Looking at the third stage, the voltage drop over R8 (47K) is 400 - 179.8 = 220.2 V. So the current through R8 would be I = V / R = 220.2 / 47K = 4.7 mA. But this current leads to a cathode voltage of V = I x R = 0.0047 x 10K = 47 V, while you measured 10.4 V. So something is not right with the schematic and/or your measurements of that stage.
Looking at the first stage, the current through R17 (4K7) is I = V / R = 3.67 / 4K7 = 0.78 mA. So the voltage drop over R4 (47K) would be V = I x R = 0.00078 x 47K = 36.7 V. This would mean a B+ for the first and second stage of 192.5 + 36.7 = 229.2 V.
Looking at the second stage, the cathode current is I = V / R = 2.43 / 6K8 = 0.36 mA.
The total current of stages 1 and 2 combined is 0.78 mA + 0.36 mA = 1.14 mA. The voltage drop over R6 (47K) would than be V = I x R = 0.00114 x 47K = 53.6 V. These numbers would lead to a B+ for the first and second stage of 400 - 53.6 = 346.4 V which doesn't match the 229.2 V found earlier.
If the current of the first and second stage of both channels run through R6 (47K), than the voltage drop over R6 doubles to 107.2 V, giving a B+ of 400 - 107.2 = 292.8 V. But this voltage is still too far off the 229.2 V calculated above. So something is wrong with the schematic and/or your measurements.
Looking at the first stage, the current through R17 (4K7) is I = V / R = 3.67 / 4K7 = 0.78 mA. So the voltage drop over R4 (47K) would be V = I x R = 0.00078 x 47K = 36.7 V. This would mean a B+ for the first and second stage of 192.5 + 36.7 = 229.2 V.
Looking at the second stage, the cathode current is I = V / R = 2.43 / 6K8 = 0.36 mA.
The total current of stages 1 and 2 combined is 0.78 mA + 0.36 mA = 1.14 mA. The voltage drop over R6 (47K) would than be V = I x R = 0.00114 x 47K = 53.6 V. These numbers would lead to a B+ for the first and second stage of 400 - 53.6 = 346.4 V which doesn't match the 229.2 V found earlier.
If the current of the first and second stage of both channels run through R6 (47K), than the voltage drop over R6 doubles to 107.2 V, giving a B+ of 400 - 107.2 = 292.8 V. But this voltage is still too far off the 229.2 V calculated above. So something is wrong with the schematic and/or your measurements.
I'm sorry I forgot that I changed the R3 from 6,8k to 2,8k. Also the B+ is 360V. All the voltages are double measured. I can't really understand why does it when scope was showing everything ok, signals were in phase and gain was also equal on both channels.
With a B+ of 360 V instead of 400 V, and with R3 being 2K8 instead of 6K8 the numbers still don't add up.
Third stage: The voltage drop over R8 (47K) is 360 - 179.8 = 180.2 V. So the current through R8 would be I = V / R = 180.2 / 47K = 3.8 mA. But this current leads to a cathode voltage of V = I x R = 0.0038 x 10K = 38 V, while you measured it being 10.4 V.
First stage: The current through R17 (4K7) is I = V / R = 3.67 / 4K7 = 0.78 mA. So the voltage drop over R4 (47K) would be V = I x R = 0.00078 x 47K = 36.7 V. This would mean a B+ for the first and second stage of 192.5 + 36.7 = 229.2 V.
Second stage: The cathode current is I = V / R = 2.43 / 2K8 = 0.87 mA.
The total current of stages 1 and 2 combined is 0.78 mA + 0.87 mA = 1.65 mA. The voltage drop over R6 (47K) would than be V = I x R = 0.00165 x 47K = 77.5 V. These numbers would lead to a B+ for the first and second stage of 360 - 77.5 = 282.5 V which doesn't match the 229.2 V found earlier.
If the current of the first and second stage of both channels run through R6 (47K), than the voltage drop over R6 doubles to 155 V, giving a B+ of 360 - 155 = 210 V, which also doesn't match the 229.2 V found earlier (but it's getting close...).
The biggest mistery (to me) are your measurements of the third stage. Are you sure about the values of the cathode and anode resistors in that stage?
Do stages 1 and 2 of both channels share R6? Or does each channel has its own R6?
Third stage: The voltage drop over R8 (47K) is 360 - 179.8 = 180.2 V. So the current through R8 would be I = V / R = 180.2 / 47K = 3.8 mA. But this current leads to a cathode voltage of V = I x R = 0.0038 x 10K = 38 V, while you measured it being 10.4 V.
First stage: The current through R17 (4K7) is I = V / R = 3.67 / 4K7 = 0.78 mA. So the voltage drop over R4 (47K) would be V = I x R = 0.00078 x 47K = 36.7 V. This would mean a B+ for the first and second stage of 192.5 + 36.7 = 229.2 V.
Second stage: The cathode current is I = V / R = 2.43 / 2K8 = 0.87 mA.
The total current of stages 1 and 2 combined is 0.78 mA + 0.87 mA = 1.65 mA. The voltage drop over R6 (47K) would than be V = I x R = 0.00165 x 47K = 77.5 V. These numbers would lead to a B+ for the first and second stage of 360 - 77.5 = 282.5 V which doesn't match the 229.2 V found earlier.
If the current of the first and second stage of both channels run through R6 (47K), than the voltage drop over R6 doubles to 155 V, giving a B+ of 360 - 155 = 210 V, which also doesn't match the 229.2 V found earlier (but it's getting close...).
The biggest mistery (to me) are your measurements of the third stage. Are you sure about the values of the cathode and anode resistors in that stage?
Do stages 1 and 2 of both channels share R6? Or does each channel has its own R6?
Well I think I solved it. I put a bypass cap to the EF86 cathode resistor and also put 3,4K instead os 2,2K and voices are back! The amp sounds brilliant. But thank you very much kind sir for your help.
I am reviving this thread as I just encountered a similar case.
I recently picked up pre-amp from a dump bin.
If both left and right input RCA cables are connected, vocals/voices are extremely hollow/low/distant/thin.
If I connect only the right RCA cable, or only the left RCA cable, then it sounds much better and vocals/voices actually find their place.
Any clues?
I'm think it has to be a basic wiring issue, but I'm not seeing it.
Posting this here, as #13 sounds like a good theory, even though I don't see what ground is missing.
I recently picked up pre-amp from a dump bin.
If both left and right input RCA cables are connected, vocals/voices are extremely hollow/low/distant/thin.
If I connect only the right RCA cable, or only the left RCA cable, then it sounds much better and vocals/voices actually find their place.
Any clues?
I'm think it has to be a basic wiring issue, but I'm not seeing it.
Posting this here, as #13 sounds like a good theory, even though I don't see what ground is missing.
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A brand and model number or chassis number of the unit would aid me in determining things.
Without that, I can't give you any answers.
I am sorry, I should have done better.
I think this was built from a kit of Chinese origin. So, I don't have a model number that would be well known.
But I can describe the wiring as I see it...
2 shielded cables go from input RCA jacks to the potentiometer.
Each of these shielded cables has 2 conductors plus the shield.
At the level of RCA jacks, the shield is clipped.
At the level of the potentiometer, the negative conductor and the shield are braided and tinned together.
From the potentiometer to the pre-amp, L/R wiper wires and L/R common wires are connected to the pre-amp PCB.
L wiper wire goes to one grid of a dual triode. R wiper wire goes to the other grid of the same dual triode.
L/R common wires go from the potentiometer to the star ground on the PCB.
The first stage is a 12AX7 with signal coming out of the plate. So, both L and R signal will come out of first stage inverted.
The second stage is a pair of 6C19Ps in Common Cathode Follower configuration. So, both L and R signal will not be inverted during that stage.
The output will be inverted compared to the input, but both L and R should be inverted similarly.
Many words for a missing picture. Does it help?
Yes, it's the most logical explanation, but I have not found the source of the inversion yet.
See my follow up answer just in case it's right in front of me and I don't see it.
Is there a phase switch on the front of the preamp? Lots of older preamps had phase or 180 degree switches.
If this is a chinese product, it's new, not older, there won't be any such phase switching.
It's no wonder someone tossed the heap into a dumpster.
I agree. Since I have much to learn, such cheap contraptions are perfect corpses for me to practice on. Granted, I learn most when I figure out something that is utterly messed up.
At the RCA jacks the shield should go to the RCA jacks and the RCA jacks should be insulated from chassis. It's nothing to do with the quality of China products rather the person who wired it up. Without this you are relaying on the mains earth between the two units, for the signal return. Take a photo of the RCA jacks if you would.
Those cables are 2 conductors + shield.
There is 1 for each RCA jack.
For each jack, 1 of the conductors goes to the center pin and 1 of the conductor goes to the outer contact of the jack.
So, the signal return should not be a problem, right?
I'll snap a picture next time I am down there.
If I'm following you correctly (maybe not), if one, or both, of the input jacks' (internal) wiring were swapped hot-for ground it would give your symptoms.
All good fortune,
Chris
All good fortune,
Chris
I suspect it is something wrong with the signal ground. If both are plugged in then you get L-R in one channel and R-L in the other. Thats your no voices. If one is plugged in you get say just L capacitively coupled through the lack of signal ground. it's something like that. It could also be broken cables too. This would fit in with the thin sound too - no bass.
I would not expect in #54 for you to be using stereo coax cable for each RCA. I would expect the braid of each cable to go to the ground tag of each RCA not the second conductor. If the negative conductor was not connected at the pot end I would expect that to produce the fault. The fault could also be on the output side.
I would take the coaxs out and use single core coax. The fault may become apparent as you rewire. A meter would also confirm the issue if you buzz RCA grounds to PCB ground.
I would not expect in #54 for you to be using stereo coax cable for each RCA. I would expect the braid of each cable to go to the ground tag of each RCA not the second conductor. If the negative conductor was not connected at the pot end I would expect that to produce the fault. The fault could also be on the output side.
I would take the coaxs out and use single core coax. The fault may become apparent as you rewire. A meter would also confirm the issue if you buzz RCA grounds to PCB ground.
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95% probability you have miswired signal cables/connectors so focus on that.
5% there is something wrong with the amplifier, and that´s only because it´s DIY, so suspect.
5% there is something wrong with the amplifier, and that´s only because it´s DIY, so suspect.
It's actually a Belden microphone cable, I think. Good quality cable that is not from China for sure. (the previous builder must have put that in)
I noticed that many of these Chinese amps float star ground, or signal ground, or everything (ahah). In this build, this has been corrected, signal ground goes to star ground and star ground goes to earth ground. So, maybe, this goes against the assumptions of this circuit.
Thank you so much for your help!
I am mystified by this problem, but it's almost certainly due to the fact I am so new at this.
But, my intuition is also that whatever it is, it has to be before the 12AX7 first stage, because left and right channels go their own way from there.
I didn’t mean to be disparaging. I know they have more intelligent people than any other country in the world, by virtue of their large population.
And they don’t hand degrees for showing up.
But they do have different views on the safety of ground lifts, as evidenced in some of their products.
And they don’t hand degrees for showing up.
But they do have different views on the safety of ground lifts, as evidenced in some of their products.