Weak left Channel - What to check? [resolved]

[jrhunter62]

Just finished my Eros 2 Build and it sounds great, but the left channel is noticeably weaker than the right.  I have swapped the L and R tube positions, no change.  Swapped cables, no change.  I hooked up my Schiit Mani phono pre and the channels were balanced.  I have looked over the connections and solder points and nothing looks suspect to my inexperienced eye. What should I check, test, re-solder to fix this? 

[Paul Birkeland]

Well, we have seen a fair number of situations like this over the years.  The difficulty of diagnosing this with a phono preamp is that the equalizer to correct for the RIAA curve should drag down the response (varying according to frequency), so sometimes people will be debugging the channel with low output when in actuality the EQ isn't connected and it's the loud channel that's causing the problem. 

Could you post the voltages on the small center PC board?  (Any of those terminals with a wire leaving that say "I" or "O" on the label)  Possibly there's something buried in the DC voltages that would give us an idea. 

Something else that occasionally happens is that the 100uF caps are installed backwards or not well soldered and that will cause a gain difference, as would a loose 10,000uF cap up on the front terminal strips. 

[Doc B.]

Make sure the EQ components are attached at the right points too. I've seen a miswire there cause this problem in the past. This would include the components on the five lug strips inboard of the two front tube sockets.

[jrhunter62]

I have gone over the passive RIAA connections and didn't find anything that looked out of place.  The voltages checked as follows:

IA - 217v
IB- 217v
OA - 159.8
OB - 166.3
OC - 98.9
OD - 99
OkA - 99.2
OkB - 99.3
OkC - 1.44
OkD - 1.49

[Paul Birkeland]

Yeah, those are dead nuts on.  That's sort of a good thing and a bad thing.  When you have voltages that are that perfect, you'll be looking at an EQ issue, or a solder joint on the input or output of the preamp.  Something like this would have me looking really hard at the solder joints on the RCA jacks and on the coupling caps. 

In the absence of something obvious, can you download a signal generator app on your phone, turn it all the way down, then up one click on the level, then us an 1/8" to RCA adapter to send signal into the amp.  With your meter set to AC voltage, try measuring the AC voltage across the input jacks with your meter at 20Hz, 1000Hz, and 20,000Hz.  This is one of those times where a cheap meter that only has a 200V and 700V AC scale will not quite be up to the task of what you're doing, but the $25 meter at Harbor Freight can resolve low AC voltage levels and I think should at least pick up the 1kHz tone without too much trouble. 

Anyway if you can feed a consistent amount of signal into the input jacks and successfully measure it (with the Eros off), then we can move on to additional testing. 

-PB

[jrhunter62]

I have the tone generator app, the RCA adapter, and a decent multimeter.  I don't exactly understand how to conduct the test you suggested.  Am I measuring the output jacks?

[Paul Birkeland]

You need an 1/8" TRS to RCA cable to plug into your headphone jack on the TRS end and the input of the Eros on the other.  You are feeding the signal from your phone into the Eros.

[jrhunter62]

Got it.  What am I measuring? The output jacks?  What should I see?

[jrhunter62]

I have measured both the input and output jacks and I don't read anything on the DMM while sending a 20hz, 1000hz, or 20,000hz tone.  I am placing the test leads on the soldering points on each jack (input, output, L, R).  am I doing this correctly?

[Paul Birkeland]

AC voltage across each input jack.  You can post a picture of your meter if you need help setting it properly.

[jrhunter62]

I have attached several pictures of what I am working with and how I am testing. 

[Paul Birkeland]

Try 60Hz.  The red clamp that you're using probably isn't a good idea.  It looks like it's touching the center barrel of the RCA jack and the outside of the RCA jack, which will short out all the signal and give you a 0V reading.

Your meter specifies a 2V scale for AC so it should work properly to take these measurements.

From what I can see of your build I would add a little solder to just about all of the solder joints and a lot more heat.

[Doc B.]

The meter should be connected to an input jack - where you show it - to measure the level you are putting into the Eros from the signal generator. The level should be set to around 5mVrms AC. I would be inclined to test the output from the generator before I even connected it to the Eros just to verify signal is present and near the correct level. 

Once you have verified the signal level is present and set where you want, move the test leads to the output jacks and measure that level. 

[jrhunter62]

At 60hz both input jacks measured .04.  I got a reading of 0 on the output jacks with the amp off.  With it on I measured 36 on the right and 38 on the left.

[Doc B.]

.04V is 40mV. If possible you want to set the signal level at the input to be about 5mV which is .005V.  Once you get that set measure the output level again. Then try 1000Hz and 10000 Hz both set at 5mV, and see what the output voltage reads.