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76.25.70.1
There's a new Hagerman Cornet2 Silver Edition vacuum tube MM phono on auction up on Audiogon. This is a new unit just built by Jim of Hagtech.
I have no affiliation or other interest in this auction. Just wanted to let folks know in case anyone is interested in bidding. It's a nice phono..... I owned one a few years ago. I paid $1000 for one of the last ones built before they were discontinued. This was the "Silver Edition" with upgraded parts.
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but looking at the schematic, the first stage cannot even drive the second stage's grid resistor let alone the first RIAA filter stage.
I don't get it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
. . . why mine sounds so good, and why many others like theirs too. Have you actually listened to one? Any theories as to why something with a dysfunctional schematic can actually work so well for its users?
"Any theories as to why something with a dysfunctional schematic can actually work so well for its users?"Define "work so well".
No, I haven't listened to one.
When I build a circuit I try to follow known engineering practices.
Would anyone load a 300b into an impedance lower than it's own plate resistance?
I think most understand that would lead to a high percentage of harmonic distortion.
Rule of thumb would be a ratio of plate resistance to plate load of 1:3 for an output stage and 1:10 for a voltage gain stage.
I'm sure there is a method to the madness, I just don't see it.
Numbers, The plate resistance of a 12ax7 with a 910 ohm, unbypassed cathode resistor is somewhere north of 150k.
That plate resistance is in parallel with the 150k plate resistor so we have an output impedance of 75k driving the RIAA filter that is in parallel with the following stage's grid resistor.
At the knee of the filter the reactance of C202 is equal to the value of R205. 6800 + 6800 in parallel with the grid resistor 150k = 12637 ohms with a 10K in series. So we have a source impedance of 75k driving 22637 ohms.
That creates a very vertical load line for the 12ax7 resulting in high harmonic distortion. Maybe the voltage swing is so low that that doesn't matter. My question is why load the 12ax7 into such a low impedance?
What's the up side?
I have a question, have you listened to a phono preamp that follows normal engineering practices such as having stages with low enough output impedance to properly drive the load and keep harmonic distortion low and also lets the RIAA filter parts do the frequency shaping alone, without having to factor in losses caused by a gain stage that's struggling to cope with the load?
I'm not saying that the Cornet doesn't sound good. Enough people like it that it must have something going for it. But what if the same designer, with all his talents, solved the issues I bring up? How good would it sound then?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/14/12 08/14/12
I am not an electronics wizard, but it seems like you're working backwards. Why break down a schematic for a piece of gear and decide it's faulty, without ever hearing one? Do you do that for all your equipment, before you buy? "This can't be any good, look at this circuit!"
I don't mean this the way it sounds, honestly. It just seems misguided. If it sounds good, it works. Could it sound better? I don't know. Does it need to sound better? Probably not.
I don't buy preamps or power amps anymore. I build my own.
But if I were going to buy something I would want to see the schematic first.
The reproduction of an audio signal is not magic. It requires circuits that are designed correctly.
I'm not saying that there aren't different ways to skin a cat but the cat needs to be skinless in the end.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
In a way, I'm glad I don't have those skills.
I can understand that sentiment.
But I'm 57 years old and have been doing electronics since I was about 13.
I burnt up a table radio doing a dumb thing. I replaced a tube with the wrong type (it looked the same, it really did :-) ) and Dwight Finuf MADE me pay attention to every move he made trouble shooting the thing.
When he was done and the thing was fixed I understood exactly what burned up and why.
Dwight (K6LOV) worked for Northrop Aerospace. He worked on the electronics that released the para-shoot for the Apollo missions and when Northrop would lose their government contracts and let everyone go, Dwight would open a TV repair shop.
There are a lot of circuits that will amplify audio signals and some can sound good to a lot of people. That doesn't mean they are amplifying the signal with the least amount of change to that signal.*
If you study the plate curves of a tube, the tube will tell you the operating point (voltage, current and load line) where there will be the least harmonic distortion.
You have to watch out for Miller and stray capacitance and make sure you have the drive current to overcome that shunt capacitance. All audio transformers in the circuit need to be the best you can afford with the highest bandwidth.
I subscribe to Lynn Olsen's design philosophy.
Make every stage as linear as you can.
That is my goal, and the way I see it, the original goal of this hobby. In a way that goal has been forgotten and a lot of people claim that the only goal it to "make the owner happy". There are a lot of what I call "Happy, Happy sound machines" out there. I mean they are sound producers not sound reproducers.
I guess that's fair enough but it's not the original meaning of Hi-Fi.
*With a phono preamp we have to restore the frequency response and phase response using an inverse RIAA filter. But even this we want to do in a precise way, not just something that "sounds good".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
A response to your earlier post from one more knowledgeable than I:
Lets look into 12AX7 datasheet and specifically at "Class A resistance-coupled amplifier" section. Here we can find the standard usage with the following params:
Ub=300V (Cornet 2 Ua=300V)
Rk=900Ohm (Cornet 2 Rk=910Ohm) - the only difference is bypassing the Rk in the datasheet
Rp=100kOhm (Cornet 2 Rp=150kOhm)
Rg=100kOhm (Cornet 2 Rg=47kOhm in the 1st stage, 150k in the 2-nd stage)
Rload=240kOhm (Cornet 2 Rload= from 110kOhm at 20Hz to 16,5kOhm at 20kHz in the 1st stage, from 8 MOhm at 20Hz to 11,5 kOhm at 20kHz in the 2nd stage)
Counted load from datasheet: around 70 kOhm; from Cornet 2: 1st stage 63kOhm to 15kOhm, 2-nd stage 147kOhm to 10,5kOhm.
It seems there is no such catastrophic differences from the datasheet usage in design. The only question left is high-frequency loads. But at the output there is only small amount (-40dB at 20kHz comparing to 20Hz) of high frequencies. Input voltage is 5mV gives us only around 200-250mV (1st stage) of the anode voltage swing total, so linearity is clearly not damaged.
Maybe I'm wrong, but my impression that the output impedance of the stages on AX7 in the design is about 170kOhm. About 2 years ago I counted the value somehow and proved by recounting the RIAA curve.
It's not the voltage or the plate resistor value. The cathode should be bypassed but the real problem is the load.
A 12ax7 will not drive (without getting into trouble) the impedance represented by the RIAA filter in that design.
Let's look at the RCA 12ax7 phono pre.
http://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htm
This is a simple phono pre with a "one shot" RIAA filter between the two triodes of a 12ax7.
R6 (R204 in the Cornet a 10k) is 470k.
The output is labeled 220k ohms min.
The operating point of the 12ax7 stages themselves are fine (well, not really. I would run twice the current but that's another story). It's the load that's the problem.
In the second stage of the Cornet the tube is playing into 3520 ohms at the high frequencies.
Again, it's not the operating point of the 12ax7 in the Cornet that is wrong. It's the load line. At the high frequencies the tube is being loaded down so you have the filter shaping the frequency response and you have the loaded down tube shaping the frequency response.
When a filter shapes the frequency response you get the response you want and the phase shift you want but you don't get added harmonic distortion.
When a loaded down tube stage shapes the frequency response you get added harmonic distortion.
If the filter is designed to not load down the tube, you get the phase and frequency response you want without the added harmonic distortion.
A phono preamp doesn't have to load down the tube to shape the frequency response. Think of a RC filter as a frequency dependent voltage divider.
The RIAA filter designer tool in the link below assumes that the gain stage has a zero ohm output impedance and is not being loaded down my the filter. The filter is doing the frequency response shaping all by itself.
If the values chosen, by the user, load down the tube then the frequency response will not be correct. I assume the designer of the Cornet took the fact that the tubes are being loaded down into account and adjusted the values of the RC filter networks accordingly.
That solves the frequency response problem but does not address the harmonic distortion problem.
I could be all wrong. The Cornet might be a great phono preamp.
I just don't understand why anyone would build it that way.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I really know very little about amplifier design and I just bought the circuit board for the Cornet2. Is there any way to make it perform properly? In other words, is there a simple modification to make it a low distortion phono stage because that is what I want. Should I simply dump the project and buy something else because I have not yet ordered the enclosure or any of the parts. What I'm looking for is a topnotch MM phono stage that I can use with a step-up transformer for my Technics SL-1200. What do you think I ought to do?
Feel free to email me privately.
Thanks,
John Elison
This is what I listen to (with some slight mods since I drew this schematic). It doesn't make a lot of gain but I use a MC step up transformer that makes up the difference.
My cartridge is .2mv output and I use a Jensen 1:36 transformer so I'm putting 7.2mv (or so) into the phono pre. The output is stronger than my CD player.
My preamp is on 3 chassis. One for the CCS feed shunt regulated B+. It uses two vr150 tubes in series.
The second chassis just for the LCLCRC DC filament supply and the preamp circuit is on the third chassis.
I use the 10m45s in cascode for the CCS's and for all tube stages I use the mu output to keep the drive impedance low.
The autoformer volume control is all I have for a line stage.
I have to say, I would bet your Pass Labs phono pre will be hard to beat but it would be an interesting comparison.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I need at least 40-dB gain with 100k-ohms input impedance. I'll be using a DL-S1 cartridge with a 1:40 SUT.
How much gain do you get from your phono stage?
Yeah, my Pass Labs XOno is pretty good, but it has only one input and I have two turntables. Furthermore, I thought it might be fun to get into a tube phono stage.
Best regards,
John Elison
"How much gain do you get from your phono stage?"
I haven't measured it. All I can do is repeat what I said in my post.
My cartridge is .2mv output and I use a Jensen 1:36 transformer so I'm putting 7.2mv (or so) into the phono pre. The output is stronger than my CD player.
You would have 6mv. I think you would have plenty of gain. You run a line stage preamp with gain, right? I don't have any gain in my AVC.
"I need at least 40-dB gain with 100k-ohms input impedance. "
In a tube circuit the input impedance is simply the value of the first stage grid resistor. I left that resistor unlabeled in the schematic as it can be any value it needs to be for cartridge loaded.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think you should hear one first. Clearly, they perform to a lot of folk's satisfaction.
I'd love to hear one. I'd love to measure one. However, I don't want to put any more money into this project if the phono stage is plagued with harmonic distortion.
I owned a Dynavector XV-1 cartridge several years ago that I bought brand new. When I measured it, it had four times the harmonic distortion of my DL-103R and my OC9ML/II using the same test records. To this day I believe the coil around the front pole piece was put there to increase harmonic distortion because Dynavector believed it made the cartridge sound better. I don't like harmonic distortion and I don't want a phono stage that produces excessive harmonic distortion simply because its designer thinks harmonic distortion makes vinyl sound better.
If you have a Cornet2 that you can lend me so I can hear it and measure it, I would appreciate that very much.
Best regards,
John Elison
Opus 104
...with the Cinemag SUT !! But the Hagerman is nice and if priced right, would be a good phono.
to the hotrodded 834P. It did everything "better" than the EAR except having a big 3D soundstage.
Now, however, my Shindo pre puts them both to shame.
Anyone want to buy a JLTi? :D
...to use the 834P in MM mode then couple it with a quality external SUT for a MC cartridge. Much much better that way. I didn't believe it until I tried it for myself. ;-)
Yes, did that too (and preferred that way on the JLTi and Shindo as well) - Jensen, Bob's Cinemag, EAR MC-4. Still, the JLTi was significantly better in my system.
Semi-octal Cornet 2, NOS tubes, all Teflon caps, copper chassis.....
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