The Two Channel Advice Thread

Discussion in 'Advice Threads' started by purr1n, Nov 10, 2016.

  1. Armaegis

    Armaegis Friend

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    @elmoe
    Maybe just get an XLR male-to-male adapter?
     
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  2. elmoe

    elmoe Friend

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    Male to female you mean? Yeah I thought about trying that first before spending $$$ on a custom cable. I'll probably go that route.
     
  3. Armaegis

    Armaegis Friend

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  4. elmoe

    elmoe Friend

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  5. fastfwd

    fastfwd Friend

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    Ground loop, most likely caused by the wiring inside the Supra. Do you have a schematic or can you easily tell what's connected to pins 1 and 3 of the XLR headphone outputs?

    Ideally, pin 1 would go directly to chassis ground and pin 3 to the negative output. But especially if the Supra also has an unbalanced headphone jack, pins 3 and 1 might be connected to some combination of nothing, each other, and/or signal ground -- which will work fine with headphones but not with a power amp.

    Like @Armaegis , I think the Supra might not actually have a differential amplifier. The fact that your XLR-to-RCA experiment didn't let the smoke out makes me think that pins 3 and 1 might already be wired together (and to signal ground) inside the Supra.

    Anyway, you can try to fix the hum by lifting the Supra's ground (by plugging its 3-prong AC cable into a 3-prong-to-2-prong adapter) or by cutting the connection between cable shield and pin 1 at one end of the cable. But if (and only if) pins 1 and 3 are wired together inside the Supra, then no connection arrangement will sound better than using those XLR-to-RCA interconnects between the Supra and the unbalanced amp inputs, so just do that.

    Good luck.
     
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  6. elmoe

    elmoe Friend

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    Thanks I'll take some pictures of the innards of the Supra tomorrow after work to get a better idea, but when I last looked it didn't seem like pins 3 and 1 were wired together for either channel. In fact there were capacitors for both channels, each wired to the XLR outputs.
     
  7. lagadu

    lagadu Facebook Friend

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    @Riotvan @Ringingears following your advice, after a busy weekend of experimentation, I ended up landing on something that I didn't expect: the speakers are aimed at a direction perpendicular to what the room's "normal" usage suggests. There's still some tweaking that I want to play around with but I just wanted to say thanks for the advice on getting me started, I'm significantly more pleased with the results so far than I thought I would be this early in the process.
     
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  8. Riotvan

    Riotvan Got lost for three weeks at Delft City Hall

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    Good stuff enjoy!
     
  9. Ringingears

    Ringingears Honorary BFF

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    Great! Have fun.
     
  10. elmoe

    elmoe Friend

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    @fastfwd here are some pics of the internals, there are 2 XLR outputs and two single ended 1/4" headphone outs as well:

    [​IMG]
    [​IMG]
    [​IMG]
     
  11. bobboxbody

    bobboxbody Almost "Made"

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    Has anyone heard the Elekit TU-8340? All I can find google searching is a wall of sound review, which says it's great, but means little to me. I have the itch to build another amp kit and want around 20 watts of tube with volume control. Other recommendations welcome.
     
  12. monacelli

    monacelli Friend

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    This might make it easier for people to help you (flipped orientation and labeled pins). As @Armaegis and @fastfwd have speculated, Pin 1 on your XLR outputs is floating. (This is hard to see from this angle, but confirmed by the other photos.) This may be why the XLR --> RCA experiment didn't let out the magic smoke.

    [​IMG]
     
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  13. elmoe

    elmoe Friend

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    Thank you, I appreciate that, I can confirm R1 and L1 above do not have anything soldered to them.
     
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  14. GanGreinke

    GanGreinke Almost "Made"

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    I have been thinking about building one of the First Watt clones (possibly an F6), and I was wondering if it was possible to build 2 of them and use them as monoblocks with XLR inputs using the stock boards from diyaudiostore?
     
  15. elmoe

    elmoe Friend

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    So would making the Supra work properly with a power amp, balanced, be an easy task or is that something I should give up on? Is it not truly balanced having floating pin 1s?
     
  16. fastfwd

    fastfwd Friend

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    The audio signals are supposed to be on pins 2 and 3, and it looks like they are. So the Supra has real balanced outputs, and your only problem is the ground loop between it and your power amp.

    Earlier, I said you could try to eliminate the hum by lifting the Supra's ground with a 3-prong-to-2-prong AC plug adapter. But I've just taken a brief look at some discussions on the web, and I gather from what I've read that the SinglePower amps weren't always put together as safely as they should have been -- high-voltage wires passed through bare holes in sheet metal, underrated components, gigantic caps with no discharge path, etc. And since the power-cord ground prong might be the only thing that keeps you from dying if your amp has those design/construction faults, lifting it would be a pretty terrible idea.

    Tying the XLR pin 1 to the Supra's chassis ground (not signal ground) might also eliminate the hum. But again, from what I saw on the web, the enormous caps in those amps are very dangerous to work around -- they hold a ton of charge, and with no bleed resistors to safely drain that charge away, they'll continue to hold it even after the power's been switched off. So unless you KNOW that your amp has been made safer by the addition of resistors across the caps, you shouldn't be poking around inside -- even opening it up to take photos may have been risky.

    Anyway, to summarize... If you're certain that your amp was built much more safely than the others, or if somebody competent has already gone through it to correct any safety issues it might have had, you could try the two things I suggested. Otherwise, please don't.
     
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  17. elmoe

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    Thanks, this is one of the earlier "safe" ones and it does have bleeders across the caps. It has also been inspected by a knowledgeable technician prior to my purchasing it. I appreciate the suggestions that is what I needed to know.
     
  18. wormcycle

    wormcycle Friend

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    Two 3 pin headphone outputs do not have ground, those are just two channel plus minus connections only.
    Before I start a bit convoluted explanation your problem may be that connecting headphone directly out to the amp you are missing the ground wire.

    If you ever opened the 2x3pin to 4 pin XLR balanced headphone adapter you will see what I mean. The ground, pin 1 on 3 pin XLR is never connected. The same applies to 4 pin TRRS.

    I have done a lot of reading recently to figure out how to DIY an idiotically expensive A&K line out cable, so here is another example of difference between the balanced headphone out and line out.
    There is a wiring difference between the balanced line out and balanced headphone jack on every DAP that offers balanced line out: A&K, Onkyo etc.
    Balanced line out requires an additional grounding connection through the SE headphone out. See the picture below. Headphone adapter would be just 2xXLR mapped to 2 pin TRRS. This is line out cable for A&K, it needs additional 3.5 connector for grounding.
    [​IMG]
     
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  19. elmoe

    elmoe Friend

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    I owe you a beer or three, I wired pin 1 L/R to chassis ground and that fixed the issue completely.

    Thanks, that was definitely the issue as wiring pin 1 on both to chassis ground killed the buzzing. I appreciate you taking the time to explain further!
     
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  20. fastfwd

    fastfwd Friend

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    Oh good, I'm pleased to hear that.

    Remember that you can no longer use an XLR-to-RCA adapter on the Supra's outputs..
     
    Last edited: Oct 25, 2019

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