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Bravo Headamp > Borbely Preamp

Really digging the modded Bravo + Grado cans and was wondering how that low voltage tube/mosfet hybrid might sound as a preamp. Looking around yielded this [Borbely balanced tube input mosfet output linestage single ch preamplifier PCB !](Borbely balanced tube input mosfet output linestage single ch preamplifier PCB ! "Borbely balanced tube input mosfet output linestage single ch preamplifier PCB !") Populated an order list for Mouser but had to source some of the older nla stuff from ebay.

It requires a +/- 24 volt power supply and 6 volt for the tube heater. Not having luck there. What are my options?

«13456

Comments

  • 16V rectified will almost yield 24VDC. So I'd get a 16-0-16 secondary transformer.

  • You might be able to wrap a few turns and get your 6 as well

  • You'll need some resistance between filter cap sections to reduce ripple. Here's a free design program that's really useful - http:/duncanamps.com/psud2//

  • Do only the 'triple' secondary winding transformers have the additional 6v. tap for the tube hearer? Is it viable to use a transformer for the 18-0-18 and another separate one for the 6v. tube heater requirement?

  • You can use two transformers.

  • edited November 2020

    You can even use 3 seperate transformers. Sometimes (rarely) it's actually cheaper that way. It's usually just way less cluttered to use one dedicated transformer that has all the taps/windings you need.

  • edited November 2020

    +200v and 6.3v for the heater but no low voltage + 6.3v. Guess multiples it will be.

  • Right/wrong/way off?

    It looks like the through hole trans are about half the price of a chassis mount. Since +/-24vdc is needed the trans will need to have dual secondary windings and the trans outputs will be wired in parallel to feed two bridges. So the parallel output of the trans needs to be ~16vac. The preamp boards are mono and draw ~100ma per not including the tube heater. The Hammond 161f34, $9.07, is 17vac@250ma and should drive both boards.

  • Woot! Found a +/-24vdc smps for $64.99 ... + $88 shipping.?.

  • Antekinc.com they have good prices and a huge selection.

  • Building this for the board's power supply. Found some small Antekinc torrids (thanks Ken) for board power and tube heaters. The torrids are $18 each, or maybe not, might may have ordered 4 of each, not sure. The add's wording was conflicting. Had to source some of the nla mosfets/regs off ebay, and the rest from Mouser. Looks like ~$47 total for a pair of board power supplys.

    kenrhodes
  • That's a lot simpler than the one I found using LM317/337 regs. I'm surprised the 7x24 regs are obsolete. I thought avionics stuff used 24 volts?

  • You can use MC7824 as a direct replacement for a current obtainable part.

    I'm not deaf, I'm just not listening.
  • @dcibel said:
    You can use MC7824 as a direct replacement for a current obtainable part.

    And mc7924 for the neg leg?

    Now you tell me ...

  • I think you can work it out without me telling you.

    I'm not deaf, I'm just not listening.
  • Be careful with electronic parts off of ebay, lots of counterfeits.

  • @dcibel said:
    I think you can work it out without me telling you.

    They are other parts (diodes, mosfets, etc.) that are now obsolete, or ebay finds; is there an easy button for crossing old mosfets, etc., with new available ones? Trying to nail one down with Mouser has proven time consuming and as @ugly_woofer said, I do not trust ebay for things like this.

  • The Mouser brouser is the best. The next thing to do would be to check the original manufacturer, sometimes they will list the replacement part.
    I can often cross parts if you need help.

  • edited December 2020

    When you look up obsolete parts on Mouser, Digikey, etc. Read the EOL statements from the manufacturer, they usually include a list of suitable replacements.

    I'm not deaf, I'm just not listening.
  • edited January 3

    $hit. It's been two weeks past due and the tracking number says I got it. I didn't get it. Got stiffed by a China seller and eBay on some L7824cv voltage regs. Seller says I signed for it ... um, right, a signature on $5 worth of electronics??? Seller then changed the date it was supposedly received, and when I try to escalate with eBay I get an error message. Lesson learned. Had similar problems with an Amazon order so I cannot say they are any better.

  • Well that sucks! I do know packages are still moving at a snails pace in some places. I sold some camera gear on Ebay right before Christmas. One package shipped out on the 14th still hasn't made it to Buffalo NY.

  • Yup shipping is crappy right now. You can call eBay. I've interacted with them on several deals gone bad and had great luck every time.

  • Is this what you are looking for?
    https://www.digikey.com/en/products/detail/stmicroelectronics/L7824CV/585981

    Sucks fighting with bad vendors. :s

  • edited January 4

    @kenrhodes said:
    Is this what you are looking for?
    https://www.digikey.com/en/products/detail/stmicroelectronics/L7824CV/585981

    Sucks fighting with bad vendors. :s

    Sorry, my bad. I need some L7924cv, the other side of the swing. Didn't see the L7924cv at digikey.

    Had already placed the order for the OEM L7924cv and L7824cv from ebay before decibel mentioned the MC version. Mouser currently has both of the MC in stock. In the shopping cart.

    OR

    Could I do this ($75) +/-24vdc, 600ma linear power supply instead? https://mouser.com/ProductDetail/784-HAA24-0.6-AG/
    But I already have the Antek torrids to build power supplys so if you guys think the power supply posted earlier is fine for the job that is the direction I'll go and place the order with Mouser.

    Still leaves 6.3v for tube heater supply. For that I have a pair of 6+6 Antek. Yep, that would be ac, and IIRC, ac tends to be more problematic in regards to noise but ... what needs to go between the torrid and tube neater to smooth the supply? KISS. Keep It Simple for Stupid. <that would be me.

    Somehwhere, someone posted to keep the power supplies separate. Can I use one power supply for both board's (need two boards for stereo) and a separate supply for both tube's heaters so I have (2) power supplies total, or do I need two separate supplies for for the boards (4 power supplies total) and two for the tube's heaters?

    Shipping is slow and that's why I gave the seller an extra two weeks before doing anything. Kinda erks me though that the seller says I signed for the package and I cannot escalate the issue because of it always says "Oops, we had a technical hiccup Please try again later" always occurs on the ebay page.

  • If you're having trouble sourcing appropriate regulators, I would be inclined to try two little SMPS that don't have fans. Just tie the Neg from one to the Pos of the other and there's your ground from the power supply circuit. Verify the outputs are isolated from the AC side, but I'll bet they are.

    Something like this - $21/pair and you'll have them in just a few days -

    https://www.amazon.com/Supply-Adapter-Superpower-100-240V-Transformers/dp/B07D332YHG/ref=sr_1_7?crid=1JWP01DV7PPMH&amp;dchild=1&amp;keywords=24v+power+supply&amp;qid=1609728954&amp;sprefix=24v+,aps,548&amp;sr=8-7

    kenrhodes
  • edited January 4

    In case anyone decides to use ebay for unobtainium electronics, do not buy from miantaxia-0.

    @Tom_S said:
    If you're having trouble sourcing appropriate regulators, I would be inclined to try two little SMPS that don't have fans. Just tie the Neg from one to the Pos of the other and there's your ground from the power supply circuit. Verify the outputs are isolated from the AC side, but I'll bet they are.

    Not really having trouble finding parts now that I know other regs will work, just a lack of knowledge on my part. Would this smps be as quiet as the linear power supply, or quiet enough? Will any buffer be needed between the smps and preamp board?

    What about another small smps for the 6.3v heater?

    Can I use one pair of power supplies for +/-24vdc, large enough to power everything, and use a buck converter to get the 6.3v heater supply?

  • Still waiting on the preamp board to arrive before ordering more parts. It shows as being in Atlanta GA since 12-21.

  • What brand/model potentiometer for the input and what 10k, 20k, or 50k resistance?

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