Hello all,
I'm new to DIY forums but thought I start off with a question that I can't find a direct answer to, and some photos of my recent tinkering which might help get me out of the moderation loop.
I recently converted an 8303-20 Magnavox single-ended tube amp to a stand alone. I'm using it exclusively for my U-Turn Orbit turntable.
After the cosmetic work and replacing all of the electrolytic capacitors it played beautifully, but had a hum/buzz at the far side of the volume knob (lowest attenuation, about 3/4 volume). I assumed 60hz hum, ground loop from the mostly original wiring, but after replacing the ceramic coupling capacitors (.01uf) with orange drops (.022uf) the noise is gone.
Can ceramic capacitors pick up noise and insert it into the signal path? Could the increase in capacitance be the reason that the hum disappeared?
Thanks for the thoughts.
Sean
I'm new to DIY forums but thought I start off with a question that I can't find a direct answer to, and some photos of my recent tinkering which might help get me out of the moderation loop.
I recently converted an 8303-20 Magnavox single-ended tube amp to a stand alone. I'm using it exclusively for my U-Turn Orbit turntable.
After the cosmetic work and replacing all of the electrolytic capacitors it played beautifully, but had a hum/buzz at the far side of the volume knob (lowest attenuation, about 3/4 volume). I assumed 60hz hum, ground loop from the mostly original wiring, but after replacing the ceramic coupling capacitors (.01uf) with orange drops (.022uf) the noise is gone.
Can ceramic capacitors pick up noise and insert it into the signal path? Could the increase in capacitance be the reason that the hum disappeared?
Thanks for the thoughts.
Sean
You mean there was a buzz coming from the vicinity of the pot, or there was a buzz from the speakers when the pot was in the 3/4 position?scorron said:
Ceramic coupling caps would be unusual in an audio amp, although common in wireless sets. Changing the value is unlikely to affect hum.
Yes, perhaps, if they are high-k ceramics. That is why they are avoided in serious audio amps. However, you have buzz not noise or microphony so unlikely to be what was happening.
Unlikely.
You probably disturbed something in changing the caps, and accidentally made good an intermittent ground contact somewhere. Another possibility is that there was stray capacitive coupling to the old caps but you happened to put the new ones in a better position - this would be consistent with the buzz being worst at just below max volume as this is when the volume pot presents the highest source impedance to the following grid.
Buzz from the speakers. More like hum but with a harsh edge. My guess is the 60 year old ceramic capacitor was picking something up.
Anyway, the problem is solved, I was just curious what fixed it. Not so curious that I'll try to duplicate it.
The orange drops sound great now, I've got no reason to try anything else. Paired with Klipsch speakers, half volume is more than enough power to bother the neighbors in my apartment.
It's an interesting topic: lots of opinions. Seems like the ceramic caps certainly could have been picking up noise or creating a signal themselves due to vibration. Worth experimenting in the future.
Has anybody ever tried to use a very large ceramic capacitor as a pickup, like a piezo mic?
If so, it almost certainly won't be because the cap is old but because of where it is placed. It probably won't be because it is ceramic, but because it is a certain physical size. If it was piezo pickup then it would not peak at just below max volume.scorron said:
My guess is that the old cap happened to be positioned near something carrying hum or ripple, so stray capacitance coupled this into the signal path. The new cap happens to be positioned further away, or is physically smaller.
I don't recall making any other changes or repositioning wiring significantly between the time I noticed the hum was gone and changing the caps, but I suppose it's possible that a small movement is responsible. The position of the capacitors changed slightly, just making sure they were as far from all other wires as possible.
The coupling caps in question connect the plate of the 6EU7 to the grid of the output tube (6BQ5).
The coupling caps in question connect the plate of the 6EU7 to the grid of the output tube (6BQ5).
Ceramic caps do indeed exhibit piezoelectric properties. Some caps are particularly bad, like some high "K" multilayer SMD parts, while others have virtually zero piezo effects. This works in both directions. Mechanical force applied to the cap will generate a voltage across the terminals, and a changing voltage across the cap, or current through the cap will generate a flexing effect, often generating sound in the process.
The voltage generated by vibrating an ordinary disc ceramic cap would be small, in the millivolt range, but it could be audible when connected directly to the grid of a tube.
I was part of the design team on the original iDEN pocket phone (Nextel, Clearnet Mike....). This phone pulsed the 1 watt transmitter at a low frequency rate. After dealing with the tantalum shortage, and exploding tantalum caps, we welcomed the new (1990's) high "K" ceramics from Murata.....Until phones started ticking. It turns out that these caps were highly piezoelectric, and the PC board acted like a speaker cone. Murata cooked us up some new caps that were much quieter, but physically bigger.....New PC board spin.
It is possible that your old ceramic caps were piezoelectric, but probably not the sole source of the hum. If you have an audio signal generator capable of several volts of output, try connecting the cap across the output and spinning the frequency dial. There might be a range of frequencies where the cap "sings." If it sings at a multiple of 60 Hz........
The voltage generated by vibrating an ordinary disc ceramic cap would be small, in the millivolt range, but it could be audible when connected directly to the grid of a tube.
I was part of the design team on the original iDEN pocket phone (Nextel, Clearnet Mike....). This phone pulsed the 1 watt transmitter at a low frequency rate. After dealing with the tantalum shortage, and exploding tantalum caps, we welcomed the new (1990's) high "K" ceramics from Murata.....Until phones started ticking. It turns out that these caps were highly piezoelectric, and the PC board acted like a speaker cone. Murata cooked us up some new caps that were much quieter, but physically bigger.....New PC board spin.
It is possible that your old ceramic caps were piezoelectric, but probably not the sole source of the hum. If you have an audio signal generator capable of several volts of output, try connecting the cap across the output and spinning the frequency dial. There might be a range of frequencies where the cap "sings." If it sings at a multiple of 60 Hz........
Hum which peaks at just below max volume (usually around -6dB) is usually caused by hum pickup on whatever is connected to the pot slider - typically a coupling cap to a valve grid. If the cap you changed is nowhere near the volume pot (in either physical or circuit terms) then it is unlikely to be responsible.
Attached is a photo of one of the coupling caps in question and the original volume pot behind it. This is a photo of the 100% stock configuration, but even after replacing the pot and simplifying the circuit quite a bit, the coupling cap remained in close proximity.
Might be related to cap geometry and orientation and as wavebourn indicated a symptom of oscillation.. It might also be an issue with the pot which I assume was also replaced when you did the rebuild. (Mine apparently was a bit different as there was only a balance control on the amplifier chassis)
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