I am in the beginning stages of building a Hagerman Cornet Octal phono stage. I attached the schematic except I will be using a CCS on the cathode of the output. Also the PSU is in a separate box, already built, with H+, B+ and ground in an umbilical and a separate wire from chassis to chassis (the IEC ground is bolted to the PSU chassis, wire from there will go to the phono stage case).
I have a question about the grounding scheme. I have ran a separate wires from each component to a buss bar or star with good results in previous builds. For this build I was thinking of grounding sections together then each section to a buss or star.
I am using 5 lug terminal strips and thought about using one lug to go to the buss/star and have the components for each section go to their lug. For example on 1st lug, input ground?, R14, R15, C7- 2nd R16, R17, C8 - 3rd R18 (coming off CCS), R19, output?- the filter caps and resistors going to the anodes would be 4th, heater filter caps 5th.
Is this a reasonable solution? It does cut the number of wires quite a bit
If I do this should the L and R channels be on separate lugs, which would be 9 wires/lugs to ground total?
I tried to attach the pdf schematic.
Thanks
file:///C:/Users/david/Downloads/cornetoctal.pdf
I have a question about the grounding scheme. I have ran a separate wires from each component to a buss bar or star with good results in previous builds. For this build I was thinking of grounding sections together then each section to a buss or star.
I am using 5 lug terminal strips and thought about using one lug to go to the buss/star and have the components for each section go to their lug. For example on 1st lug, input ground?, R14, R15, C7- 2nd R16, R17, C8 - 3rd R18 (coming off CCS), R19, output?- the filter caps and resistors going to the anodes would be 4th, heater filter caps 5th.
Is this a reasonable solution? It does cut the number of wires quite a bit
If I do this should the L and R channels be on separate lugs, which would be 9 wires/lugs to ground total?
I tried to attach the pdf schematic.
Thanks
file:///C:/Users/david/Downloads/cornetoctal.pdf
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Each circuit section (three per channel) should have one ground wire. Those all could go to a lug
on the ground strip, and then to the main ground. Separate grounds for each channel is much better.
A buss bar ground or a single star ground is not good for a phono preamp.
on the ground strip, and then to the main ground. Separate grounds for each channel is much better.
A buss bar ground or a single star ground is not good for a phono preamp.
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Yes, each section for a tube consists of the cathode resistor to ground, and all the grid components
going to ground for that tube only.
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Thanks, i went to electronics school in the 70s, passed the associate CET and license exams, then got out after a year. I remember some of what we were taught but most of that was repair, not design.
Tube phono stages are very sensitive to hum and noise, due to the high impedance levels present.
Also keep all the ground wires as short as possible. The HV should at least have a CRC filter in
the power supply cabinet so most of the hum will stay there.
The phono grounds should all go first to the ground lead of a large decoupling capacitor (audio ground),
which then goes to chassis ground. There should be only one connection from audio ground to chassis ground
per chassis. Sometimes there is a small impedance added between the audio ground and the chassis ground,
to deter ground loops.
Also keep all the ground wires as short as possible. The HV should at least have a CRC filter in
the power supply cabinet so most of the hum will stay there.
The phono grounds should all go first to the ground lead of a large decoupling capacitor (audio ground),
which then goes to chassis ground. There should be only one connection from audio ground to chassis ground
per chassis. Sometimes there is a small impedance added between the audio ground and the chassis ground,
to deter ground loops.
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Typically they range from 20uF to 100uF, but the audio circuit's low frequency stability can depend
on this value. The audio circuit time constant(s) should be much smaller than the power supply
time constant(s), to avoid forming an oscillator at subsonic frequencies.
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I have had good success with phono preamps by using a ground buss for each channel, having both originate at the ground post by the input jacks. This presumes a dual mono power supply, which I would suggest using if at all possible. If you don't have a dual mono power supply, then I would go with a shared ground buss for both channels. I have done this as well with zero issues.
A constant current source on either of the first two stages will give you far better mileage than using it in the cathode follower. I have no idea why that cathode follower runs ~15mA, that will be brutal in terms of thermal dissipation for a CCS!
R7 and R6 can be omitted from that schematic.
36dB of gain seems really low for a phono preamp. Your 5mV cart will put out ~300mV of signal, that's not all that great. Bypassing the cathode resistors under the 6SL7s would bring that gain up, as would CCS plate loads. EQ adjustments may be required for that though!
A constant current source on either of the first two stages will give you far better mileage than using it in the cathode follower. I have no idea why that cathode follower runs ~15mA, that will be brutal in terms of thermal dissipation for a CCS!
R7 and R6 can be omitted from that schematic.
36dB of gain seems really low for a phono preamp. Your 5mV cart will put out ~300mV of signal, that's not all that great. Bypassing the cathode resistors under the 6SL7s would bring that gain up, as would CCS plate loads. EQ adjustments may be required for that though!