Question about DC blocking cap. Yamaha P2200 - diyAudio
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Old 8th February 2017, 05:31 PM   #1
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Default Question about DC blocking cap. Yamaha P2200

Howdy folks,

I happened upon a pretty crazy craigslist deal where I found 5! Yamaha P2200/P2201 amps.

Crazy. Road abused yes.

And only one of them works properly. My question here is, assuming it's powering JBL 4412's, 8 ohm impedance, what would be a good value for DC blocking caps on the output of the amp?

I just want to be absolutely sure when I try it with the JBL's that I'm not going to be putting +/-80V on their coils or that a transistor failure down the road won't do the same.

Something like two 4700uF back to back to create essentially a non polar cap? That's a value I've heard before in regards to an 8 ohm load, but I'm not entirely sure.

???
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Old 9th February 2017, 01:13 AM   #2
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That will work for 8 ohms, but on a split supply amp it will sound a little funny. At least the one I tried did on the CS800s I tried it on.
The one I tried was two 10000 uf back to back, since that was what I had used.
The CS800s has competent DC detection and relay disconnect, Enzo tells me. I don't have any knowledge of what is in a Y P2200.
Single supply amps where the output stays above zero can use a single e-cap of proper polarity in series with the speaker, which is what I tend to build. There is no center tap transformer connection to the speaker.
DC detect and disconnect circuits using two FETs source to drain is what is done in a more modern fashion. See the various protection threads on SS forum for schematics of other Solid state protection circuits. I don't know any place to buy one. The protection circuit diyaudio sells has an old fashioned hard contact relay, which is slow and prone to contact oxidation and silence in the 6 to 10 year realm. E-bay units, the pictures show a dinky relay probably not rated for the 1000 A DC rush they undergo when the rail caps dump through a shorted transistor. DC tends to weld contacts together, hardly any relays are rated for it.
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Last edited by indianajo; 9th February 2017 at 01:18 AM.
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Old 9th February 2017, 01:51 AM   #3
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Thanks indianajo

Yeah, from researching it seems like the relay protection circuits may or may not work, depending on the robustness of he relays? I haven't invested enough time reading up on them.

I was on my way to placing a mouser order so I thought I'd at least try the blocking cap(s) at first to be sure not to screw up my speakers. I'm not familiar yet with this amp combo, so I suppose I won't know what "funny" sounds like.
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Old 10th February 2017, 02:12 PM   #4
AndrewT is offline AndrewT  Scotland
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4m7F back to back results in 2m35F

F-3 dB = 1/{2 Pi R C} = 1/{2 *3.14*8*0.00235} = ~8.5Hz
F-1dB ~17Hz.
That indicates you won't lose much low bass signal with a pair of back to back 4m7F.
You will hear it.
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