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I've had my LX120h and the LX412 for a while now and never really questioned the power output since i'm only running one 8 ohm cab so i assumed that why it's not that loud (especially the clean channel), anyway...I decided to have a go by connecting the LX head to my carlsbro 4 ohm cab and to my surprise there was absolutly no f##### difference, I'm sure my laney HCM65R is louder on the clean channel. I've taken the LX head apart, checked what i could with a multimeter (diode junction, transister junctions and resistors etc) and they seem fine. Am I expecting too much from the 120 watts? ......My Carlsbro GLX 150 is ermmm....150 watts into 4 ohm and that really is mind-blowingly loud (and i mean....insanely loud). Does Anyone else here have the LX120h and cab and feel dissapointed with it's volume?
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Time to put the Laney on the bench and see what type of voltage you get into a 4 ohm load - EQ may also be a complicating factor. Then there always hearing loss
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Thanks for replying
Before i delve deeper into the amp, i need to find out whether it's a pre-amp or power amp issue, so......, is it possible to connect a guitar to an FX pedal then connect the FX pedal to the "FX return" so as to by-pass the pre-amp stage? I know messing around with the FX send/return can cause other problem as the signal path isn't always re-stablished after removing the Fx pedal
Also...if it is a pre-amp issue i need to remove the control knobs to remove the pre-amp, are the just pushed on or are they glued on?
As a way of getting accross the sort of output i'm getting on the clean channel......i can put all the EQ's up to 10, the volume up to 10 have the guitar about 2 feet away and i dont get any feedback. With my HCM65R i'll get feedback with the volume at about 5.Comment
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Try the HCM send into the LX return. Try the LX send into the HCM return. Should give you an idea whether it's the pre or power amp. The knobs are usually push on, but they can be so tight they will break the pot shafts before they slide off. Sometimes a heat gun or hair dryer will help them come off.
As far as the difference in power from the LX to an 8 or 4 ohm load, it may not be all that noticeable, probably less than 3db difference to the speaker. A bit of volume increase, but not like night and day or anything.Originally posted by Enzo
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Thank you g-one for that tip, had a go and by the sounds of it, it's the power-amp of the LX thats the problem. The pre-amp of the LX drove the power-amp of the HCM fine while the pre-amp of the HCM driving the LX power-amp showed a much lower output so much that the HCM output speaker was drowning out the LX.
On the circuit diagram there's two pre-set pots....."offset adjust" and "bias adjust" any one know how to make use of these? A line of text on the circuit diagram tell you to set 20ma between the two "testpoint" using VR5 (one of the volume pots i think) before adjusting bias but nothing about what to do next? Anyone any ideas?Comment
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You are confusing gain with output power, it´s not that way..i can put all the EQ's up to 10, the volume up to 10 have the guitar about 2 feet away and i dont get any feedback. With my HCM65R i'll get feedback with the volume at about 5.
You are comparing apples to oranges.
Yes, do not even look at them.On the circuit diagram there's two pre-set pots....."offset adjust" and "bias adjust" any one know how to make use of these? A line of text on the circuit diagram tell you to set 20ma between the two "testpoint" using VR5 (one of the volume pots i think) before adjusting bias but nothing about what to do next? Anyone any ideas?
Don´t mess with them.
They are absolutely not related to what you call a "problem" , and by messing with them you may easily overheat and burn your amp.Juan Manuel FaheyComment
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"Offset adjust" is for setting the Vdc offset at the output.(ie: speaker)
Ideally this can be zero volts.
"Bias Adjust" sets the idle current going to the output.
Measured at the stated test points.
(are you sure it is mamps, and not mvolts)
(if the TP is stated in ma, then you can measure in mv, & use ohms law to convert it to ma)
VR5 is not the volume pot.
It is a trimmer pot on the power amp board.
In my opinion neither of these adjustments is going to "fix" a low volume problem.
They very well may point a tech to THE problem, but they are not capable of repairing anything like a low output problem.Comment
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Ok, thanks for replying, so in your opinion, given the symptoms (the EQs @ 10 and volume @ 10 and no feedback) do you think I have a problem with my amp? I know it's impossible to quantify the output i'm getting on here. I was hoping some here had an LX120 to see if they are able to turn everything to 10, stand a few feet away and still able to convers with someone standing next to them. I am comparing the LX120 to my Carlsbor GLX150 which is 150W into the same 4 ohm cab and there is a massive difference in "Volume".Comment
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Ah yes...it's in mV not mA, I was flicking between the circuit diagram for the amp and the datasheet for the SAP15N/P (the darlington transistors used in the amp). Also........the circuit diagram specifies SAP16N/P but the parts list shown SAP15N/P (and with STD03N/P as replacement). Could this be the issue?Comment
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You really should quantify the output power.
Hook up a volt meter set to read Vac to the amps output, with the cab connected.
Wail away & read the Vac indicated by the meter.
Voltage squared divided by the cab resistance= watts power output.
Measure both amps & you will have a base line to work from.Comment
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All three transistors are basically identical.Originally posted by RODNEY View Post
If there is a problem with the power amp a few basic voltage measurements would help to localize the problem.
First item to be checked is the power rails.
Are they similar voltage levels (with opposite polarity). Vdc.
Is there a minimum of Vac ripple on each rail (tens of mv's)
The base connections of the darlingtons should have the same voltage level, with one being positive & the other being negative.Comment
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Just done the measurements on both amps ..clean channels, volume and EQs on 10.
HCM65R = close to 20V... so (20 x 20)/8 = 50 watts, close enough to 65W.
LX120 = close to 8.5v..... so (8.5 x 8.5)/4 = 18watts.......good grief....worse than I thought.
Right then...back to the schematic it is.Comment
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ok, thanks....one thing I'm not sure of though, the bases on both darlingtons are connected together via VR2 in parrallel with a resistor so I'm to measure the both of the base voltage relative to...what.... test-point 1/ test-point 2 as appropriate?Comment
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Well now, it seems that you have me at a decided advantege.
You have the schematic.
The forum does not..
If you can, please post it.
As to the darlingtons, I can only go by what I have.
There is an app. note in the SAP15 data sheet that shows a recommended output configuration.
There is indeed a VR in between the bases.
That will not affect any measurements that you take on the bases. (unless it is shorted or, EEEK!, open)Comment
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