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Old 13th April 2015, 12:54 AM   #1
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Default DIY 12AU7 Hybrid Amp voltage variances

I know I should be better at finding answers in forums, but I have tried for weeks and cannot find an answer to my simple problem... So I apologize, because I'm sure this issues has been addressed a million times!

I ordered a Tube Hybrid Amp board from Fred's Headphone Amplifiers, and have put it together, fired her up and....

When using the amp with headphones the right channel sounds great, but the left channel is very faint sounds with minimal volume level

When I take voltage readings at the capacitors + points, I get 10.9 on on channel and 8.2 on the other.

I have tested all resistors for both channels and they show very little variance.

I have swapped out the op amp and same issue.??????
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Old 13th April 2015, 01:15 AM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by bconnelly View Post
the left channel is very faint sounds with minimal volume level.
It may be that the faint sound in the left is only crosstalk, and the left input is not working at all.
Have you tried another input cable to see if the problem is the cable? Can you measure the left tube voltages?
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Old 13th April 2015, 01:21 AM   #3
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The first thing to check is for dry soldered joints or joints missed completely.
A good visual check.
Then check right components have gone in the right holes.
Use a DMM to measure resistor values.
A circuit diagram would be useful so we can see any obvious mistakes in that.
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Old 15th April 2015, 09:09 PM   #4
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Ok,
So, looked over my solder joints real good, and I did find 1 joint that definitely was not good. I fact, I think I had over heated the joint and the metal ring round the hole of the PCB appears to be melted or at least had lifted up and was not making a good connection.

I scraped a little of the protective coating on the back of the board to reveal some copper by the joint and re soldered the joint. I touched up two other joints that looked suspect.

Viola, Both channels were working great, not perfectly balanced but close. I put a reading on the + leads of the 2 caps. The readings were 9.8 vs. 8.9. Definitely allot closer!!! Sounded great!!!

Well, I thought I had it fixed. Now, the amp plays both channels intermittently. The right channel always puts out a good signal. I will need to do some more observing, but it seems that when powering on the Headphone amp both channels play great for a period of time, and then the left channel goes out. Until I observe further, I cannot say for sure if it is related to me power it on initially, and goes out after a period of time, or if it's just coincidence

It seems that once the left channel goes out, it will not kick back in until it has be powered down, and sit for awhile...at least 10-15min.

Does this sound like a symptom that might point to an issue, or is it more likely just a coincidence, and I just haven't got a good solder joint???

B
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Old Yesterday, 11:09 AM   #5
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Join Date: Aug 2009
Did you check your tube socket connections ? Maybe the mechanical contact to the tube pins is not strong enough. Another idea would be that your tube is faulty ( eg. weak joint inside the tube which disconnects after a while ).

Also make sure to check with your eyes if both heaters of the tube are glowing.

Best regards,
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Old Yesterday, 11:16 AM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by bconnelly
It seems that once the left channel goes out, it will not kick back in until it has be powered down, and sit for awhile...at least 10-15min.
Could be something overheating.

Could you point us to a circuit diagram?
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Old Today, 12:36 PM   #7
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Join Date: Feb 2015
Location: North Carolina
I purchased the amp kit from Fred's Amp's

Here is a link
12AU7 Instructions : Fred's Headphone Amplifiers

Thank you to everyone who has given their input!

B
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