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Old 5th March 2007, 09:38 PM   #1
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Default 2 lm3886 based amplifiers with different sounds

I'm seaching for answers about why my two gainclones amplifiers sound so different.

The first one as been builded by myself. It use :

-Point to point wiring
-lm3886t, good heatsink
-400va (8 X 50va)
-4 diodes bridges mur 860
-regulated using lt1083
-8000 uf per rail X 4 panasonic FC before the regulation
-47uf panasonic FC after regulation

The voltage after regulation is +- 16.5volts with plenty of current. Everything is high current including switches, everything is dual mono with ps is a seperated enclosure.

On the chips, i use

-vishay s102k 0.05% everywhere
-no zobel
-no capacity under the - input for dc offset

My second gainclone :

-chiamp.com kit pcb
-1 transformer 250va 25vac (arround +-34v dc)
-1 rectification board (2 bridges mur 860)
-10 000uf per rail (two rails)
-snubbers on the PS
-lm3886tf
-small panasonic fc on the chips boards
-zobel at the output
-caps in // with the mute resistor

In fact I think it's the complete chiamp.com kit with all feature installed but with one transformer and two rails only.

My system is a sony scd-1 source, a pair of tx-102 mkIII as a preamp and the speakers are bigfunhorn using lowther dx4 (16ohms).

I was really surprised of hearing such difference between two amplifiers based on the same chips each having more power than I need for my application. My firsts gainclones where using lm3875 chips so at the time of building my regulated lm3886 gainclone I blamed the differences on the chips ... maybe I shouldn't have.

The differences between the two are that the chipamp kit is way less contolled in the bass, it's less neutral, it got a 120hz small noise (consider that I get arround 103db eff at the listening position) but the noise is almost irrelevant. Also, even if the chipamp kit got more power with the 34 volts dc, at high volume the sound become blurry and finaly the highs are a little harder and granular.

Also the chipamp kit amplifier heat up, it's warm but not dangerously hot, where the other never heat at all. Is it the results of running it at +-34vdc compared to +-16.5vdc ...

So my questions are :

where would you think thoses differences are coming from ?

Is it the zobel network, the parts, the voltages, the pcb, the snubber, the regulation, the small caps on - input legs of the opamp, the 10 000 uf used without regulation ... ?

Is the heat really function of the voltages when I only use a watts or two in worst situation ?

Keep in mind that I only need 1-2 watts of power for loud listening.

I will start my investigation soon but I would like to have some feedback of people who builded different versions of thoses amplifiers and have developped knowledge about which parts affect the sound is which way.
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Old 5th March 2007, 09:45 PM   #2
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Are you really sure you could pick the amp in a blind test? The mind is powerful.
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Old 5th March 2007, 09:52 PM   #3
BWRX is offline BWRX  United States
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Default Re: 2 lm3886 based amplifiers with different sounds

Quote:
Originally posted by franksmith
I'm seaching for answers about why my two gainclones amplifiers sound so different.
where would you think thoses differences are coming from ?
Regulated supply vs. Unregulated supply
+/-16.5V vs. 34V
I don't know what topologies (non-inverting vs. inverting) your two amps are but that is a factor as well.

Quote:
Originally posted by franksmith
Also the chipamp kit amplifier heat up, it's warm but not dangerously hot, where the other never heat at all. Is it the results of running it at +-34vdc compared to +-16.5vdc ...

Is the heat really function of the voltages when I only use a watts or two in worst situation ?
Yes. For the same amount of output power the amp with the higher rails has to drop all that extra voltage across its output transistors.

Quote:
Originally posted by franksmith
Keep in mind that I only need 1-2 watts of power for loud listening.
If you only need a few watts then I'd use well regulated lower voltage supply rails, like you have already done using the LT1083s.
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Old 5th March 2007, 09:53 PM   #4
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Hi peranders,

I'm pretty aware of subjective impressions and like the blind testing. Sure it would be easy to pick them up, specialy at high spl. Also the small 120hz would help

I'm not a tweaking freak playing endlessly with cables or spikes ... I don't think thoses differences are at the level of cables changes but more at the level of component changes.

I had pretty much the same comments from 2 differents friends.
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Old 5th March 2007, 09:57 PM   #5
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Thanks for the answers,

My regulated one is non inverting, I believe that the chiamp.com kit is non inverting also.

For the voltages differences, I will use a variable transformer at the input of the +-34vdc one to lower it at the same level of the other one for comparison. It sould be a good start.

After I was planning to remove the zobel to see if it can have bad effect with high impedance drivers.
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Old 5th March 2007, 09:58 PM   #6
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It seems to me you have some unresolved construction problems to deal with before you can make a valid comparison of the sound.

The amp that is getting hot may be oscillating. That would certainly affect the sound because it would generate IM products all over the place. You say there is some audible noise coming from that amp - another symptom of a problem that should be addressed. There should be no audible noise no matter what the operating voltage is. Check the rectifiers in the power supply, grounding, etc.

If the temperature difference is merely due to higher operating voltage and different (Inadequate?) heatsinking, the thermal protection circuits in the chip may be turning on and affecting the sound.


I_F
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Old 5th March 2007, 10:20 PM   #7
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Hi I_F,

It's one of the first thing I checked on the amp when I doscovered the small heat.

It doesn't seem to get in oscillation, it's something I had to deal before including when building my phonoclone.

For the noise, on regular efficiency speaker I have absolutly no noise (120hz) that I can hear but with the horns I have to deal with high sensitivity and it's something I can get from the other gainclones I've builded when messing up with grounds.

The gainclone which show some 120hz noise got a ground star configuration that seem ok.

I have to admit that I often run my system without ground at all, else my amplifier is the only ground point.

Something I forgot to say is that my regulated gainclone use different enclosure for transfos / bridges and the amplification section. The gainclone showing some 120hz is all builded in the same aluminium enclosure.
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Old 6th March 2007, 12:35 AM   #8
digi01 is offline digi01  China
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exchange transformer each other watch what happen?
i think the voltage and the resistance of transformer's second coil is is powerful.

P.S.
you use a preamp to control the gc.notice brain's design have a input balance resistor.

Z
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Old 6th March 2007, 10:42 PM   #9
ttan98 is offline ttan98  Australia
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My 2 cents opinion,

Comparing 1st amp and 2nd amp,

1. regulated supply, the output resistance of the
regulated is very low, my experience power supply
with less o/p impedance than conventional one with no regulation will produce a better bass response.

2. Less supply voltage hence less bias current
hence less heat disspated hence runs a little cooler.
your 2nd amp runs a little hotter.

3. point to point wiring in most cases produces cleaner
sound that is why most tube DIYers do that. There are only a few connections for chipamp, point to point wiring is really not a big deal.

4. 1st amp has less o/p power due to lower supply
voltage. 2nd amp gives you more power. This is not that important to you as I believe you are running efficient speakers.

cheers
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Old 7th March 2007, 12:14 PM   #10
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Default Which one SOUNDS better

Which amp you find better at normal music reproduction. Whether point-2-point wiring amp sounds more dynamic or the one from chipamp.com
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