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Old 24th January 2012, 11:27 PM   #541
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Quote:
Originally Posted by terranigma View Post
A new contender comes alive step by step.. This build has been almost a full "Dutch" Circlophone. I'm thinking about an elegant way for attaching heatsink for Philips BD140 drivers. Servo transistors not soldered yet.

Its companion will be the next.

Elvee: May you suggest a Philips made npn power transistor please?
Nice! I like the polystyrene caps and the bonded input pair.
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Old 24th January 2012, 11:28 PM   #542
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Originally Posted by danielwritesbac View Post
Well, this thing really rocks. Only problem is not nearly enough gain, and if I set the feedback resistor up to 18k, then there is enough gain but far too much upper treble (distorts snare drum). I'm using the Circlophone pictured in post 504. Perhaps one or more values need altered to bring down that treble a bit? Otherwise, it is an impressive amplifier.
Daniel, did you change the feedback resistor without changing the feedback capacitor?
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Old 25th January 2012, 06:15 AM   #543
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Originally Posted by odysseybmx414 View Post
Daniel, did you change the feedback resistor without changing the feedback capacitor?
Yes.
I did not change R16 or C3 or C4.
The treble errata exists no matter if R17 is at 10k or 22k.
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Old 25th January 2012, 08:38 AM   #544
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Originally Posted by odysseybmx414 View Post
Nice! I like the polystyrene caps and the bonded input pair.
I have to twist (and isolate for preventing short-circuit) C-E legs of Q4 (left one) for doing that. Otherwise, it is not possible bond them face-to-face as shown in photo.
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Old 25th January 2012, 10:52 AM   #545
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by terranigma View Post
Elvee: May you suggest a Philips made npn power transistor please?
In TO220: BDT85
SOT93: BDV95

In their new line of products, NXP doesn't seem to have power Bjt's anymore.

Quote:
I did not change R16 or C3 or C4.
Gain adjustments should be made by altering R17 only. R20 follows R17, and R16, C3, C4 remain identical.
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♫♪ My little cheap Circlophone© ♫♪
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Old 25th January 2012, 02:23 PM   #546
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Originally Posted by Elvee View Post
In that configuration, the full power will be reached with less than 1V input.

With 18K, only 0.5V will be sufficient to reach the clipping limit.

That is probably what you're hearing. Otherwise, the amp is flat, and the characteristics are practically unaffected by the gain.
If anything, the high frequency response might just be infinitesimally reduced at a higher gain.


You could increase the input filter capacitor C5, but as I said the real issue is probably that you have reached the clipping limit.
You could try to fit one of the clipping indicators I have described. If you see the LED flashing when "there is too much treble", you'll know for sure the reason.
I was referring to the upper border of the audio band being disproportionate regardless of playback at modest levels.

Clipping? Must be kidding? Because of curiosity I managed to turn up the cute little Circlophone until clipping. Perhaps I should have either put the 94db efficient speakers far away from the tall bookshelves or used a helmet. Have you built a higher voltage edition of Circlophone? Gosh man, clipping isn't a secret because everything in the house will be shaking violently with audio like a gale force wind. My sample also has the weird errata of power output depending on operating temperature as it is not very strong when cold, eventually warms up to prodigious, and after that, it will change output by itself during normal/modest levels playback concurrently to when the orchestra decided to play a bit louder but instead you may get concert levels by surprise. It does this reliably, but I think it is unusual. A concert actually sounds like that if you're on the front row.

I hadn't tried it extremely loud before, but apparently the cute little amplifier is unexpectedly capable.
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Old 25th January 2012, 04:19 PM   #547
Elvee is offline Elvee  Belgium
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If it isn't clipping, there are two possibilities: either there is some malfunction, but I doubt it because it would impact the reproduction negatively and unpleasantly, or that's simply the sonic signature of the amplifier.

If you have some test equipment, put a 1KHz sinewave at the input, and check the output on a scope, just to make sure everything is healthy.

I didn't build high power versions, my neighbours are too close for that....
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Old 25th January 2012, 10:05 PM   #548
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I would put the feedback resistor back to normal value. It, and the feedback capacitor, are calculated. Changing them effects stability margins.

Also i would go over your circuit again. Doesn't sound at all like what I hear from mine or from the other's descriptions. Maybe you have a capacitor the wrong value somewhere?
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Old 25th January 2012, 10:12 PM   #549
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Thank you.
I was worried about the possibility of a construction error. I will gently filter this one (a bit of extraneous RF RC's will get it) and attempt increasing the onboard compensations, and then build another somewhat differently. I've got to learn how to make PCB's. The little compact perfboard Circlophone is mighty entertaining, but it was laid out tight to avoid trace length, so I can't swap components without a partial rebuild. Right now it has an 3R+150nF (polyester dip cap) zobel added at the speaker jack and while the treble is a bit noticeable, it isn't overwhelming.

The ratio between transformer voltage and output power at RMS is similar to a standard bridge amp. I did manage to learn that my big radio project probably needs a 250w rated output device, the transmitter heatsink, and less voltage.

A violin sounds perfectly realistic and you could estimate the brand of violin and the brand or at least the gauge of strings on it, but a cello sounds like a voila and a piano like a keyboard, and so 100uF is too small. Terranigma latest photo shows 470uF, and probably for a "big enough" approach. Next, I will attempt "the middle ground" with 220uF per rail, although may actually do something more creative in addition to those. In any case, 100uF is too small.

Last edited by danielwritesbac; 25th January 2012 at 10:17 PM.
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Old 25th January 2012, 11:07 PM   #550
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Daniel,

Regarding my experiences, I may add some trial list for you as follows:

* Try many ac-coupling caps including non-polar polyester ones. Mine is 5uF 350V polarised electrolytic.
* Trial with other brand input pairs with minimum 300-350 hfe.
* Use bigger sized and valued decoupling caps. Bigger caps comparing small ones with same rating/value generally means more ripple current. This approach prevents some bottlenecks on supply side. My previous builds were using other brand, also bigger 470uF's. In photo, the small black cap below is also 470uF/50V..then compare their size
* Take attention for if there is any compensation cap heated by driver and servo transistors.

I'm just another witness for how this amplifier capable of things.

Last edited by terranigma; 25th January 2012 at 11:30 PM.
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