Autocostuire Fenice 20a & hiss problems

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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?

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.
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?).

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?



Paid Member
2004-10-07 6:05 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.


Paid Member
2004-10-07 6:05 am
plovati said:
Who is the real manufactor of these boards and where I can get documentation about it?

I will ask the manufacturer if they wish to be identified.
Autocostruire is the dealer. I do have the schematic, I'll ask if I can publish it. Shouldn' t be a problem.

There clearly is/was something wrong with this board to start with. Maybe the missing parts were the cause, maybe not.

Maybe not, none of my Fenice boards had any problem other than the hiss. This was simply due to the missing cap C31* that decouples the +5V. Once that cap is installed, the hiss goes away and the board is as silent as the Sonic Impact, perhaps more so.

*C32 is also missing, it decouples the Sleep function.
Same problem with Class-t-amp-2020-m ?

Hello !

I seem to have a similar problem with my Class-t-amp-2020-m kit from Autocostruire (a very loud hiss from the speakers). Since I am not exactly experienced in these things could anybody please tell me what is happening and what I can do about it in not-too-technical terms ?

Thanks !

Initial Fenice20 Impressions

I received a Fenice20 yesterday and hooked it up using a lab grade regulated DC power supply driving my cheap bench speakers.

No mods yet and very bad connections it sounds better then my gainclones. Not as much volume of course.

I am going to use them as the biamp to drive my Raven3s. 800hz crossover using a Marchand I built.

They are rated at 100db so they should crank. I tried it with a frind'd tricked out Red Wine and and the sounded incredible.

Id like to hear what type of input mods people have done with this board.

Bill Strum
Fenice 20 Mods.


I've moved to stage two. I have mounted the Fenice on a perf board with two 3/4" brackets I drilled out for RCA jacks and bining posts.

Power is comming from a lab quality commercial power supply.

Batterys come later.

Panomaniac Says Replace the 2 onboard electrolytics, they're junk.

I agree. I measured the diameters of the current electrolytes they are .3 " ~ 8.5mm This is a lot samller then the Black Gates or Elna Cerafines.

Is the answer ugly fit or do you have a recomendation for another replacement?

For input mods it appears C5 - C8 get replaced with 2uf each of a film or other audio friendly cap.

Any recomendations here?

By the way the Fenice sounds good as it sits.


Paid Member
2004-10-07 6:05 am
For the electrolyitcs I like Panasonic FM series 680uF 16V. You can get them at Digikey. Get the thin ones (8mm diam), I don't think they stock the fat ones any more.

You can remove the SMD inputs caps from the pads and solder something like a Blackgate in place. Each input has 2 SMD caps in paralelle. You can bridge either pad, or both.

If you want to use a big film cap, then remove and bridge the SMD caps, remove the jumper tabs where it says "close no pot", and connect your input cap to the center hole of the pot pads. Input ground can stay where you have it.

Be sure to check for DC at the outputs before you connect speakers!
Proposed mods to the Fenice

Original Feince20 Input circuit

Proposed modifications

I am not 100% sure of my approach and invite suggestions:

a. Remove C6 and C7 and Jumper, this gets rid of the 1uf SMD caps

b.Remove the pot jumper and use the pot thru wholes for mounting a 2uf "audio" (non polarized) cap Suggetions??

Not shown are bypass caps on the output side (C10 and C11) they would be replaced with Pansonic FS caps. the Blackgates won't fit.

And now my big question. C25 is a 10uf cap the forms a high pass filter (DC blocking) this is obviously in the signal path. should I ( think I should) pull it and the 100K resistor and use a good film cap and a quality resistor. They would be point to point wired and with the cap lead being the be input and the resistor soldered to the ground input pad.

All suggestions are appreciated.
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