How about posting a schematic?
Hum is sometimes caused by ground loops of the first and or 2nd B+ filter cap.
Those capacitors return wires should only return to the secondary CT, or bridge as the case may be (transient current has to be a local loop).
Only after those transient currents have been returned, should you connect a separate wire from the 2nd cap return to the amp central ground point.
Then there are the details of how you handle the RCA input connector. The RCA connectors should be insulated from the chassis. The RCA connector returns to the bottom of the input stage cathode circuit. Then from there the cathode circuit is returned to the central ground point.
If there are stages in the amp that has a floating cathode (concertina phase splitter; an SRPP stage, or an unbypassed cathode), then the filament secondary may be transferring noise from other windings of the power transformer. The filament secondary should either be grounded through the center tap, or a pair of 'pseudo center tap' resistors to ground;
or to an elevated voltage in the case of the concertina or SRPP stage, and bypassed with a capacitor.
Attention to details is required to get the hum down to less than 100uV (or perhaps lower since I can not test for less than 100uV with my test equipment).
And all that with my single ended and push pull amps that do not have global negative feedback.
I usually like to do near field listening at my desk or near field listening in the bedroom. I mostly use the living room stereo when I watch DVDs.
And just how much is your hum?
Hum is sometimes caused by ground loops of the first and or 2nd B+ filter cap.
Those capacitors return wires should only return to the secondary CT, or bridge as the case may be (transient current has to be a local loop).
Only after those transient currents have been returned, should you connect a separate wire from the 2nd cap return to the amp central ground point.
Then there are the details of how you handle the RCA input connector. The RCA connectors should be insulated from the chassis. The RCA connector returns to the bottom of the input stage cathode circuit. Then from there the cathode circuit is returned to the central ground point.
If there are stages in the amp that has a floating cathode (concertina phase splitter; an SRPP stage, or an unbypassed cathode), then the filament secondary may be transferring noise from other windings of the power transformer. The filament secondary should either be grounded through the center tap, or a pair of 'pseudo center tap' resistors to ground;
or to an elevated voltage in the case of the concertina or SRPP stage, and bypassed with a capacitor.
Attention to details is required to get the hum down to less than 100uV (or perhaps lower since I can not test for less than 100uV with my test equipment).
And all that with my single ended and push pull amps that do not have global negative feedback.
I usually like to do near field listening at my desk or near field listening in the bedroom. I mostly use the living room stereo when I watch DVDs.
And just how much is your hum?
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I meant that 5R4 requires a 4uF cap instead of 1100uF that Cary has put in the circuit.When I turn the safety switch on, both of the 5R4 tubes flash instantly but they keep working fine since 2004. One week ago I put a 2uF NOS Sprague military capacitor before the 1100 one and a 50ohms/20watts Visay resistor after the 2uF cap. Results= No flashing,B+ 320V instead of 380 and 1 volt output loss on the oscilloscope. If I lower the resistor with a value of 5 to 10 ohms, it will be just fine.I placed the power resistor outside the chassis, below the power transformer because there was not enough space in there.
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6A3: thank you for your answer. You can find the schematic in another post:
https://www.diyaudio.com/forums/att...ilding-cary_audio_design_sla-70_schematic-pdf
My layout is identical to the one in the pdf.
As for your request, I've verified that the RCAs are isolated from chassis.
Since I don't have an oscilloscope (not yet) I've tried to measure the noise with a pretty good quality USB soundcard, connected to the right channel speaker out.
Here are the results:
The wave has been captured with input RCA short-circuited and an 8 Ohm speaker on output.
It is clear that what I was hearing wasn't actually hum but 100Hz ripple (with a tad of hum). The ripple charge/discharge exponential profile is evident.
I believe that I have to work on the DC filtering. Heaters lines do not seem to be the problem. Those big 1100uF capacitors alone cannot give an acceptable (at least for me) ripple filtering.
Metanastis: I do not suffer from rectifiers flashing. The only time I saw that happening was because of my fault, cycling ON-OFF-ON too fast; the heaters was still hot and the rectifiers in conduction, leading to a fast recharge of filter capacitors. At cold poweron there are no flashes at all.
https://www.diyaudio.com/forums/att...ilding-cary_audio_design_sla-70_schematic-pdf
My layout is identical to the one in the pdf.
As for your request, I've verified that the RCAs are isolated from chassis.
Since I don't have an oscilloscope (not yet) I've tried to measure the noise with a pretty good quality USB soundcard, connected to the right channel speaker out.
Here are the results:
The wave has been captured with input RCA short-circuited and an 8 Ohm speaker on output.
It is clear that what I was hearing wasn't actually hum but 100Hz ripple (with a tad of hum). The ripple charge/discharge exponential profile is evident.
I believe that I have to work on the DC filtering. Heaters lines do not seem to be the problem. Those big 1100uF capacitors alone cannot give an acceptable (at least for me) ripple filtering.
Metanastis: I do not suffer from rectifiers flashing. The only time I saw that happening was because of my fault, cycling ON-OFF-ON too fast; the heaters was still hot and the rectifiers in conduction, leading to a fast recharge of filter capacitors. At cold poweron there are no flashes at all.
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Not only inside the amp but also with the connection of the ground on power cable ( when present)
If you have a Ac cable with ground connected to a common panel where other stuff are connected try to remove temporarly the ground of the amp or use a power cable without ground
In case write me a pvt message
Walter
If you have a Ac cable with ground connected to a common panel where other stuff are connected try to remove temporarly the ground of the amp or use a power cable without ground
In case write me a pvt message
Walter
Try shorting Pins # 5 of both output tubes to ground.
If the hum goes away, then the problem is hum is getting into the 6SL7 stages.
The schematic showed either filament windings with a center tapped to ground, or a pair of resistors to ground (pseudo center tap). Make sure you have one or the other of those.
The schematic did not show the capacitance of the 2nd B+ filter cap. What is that value?
I suspect a pair of ground loops:
1. The B+ secondary center tap to the 1100 uF filter cap
2. The input tube's cathodes circuits, and also the 50k Ohm pot in the splitter gain balance circuit.
It is all about how and where you wire the placement of grounds.
# 1. is coupling into # 2.
Lets talk about the placement of grounds. If you look at the schematic, the B+ center tap and standby switch (when closed) are connected to the cathode circuit of the input tube . . .
Wrong! Do not do it that way.
Instead, the switch is supposed to be directly connected to the negative terminal of the 1000uF cap. Then you connect the negative of the 1100uF cap to the negative of the 2nd cap. After those wires have been properly connected (with the ground currents traveling in that order), then a 3rd wire connection is from the negative of the 2nd capacitor to the bottom of the output tubes cathode circuit (the bottom of the fuse that is in the the bypass cap and bias resistors circuit).
In the interest of keeping rectifier tubes happy, both at amp turn on, and during a hot-start (like a momentary power drop-out and return to on),
I would get a 100 Ohm 25 Watt power resistor and connect it between the rectifier outputs, and the 1100uF cap positive terminal.
That 10 Ohm resistor is not large enough.
You will have a slight reduction in power out, but there will be a little less hum to (less hum after you fix the major hum problem which is the ground loops coupling to each other.
What is the voltage at the plates of the output tubes (careful when measuring)?
What is the voltage at the cathodes of the output tubes?
Are you using 6L6, EL34, or what?
Lets get this amp up and running, and putting sweet sounds to your loudspeakers.
If the hum goes away, then the problem is hum is getting into the 6SL7 stages.
The schematic showed either filament windings with a center tapped to ground, or a pair of resistors to ground (pseudo center tap). Make sure you have one or the other of those.
The schematic did not show the capacitance of the 2nd B+ filter cap. What is that value?
I suspect a pair of ground loops:
1. The B+ secondary center tap to the 1100 uF filter cap
2. The input tube's cathodes circuits, and also the 50k Ohm pot in the splitter gain balance circuit.
It is all about how and where you wire the placement of grounds.
# 1. is coupling into # 2.
Lets talk about the placement of grounds. If you look at the schematic, the B+ center tap and standby switch (when closed) are connected to the cathode circuit of the input tube . . .
Wrong! Do not do it that way.
Instead, the switch is supposed to be directly connected to the negative terminal of the 1000uF cap. Then you connect the negative of the 1100uF cap to the negative of the 2nd cap. After those wires have been properly connected (with the ground currents traveling in that order), then a 3rd wire connection is from the negative of the 2nd capacitor to the bottom of the output tubes cathode circuit (the bottom of the fuse that is in the the bypass cap and bias resistors circuit).
In the interest of keeping rectifier tubes happy, both at amp turn on, and during a hot-start (like a momentary power drop-out and return to on),
I would get a 100 Ohm 25 Watt power resistor and connect it between the rectifier outputs, and the 1100uF cap positive terminal.
That 10 Ohm resistor is not large enough.
You will have a slight reduction in power out, but there will be a little less hum to (less hum after you fix the major hum problem which is the ground loops coupling to each other.
What is the voltage at the plates of the output tubes (careful when measuring)?
What is the voltage at the cathodes of the output tubes?
Are you using 6L6, EL34, or what?
Lets get this amp up and running, and putting sweet sounds to your loudspeakers.
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The wave you see in the above post has been taken with the amp alone. RCA inputs shorted. How could I have ground loop in that conditions?
Milanraf,
Check my posts #29 and #30.
You may have been editing your post # 31 at the time.
Another clue: the hum is 100Hz, 2X line frequency.
External ground loops tend to be either 1X line frequency, or very high switcher frequencies.
If the problem is poor filtering of the output tube B+, that would all come out as common mode, and common mode will mostly cancel in the output transformer (but will rob power unnecessarily). The output tap hum from that would be the imperfect balance of the output tubes and imperfect balance of the output transformer.
Let men know what grounding pins 5 and 5 tells us.
Check my posts #29 and #30.
You may have been editing your post # 31 at the time.
Another clue: the hum is 100Hz, 2X line frequency.
External ground loops tend to be either 1X line frequency, or very high switcher frequencies.
If the problem is poor filtering of the output tube B+, that would all come out as common mode, and common mode will mostly cancel in the output transformer (but will rob power unnecessarily). The output tap hum from that would be the imperfect balance of the output tubes and imperfect balance of the output transformer.
Let men know what grounding pins 5 and 5 tells us.
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Now you say you check the amp in stand alone and rca grounded.
With the rca free, without shorting?
What happen?
Regarding the filtering it is ok also if the cap capacity is very high for GZ34.
But you have to consider the age of that caps, maybe they need to be changed.
After the 500 ohm resistor to supply the input tube, which is the value of cap?
In case can be interesting to double the value to 1K and see what happen
Walter
With the rca free, without shorting?
What happen?
Regarding the filtering it is ok also if the cap capacity is very high for GZ34.
But you have to consider the age of that caps, maybe they need to be changed.
After the 500 ohm resistor to supply the input tube, which is the value of cap?
In case can be interesting to double the value to 1K and see what happen
Walter
The noise worsens considerably. The dominant is no more 100Hz but 50Hz. Of course the open input catches environmental noise.
There is no way to read the capacity on the cap. It has been painted by Cary. Actually filter caps do not have covers but are exposed to the outside and Cary covered them with the same paint used for output transformers' cases.
I cannot measure the capacity with my MM right now: battery dead.
Anyway I can make assumptions: the two caps have exactly the same size, the same screw-in terminals and are working at almost the same voltage. May them be the same capacity?
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