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Old 15th February 2003, 10:29 PM   #1
gpapag is offline gpapag  Greece
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Default "Directional" Inverted LM3866T

Split from "just another Gainclone"


Dear fellows
It is not important if an other set of inverted chip power op amp has been built. OK. But i have to write to this thread again, this time for to thank all the contributors, as well as to admit in public that i have taken a lesson.
What i have built is seen on the attachment. LM3886T inverted. 11K to In-, 110k feedback (gain x10), +/- 37V DC unregulated. Initial operation with regulated bench PSU showed oscilation above 1W ouput if no bypass capacitors were used on the supply pins of the IC. So 100uF// 100nF were soldered directly at the +/- pins. DC offset with V+ grounded was ~ 150mV, but this varied enough, depending on the load seen by the input. I settled for 7K5//100nF from In+ to ground for 15mV DC offset at the output (+/- 5mV depending on the input load). I housed the amps on a small enclosure together with the PSU. The usual hum problems were dealth succesfully by carefull return path cabling (see attachment). 10mV/100Hz spikes at the output, not heard thru the speakers. Very stable operation, superb HF square wave output, burned in for a night, next day listening test. Jaw falling bass, but treble were like hidden behind a curtain. Quite the opposite from what i was expecting. More burning in with the dummy load, and i started doing my visit to this site. Somewhere, i fall on the issue of "component sound", then on "cable directivity" and finally on "resistors directivity". I didn't have enough trangualizing pills, so i switched off the PC. Next day some more listening test. Not much of a change. The dillema was raised. Either i will rebuild them as non-inverting, or i will turn to the TDA7294, both tested in the past with better results. Being quite lazy to do any of them, i tried something really easy, something that i didn't believe in. I tested the theory of the "resistors directivity". Each of the three resistors of the circuit was replaced with an antiparallel combination (2// resistors, but with their color coding on opposite direction). The result? I turned to listen to a totally different amplifier. What a clarity, what a detail! Now i had to swallow all the remaining pills. Me the subjectivist, me the orthologist to be broven wrong by three resistors and their idiosyncratic behaviour. This WAS a lesson for me, and i think i have to share it with you. It is not a joke.
Now this small amplifier has replaced my much beloved tube PP/class A/no global feedback. It is better indeed, very open, more clean, more detailed, faster, equally balanced, only it doesn't warm up my room.
It is driven equally well from my 1K CD or my <100R preamp. It's sound does change though with different interconnects.
Regards
Geor
 
Old 15th February 2003, 11:19 PM   #2
gpapag is offline gpapag  Greece
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Submitt attachment
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Old 16th February 2003, 05:14 AM   #3
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Ah, but was the direction the issue or was it a thermal effect due to having two resistors in parallel and thus able to dissipate twice the power for the same temperature rise? Or was it that you started with a cold solder joint, and when you put in the new resistors you made a good joint? Or is the physical position of the new resistor pair different from the original so the stray capacitive coupling has changed? Or was there some defect in the original resistor which was eliminated when you put in two new resistors?

Some have expressed an opinion that resistors are "directional". My opinion is that I'd like to get some of whatever they've been smoking...

MR
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Old 16th February 2003, 06:43 AM   #4
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Quote:
Originally posted by MRehorst
Some have expressed an opinion that resistors are &quot;directional&quot;. My opinion is that I'd like to get some of whatever they've been smoking...
Just don't be a bogart.

se
 
Old 16th February 2003, 07:50 AM   #5
gpapag is offline gpapag  Greece
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Originally posted by MRehorst
"Ah, but was the direction the issue or was it a thermal effect due to having two resistors in parallel and thus able to dissipate twice the power for the same temperature rise? Or was it that you started with a cold solder joint, and when you put in the new resistors you made a good joint? Or is the physical position of the new resistor pair different from the original so the stray capacitive coupling has changed? Or was there some defect in the original resistor which was eliminated when you put in two new resistors? "
===========================================
Dear MRehorst
You are right in doubting the directivity effect, because it is a bit of funcy. (me also, didn't like it. I have to go through some of my other equipment, do there some resistor changes, see of any differences and report back). I myself have thought of all of your concerns, but nothing seems to be the issue. The resistors i had initially installed were R6, R7: carbon 1/2W. Replaced each with 2 carbon 1/4W. R5:metal film 1/2W (0.6W). Replaced with 2 metal film 1/4W. Parallel combination was placed at exactly the same location as single resistors. Cold solder joint and resistor defect was not either. The first trial was with a gain of X20. Then i went for a gain of X10 (resistors changed). The same sound character in both cases, on both channels. Then i did the paralleling and heard the difference, which is obvious at all level settings, amp cold or warmed-up. My room is small (4mx4mx3m) and the amp does not have to play higher than approx a mean of 3-4 WRMS (Speakers 8R, 89db/1m/1W). When i did some auditing to a friend's house (room 10mx6mx3m), the amp behaved very well at normal listening levels. At high levels it started to distort (heatsink not more than 45 d. Celcius), something i was expecting, as his speakers are 4R nominal, 89db. The LM3886 data sheets don't make any firm statement about, except that the power dissipation increases, but the supplied graphs do imply that for 4R load, power supply has to be less than +/_ 30V DC. My PSU is +/_ 37V no drive, +/_ 30V full continuous drive.

By the way, i did some measurements which might be of interest to some.
The 100Hz ripple up and downstream of R1 are 2 to 1 (200mVpp/100mVpp no signal, 2500mVpp/1200mVpp at 18WRMS out).
The modulation of Vsupply by audio signal at 18 WRMS out (both channels driven) is as follows:
Freq. (Hz) // Modulation (Vpp)
10 // 10
20 // 9
30 // 7
40 // 6
50 // 5
100 // 2.8
250 // 1.6
500 // 0.8
1000 // 0.4
2500 // 0.15
5000 // 0.1
10000 // 0.1
. .
. .

The modulation of the Vsupply of the undriven LH amp by the 18WRMS driven RH amp., although they share the same x-former was les than 20mVpp at all frequencies. Not bad at all.

I would be helpfull if somebody was doing some "resistors directivity" verification test with his inverted gainclone and report the results here. I would be more than happy if my findings were proven wrong. Then, things would seem to be normal again!
Regards
George
 
Old 16th February 2003, 09:37 AM   #6
Bobken is offline Bobken  United Kingdom
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Hi gpapag,

Don't be deterred by any of the usual comments which always come out of the woodwork if you post anything like this. It is all too easy to sit in a chair and ridicule what others have discovered and it is merely a cover for their own lazyness in many cases.

If I may offer a suggestion based on my own long-term listening experiences, I would suggest that you should try to 'reverse' the experiment, and ideally do this change back and forth, several times.

Doing this will eliminate any other unwanted variables of which you may not be aware, and provided that the results are always consistent each way, then you will know *without any doubt* that it is the reversal (or whatever) of the resistors which is responsible for any changes you hear.

I have been 'listening' to components for about 30 years now, and I don't mind admitting that, in the early days, I made many mistakes in reaching some of my initial conclusions, due to inexperience.

One of these times was after making some change, and I was initially pleased to hear an immediate improvement, but later on after subsequently reversing the change (because I wished to try the 'new' cap in another circuit to see if its apparent beneficial effect was universal) the improvement remained!
Therefore it couldn't have been the change of cap which was responsible for any sonic difference, but I knew that the change was real and not illusory.

It was therefore clear to me (probably as I had not made any other changes for some months) that most likely the 'cleaning' effect caused by disconnecting and reconnecting the cables (which had remained undisturbed for so long) was actually responsible for the 'improvement' which I heard.
All connections (whether gold plated or whatever) will tend to deteriorate over time due to tarnishing of connectors etc., and simply cleaning them (either accidentally, or deliberately!) will generally effect some improvement to the overall sound quality.

Similarly, a previously undetected poor (dry) soldered joint can be remedied during the 'substitution' process, but if you do as I suggest and reverse the change a couple of times, it overcomes any of these possibilities.

Since becoming aware of the likelihood of being lead astray by any of these false effects, I will always now do the 'reversal' trials, even though it is an added aggravation and takes up more time, as this is the only way of being absolutely certain over the reasons for any perceived differences in the sound.
You do need to eliminate any extraneous influences, and if an experiment can be done and reversed repeatedly, and consistently, then you are guaranteed that this is the cause of whatever differences you hear.

I would be very interested to hear some more about your trials, as, most unusually, directionality in resistors, is one of the very few tweaks which I haven't yet tried for myself.

Regards,
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Old 16th February 2003, 12:13 PM   #7
gpapag is offline gpapag  Greece
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Dear Bobken
Thanks for the encourangement. I was very sceptical posting my findings. I too hate black art, magic or whatever is not explainable (let alone if it costs an arm and a leg). The reason that i did not any reversing, is that this amps are hard wired and all the components are wired on the pins of the ICs. So it is not easy to change a component withought damaging something. But I HAVE to do extensive testing as you suggest, and the only way to do it is to build in the open 2 LM1875 which have only 5 pins. As long as i have some new evidence, i will report back.
Regards
George
 
Old 16th February 2003, 02:58 PM   #8
fedde is offline fedde  Netherlands
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Maybe a very obvious point, but do you realize that the value of the resistors is halved by putting two resistors parallel ?

And I wonder how you manage to get the exact same value from the single resistors for the parallel resistors...

Fedde
 
Old 16th February 2003, 03:09 PM   #9
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I guess to obtain 110K he used 2x 220K in //. To get 11k he used 2 x 22k in //.
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Old 16th February 2003, 03:18 PM   #10
fedde is offline fedde  Netherlands
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I hope so...
 

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