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Stoatgobbler 28th August 2005 06:32 PM

Autocostuire Fenice 20a & hiss problems
Hi there,

I am using a T-amp driven by a regulated power supply and with a pair of Lowther Acousta's (EX3 drive units) with little or no noise (hiss or hum) from the speakers. These speakers are 96-97dB/w/m efficient so take little driving.

I've bought a Autocostuire Fenice 20a, boxed it, and connected it to the same power supply and Lowther's and used a 50kohm stepped attenuator I had lying around (from Ebay seller KYC111).

The Fenice has a 10u input capacitor, a 100Kohm resistor in parallel with an expected 100Kohm input pot then the standard 2uF input capacitor and R1/R2 gain resistors at 22Kohm each.

The problem I have is one of hiss from the speakers, considerable, obvious and loud at the listening position, it makes the unit un-listenable to.

I shorted the inputs to ground (pre input capacitor) with no effect of the volume of the hiss.

I've cranked the volume up and the hiss lessens very slightly with increase in volume but only slightly.

I've open circuited the inputs (removed the jumpers) and the unit quitens down acceptably.

I've checked the R1/R2 resistors and they are 22Kohm so the gain of the chip is "standard".

The first notch on the volume gives me a audible volume (at the listening position) with three steps or so in being "normal" listening levels.

Unfortunately with SMD components I can't reduce the gain so easily, does anyone have any experience or advice for me please?


I_o 28th August 2005 11:59 PM

Try to change source (cdp).

Finnegann 29th August 2005 11:26 AM

Yes, I had the same hiss, loud and unacceptable. I checked a couple of things with my multimeter and... now its dead. No sound at all. There clearly is/was something wrong with this board to start with. Maybe the missing parts were the cause, maybe not. I can't return it, because I modded it. What a waste of money.

Stoatgobbler 29th August 2005 11:26 AM

The hiss is from the Fenice and not the source (hiss is present with or without the "source impedance" (Meridian 507 CD) connected).

What causes hiss?, I've always presumed noise from resistors, particularly carbon and even more particularly old ones. I am also presuming that this noise is normally unobtrusive to those with insensitive loudspeakers and is due to the high gain of this amplifier (but why is the T-amp whisper quiet?).


Stoatgobbler 29th August 2005 04:16 PM

Hi there Finnegann,

Not a complete waste, at least the air cored inductors can be re-used in the T-amp.

I have stripped the T-amp out of its box and mounted it into my case alongside the Fenice. The T-amp is (almost) completely silent so the hiss has to be generated directly from the Fenice (every other component is the same).

I can't understand how it can hiss though unless the gain is so high that you are amplifying the components noise, however, the music volume levels from the Fenice are comparable to the T-amp at the same attenuator settings so the Fenice is not ultra high gain.

Maybe Autocostuire are using cheap components?

I don't like giving in so easily, anyone any suggestions about what components to try and replace, I've never tried surface mount soldering but there's little to lose?


krungthepchalee 2nd September 2005 07:20 AM

problem with hiss
Hi there,
if you have troubles with hiss on FENICE 20a there is a solution
to be satisfied with this really great sounding module.

Stoatgobbler 4th September 2005 05:33 PM

Fenice 20a fix

Thankyou for this, I had completely missed it, again, many thanks.


Pano 7th September 2005 11:33 AM

Aloha fellow DIYers

I haven't posted here in a bit, but I have been busy with the amps.

As for the hiss on the Fenice, I have several of these boards and just ordered 10 more. Yes, the hiss is a problem. But as shown in the posted link above, the fix is simple, the +5V decoupling cap was left off.
"Stealing" one from the input is an easy solution and cures the hiss.

I get my boards from the manufacturer, not from Autocostruire, the recent batch of Fenice boards all had the same missing caps.

FWIW, I like this board a lot; it is now my board of choice. Very easy to mod, nicely built. It has a few problems, but no more than any other Tripath board out there. Some of the problems are easy to fix (e.g. the onboard electrolytics) some others are not.

The Fenice board seems to have a much lower DC offset than the typical Sonic, as well as much less transistor bounce. The Fenice uses a more aggressive output filter which results in much less remaining ultrasonic noise than the Sonic. Nice inductors on the board, too.

I'll post more in another thread, if anyone is interested.
There will be major updates to the Sonic modding on my site in a few days. Stay tuned.

plovati 7th September 2005 11:45 AM

I got some Fenice 20 boards, they reports MOKSdesign as manufactor. No other information. What is it?
Who is the real manufactor of these boards and where I can get documentation about it?

Finnegann 8th September 2005 03:33 PM

"...we are contacting our customers", they say. My fenice is in the dump, but I'm waiting...

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