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Modification of the S.M.S.L. sAp VI headphone amplifier

Posted 14th January 2013 at 12:29 PM by rjm
Updated 27th December 2016 at 02:38 AM by rjm

I couldn't even build it myself for the price they are selling it at, $50 on ebay. It caught my eye for the linear, split power supply. I love the chrome 3-pin power connector, too.

As is usual when you buy cheap ultra-Chinese audio gear via eBay some adjustments are needed, however.

The basic problem seems to be a little mix up with the input coupling capacitors. It came with polar electrolytic capacitors, following the markings on the circuit board. If you lstudy the schematic I sketched up below you will notice the input signal and DC offset can swing in both positive and negative directions relative to the op amp inputs - polar capacitors in this position are a bad idea.

I recommend anyone buying this to replace the input caps with Nichicon Muse ES or similar 4.7 uF or 10 uF bipolar electrolytic caps. (I have some extra I can mail out. pm me if interested.)

Other than that it's a pretty solid circuit as far as I can see.

Non-inverting NJM4556 gain 6. LM7812,7912 voltage regs. The 50k input pot is loaded by 10k. This is bad practice but it works here together with the 10 ohm output resistor to keep the volume position reasonable over a wide range of headphone impedance. 10k loads on both inverting and non-inverting inputs insure the DC offset is minimal while maintaining a sufficiently high input impedance. Resistance values are an intelligent balance of noise and offset/loading considerations.

I also made a few other changes while I was under the hood.

- 1000 uF filter caps changed to Nichicon KW.
- 470 uF filter caps post reg replaced with Nichicon KW 100 uF.
- 0.1 uF ceramic cap soldered between op amp V+ and V- pins.

I can't say any of those mods were vital, but they sure didn't hurt.

I'm quite pleased how well this came out, actually. Not bad at all.


After more extensive listening, I'm growing ever more impressed. Given the asking price, it runs far and above expectations. This little amp sounds good without ever drawing attention to itself. The operation is quite flawless, with no quirks, and it handles 16 ohm or 300 ohm headphones equally well with no fuss. The overall fit and finish is a little rough, but not in the least bit sloppy.
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Total Comments 8


  1. Old Comment
    KMossman's Avatar
    Is there a manufacturer's name, label, or something?
    Posted 5th February 2013 at 12:13 AM by KMossman KMossman is offline
  2. Old Comment
    rjm's Avatar
    It's the model sAp VI, made by the company S.M.S.L. (homepage).

    The board, back and front plates are all properly marked and labelled, though the etching of the front panel is very light and somewhat difficult to pick out.
    Posted 5th February 2013 at 12:17 AM by rjm rjm is offline
  3. Old Comment
    rambojj's Avatar
    A dealer hear in Germany offers it for 49 Euro. How far away ist this HP-Amp from your sapphire?
    Posted 1st March 2013 at 12:23 PM by rambojj rambojj is offline
  4. Old Comment
    If you still check this blog, is there anyway to lower the output impedance? Can I bypass the 10 ohm output resistor with no adverse effects, say, by replacing other parts?

    The reason why I ask this question is that I am interested in using this amp for low impedance headphones, but the amp's output impedance bothers me (i.e., low damping factor).

    I am also looking at another similar Chinese headphone amp that uses JRC4556AD:

    HiFi Mini Rechargeable Battery JRC4556 Ad Portable Headphone Amplifier Amp | eBay

    Any comments?

    Posted 4th January 2014 at 03:05 AM by Jay_WJ Jay_WJ is offline
    Updated 4th January 2014 at 03:18 AM by Jay_WJ
  5. Old Comment
    rjm's Avatar
    If it bothers you, just bypass the 10 ohm resistor. It linearizes the output level for low impedance headphones.
    Posted 4th January 2014 at 06:54 AM by rjm rjm is offline
    Updated 4th January 2014 at 06:59 AM by rjm
  6. Old Comment
    Thanks for your reply. Just one more question. Any undesirable effects if I bypass it? For example, noise increases or volume range becomes too sensitive, etc?

    I believe for low impedance headphone use, after bypassing the resistor, I can change the gain from 6 to 2. I am also thinking of adding a gain switch.

    Posted 4th January 2014 at 02:48 PM by Jay_WJ Jay_WJ is offline
    Updated 5th January 2014 at 08:24 PM by Jay_WJ
  7. Old Comment
    Hello RJM, Greetings!
    Looks like this little baby is dandy for some mods. I have a few questions if you still monitor this section. 1.) What is the function of R13/14? 2.) Would replacing the Vregs with their OnSemi counterparts be beneficial at all? 3.) I notice that there are no snubbers or bypassing on the rectifier diodes and V pins for the chip. Would any of that help?

    Posted 7th March 2014 at 09:03 PM by FLAudioGuy FLAudioGuy is offline
  8. Old Comment
    jean-paul's Avatar
    Wima MKS2 XL 4.7 F would be a better choice for input caps.
    Posted 2nd April 2014 at 10:17 AM by jean-paul jean-paul is offline

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