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Tube Phono Preamps
Tube Phono Preamps
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Old 14th October 2016, 07:26 PM   #211
pchw is offline pchw  United States
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Tube Phono Preamps
Quote:
Originally Posted by Sodacose View Post
Glad there's another one in the wild, Fred! Thanks for sharing your build. Any tweaks or tricks you'd care to share?
I have to admit that it was really a "follow the recipe (the excellent blog)" and then tweaked according to what were in my possession.

First trick (not so much a trick, more like a common sense ) is not to use an undersized chassis. The decent size Hammond chassis allowed me to use double adhesive tape to play with various placements with the parts. Still I didn't place the 12AX7 as best as I could. I was too obsessive to put them far from the iron and ended up with some challenge to solder on one side - too close to the side of the chassis.

Second tweak was to figure out how to use this Healthkit 54-950 transformer in this build. I measured the ESR and unloaded VAC and used the data to model the PSU in PSUD2. I had to go bridge full wave to get the voltage. Therefore, SS diodes were used. The beauty was I could model the psu according to the parts that I found in my wife's junk box

I used PSUD2 to model the blog's PSU up to the VR resistor at 25ma load. The idea is, if I manage to reach the same "entry" point, I can very much follow the same circuit beyond the VR resistor. I then tweaked the PSUD2 model of my transformer to roughly match the same voltage at the same point (approx. 330VDC). In my live preamp, I measured 24ma consumption from both channels.

Since this transformer has a 14.5 (measured 16.2 unloaded!!) VAC secondary, it was a no brainer to go DC filament supply using a savaged L7812CV which is bolted and isolated to the chassis. This likely contributes a great deal to the quietness of the phono-pre.

For the RIAA resistors, I essentially series and parallel resistors to get to the values that I wanted. Online parallel resistor calculators were my friends. The RIAA caps are Russian polystyrene (0.01uf) 0.1% and Wima (3300pf) 2%, matched between channels. I wired one 12AX7 per channel; hence, it was easy to visualize whether I wired both channels right or wrong :-) I had the grounding of each channel converged into one point and then to the 1st cap's ground. That's why there are a lot of black wires.

If you look at the 2 new pictures, you will see that none of the filter caps protrudes to the surface. They are all stuck to the chassis using the 3M VHB double adhesive tape which sticks like cement after overnight cure. I can lift the whole chassis by holding the 2 450/220uF caps!!!! This tape is really handy to avoid punching and drilling in certain occasion.

The only little drama that I had was one of the socket pins had solder inside. I didn't know until I had trouble to insert a 12AX7, doh!! The lessen is, if I reuse used sockets, I better check the socket to make sure this will not happen again. The oh $h|t moment was just no joy :-)

2 more pics from the front and the back, it is kind of some what "empty" .....

Cheers,
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Old 16th October 2016, 08:13 PM   #212
adam1016 is offline adam1016  United States
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Hey, I've got all the parts to build this amp, but have a couple questions, as this is my first project of this caliber.

1. Should I try and layout all the parts in the chassis, then wire? Or other way around? I'm struggling with the build order a bit.
2. I'm sure this will make a bit more sense once I've got the transformer in hand, but just to confirm, the heater for the 12ax7 will come from the 2 green leads on the transformer, which are +/- 6.3V, giving me a heater voltage of 12.6V?
3. I sorta see the grounding going on, but the one big thing that confuses me is are all grounds the same? Like is signal ground and power ground the same point? From what I can tell, there are two grounding points in Sodacose's build, one on the right terminal of the motor run cap, and the other one the screw in the chassis to the left of that. Why not ground everything to the chassis?
4. I'm sure there's a way to say this properly, but the resistors between the cathode and ground, do those just serve to reference the cathode to above ground? What does that even mean?
5. One other thing, when testing all of this, I'd like to test the power supply first, especially as I build it. Can I test the power supply with just a multimeter? I'd like to test it after the 5y3gt stage, after the filtering stage, and after the voltage regulation stage?
Sorry, I'm not too solid on my EE and tube theory. I sorta get what's going on, having build a gainclone in the past, but this is just so much more complicated. Are there any good, short, primers on what each component in an amplifier does?

Last edited by adam1016; 16th October 2016 at 08:28 PM.
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Old 18th October 2016, 03:57 PM   #213
Sodacose is offline Sodacose  United States
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Quote:
1. Should I try and layout all the parts in the chassis, then wire? Or other way around? I'm struggling with the build order a bit.
Yep, this is how I would do it for sure. Get all your sockets, transformers, and caps mounted first. Then begin the actual wiring process. You'll want to position the 12AX7s further away from the AC inlet, the power transformer, and filtering bits so that it is physically separated from the noisy bits.

Quote:
2. I'm sure this will make a bit more sense once I've got the transformer in hand, but just to confirm, the heater for the 12ax7 will come from the 2 green leads on the transformer, which are +/- 6.3V, giving me a heater voltage of 12.6V?
The transformer in the BOM has a 6.3V winding with a center tap. You'll need to wire the 12AX7 heaters in parallel (ie one end of the heater tap to pin 9 and the other end to pins 4 and 5). The center tap will be grounded, usually through a simple resistor voltage divider (I think the explanation for this is on my write-up). Heater leads are probably green for the ends and green/yellow for the center tap (again, going by memory).

If this is your first tube project, I highly suggest going with DC heaters. My preamp had AC heaters and it was silent at normal listening levels, but it is not hard to end up with heater noise in such a high gain circuit. Merlin has a really good explanation of DC heaters on his website here: The Valve Wizard

Quote:
3. I sorta see the grounding going on, but the one big thing that confuses me is are all grounds the same? Like is signal ground and power ground the same point? From what I can tell, there are two grounding points in Sodacose's build, one on the right terminal of the motor run cap, and the other one the screw in the chassis to the left of that. Why not ground everything to the chassis?
All the grounds are connected at only one point (star ground at cap). The TT ground post is also the chassis ground, IIRC. I ran all of the signal grounds to the star ground and all of the power grounds to star ground without mixing "types." Running all of the grounds directly to the chassis would almost guarantee a ground loop and lots of noise.

Quote:
4. I'm sure there's a way to say this properly, but the resistors between the cathode and ground, do those just serve to reference the cathode to above ground? What does that even mean?
You got it! A tube is biased by creating a voltage difference between grid and cathode. The grid is at ground potential and the cathode is above ground potential by virtue of the current through the cathode resistor (volts is ohms times amps).

Quote:
5. One other thing, when testing all of this, I'd like to test the power supply first, especially as I build it. Can I test the power supply with just a multimeter? I'd like to test it after the 5y3gt stage, after the filtering stage, and after the voltage regulation stage?
Yes you can use a DMM to test this. Just be sure it can handle high voltage. You'll need to test each of these points one at a time and you need to do so safely! Don't move the probes around in a powered up device. Hook the DMM up, power on, observe, power down. The write up includes a discussion of bleeder resistors but be doubly sure all caps are discharged after powering down and before poking around.
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Old 18th October 2016, 08:30 PM   #214
adam1016 is offline adam1016  United States
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So the heater winding has 3 taps, a top, bottom, and center tap. The voltage between the two terminals is 6.3VAC, and the difference between the center tap and each terminal is about 3.15VAC. I have 6.3VAC coming from both terminals, which is the proper heater voltage, that I can then wire in parallel, as wiring in series would cause a voltage drop across all the heaters.

What I don't understand, even after reading the Valve Wizard article, is how referencing the center tap to the heater voltage helps reduce hum. So I have that clean. filtered, and rectified B+ coming out after the rectifier, filter, and regulator section. I would then be putting +32VDC into the center tap of the heater coil. If I understand what this does, this makes the AC voltage float on top of this DC voltage, making it an AC wave between about 29 and 35 volts, which is more than the heater needs, which I know is wrong. What does having that floating voltage actually do??
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Old 18th October 2016, 08:51 PM   #215
rongon is offline rongon  United States
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Tube Phono Preamps
Quote:
What I don't understand, even after reading the Valve Wizard article, is how referencing the center tap to the heater voltage helps reduce hum. So I have that clean. filtered, and rectified B+ coming out after the rectifier, filter, and regulator section. I would then be putting +32VDC into the center tap of the heater coil. If I understand what this does, this makes the AC voltage float on top of this DC voltage, making it an AC wave between about 29 and 35 volts, which is more than the heater needs, which I know is wrong. What does having that floating voltage actually do??
There are interactions between the tube's cathode and heater.

First, there is a maximum DC voltage difference allowed between the heater and cathode. For most common tubes this is 100V DC. So, if the cathode of a tube has a high voltage, for instance as would happen with a cathode follower, then you need to make sure the heater's voltage is 'biased' to a DC voltage that keeps the heater voltage less than 100V different from the cathode voltage.

There is some evidence that having the heater sit on a DC voltage of about +40V from the cathode reduces hum and noise.

A common point of confusion is caused by the use of the word 'ground.' Most will assume that ground has to be at 0 volts DC. This is actually not the case. 'Ground' can be lifted above 0VDC (a positive DC voltage), or it can be a negative DC voltage.

The heater's voltage (or the filament voltage, same thing) is referenced only between the tube's heater pins. It will need a path to 'ground', but that ground can be lifted above 0 volts.

Let's say we have a power supply transformer with a 6.3V AC secondary winding that is center-tapped (written '6.3VCT'). The center tap of that winding can be connected to +40VDC, while the AC supply to the tube's heaters is still 6.3VAC.

The secondary winding's center tap has been 'grounded' to +40VDC. Sorry about the sloppy use of the word 'grounded' -- that's how people often use it. The more correct term would be that the secondary winding center tap has been referenced to +40VDC. However, the heater supply is still 6.3VAC.

This can be done with a DC heater supply too, as long as the 'ground reference' is 'lifted' to +40VDC. The DC heater supply is still a 6VDC supply. It's just referenced to a higher DC voltage, often said to be 'lifted above ground.'

I hope that's not confusing.
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Old 18th October 2016, 09:50 PM   #216
Sodacose is offline Sodacose  United States
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Yeah, what Rongon said

It should also be added that "grounding" the center tap either to 0V or some DC reference and twisting your heater leads collapses the magnetic field created by the heater current (because it is then flowing in equal but opposite directions). This reduces the chance that 60hz hum will leak into other parts of the circuit. Not having a center tap (or not using a virtual center tap) with AC heaters increases the possibility of heater induced hum.
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Old 13th June 2017, 05:29 PM   #217
adam1016 is offline adam1016  United States
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https://imgur.com/gallery/bjgKz

Here are the guts of my build of the phono preamp posted by Sodacode

I am having some issues with the sound, specifically with the dynamic range, and having rolled off mids and highs...
I followed the schematic and BOM to the letter. The MOSFETS test fine, and it's making sound. I think the dynamic range might be a result of high noise floor from 60Hz hum, which I plan on elevating right now, and probably rectifying and regulating later.

For more records of the troubleshooting, see my reddit thread here:
https://www.reddit.com/r/diytubes/co...ds_underwater/
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Old 30th October 2017, 06:54 PM   #218
pelayostyle is offline pelayostyle  United States
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Hi Sodacaose, love your website btw. So I'm going to be building your phono preamp soon. Is the schematic on post #81 the newest or should I stick to whats on your website?
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Old 30th October 2017, 07:03 PM   #219
Sodacose is offline Sodacose  United States
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Hi pelayostyle,

Go with the website please. That is easier for me to keep updated with info. One thing I did not include is regulated heaters. I definitely recommend that route to nip any hum in the bud before you build.

Best of luck on your build!
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Old 30th October 2017, 08:09 PM   #220
pelayostyle is offline pelayostyle  United States
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Quote:
Originally Posted by Sodacose View Post
Hi pelayostyle,

Go with the website please. That is easier for me to keep updated with info. One thing I did not include is regulated heaters. I definitely recommend that route to nip any hum in the bud before you build.

Best of luck on your build!
Yup, I have a Pete Millet filament regulator board that I'll be using for the build. I'll post pics here and on r/diytubes (Hello fellow redditor) once I'm done.
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