| Algar_emi |
Hi. I side track a little bit on the Toccata phono preamp. I was having problem with my current test setup. I think the assembly method that I used had to much parasitic coupling between parts. It is causing a degradation of the hi freq slope. I will resume this project soon.
In the mean time, I wanted to have a base tube phono preamp to compare my other projects to. I also want to try a phono preamp with tube HV supply and check if indeed it can impact on the preamp sound.
A little search pointed in the direction of the Cornet2 tube phono preamp kit. This very nice preamp made by Hagerman Technology, has everything I was looking for. It also received very good review from other users. It contains all the nice tricks to get a nice sound. It has no feedback Pure Class A single-ended amplifier stages, split RIAA Passive equalization, Cathode follower output with CCS loading, Vacuum tube HV rectification and DC heaters.
This preamp is available as a kit. Futhermore the user manual includes the parts list and very detailled schematics. In the pure DIY tradition, I used the schematic and design my own PCB, reinventing the wheel again. I used the general layout of the original PCB.
However it is my own version since I don't know the exact PCB layout, grounding arrangement, etc. It may well not be as quiet or as well layout as the orignal Cornet2 that was obtimized for best result. By the way, I must say that the kit parts prices are very raisonnable. I cannot do my own for a better price. I didn't it as always for my own use and just to have fun building some great equipment from scratch.
But it is a lot of work. Don't ask for my own PCB. Buy the original one for best results, and at this price, it is a steal. It took me more than a week of work at night just to layout the PCB...
My own board is smaller and more compact because I had only access to 6"X9" PCB. I use the less expensive 270BX Hammond transformer and added a few things like a Gnd Lift switch and a Mute relay with switch and led. I kept the Mono switch but mounted on the side. I also installed AuriCap and Solen Caps Upgrade and using PRP 1% resistors for the RIAA network.
It is completed and fully functional. I will start to test it, maybe tomorrow. |
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| Algar_emi |
| This is my PCB. The original PCB used heatsinks installed in a V-Shape to shield the Input/Output connectors from the supply. I used instead bare copper PCB plates. It was much longer to install but more fun too :D |
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| Algar_emi |
First listening impressions.
Setup: Michell Tecnodec with Benz Micro Ace low output MC cartridge and Denon AU-320 Step-Up Transformer.
Just one word - WOW :eek: It is incredible. The preamp is brand new and it is already just incredible. Very musical and balance, the highs are silky sweet and clear, the mids are sweet and the bottom end is tight and precise. No trace of tube sound coloration. Instruments just pop out in the air, timing is precise. Distorsion seems very low, I cannot hear any. It just makes nice music. I recommend this very nice kit to everyone looking for a nice tube phono preamp :cool:
It seems that my PCB is not bad at all. The hum noise level is already very low even using my very bad test setup. Just mounted in the air, it can pickup any stray field, like my main preamp supply transformer. Once mounted into a metal enclosure it should be pretty good. The noise level is already hidden by the music.
I used extensive ground arrangement on the PCB with separate power and signal grounds, separate ground nodes for each stage left and right. All the sub-nodes meet only at one main star ground point. This way the power ripple noise don't mix with the sensitive signal ground. The input signal ground goes directly to the star ground too. It seems to work rather well. I also used as possible curved pcb traces (like Mark Levinson pcb). Probably overkill but looking good...
I'll do some accurate measurements tomorrow but I already expect very good numbers :angel: |
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| arnoldc |
I know a lot of people like the Cornet/2 and I babysat a 2 once...
My comments-
- I find it colored, not the most transparent tubed phonostage I tried. The Wright Phono (really old one) is even more transparent, or the Opera Audio Cyber 40.
- I find it slow and lacking in dynamics compared to my SS clearaudio Symmetry. I used a Peerless/Altec step up 1509A on the Cornet 2 for MC.
- it definitely is a step-up compared to my Musical Fidelity X-LP.
I do have a different setup though- clearaudio Champion Magnum, Tangent arm, Concerto cartridge.
- I always wondered why people use AuriCaps for the RIAA when they don't have the best tolerance.
Congratulations on your new baby! |
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| Algar_emi |
Hi. I use the Auricap only for the the coupling caps. I kept the Panasonic polypropylene 5% caps for the RIAA. This is why I replaced the RIAA resistors with PRP 1% instead of the original 5% resistor.
I agree that is not the best preamp in the world, but that for the money it is quite good. I have also a PassLabs XONO clone and it is way superior, but much more complex, big and expensive to build.
Bye... |
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| Algar_emi |
| Looking at some Acoustic Research phono preamp PCB, I think I can improve my PCB by connecting the input ground to the first stage signal ground node instead of the main star ground point that is probably too poluted with noise. I'll try that tomorrow. |
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| Algar_emi |
| Voilą! Much better now :D. By connecting the input connectors ground to the input stage ground node, it reduces the hum level to almost nothing. It was a huge improvement. There is still a slight hum on one side of the preamp. I'll try to get this channel as good as the other ;) |
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| Algar_emi |
I found why there was a difference of noise between both channels. One channel had the last 47uF supply filter capacitor not correctly soldered. Now both channels are the same. I'm still stuck with this noise, square wave shape on both channels. Amplitude is about 10mv pk-pk with the inputs shorted. The pcb is perfectly clean, no flux deposite between pins.
I tried numerous things, exchange all the tubes, connect the ground to the safety ground using a 100R resistors and cap in parallel, replaced the heater supply with a good laboratory power supply with almost no ripple at all, Try other ground routing, different power outlet, shutdown of all electric noise sources, neon, soldering iron, etc, but no improvement.
My actual setup is not mounted into a metal box. Can this explained why I got this noise? Why a square shape? |
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| Algar_emi |
| I built an Inverse RIAA filter to test and burn-in my phono preamp. Here the result. |
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| Algar_emi |
| Interesting findings. First, I refined the main star ground connection points. My original design was to connect 0.086'' traces all together in a star pattern. But these large traces produce a large connection area. Using an sharp blade, I cut the traces end to get a connection on just a small point. Take a look at this star ground from a Naim preamp. It is looking more like this now. The noise shape became more like normal random noise. |
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| Algar_emi |
| Then, I installed the short directly at the input resistor on the PCB instead that on the RCA. Bingo. Now the noise is much lower, less than 5mv pk-pk. I guest that if I mount the preamp into a metal enclosure, the noise would be lower. I may use coax cable to connect the input RCA directly at the tube input resistors. |
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| Algar_emi |
I installed RCA input jacks directly at the tube inputs. Then I listen and the noise is gone at normal listening level. I have to crank the preamp to 3/4 full volume to hear only a slight hum. So the noise issue is solved.
Using the inverse RIAA filter I injected a square wave, here it is at 100Khz, still prefect on both channel. I tested the -3dB bandwith at 530kHz! Very good indeed :D |
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| sorenj07 |
| wow - i knew that wiring affected signal quality a lot, but not THIS much 0_o. my linestage is a chicken's nest in comparison. Maybe if I worked on the ground wiring in that... |
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| Algar_emi |
| WOW indeed, it looking even better than my Xono PassLabs clone, that is, by the way, much more complex to build. The Xono bandwidth is limited to 43Khz. ;) |
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| Algar_emi |
I just completed the case. It is very strong and has a few nice features. The first image is showing the front and back of the preamp.
On the front, there is the Mono and Mute switches, the dual colors power On and the Mute On leds.
On the back, the usual AC input socket with the main power On switch, a case ground lift switch (throught a 100R resistor with a 0.1uF, 250V X2 capacitor in parallel), the Cardas In/Out RCA connector and the ground post. |
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| Algar_emi |
| Now the little details :D This is the top plate, made of 1/4 inch aluminium (by the way the case is all made of 1/8" alu). I created a nice pattern of ventilation slots. The rectifier tube goes out in the middle. I used this approach because I didn't want to have a case with exposed tubes on the top. It is difficult to fir into a rack and more dangerous for the tubes. It also adds shielding to the preamp. |
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| Algar_emi |
| This is a detail of the ventilation slot. I created a pattern of small square holes by doing numerous cuts under the plate. |
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| Algar_emi |
| For the side panel I used flat head hex cap screws flush mounted, not bad ;) |
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| Algar_emi |
| This is the inside view. There is a metal plate where the PCB is mounted from under, as the original Cornet case. On the right we can see the power transformer vented enclosure. It isolates the transfo from the preamp. These 2 inside plates add a lot to the enclosure strength. |
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| Algar_emi |
| This is the transfo cavity on the rear. The other holes will be use by the In/Out coax and the ground wires. |
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| Algar_emi |
| To reduce tube microphony, the transformer is mounted on rubber o-ring. The PCB is suspended from the metal plate using rubber washer and o-ring. Finally the metal plate is also decoupled using the same way. |
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| Algar_emi |
| This is a side view, the bottom holding bracket is removed for clarity. We can see the suspended PCB and the transfo on the back. |
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| Algar_emi |
| On the bottom there are ventilation holes and three cones as feet. |
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