300B Headphone Amp

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Ray P
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#61 Re: 300B Headphone Amp

Post by Ray P »

A couple of pictures.

Image

Image
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Ray P
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#62 Re: 300B Headphone Amp

Post by Ray P »

Wahooo! I'm in business. Worked first time and I've been enjoying a quick canter through various tracks from my music library.

First thing I noticed is that it's completely silent, such that I could hear the PC fan (not particlarly loud) even with my headphones on. Initial impression of the sound quality is extremely positive. I have some chores now but I'll have an extended listen later and hopefully try it using the USB Audio Pro app on my smartphone too.

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Last edited by Ray P on Sat Apr 18, 2020 12:43 pm, edited 1 time in total.
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izzy wizzy
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#63 Re: 300B Headphone Amp

Post by izzy wizzy »

Very impressive build
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#64 Re: 300B Headphone Amp

Post by Cressy Snr »

That’s well shoddy Ray. Well done sir!
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Ray P
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#65 Re: 300B Headphone Amp

Post by Ray P »

Thanks chaps. Now I know it works OK I'll have to shell out and get the top plate machined to finish it off.

The man from Amazon was true to his word and delivered today. One of the items was a USB OTG connector to fit my new smartphone so I've had a quick listen using the phone as the digital source via the USB Audio Pro app - sounds very good, if not quite up there with HQPlayer, and is a handy option to have.

USB Audio Pro is worth checking out if you've got an Android smartphone and a USB DAC - it plays FLAC files and the like in their native format and bypasses loads of the OS stuff - essentially turns your phone into a decent hi-res capable digital music player that'll connect to USB DACs.
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Dave the bass
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#66 Re: 300B Headphone Amp

Post by Dave the bass »

Nice! Ooops, I mean Bill Oddie, nope, I mean shoddy.

I use VLC on an Android phone for FLAC file playback, s'good.
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Ray P
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#67 Re: 300B Headphone Amp

Post by Ray P »

I've been having a more extended listen and it really is very good, I'm hearing more into the music, little details emerging and fantastic seperation of all the strands. Nicely balanced with no aspect overtly emphasised - very listenable.

One slight concern is the temperature of the heatsinks on the Andrew DHT filament supplies, it's getting up to just over 80deg C so probably not good for long trouble-free service. I've checked and although the transformers are nominally 9VAC I'm seeing 10.5VAC on the supply inputs so it looks like I'm having to burn some extra volts. The transformers are 30VA with twin secondaries connected in parallel so one option is to try on just one secondary (1.25A x 5V x 1.8 is around 11VA so a 15VA secondary should suffice and bring the voltage down nearer its rating). Alternatively, I could insert some resistors before the supply inputs to drop a volt or two.
Last edited by Ray P on Sat Apr 18, 2020 9:48 pm, edited 1 time in total.
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pre65
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#68 Re: 300B Headphone Amp

Post by pre65 »

I use 1R on each lead on some of mine.
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#69 Re: 300B Headphone Amp

Post by Max N »

Really nice work Ray, I’m glad you’re enjoying the sound.
With regard to the transformer voltage, I think just using one secondary should be the first thing, and then if you still need some dropper resistors, they won’t generate as much heat.
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Nick
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#70 Re: 300B Headphone Amp

Post by Nick »

As you are using toroids I was going to suggest a much more elegant solution would be some bucking turns to bring the voltage down to whatever you want with no wasted energy as heat, but I notice your transformers have filled centres.
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Ray P
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#71 Re: 300B Headphone Amp

Post by Ray P »

Nick wrote: Sun Apr 19, 2020 2:22 pm As you are using toroids I was going to suggest a much more elegant solution would be some bucking turns to bring the voltage down to whatever you want with no wasted energy as heat, but I notice your transformers have filled centres.
Yes, they're epoxy filled centres, plus it would be quite a lot of work to get them out so I'll go for the easy options and try just one secondary first. I've got a 'day off' tomorrow so will give it a try.
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Ray P
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#72 Re: 300B Headphone Amp

Post by Ray P »

So, using only one secondary reduced the voltage by 0.2VAC and didn't really lower the temperature of the heatsinks to any degree - the toroids are clearly well over-specced. I'll have to sort out some resistors too.
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#73 Re: 300B Headphone Amp

Post by izzy wizzy »

You'll get minimum heat dissipation with 1.5v drop (leaves a bit in hand) across the 1085. Measure from the caps at the input to the tab on the heatsink.
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Ray P
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#74 Re: 300B Headphone Amp

Post by Ray P »

This afternoon I've connected up a spare 25VA 7VAC toroid to the Andrew DHT filament supplies, one secondary per supply. After 15mins I'm seeing;
  • 7.75VAC on the input
  • Rock Steady 4.97VDC on the output
  • Heatsink temperature 56C
  • Toroid temperature 49C
I think I need to remove the back panel and replace the two 9VAC toroids with this one. That'll be a pain, but quite doable.
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Ray P
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#75 Re: 300B Headphone Amp

Post by Ray P »

Today the two 35VA 9VAC toroids came out and in went the single 25VA 7VAC in their place - fiddly job and I hate cleaning the varnish off transformer leadouts.

No detriment to the sound quality but it's running cooler. Just having a listen to a selection of music at the moment...
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