300B Headphone Amp

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Ray P
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#106 Re: 300B Headphone Amp

Post by Ray P »

So with the 300B filaments needing 5V (@1.25A) and allowing a 'generous' headroom I need to target around 10V after the rectification/filter caps.

Assuming a drop across the diodes of 0.7V x 2 = 1.4V

My target VAC in = (10+1.4)/1.414 = 7.9VAC

That means none of my spare transformers is suitable so I'll go with the dropper resistors. My extant transformers deliver about 10.5VAC so I need to drop about 2.5V, current is 1.25A so a 2ohm resistor should be about right - power wise I'll be conservative and go with at least a 10W rating.
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Nick
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#107 Re: 300B Headphone Amp

Post by Nick »

The ideal solution is to wind extra turns on the toroid to add or subtract voltage. but I seem to remember you have them with a filled centre.
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Andrew
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#108 Re: 300B Headphone Amp

Post by Andrew »

Nick wrote: Mon Jul 26, 2021 9:25 pm The ideal solution is to wind extra turns on the toroid to add or subtract voltage. but I seem to remember you have them with a filled centre.
That's a good point, Nick. I'd forgotten about that little trick.
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Ray P
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#109 Re: 300B Headphone Amp

Post by Ray P »

The 9V transformers I'm using are encapsulated so no options there beyond swapping them out.
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pre65
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#110 Re: 300B Headphone Amp

Post by pre65 »

The resistors I normally use are 1R Welwyn 7W, the green ones.

One in each leg of the transformer output.
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Ray P
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#111 Re: 300B Headphone Amp

Post by Ray P »

I've been experimenting with dropper resistors this morning - looking to reduce the input voltage to the DHT filament supplies in order to reduce the heatsink temperature a bit as it's high enough to have an affect on the filter caps located next to the sinks.

With no dropper resistors in place the input voltage this morning was 10.1VAC

Three 0.47R 10W resistors in series dropped the input voltage to about 7.8VAC however, when I listened there was a low level buzz on the channel I had the dropper resistors on.

Reduced it to two 0.47R resistors and the input voltage is about 8.5VAC and the buzz has gone. I'm assuming the buzz was the result of insufficient headroom in the DHT supply as nothing else has changed.

After about half an hour of playing the heatsink temperature of the module without the dropper resistors is 89°C amd the one with is 70°C and those look stable.

I'll get some suitable chassis mountable resistors and work out how I can do a permanent installation.
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