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Old 13th November 2005, 08:38 PM   #11
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I have made the second module work today, still in "prototype mode". It is much better in stereo !


It exhibits 2.2mV of DC offset, still an excellent value IMO.

It's hard to tell what configuration is the best, as the chips are still improving with the breaking-in.

I got some fears with it : a not well soldered capacitor made intermittent contact with the PSU rail and caused some lightning ! At first I thought I burnt the IC !


I also tried the 6 other capacitors, and what I can say is that with them, bass is quite rock solid, even on my little FX-120 speakers. Never heard that before, definetely !

But without any snubber, it definetely destroys the highs.

I put some 100nf caps on the chip's PSU pins and on the output of the PSU board.

It does improve the sound but right now everybody is sleeping so I can hardly tell what configuration is the best, low cap, high cap, snubberized, etc...

I have many trials to do with all the schematics of Carlosfm and P-A !!!

Got to get some more little snubber caps
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Old 13th November 2005, 09:52 PM   #12
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Have fun experimenting just one suggestion though..... some insulation tape or probably better heat shrink around your mains terminal block would probably be a good idea whilst you don't have this in a case, don't want to end up

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Old 14th November 2005, 03:23 AM   #13
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Quote:
Originally posted by wintermute
Have fun experimenting just one suggestion though..... some insulation tape or probably better heat shrink around your mains terminal block would probably be a good idea whilst you don't have this in a case, don't want to end up Tony.
Good point Tony,

I always assume the nephews, neighbour's kids or the cat is going to stick something where it doesn't belong. In truth, it would more likely be me after a long day or a few wines.

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Old 14th November 2005, 04:28 PM   #14
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Lol you are right, I didn't insulate the mains !

the switch is quite far from the experimentation zone, and the only risk would be if someone else would plug my amp and put his fingers on the main switch, which I stricly forbid because my amp is very fragile right now

Hope my little sister doesn't know how to plug the main connector in the wall and put her fingers in the switch, or hope my mother won't end doing a crisis seeing all this stuff on my desk (which is scrap in her opinion ) and throw it on the floor, or hope my too curious father won't plug it in and touch everything . Hope also that my angry brother won't come with a wood stick and strike everything arround !

Lol no, I don't live in a mad family

I'll surely put some tape on the main swith, as you suggest it, it is way safer, even if I rarely drink liters of Vodka in my bedroom


Concerning the amp itself, I've put snubbers on the PSU board (100nf on the connexion between the amp and the PSU) and on the chip's PSU pins (100nf on the 1500mfd capacitors legs).

I let it burn in and try to hear differences between the two schematics I designed. It is quite impossible right now to tell them apart.

I also get a very very small hum noise (audible at 5cm) when both amps are connected on the PSU, it must come from the "temporary" connexion because both module alone don't have any hum.
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Old 14th November 2005, 08:20 PM   #15
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hehehe don't underestimate your ability to do something silly just as greg said it is more likely to be him than anyone else, and I can speak from personal experience, it doesn't have to involve alchohol

on the not hearing any difference IMO if you don't hear a difference straight away, any difference you do hear (later) is quite probably your brain adjusting to the sound of the amp. I had a pair of bolle sunglasses years ago that had very yellowish brown lenses. When you first put them on everything looked the wrong colour. Within a few minutes everything looked "normal" again, your brain compensates for anything that is not "normal". A more extreme example was tests done years ago, where they made people wear glasses that turned everything upside down. after a certain amount of time wearing the glasses everything went right way up again, and was upside down if they took the glasses off!!!

Unless you have two identical amps and do A+B comparisons between the "broken in" one and the just built one IMO you can't trust that any difference you are hearing isn't purely your brain getting used to the sound and tailoring it to the way it thinks it should be

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Old 15th November 2005, 04:01 PM   #16
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The difficulty to compare them comes from the fact that my speakers are placed assymetrically in my room : my listening room is far from being optimal : first, both speakers are placed at 75cm from the floor, and then, one is placed under my bed (my bed is 1m80 from the floor), so they don't present the same frequency response and bass reproduction.

All I could say is that the amp gets more refined : i heard yesterday things I never heard before in my music : it is not accomodation ! (or I hope so).
It's true that you accomodate to sound. But when you hear something realistic, it immediately appears better. Before I had my FX-120 speakers, I had my father's old speakers, and they are not good (veiled, no bass...). When I came back from holiday, I had listened to music through headphones, that are better. And yes, I needed a period to accomodate to the sound of the old speakers. On the opposite, when I got my FX-120 for the first time, they appeared me natural from the 1st second I heard them, and I discovered real "hifi".

There is probably very little difference between the amps anyway. Maybe the one with Ri = 1k has a lower noise floor (less hiss), but I'm not totally sure of that.

Is it normal that my amps get at body temperature (35C on the heatsink) when idle ? Or is it showing that they are oscillating ?
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Old 17th November 2005, 08:31 PM   #17
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Default Feedback comparisons

Well, I built a little switch circuit to allow direct comparison between both configuration ( Ri = 1k and Ri = 4.3k). The soundcard was used in mono. The tests where done on only one speaker. I used different music materials (techno, classical, pop, metal...)

I can confirm that there is very little difference between both amps :

- the amp with 1k Ri has :
- a little less hiss noise
- It has brighter and more detailed highs (the difference is still very small)
- voices look a little brighter and feel more alive (very little difference )
- impulse transcients have a bit more impact
- the sound is overall "sharper" and faster
but :
- it sometimes feel that bass is a bit "shorter" and "dry"
- test tones reveal a bit more harmonic distorsion between 10Hz and 40Hz (estimated by ear, my speakers only go down to 45Hz)


- the amp with 4.3k Ri has :
- rounder sound overall, softer and hence a bit less crisp. It sounds more "shy"
- a bit warmer bass
but :
- a little more hisss
- maybe a bit more distorsion above 50Hz
- highs sometimes feel less "complex", the rounder sound makes them a bit hidden


I overall prefer the Ri = 1k version, and I will have to do further listening test to figure out if this version really has less bass (when my parents won't be there ).

Some people may probably prefer the Ri = 4.3k version, if they have less refined speakers, or don't like "fast" sound.

My Fostex drivers are very revealing and I like sound a bit analytic to hear the best from them, while some would find it harsh.

Hehehehehehehe now, let's beef up the PSU !!
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