EJ Jurich Preamp build.
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Ben Brearley
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Geoff, have you built the preamp? Was it a success? I am asking because I too have the Jurich book. I built the buffer circuit w/ the turntable preamp. For some reason which I don't understand, the volume was always at maximum. I used a pot w/ audio taper. I assembled the preamp following Jurich's component layout illustration. It irritated me immensely. So I rebuilt it, this time using my own layout on turret boards. I hooked it up to my amplifier last night (a dynaco st 70), and once again, maximum volume. I can't find any contact address or email for Mr. Jurich, and I have a few questions I want to ask him.
Since Mr. Jurich has apparently gone into hiding, I am hoping someone here can tell me what I did wrong. I am not an expert on tubes or electronics. It could easily be a stupid mistake on my part. Or it could be a problem w/ Jurich's schematics. I am posting a scan of the schematics. If necessary (and anyone cares) I could post some photos of my recent attempt. But any help would be appreciated. I admit to being a novice and will not be offended if someone points out my obvious stupidity/incompetence. I just need to know what is going wrong here. Thank you.
- Ben
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There's no reason the schematic for the line stage should result in constant volume. I wonder if you might have somehow connected the volume control(s) incorrectly. Maybe a closeup photo would be helpful in figuring it out.
Jack
Jack
Ben Brearley
New Member
Thank you so much for responding. I will upload a few close up photos in the next hour or so.
Ben Brearley
New Member
That took longer than expected. I can't get the battery on my wife's camera to charge, so I took video of the preamp using these ridiculous "spy" glasses. I could explain but it's not worth it. On to the photos. There are two of them.
In the first one you can see the pots, right above where I helpfully wrote "potentiometers." The purple wires attached to the right tabs are the signal, coming from the TT preamp via a switch and, before that, through 2.2 uF 100 volt capacitors. That right there may be the problem? Missing 220k resistors? You think?
The middle tab directs the signal to the buffer circuit. The final tab goes to ground, through a 10K resistor.
I am going to add a 220 K resistor to each side and what that does. Could this be the problem? I will return with some hopefully better photos once my wife is conscious. Thank you Jack for the suggestion.
In the first one you can see the pots, right above where I helpfully wrote "potentiometers." The purple wires attached to the right tabs are the signal, coming from the TT preamp via a switch and, before that, through 2.2 uF 100 volt capacitors. That right there may be the problem? Missing 220k resistors? You think?
The middle tab directs the signal to the buffer circuit. The final tab goes to ground, through a 10K resistor.
I am going to add a 220 K resistor to each side and what that does. Could this be the problem? I will return with some hopefully better photos once my wife is conscious. Thank you Jack for the suggestion.
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Be sure those two 10KΩ resistors connected to ground are really 10K. In fact, those aren't needed, you could connect that lead of the two pots directly to ground. Also be sure the other grounds of the preamps - such as the two 1KΩ resistors connected to the cathodes of the first tube - are connected to the same ground.
Jack
Jack
setup1
Super Member
I just noticed the E J Jurich phono section schematic looks very much like the RCA phono with passive RIAA EQ found in the back pages of the RCA tube manual which requires a minimum 220K load! If used with the linestage/buffer with 100K input level controls, the frequency extremes of the phono section will prematurely roll off and sound anemic.
This is correct, 220KΩ is the minimum recommended load. The simplest solution would be to use a dual 250KΩ pot in place of the 100K. The OP could also eliminate the 0.1uF caps and 470K grid resistors at the inputs to the line stage.
Jack
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Ben Brearley
New Member
Thank you for the replies and the interest. In the past I wasn't able to find anything about E.J. Jurich on the internet. Yesterday, after posting my reply to Geoff, I decided to consult Google one more time: I found a website and contact information. So I wrote to Mr. Jurich explaining my problem and he wrote back, saying I should eliminate the 10K resistors which go to ground off the left tabs of the potentiometers. I have not yet done this, but soon will.
I will also try replacing the pots with 250K pots, if I have any on hand. I have a feeling I don't. If I don't, I will order some, and investigate this idea further.
The grounding situation is as follows: since I built everything on turret boards, the grounding goes from turret board to turret board and finally to the power supply area, where it meets up with the safety ground from the power plug and both are attached to the chasis. Is this a good strategy, or no?
I will also try replacing the pots with 250K pots, if I have any on hand. I have a feeling I don't. If I don't, I will order some, and investigate this idea further.
The grounding situation is as follows: since I built everything on turret boards, the grounding goes from turret board to turret board and finally to the power supply area, where it meets up with the safety ground from the power plug and both are attached to the chasis. Is this a good strategy, or no?
I try to build so that the ground from each stage has its own wire back to the main power supply ground. This creates a "star ground" and helps to eliminate ground loops around those stages. In your case for example (the line stage), I would connect the input jack grounds, the bottom of the level pots, the bottom of the cathode resistors and the bottom of the grid resistors together, and run a wire from that point (which would not be connected to the chassis) to the main power supply ground. This prevents the ground current that's drawn by the 12AU7 cathode followers from modulating the ground current flowing to the input stage. The two flow on separate wires, rather than mingling together in a common path through the chassis. This probably isn't so critical in a low gain preamp, but it can cause big issues in an integrated amp where the ground current supplied to the outputs is so substantial.
Jack
Jack
Ben Brearley
New Member
Thank you Jack, that is good information. By the third time I read your response I was able to visualize what it is you are saying. The use of the turret boards encouraged me to see things in a modular type of structure. Although that is not compatible with the ideal grounding set-up you outline. I enjoyed the puzzle-like work of making everything fit on the turret boards. This adds a further piece to the puzzle, which I will modify.
As far as puzzles go, Mr. Jurich said I could get rid of the 10K resistors. He said their purpose was to provide for "unity gain." Am I correct in thinking that unity gain means the signal entering the device will have the same voltage as the signal exiting the device? If so, what is the potentiometer for? Do I not understand "unity gain"? Since I'm using the st70, which has no volume control, why would unity gain be useful, for me? What I'm trying to build is not just for a turntable. There are two inputs, one of which goes to the turntable preamp and, exiting that, to a switch; the other goes directly to the switch. That switch then sends whichever signal to the pots, and onward. Are all my ideas about what I'm creating just confused?
As far as puzzles go, Mr. Jurich said I could get rid of the 10K resistors. He said their purpose was to provide for "unity gain." Am I correct in thinking that unity gain means the signal entering the device will have the same voltage as the signal exiting the device? If so, what is the potentiometer for? Do I not understand "unity gain"? Since I'm using the st70, which has no volume control, why would unity gain be useful, for me? What I'm trying to build is not just for a turntable. There are two inputs, one of which goes to the turntable preamp and, exiting that, to a switch; the other goes directly to the switch. That switch then sends whichever signal to the pots, and onward. Are all my ideas about what I'm creating just confused?
That's an interesting comment by Mr. Jurich. There are a couple oddities in this circuit. I'm going to simulate it in SPICE and let you know what I see for total gain. I have standard gain blocks and followers already modeled. I'll get back to you in a bit...
OK, a simulation with the line stage driven by the last 12AX7 stage of the phono preamp indicates a gain of about 7dB with the level pot at minimum. With the pot at maximum, with or without the 10K resistor, gain is about 28dB. The latter is equivalent to roughly X25 in voltage, and I consider that way too much to be usable. Unless there's a compelling reason why you would need that much gain, I really think a different line stage configuration would be advisable. That's a whole different conversation. 
Jack
Jack
Just thinking out loud here, I think you would be better served with a preamp like this. Gain is about 10dB, or approximately X3. This is much more usable for driving the ST-70, and it can be tweaked if necessary using the resistive divider between the two stages. That divider serves to both reduce AC gain and set the DC bias of the follower.
Jack
Jack
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Ben Brearley
New Member
Thank you very much Jack. Your help (esp. by running SPICE and sharing the results) went above what I was ever expecting. I see now that the entire project was doomed, at least for my needs. It might have been fun twiddling with turret boards, but the frustration of going over everything multiple times without resolution was really getting me down. It may seem melodramatic, but tossing that Jurich book in the trash and switching my focus to something totally different is a damn good feeling.
I was thinking of pulling out my old PAS 3 and maybe updating that w/ some of the stuff I found online. But the schematic you provided above might be a better idea. Plus, it wouldn't leave me confined to PCBs, which I don't like.
I was thinking of pulling out my old PAS 3 and maybe updating that w/ some of the stuff I found online. But the schematic you provided above might be a better idea. Plus, it wouldn't leave me confined to PCBs, which I don't like.
Ben Brearley
New Member
Despair is not the right attitude.
The power supply has never been a problem, and if the phono section is from an RCA manual, I have to assume it is sound. So I will insert the circuit you suggest and get back to you with the results.
The power supply has never been a problem, and if the phono section is from an RCA manual, I have to assume it is sound. So I will insert the circuit you suggest and get back to you with the results.
Most preamps have too much gain IMO. In fact, I only recently began to consider this in earnest and to address it in my own system. The heavily modified SS preamp I'm using for backup (while another project is underway) exhibits about 6dB maximum gain (X2). Under most conditions, this results in a volume control setting around 10 or 11 o'clock. Regardless of all the factors that might influence the exact setting (speaker sensitivity, room size, program material, etc.), the bottom line is that the preamp easily can drive my amplifier (roughly the same sensitivity as a ST-70) well into clipping with all sources.
One aspect of this that I find advantageous is that it positions the volume control much farther off its counter-clockwise stop than would otherwise be the case. This means it's usually operating outside the area where volume controls tend to track more poorly channel-to-channel as they age and wear. In my case, the 30-something year old volume control is now using a portion of the pot that was likely seldom used by previous owners, so that potential issue is avoided entirely. Also, at any given system volume, much less of the carbon track is in series with the signal. This can result in a major improvement in clarity and overall signal quality.
The only potential downside to all this is that it also moves the operating position of the pot farther up on the pot's taper. If carried to extremes, it could result in the user losing the benefit of the low-level adjustment vernier made possible by the log (audio) characteristic. However, the curves for potentiometer tapers indicate this won't occur to a substantial degree until the pot is set around the 3 or 4 o'clock position. With a 1V signal source and typical amplifier sensitivity (and 6dB available gain in the preamp), that would be way past the point of heavy clipping. A Loudness tap might also be rendered ineffective under these conditions, but my preamp doesn't have that function. Overall, I think you'll find that the circuit above performs well and is very transparent when used ahead of the Dynaco.
Jack
One aspect of this that I find advantageous is that it positions the volume control much farther off its counter-clockwise stop than would otherwise be the case. This means it's usually operating outside the area where volume controls tend to track more poorly channel-to-channel as they age and wear. In my case, the 30-something year old volume control is now using a portion of the pot that was likely seldom used by previous owners, so that potential issue is avoided entirely. Also, at any given system volume, much less of the carbon track is in series with the signal. This can result in a major improvement in clarity and overall signal quality.
The only potential downside to all this is that it also moves the operating position of the pot farther up on the pot's taper. If carried to extremes, it could result in the user losing the benefit of the low-level adjustment vernier made possible by the log (audio) characteristic. However, the curves for potentiometer tapers indicate this won't occur to a substantial degree until the pot is set around the 3 or 4 o'clock position. With a 1V signal source and typical amplifier sensitivity (and 6dB available gain in the preamp), that would be way past the point of heavy clipping. A Loudness tap might also be rendered ineffective under these conditions, but my preamp doesn't have that function. Overall, I think you'll find that the circuit above performs well and is very transparent when used ahead of the Dynaco.
Jack
