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Old 10th March 2012, 10:39 AM   #751
Piersma is offline Piersma  Netherlands
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Default Error

hi Tonedeaf,

If you are ready to assemble the PCB designed by Alex, note that C2 is lost on the board. You can solder the 470pF capacitor underneath the board.

have fun with this great amp!
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Old 10th March 2012, 12:11 PM   #752
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Default CFP PCB status

I have finished CFP PCB with bc546/bc556's which differs from my very first CFP build with bc550/bc560's. I faced with a dancing dc-offset problem. Then I have to do some more to fix this issue.

Quote:
Originally Posted by TonedeafDIY View Post
Any opinions on MJ15003's in this circuit?
I have tested MJ15003's and they are OK. Good devices.
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Old 10th March 2012, 05:11 PM   #753
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Thanks guys.
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Old 11th March 2012, 12:02 AM   #754
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Quote:
Originally Posted by terranigma View Post
I have finished CFP PCB with bc546/bc556's which differs from my very first CFP build with bc550/bc560's. I faced with a dancing dc-offset problem. Then I have to do some more to fix this issue.
What happens when R21 is too high or too low? I do no know. Perhaps too low causes too hot, but too high inspires a spurious noise which "may" be accompanied by an offset fluctuation?

What happens when D8+D9 is too high or too low? It is likely that too low would be a slightly warmer driver but I do not know what happens if too high. I think that D8+D9 should represent "1/2 the total voltage span" "in fully loaded condition" and that way the value is never too high.

What happens when C3 is too small and/or C10 is too large? Well, in that case, the largest signals, which are subharmonic, can be tall enough to switch on (blink on) D10, D11, creating, temporarily either a DC coupled or mixed coupled amplifier, but only for the duration of the extra low bass beat and only if there is enough input voltage to do it. The offset would dance to the bass beat. A big enough C3 keeps D10, D11 off.

Constraining the input:
Computer sources (any digital single rail source) can output a lot of crazy, so it is probably good to restrain that a bit. Did you try my back to back (inversely series) zener device put parallel to R20? That is a general purpose overload blocker that "blocks crazy" since it is capacitive to block RF and also you can set its clipper (Zener+0.7) to limit overload at the input.
I used 3.6v zeners, but I guess that larger value zeners, like 5.1v will work. The zener value is to abruptly shorten all unexpectedly overlarge signals that you didn't want to amplify. If the zener value is too small, that would sound exactly like clipping but if it is unnecessarily too large the effectiveness is diminished. The standard overload blocker is enough to stop most oddities, but you have to decide how much input voltage is useful and get zeners barely bigger than desired signal. The photo below (it is 5.1v+0.7v) is only an example, so use a value suited to your source device(s).
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Last edited by danielwritesbac; 11th March 2012 at 12:20 AM.
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Old 11th March 2012, 10:14 PM   #755
gfiandy is offline gfiandy  United Kingdom
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Daniel have you tested your overload circuit as there is nothing to limit the current once the Zenner conducts. You may get away with this as it is AC coupled but if you have a really big AC input it will destroy the Zenners.

Andrew
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Old 11th March 2012, 10:27 PM   #756
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Default Fake Hitachi!

Thanks to Elvee for saving me from counterfeit transistors!
Here's a photographic comparison of fake versus real 2SB649.
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Old 11th March 2012, 10:40 PM   #757
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Quote:
Originally Posted by danielwritesbac View Post
What happens when R21 is too high or too low....
On first run offset falls down to 1mV Ievel and begins to raise up to many volts. I checked all semiconductors and some resistors including R21. It is not marginal, 46K.. D8+D9 is a single 1W 30V zener.. I used carbon film resistors everywhere excluding 330R, 1R, 0R47 which are metal oxide. I altered input transistors also.. Actually I don't want to struggle much more with this build because I have altered the pcb again and I want to build that one from scratch.

I haven't tested your overload blocker..there is no reason to not to give a try. Since that it is a very easy and specific addition, it may best to solder under the pcb.

Any comments welcome.
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Last edited by terranigma; 11th March 2012 at 10:48 PM.
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Old 11th March 2012, 10:45 PM   #758
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Quote:
Originally Posted by gfiandy View Post
Daniel have you tested your overload circuit as there is nothing to limit the current once the Zenner conducts. You may get away with this as it is AC coupled but if you have a really big AC input it will destroy the Zenners.
Andrew
Thanks man!
Are you thinkin' what I'm thinkin'? This zener device might be a useful softer clip limiter if put series to (intercepting) the ground side leg of a voltage divider. Then, the + leg of the voltage divider also serves up some useful current limiting. Awesome! The sources will withstand 62R/62R+D+D (regardless of zener value). But, I cannot guess optimum for the 2 resistor values. What values should I use?
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Old 11th March 2012, 11:02 PM   #759
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Quote:
Originally Posted by terranigma View Post
Any comments welcome.
BC547 Q12 or Q11 will fail. VCEO = 45v and if 61v, it would go all unexpected.

BC546 Q12, Q11 works up to 65v, aka pair of 32v rails; however, a power surge will knock one out.

2SC2240 Q12, Q11 could stand up to whatever voltage and still work as a transistor.
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Old 12th March 2012, 12:16 PM   #760
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Default Miniature Circlophone?

I am curious to know of what resistor and other values may change to optimize BC639 for Q5, Q6 (esp lower heat) and what is the maximum permissible rail voltage without overheating the BC639?

P.S.
I spotted some cute little MJE3055 (to220) to use for a small scale Circlophone.

Last edited by danielwritesbac; 12th March 2012 at 12:32 PM.
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