Hello everyone!
I've just noticed that when my tube preamp is plugged into my computer (giving it a ground connection) that even with the computer sound muted (I'm sure that it is NOT the computer or a virus making this music) If I turn up the volume of my amplifiers connected to my tube preamp (all of which are solid state) I can hear the faintest bit of music, it's clear but so so faint, and almost drown out by the pink noise that my tube pre-amp makes (it's a homemade, self designed low voltage line level preamp, it's not perfect).
Here's a schematic that I made a bit ago of my Low Voltage Line Level Tube Preamp (I believe the only change is R4 is now 10K)
Any help or information is appreciated, thanks!
EDIT: I've just found out that it's actually picking up an AM station, which is actually located about 3 miles from me...
I've just noticed that when my tube preamp is plugged into my computer (giving it a ground connection) that even with the computer sound muted (I'm sure that it is NOT the computer or a virus making this music) If I turn up the volume of my amplifiers connected to my tube preamp (all of which are solid state) I can hear the faintest bit of music, it's clear but so so faint, and almost drown out by the pink noise that my tube pre-amp makes (it's a homemade, self designed low voltage line level preamp, it's not perfect).
Here's a schematic that I made a bit ago of my Low Voltage Line Level Tube Preamp (I believe the only change is R4 is now 10K)
Any help or information is appreciated, thanks!
EDIT: I've just found out that it's actually picking up an AM station, which is actually located about 3 miles from me...
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Tubes are like jfets make R5 100k and c2 1uf
c3 = .22 uf and R4 = 330k
R1 and R2 may have to be 47 to 100k due to the low plate voltage
60 to 80 volts on plates may work better
c3 = .22 uf and R4 = 330k
R1 and R2 may have to be 47 to 100k due to the low plate voltage
60 to 80 volts on plates may work better
Metal shielding of whole preamp...100uF elko+1uF PP+.1uF PP to V+...from 100pF to 1nF to input...from 10R to 100R between R3, R5 common point and GND... correct shielded cable between computer and preamp... etcetera..etcetera...
12AU7 tubes do not operate properly with very low plate voltage.
You are using solid state resistor and capacitor values on this tube stage.
The tube is detecting the AM radio signal because the tube is very close to cutoff.
It's rectifying the RF and producing audio.
This is a very poor design.
You are using solid state resistor and capacitor values on this tube stage.
The tube is detecting the AM radio signal because the tube is very close to cutoff.
It's rectifying the RF and producing audio.
This is a very poor design.
Here's an article that describes using a gridstopper to filter redio frequencies from the grid. The Valve Wizard
I built a low voltage (12v) 12au7 based headphone amp from a design on another diy site I was getting radio reception as well. I played around with a few different resistors values on the grid until I got the right balance of enough volume and RF interference low enough.
I built a low voltage (12v) 12au7 based headphone amp from a design on another diy site I was getting radio reception as well. I played around with a few different resistors values on the grid until I got the right balance of enough volume and RF interference low enough.
Are your input RCA jacks connected directly to the metal chassis, as they should be?
When all else fails, read the instructions:
"A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing"
by Jim Brown K9YC
Audio Systems Group, Inc.
The basis of this tutorial is a combination of my engineering education, 55 years in ham radio, my
work as vice-chair of the AES Standards Committee working group on EMC, and extensive research
on RFI in the pro audio world where I’ve made my living.
http://www.audiosystemsgroup.com/RFI-Ham.pdf
*******************************
"RF Interference in Audio Systems"
These are my Power Point slides for a tutorial lecture held at the 2008 Fall Convention of the Audio Engineering Society in San Francisco. The presentation describes the fundamental mechanisms by which RF interference occurs, then shows how interference can be avoided by proper system design and how it can be eliminated from existing systems.
http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf
When all else fails, read the instructions:
"A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing"
by Jim Brown K9YC
Audio Systems Group, Inc.
The basis of this tutorial is a combination of my engineering education, 55 years in ham radio, my
work as vice-chair of the AES Standards Committee working group on EMC, and extensive research
on RFI in the pro audio world where I’ve made my living.
http://www.audiosystemsgroup.com/RFI-Ham.pdf
*******************************
"RF Interference in Audio Systems"
These are my Power Point slides for a tutorial lecture held at the 2008 Fall Convention of the Audio Engineering Society in San Francisco. The presentation describes the fundamental mechanisms by which RF interference occurs, then shows how interference can be avoided by proper system design and how it can be eliminated from existing systems.
http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf
I know many of the resistor values are not correct but I've done that temporarily to tone this down until I get a nice volume POT.
I also didn't use the common B+ voltages because I wanted this to be sort of unique. I'm not one of the people who can pick up a schematic online, build it, and then feel happy inside, if I do that all I feel like is that I copyed someone elses work, something that I could've just had someone else make. I like to be unique.
I like the look of my preamp as it is ( <== my profile picture) so complete metal shielding is not possible.
I don't really want to go to smaller capacitor values because for whatever reason, when I do the sound seems very tinny and bland, I'll leave all my coupling caps as they are. And yes I realize I need better cables, which will be coming soon.
Everyone tells me that, but then why is it you can find many hybrid designs with the 12AU7 that also use them at such a low voltage, and why does mine work, and even sound good.
And really I'd not say it's very close to cutoff as it works with less than 12V.
I find it a bit bold for you to call it a poor design, that is an opinion, to me it amplifies audio in as nice a matter as I want it to.
Thanks for the helpful link, I'll check it out.
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Any significantly non-linear circuit element will detect AM radio. You have two significantly non-linear elements: the two triodes (because they are being operated at too low a supply voltage). An input filter might help, but the only real solution is a complete redesign.
Hello,
I'd prefer not to redesign the whole thing, because it's unique as it is and is good enough in every other aspect. Your explanation is very clear of the problem at hand, I realized long ago that I wasn't going to achieve much linearity out of these with such low B+ but I never though that might cause it to pick up AM radio of all things!
I think most of the problems will be eliminated when I get a proper cord to connect this to the computer. It's only about 4 feet away but all I had left is some 15foot junk probably unshielded cord. I think a proper shielded cord will right this problem, but at least now I have an explanation as to how in the world this can happen.
Thanks.
A screened input cable may help. However, you might be surprised just how easy it is to pick up AM on the circuitry itself. AM field strengths can be quite strong. Linear circuits ignore it, so people don't even know it is there.
Well if I unplug the input to my tube preamp I can no longer hear the station, however, the pink noise increases when I do this so It might still be there but just covered up.
There is zero bias on the second stage. RF is passing thru the first stage and being rectified by the second stage. You will get far better performance with more B+ voltage.
An RF filter on the input? Why not re-design the circuit so that the tubes operate within the linear part of their curve? Increase the plate voltage and bias both tube sections properly. As it's designed, the circuit is a pretty good AM RF detector.
It's just a bad circuit design for a preamp.
It's just a bad circuit design for a preamp.
As I said, I want to keep it as it is! I'm no genius, I'm still trying to figure out how to do simulations in LTspice for another amp I'm building, but not telling anyone else about (reason being, I hate it when someone trys to tell me what to do, I design it how I want, I love suggestions but when people keep pushing me to do something just because it's not EXACTLY right, I get annoyed.)
I don't always want to take the easy way out, if I did I'd pull off the most frequently used schematic of a bone stock standard tube preamp and reproduce it word for word component by component and then get that dry feeling of almost accomplishment instead of having something truly unique, and perhaps not so perfect.
I WILL keep this amp mostly as-is, mostly because it's the first I've made, and should stay like that so I can always look back on it.
I know that getting the voltage up would make it much better but it also removes the originality. Then all it'd be is the normal gain stage out of every single amp that ever used a 12AU7 gain stage.
Yes thank you, we can all see it's pretty good at picking up RF and apparently not good (I disagree) at being a preamp.
Is this going to cause the sound to be tinny and bland (sort of like turning off Loudness on a solid state amp)?
I've not yet wrapped my head around the different grounding concepts, but I know I should learn, it'd probably help me with the monoblock I'm making too (12AU7A driving a 6CU6).
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You really should learn the basics of electronics before you attempt to design something.
If you "hate it when someone tries to tell you what to do" ... why are you asking for help?
If you "hate it when someone tries to tell you what to do" ... why are you asking for help?
I definitely do know the basics of electronics thank you very much, and most of what I learned was from me trying huge projects and then slowly learning as I complete them.
Oh and since when does the basics of electronics include tubes? Or rf filtering? Or how a "Poor" preamp can pick up AM?
I ask for help because there are some folks (like DF96) who really do want to help, and do so nicely.
Then there are others that just say no no your wrong, don't even try to fix it, just start over, that was horrible, bla bla bla........
I mean really, do you see how hostile and pushy your being towards me? Would you be happy if I told you to learn how to hold a conversation before trying to help someone?
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Guys take it down a notch or two, and to the OP if you post a design that others identify obvious shortcomings in you need to expect and take the criticism with some grace. Guys, don't make me go further ok?- Status
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