Digital Multi-Meter for Troubleshooting Pass B1

[parodielin]

I built the Pass B1 buffer preamp (http://www.passdiy.com/store.htm) and used it with Class D amp. The sound is very good.

However, the B1 has an unbalanced loudness between the right and left channel. The left channel is a bit louder than the right. I first thought it was an issue with the volume pot so I changed a new vol pot - that's not the issue.

So someone suggested that I used a digital meter to test the resistors. I never used a digital meter before and here is the suggestion:

"You need 600Vdc to 200.0mVdc and 300Vac to 200.0mVac as maximum scale on a 2000count meter.
A wide range of resistance 20M00 down to 200r0 is usually good enough.

Any extra ranges are simply nice to have but not essential.
Expect to pay <$10 or <£7 for a basic DMM that reads to those scales and guarantees <0.5% +-1LSB on it's most accurate DCV scale."

I found this one: http://www.harborfreight.com/general...ter-37772.html

The comment was: "one is missing both Vac and Vac lower sensitivity scales." I wasn't clear on what this means. Any suggestions?

Thanks!

[Æ]

VAC means Volts Alternating Current.
You get what you pay for. Best if you get a decent one, stay away from Harbor Freight junk.
You want a True RMS digital meter.

There was a thread a few weeks back, asking what brand of meter(s) we preferred/owned.
Quite a lot of us like/own American made FLUKE.

http://us.fluke.com/fluke/usen/Digit....htm?PID=56135

I built the Pass B1 buffer preamp (http://www.passdiy.com/store.htm) and used it with Class D amp. The sound is very good.

However, the B1 has an unbalanced loudness between the right and left channel. The left channel is a bit louder than the right. I first thought it was an issue with the volume pot so I changed a new vol pot - that's not the issue.

So someone suggested that I used a digital meter to test the resistors. I never used a digital meter before and here is the suggestion:

"You need 600Vdc to 200.0mVdc and 300Vac to 200.0mVac as maximum scale on a 2000count meter.
A wide range of resistance 20M00 down to 200r0 is usually good enough.

Any extra ranges are simply nice to have but not essential.
Expect to pay <$10 or <£7 for a basic DMM that reads to those scales and guarantees <0.5% +-1LSB on it's most accurate DCV scale."

I found this one: http://www.harborfreight.com/general...ter-37772.html

The comment was: "one is missing both Vac and Vac lower sensitivity scales." I wasn't clear on what this means. Any suggestions?

Thanks!

[philiparcario]

changing volume pot does not solve the pot issue. you need the same pot and need to reverse its channels. if the problem moves it is the pot. you can have 10 pots and they all can track 60 40 or 65 35 instead of 50 50 . if your pot had a 60 40 bias and you changed it to a second pot that has a 58 42 bias . you would think the one channel is bad with both pots. the truth is 2 bad pots. take the pot and reverse channels with the pot. if the problem reverses you just had two bad pots. all this costs you is labor. buying a good meter is 30 to 50 bucks minimum.


by the way a pair of mono pots lets you balance perfectly .


http://www.partsconnexion.com/product7381.html



now remember even though my point is true.

you may have had 2 good 50 50 pots so to speak and you need the meter. it is just that my trick of flipping the pot will let you know if it is the pot and it is a "free" test.

[jonpike]

It is worth spending more on a meter... then you know you have something to trust.

That said... a $400 multimeter isn't something everyone is willing to afford for a home bench lab. A basic one will have the things you need... most tend to cover the same aprox ranges. I'd try for a 4 1/2 digit, but 3 1/2 can be workable. For your balancing task... look for how many decimal places you end up with when measuring about 1VAC. You want at least two, so you have some good resolution for checking left and right balance. Also look at how much resolution (digits) you have on the resistance scale, in the range of your pot value, and other gain determining resistors.

Another approach on your volume match problem... It occurred to me that you likely have a pot that's not matched well, as has been discussed. If it's more a L-R balance thing than a L-R tracking thing, (i.e. one side of the pot is not the same value as the other, rather than one side changes faster than the other as you adjust it) you could add two small trim pots to tweak the gain and trim the two channels to balance. Even if you have a tracking linearity error, tweaking it into balance on the low end might be helpful, since it's most noticeable there. When you turn it way up, and only then it offsets a bit, it might not be as apparent. If it is an offset error, you will fix it entirely.

So, basically you'd need to put small trim pots in series with the main volume control pot, and adjust them till you get the same voltage out in both channels. (with the same voltage driven in, of course)

The other thing to do with your DVM, is to measure the resistance of the volume pot at various positions. Set it at a point near one end, check the L and R sides, move it a bit, check again, till you have a bit of a graph of the pot, and how closely the two sides track. Then you will see if it's a offset or a linearity problem, or if it's even the pot at all. Could be you have a slightly off value fixed resistor in one of the channels causing an offset in gain. And you can then go hunting for it, but remember that resistors don't often measure right when installed in the circuit, you probably need to lift one end.

Sorry about all the rambling, and I hope that helps.. but there's many things that could cause this, and many ways to use the multimeter to help you find them... Good luck!

[parodielin]

I am trying to avoid two pots if possible. My thinking has been there are so many stereo pots on my electronics and all work great.

I'll reverse the pot connections. But given that I've been using two pots, I randomly select which one goes to which, and the left channel is consistenly louder the chances of bad pots, IMO, is small.

A meter of $40-50 is my budget limit. The whole preamp costs $100 and getting a meter too expensive doesn't make sense. During my speaker building journey, I haven't come to the ocassion that I need a meter yet. Plus, I don't know how many electronics that I'll be DIYing. So any suggestions on the sub $20 meters that would do this job - getting my Pass B1 right?

I found this one on Amazon: http://www.amazon.com/Mastech-Digita...5869219&sr=1-1

This one seems to have all scale suggested from the 1st post. If there are cheaper ones, please advise. Sub $20-30 preferred.

Another interesting observation I'd share here. I have two sets of speakers in that configurations: RS TMWW and Continuum. When I use HK 635 as preamp, the image is centered. When using Pass B1, the image shifted left. When I switched channel connections (Left -> Right Spk, Right -> Left Spk), the image shifted to right. The intesting thing is the "degree of shifting" is different on two sets of speakers. With RS TWMM, it shifts further left than Continuum. What's the reason?

changing volume pot does not solve the pot issue. you need the same pot and need to reverse its channels. if the problem moves it is the pot. you can have 10 pots and they all can track 60 40 or 65 35 instead of 50 50 . if your pot had a 60 40 bias and you changed it to a second pot that has a 58 42 bias . you would think the one channel is bad with both pots. the truth is 2 bad pots. take the pot and reverse channels with the pot. if the problem reverses you just had two bad pots. all this costs you is labor. buying a good meter is 30 to 50 bucks minimum.


by the way a pair of mono pots lets you balance perfectly .


http://www.partsconnexion.com/product7381.html



now remember even though my point is true.

you may have had 2 good 50 50 pots so to speak and you need the meter. it is just that my trick of flipping the pot will let you know if it is the pot and it is a "free" test.

[Æ]

A meter of $40-50 is my budget limit. I haven't come to the ocassion that I need a meter yet.

Parts Express sells a few inexpensive multimeters, so does Radio Shack.

Don't underestimate the uses of a multimeter. You can use it all around the house and car.

You can use it to check a wall outlet/circuit for proper AC voltage, is the circuit dead? Check a flashlight or car battery for proper DC voltage. If your car battery is dead, is it the battery or is it the alternator not charging the battery? Check for blown fuses, bad switch contacts, light bulb filaments, broken wires. Voice coil resistance. Later on maybe use it to check diodes and resistors etc.

[parodielin]

That works better from a budget justification to my wife. My house has a few wall outlets without power.

Parts Express sells a few inexpensive multimeters, so does Radio Shack.

Don't underestimate the uses of a multimeter. You can use it all around the house and car.

You can use it to check a wall outlet/circuit for proper AC voltage, is the circuit dead? Check a flashlight or car battery for proper DC voltage. If your car battery is dead, is it the battery or is it the alternator not charging the battery? Check for blown fuses, bad switch contacts, light bulb filaments, broken wires. Voice coil resistance. Later on maybe use it to check diodes and resistors etc.

[Æ]

That works better from a budget justification to my wife. My house has a few wall outlets without power.

And if you are ever doing anything with audio, you'll certainly benefit from having a True RMS multimeter as opposed to not having True RMS.