KEL84 - Slight channel imbalance

[Sid and Coke]

Hello folks,

I was just about to do a few A-B demos between two Phono stages that i have at my disposal for a few days; a brand new Cambridge Audio 640P and my existing Rotel RQ970BX (with my version of the HFW Andy Grove DIY mods). The CA640P has less gain than my RQ970 so i wanted to set the volume level accurately for a fairer comparison between the two. I used a couple of test records and a cheap Tandy analogue SPL meter on a tripod.

During this set-up phase i noticed that my KEL84 has a channel imbalance between the L and R channels, the RH is about 2dB less than the LH. So far i have;
i) Transposed my R & L ECF80's - no change
ii) Transposed V3 & V4 - no change
iii) Transposed V5 & V6 - no change
iv) Transposed L & R speakers - no change
v) Transposed L & R speaker wires - no change

Therefore I don't reckon that my channel imbalance is caused by my speakers, speaker wires ( slightly different lengths), ECF80 valves or EL84 valves.

This isn't a major problem and I didn't actually notice it until i got the SPL meter out. I actually sit slightly closer to the RH speaker than the LH one anyway so for real life practical purposes it is probably doing me a slight favour.
Test equipment can give you too much information sometimes.

[david counter]

could be the volume control?

all pots are not symmetrical in their response , normally it's more noticeable at lower volumes

[hooger]

Just a hunch, but is the room / loudspeaker placement perfectly symmetrical? If not then then the chances are that each loudspeaker will couple in differently with the room and hence give a slightly different level for each. This will be particularly apparent for single frequency testing. Try swapping the outputs so that the left ch goes to the right LS and vice versa.
Hooger

[Sid and Coke]

I've just repeated the test using my CD player and a test disc with tones on it and one channel is definately lower than the other. So it isn't my Cartridge/ tonearm wire, or TT interconnect either.

[Sid and Coke]

Originally Posted by hooger

Just a hunch, but is the room / loudspeaker placement perfectly symmetrical? If not then then the chances are that each loudspeaker will couple in differently with the room and hence give a slightly different level for each. This will be particularly apparent for single frequency testing. Try swapping the outputs so that the left ch goes to the right LS and vice versa.
Hooger

I've done all the easy/obvious swaps Hooger, as listed above.

The RH Speaker is slightly more into a corner, however I would say they are fairly evenly placed in the room. If anything I would have thought this would actually boost the signal from the speaker.

I'm not going to loose any sleep over this as the amp still sounds fab. I have a DMM and an oscilloscope at my disposal.

Where to plug what and what to look for would be the question though

These DIY valave amps definately are fit and forget thats for sure

Or should i just ditch the test equipment put some AC/DC on and relax - ( don't answer that one ...)

[hooger]

Sorry, just trying to help.

[andrew ivimey]

but but but, you didn't notice this 'imbalance' till you got out a cheap analogue SPLmeter ? 'nuff said.

Of course it never is 'enough said'. Many text books describe 1dB something like 'the smallest amount of sound difference that is reliably perceived by the human listener' So, 2dBSPL should be pretty easy hear. But you didn't, until you got the meter?

To go further then, (if the room is reasonably symmetrical with as little as possible to mess up the balanced soundfield and no reverberation) .... then,
you are putting out 1kHz tone or a steady stream of broad band noise and the analogue scale of the meter is large enough to see a 2dB steady (mean?) diffference, is it? Playing music is useless for this sort of test.

The SPLmeter is in exactly the same place (suggest horizontal to the speaker cone and one meter away, for the test weren't you?

One CD player as reference.

The above gives a fair test.

You gave five conditions you didn't change the interconnects right to left, left to right. That might give you piece of mind ... or drive you nuts.

If it ain't the above then it might be something inside the amp.

later,

hm, seems like most of the above is covered.

[Shian7]

Why not fire up your scope and try this on each channel:

http://www.wduk.worldomain.net/forum...read.php?t=155

[John Caswell]

Hi S&C et al
As you have a test DVD with 1 KhZ tone on L & R channels, set it up into the amp and measure, with a DMM set to 2V ac, across an 8R resistor instead of the LS, and with the output set to about 1 - 2 volts. This at least give you a definitive idea of levels.
Don't use an SPL meter for this, as it is affected by all things extraneous.
If the levels are drastically different then there is likely to be a problem with the amplifier. Obviously report back here and we should be able to sort it out

John - Dr John

[Shane]

Try swapping L & R inputs and L & R speaker leads at the amp input and output terminals.

If the problem changes channels, it's the amp. If it doesn't, it isn't.