Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: ScottAstroNut on February 16, 2021, 10:15:57 PM
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Having just newly completed my BeePre 2, with the generous help of another forum member, I am pleased with the quality of sound produced. Except that I, too, am experiencing a 120 Hz hum. Rather than hijack the ongoing thread on this same topic, I thought that I would start this new one.
I have followed the other thread closely, and have tried the fixes proposed there. I have checked the transformer assembly and connection. I have reflowed the grounds around the circuit. I have tested the unit for hum both with and without sources attached. I have tried to hear a difference switching the balance from left to right, with no effect at all. The hum does grow louder as the volume is turned up. I've checked, and the hum is in the preamp, not the amplifier (I am using both a Kaiju for speakers and a S.E.X. for headphones). I am running all my equipment through a Panamax power manager and have never had a problem with outlets before.
I've had no luck in finding a cause for the hum, which I am very sure is 120 Hz, not 60 Hz.
Ideas?
Thanks!
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We sent out a box for the first BeePre so it can be returned to me to have a look at. I have a sneaking suspicion that it will work perfectly here, but it's always a good idea to check!
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Hi, Paul! Thanks for the response.
Are you suggesting that I should request the repair service for my BeePre 2 and send it in? Or, are you suggesting that I wait until you've had the chance to look at the first BeePre before making such a decision?
I am willing to send the unit in if you think that best.
Scott
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PB is out of his office for a few days. But I'm pretty sure he means let him get the other preamp in and see what is going on. Meantime I'll suggest plugging your BeePre directly into a wall outlet instead of the power generator to see if it makes a difference.
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Thanks! I will give that a try.
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I may have found a clue to the hum. I changed the tubes I had been using with another set of new tubes and the hum seems to have quieted down significantly. The hum is still there, but only when I turn the volume up beyond normal listening levels. That's using either headphones or speakers, although the hum with speakers is evident only when the volume is turned up close to maximum.
I would say that with my BeePre 2 the hum is something I can live with, although I must say I'm a little surprised that it is there at all. I would think that with the well regulated DC filament supply, one would hear very little noise at all.
I must say, however that the BeePre 2 sounds as good as the original version it is replacing. Very musical with great extension both in bass and treble. Can't wait to see what upgrades you will be coming out with!
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There will indeed be some noise if you turn the preamp all the way up, and especially if there's nothing connected to the input you're on.
If this is somehow tube related, it may help to let the tubes cook in for a few days, though on the exterior this seems unlikely.
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I wonder if the hum is caused by mechanical vibration from the power transformer. In an earlier thread, PB mentioned that the BeePre 2 transformer is the same as the Kaiju. IIRC, there was a batch of Kaiju transformers that had some manufacturing issue that resulted in buzzing from vibration. If this issue has resurfaced in the BeePre 2, the transformer vibration may be amplified via microphonics.
Can you detect any vibrations from the transformer or chassis?
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I wonder if the hum is caused by mechanical vibration from the power transformer.
This was an issue that popped up in 2018 and was resolved with some manufacturing changes.
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I think I can hear the transformer humming, but only very, very slightly.
The hum is not intrusive while music is playing, but it is present in between tracks. Not that loud, but it is there at normal listening levels. Barely.
Scott
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I should clarify that I hear the hum only when using headphones. I am using a S.E.X. 3.0 as my headphone amp with a pair of Audeze LCD-XC headphones. When using sensitive speakers I don't hear the hum at normal listening volumes.
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Do you have the SEX turned all the way up?
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Is this perhaps just normal operation for the new BeePre 2? I don't remember hearing this with the BeePre 1.
One thing that has improved greatly are the microphonics.
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The noise floor of the BeePre 2 will be at worst (with the volume turned all the way up) as bad as the BeePre 1. The noise and microphonics scale down as you turn the volume pot down.
If you are feeding your BeePre into your SEX amp and you don't have the volume control all the way up on the SEX amp, that would potentially cause some issues.
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Hi Paul, I have tried it with the S.E.X. turned all the way up and with it turned up only part way. I couldn't hear a difference.
One thing I will say is that my S.E.X. is dead quiet. With no active sources I can turn it all the way up and not hear any hum.
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With the BeePre and SEX cabled together, is the noise there with the BeePre turned all the way down?
How about if you touch a bare wire between the chassis plate of the SEX and the BeePre?
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With the S.E.X. turned all the way up I hear the hum much more intensely. It then becomes clearly audible at all but the lowest listening volumes. If the S.E.X. is turned all the way up, then then I am barely turning up the volume knob much at all. In that region I notice that the volume pot becomes very imbalanced.
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The hum goes away when the volume pot is all the way down.
Connecting the two chassis plates together did nothing.
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Might this be a problem with the high voltage supply? Would it help if I re-flowed the joints in the high voltage supply?
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It could be an issue with the high voltage supply. With the SEX amp turned up all the way, the noise floor of the BeePre shouldn't be a problem.
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I will re-flow the power supply and get back to you later.
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I just re-flowed all of the solder joints in the HV supply. Absolutely no change at all.
Next idea?
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Something new... comparing the "hum" to test signals, I don't think that the hum is 120Hz. Something closer to 200 to 240Hz. And more of a buzz than a hum.
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Discerning the frequency of noise is not easy. With my current noise problem I listened to 60 and 120 Hz signals on two different online tone generators for an hour, going back and forth, and at the end I didn’t know what the hell I was hearing. I think it’s the kind of thing that requires a lot of experience to determine. It is said that 60Hz is hum and 120 Hz has a buzz but when I listened to the tone generator samples both frequencies had some buzz, 120 just had a slightly more buzzy character. The other thing that I wonder about is: is the AC current coming out of our plugs a perfect sine wave? If it isn’t, any anomalies in the wave shape could produce a buzzy sound at any frequency, I guess. Scott, I know you and PB will figure this out. I am watching with great interest.
Jamie
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I’m following too very closely. I’m shipping my beepre2 to PB as soon as I get the box.
Best
AB
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What you need is an oscilloscope or a spectrum analyzer, if you want to really know the character of the hum/buzz. PB has those tools, so we may know more soon.
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Rather than playing tones, I've had decent luck with using an app called spectroid that displays a live fft plot. Last time I had noise to diagnose I couldn't tell by ear, but with the app it showed nearly equal peaks at 60 and 120 hz.
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I have access to test equipment in my lab at school. I will grab an oscilloscope and frequency generator to see what I can see. I can create a spectrum analyzer if needed using a voltage probe I can hook up to a computer. I had been holding off in the hopes that there might be an easy solution to this problem. I might bring in the help of a good friend who is a retired acoustic engineer who used to design speakers and amplifiers. My background is in electrical engineering and physics, but my area of expertise is digital logic systems. This analog, tube stuff is very different from that! I am having fun learning this new/old stuff.
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Am l looking at the output using an oscilloscope and the signal is confusing. Not very clean at all. My friend the audio engineer is coming over in a little while. Perhaps he can make sense of what I am seeing.
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My friend the audio engineer and I explored the BeePre in some more detail. With an oscilloscope on the output, some irregularities that would create a humming or buzzing noise were clearly seen. However, I did not get good photos of the traces. We then turned the BeePre over in its test stand to probe further the high voltage supply and the filament regulators. However, before we could take any measurements, and immediately upon powering up the BeePre 2, there was smoke and one of the 270 ohm resistors in the high voltage supply died. I've attached a photo showing which resistor blew. As far as we could tell, nothing we did would have resulted in the death of that resistor. Nothing about the circuit was modified in any way. All I did was turn the power on. That particular 270 ohm resistor was extremely hot to the touch, whereas none of the others were.
Perhaps just a bad component?
At this point I am bewildered.
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Perhaps just a bad component?
No, that's not how this works unfortunately.
A resistor will smoke and blow up because you drew excessive current through it. I have circled in green one possible cause. If the lead circled and the middle terminal strip lug circled touch each other, you'll send that power supply dropping resistor up in smoke.
The leads circled in red should be bent straight up so there's no risk of them touching the chassis.
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Thanks, Paul,
It's possible that there could have been a short where you have circled the green, and I had thought of that, too, but upon close inspection I saw a gap between the grounded terminal and the lead off the 22 ohm resistor. I also didn't see a spark, just saw smoke. But again, I could be mistaken.
Regardless, the 270 ohm resistor is toast.
Will do on the other circled red items. That was my mistake. Being careless.
What would you recommend at this point? Make the adjustments you suggested and replace the 270 ohm resistor?
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I would make your amp look as much like the one in the manual as possible. I wouldn't leave that resistor lead like you have it, that may continue to smoke your power supply randomly if you don't address it.
I would replace the 270 ohm 5W resistor after you've made the adjustments and give it another shot.
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Will do.
Do you recommend I contact Eileen for a replacement or simply purchase one from one of the electrical supply companies?
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Whichever one is more convenient for you. For a single part order, it tends to be far less expensive to just use us.
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Great. Will contact Eileen.
Do you think that the proximity of the 22 ohm wire to that ground lug could have been a source of the hum?
Thanks for your help!
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I don't really know, it's certainly worth the effort to correct that area and see if that helps.
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I was mulling this over while doing some other work, and that short wouldn't blow out that resistor, but shorting that connection to ground will remove one of the filtering elements in the power supply and increase the noise floor. This wouldn't be a huge increase in noise, but it would be audible, and it would come with ever so slightly higher than expected voltages.
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Paul, I was thinking about that, too. The combined resistor/capacitor/diode assembly is essentially both a high pass/low pass filter, isn't it? So in one sense it is already a short to ground as seen at terminal 36? Shorting the 22 ohm resistor to terminal 35 therefore shouldn't be able to take out the 270 ohm resistor between terminals 36 and 39. In which case, what did? All I did was turn the power on to a circuit that was already working, albeit with a slight hum.
Also, the readings I measured at terminals 6 and 16 in the last voltage test measured lower than the expected 100VDC by 7%, not higher.
What I can say is that the oscilloscope trace of the output with no input signal was not clean. My friend the audio engineer helped me with the oscilloscope readings. What we saw was baffling enough to him that he wanted to trace the signals through the circuit. That was what we were getting ready to do when the 270 ohm resistor blew. We hadn't touched anything in the circuit, however, before the resistor went up in smoke, so it wasn't caused by a slipped probe. All I had done was invert the circuit in the building jig I use during construction and testing. Rather than use the wooden base provided as a building support, my jig allows me to invert the circuit with tubes and transformer attached without bottoming out. Just more convenient and secure.
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If you flip the amp over and a resistor goes up in smoke, something may have been really close to touching where it shouldn't have, or barely touching but then touching too much when you flipped it over, so in a way you have a more solid hint at what's going.
I repaired an old Pilot EL84 amp that relied on ground contacts built into the 9 pin sockets. It would only work upside-down, when you flipped it back over the sockets were loose enough that those connections would open up and the amp would do strange things. This isn't all that related to what you're experiencing, but similar in the sense that you have a hint now that something may be moving around more than it should.
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My next question, then, is how to go about diagnosing that without blowing another resistor once I get a replacement and have repaired the circuit?
As I mentioned, at one time I did electrical engineering, but it was all digital logic systems. I did my circuit design on breadboards or perfboards with lots of wire wrap. If I mis-wired something, usually it would not go up in smoke! I did learn about regulated power supplies, but only to the extent that I could build one that would provide the 6 to 12 VDC I needed for my digital IC's. You analog (and especially you audio tube-analog!) guys work in an entirely different world. One that I am very much enjoying getting to know a bit better by building your kits. I have a lot of respect for the expertise and experience you need to do your kind of job.
Having circuits that work only upside down or right side up is amusing to me! I love EE war stories!
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I think you have to look at this from two different perspectives. Yes, you can analyze the circuit and peruse the schematic to see if you can imagine a scenario where something that isn't exactly like the circuit drawing is causing the issue. But from a practical standpoint it's a lot easier to say "crap, something shorted" and look at the amp for signs. There are various ways this can happen. Gravity - when you flip the amp over something moves and shorts out. Gravity, part 2 - when you flip the amp over an offcut piece of wire that was hiding somewhere falls down and shorts something. Time - there was a miswire from the beginning that finally made the resistor give out from being overloaded.
If I was stuck I would measure across each filter cap and look for a low resistance reading to ground. If I found one I would start to investigate the components nearby. When a big resistor smokes there is a short somewhere. Miswire, leads touching things they shouldn't, caps/diodes in backwards - the list of possibilities is not too long.
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Hi, Doc! Thanks for the helpful insights.
I did check the caps in the HV supply, and did find that the 220 microfarad cap closest to the transformer has a very low resistance reading. So I guess I am going to need a new capacitor, then. I don't immediately see anything amiss in the vicinity of the cap, however. The diodes look to be OK. None appear to be oriented the wrong way. The other three 270 ohm resistors don't appear damaged when measured. The other two large caps seem fine.
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I did check the caps in the HV supply, and did find that the 220 microfarad cap closest to the transformer has a very low resistance reading. So I guess I am going to need a new capacitor, then.
No, lift one leg out of the circuit then check again. If the cap is still soldered into the amp, you can't say much about its condition. The caps we provide are of exceptional quality, so it would be surprising to randomly get a dead one.
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That seems like a really weird place for a resistor to fail: the last resistor in the return (-ve) rail of the B+ supply. Granted, I don't know much about this stuff, but I can't think of a way that could happen that doesn't involve an obvious serious wiring error - and I don't see one in your photo.
So a shot in the dark: how much physical tension do you have on those white cement capacitors? I can imagine that the internal connection between a lead and the internal wire coil (or whatever resistive structure is inside) might crack if you soldered one lead and then pulled the other really tightly before soldering it -- or installed the resistor suspended by the leads and then tried to push the resistor towards the chassis. A cracked/flaky connection inside the resistor might easily overheat with even normal current running through it and then pop/smoke when it inevitably fails. I can also imagine it being noisy when it was still capable of passing current. Just a thought.
cheers, Derek
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Oh, right about removing the cap from the circuit to test it. Given the lateness of the hour here and the fact that I am tired, I think I will leave that for tomorrow.
The caps are of exceptional quality, and they worked just fine before, so they weren't dead on arrival.
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When you get the replacement resistor, can you remove the 22 ohm resistor with the parts installed on it and try putting it between 33L and 35L?
-PB
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Sure. No problem.
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I think I found it! The reason for the blown resistor, that is. Upon careful inspection of the HV supply, I found a solder run that looks like it shorts one of the 270 ohm resistors to ground. It looks to me like the excess solder barely touches the chassis, so that if the chassis were placed right side up, gravity might very well pull the assembly down enough that the solder run doesn't short. Upon inverting the chassis plate, gravity pulls the solder run to the chassis and a short occurs. This must have occurred when I re-flowed all the solder joints in the high voltage supply, and simply didn't catch this bad joint. Dolt!
I have also made the change Paul suggested of moving the 22 ohm resistor to terminals 33 and 35. We shall see what happens once I get a replacement 270 ohm resistor.
I haven't received a reply for a parts request I sent to [email protected], so have gone ahead and ordered parts from Mouser.
Scott
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... so have gone ahead and ordered parts from Mouser.
Scott - Mouser is way behind on orders b/c of the Texas storm. I have an order from Feb 12 that still hasn't shipped, and two more in queue that haven't even been processed yet. If you want something fairly quickly, I suggest a local electronics hobbyist store. Or Digi-Key. And then cancel the Mouser order.
And congrats on potentially having found the cause of the problem.
cheers, Derek
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In your photo I see UF4007 leads that look like they will short out the two power transformer terminals. I would provide some separation for those while you're waiting.
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I think that the diode leads looking to be too close together may be a perspective thing with the photo. Here's a photo from the other side showing that the leads are not near each other and are constrained well enough, I think, that there is no chance of them touching. Please let me know if this looks OK. If not, then I will definitely make adjustments. Thanks!
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Derek, thanks for the heads-up on Mouser. I've gone ahead and placed an order with Digi-Key for the parts.
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I have a question on how to proceed once I get the replacement 270 ohm wire wound resistor...
Once I have the part in hand, do you recommend that I simply insert the new resistor and "give it a go?" Or, do you think a more deliberate approach would be better? For example, should I disconnect the UF4007 diodes from the transformer, then test the transformer output to make sure it is OK. Then, reattach the diodes, but disconnect the HV leads to the rest of the circuit and test the output of the HV supply before attaching the rest of the circuit? This latter approach seems safer to me, but more work. I love soldering parts. I am not so fond of de-soldering!
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That's some good photo work, those look great.
You will need to put the 270 ohm resistor back in place and fire the BeePre back up.
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Well, the BeePre 2 is back up and running. I replaced the 270 ohm resistor in the HV power supply after fixing the other minor issues and sweet music is playing again. Unfortunately, the hum/buzz issue is still present. I think it is slightly lessened by having moved the 22 ohm wire wound resistor to its new location as requested, but it is still there. Slightly, but still audible at normal listening levels through headphones using my S.E.X. 3.0 as the headphone amp. I wouldn't be too concerned about this issue except that my Moreplay, when used in the same location, is extremely quiet. I am pretty sure at this point that the BeePre 2 is correctly constructed. Also, it doesn't seem to matter which headphones I am using, from 32 ohm Grado PS 500e's to 600 ohm Beyerdynamic DT880's.
Thoughts? Suggestions? Again, this is a non-issue when listening with speakers. Thanks.
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On a positive note, I do have to say that the overall sound quality of the BeePre 2 is outstanding. Very sweet and musical while also possessing amazing clarity and resolution. Makes me wonder what improvements I might hear if using a Mainline instead of the S.E.X.?
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I did the changes suggested by Paul and I found myself in your same situation. Hum still there but to a slightly less extent.
I shipped the beepre2 to Paul yesterday. Hopefully he can find the problem.
Best
AB
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It's somewhat possible that the BeePre just isn't suitable to sit in front of a headphone amp. It would not surprise me at all seeing that you have Grado headphones in your lineup which are incredibly sensitive.
Yes, a Moreplay will have a lower noise floor since the 6V6 is an indirectly heated tube.
If you use the BeePre as a preamp in front of a power amp driving speakers, how well does that work?
The upcoming upgrades will drop the noise floor a bit more as well.
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Paul, the BeePre 2 sits in front of my Kaiju just fine. No hum at normal listening volumes. Sounds great.
Too bad to hear that the BeePre won't work with a headphone amp. I listen to headphones much more often than I listen to speakers. Guess I'll keep using my Moreplay.
Thanks for all your help.
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I was also wondering... any word yet on when the upgrades will be released? If they drop the noise floor even some, that would really help with headphones.
Thanks!
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You could add a tape output to your BeePre and send that to your SEX amp.
Even the SEX amp required a ton of deliberate work to be quiet enough for IEM and Grado use. I did not spend any time putting a power amp in front of the BeePre and plugging Grados into it. It's fortunate that a Moreplay will work under those conditions, but even that is mostly accidental.
I'm currently working through the first upgrade manual on my own BeePre to double check the steps.
-PB
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Paul, no problem at all here. I very much appreciate the effort you and the others go through in putting these kits together for us. Testing under all circumstances isn't possible. The Moreplay will work fine for now with the SEX. The SEX is a veritable work of art! It's an amazing amplifier!
Thanks again!
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If you want to add the tape output, let me know. That would let you send the signal to your SEX amp without going through the BeePre active circuitry.
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Paul, I'd love to know about adding a tape out. Please advise.
Thanks!
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The center four pins on the selector switch are a tape output. You would send those to the back of the chassis and you could add some conventional RCA jacks in the input transformer holes. It is a little tricky to fit extra wires in the selector switch holes, but it's doable.
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Paul, while I think about it... another option I was curious about was that of adding a second set of single-ended RCA outputs so that a subwoofer could be used with the BeePre. I'm assuming that this wouldn't be difficult to do, either? The Moreplay does it quite easily.
In fact, might it not be too difficult to add both a tape out and a second set of SE RCA outs?
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Paul, I see exactly what to do to implement the tape output. Thanks!
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Paul, instead of attaching directly to the selector switch, couldn't I simply take the leads off the five-lug terminal strip in between the selector switch and the balance pot? Seems like that would be easier.
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Sure, you can go from that strip also.
I would hold off on the second set of outputs, as that gets populated in the second upgrade. A set of Y cables will split the outputs.
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Hmmm... second upgrade? Sounds intriguing! I will hold off on implementing a second SE output.
Thanks!