Need help/recommendations on rebuilding phono stage

  1. ashpool
    Quote:

    Originally Posted by Garbz
    You're welcome. Though the thought that the wallwart is putting a hum on the ground plane is worrying. You sure you didn't connect it to a supply rail [​IMG], sorry had to be said, stupider things have happened.



    The voltage coming from the wall wart has something like a 0.04mv-0.08mv AC component between the ground pin and either of the rails when there is no load. There is a roughly equivalent AC component between the two rails. When actually plugged into to PH-1, the ripple between the two rails increases to 0.175mv. I assume when there's an actual signal going through, it gets even worse.

    After more investigation, it looks like the hum is louder in one channel than the other. I made a makeshift battery pack for the PH-1 and now the noise has gone down quite a bit, but I can still hear a slight hum in the same channel. Next step is to check the solder joints on the inputs. Any ideas what else could be causing this hum?

    Quote:

    Originally Posted by Garbz
    When it's all up and running apply a Class-A bias to the chips. The circuit can be copied directly from the PPA or MINT designs. All you need is 2 FETs. Personal opinion is it made it well worth it.



    I'll probably look into this once I've built the PSU. How much bias current do you recommend for OPA627s?
     
  2. ashpool
    I checked the connections on the input/output RCAs and they're pretty solid. With the battery packs in place, one channel is basically free of hum, while the other still hums quite a bit. There is no hum if I don't plug in the output from the turntable, but I've verified that the hum is tied to that particular channel of the PH-1 and is not coming from the turntable. I'm going to do one more check of the solder joints in the affected channel, but I'm a little at loss for what to check next.

    Any suggestions?
     
  3. ashpool
    *sigh*. I found the problem. The hum was indeed 60Hz hum, coming from *other* nearby components. The transformers from my preamp and power amp were inducing hum in the wires leading from the input RCAs to the pads on the PCB. I was able to greatly reduce the hum by making the wires between the RCAs and the PCB as short as possible, but the only solution that totally eliminates the hum involves moving the PH-1 as far away from other transformers as possible.

    I guess wooden housings don't do a lot to eliminate interference... This could explain why the inside of the PH-1 is coated in a layer of gold paint[​IMG], but the paint doesn't seem to do anything at all for the transformer hum, even with the housing closed.
     
  4. Garbz
    Gold is not a mu-metal. The only thing that'll eliminate magnetic hum is ferrite or steel or similar such metals. Even then chances are the hum is entering through the cords themselves. Remember the turntable output is a few mV, 4-5 for a MC cartridge less for a MM IIRC. At a gain of several thousand this is quite audible as you've noticed.

    Make sure you powercords don't run parallel to any interconnects either.

    um a couple of mA like 1-2 should be fine to bias the opamps. Remember that they are driving a load often much greater then 10k. Very little bias current is needed to ensure the output doesn't drop into class-AB. This is much harder to do when driving headphones directly.
     
  5. Desi
    On question?
    Whats the audible differrence if i add bigger value like some uF filter caps??
    John G on his RA and PH has 0.1uF and on the opposite Joseph G on his HPA has 5uF (or maybe more)!
    They both use 4556 ic anyway...
     
  6. rellik
    My suggestion is to not use the dynahi psu if you want to enjoy it now. They are good for burn in but are kinda like a brick wall, a really naughty one due to the use of specific valued tant caps and the hhh heavy monolithic bipolar opa541's.

    The circuit is so freaking small that too much caps will just suck that little vinyl signal right into em, straight back into the psu. A battery would be great with the use of some really small caps, think like the size of the pickup needle. Small mettalized films like Wimas or VishayDale will be nice too but film and foils will loose the dusty grainy nature that makes vinyl so cool. Dried up electrolytics would be fun too to impart the "Nirvana" style grunge. Along the lines of Roderstein Bakelites.

    Either that of pump up the volume bass (amp) with some really nice sensitive mettalized or doped junction fets and pump them right up into the serrrious monolith Excalibur lines from TI. Something like a LME49860 to LME49600 to TLE2301. The whole damn thing is open for bandwidth control of the internal oscillator. The substrate is even directly connected to 4 dedicated pins for that "Ged Dow! Ged Dow!" The output pull up and pull down are seperate for nice juicing as well, like drop some coupling coils (driver/in/out) into some nanometal alcohol.

    EDIT: Biasing OPA621/631 kinda makes em sound like a old portable cassette player...biasing buffers can work better for that "Bwhomp, shoop, Bump, bump effect"
     
    Last edited: May 17, 2017
  7. Desi
    Built by a friend!
    Made to match his Grado Statement cartridge!

     

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