Modification of the S.M.S.L. sAp VI headphone amplifier
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.
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.
Total Comments 3
Comments
-
Is there a manufacturer's name, label, or something?Posted 4th February 2013 at 11:13 PM by KMossman
-
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 4th February 2013 at 11:17 PM by rjm
-
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 11:23 AM by rambojj
