Just found this on "shorting RCA plugs".
Building an RCA Input Shorting Plug -
Is this the best way? Never heard anyone suggest a 470ohm resistor used.
Building an RCA Input Shorting Plug -
Is this the best way? Never heard anyone suggest a 470ohm resistor used.
Ideally the wire should be minimal in length. I just used the legs of a resistor to short them right on the pcb.
My "audiophile" LM3886 approach
Shorted with a 470-ohm resistor and no hum on that channel.
Also check the RCA's and they are isolated from the chassis.
I've seen where some put a zobel at the input, would that be the next thing to try? If so what values for the Cap and Resistor?
Edit-also same offset with and w/o the input shorted -3mV right and -1mV on left channel.
Shorted with a 470-ohm resistor and no hum on that channel.
Also check the RCA's and they are isolated from the chassis.
I've seen where some put a zobel at the input, would that be the next thing to try? If so what values for the Cap and Resistor?
Edit-also same offset with and w/o the input shorted -3mV right and -1mV on left channel.
-1 and -3 offset is very good. If when you short the inputs the channel is silent and when you have a source connected it's silent why would you add the zobal?
Do you intend to leave the amp on with no inputs connected?
Do you intend to leave the amp on with no inputs connected?
Good point Troystg, I will tidy the wires a little more and call it done and just enjoy the music.
Let warm up.
I have built a few versions. They seem to sound a little better after being powered up for a while. Overnight is great but I think a hour or so is also good.
The 1 - 3 mV dc offset is also fine. Try measuring the AC voltage. This is your hum. The layout of the wiring will make a difference.
Look at Mauro’s orginal wiring diagram. It is shown in the build sheets. He was very detailed here. This was to minimize hum pickup from power transformer leads.
Lastly he did not recommend using over 0.33 ufd for the input coupling cap. Said it could lead to low frequency instability. Lectured me several times on this. I replaced my 1 ufd caps with 0.47 ufd to make sure.
Now use a four channel system and crossover at 65 Hz so this may not be a factor. Helps with the power output limitations also.
I have built a few versions. They seem to sound a little better after being powered up for a while. Overnight is great but I think a hour or so is also good.
The 1 - 3 mV dc offset is also fine. Try measuring the AC voltage. This is your hum. The layout of the wiring will make a difference.
Look at Mauro’s orginal wiring diagram. It is shown in the build sheets. He was very detailed here. This was to minimize hum pickup from power transformer leads.
Lastly he did not recommend using over 0.33 ufd for the input coupling cap. Said it could lead to low frequency instability. Lectured me several times on this. I replaced my 1 ufd caps with 0.47 ufd to make sure.
Now use a four channel system and crossover at 65 Hz so this may not be a factor. Helps with the power output limitations also.
I think you meant to write less than 0.33. Larger than 1.5 will yield no sonic improvement. Less than 0.33 will affect low frequency, unless that's what you want to do.
In my amps, I use a .022uf input cap and 18k R13 as a passive 6db high pass crossover at 400Hz, which works very well.
Peace,
Tom E
You are right, it was the Evolution where the coupling cap was specified as a 0.22 ufd. The My_Ref was a 1 ufd coupling cap.
Ran a few more tests and I'm still perplexed by my findings.
Both RCA's plugged in - no hum
both RCA's unplugged - hum
either RCA plugged in - no hum
It is certainly AC type hum - measuring 0.002 on Left channel input and 0.006 on right channel input.
Is it possible it is picking up transformer hum? Should I try a shielded cable and see if that helps?
Also see a lot of people (Rev D version) that uses a CL60 on teh PGND connection. Would that do anything?
It'a annoying and I want to try and figure out why the hum is there, as much for learning as just to have a quiet amp at all times.
Both RCA's plugged in - no hum
both RCA's unplugged - hum
either RCA plugged in - no hum
It is certainly AC type hum - measuring 0.002 on Left channel input and 0.006 on right channel input.
Is it possible it is picking up transformer hum? Should I try a shielded cable and see if that helps?
Also see a lot of people (Rev D version) that uses a CL60 on teh PGND connection. Would that do anything?
It'a annoying and I want to try and figure out why the hum is there, as much for learning as just to have a quiet amp at all times.
an open interconnect attached to an input becomes an aerial feeding the Rin resistance.
That will pick up a lot of interference.
Short the far end of that interconnect and now the amp input sees the short as it's input load and the noise pickup is very much reduced.
Question,
do you ever listen to audio files with the interconnect open ended?
Now to your non results:
what is the AC voltage on the amp output when you short the inputs with dummy zero ohms plugs? measure using a DMM that has a resolution of at least 0.1mVac
what is the AC voltage when you fit one long interconnect with an open end. measure both output channels.
what is the AC voltage when you fit two long interconnects both with open ends.
Short the barrel/outers together at the open ends. Short the inner to the outer of both interconnects. Measure the AC voltage.
short the inners to the outers, leave the barrels/outers not connected. measure the AC voltage
Trying to measure 0.001Vac on a scale with a resolution of 0.001Vac is nearly impossible. Read your instruction booklet. The final digit has a tolerance of typically +-0.5% of reading PLUS 2 or 3 digits.
That will pick up a lot of interference.
Short the far end of that interconnect and now the amp input sees the short as it's input load and the noise pickup is very much reduced.
Question,
do you ever listen to audio files with the interconnect open ended?
Now to your non results:
what is the AC voltage on the amp output when you short the inputs with dummy zero ohms plugs? measure using a DMM that has a resolution of at least 0.1mVac
what is the AC voltage when you fit one long interconnect with an open end. measure both output channels.
what is the AC voltage when you fit two long interconnects both with open ends.
Short the barrel/outers together at the open ends. Short the inner to the outer of both interconnects. Measure the AC voltage.
short the inners to the outers, leave the barrels/outers not connected. measure the AC voltage
Trying to measure 0.001Vac on a scale with a resolution of 0.001Vac is nearly impossible. Read your instruction booklet. The final digit has a tolerance of typically +-0.5% of reading PLUS 2 or 3 digits.
Ideally it should be a potentiometer at the input and that terminate the input and would stop your hum.
To put this in perspective, EVERY MyRef amp ever built hums with open inputs. If the hum bothers you while listening to your system with no inputs, 😉 just turn it off. No more hum.
Peace,
Tom E
I get it, I'm being stupid...but I thought I read every page of this and the 4 other My Ref threads and never saw a single mention of this type of humming. Granted who would judge hum with an open input as a problem except me.
OCD is a disease and not understanding "why" makes it twice as irritating to my brain.
I will stop trying to figure out why and enjoy what I have when the inputs are plugged and the music is playing.
OCD is a disease and not understanding "why" makes it twice as irritating to my brain.
I will stop trying to figure out why and enjoy what I have when the inputs are plugged and the music is playing.
Yeah, we're all a little obsessive about our babies. You've done nothing wrong and there's nothing to fix. Hope you can relax and enjoy. You have really nice amplifiers to make it easier!
11th My_Ref Fremen Edition GB is open!
http://www.diyaudio.com/forums/group-buys/314205-my_ref-fremen-edition-gb-11th-run.html#post5230242
http://www.diyaudio.com/forums/group-buys/314205-my_ref-fremen-edition-gb-11th-run.html#post5230242
Can the myref run tweeters? I'm planning to tri-amp since the fullrangers sound a little bit absent on the top-end. Do the tweeters still need a protection cap? I know I can use a lower power amp but I just want to use myref.
Yes you can - but YES you will need a cap (1st order XO) at a minimum to protect your tweeters.
As long as you have an adequate (properly designed) crossover, your tweeters will be fine with the MyRef as an amp.
As long as you have an adequate (properly designed) crossover, your tweeters will be fine with the MyRef as an amp.
Or use the cap as the first one pole stage of a multi-pole filter.
If you have a natural roll-off that is at the correct frequency and is 2pole and you want 4 pole at that same frequency then you could use the capacitor to add one pole passively and add in a second one pole active but with a Q = 1 so that you end up with a Butterworth 4pole acoustic roll-off.
But it's much better to have an octave spare in the natural roll-off and then you have a 3pole active with Q=1 and the passive 1pole from the capacitor. This gives a cleaner performance at the turn-over frequency.
But that active 3pole may need to be shelved into a 1pole, where it reaches the natural roll-off of the driver.
If you have a natural roll-off that is at the correct frequency and is 2pole and you want 4 pole at that same frequency then you could use the capacitor to add one pole passively and add in a second one pole active but with a Q = 1 so that you end up with a Butterworth 4pole acoustic roll-off.
But it's much better to have an octave spare in the natural roll-off and then you have a 3pole active with Q=1 and the passive 1pole from the capacitor. This gives a cleaner performance at the turn-over frequency.
But that active 3pole may need to be shelved into a 1pole, where it reaches the natural roll-off of the driver.
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My speaker protection isn't kicking in. I just noticed a DC offset of around 10V because R43 (Rev C) was 22M ohms. Although I've fixed it and there's acceptable DC offset now. One day the old speaker cables just shorted when I was twisting them straight. The 200w bulb in series went very bright for some seconds when I just turned the thing off and fixed the wires before powering on.
I think rev C has slightly modified values of the protection circuit than A. Attached are schematics and Mauro's inputs.
What's the right way to troubleshoot.
Included:
1. Original schematic
2. rev C.
3. rev A (improved speaker protection that everyone is supposed to be using in the latest version / kit ? )
I think rev C has slightly modified values of the protection circuit than A. Attached are schematics and Mauro's inputs.
What's the right way to troubleshoot.
Included:
1. Original schematic
2. rev C.
3. rev A (improved speaker protection that everyone is supposed to be using in the latest version / kit ? )