DG300b Build Thread

Discussion in 'DIY' started by Jh4db536, Jan 3, 2017.

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  1. Hayo1991

    Hayo1991 Rando

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    I recently got my boards for the DG300B and was initially planning to build it for the HD800, with either 45 or 2A3 tubes and output transformer impedance matched with the HD800.

    But since I got speakers with a sensitivity of 92dB I started wondering if it wouldn't be a shame not to use it as a speaker amp also. Then it would be 300B with output transformer impedance matched with the speakers and a LAPD circuit for the HD800.

    What do you guys think?
     
  2. colhd

    colhd Rando

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    And I thought I was the only one building another DG300B primarily for HD800 use LOL.

    Like you I am thinking of using a speaker (4 / 8 ohm) transformer with a resistor as opposed to a dedicated headphone transformer. Difference is I am definitely using the NOS 6s4s (or 6c4c depending on what you read) as the output tube which is electrically pretty much identical to the 2a3. Using a higher than normal 5K primary on the OPTs (so more like a 300B OPT) will further reduce power but also reduce distortion. Should be around 2W but for me this would still work with 92dB speakers (my room, my music, my listening levels etc). Saves me a lot compared to 300B tubes so more money for the OPTs and less money to replace tubes when that comes around.

    Might come down to how difficult your speakers are to drive; I believe that the sensitivity is not the only aspect that matters.
     
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  3. Hrodulf

    Hrodulf MOT - Sonarworks

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    Don't want to poop the party, but 2W won't be enough for 92dB/W/1m. For 2W you need 96+dB, unless you're listening very at a very low level.
     
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  4. Hayo1991

    Hayo1991 Rando

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    Good to read I'm not the only one. I like the tip on the primary impedance, I'll have to investigate that.

    Yes, I thought so. That's why I'm considering the 300B option, which should be 10W. But I'm not sure if it would work that well with the HD800
     
  5. Hrodulf

    Hrodulf MOT - Sonarworks

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    You need to look at gain to find out what works and what doesn't with your HD800.
     
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  6. colhd

    colhd Rando

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    Yes, low level in a small room in almost nearfield conditions. But it's mainly for headphones which is why I didn't choose the 300B route.
     
  7. colhd

    colhd Rando

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    I know very little about this stuff but there are some good threads on DIYAUDIO. In particular there is a long, rambling and ultimately indecisive thread about an all DHT headphone amp. Lots of good information. The idea for a higher than usual primary winding for a 2a3 came from that thread and Tom at Neurochrome as well as Monolith Magnetics seem to think it makes sense. If they are happy then so am I.
     
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  8. tomchr

    tomchr MOT - Neurochrome

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    Higher primary impedance gives you lower THD at the expense of lower output power. I practice, I find the difference between 3.5 kΩ and 5 kΩ for the 300B to be negligible in terms of output power, but the difference in THD is significant, hence, I optimized for THD.
    Note that I tend to run the 300B at 400 V, 85 mA rather than the more common 360 V, 60 mA. The latter gives ridiculously low power output (2-3 W versus 10 W for the 400 V, 85 mA operating point).

    I specify max power at 3% THD at 1 kHz. Many use 5% or even 10% for tube gear.

    Tom
     
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  9. tomchr

    tomchr MOT - Neurochrome

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    The DG300B will produce 10 W, so you'd get up to 105-108 dB SPL at 1 m with your 92 dB efficient speakers. That should be plenty loud for you... :)

    To drive your HD-800 with that amp, just put 12 Ω across the 8 Ω output tap on the output transformer and use that to drive your headphones. That'll have enough oomph for 270 mW into 300 Ω and 2.5 W into 32 Ω.

    Tom
     
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  10. msommers

    msommers High on Epipens

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    @tomchr Do you happen to have a DG300B built in Calgary?
     
  11. tomchr

    tomchr MOT - Neurochrome

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    I do. Two stereo amps, in fact. They're built for driving speakers, but that's easily changed if you'd rather drive phones with it. Classic-Tone power transformer. Electra-Print OPTs. JJ300B tubes + E88CC drivers.

    I'd be willing to part with them if the price is right. A listening session can be arranged too. I do get that listening is believing. :)

    Tom
     
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  12. Hayo1991

    Hayo1991 Rando

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    Thanks Tom! Do you maybe have a schematic for this?
     
  13. tomchr

    tomchr MOT - Neurochrome

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    I do have a schematic. For some reason I can't attach images here and I don't have it hosted anywhere. I'll be happy to share if you toss me an email, though. Just add @neurochrome.com to my my user ID here and you have my email address.

    Basically, you connect a 12 Ω, 10 W resistor across the secondary and then connect your phones in parallel with that. If you use an 8 Ω secondary, you can drive speakers with the amp that way too - just remove the 12 Ω resistor. If you wish, you can implement a speaker/phone switch as well. If you go with the switch, I recommend adding a Tranzorb across the secondary to control the flyback voltage when the switch opens.

    Tom
     
  14. tomchr

    tomchr MOT - Neurochrome

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    Those who subscribe to my newsletter or follow Neurochrome on Facebook know already, but I thought I'd made it more widely known that I have decided to discontinue the DG300B once the current board stock runs out. I'm spread too thin and have decided to focus on my solid-state designs. I have 15 DG300B board sets left. You can find them here: Neurochrome DG300B Product Page.

    Tom
     
  15. tomchr

    tomchr MOT - Neurochrome

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    Six board sets left.
     
  16. Cspirou

    Cspirou They call me Sparky

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    Shoot, i guess I missed out on the spud amp PCBs
     
  17. tomchr

    tomchr MOT - Neurochrome

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    Yeah... I do have four board sets left of the DG300B, though. :)

    Tom
     
  18. tomchr

    tomchr MOT - Neurochrome

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    Make that three board sets left.

    Tom
     
  19. purr1n

    purr1n Finding his inner redneck

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    If any of you are into DIY, this is an excellent 300B amp. Use good iron and it will be x100 better than WA5.

    Here is an EXCELLENT 300B OPT:

    http://www.mableaudio.com/en/productview.asp?sid=2573

    Not cheap for China, but it's better than anything I've heard from Lundahl, Electraprint, Cinemag, and on par with the entry level (that is non exotic silver nano whatever) boutique stuff from Monolith or Audio Note. Yes, I've done direct comparisons.

    This is kind of a secret I've been keeping, so I will never mention this again publicly.
     
    Last edited: Mar 24, 2019
  20. Cspirou

    Cspirou They call me Sparky

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    I've actually had MableAudio on my radar for awhile. Glad to get some confirmation.
     

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