Stopping preamp hum on Extreme Plat

[blessingx]

I suspect everyone it tired of Singlepower issues, but quick question if you don't mind. Any suggestions for eliminating hum from an Extreme Platinums preamp out? Have moved outlets, swapped variety of tubes, tested other preamps, used with headphone and speakers, to no avail. For a decent chuck of music I like the Singlepowers punch with the Emotiva RPA-2 and K1000s over other pres around the house (GS1 or Parasound P/SP 1500), but of course the slight hum is killing it. Headphone out has always been good, though the three inputs have never been able to be used simultaneously (all work, just not cables plugged into to any two at one time - goes very quiet even if only one source is on). So there may be an electrical issue, but I pray not.

Any suggestions (besides replacing it)?

[spritzer]

Open it up and post pics. With SP one never knows what to expect so it could be something simple or very complicated but pics would be the best way to go.

[tkam]

Have you tried a cheater plug yet?

[blessingx]

tkam: I hadn't tried a cheater plug, but just did and no luck.

spritzer: Attached are a few photos. Could take more, but these were quickies. Nothing looks disconnected, burned out, etc. though not sure if there are problems with the design.

That was a gimme in the joke department.

If anyone has an ideas I'm be much appreciative.

[mulveling]

Hum/buzz problems have popped up for me quite a few times in various incarnations of my speaker rig. The added component for a speaker rig (amp/preamp), and any multiple sources can really complicate things. Generally if the following doesn't eliminate all hum/buzz for me (within reason), then the problem is in a component:

Source: 2-prong plug (phono stage in my case, though a PS Audio GCPH did just fine with a 3-prong plug)

Preamp: 3-prong plug

Stereo Amp/ Monoblock amps: 2 prong plugs

I haven't had multiple 3-prong plugged sources in my system, which could complicate things further. The entry-level PS Audio cables are really nice for their removable ground pin. I've not yet been able to achieve absolute silence with a 3-prong plug on a power amp. Interestingly enough, the GCPH had a hum of its own until the internal toroidal transformer was rotated into a more optimal position (must have gotten out of whack) - that hum was a bit different than usual as it varied with system volume level (since it originated inside the box of the source). Good luck!

[luvdunhill]

what tubes are you trying? I'd try to get the gain as low as possible, by tube selection...

[spritzer]

Ok, better then most but R53 will not last much longer and might even be way out of spec by now. It's a 100R so any 2W resistor would work there as 1/2W is clearly not enough.

The preamp output is taken from the headphone jack but the ground is shared with the input sockets so you might be dealing with a ground loop. They are more likely in the P2P wired amps but certainly possible in these amps since Mikhail never could solder worth shit. If you a 1/4" to RCA cable then you test if it is the ground that is causing the hum.

You also have no input selector so the inputs are all connected together. You need a 3P4T switch for that which is dirt cheap and simple enough to wire up. I would also cut the loudness taps on the volume control if there is any danger they are shorting to ground.

[blessingx]

luvdunhill - Been all over the place (including with and without ECC adapter), but I still have a few combos I can try. Not aware of their individual gains, but will definitely exhaust the options and report back. Hum has been pretty consistant so far though.

Spritzer - That for taking a look and giving feedback. Have always been surprised about the lack of input selector. As traditional dynamics aren't used a bunch here I was trying to keep the spending down on that side of things and my skills there are near zero. To work on and keep or sell and start over (or just use GS1)? That is the question.

Thanks again guys.

[luvdunhill]

can you try 3x 6SN7 for example?

[guzziguy]

Nope. Output tubes are limited to 6as7/6080 type tubes.

[spritzer]

You can see the nominal gain of a tube type in the data sheet under u or mu. It's only a guide though as for instance the nominal impedance of a speakers but it's fine for this purpose. The ECC tubes are high gain tubes so and not really something you need for this role.

Edit: A 6SN7 as the input tube would indeed be a good place to start as Marc pointed out.

If you know how to solder then I'd definitely replace that 100R resistor (just make sure all the caps are discharged before doing any work inside the amp) and fitting a simple DPDT toggle switch on the back would give you two inputs and the third pair of RCA's could be wired as a loopout. It would be a great beginners project as some older switches are huge and easy to work with. From the looks of the lead wires this amp has a Plitron transformer and with the birdge rectifiers mounted on the chassis this amp could give you years of trouble free service.

[mrarroyo]

Could the second set of RCAs be a pass-through instead?

[blessingx]

Been out of town. Tried a few more variations and 6SN7 & especially 5998s decreased the buzz, but still there.

[spritzer]

The 5998 is a low mu tube so that would make sense.

[nikongod]

can you try 3x 6SN7 for example?

Edit: A 6SN7 as the input tube would indeed be a good place to start as Marc pointed out.

NOOOOOO!!!!!!!

DO NOT DO IT!!!111

The output stage runs too much current for a 6sn7, even with both sections paralleled.

The 5998 is a low mu tube so that would make sense.

Its a cathode follower. The gain of the output stage is ~1 no mater what tubes you use, output impedance should (theoretically) be lower with 5998.

Im pretty sure the hum comes from the B+ which is not as well filtered as it is on the PPX3. PPX3 has larger resistors (more ohms) in its CRCRC supply.

From screwing around with my Extreme, I would swap the plate resistors in the gain stage with SS CCS's. The good news is it can PROBABLY be done without pulling the PCB. The bad news is my amp hisses and I dont know why (Its not oscilation, I just dont know where the noise starts). It may be that I have noisy cathode LED's (another mod I did to the amp) and I am too lazy to swap them back to resistors & caps to test that theory. Something to note if you do decide to do this is that the amp must NEVER be tube rolled outside of a certain family.

[spritzer]

I've lost count on how many times I've reviewed the Extreme schematic so I should have got that.