Test & Measurement interface for Soundcard

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shining31

Member
Joined 2018
Hi!

I've received recently a year ago an used PM int.
It's my first electronic measurements jig :) but I use my Scarlett solo SC and arta/rew softwares for a while now as I'm into speakers DIY building.

Yesterday, for a quick & dirty health check I've played with the interface in loopback mode and.. OMG ... An distortion factory!
Thd is 100% (more or less) whatever attenuator position :(
I've crossed checked my sc with a direct loopback, she's working fine.
I think I'm good for connections and knobs positions.

It was too late last night to investigate further, but to prepare next session, do you think it can be due to the fuse? Or maybe an dead IC?
 
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shining31

Member
Joined 2018
Yes! Another evening, another try :)
So I've tested some use-cases with Pete Millet Interface (PMI) fed with computer USB or USB bank, it doesn't change a lot (just 50hz hum etc...)

SC out (-1dBfs / 1Vrms) -> SC in
https://imgur.com/dLCg9Eo
Seems fine to me

SC out (-1dBfs / 1Vrms) -> PMI in -> SC in (pre-amp a bit adjusted)
https://imgur.com/Blu3tB1
So far so good, but the PMI pick some noise

SC out (-1dBfs / 1Vrms) -> PMI out -> PMI in (switch genMon) -> SC in (pre-amp a bit adjusted)
https://imgur.com/q0ucBAL
Et patatra !

So it should be around the out amplifier. I've opened it
https://imgur.com/j9IkUZW
Nothing seems bad. Just some resistors soldered in parallel, but it's in the input path.

I'll start by checking the components values with my magnifying glass :)
 
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dch53

Member
Joined 2015
I've been using a Juli@ internal soundcard with my Soundcard Interface for all my testing for a few years.

I'm over carting heavy SE tube amps to my study from my workshop (for which read " formal dining room table!") and I really need to be freed from my desktop computer.

Can anyone recommend an external usb soundcard for use with the Soundcard Interface please?

Focusrite Scarlett? Behringer UMC202HD?
 
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shining31

Member
Joined 2018
bohrok2610 said:
It seems to be clipping the soundcard (peak level 0dBFS). Try another range in the soundcard interface.
Click to expand...
Yep, it's just screenshot taken too fast, but even with pre-amp adjustment it change nothing.

I've done some other test after my post, I've tried to flip both INA134 and surprise, the distortion factory is gone from PMI output to appears on PMI input.
I've switched them multiple times and that seems to correlate.
So maybe it's a faulty op. I'll order a new one.
 
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bohrok2610

Member
Joined 2005
Here is a measurement of my PM interface with ES9038Q2M output and AK5394 input. I normally use Jan Didden's Autoranger as it is more flexible and has lower noise floor and distortion. But for tube measurements PMI works fine.

ES9038Q2M-L_-20dBFS_PMI-200mV_AK5394-L_1k@176k4.png
 
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jackinnj

jackinnj

Member
Joined 2002
tech-diy.com
celica-fiend said:
Im currently trying to find a sub or a work-around but just so folks are aware.. two of the more expensive parts, the AD536AJQ RMS->DC IC to drive the display and the MDMV0000 display are both either discontinued or have long enough lead-times that they may as well be.
Click to expand...
The AD536AJQ RMS converter is going to be a long term problem, but you can make a daughter card and use a LTC1966 -- the device is MSSOP single supply so you will have to pull the voltage off the 5V node and it is available from the usual suspects. The LTC1966 is best used with a "post filter" -- see figure 13 in the datasheet. (Note that Pete also filters the output of the AD536.

There is a substitute for the MDMV000 display -- CTon DK201 (at DK). You may find something less expensive at Jameco or MPJ. https://www.digikey.com/en/products/detail/c-ton-industries/DK201/263483
 
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celica-fiend

Member
Joined 2009
jackinnj said:
There is a substitute for the MDMV000 display -- CTon DK201 (at DK). You may find something less expensive at Jameco or MPJ. https://www.digikey.com/en/products/detail/c-ton-industries/DK201/263483
Click to expand...

OK that is weird.. I have two identical Digikey.ca windows open.. one shows the DK201 as 'available to order, lead time 35 weeks' and one shows 16 in stock.
Identical URL's.. If I refresh the one showing 16 in stock, it goes back to 'available to order' haha.

Fingers crossed, I'll order and see what happens.

Thanks for the tip on the AD536, I was looking at adapter boarding the SMT version.. I've just hit purchase on a DIP version from eBay where the seller has a few positive feedback on exactly this part.. if that fails, it'll be adaptation time.
 
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celica-fiend

Member
Joined 2009
Got it all assembled a couple weeks ago after waiting for some back orders. The DK201 display requires modification to work and, at the moment, I can't confirm proper operation until I get the issue below sorted:

Im hoping someone with more recent experience than I can help with some troubleshooting. I suspect my eBay AD536 is either counterfeit or defective.

Upon first power-up, the display showed ~150mV and climbing at ~1mV/s with the input grounded at the BNC. I powered the unit down, unplugged the display to avoid damage and upon powering back up, found the AD536 had ~550mV and climbing on pin 8, pin 5 on display was ~220mV. If left for a couple minutes, pin 8 will climb to ~640mV. I removed U7 from its socket to isolate the AD536 and have measured a steady ~60mVDC / 1.6VAC @ pin 1 (input) of the AD536.

I've checked component values and orientation in the measurement circuit and that all looks ok.. any other suggestions?

thanks..
 
Materialsguy

Materialsguy

Member
Joined 2017
shaunmerrigan.info
Paid Member
Just finished building Pete's interface (for use with tube gear) I have a couple of points to offer:
  1. As of this writing there is a seller on eBay who has NOS Red Lion MDMV0000 for under $USD 20 + shipping. PM me for the name if interested.
  2. When looking for the AD536AJQ (CERDIP package), I noted that Tektronix and HP used the AD536AJD (brazed ceramic DIP package) in some of their older equipment (1980's). For example, I had a parts unit Tek DM501A DMM (bad switches) with a working AD536AJD that I used for the interface.
--materialsguy
 
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keith956

Member
Joined 2020
I've bought a PCB from Pete's eBay shop, and have managed to order most of the parts from Mouser/Digikey. I've also found a AD536 that is used but is in working order. For the meter I've ordered a DK201 which should work with a little effort. Still missing the INA134PA, they are listed by Digikey but on backorder with an unknown delivery date.

Is there a source for the front/rear panel? I've got the drawing files but not sure who could make up the panels from them. Any help appreciated.

- Keith
 
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tubekiddo

Member
Joined 2022
I am in the process of redesigning the whole thing —for me…
Changes:
  • no SC output, only 2ch (stereo) input
  • all EOL parts replaced, different RMS measurement with modern parts, incl. new panel, cost effective

If you’re interested, PM me and I’ll send you my KiCad and LTSpice files. The build is not yet confirmed, I am in the process of putting it together and testing…

P.s. I have agreement from Pete to modify his design w/o commercial interest! Gerbers attached.
SpiceSIM.png
 

Attachments

  • SouncCardIF.zip
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  • SoundCardIF.pdf
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K

keith956

Member
Joined 2020
I believe places like PCBWay can create panels machined and printed on a variety of materials, I guess the question is:
  • what material? There is a choice of plastics/metal
  • I have .dwg/.dxf/.cdr files Pete sent me, which seem to have 2 layers, one for the panel outline/holes and one for the text. Will these be suitable to give to e.g. PCBWay? Do they need any more info in order to make up a front/back panel?
Sorry to ask what are probably dumb questions but I've never tried this before.
 
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