audio-talk

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.

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.

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.

The 9V transformers I'm using are encapsulated so no options there beyond swapping them out.

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

One in each leg of the transformer output.

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.

With Owston fast approaching I thought I ought to pull my finger out and make some progress with projects!

Today I finished off my 300B Headphone amplifier, a scratch build of the Transcendent Masterpiece preamp.

Today's finishing touches were to properly install (bolted to the chassis top plate for cooling) the 1R0 dropper resistors to reduce the input voltage, and thereby the the heatsink temperature, of the 'Andrew' DHT filament supplies, and then bolt the lid of the chassis in place.





Listening to it at the moment with input from my DSD decoder - native DSD256 recording of guitar and violin pieces - and it sounds superb. I'll definitely bring it (and the decoder) to Owston.

Ooo - what 300B's are those?

Thermionic Idler wrote: ↑Sun Oct 31, 2021 8:17 pm Ooo - what 300B's are those?

PSVANE 300B-T - black glass

A very worthwhile step up from ElectroHarmonix golds.

I treated myself to a pair of HiFiMan Sundara headphones and have just been lstening via the 300B HPA - I have to say they're sounding very good straight out of the box. They're physically quite large but seem very comfortable. They use planar drivers

I'll put some more hours on them and then try to have an objective comparison versus my BeyerDynamic DT-880s (600ohm) and Goldring DR-150s, both of which use conventional drivers.

All being well I plan to bring the 300B HPA to Owston and will accompany it with the Sundaras and DT-880s.