Ah, I see, you’re using the line output on the Power Station. I’m using the speaker output, into a mic’ed iso-cab.
This means the hum is present at both the speaker output and the line output, so a bad connection/socket at the speaker output does not seem to be the problem after all. I still don’t understand what happened when I gently wiggled the speaker cable though…??? Why would this reduce the hum, even if it was only temporarily? I made sure I wasn’t moving the jack plug inside the socket when I was doing this. The only thing I can think of is that wiggling the speaker cable may have affected the cable’s resistance or something…? I did this with three different, good quality speaker cables that I know to be in good condition. The result was the same with all three cables.
Anyway, I though you were using a mic’ed cab so I didn’t understand where the Lehle Split would fit into your signal chain. The only possibility seemed to be between your guitar/amp, and I couldn’t understand how that could reduce the hum at all. Please disregard my comment about inserting a Radial Isolator between my guitar/amp. That clearly won’t do anything as the hum is present whether there’s an amp connected to the Power Station or not. I just figured I should give it a try, because it seemed like this had somehow mysteriously reduced the hum with your setup. My mistake.
Using an isolator between Power Station > interface makes a lot more sense, but that’s unfortunately not gonna work with my setup (Power Station > iso-cab).
This may also explain why the hum seems to be slightly worse with my setup. The level of hum I’m getting is really not acceptable for ANY application.
It seems to me that the transformer may be injecting hum into the tubes or other parts of the circuit. Or could it be related to the fan? I mean, the entire unit is practically vibrating at a frequency close to this hum. Actually, it seems to me that the tube circuitry is unusually susceptible to microphonics. Even just tapping the top lid gently will produce a surprising amount of noise. I don’t get anywhere near this level of noise/microphonics when gently tapping any of my other tube gear. I would have to smack the output tubes with a chopstick to get a response like this from any of my other amps (not that I normally do this of course!!!). It’s not the tubes themselves, I already tried replacing them with tubes I know to be in good condition. The result was exactly the same with new tubes, so I ended up re-installing the factory tubes.
One thing that can definitely introduce hum is unbalanced output tubes. I tested the tubes (as well as the replacement tubes I tried) and they were perfectly balanced. The 12AX7 has perfectly balanced triodes, and the 6L6GCs are perfectly balanced as a pair. But could some OTHER part of the tube circuitry be unbalanced? And could this introduce 150 Hz hum, rather than 50 Hz?
I’m using both (as you can see in the measurements as well - see last two sets of measurements, which are mic’d). I actually started this quest and found your topic because I noticed the hum in the speaker cab.
No, I tested disconnecting the fan for a brief moment: didn’t do anything to the hum or the frequency spikes in the spectrum analyser.
Yes, but if you tried changing valves already and it’s also present in the line out (which I would guess its subcircuit is entirely solid state and situated before the power amp section), I’m happy to remove this one from the working hypothesis.
In any case, powering the PS-2A through a Morley Humno seems to have the same effect as the built-in ground lift (i.e.: the hum is still present and toggling the ground lift has no effect anymore).
What is baffling to me that in the measurements of the unbalanced line out with ground lift off, there doesn’t seem to be the same correlation between the 50Hz and 150 Hz as in the other measurements, making me wonder if it really is the third harmonic of the grid or ground loop that we’re hearing/seeing. It could be a measurement outlier of course.
Something surely seems to be resonating at the third harmonic of mains frequency. I’m doubtful it’s caused by a grounding issue (ground loop, bad grounding) or bad valves and I am indeed suspecting the transformers. Going by the microphone measurements above the unit it could be the one on the left, but then again what do I know; I’m not an EE. It could equally be a cap or some other component out of spec, or in fact magnetic coupling between the transformers, which gets worse when a big piece of metal is present, such as the top lid. We could try to further isolate this out by removing the valves and then look at the output on the line out.
But again, not an EE, so before I proceed with experiments I first would like some expert guidance from the Fryette guys. Thomann gracefully extended the return period to allow and work this out with you guys.
It’s on my list to do some debug - I need some magnetic field probes to help debug this properly. But I need to buy or borrow those first! I’ll first try to replicate what you have done then see if I can determine the source - but yes I agree with the conclusions so far (looking at you power transformer).
Steve did a lot of engineering to get the noise floor so low already regarding placement of the major components. It’s a small box with a lot of magnetic field energy all over the place (inductor for the reactive load, power transformer, audio output transformer, cabling carrying high current). But we do like a challenge!
Sounds good, thanks! I have notified Haar Guitars that I will put the return on hold for a couple of weeks, to give you the time to hopefully come up with a solution.
Of course I would be very pleased to come up with a solution in a short amount of time. But I just want to set some expectations. I need to confirm the results, understand the issue and figure out a mitigation that can be implemented, get advice from the shop. That’s quite a lot to achieve.
In a nutshell; don’t let this progress hold up your return, if that is what you want to do! I just want you to enjoy your gear and if the current PS-2 is not cutting it then that is what you should do. That said, I will certainly post progress, so let’s see what happens.
The thing is, when I say “return” I mean “return for repair” rather than “return for refund”, which is probably too late at this point. If this isn’t a fault with my unit specifically, but more of a design issue that affects ALL these units (which seems more and more likely at this point) it won’t be worth the hassle and the shipping costs to return it. Unless a fixable problem has been identified of course.
So basically, if the Power Station has an inherent noise issue that Haar Guitar’s tech won’t be able to fix, there’s no point returning it. If your techs can find a way to fix this, I strongly doubt if Haar Guitars will be able to fix it either. Fully insured shipping to The Netherlands would be close to $200 one way, and at this point it seems like this could just be a waste of money.
If a fixable flaw can’t be identified I guess I’ll just have to find a way to use it safely without the lid. Maybe get a new lid made from a non-magnetic material that can handle heat. But I guess there must be a reason why it wasn’t designed that way to begin with?
Understood, sounds good @Trerik I agree with all of that. I should have aimed my above reply more at @magchiel than you, I think he is waiting for a warrant return.
Can you send me or post your serial numbers so I can identify the exact revision of your power stations?
I managed to do some measurements today. I played a test signal (around 1kHz) into the Line In of my PS-2A, connected the Spkr Out to a dummy 16 Ohm resistive load and recorded the voltage across it.
When in use there is 12dBV – -50dB > 60dB power supply rejection ratio (PSRR) difference between the signal and loudness noise component, which for me is the 50Hz mains hum. This is pretty good! Let me know your serial numbers and we can take it from there.
Regards,
Dan
PS. I have some magnetic field probes! But did not poke around yet nor take the lid off…
I have confirmed with the shop that you models have the latest power supply revision. Here are some items that typically increase hum:
Power tube (bias) running to hot
Power tubes out of balance (push-pull circuit issue)
Cathode leakage in phase splitter/preamp tube
Ground loop: amp and PowerStation must be connected to single wall socket. Moreover, get an extension plug, connect it to one wall socket and then plug all equipment into the single extension.
If still under warranty, you could ask your dealer to for a service. If out of warranty, I would still get in touch with them and ask them for contact details of the repair tech. they use in your country. Usually, repairs outside of US are handled locally.
I have a couple of questions/comments, just to rule out any potentially issues that I can maybe eliminate myself (before I spend a fortune on insured, international shipping):
**-Power tube (bias) running to hot**
What is the correct bias for the output tubes in these units? I have bias probes and can easily check this. I did try to replace the power tubes, but I replaced them with tubes that measured almost exactly the same, so the bias would still be more or less the same. There is no visual indication that the tubes are running too hot though, no redplating or any unusual amount of heat.
**-Power tubes out of balance (push-pull circuit issue)**
This can also be ruled out (or confirmed) if I check the bias. If there is a balance problem it must be the circuit rather then the tubes. The power tubes measure perfectly balanced.
**-Cathode leakage in phase splitter/preamp tube**
I tried replacing the phase splitter tube with a good quality NOS tube with exactly the same result, no difference whatsoever, the hum was still the same.
**-Ground loop: amp and PowerStation must be connected to single wall socket. Moreover, get an extension plug, connect it to one wall socket and then plug all equipment into the single extension.**
Ground loop is not the issue here. I get the exact same hum no matter where I connect the Power Station to power. Whether I use a single wall socket or the same extension as my interface/laptop, the hum is always the same. I even did a test where I ran my laptop on battery power with a usb-powered Behringer interface instead of my RME interface, which means the Power Station was the ONLY equipment connected to mains/ground. The hum was still the same. Also, like we have already established, the ground lift switch on the Power Station does not affect the hum at all (well, maybe MARGINALLY) so a ground loop seems highly unlikely to be the problem here.
The hum is 100% definitely originating from inside the Power Station itself.
I know you have said before, but what country are you in? If you have a Fryette dealer then there is a good chance that it can be done locally.
Thanks for the detailed reply and repeating yourself, I loose track for the details sometimes!
If you don’t have a dealer for a local repair then sending it to Fryette will be the best option, with the unit in hand they can figure out what is going on.
I’m not sure, this in the info that the shop would share with local tech during a repair. I don’t see why we cannot share with you – let me get back to you.
@Trerik do you see any information indicating the target plate current on the inside of the unit? This will normally be a number written on a Fryette sticker saying I_p. For example, on my PS-2A, the plate current is set to 46mA.
I’m in Norway. There’s no Fryette dealer here as far as I can tell…? Which is the reason why I had to purchase it from the EU. I’ve used some cheaper attenuators in the past with less than satisfactory results, so I wanted to get a better one this time, and from the ones that got decent reviews from actual users, the Power Station was actually the easiest one for me to get. Apart from the hum I have been very happy with it. Tonally/dynamically it has certainly been an improvement over the cheaper ones I ended up selling.
Bias probes:
I’ve got one from TAD with four probes. With this one I can measure a full quad simultaneously, which very convenient to rule out any bad tubes when installing a brand new quad. This has actually saved me some potentially costly repairs. The quality of current production tubes is absolutely SHOCKING. On more than one occasion I’ve had ONE tube in a matched/tested quad “run away”. If I had only used one tube on one of the other tubes I might not have noticed this before the faulty tube redplated and BOOOM… I have now started to check brand new tubes on a tube tester before installing them.
The TAD probes can only measure plate current though, so I’ve also got a single probe from Eurotubes that can measure both plate current and plate voltage. On many occasions this can be a lot more convenient than opening up the chassis to use a multimeter when I need to measure the plate voltage.
I don’t have the Power Station in front of me right now. I will check this in the next day or two.
46 mA, isn’t that quite high for a 6L6GC? It will depend on the plate voltage of course… I’m used to dealing with guitar amp bias, and I guess the output stage in the Power Station must be designed a bit different than in a guitar amp?
If might be a good idea then to get in contact with a dealer. It might be cheaper to send it to EU rather than US. They should be able to put you in contact with a local repair person and you can deal directly with them, or maybe via the dealer if easier for an out of warranty repair.
Wow, that is certainly “hotter” than I would bias a guitar amp. I would never go higher than 70% of maximum dissipation with guitar amps. And that would be the absolute maximum I would risk. In fact, I often set the bias slightly lower with certain amps that I know can be electronically unstable (for example vintage Marshalls), to stay on the safe side. But i guess these Power Stations aren’t electronically comparable to guitar amps.
