I would think with a circuit that's running in class A at very low plate currents (15mA for four triodes and two source followers) that holding voltages rock-steady is not the issue.
I understand the shortcomings of using chokes in a passive supply, namely resonances(!), but this is not an issue in a properly done supply with RC decoupling/filtering.
I ran my built CRCRC in PSUD2 and it says I should expect 40uV (microvolts) of ripple at the DC IN of the audio boards. There are two more stages of RC filtering for each channel after that.
I understand the shortcomings of using chokes in a passive supply, namely resonances(!), but this is not an issue in a properly done supply with RC decoupling/filtering.
I ran my built CRCRC in PSUD2 and it says I should expect 40uV (microvolts) of ripple at the DC IN of the audio boards. There are two more stages of RC filtering for each channel after that.
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Thanks, I see. What kinds of problems would this cause in a B+ supply for a low-current, high impedance audio circuit?
In my case they ended up on the output to some extent. That was a bit of a nuisance when measuring low-level signals with an oscilloscope (the signal randomly moved into and out of the picture). Theoretically they could cause extra intermodulation distortion in the main amplifier or the loudspeaker, but I can't say I ever noticed that. To me my amplifier with CRCRC filter just works fine.
Today is the first day I've had this preamp up and running long enough to have a real listening session.
It seems that after warming up a while, there is some hum in the output, but I have to stick my ear right up into the speaker to hear it.
I'm thinking that the culprit is probably not AC ripple riding on the B+ but inducted electromagnetic noise from the two power transformers on the same aluminum chassis with the audio circuits. I figure that's unavoidable. You can't hear any hum if you move just a few cm away from the woofer, and this is with the volume up at a moderately loud listening level. So I'm satisfied with the results.
It seems that after warming up a while, there is some hum in the output, but I have to stick my ear right up into the speaker to hear it.
I'm thinking that the culprit is probably not AC ripple riding on the B+ but inducted electromagnetic noise from the two power transformers on the same aluminum chassis with the audio circuits. I figure that's unavoidable. You can't hear any hum if you move just a few cm away from the woofer, and this is with the volume up at a moderately loud listening level. So I'm satisfied with the results.
It sounds like 120Hz, which points to ripple, don't it...
There is still something really weird going on in the right channel. If I disconnect the inputs, the hum goes completely away BUT there is a faint crackling/popping noise coming from the right channel only. The left channel PCB is behaving really nicely. The right channel PCB is a mess.
I'm going to see if replacing some passive parts on the right channel PCB clears that up. I might even remove the tube socket on that side and replace it with a new one.
There is still something really weird going on in the right channel. If I disconnect the inputs, the hum goes completely away BUT there is a faint crackling/popping noise coming from the right channel only. The left channel PCB is behaving really nicely. The right channel PCB is a mess.
I'm going to see if replacing some passive parts on the right channel PCB clears that up. I might even remove the tube socket on that side and replace it with a new one.
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Yeah.
What's weird is that when you first turn on the preamp, the hum is practically nonexistent.
After a few hours the right side gets microphonic and weird, and there's some hum.
But the left side is still fine, although now with a bit of hum from there too.
Rapping on the chassis provokes a loud crackle from the right channel *only*.
The right side PCB has something wrong. Maybe a leaky/dirty tube socket, or a bad passive component. More work will be required.
What's weird is that when you first turn on the preamp, the hum is practically nonexistent.
After a few hours the right side gets microphonic and weird, and there's some hum.
But the left side is still fine, although now with a bit of hum from there too.
Rapping on the chassis provokes a loud crackle from the right channel *only*.
The right side PCB has something wrong. Maybe a leaky/dirty tube socket, or a bad passive component. More work will be required.
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Well, I am an idiot.
Oh man. Now I'm going to publicly humiliate myself.
I started unsoldering the precious tightly-matched RIAA parts from the right channel PCB to re-use them in a fresh PCB (I have two spares). In the course of doing that, I discovered that even though I had thought I had re-wetted every solder joint on the board, I had managed to leave the pad for the 2nd stage cathode resistor completely unsoldered.
Now, the 2nd stage 12AX7 plate is DC-coupled to the gate of the MOSFET source follower. The big fat clue that I missed was when I saw that as the preamp warms up the voltage at the MOSFET source would start creeping up, and up and up. Of course. Now it all makes sense. The 2nd stage 12AX7 was turning off because its cathode was lifted from ground. As it did, its plate voltage drifted closer to the B+ potential. That explains why the right channel would mute after it got warm.
So in the end, it was a bad connection to ground.
D'OH!!!
Oh man. Now I'm going to publicly humiliate myself.
I started unsoldering the precious tightly-matched RIAA parts from the right channel PCB to re-use them in a fresh PCB (I have two spares). In the course of doing that, I discovered that even though I had thought I had re-wetted every solder joint on the board, I had managed to leave the pad for the 2nd stage cathode resistor completely unsoldered.
Now, the 2nd stage 12AX7 plate is DC-coupled to the gate of the MOSFET source follower. The big fat clue that I missed was when I saw that as the preamp warms up the voltage at the MOSFET source would start creeping up, and up and up. Of course. Now it all makes sense. The 2nd stage 12AX7 was turning off because its cathode was lifted from ground. As it did, its plate voltage drifted closer to the B+ potential. That explains why the right channel would mute after it got warm.
So in the end, it was a bad connection to ground.
D'OH!!!
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I've had the preamp running for a couple of days. Got to play a few records over the weekend, and tried it out with a few 12AX7 types I have around. A couple of observations:
- With a pair of used vintage RCA 7025 and with the volume cranked, I heard 60Hz hum out of the right channel and a higher pitched hum out of the left channel. Swapped tubes side to side, the noise followed the tubes. With the inputs shorted and those tubes in place, the hum pretty much goes away but is still there, just reduced in level.
- Swapping in a pair of used vintage Sylvania 7025 and with the volume cranked there is no hum, but there is some popcorn noise from one of the tubes. It's not bad, though.
- Swapping in a pair of used vintage Telefunken ECC83 (and again with the volume cranked), there is a milder bit of hum and some hiss/sizzle noise. Not bad.
The noise from this thing is very tube-dependent. I'm surprised how much variation I'm seeing/hearing. A tube might give you noticeable hum, or it might give you no hum at all. Therefore, I think the B+ supply itself is quiet. The search is on for the quietest pair of 12AX7s I can find around here.
Would a voltage regulator have any influence on this behavior? Could super low output impedance make a difference for this?
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- With a pair of used vintage RCA 7025 and with the volume cranked, I heard 60Hz hum out of the right channel and a higher pitched hum out of the left channel. Swapped tubes side to side, the noise followed the tubes. With the inputs shorted and those tubes in place, the hum pretty much goes away but is still there, just reduced in level.
- Swapping in a pair of used vintage Sylvania 7025 and with the volume cranked there is no hum, but there is some popcorn noise from one of the tubes. It's not bad, though.
- Swapping in a pair of used vintage Telefunken ECC83 (and again with the volume cranked), there is a milder bit of hum and some hiss/sizzle noise. Not bad.
The noise from this thing is very tube-dependent. I'm surprised how much variation I'm seeing/hearing. A tube might give you noticeable hum, or it might give you no hum at all. Therefore, I think the B+ supply itself is quiet. The search is on for the quietest pair of 12AX7s I can find around here.
Would a voltage regulator have any influence on this behavior? Could super low output impedance make a difference for this?
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Since it goes away when the input is short, I think you might be picking up radio... What value of grid stopper did you use? Got any ferrite beads or similar? RF choke?
I once build a phono stage that used 12SL7 as input tube. RCA was quiet, Westinghouse picked up 98.1MHz FM radio. I now use RG174 to connect the RCA jacks to the circuit in my builds.
I once build a phono stage that used 12SL7 as input tube. RCA was quiet, Westinghouse picked up 98.1MHz FM radio. I now use RG174 to connect the RCA jacks to the circuit in my builds.
Beyond coax, beads, grid stoppers, grounded shield caps are good against RFI in phonos too. High impedance Riaa network does not help with avoiding interference. BTW don't use a grid stopper and unbypassed Rk more than three times greater in total value than the cart's source impedance, because extra noise will be added.
The heaters are DC regulated, adjusted to 12.56VDC (that's where the trimpot seems to stabilize it).
The heater supply uses a separate 12.6VAC transformer with Schottky diode bridge, resulting in a rectified DC of just under 17V. That goes to two 10,000uF caps in parallel, then to an LD1085 with OUT pin bypassed.