I have a vintage Dynaco Stereo 400 that exhibits an annoying hum. It's only noticeable at quiescent (idle) and at low listening volumes. I don't know the frequency of the hum.
If I measure the power supply output, the rail is 150 volts DC with an AC component of 160 millivolts at 120 hertz. So the ripple is 0.106%.
Questions:
1) Is the magnitude of this ripple enough to cause the hum I'm hearing?
2) What is an acceptable level of ripple for an solid state audio amplifier?
3) If I replace the large capacitors, will this cure the hum?
Thank you all.
Gary
If I measure the power supply output, the rail is 150 volts DC with an AC component of 160 millivolts at 120 hertz. So the ripple is 0.106%.
Questions:
1) Is the magnitude of this ripple enough to cause the hum I'm hearing?
2) What is an acceptable level of ripple for an solid state audio amplifier?
3) If I replace the large capacitors, will this cure the hum?
Thank you all.
Gary
The ripple you report is typical. Replacing the power supply caps won't have much, if any effect, on the hum delivered to the speakers.
RMS value of the noise at the speaker terminals, assuming mostly hum...
1 mV RMS is about the upper end of acceptable, though kind of noisy
0.25 mV RMS is pretty quiet.
0.1 mV RMS and below is very good...
What is the hum like if no inputs are connected? In that state, does it change if you fiddle with the level controls? Are both channels the same amount of hum? Does the hum change in either left side ON or right side ON position?
Is the hum electrical, or actually mechanical buzzing of the transformer?
RMS value of the noise at the speaker terminals, assuming mostly hum...
1 mV RMS is about the upper end of acceptable, though kind of noisy
0.25 mV RMS is pretty quiet.
0.1 mV RMS and below is very good...
What is the hum like if no inputs are connected? In that state, does it change if you fiddle with the level controls? Are both channels the same amount of hum? Does the hum change in either left side ON or right side ON position?
Is the hum electrical, or actually mechanical buzzing of the transformer?
djoffe - Thanks for the prompt reply! To answer your questions...
The hum is always present and remains constant in both channels regardless of input signal, level control settings or power switch ON position (left or right). The transformer is not buzzing, it's strictly an electrical hum, coming only from the speakers.
I have checked for loose ground connections and cannot find any.
Also - and this may be an unrelated problem - when operating at very high power levels and during certain loud musical passages, the speaker relay opens and will not reclose. It didn't do this in the past.
Thanks in advance for any advice.
The hum is always present and remains constant in both channels regardless of input signal, level control settings or power switch ON position (left or right). The transformer is not buzzing, it's strictly an electrical hum, coming only from the speakers.
I have checked for loose ground connections and cannot find any.
Also - and this may be an unrelated problem - when operating at very high power levels and during certain loud musical passages, the speaker relay opens and will not reclose. It didn't do this in the past.
Thanks in advance for any advice.
need to work out if it's mains 50/60Hz or rectified 100/120Hz hum. Mobile phone app, audacity freeware or others to measure.
Do you hear the hum when seated in your normal listening position? If hum is only noticeable within say 1meter of spkrs then I'd live with it (with such a vintage amp)
What is the amplitude of the hum at the spkr posts(oscilloscope)
Do you hear the hum when seated in your normal listening position? If hum is only noticeable within say 1meter of spkrs then I'd live with it (with such a vintage amp)
What is the amplitude of the hum at the spkr posts(oscilloscope)
I haven't worked on a Dynaco 400, but have upgrade a few other pieces from Dynaco, such as Stereo 120, PAT-4 and FM-5. All of these models have power supply noise on the outputs because of poor earthing layouts.
The circuit boards are designed as though all earth tracks are equipotential, but if current is flowing down an earth track there must be a voltage drop along that track. The voltage drop will mimic the ripple currents of the power supply. Where the track layout is poor, audio grounds can be infected with the ripple current induced noise.
In the case of the Stereo 200, implementing a proper star earth layout where all ground currents are returned to a common point, being the joint between the earth (chassis) ground the the filter capacitors solved the noise problem completely. It's a bit more complicated with the PAT-4 and FM-5 because you need to cut some PCB tracks and replace with wire straps in place to get noise currents away from audio grounds.
The circuit boards are designed as though all earth tracks are equipotential, but if current is flowing down an earth track there must be a voltage drop along that track. The voltage drop will mimic the ripple currents of the power supply. Where the track layout is poor, audio grounds can be infected with the ripple current induced noise.
In the case of the Stereo 200, implementing a proper star earth layout where all ground currents are returned to a common point, being the joint between the earth (chassis) ground the the filter capacitors solved the noise problem completely. It's a bit more complicated with the PAT-4 and FM-5 because you need to cut some PCB tracks and replace with wire straps in place to get noise currents away from audio grounds.
To reply to some of these posts...
The speaker hum is not subtle. It's noticeable from anywhere in the room, even in an adjacent room.
I did rework the terminals to the large caps and this didn't have any effect.
If I remove the input cables and install RCA shorting plugs, there's no change in the hum. I still hear it and it appears to be the same amplitude.
I will measure the signal at the speaker terminals to see what the frequency is and post again.
The speaker hum is not subtle. It's noticeable from anywhere in the room, even in an adjacent room.
I did rework the terminals to the large caps and this didn't have any effect.
If I remove the input cables and install RCA shorting plugs, there's no change in the hum. I still hear it and it appears to be the same amplitude.
I will measure the signal at the speaker terminals to see what the frequency is and post again.
try to remove the wire connect to pin 1 (input pin) of the main amp board, to isolate the signal from the input filters and protection board ,
see if this would lower the hum.
check R307, R308, Zeners D304, D305, C302, C303, see if they are in good condition.
wish you solve the hum problem.
see if this would lower the hum.
check R307, R308, Zeners D304, D305, C302, C303, see if they are in good condition.
wish you solve the hum problem.
Nothing wrong with post 10. All above are high failure parts.
This unit is over 40 years old. Rubber cap seal deterioration is a possibility.
2 ways to check the mains caps. C304 305 306 307 One is buy a Peak ESR meter, about $120 plus freight. After disconnecting, compare esr to tables in the manual.
Other way, maybe cheaper, is to check the maximum power out of the amp. Buy or assemble 8 ohm resistors sufficient to handle 200 w. Better a pair of them to check the at at a full 200 W. If your speakers will handle 200 W each for a while, use those. (Many home speakers will not). Buy a $25 analog VOM. DVM lie a lot on music frequencies. Energize with a compressed radio station, say a pop rock station. Turn up to clipping, a funny dull sound. Back off a little. Listen to sound on a 32 ohm speaker or similar parallel to 8 ohm speaker. Measure AC volts on 8 ohm resistor right before clipping. P=(V^2)/Z where Z is speaker or resistor impedance. P<200 w/ch, you have dried up mains caps. When those go, I usually change all the other e-caps, doing 2 at a time then checking sound to make sure I didn't inject a mistake. If sound is worse, I know just where the problem is, what I just did.
I would not buy extremely expensive screw terminal mains caps. I assemble 10000 or 20000 uf caps out of 3300 or 4700 uf snap in caps, glued to a piece of Nema CE garolite. I bus the caps together with 18 ga wire, then connect as normally. Board can be screwed to case with #6 machine screws, or 4 mm if you are metric. I use elastic stop nuts so they don't come unscrewed.
I haven't found my ST120 to hum much by contrast with johnmath. Neither did my ST70. I had sensitive speakers, 101 db 1w1m so that should be a good test. I listen at 1 w usual with 70 w peaks occasionally in classical music.
This unit is over 40 years old. Rubber cap seal deterioration is a possibility.
2 ways to check the mains caps. C304 305 306 307 One is buy a Peak ESR meter, about $120 plus freight. After disconnecting, compare esr to tables in the manual.
Other way, maybe cheaper, is to check the maximum power out of the amp. Buy or assemble 8 ohm resistors sufficient to handle 200 w. Better a pair of them to check the at at a full 200 W. If your speakers will handle 200 W each for a while, use those. (Many home speakers will not). Buy a $25 analog VOM. DVM lie a lot on music frequencies. Energize with a compressed radio station, say a pop rock station. Turn up to clipping, a funny dull sound. Back off a little. Listen to sound on a 32 ohm speaker or similar parallel to 8 ohm speaker. Measure AC volts on 8 ohm resistor right before clipping. P=(V^2)/Z where Z is speaker or resistor impedance. P<200 w/ch, you have dried up mains caps. When those go, I usually change all the other e-caps, doing 2 at a time then checking sound to make sure I didn't inject a mistake. If sound is worse, I know just where the problem is, what I just did.
I would not buy extremely expensive screw terminal mains caps. I assemble 10000 or 20000 uf caps out of 3300 or 4700 uf snap in caps, glued to a piece of Nema CE garolite. I bus the caps together with 18 ga wire, then connect as normally. Board can be screwed to case with #6 machine screws, or 4 mm if you are metric. I use elastic stop nuts so they don't come unscrewed.
I haven't found my ST120 to hum much by contrast with johnmath. Neither did my ST70. I had sensitive speakers, 101 db 1w1m so that should be a good test. I listen at 1 w usual with 70 w peaks occasionally in classical music.
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disconnecting the input wire between the main amp and the filters is intended to find the humming source.
If the hum is lower then the hum comes from the filters in front.
if the hum remains the same we narrow down the humming source area to the main amp.
It is good to check the values of the resistors, solder joints. bad connections, pads, etc.
If the hum is lower then the hum comes from the filters in front.
if the hum remains the same we narrow down the humming source area to the main amp.
It is good to check the values of the resistors, solder joints. bad connections, pads, etc.