[holland]

Gain too high (hisssss)? Turn on pop annoying? Easy fix.

First, I replaced the batteries from 2 2600mAh AA to 3 800mAh AAA, to get the voltage up a bit (not related to title) from 2x1.2 to 3x1.2. Even with that change, I've been running the amp in unattended mode since Sunday afternoon non-stop, and it's still not dead. The charge LED will become useless though. There's not enough voltage variance with the charger to turn on the LED when the batteries aren't dead.

I recapped the electrolytics and lowered the gain on my amp. No more turn on pop and I have more usable volume control.

Original items:

(2) 1000uf 6.3V Nichicon VR(M)
(2) 2.2uf 50V RSA
(1) 10uf 50V
(2) 100uf 10V caps
(2) 40.2K SMD resistors

I replaced it with Panasonic FM/FC and Panasonic 0805 SMD resistors.

(2) 1000uf 16V FM (doesn't quite fit into the case so I did a naughty and bent the leads and stuck them out). I'll look for better a fitting cap some other time.
(2) 330uf 6.3V FM or 220uf 10V FM (you'll see that changing the gain would allow for a smaller bypass cap).
(1) 10uf 50V FC
(2) 100uf 10V FM
(2) 24K SMD resistors for approximately 5x gain.

Why did I change the 2.2uf to 330uf?

The turn on pop is directly related to

10/(Cbypass * 250K-Ohm) <= 1/ ((Rfeedback + Rinput) * Cinput). This means the Cinput is charged before Cbypass is charged. With the original values it wasn't. With the higher value cap though, the startup time is faster as well, I believe (and definitely seems that way).

Original
Cbypass = 2.2uf
Rfeedback = 40.2K
Rinput = approx 4410 (measured)
Cinput = 100uf

New
Cbypass = 330uf
Rfeedback = 24K
Rinput = approx 4410 (measured) -- untouched
Cinput = 100uf


Why didn't I change Cinput.

Rinput and Cinput forms a high pass filter for the input, so I didn't want to mess with that. The center frequency is 1/(2*pi*R*C).

The 1000uf on output forms a lowpass and is a DC blocking cap. It is mandatory. The chip has a default DC offset of Vdd/2.

Again, same center frequency calculation. 1000uf is fine there for me, R is your headphone R, but that changes with frequency.

All the electrolytics listed, except for the 10uf is in the signal path. The 10uf is a power bypass cap. I had a tant on hand, but I didn't replace it with that.

What's it sound like? I don't know, it seems somewhat similar to pre-mod, but I must admit I didn't use the amp much, the hiss and pop was too annoying. It seriously needs a grounded metal/conductive plastic case to reduce RFI. I have to stick the amp into an anti-static bag at work to be anywhere useful. I think the lowered gain would help there.

I just did the mod and everything seems to be OK. My primary goals were to eliminate the pop and add one more battery for a voltage increase. Needless to say, any warranties are null and void, but that's insignificant to me.

Gain is Rfeedback/Rinput.

Original is 9x:

Rfeedback = 40.2K
Rinput = approximately 4410

New is approximately 5-5.5x

Rfeedback = 24K
Rinput = approximately 4410

 

[mandrake50]

Thanks for the great writeup!
I think I will do, at least, the gain mod.

Phil

 

[holland]

Quote:

Originally Posted by mandrake50 /img/forum/go_quote.gif Thanks for the great writeup! I think I will do, at least, the gain mod. Phil

I did the caps as well, since I had to remove a few of them to get clear line of site for the resistors.

I wish I could give a better review post-mod, but my ears are hurting and I don't feel like listening to anything for a while.

Edit: I was wrong, it didn't help the turn on pop at all.

 

[daveyostrow]

quick help question. i by mistake put the charger plug into the in output (just touched inside), and now the red light wont turn off no matter what... is there a way to manually turn the power off?

 

[holland]

Quote:

Originally Posted by daveyostrow /img/forum/go_quote.gif quick help question. i by mistake put the charger plug into the in output (just touched inside), and now the red light wont turn off no matter what... is there a way to manually turn the power off?

Where? Input or output? It probably doesn't matter though.

There's no power switch. The battery is always live. The voltage is always going through the circuitry and the amp is in shutdown state.

You have 2 choices. 1) Utilize the warranty of Microshar, but I bet it won't be supported (user error, etc.). 2) Debug it yourself.

I would guess that you fried the transistors that are part of the "shutdown" circuit.

Your best bet is to open it up and probe pin 1 of the amp chips (2 of them) relative to battery ground. It should be at battery voltage, ~2.4V-2.6V. The pin is connected to 2 transistors in emitter-follower configuration, which is then routed to the 2 jacks (there's stuff in between, but they are passive components), which is connected to grounded when there is no headphone jack in there.

 

[daveyostrow]

was in the input. it should shut off if both the in and out are note connected at the same time, now it stays on. ill send them an email cause im not electrically inclined.

 

[daveyostrow]

they took care of it, no problem.

they wouldnt tell me if it would happen again though...