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Summing channels;Why? JE EL34

9050lx

Member
2010-08-29 1:20 pm
fl
I built an JE labs EL34 amp recently.while checking for output from my source I disconnected one RCA channel only to get a summing through both speakers(both speakers play when either right or left rca jack is disconnected..I have checked and rechecked my wiring and can not discover why the amp is doing this.Any old pros here point me in the right direction? I have checked the output connections and verified they aren't crossed.I have rechecked the 6sl7 and EL-34 connections and checked they are exclusive to each side.Any help to point me in the right direction is greatly appreciated.
 

rayma

Member
2011-04-29 8:37 pm
I disconnected one RCA channel only to get a summing through both speakers(both speakers play
when either right or left rca jack is disconnected.

If both outputs are the same with only one input, there must be a connection between the two inputs.
When both inputs are connected, do you get stereo, or mono (just a center image)?
 
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rayma

Member
2011-04-29 8:37 pm
I think I get stereo when both inputs are connected, as you can hear an additive component to the sound.

I would look carefully at the input sockets and level controls, for a wire or resistor connection
(or wrong connection) between the channels. Since this was just built, the odds are that it's a wiring error.

Or, maybe the speakers are wired wrong. It looks like the amplifier's outputs are floating.
 
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rayma

Member
2011-04-29 8:37 pm
What if one of the outputs b+ and plate wires were miswired? Could this cause an issue?
I have looked at this circuit so much I can't see objectively right now.

Both channels share the power supply. Even if the plate leads of the output transformers were swapped,
that would not cause sound in both speakers. Do you have a connection pictorial diagram?
Can you post a photo of the chassis wiring? Take a day off and look at it again.
 
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9050lx

Member
2010-08-29 1:20 pm
fl
jeel34.jpg
My cell phone will not take a good pic to save my life.
 

rayma

Member
2011-04-29 8:37 pm
My cell phone will not take a good pic to save my life.

Notice that the two channels are completely independent except for a connection to the power supply.
So, if you have one input that goes to both speakers equally, there must be a wrong connection.
A very small amount of crosstalk can be normal though, especially with one input open.

Take a break for a day or two, and then look at the wiring again.
 

9050lx

Member
2010-08-29 1:20 pm
fl
Inputs are separate, I swear.The outputs are separate connection wise except where I Y'ed the B+ with separate fuses for the output transformers.The voltages seem to be correct.Using EL34-A Shuguangs to save my Mullards.700 ohm cathode resistors and 406 volts B+ via monster 10 lb potted milspec transformer with bridge rectifier and solid state UF5008 diodes.Separate filament transformer for 6 volts from Edcor. (2) chokes with cap input and (2) 560 ufd 450 volt caps (got them cheap)10 watt Transcendar SE 5K outputs.Sounds pretty good but should function correctly!
 
If you have a function generator you can determine whether the coupling between channels is capacitive or an actual short between the inputs. If capacitance the separation between channels will decrease with increasing frequency, if not it will be constant.

Look for wiring that is very close together, and bear in mind if input related the problem becomes more significant the higher the source impedance.

One other test to perform would be to short the input to one channel and drive the other and see how much cross-talk you get.

Remember to load both channels with appropriate load resistors or in a pinch, the speakers.
 

9050lx

Member
2010-08-29 1:20 pm
fl
Check the connection of the 40uF filter/decoupling caps. If they are not connected correctly they could increase the cross-talk. (They are shown connected correctly in the schematic.)
I even replaced these capacitors this AM on a hunch and it made no difference.Maybe I need to step away and make sure I am not seeing things incorrectly.I must be missing something obvious.
 
... .Maybe I need to step away and make sure I am not seeing things incorrectly.I must be missing something obvious.
It's very easy to miss something - we've all done it!
One suggestion would be (with power down and caps drained) to check for continuity between every connection point in the left channel and the corresponding connection point in the right channel. Apart from the B+ point there should be no continuity, of course. (Although you should read some DC resistance between the 'left' EL34 plate and the 'right' EL34 plate, via the two output transformer primaries. Also some DC resistance via the power supply dropper resistors at the tops of the 40uF caps.)
 
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