I bought a set of the Pearl boards from a member of the diyaudio site on the Pass Labs board. The person I bought them from had already stuffed the boards with the components (using a q pack from passdiy), but, with other demands on his time, he didn't think he would be able to build the chassis and ps. He said that he had powered them up and tested the voltages at the points on the boards and they were fine. I have no reason to doubt him - the assembly job was first-rate, and all the components are correct in value and orientation. But now, after having built my ps (like the one suggested in the passdiy article) and chassis and powering the boards, I am having problems with the voltage measurements.
Specifically, looking at the boards and the schematic with the voltages on it (http://www.passdiy.com/images/projects/pearlfig5.gif), with all voltages measured ref to PAD2, and the schematic's suggested value in parentheses, here are some of the readings:
PAD1 = 44.6 (45.7)
PAD4 = 9.0 (8.95)
emitter Q2,Q12 = 8.4 (8.4)
R8 = 29.5 (28.8)
base Q3 = 25.6 (24.9)
emitter Q3 = 29.2 (24.2)
base Q5 = 28 (11.6)
R23 = 28.5 (15)
and, lastly, PAD7 (output ground) = 29.3
I didn't write down any other voltage readings, but everything else was reading roughly 29. Strangely, *both boards* exhibit the same behavior (my voltages above were for my 'right channel' board, but the left was doing the same. For a while I was thinking that one board was dumping 29v on the ground and that was causing both to be off (I had the grounds connected at the output), but after I separated the grounds for each board, and then powered one board at a time for measuring, I still got the same results.
So, my best guess is somehow I've toasted one or more of the Qs (3 and/or 5?) on both boards, but I am really a novice at this, and I'd appreciate any advice before I start replacing components.
Todd
Specifically, looking at the boards and the schematic with the voltages on it (http://www.passdiy.com/images/projects/pearlfig5.gif), with all voltages measured ref to PAD2, and the schematic's suggested value in parentheses, here are some of the readings:
PAD1 = 44.6 (45.7)
PAD4 = 9.0 (8.95)
emitter Q2,Q12 = 8.4 (8.4)
R8 = 29.5 (28.8)
base Q3 = 25.6 (24.9)
emitter Q3 = 29.2 (24.2)
base Q5 = 28 (11.6)
R23 = 28.5 (15)
and, lastly, PAD7 (output ground) = 29.3
I didn't write down any other voltage readings, but everything else was reading roughly 29. Strangely, *both boards* exhibit the same behavior (my voltages above were for my 'right channel' board, but the left was doing the same. For a while I was thinking that one board was dumping 29v on the ground and that was causing both to be off (I had the grounds connected at the output), but after I separated the grounds for each board, and then powered one board at a time for measuring, I still got the same results.
So, my best guess is somehow I've toasted one or more of the Qs (3 and/or 5?) on both boards, but I am really a novice at this, and I'd appreciate any advice before I start replacing components.
Todd
Grounding
I had a similar problem, and it turned out to be the grounding that was at fault.
Make sure that you are tying the signal and dc power grounds properly.
Search the archives for tons of grounding advice.
I finally got my case built (no pics yet), but am still trying to track down a low level hum. My cart might just not have enough output, I guess.
Good luck.
Cody
I had a similar problem, and it turned out to be the grounding that was at fault.
Make sure that you are tying the signal and dc power grounds properly.
Search the archives for tons of grounding advice.
I finally got my case built (no pics yet), but am still trying to track down a low level hum. My cart might just not have enough output, I guess.
Good luck.
Cody
Thanks for the advice. Believe me, I did search the archives before building the chassis and making the connections, and found loads of grounding tips. Most of these are in regards to eliminating loops and associated hum.
Seems like if this is a grounding problem here, there is something fundamental to the scheme that is not connected. It's possible I misinterpreted some of the advice, and I will review the whole grounding setup yet again tonight.
When you say:
"Make sure that you are tying the signal and dc power grounds properly."
Do you mean as in the passdiy pearl article?:
"The two isolated supplies will ultimately have their Grounds meet each other, and the best place to accomplish this is at the output. If this preamp is constructed with two channels in the chassis and an external supply, then the grounds should meet at the output connectors. These output connectors should be close together, and attach to chassis ground at this point."
Todd
Seems like if this is a grounding problem here, there is something fundamental to the scheme that is not connected. It's possible I misinterpreted some of the advice, and I will review the whole grounding setup yet again tonight.
When you say:
"Make sure that you are tying the signal and dc power grounds properly."
Do you mean as in the passdiy pearl article?:
"The two isolated supplies will ultimately have their Grounds meet each other, and the best place to accomplish this is at the output. If this preamp is constructed with two channels in the chassis and an external supply, then the grounds should meet at the output connectors. These output connectors should be close together, and attach to chassis ground at this point."
Todd
If I read the article correctly, the following would be my intrepretation. Channel A power supply should connect to the the Channel A circuit board at Pad 1 and the negative at Pad 2. The same should be done for Channel 2. The inputs should be isolated RCA connectors with the center conductor going to the + IN and the shield to the - IN (I can't read the PAD #'s on the PDF). Do the same for the other channel. The output +OTL should go to an isolated RCA connector center conductor and the -Output should go to the shield. Do this for both channels. Then run a short wire from each output RCA connector shield to the same point on the chassis. Make sure the ground on your power cord is connected to the same point on the chassis as the short wires coming from the output connectors shields. This is my interpretation, for what it's worth, I hope it helps.
thanks, and more...
thanks for your interpretation, kilowattski. I appreciate the point by point connection description - language I can understand, especially with all the different 'grounds' involved.
after checking over everything again last night, your interpretation is in line with what I set up, but I definitely screwed up and forgot one ground connection - the connection of the 'earth' (powercord) ground to the chassis. funny how you can stare at something over and over and still miss the really big mistake. unfortunately, even with this, I don't think this scheme is entirely correct, because the online article does not cover the latest revision of the pearl boards, which have separate power and signal grounds. these have to be connected at some point on each board, with a jumper at a specific location suggested by Wayne at Pass Labs. (thanks for the email reply, Wayne.)
while I think I can turn the corner on getting this solved - connect the signal and power grounds by jumper on the board and connect both board grounds from output rca to chassis and earth ground - I need to look over the grounding recommendations I collected and see if I can get back to understanding some of the implementations. Like this one:
""I suggest that both chassis be connected directly to "earth ground" (i.e. AC third wire). Then connect the PS "ground" to the chassis through a power NTC thermistor (a.k.a. inrush limiter) like Digikey KC011L-ND. This keeps the PS ground (which gets connected to signal ground near the input & output RCA connectors) isolated from earth ground by a few ohms under normal conditions." - Ren
And this:
"Had a hum problem when i first fired my pearl up.
The solution was to remove the connection between ground points on the pcb of each channel and connect them separately to the common ground point between the output jacks of the pearl.
Now, if you use a separate outboard ps you can connect AC ground to its chassis but do not connect any AC ground to the chassis of the pearl itself.
Connect ps ground to the ground point between the output jacks and if the hum doesn't go away try connecting this ground point to the chassis, but as close to the output jacks as possible.You could even insert a 100R resistor between chassis and ground point to avoid any further loops.
The whole idea is to create a single grounding point as close to the output as possible and ground all elements independently to this point.
If you are not using an outboard ps, you have to connect AC ground to the chassis for safety but with the use of a thermistor like NP normally does. The rest is the same.
" -Nickolas K.
thanks for your interpretation, kilowattski. I appreciate the point by point connection description - language I can understand, especially with all the different 'grounds' involved.
after checking over everything again last night, your interpretation is in line with what I set up, but I definitely screwed up and forgot one ground connection - the connection of the 'earth' (powercord) ground to the chassis. funny how you can stare at something over and over and still miss the really big mistake. unfortunately, even with this, I don't think this scheme is entirely correct, because the online article does not cover the latest revision of the pearl boards, which have separate power and signal grounds. these have to be connected at some point on each board, with a jumper at a specific location suggested by Wayne at Pass Labs. (thanks for the email reply, Wayne.)
while I think I can turn the corner on getting this solved - connect the signal and power grounds by jumper on the board and connect both board grounds from output rca to chassis and earth ground - I need to look over the grounding recommendations I collected and see if I can get back to understanding some of the implementations. Like this one:
""I suggest that both chassis be connected directly to "earth ground" (i.e. AC third wire). Then connect the PS "ground" to the chassis through a power NTC thermistor (a.k.a. inrush limiter) like Digikey KC011L-ND. This keeps the PS ground (which gets connected to signal ground near the input & output RCA connectors) isolated from earth ground by a few ohms under normal conditions." - Ren
And this:
"Had a hum problem when i first fired my pearl up.
The solution was to remove the connection between ground points on the pcb of each channel and connect them separately to the common ground point between the output jacks of the pearl.
Now, if you use a separate outboard ps you can connect AC ground to its chassis but do not connect any AC ground to the chassis of the pearl itself.
Connect ps ground to the ground point between the output jacks and if the hum doesn't go away try connecting this ground point to the chassis, but as close to the output jacks as possible.You could even insert a 100R resistor between chassis and ground point to avoid any further loops.
The whole idea is to create a single grounding point as close to the output as possible and ground all elements independently to this point.
If you are not using an outboard ps, you have to connect AC ground to the chassis for safety but with the use of a thermistor like NP normally does. The rest is the same.
" -Nickolas K.
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