This Creative Labs SoundBlaster USB, model SB0490 external sound card I have. Hopefully it still works. Would that be sufficient one?
Yes, I guess so.. this one is three gang pot, two are for left and right separate, but turning together and third gang is for balance.
I somehow tend to agree 🙂 I find it very useful, and when looking at amp, always looking for that choices including balance..
And this JVC has also variable loudness, quite cool 👍
Morning thoughts:
a) you need to verify that the volume pot is in acceptable technical condition. Because if it is not then a replacement should be obtained (defect amplifier or pulled component from eBay etc) before investing time into further reconditioning.
The simple check would be to solder pieces of plain wire to earth and wiper terminals (from below the print board) and use two digital multimeters to monitor the resistance changes in both channels when turning the volume knob (slowly). The measured resistances have to change "smoothly" (without jumps and breaks) and in a similar tempo (they will never be equal, there will always be some difference between them).
b) you should "work" the filters, loudness and mute push-switches perhaps twenty-thirty times. These switches are in practice rarely used and their contact quality may deteriorate over the decades (from not being moved mechanically into another position).
And they are all in the signal path:
a) you need to verify that the volume pot is in acceptable technical condition. Because if it is not then a replacement should be obtained (defect amplifier or pulled component from eBay etc) before investing time into further reconditioning.
The simple check would be to solder pieces of plain wire to earth and wiper terminals (from below the print board) and use two digital multimeters to monitor the resistance changes in both channels when turning the volume knob (slowly). The measured resistances have to change "smoothly" (without jumps and breaks) and in a similar tempo (they will never be equal, there will always be some difference between them).
b) you should "work" the filters, loudness and mute push-switches perhaps twenty-thirty times. These switches are in practice rarely used and their contact quality may deteriorate over the decades (from not being moved mechanically into another position).
And they are all in the signal path:
Another quick test - to verify that the problem is not in the power amplifier section - is to feed a signal of one (better behaving) channel from preamplifier output via Y-splitter to power amplifier both inputs. If the power amp is ok (more or less) then the output sound should be the same in both channels.
A lot of posts overnight 🙂
I'm not sure where you are up to with it now... re the distortion at low volume etc. Are we saying that a clean of the relay did not fix that? The standout clue earlier was the problem righting itself as you increased volume up and then back down which is classic relay symptoms. Are there any speaker selector switches as they can do the same?
An easy check is simply to use a fresh music source recorded at much lower level meaning you need to turn the volume up much more. The relays and switches won't know the difference 😉 and so if it still behaves the same its probably not the volume pot but could well be a contact problem. I would at that point want to 'prove' the relay by simply linking it out as a test... and any speaker switches if it has them.
I'm not sure where you are up to with it now... re the distortion at low volume etc. Are we saying that a clean of the relay did not fix that? The standout clue earlier was the problem righting itself as you increased volume up and then back down which is classic relay symptoms. Are there any speaker selector switches as they can do the same?
An easy check is simply to use a fresh music source recorded at much lower level meaning you need to turn the volume up much more. The relays and switches won't know the difference 😉 and so if it still behaves the same its probably not the volume pot but could well be a contact problem. I would at that point want to 'prove' the relay by simply linking it out as a test... and any speaker switches if it has them.
Thank you for all suggestions 🙂 .. what first?
Today connected the amp.
The right channel was a bit weaker. While playing, many times distorted even at higher volumes (still using just headphones). Then the speaker relay again reacts to tapping. (I again might need to remove it and check it.)
Then I did repeatedly pressed and depressed many buttons many times - low and high filter don’t react in bad way, loudness and muting perhaps yes, speakers selectors also perhaps yes.
When all seemed to be fine, I removed rca jumpers and applied rca splitter on left preamp output and connected both power amp inputs. The volume pot reacted on each step from 0. Then I moved pre out to right channel out. The volume pot reacted well too, from 0. That may suggest volume pot is still ok.
Now I am listening a few songs on lowest volume. Both channels work fine. Moving volume pot up down, randomly, seems ok too. Listening now half an hour or more, remaining at lowest volume, and after 5 minutes or so right channel started to fluctuate - disappearing and reappearing. The right channel is always weaker.
FF
Another half an hour. The lowest volume, both channels seem stable and probably equal..
Today connected the amp.
The right channel was a bit weaker. While playing, many times distorted even at higher volumes (still using just headphones). Then the speaker relay again reacts to tapping. (I again might need to remove it and check it.)
Then I did repeatedly pressed and depressed many buttons many times - low and high filter don’t react in bad way, loudness and muting perhaps yes, speakers selectors also perhaps yes.
When all seemed to be fine, I removed rca jumpers and applied rca splitter on left preamp output and connected both power amp inputs. The volume pot reacted on each step from 0. Then I moved pre out to right channel out. The volume pot reacted well too, from 0. That may suggest volume pot is still ok.
Now I am listening a few songs on lowest volume. Both channels work fine. Moving volume pot up down, randomly, seems ok too. Listening now half an hour or more, remaining at lowest volume, and after 5 minutes or so right channel started to fluctuate - disappearing and reappearing. The right channel is always weaker.
FF
Another half an hour. The lowest volume, both channels seem stable and probably equal..
Try this as a definitive test.
Locate the appropriate pins on the relay and link them out under the board. This bypasses the relay completely. Continue to use only headphones for your testing while the relay is shorted, or should you want to use speakers then turn the amp on first and then switch the speakers on.
Make sure you get the correct pin groups. You do not want to short one channel to the other accidently. Having opened the relay it should be straightforward to determine the grouping.
The headphone output is at the top right (circled just for reference)
Locate the appropriate pins on the relay and link them out under the board. This bypasses the relay completely. Continue to use only headphones for your testing while the relay is shorted, or should you want to use speakers then turn the amp on first and then switch the speakers on.
Make sure you get the correct pin groups. You do not want to short one channel to the other accidently. Having opened the relay it should be straightforward to determine the grouping.
The headphone output is at the top right (circled just for reference)
Ok, so I connected the violet paths. So far so good. No loosing channel. The pot reacts well. Connected the speakers. Still everything fine. During next week I will continue with preparations for those measurements..
By the way it sounds great even now. 👍
By the way it sounds great even now. 👍
I searched for this kind of manual for dummies a decade ago - unfortunately failed to find one and had to take the learning curve the "hard way" (with support and help from local forums) 😉
But the good news is that if you take the easiest route then - in the end of the day - it is very easy (only the first steps seem difficult).
To start from the beginning - there is more than one software that you can use for this purpose (RMAA, REW, ARTA etc) but I have found RMAA the easiest one to use on entry level.
So get RMAA from https://audio.rightmark.org/index_new.shtml and install it.
Regarding soundcard - your SB seems to be too complex from cabling perspective for this purpose, I would recommend Behringer UCA222 as starting point (cheap, has RCA sockets so need for special cables, can be bought from Amazon, eBay, Thomann etc etc). Drawback is the sampling rate limitation so you will get results to ca 22kHz. But once you need data from that frequency upwards then you will invest into a better soundcard.
Outputs of the sound card are to be connected to amplifier input (tuner, aux etc).
Inputs of the sound card are to be connected to the amplifier pre-out sockets. Yes later on you can connect them also to speaker terminals but you need to provide a voltage divider (attenuator, potentiometer) to avoid accidentally frying your sound card by sending e.g. 40V AC to it (standard input level 2V).
Yes, I know, I have fried one...
Launch the software, select sound card devices for input and output.
Pull down the signal output level from computer mixer to minimum (max output level from the soundcard should be ca 2V AC which is too hot for the amplifier anyway).
Turn the amplifier volume knob down to minimum, bass and treble controls to neutral and power on the amplifier.
Tick the "Adjust playback/recording" field.
Click on the leftmost button in Run tests area - spectrum analyzer screen appears.
Start increasing the windows volume mixer level for increasing the output signal and also turning up the volume knob. Target is theat indicator bars on the left side will show e.g. ca -3dB.
Once done with level adjustment, click the Start test button.
Tests are run and you can save the results in a summary window and also in a data file.
In a summary window view you will see numeric results and click on graph window opening buttons to see differences between channels.
As first step I usually look at frequency response, noise, THD and crosstalk graphs.
Walk in the park - isn't it? 😉
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That sounds promising then. Give it an extended test to be sure. Cold starts and all that. If it stays good then the relay is playing up.
When you cleaned the contacts did you pull the paper soaked in iso through the contacts with enough force? You need to manually close the relay with the paper between the contacts and pull it to and fro.
Thank you very much for the steps!
I am going to order that Behringer UCA222 today.
22kHz is more than I can hear..
The summary for the connections:
-UCA out to AUX
-UCA in from Pre Out
-UCA USB to PC USB
For later, speaker outputs I could connect through passive preamp - it will act as attenuator. Good idea?
Then from measurment general understanding perspective:
Is that going to be multiple measurements based on volume pot of the amp?
In that case I will do adjusting playback/ recording levels every time based on volume pot position, is that correct assumption?
Also is AC voltage on UCA out increasing during the test?
At some point I should check the UCA out for voltage, which should be less the 2V as you mentioned..
Thank you
Thank you, yes, it is positive result for now 🙂
Unfortunately I didn’t apply force while cleaning the contacts, I can do that again.
Also, maybe there is a possibility that springs inside lost their force to keep the contacts fully engaged.. perhaps when voltage drops while lower and lower level of amplification?
Connection plan is ok, leave the speakers part to the time when you are comfortable with the process.
You do one measurement (set of different measuerements) at a time (after clicking Start test) - the combination of input signal level and volume pot position have to provide the signal bars at ca -3dB level.
You can perform as many measurements as you want with different vol pot positions - but you need to adjust then the soundcard output level from Windows sound mixer (yes, every time you start a new measuring session).
RMAA measuring signal is a set of different signals with different levels - if your tone controls are in neutral position (or amplifier Sound direct switch is ON - if it has such) then you should be ok once the initial level has been set - no need for any further intervention.
If you start receiving error messages about clipping then your amplifiere frequency response is more off the horisontal line than the headroom you left when adjusting the levels (in this example 3 dB).
You do not actually need to check the UCA output voltage - but if you want to you should visit your friend with an oscilloscope. RMAA is sending out bursts of calibration signal and DMM will not show these levels correctly (I tried that, no useable result).
You do one measurement (set of different measuerements) at a time (after clicking Start test) - the combination of input signal level and volume pot position have to provide the signal bars at ca -3dB level.
You can perform as many measurements as you want with different vol pot positions - but you need to adjust then the soundcard output level from Windows sound mixer (yes, every time you start a new measuring session).
RMAA measuring signal is a set of different signals with different levels - if your tone controls are in neutral position (or amplifier Sound direct switch is ON - if it has such) then you should be ok once the initial level has been set - no need for any further intervention.
If you start receiving error messages about clipping then your amplifiere frequency response is more off the horisontal line than the headroom you left when adjusting the levels (in this example 3 dB).
You do not actually need to check the UCA output voltage - but if you want to you should visit your friend with an oscilloscope. RMAA is sending out bursts of calibration signal and DMM will not show these levels correctly (I tried that, no useable result).
One last thing - before starting to measure the amplifier you should do the loopback test i.e. connect the sound card outputs and inputs via the RCA cables you are going to use (using F/F connectors between the cables) - this will give you the actual capabilities of your test rig.
RMAA does not use calibration files but the result should be pretty straight and a magnitude better than the results of your amplifier.
RMAA does not use calibration files but the result should be pretty straight and a magnitude better than the results of your amplifier.