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PostPosted: 10 Jun 2019, 07:18 
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I’ve built quite a number of the Oatley K272? series HP amps. They ‘tend to favour the PT2308 chip HP driver (Class AB). You can drop in the OPA2134 and often I did. But after building “LeManja” tube and chip HP amp I realised what an exceptional chip the PT2308 is. Extremely low noise and a distortion of 0.001%.

Oatley have this bend to use a 9V battery, the chip max VDD is 7V and to not let the smoke out they use a 5V chip or three pin reg. Silly. And a waist of “V”. Mine will employ 4 X 1.5V lithium batteries for 6V. Perfect.

Cct. brd. is on Vera-brd or strip board and enclosure is ABS.
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PostPosted: 10 Jun 2019, 14:47 
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Looks good, transitors do make a diffrence easily kill the whole amp.
Could be using better transitors the PT2308


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PostPosted: 10 Jun 2019, 18:41 
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ILoveHiFi wrote:
Looks good, transitors do make a diffrence easily kill the whole amp.
Could be using better transitors the PT2308

Whole idea is compact and lite. You evidently haven’t heard the PT2308.Better than a CMoy amp.

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PostPosted: 11 Jun 2019, 03:11 
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Being better than a Cmoy which is ocl direct drive the phones is very impressive. Considering the current one is single ended


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PostPosted: 14 Jun 2019, 05:54 
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Completed. The engraving (name plate) is due to picked-up.

I made it really hard on myself here building this smaller than I have in the past. Both the enclosure and especially the the tiny bit of Vera-brd. To fit the large parts, phone sockets and battery pack this was the only way I could do it.

I couldn’t use a 9V battery which would have saved some pace because the PT2308 has a max VDD of 7V. Oatley use this chip and add either a 5V three pin reg or a 5V chip reg. Firstly you are losing battery power in the reg and secondly there is a little loss in head room at the lower voltage. You could lift the reg ground with a diode but then you add noise and still battery power loss.

Best solution is 6V battery setup as here. Also with a capacity of around 1000mAh for cheap alkalines and a draw of 7.8mA I will get well over 100hours of play between battery replacement. Slip in some Lithium AAAs and you’ll forget which century you changed the batteries.

Al foil lines top and bottom and is earthed for shielding and a Dacron pad helps control resonance. It sounds glorious and now getting some hours to burn in all parts mainly caps. Caps are NP and LESR. Chip is the PT2308. Wiring is silver plated fine copper and solder WBT silver. Safe on with input phone socket switch to cut battery supply when nothing plugged in.
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PostPosted: 15 Jun 2019, 19:55 
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Sounds like you got the case professional done


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PostPosted: 15 Jun 2019, 20:42 
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ILoveHiFi wrote:
Sounds like you got the case professional done

No just off the shelf. There is about four or size sizes. This one is the second largest.

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PostPosted: 15 Jun 2019, 20:45 
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The completed and assembled “retro-2308” looks good and sounds even better. Also the cheapy freebie CD players plays far better than it should. Ear and line out plus bass boost and hold. Not bad for a give away with a magazine subscription twenty or so years ago.
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PostPosted: 15 Jun 2019, 21:42 
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Hi, Nice application. 4 batteries should last a long time in it. I use similar cases that are really nice from Context Engineering out of Santa Clara California. Not real cheap, but really well made and will accept standard size PCBs in slots. BTW the fully built HP version using the tda1308 is available on Amazon for $8US. What is the source of the PT2308. Everyone here seems to want to sell you 50 or more.

EDIT: My current HP amp project is not SS so really doesn't belong here...but FYI it uses 6AS7s in SRPP for a capacitor coupled output. Not tested yet, but the math says it will deliver +/- 17 VRMS into 62 ohms. A bit of over kill really :evil:

Good listening
Bruce

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PostPosted: 15 Jun 2019, 22:04 
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gofar99 wrote:
Hi, Nice application. 4 batteries should last a long time in it. I use similar cases that are really nice from Context Engineering out of Santa Clara California. Not real cheap, but really well made and will accept standard size PCBs in slots. BTW the fully built HP version using the tda1308 is available on Amazon for $8US. What is the source of the PT2308. Everyone here seems to want to sell you 50 or more.

Good listening
Bruce

I agree. I can’t find reasonable quantities anywhere. I think I saw 30 some where but I’d be more happy with10. Not sure how the 1308 compares but very pleased with this build. The ABS enclosures aren’t expensive here and I’m still on staff discount so even cheaper.

I may try Oatley Electronics here who use the chip in a number of headphone amps. Not sure they will sell seperate but I’ll find out.We could do a group buy but the postage will be dearer than 50 of them.

http://retro-thermionic.blogspot.com/20 ... it-in.html

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