There are several places where the schematic is clear and those who have built this mono block before seem unwilling to help now that I have put significant time into the pre build. So, I am not going to be deterred, simply see it as a learning opportunity to understand amplifier design and specification (I see no point wasting energy getting upset, raised to keep smiling and laughing). This is the highest resolution available, glad there is anything.
What is the purpose of C9 capacitor, please?
What about C2 capacitor, please?
What about C7 capacitor, please?
Onto the resistors.
I think R1 is 150kΩ, R2 a stab in the dark of 390Ω, R3 guessing 100Ω, and forget trying to guess R4. If know the purpose of this arrangement and preferably how to specify, can figure it out.
Lastly, the purpose of R8.
Thanks in advance!
What is the purpose of C9 capacitor, please?
What about C2 capacitor, please?
What about C7 capacitor, please?
Onto the resistors.
I think R1 is 150kΩ, R2 a stab in the dark of 390Ω, R3 guessing 100Ω, and forget trying to guess R4. If know the purpose of this arrangement and preferably how to specify, can figure it out.
Lastly, the purpose of R8.
Thanks in advance!
C9 limits the high frequency response (and possibly stops oscillations).
C7 does a similar thing (in tandem with R8).
C2 is the capacitor for the screen grid - most of the time you see then return to ground instead of the cathode.
"I think R1 is 150kΩ, R2 a stab in the dark of 390Ω, R3 guessing 100Ω, and forget trying to guess R4." Correct, and I think R4 is 560k.
R8 along with C7 make a low pass filter if I'm not mistaken.
All for stability with global negative feedback most likely (to avoid phase shifts).
C7 does a similar thing (in tandem with R8).
C2 is the capacitor for the screen grid - most of the time you see then return to ground instead of the cathode.
"I think R1 is 150kΩ, R2 a stab in the dark of 390Ω, R3 guessing 100Ω, and forget trying to guess R4." Correct, and I think R4 is 560k.
R8 along with C7 make a low pass filter if I'm not mistaken.
All for stability with global negative feedback most likely (to avoid phase shifts).
So what would happen if left out?
If necessary, know a resource on how to select?
Okay. 4μF? Any advantage of one over the other?
Okay.
And just happen to know what a low pass (high cut) filter is. This case my understanding is to reduce or illuminate mains hum.
All of the capacitors and resistors?
If you leave it out, the amplifier might not be stable. Go with the suggested value in the schematic...
4u7 is find for a screen grid cap.
In this case the low pass filter keeps RF (and HF) out of the amp.
Yup. Those parts all work together to define the frequency poles of the amplifier.
R16 and C8 are the gNFB loop parts... If you don't connect them, the amp will have far too much gain, but it will be stable even without those parts. Unusably sensitive though.
4u7 is find for a screen grid cap.
In this case the low pass filter keeps RF (and HF) out of the amp.
Yup. Those parts all work together to define the frequency poles of the amplifier.
R16 and C8 are the gNFB loop parts... If you don't connect them, the amp will have far too much gain, but it will be stable even without those parts. Unusably sensitive though.
I forgot to say thank you for such extensive help, certainly appreciated! 😀
Oh dear. Well, I can't read the specification, can you? Worse case, has to be information out there. Just remembered have the original design, though so different and in Chinese of little help to me, maybe it will to you?
Ah, I thought the seven was a typo, see I learned something. 🙂
Appreciate the correction. Now that makes me wonder if something was amiss with this circuit on my console, at the prior house if turned on a lamp on the same circuit the radio buzz; I solved the issue by not turning on the light.
Frequency poles?
Glad you asked for the schematic, missed them way at the top of the page... Is this the feedback loop? R16 12kΩ and C8 39μF?
Mind me asking a curiosity question? Seems I ask too many questions, why ask. Originally it was 10kΩ, what happen if this was adjustable?
Oh dear. Well, I can't read the specification, can you? Worse case, has to be information out there. Just remembered have the original design, though so different and in Chinese of little help to me, maybe it will to you?
Ah, I thought the seven was a typo, see I learned something. 🙂
Appreciate the correction. Now that makes me wonder if something was amiss with this circuit on my console, at the prior house if turned on a lamp on the same circuit the radio buzz; I solved the issue by not turning on the light.
Frequency poles?
Glad you asked for the schematic, missed them way at the top of the page... Is this the feedback loop? R16 12kΩ and C8 39μF?
Mind me asking a curiosity question? Seems I ask too many questions, why ask. Originally it was 10kΩ, what happen if this was adjustable?
Almost... C8 is 39 picofarad....
C9 68pico
C7 33pico
C2 4u7
C8 39pico
The R and the C work together. Make R16 a value you like (too low = too much gNFB = unstable) and then adjust C8 on test (oscilloscope, 10kHz square wave, too big of a cap will round off the first square, too little and it'll bounce or have a spike like in this trace:
C9 68pico
C7 33pico
C2 4u7
C8 39pico
The R and the C work together. Make R16 a value you like (too low = too much gNFB = unstable) and then adjust C8 on test (oscilloscope, 10kHz square wave, too big of a cap will round off the first square, too little and it'll bounce or have a spike like in this trace:
Appreciate the extra effort for clarity. 🙂
The R and the C work together. Make R16 a value you like (too low = too much gNFB = unstable) and then adjust C8 on test (oscilloscope, 10kHz square wave, too big of a cap will round off the first square, too little and it'll bounce or have a spike like in this trace:
I don't think want to get this too into it. The original was 10kΩ, how much a difference does going up to 12kΩ?
Not much?
So less resistance be better? How low?
Appreciate indulging my desire for understanding. 😀
Some reason thought gNFB was gain, clearly have no understanding, oops.
So a higher value produces better results, to a point?
I sense an impending rabbit hole... Speaking of late, high time I take up the horizontal position. Will do so with a smile knowing this monoblock is in fact can be built (kind of hard when don't know what all the parts are).
Yes, building an amplifier with global feedback is complicated, and you have to observe the schematic exactly. The question here is--what is the output transformer? What model? Without that knowledge, the specific stabilizing components become meaningless, because they will be different for every output transformer. If this is designed for an Audionote transformer, then that is what you must use. Otherwise you must use an oscilloscope and know how to tune the circuit for a different transformer.
Unable to sleep so might as well be productive.
I appreciate this question, seems really important. Thank you. 🙂
This is another concern I have that had forgotten about. The schematic states "OPT ZPP 8kΩ to 10kΩ." I have found no explanation for a z push pull, nor an OPT at this impedance.
Edit
This is interesting, the difference between the two, particularly the recommendation for lower impedance.
"U064 PP output for EL84 UL 8000 ohms Rla-a. 10W at 30 Hz. 80 mA per side. Choice of 43% or 20% UL taps. 25 Hz to 30 kHz. Size F"
Versus two EL84s in parallel, which this PP is.
"U065 PP Output for 2 pairs EL84 UL 4000 ohms Rla-a. 30 W at 20 Hz. 100 mA per side. For 2 pairs parallel connected EL84. 43% UL taps. 20 Hz to 30 kHz. Suitable for Vox AC30. Size M"
I appreciate this question, seems really important. Thank you. 🙂
This is another concern I have that had forgotten about. The schematic states "OPT ZPP 8kΩ to 10kΩ." I have found no explanation for a z push pull, nor an OPT at this impedance.
Edit
This is interesting, the difference between the two, particularly the recommendation for lower impedance.
"U064 PP output for EL84 UL 8000 ohms Rla-a. 10W at 30 Hz. 80 mA per side. Choice of 43% or 20% UL taps. 25 Hz to 30 kHz. Size F"
Versus two EL84s in parallel, which this PP is.
"U065 PP Output for 2 pairs EL84 UL 4000 ohms Rla-a. 30 W at 20 Hz. 100 mA per side. For 2 pairs parallel connected EL84. 43% UL taps. 20 Hz to 30 kHz. Suitable for Vox AC30. Size M"
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Thank you for your concern. I have worked on a tuner above a live tube amplifier, no protection, well aware of the shocking results if made one wrong move.
If it has never been powered up, why then the concern?
Discharge, how?
Of course there is no risk if the power has never been turned on! But presumably you will want to test it....
It's normal for a high value resistor to be placed across the reservoir capacitors which will bleed off the voltage in a minute or so but none shown on your schematic so use an insulated 10k 1wat resistor on a croc clip lead to 0v.
It's normal for a high value resistor to be placed across the reservoir capacitors which will bleed off the voltage in a minute or so but none shown on your schematic so use an insulated 10k 1wat resistor on a croc clip lead to 0v.
My excuse for not understanding your warning is I took melatonin cap and sleep medicine, so a bit groggy... 😛
Yes, now I see what you mean, hadn't noticed that before, interesting. Okay, will add those to my order.
If adding the resistors to the circuit, use a higher value such as 1Meg 1watt. I suggested 10k for non permanent connection and for fast discharge.
The OPT impedance is of the primary and has to match the tube, so higher power ones would use a lower impedance transformer (I think it's derived from the turns ratio squared multiplied by the output impedance but don't quote me!)
The OPT impedance is of the primary and has to match the tube, so higher power ones would use a lower impedance transformer (I think it's derived from the turns ratio squared multiplied by the output impedance but don't quote me!)