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Old 23rd May 2011, 06:43 PM   #1
E.T. is offline E.T.  United States
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Cool output caps -- your observations, pleeze

I'm looking to buy some polypropylene (or better) output capacitors for a 6as7 headphone amp. 100 uF or larger, 250VDC or higher.

Has anyone done A/B/C comparisons of $10 - $30 ones such as Dayton, Solen, Jantzen, &c?

All I've gleaned from the net so far is that the cheap ones don't necessarily sound cheap, and they all sound different. So far the Daytons look like a the lowest price for a real cap.

As output caps, can you parallel these things per usual, eg 2 @ 100 uF = 200uF? This would certainly lower the total resistance. My understanding is that higher values get you lower bass. Must the valeus of output caps be carefully tuned to the circuit, or is lower generally better?

I've been looking for affordable OT's at 300 : 300 ohm, or 300 : 600, but I haven't found any that can handle tube plate voltages on the primary. Maybe I'm looking for the wrong thing. Even cheaper trannies usually sound smoother than caps, que no?
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Old 23rd May 2011, 09:27 PM   #2
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One danger of using a cap coupled tube headphone amp is that if the caps fail, not only can you end up with a lot of voltage on your head, but the pop from a failing capacitor could damage your hearing. That said, most capacitors made by reputable manufacturers when used within their specifications will be fine. However, I have had at least one of the brands you mention fail in other applications. Perhaps looking for the cheapest capacitor is not wise. I'd look at motor run caps myself. Allied Electronics carries several.

As for transformers, the tube wants to see a load of several times higher than its plate impedance, regardless of whether you are loading the plate or the cathode (true in the cap coupled case, too, b/t/w/). So, for a 6AS7 with an rp of ~280 ohms, you want a transformer with a primary of at least 1K. Use a diver with some gain and you can use a transformer with a higher primary. And, they will wind them with a 300R secondary if you ask.
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