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-   -   what to do with 1uF??? (http://www.diyaudio.com/forums/tubes-valves/186733-what-do-1uf.html)

paku5535 9th April 2011 05:01 PM

what to do with 1uF???
 
http://gaussmarkov.net/layouts/opbmp/opbmp-schem.png
hello
it has used c7,c11 as non polar 1uF capacitor. but i am unable to find this nonpolar cap with so high value in the market. i have also read the technique to use two polar electro cap can be used in series to behave like a non cap(++ --). can i apply this here??especially at the c7 positition. if not then plz say if there any alternative with out loosing any tonal effect. can i use any smaller value of c7 instead.
thanks for reding my question. plz help.

HollowState 9th April 2011 06:13 PM

Why not just put two .47uf in parallel. You don't need much voltage rating so 100 volts, or even 50V, would be more then enough.

Mooly 9th April 2011 06:44 PM

C11 is not critical and can be a small electroylitic of around 1 to 10uf. The same goes for C12. Both these caps just decouple the voltage provided by the resistive dividers. Larger cap = slightly longer time for circuit to stabilise from switch on.

The 4558 and the 741 are both ancient devices... I would replace with TL072 for the 4558 and TL071 for the 741. Straight swap and they also have low power consumption if you are by chance using a 9 volt battery.

C7 has to be 1uf or thereabouts. Again a modern electroylitic would be fine here (+ end to opamp) or as suggested, two small 0.47 in parallel.

mashaffer 9th April 2011 07:27 PM

Lots of vendors have 1uf film caps in a variety of price ranges depending on how much you want to spend. e.g. Axial

digits 9th April 2011 07:29 PM

You could also connect two 2.2uf caps in series, but antipolar, I.e. two of the same poles tied together.

paku5535 10th April 2011 06:41 PM

thanks for the replies....

can u say the tonal difference between film and mylar capacitor??

Mooly 12th April 2011 04:57 PM

In a circuit such as this I doubt you would hear any difference between caps tbh

The only real question is whether you can hear a difference between them, and without that decision being influenced by anyone telling you this or that is better before the listening test. In many cases you will "hear" what you want to hear and believe what you want to believe.

jean-paul 12th April 2011 05:20 PM

Hi Paku, no idea why you would want to use the 741 for audio unless you really like bad quality !?! When you send me a PM with your address I will send you 4 x 1 uF MKT caps with 5 mm pitch ( if that is what you need ). C5 should be a 4.7 uF MKT too for optimal results when not thinking of the used opamps.

I don't agree with Mooly, good caps give better results than bad caps. This is not a question of belief. You can learn this with some simple tests.

However, modern electrolytic caps can be very good but film caps are just better when they're in the signal path. No need to replace them after some years.

But boy, a ua741 and a 4558 for audio.....


BTW film is the common word for "plastic" film caps, there are several different film materials of which mylar (MKT) is the cheapest/smallest and MKP (polypropylene) is one of the better but physically much larger types.

tomchr 12th April 2011 05:52 PM

Quote:

Originally Posted by jean-paul (Post 2536053)
But boy, a ua741 and a 4558 for audio.....

No kidding. But then, the circuit is from the 1970'ies... I'd upgrade those opamps to something more sensible. OPA134, LME49710, etc. Although, it could be argued that using an ultra-low distortion op-amp in a circuit that's designed to give some distortion (caused by the diodes in the feedback path on the '741) would be overkill. But at least use a NE5532 or similar.

The circuit as-is will give pretty high odd-order harmonic distortion. If you want more even-order distortion ("triode sound"), change the diode arrangement D1~D6 so it becomes asymmetric. E.g. have four diodes in series in one direction and only two in the other. You can experiment with that until you get the tone you like.

~Tom

jean-paul 12th April 2011 08:15 PM

Totally missed the diodes in the schematic but I already suspected distortion was deliberate (of course after I posted my previous post) ! I obviously need to look somewhat longer at schematics, sorry. If distortion is wished then do not change a single opamp as they were likely chosen for their distortion and/or sonical characteristics.

Still I would use a 1 uF film cap for C7/C11 just as the schematic says to have the meant sonical character. If the design is tested to sound as it does now any change will make it sound different, especially when you change values. I one made the mistake by changing a coupling film cap in a Fender bassman amp that was too small in value in my opinion and it was so wrong in sound that that I quickly changed it back. Lesson learnt, won't make the same mistake again ;)

off topic: just asked myself what would happen if you have a Roland 808 and replace the old caps in it for new ones ? Will it still sound just as phat as we know ?


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