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Old 18th September 2004, 12:05 PM   #1
@mot is offline @mot  Croatia
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Default Tube preamp problem

Hi,

I am building tube preamplifier with ECC82 valves. It is SRPP desing with 350V power suply voltage. Well, my problem is that I have buzz mixed with sound and I don't know how to solve that problem. Buzz is louder with more opened volume pot. I did everything to solve it: remove input selector(I left only one input without any relays and output directly connected to power amp), stabilized voltage using TL 783, star ground scheme(left and right channels are separated completly-I am using one power suply so it is only point of connection). All of this didn't help. Is it possible that my suply voltage is too high? or it can't deliver enough current? Transformer is toroid with 300V 50mA secondar.
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Old 18th September 2004, 12:45 PM   #2
SY is offline SY  United States
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Have you scoped the power supply rail to see what's going on there?

And I wouldn't discount a ground loop- there are star grounds and star grounds.

It's often useful to have someone else go through your wiring to see if you've made an obvious error; they're the ones that will drive you crazy because you're soclose to the problem, you can't see them.
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Old 18th September 2004, 01:00 PM   #3
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Hi,

With a B+ of 350V on the top triodes you're very close to exceeding the max. design ratings for the heater/cathode insulation of the ECC82/12AU7.

As you're using two ECC82s you should use one for the top triodes and the other for the bottom ones.

The heater supplies should be independent of each other so the top triodes heaters can be bias upwards by injecting a portion of the B+ into the heater circuit.

If you must use a single heater supply or have to use a single valve per channel you can average the voltage you inject into the heater supply so that you still stay within limits for both valves.

Cheers,
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Old 18th September 2004, 01:21 PM   #4
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A little more on Frank's comment, as a fan of SRPP stages myself, I float the heater supplies and bypass them AC to ground through a 10uF, 250V 'lytic. Drops heater noise by 80dB
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Old 18th September 2004, 01:26 PM   #5
PassFan is offline PassFan  United States
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Do you have a coupling cap on the input? Perhaps the buzz is coming from your source. Maybe this is too obvious but one never knows.
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Old 18th September 2004, 01:28 PM   #6
@mot is offline @mot  Croatia
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Default Re: Tube preamp problem

Quote:
Originally posted by SY
Have you scoped the power supply rail to see what's going on there?

It's often useful to have someone else go through your wiring to see if you've made an obvious error; ...
Well, I don't have scope, and I don't know anybody that colud check my wiring. That colud be the problem
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Old 18th September 2004, 01:31 PM   #7
@mot is offline @mot  Croatia
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Quote:
Originally posted by fdegrove
Hi,

With a B+ of 350V on the top triodes you're very close to exceeding the max. design ratings for the heater/cathode insulation of the ECC82/12AU7....


I am using a single heater supply arranged to "float" at aobut 90V with respect to the ground to ensure that the upper triodes does not exceed the safe limit, and I also use a single valve per channel. I built printed circut board so I would like not to change that.
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Old 18th September 2004, 04:14 PM   #8
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If your "buzz" is volume-dependant, then it is most probably a wiring/grounding problem between your input and your circuit...or you are picking up noise from another component. Have you tried shorting your inputs to make sure that your noise isn't coming from outside your preamp?
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Old 18th September 2004, 06:58 PM   #9
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Like suggested I looked ground wiring,did some changes and buzz is reduced. On one input (no comopnents connected to preamp inputs) there is no buzz at all, but on other three it still stays...
It's going to drive me crazy
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Old 19th September 2004, 01:07 PM   #10
PassFan is offline PassFan  United States
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Perhaps it is a question of layout? Depending on what type of preamp you could consider it as a high gain device and more susceptable to field induced hum. Can you move any power supply components or wiring away from the signal wiring, or visa-versa? Have you neatly ordered all you wiring and kept heater wires spaced out into the case corners? Sometimes the buzz comes from a choke or transformer mounted a little too close to the signal path. It sounds like you are moving closer to the end so don't give up.
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