BUILD THREAD - A New Take on the Classic Pass Labs D1 with an ESS Dac - Page 67 - diyAudio
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Old 8th February 2014, 07:56 PM   #661
opc is offline opc  Canada
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Perfect! Those will work just fine. That resistor isn't in the signal path, and isn't tolerance critical, so anything will work.

Let us know how it goes!

Cheers,
Owen
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Old 8th February 2014, 08:13 PM   #662
Crom is offline Crom  United Kingdom
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Thanks for your help Owen - I'm biasing just perfectly!!

Q1: 1.65v
Q3: 1.65v
Q4: 1.65v

I've still got that problem with Q2 though. V strange voltages compared to the other 3:

Q1
Source > GND = 1.65
Drain > GND = 16.43
Gate > GND = 5.08
Drain > Source = 14.9 (positive terminal to Drain)

Q2
Source > GND = 0
Drain > GND = 32
Gate > GND = 1.621
Drain > Source = 32.15 (positive terminal to Drain)

Q3
Source > GND = 1.65
Drain > GND = 16.43
Gate > GND = 4.97
Drain > Source = 14.83(positive terminal to Drain)

Q4
Source > GND = 1.65
Drain > GND = 16.32
Gate > GND = 5.05
Drain > Source = 14.7 (positive terminal to Drain)

any ideas?
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Old 8th February 2014, 09:11 PM   #663
opc is offline opc  Canada
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Hmm... looks like he gate voltage isn't high enough to get the fet to conduct.

Check:

- Voltage between new (old) resistors and pot
- Voltage before gate resistor
- Voltage after gate resistor
- Voltage on negative supply right on the source resistor

Could be a faulty pot, hopefully not a damaged fet. Reflow all the joints and make extra sure the isolation pad is working between the fet and the heatsink.

Cheers,
Owen
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Old 8th February 2014, 09:52 PM   #664
Crom is offline Crom  United Kingdom
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Hi Owen,

Slightly unorthodox but I wanted to make sure that I was interpreting what you asked correctly so I did it visually - all voltages relative to ground.

I have to say this is great learning for me - someone in another thread said that you only learn when something goes wrong...very true!

Isolation pad is doing its job correctly.
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Last edited by Crom; 8th February 2014 at 09:55 PM.
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Old 8th February 2014, 11:04 PM   #665
opc is offline opc  Canada
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Very nice diagram! Makes troubleshooting much easier

It pretty much has to be one of the following:

- The resistor you added at R12 is faulty, or the wrong value.
- The pot is faulty or not correctly adjusted
- C35 is damaged or faulty and is presenting low impedance to GND.

Of the above three, my money would be on the pot. Try pulling it out and measuring the resistance from end to end, and wiper to each end. You might be able to do it in-circuit with everything off, so try that before pulling it.

My next guess would be the cap, but it's less likely.

With only 1.6V on the gate, the fet will not even begin to conduct, hence the 32V on the drain. With no current flowing through the drain resistor, there will be no voltage drop across it.

What doesn't make sense is that you have 0V on the source, when you should be seeing some negative voltage a little higher than the zener voltage. Not sure what's going on there.

You might want to adjust one of the other legs to have 1.6V on the gate, and see if the voltages match for troubleshooting purposes.

So close! You're 75% up and running

Cheers,
Owen

Last edited by opc; 8th February 2014 at 11:10 PM.
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Old 9th February 2014, 03:41 PM   #666
Crom is offline Crom  United Kingdom
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Owen, with your help I think I'm there!!! :-))))

What it turned out to be was a tiny spec of solder lodged between the ground and the negative input which leads to Q2 - it wasn't even soldered in place...I just brushed it out the way. This was clearly left over by me when I removed the pins I had in there originally and decided to solder wires into the more widely spaced inputs (see pic below). This was after I took the whole thing off the heatsink of course...ha, but it doesn't matter, we're all up and running at the correct voltages and I'm going to put it together and test it with the DAC

Just in case it's of any help to anyone in the future this is the logic (AKA, random guess-work) that I worked through to get there:

Following your instructions:

- I removed R12 and tested: 19.9K.
- With R12 out the way I tested the pot between the GND rail and one of the R12 terminals. The resistance tracked perfectly from 0.2R to 5.1K....looked good to me.
- I found that resistance across C35 and ground = 30R which looked a bit odd (I know the solution is probably obvious to you guys but bear with me)
- I removed C35 and tested - all fine there. When capacitor is removed 30R remains across terminals??
- I then removed C34 to compare to the same place on the L channel. Resistance there is 3.4k
- I then looked at the circuit diagram and realised that the resistance across those capacitors is going to be controlled by the pot and that the left channel was already biased to 1.65v whereas the other wasn't. This explained why one side was 30R and the other much more (big DOH! moment)
- OK, so back on track. I worked through reasons why there would be 0v at source and the most logical was that there was a short to GND somewhere nearby....and sure enough I found one!!


Q1
Source > GND = 1.65
Drain > GND = 16.44
Gate > GND = 5.11
Drain > Source = 14.65 (positive terminal to Drain)

Q2
Source > GND = 1.65
Drain > GND = 16.42
Gate > GND = 5.10
Drain > Source = 14.73 (positive terminal to Drain)

Q3
Source > GND = 1.65
Drain > GND = 16.41
Gate > GND = 4.99
Drain > Source = 14.70 (positive terminal to Drain)

Q4
Source > GND = 1.65
Drain > GND = 16.41
Gate > GND = 5.06
Drain > Source = 14.75 (positive terminal to Drain)


Readings look good :-)


I had to rig up some larger heatsinks onto the power supply as those regs aren't going to last long trying to produce 32v dc from 40vac. Even these larger ones get pretty toasty. I'll dig out some lower voltage transformers before long.


Thanks very much for your help. I'm going to start to connect it into the dac boards now...see what happens!
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Old 6th March 2014, 08:49 PM   #667
Crom is offline Crom  United Kingdom
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I have to say that it's sounding the best that my Buffalo DACs have ever sounded!!!
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Old 27th August 2014, 10:15 AM   #668
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Hello,

Just a little word from a newbie that will try to build a ESS9018 with NTD1 I/V stage. I am waiting for PCB order to start. In the meantime, I will try to check this build thread to understand how it works and how to build it correctly.

I hope I won't kill your patience..
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