super-low frequency PSU oscillations + AC coupling with electrolytics - Page 1
Forum section: electronics / beginners

author: pepelevamp
author_rank: Regular Contributor
timestamp: July 18, 2019, 09:54:14 am
content: Hi guys. Riddle me this one.My buddy and I (noob #1 and noob #2) are trying to figure out how to fix this awful noise floor of this audio tube amplifier: There is low frequency oscillation of the signal all by itself when nothing's plugged in. We are seeing these wiggles when simply recording silence on the PC (we know its not the audio interface, that goes past -120dB).We're seeing sub and super-harmonics of 50hz, and at around 4hz its @ -84dBFS. When we turn the gain up & down on the amplifier, the wiggles/oscillations remain at constant amplitude.Thus we suspect power supply in the amp and we suspect it is _after_ signal-amplification somewhere. We suspect something like the mains rectification diodes.Now we don't have much gear. We just got ourselves a wee pocket oscilloscope. We want to probe around & find the oscillations in the circuit. But here is the problem:The power supply inside the amp is 400 volts by the time it touches anything. -84dB wiggles of 400V are 40mV. So we're like ok cool - bring on AC coupling. No wakkas.BUT:We are uncertain if the AC-coupling in our wee DSO-112A pocket oscilloscope is cutting off all the low-frequencies we wanna see. And with a 10x probe to bring the DC into the scope's 50v tolerance, we fear we've attenuated the wiggles to nothing. We're not even certain if -84dB is the true value since its so close to DC that we fear its being attenuated by the AC-coupling of both our audio interface, the AC-coupling of the amplifier, and the DSO-112A pocket scope. My mate is the one doing the legwork here & can't make heads or tails of what the scope shows.So I came up with an idea. Lets make our own AC-coupling which reaches unity gain at 1hz and above. Put this on the nose of our oscilloscope & go probing. But to make this AC-coupling / low cut front-end I need large capacitor values, namely whats provided by an Electrolytic capacitor.My questions to the audience are:* Is it safe to do AC coupling like this with an Electrolytic capacitor (even if we put several in series to achieve 400V tolerance)?* Is Star Trek Picard going to be good?* Is there any credibility to my claims of attenuated dB from the low-cut?* Does anyone have a better approach?The amp is the Sebatron vmp 4000e.Yes we have changed the tubes & it hasn't resolved the problem.Attached is the signals we can see (along with a different amp for comparison. Everything is at unity gain)Thank you guys I know this is a bit of a corky one because its not actually me doing the arm/leg work but my mate is with us via 'tired-after-work' online comms. We are looking at building audio gear ourselves so this is a great exercise handed to us for our learning.Thanks again everyone.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2019, 04:15:02 pm
content: During the vacuum-tube era, many power supplies used a relatively high-inductance filter choke between two filter capacitors (usually less than 100 uF). I have seen resonant frequencies of such circuits below 10 Hz, with Q > 1, that can magnify power-line fluctuations or interact with the amplifier. What values are found in your amp’s power supply?

author: mike_doom
author_rank: Newbie
timestamp: July 19, 2019, 09:16:58 am
content: Hi Tim,I'm the guy with the Sebatron VMP Mic preamp. *See pics below*. I don't believe there are any chokes in this psu that i can see. I'm wondering if I just start replacing parts in the psu like the rectifying diodes and so forth and so on.

author: exe
author_rank: Unknown
timestamp: July 19, 2019, 10:13:20 am
content: Afaik, an RC-snubber reported to be able to cause this. If so, it's fine. The article was on Rod Elliot's website. Can't find it atm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2019, 01:02:47 pm
content: Motorboating is due to a low frequency feedback path in the amplifier circuit. It has little to do with power supply, though the feedback can be introduced through accidentally poor value RC or LC filters between stages (much as a phase shift oscillator works).Just because an amp cost something, or has some weight in its name, means absolutely nothing. The highest end amps are just as often, just as trash as the lowest. This is not my experience directly, but a close friend does tube amp repair (among other things) regularly, and this is his experience.He often fixesdesignfaults in these things, as well as incidental repairs (burned parts, etc.). (Most of which are caused by design faults anyway, i.e., that could've been reduced or eliminated through proper design. Electrolytics placed beside stinking hot vacuum tubes, for example.)(The low level 100Hz ripple also suggests poor design, either in filtering, balance or shielding.)Tim

author: pepelevamp
author_rank: Regular Contributor
timestamp: July 19, 2019, 09:15:44 pm
content: Hello.I appreciate the help - but can we answer the question at hand? About AC-coupling and frequency response attenuation.So we can continue investigating.

author: pepelevamp
author_rank: Regular Contributor
timestamp: July 19, 2019, 09:18:15 pm
content: Quote from: T3sl4co1l on July 19, 2019, 01:02:47 pmDon't give us this. This isn't a helpful contribution. This amp is in use because it has a purpose and we like what it does. We have our own reasons for fixing it - not for money. We also understand it could be a design not an introduced fault. We don't need to be told about something being great because of its brand name or whatever - that is condescending. Appreciate that we have our own reasons and offer us help - or not. But don't be demeaning like this.

author: pepelevamp
author_rank: Regular Contributor
timestamp: July 19, 2019, 09:30:36 pm
content: Quote from: exe on July 19, 2019, 10:13:20 amThank you this is awesome. Gonna go have a look at this. Thanks for your help

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 19, 2019, 10:01:13 pm
content: Quote from: mike_doom on July 19, 2019, 09:16:58 am1. Need a larger overview picture.2. Where does the Brown/Red pair of wires go to / come from?3. Where does the Yellow/Green/Blue set of wires go to / come from?4. What are the circuit traces on the bottom of that power supply board?5. Are there any other power supply circuits?Quote from: pepelevamp on July 19, 2019, 09:18:15 pm6. Tim just saved you a few days of pain.7. If this forum has a fault, it is a tendency to give more information than is requested, or less information than requested, but seldom just the information that is requested.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2019, 10:14:36 pm
content: I would gladly give a more specific assessment, if a schematic can be provided.mike_doom: you can, sure. Diodes are the least likely culprit though. More likely reactive components (like the capacitors in the possible RC filters I alluded to), or failings in the topology (too many capacitors, in a sense; but more correctly, in an unlucky combination), or effectively failings in the topology but caused by failed components (that is, an open or short circuited component effectively changes the circuit).This is why a schematic, and preferably layout and wiring diagrams, are critical information to provide. I guess it would be condescending of me to assume you knew that, and for that I apologize.Tim

author: pepelevamp
author_rank: Regular Contributor
timestamp: July 20, 2019, 01:21:58 am
content: Quote from: Tomorokoshi on July 19, 2019, 10:01:13 pmIts not the electronics tips, its the going on about it brand names of amps or not being worth anything etc. Its irrelevant & condescending.Besides we asked aboutwhether it was safe to construct an AC-coupling with an electrolyticso we could continue investigating, because its complicated trying to relay details over the forum like this.My gut feeling is no. But it wont be till monday or so until we have an opportunity to get some help with it by someone who knows more about power electronics and has more capable gear. But we are still weary of even high-end scopes AC coupling frequency response which is why we asked.

author: mike_doom
author_rank: Newbie
timestamp: July 20, 2019, 04:41:51 am
content: I wish I could provide a schematic but i have been unable to get one from the manufacturer or anywhere for that matter. *See a zoom out pic. I'll try get some more information on the components.

author: mike_doom
author_rank: Newbie
timestamp: July 20, 2019, 04:53:11 am
content: 1*see new pic2 led next to power switch on front panel3 an input transformer I believe which is switchable 230v/120v etc4 ill have to get back to you on that.5 no just that one there-The black molex socket provides power to different rails on the main mic pre-amp board. *labeled voltages Ive currently unplugged the rainbow coloured cable going to the main board.-Blue and brown are attached to power switch on front panel-the small transformer *as shown in pic here which is just after the main input transformer is this guy here:https://www.altronics.com.au/p/m7064a-powertran-5va-12+12v-pcb-transformer/

author: mike_doom
author_rank: Newbie
timestamp: July 20, 2019, 04:55:05 am
content: see labeled rails

author: mike_doom
author_rank: Newbie
timestamp: July 20, 2019, 08:02:25 am
content: I Should also make note that those sub harmonics still show even when the 12At7s tubes are removed.