltage across resistor R3 to get a value of the bias current across the JFET.

Are you using a genuine BF862 JFET? Or maybe 2SK209GR or 2SK209BL? They all have different values for R3 to allow them to work well.

Also, what is the impedance of your IEM? What rechargeable battery are you using and is the voltage full? If you use a 9v alkaline, battery is going to be dead after 2-3hrs.

Cheers,
X

Here are relevant test points:
1680264964445.jpeg
 
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#2,170
Oops - I didn’t read that very carefully then.

Yes, this amp would definitely improve the sound and I have used it with IEMs as low as 16ohms and 8ohms. Although the 8ohms was very high sensitivity (114dB/mW) so not much power was needed. In either case, they worked very well. The amp was able to more than enough drive the sound levels to high levels without distortion.

I would not use it for conventional headphones lower than 60ohms if sensitivity was below 96dB/mW. The desktop version with 120mA bias current t would be the way to go. The pocket version has about 50-60mA bias current and more suited for high sensitivity phones or higher impedance cans (ideally 250 ohms).
 
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#2,171
Peter was correct in his interpretation. I am feeling underwhelmed with the abilities of my Ipod touch and was hoping that this amp might be something that could inject a bit more "life" into it.

It sounds like it can handle the IEM's ok, which is good news.
I am a relative beginner with more of a mechanical background rather than electronics but I have built the x-overs for Audio excites revelation 2 speaker and also more recently the Nelson Pass Korg B1 pre-amp from the diy audio store. Would your pocket class A amp be a suitable build for someone with my limited experience?

If so can you point me in the direction of the BOM for the NHB version with any changes required for the low impedance IEM's? I have seen there are a few subtle variations on here, so I am hoping to be pointed in the right direction for my needs.

Many thanks

Paul
 
#2,173
Wonderful to see some renewed interest in this great little head amp. This PCA might be small in size or low component count, but it packs a serious punch with a fantastic Class A signature sound. I am convinced it will inject life into your IEM's. Even more so if you can bypass the Ipod's internal dac with a usb dac.
Should be an easy build and you don't have to squeeze it into an altoids tin if you will be using it as a desktop headphone amp. There is a desktop version of this pocket amp called the DCA. X can point you to it if you are interested.
 
#2,174
BOM is on post 1.

All resistors 0805 package, metal thick film, 1% on specified parts, else 5%. Yageo 1% 0805 SMT from Digikey are $0.77 for 100
R1 100k (5%)
R2 47k (1%)
R3 1k (1%)
R4 33R (1%) ** try experimenting with 39R and 47R to give higher bias current but more H3 and less H2 but overall lower THD **
R5 270R (1%)
R6 47R (5%)
R7_1 470R (1%)
R7_2 470R (1%)
R7_3 470R (1%)
R7_4 470R (1%)
RL 270R (5%) User select (optional, use for higher current draw on amp)
RLED 15k (5%) Current limit for LED, 15k or value according to supply voltage

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 Nichicon 2200uF 16v lay on its side
C4 Rail cap 100nF 1206 package (50v X7R)
C5 Additional 100nF rail cap 1206 package (50v X7R)

Q1 BF862 (771-BF862-T/R)
M1 ZVN4306GTA * a great alternative is ZXMN10A08GTA now that 4306 seems to be out of stock or EOL? **

Switchcraft 3.5mm stereo jacks x2 (35RAPC4BV4)
Alps 10kOhm stereo pot with power switch (RK0971221Z05)
3mm round LED (red or green)
4-pin 2.54mm pitch power connector (see post #44)

You might need to get an equivalent pot from Aliexpress or eBay as RK09 is EOL.

Make sure you use the dual 1000uF 16v Panasonic SEPF output caps for 2000uF for low impedance headphones.
 
#2,175
Hi XRK

I can only find 2SK209GR online, so I am having to follow your revised BOM for this FET (post #2015 - page 101) which calls for a revised R4 value of 67ohms.
Unfortunately I can only find 66.5ohms( RC0805FR-0766R5L) or 68ohms in these Yageo SMT resistors. Would either of these values be close enough?

Paul
 
#2,177
More parts suitability questions....

Would This Alps pot be suitable?

https://uk.rs-online.com/web/p/pote...VWdTtCh1_egUCEAQYASABEgLX7_D_BwE&gclsrc=aw.ds

If it's not, then can you suggest an alternative please.

Apologies for the all the questions. I am still a beginner with building electronics so I am a little unsure when it comes to sourcing alternative parts without part numbers to look for.

Thanks for the help

Paul
 
#2,178
The 66R5 (RC0805FR-0766R5L) for R4 will be close enough, I'm sure. They have 1% tolerance, so if you buy 10 and measure you will probably find 2 that are very close to 67R.
For the LED any standard 3mm led should do. I would choose one that has a diffused lens, like this one: VAOL-3MDE2. For the RLED - the higher one (15K) in ohm, the less bright the LED will be.
 
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