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Old 29th March 2011, 02:05 AM   #1
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Default Need help building a tube linestage for my gainclone

I am trying to build a linestage for to connect between my DAC and my four channel TDA7294 amplifier to add the tube like warmth to the sound.
The tubes i have are 3 12ax7's 4 7025's 1 12at7

i built this http://www.hagtech.com/images/clarinetschem.gif and there was hum on all volumes levels even when the preamp was off, and there was i high level of noise. i am confused about making a srpp do i use 1 12ax7 for the top and bottom of the circuit and how do i elevate the heaters. do i ground the heater. or do i use a seperate tubes for each section that would be four tubes for 2 channels?????

i bulit this circuit http://diyaudioprojects.com/Schemati...-Schematic.png i built it just like it said and for the power supply i used 280v from CRC filter and 12Vac for the heater. it turned on with super loud hum and became slightly less when the tube warmed up. i unplugged the ipod and it hums supper loud. sound does come out though.

is there any simpler circuits out there i don't need any/much gain but i want low distortion.
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Old 29th March 2011, 02:33 AM   #2
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None of the tubes you have on hand are really appropriate for a low gain line stage unless you want to use feedback. How about a 6CG7 or 6SN7 in standard common cathode?

Super loud hum sounds like bad filter capacitors or grounding problem. If the hum doesn't change with volume I suspect a power supply problem. Heater supply should be either referenced to ground or some elevated voltage.

Can you post some pictures of the build? maybe something obvious will be visible.
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Old 29th March 2011, 02:43 AM   #3
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The tubes you have are all high gain and not suited to the job at hand.

A "unity" gain buffered volume control has the proper gain structure, but will not add that touch of 2nd order harmonic distortion you crave.

IMO, you need low net gain common cathode topology and the ability to drive a relatively low downstream impedance. There are several ways to get those characteristics, but (IMO) the most economical is circuitry based on 12B4s. Any number of good 12B4 designs are available to you. I [naturally ] have uploaded my own design. You don't have to use the pseudo choke I/P filter PSU shown. A well filtered and reasonably well regulated 150 VDC supply of 50 mA. will do quite nicely. Also, a costly, "boutique", part need not be used in the O/P coupling cap. position

BTW, the schematic shows how you "raise" a heater to suppress noise and/or avoid exceeding heater to cathode potential limits. Please observe it's done via a resistive voltage divider across B+ and ground.
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File Type: jpg 12B4 Line Stage.jpg (24.4 KB, 230 views)
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Old 29th March 2011, 03:01 AM   #4
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Thank you for trying to help me

How do i add feedback to use 12ax7

How do i make the constant current source in
The circuit you sent
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Old 29th March 2011, 06:33 AM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by benproiii View Post

i bulit this circuit http://diyaudioprojects.com/Schemati...-Schematic.png i built it just like it said and for the power supply i used 280v from CRC filter and 12Vac for the heater. it turned on with super loud hum and became slightly less when the tube warmed up. i unplugged the ipod and it hums supper loud. sound does come out though.

is there any simpler circuits out there i don't need any/much gain but i want low distortion.
The above is exactly what you want. But what you found is that layout and construction details matter a lot. Following the schematic is not enough. You must follow a good star ground plan and I like to use shielded coax wires for anything going into a grid. (You only ground one end of the shield. The 12AU7 is a good tube for this. I'd never build a new amp using out of production tubes, They are still making the 12A*7 tubes.

If you are very concerned abut hum build a regulated 12V DC supply

You said "low distortion" then why bother? The entire point here is that you like the sound of a tubes distortion products. Do you really need a volume control?
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