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Old 6th March 2008, 08:14 PM   #1
tmhajw is offline tmhajw  United Kingdom
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Default cathode bypass caps suggestions please for newbie

Hi There
I've now built a phono stage, pre amp and a pair of Loftin Whites (from parts & schematics from Bluebell)- now I'm wondering about some 'improvements'... First up must surely be all those 100 micro farad cathode bypass caps in the phono & pre- they're all rifa electrolytics at the moment. Any suggestions please folks?!
Also what does bypassing the p/s electrolytics (pi filter) with small value polyester caps do??
thanks in advance....
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Old 6th March 2008, 08:31 PM   #2
Jeb-D. is offline Jeb-D.  United States
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Bypassing electrolytics with smaller film capacitors (.1uF seems to be the most common) is in theory used to better the high frequency performance. But some will debate that it can cause undesireable side effects if the value isn't chosen properly. I am unsure which to believe.

If your chathode bypass caps only have a couple of volts across them, you can try replacing the caps with OS-CONS. OS-CONS are a electrolytic-like capacitor that also has good high frequency characteristics.
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Old 6th March 2008, 08:33 PM   #3
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Go with a bypass cap. Something around 1uf and possibly a 0.1uf bypassing that again. What it achieves is to lower the impedance at high frequencies where inductance in the electro comes into play. Effectively the bypass takes over and the performance is likely to be almost the same as if the quality bypass were the 100uf cap.

If you go with caps designed for switch mode power supplies, I can't imagine you will hear a lot of improvement, but thats a matter for experimenting.

Shoog
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Old 6th March 2008, 08:34 PM   #4
jnb is offline jnb  Australia
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Bypassing the cathode resistors in the phono stage reduces noise (and may be a necessary part of the equalisation). Increasing their value would be good in theory but bigger electrolytics are not the desired solution here. You might consider LED bias or battery bias.

Bypassing the power supply caps is a process itself based on compromise. For good and trouble free performance you should put your best electrolytics here and not bypass them.
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Old 6th March 2008, 08:37 PM   #5
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To avoid using electrolytics as cathode bypass caps, you need to use a method for obtaining the bias that doesn't involve a cathode resistor. One possibility is using LEDs. I have no experience of LED bias but there are many others here who can probably help you. Morgan Jones's book Valve Amplifiers is useful on this subject.

Another possibility is fixed bias, requiring a negative supply for the grid (usually applied to the 'earthy' end of the grid leak resistor). The negative supply can be a battery or can be rectified and smoothed from the power transformer if it has a suitable winding.
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Old 6th March 2008, 08:43 PM   #6
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LED bias works very well and produces a cleaner tighter sound. Sometimes however this can be a step backwards and careful comparison of any change should be made before a final conclusion can be drawn.

Shoog
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Old 7th March 2008, 03:07 AM   #7
tmhajw is offline tmhajw  United Kingdom
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The pre ( ecc82) in particular sounds a little bit coarse, the L-Ws are lovely, the phono stage not too bad- these are my first builds, and I feel LED biasing might at this stage be rather beyond me..! Adding smaller value caps to each cathode bypass cap seems like an easy quick fix,so which variety of cap?!
For the p/s electrolytics are ok? I was wondering what difference a large polyprop as the last cap would make?
Sorry if all this is a bit basic..!
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Old 7th March 2008, 07:49 AM   #8
jnb is offline jnb  Australia
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The bypassers will not be a quick fix if they cause an unwanted resonance.

A polypropylene power supply capacitor on the other hand is a great idea if you have one large enough in value.
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Old 7th March 2008, 08:19 PM   #9
Jeb-D. is offline Jeb-D.  United States
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Here is a link to quite possibly the best thread on paralleling film caps with electrolytics.

paralleling film caps with electrolytic caps
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