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Post by DavidR on Apr 22, 2020 17:53:48 GMT -5
Is it suitable to stack transformers as in the attached picture ? The larger is for the Pre amp board and the smaller for the Phono.
It would seem to me that the eddy currents from one would affect the other.
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Post by audiobill on Apr 23, 2020 11:06:50 GMT -5
Ther's room to put one behind the other....
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DYohn
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Post by DYohn on Apr 23, 2020 12:49:11 GMT -5
There is nothing wrong with stacking toroidal transformers. The magnetic fields are well contained, which is one advantage of this design. That setup looks good to me, and you only need one mounting bolt.
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Post by DavidR on Apr 24, 2020 18:47:38 GMT -5
Ther's room to put one behind the other.... Except the wires were trimmed.
I have a hum in The Phono that is driving me crazy. Its only noticeable on soft passages. The Pre is dead quiet with CDs. The tubes for the Phono are a (V1) 1961 Sonotone 12AX7A/7025, perfect balance, supposedly not a drop of noise. They are known for being the best of the best. (V2) 1962 Mullard 12AX7A Voice of Music and (V3) 1960s era Telefunken 12AX7.
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Post by donh50 on Apr 25, 2020 12:04:16 GMT -5
I agree with DYohn it is unlikely to be the transformers themselves, stacked or not. You could try a mu-metal shield between transformers and the circuit boards. It could still be the tubes but if you swap them around and it is in both channels probably not. Vintage tubes can still be noisy and their gain may be lower. I went through a few "good" tubes at times trying to find the best ones for the phono stage. But, IIRC the issue was usually noise and not hum. Hum usually comes from different things (again IME). Do you have hum if there are no other inputs connected? Phono ground wire attached (chances are it would be really loud if not)? Are the filaments AC or DC? I did switch to DC in one design. Bear in mind the last time I rolled my own tube preamp was ca. mid-1980's, with a few designs in the 1990's, but not since. Tubes are single-ended so limited CMRR/PSRR, and phono circuits are high-gain, so it can be hard to eliminate hum. It can be insidious... Is it fundamentally 60 Hz or 120 Hz? That would tell you if it is before or after a full-wave rectifier circuit. IME experience grounding was the biggest culprit but it can be ridiculously hard to nail down for such a simple circuit. Isolating signal ground from chassis and safety ground with say 0.1 to 2 ohm resistors often helped (I had a bin of them from 1 W to 5 W or so; do not recall the UL requirements off-hand). Making sure you don't introduce a ground loop in the preamp helps. I once spent days troubleshooting only to discover I had tied shield grounds together at the input selector switch, which was after the phono stage, but made a nice little ground loop among phono stage, main preamp stage, and power supply. Clipping the phono shield wires at the switch cured the problem (grounding was handled elsewhere and the shield was still connected at the board side). Similarly isolating the phono inputs from chassis or other input grounds can help (or not). It is tricky since you have to be careful to keep a good ground for safety and playing around with grounds can cause some nasty issues at the outputs. I usually used a 'scope and spectrum analyzer on the outputs (with a 50k ~ 100k resistor to make sure the outputs were laoded and not floating) instead of debugging with it in the system. HTH, good luck! - Don |
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DYohn
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Post by DYohn on Apr 25, 2020 12:26:50 GMT -5
Could also be a ground loop between devices, which can be incredibly difficult to eliminate in phono pres. Make sure your turntable is grounded to the chassis ground of the preamp.
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Post by DavidR on Apr 25, 2020 14:35:28 GMT -5
The TT is grounded to the Pre/Phono but it might be my older PowerStat variac. It only powers the amp but is on the shelf directly below the Pre/Phono. The variac usually hums and sometimes has a slight buzz.
I used to use the ST120 amp with my Emo XSP-1 G2 but the variac was about 5 ft away. Plus I don't know if the variac is sending 'noise' thru to the amp.
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Post by donh50 on Apr 25, 2020 15:07:08 GMT -5
Magnetic fields don't travel far in terms of large values (the field strength falls off rapidly) but could you just move the variac to the side and see if it changes anything? Still suspect something else, but that might be an easy check.
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Post by DavidR on Apr 25, 2020 17:36:07 GMT -5
Yeah, I can move it and see. I also have a speaker next to it. Good thing its a temp area.
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DYohn
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Post by DYohn on Apr 25, 2020 17:46:35 GMT -5
Variac? Why are you using a variac?
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Post by DavidR on Apr 25, 2020 18:03:34 GMT -5
Its recommended to keep the voltage to the amp at 117V. My house voltage is normally slightly above 122V.
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DYohn
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Post by DYohn on Apr 25, 2020 18:19:49 GMT -5
Its recommended to keep the voltage to the amp at 117V. My house voltage is normally slightly above 122V. That's really not necessary. |
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Post by DavidR on Apr 25, 2020 18:31:22 GMT -5
It's not self regulated. No auto bias. The guy (Bob Latino) who sells them said to use a variac especially if the house voltage is high like mine (i.e. >122V). So did audiobill who sold me the amp. The bigger question for me is could it be may problem with hum in the speakers. I haven't had a chance to move it. Maybe tomorrow morning. |
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Post by DavidR on Apr 25, 2020 20:00:48 GMT -5
What the designer and builder of the Premp and Phono said about the variac being in close proximity.
Me: "I have an older PowerStat variac that hums and sometimes buzzes. It sits on the shelf directly below the Pre/Phono but only feeds the amp.
Is it possible that is causing the hum thru the Phono section to the speakers?"
Roy: "Yes, very definitely!! If you have a variac or other transformer near the phono section, it will pick up electromagnetic radiation and amplify it.
There's 40db of gain in phono and another 15db of gain in the line section. That's about 560x of amplification.
Also anything like florescent or halogen lamps, same problem."
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Post by DavidR on Apr 26, 2020 11:20:59 GMT -5
Today I set up the variac on the other side of the room (over 12ft)and ran an extension cord to the amp. Its NOT a proximity issue. I then measured my voltage at an outlet and as typical on Sundays it was below 120V so I hooked it up without the variac. I still had the hum! My next step might be to order some tubes suggested by the designer/builder and pay the fee for low noise testing/selection. They aren't too expensive. If that's not it I'm done with it. I'll go back to using the Emotiva XSP-1 G2 and have the amp sit on the small table next to the rack.
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KeithL
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Post by KeithL on Apr 26, 2020 12:04:47 GMT -5
The signal coming from a phono cartridge is tiny and so requires a massive amount of gain... hundreds of times more gain than a line level signal from a CD player.... And, just to add insult to injury, the RIAA equalization that is required for phono cartridges applies an extra 20 dB of boost at low frequencies....
The reality is that, on top of that, most tubes just... plain... aren't... all... that... quiet. It is quite normal to hear a little bit of hum or hiss during quiet passages from a tube phono preamp...
(You're never going to achieve the S/N of a CD with vinyl anyway.)
And, because they operate at very high impedances compared to solid state devices, tubes are far more sensitive to picking up hum from external sources.
And, no, 12AX7 tubes are NOT the quietest tubes on the planet...
In terms of noise 12AX7's are merely "pretty good" for applications like microphone and phono preamps... and no fancy branded ones are going to be significantly better. (People buy vintage Mullard and Telefunken tubes "because they like the way they sound".... NOT because they are especially quiet compared to other decent brands.)
(If you want a quiet tube, for low-noise high-gain applications, look at something like a 6267....)
Toroidal transformers are very low in magnetic flux leakage (compared to other types).... but we live in a sea of electromagnetic noise...
But, because the signal levels in a phono preamp are so tiny, noise control and proper layout is super SUPER critical...
Look at your grounding... very carefully... Make sure ALL of the low level signals going around inside the amp are using shielded cables... or twisted pairs... And make sure they don't run next to wires carrying AC current to light tube filaments...
And try moving some of the wires around a bit... sometimes even a tiny change will result in a major improvement in noise.
And, are all your tubes using DC power for the filaments?
AC power on the filaments, especially on phono preamp tubes, will definitely contribute hum...but there are ways to at least minimize it...
If your amp uses AC power for the tube filaments even a hum balance potentiometer will probably help...
Consider tube shields... metal ones... that are grounded... (I didn't see any in the picture. Tube shields aren't for vibration... they're for blocking noise.)...
Consider rotating the angle of the phono preamp board... which changes the angles between the traces on the board and the magnetic fields around them...
Consider a phono preamp on a separate board... with a metal shield between it and the other circuitry... preferably ferromagnetic metal like iron or steel.
Consider putting that tube phono preamp in a separate box... with no AC coming into it at all.
Making a very low noise phono preamp is difficult... and it's a lot MORE DIFFICULT with tubes... The reality is that a little bit of hum is just to be expected in all but the most exceptional tube phono preamp. If you want a super-quiet phono preamp then tubes just aren't the best way to go about it... (But you should at least be able to get the noise level below the level of the surface noise you're going to have with vinyl....)
Ther's room to put one behind the other.... Except the wires were trimmed. I have a hum in The Phono that is driving me crazy. Its only noticeable on soft passages. The Pre is dead quiet with CDs. The tubes for the Phono are a (V1) 1961 Sonotone 12AX7A/7025, perfect balance, supposedly not a drop of noise. They are known for being the best of the best. (V2) 1962 Mullard 12AX7A Voice of Music and (V3) 1960s era Telefunken 12AX7. |
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Post by KeithL on Apr 26, 2020 12:16:27 GMT -5
You're getting precariously near a very expensive rabbit hole... with nary a rabbit, or a pocket watch, in sight... Unless there's something wrong with it a tube it will not HUM and a "low noise select tube" will not hum less. A noisy tube might hiss... or it might crackle... and a tube might be "selected" for this... but tubes are not "selected for low hum".
And a shielded tube could pick up hum if its internal shield had a cracked connection... but 12AX7's aren't shielded.
It is remotely possible that a faulty tube might pick up excessive hum from its filament... if the filaments are AC powered... and the cathode insulation is failing.
And some very sophisticated balanced circuits may be quieter if you supply them with matched tubes. But, by and large, buying more expensive tubes is not a good way to try to reduce hum. Hum is almost always a circuit issue.... or a wiring or layout issue... and not a bad tube issue.... and certainly not a "tube quality" issue... (But, yeah, I VERY much doubt you're going to get a tube phono preamp to be anywhere within a mile of as quiet as the phono preamp in an XSP-1.)
Today I set up the variac on the other side of the room (over 12ft)and ran an extension cord to the amp. Its NOT a proximity issue. I then measured my voltage at an outlet and as typical on Sundays it was below 120V so I hooked it up without the variac. I still had the hum! My next step might be to order some tubes suggested by the designer/builder and pay the fee for low noise testing/selection. They aren't too expensive. If that's not it I'm done with it. I'll go back to using the Emotiva XSP-1 G2 and have the amp sit on the small table next to the rack. |
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Post by DavidR on Apr 26, 2020 14:45:19 GMT -5
Thank you Kieth for that explanation. The Pre/Phono tube unit was built by the designer so I assume he knew what he was doing.
Perhaps I'll just go back to using the XSP-1 G2 with the tube amp or call it quits with tubes all together. I had the tube amp on a small table behind my large AR90s but need to move all that SS gear downstairs to the Living Room to power my AR9s. I need the 500wpc from the SA-250 to drive them. I had wanted some gear to remain in my office for my smaller 3-way ARs. Since I was enjoying the tube amp with vinyl I thought "why not a Pre/Phono tube unit". Well, NOW I know why the answer should have been "No". My XSP-1 + SA250 are DEAD quiet even with the volume all the way up. Its easy to get used to that. Also, I find tubes addictive and not because of SQ but more curiosity to what it will sound LIKE compared to another. I will add that the tube amp and AR speakers are a wonderful combo for vinyl.
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Post by DavidR on Apr 26, 2020 15:32:17 GMT -5
You're getting precariously near a very expensive rabbit hole... with nary a rabbit, or a pocket watch, in sight... Unless there's something wrong with it a tube it will not HUM and a "low noise select tube" will not hum less. A noisy tube might hiss... or it might crackle... and a tube might be "selected" for this... but tubes are not "selected for low hum".
And a shielded tube could pick up hum if its internal shield had a cracked connection... but 12AX7's aren't shielded.
It is remotely possible that a faulty tube might pick up excessive hum from its filament... if the filaments are AC powered... and the cathode insulation is failing.
And some very sophisticated balanced circuits may be quieter if you supply them with matched tubes. But, by and large, buying more expensive tubes is not a good way to try to reduce hum. Hum is almost always a circuit issue.... or a wiring or layout issue... and not a bad tube issue.... and certainly not a "tube quality" issue... Wouldn't that show up as grid leakage on a tube tester? My tube tester does emission strength, shorts and grid leakage. They all showed NOS emission value and no shorts and no grid leakage.
Tomorrow I'll put the tube amp back with the XSP-1 and see what happens.
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Post by DavidR on Apr 30, 2020 15:52:15 GMT -5
I may have found the problem tube. Attached is a picture of the Mullard/VM 12AX7A where the upper insulating spacer plates have separated and several pieces of mica are floating about the tube.
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