Topic: Greetings... my new mic preamp.

[fivefishdiy]

I'm not talking about the preamp chips.... The preamp chips will remain socketed. (But I'm using da best in da bizness... why replace the chips

I was talking about the DC-DC converter chips causing the popping sound when phantom power is ON.

[Chuck]

I'm not talking about the preamp chips.... The preamp chips will remain socketed. (But I'm using da best in da bizness... why replace the chips

I was talking about the DC-DC converter chips causing the popping sound when phantom power is ON.

Ah... OK..
I was just thinking that the best chips in the business today, may not be the best in a few years. For example, The INA217 replaced the SSM2017. There's lot's of debate here over the sonic differences between various op-amps and/or instrumentation amps. Some resourceful guys have made a living out of replacing stock components, op-amps for example, with different op-amps, in audio gear.

[Church-Audio]

Just doing some abuse testing... hitting and tapping it with a pen while turned on, dropping it with phantom power ON, etc... just to see if any problem comes up.

(PLEASE NOTE: I'm NOT suggesting you treat the pre with abuse.)

I think I found one issue.... mainly on the phantom power DC-DC section. I need to remove anything "mechanical" in the PSU section that is prone to minute vibration... so the IC sockets will have to go.  It's causing some "popping" sounds when PP is on, and the PSU/board is tapped/dropped.  (The IC sockets just rely on mechanical pressure/force to make contact with the IC chip's legs.) ... the trimmers seems to be just fine... so they'll stay for now... I think.

The popping sound doesn't happen if PP is OFF. So I know the problem is caused by something in the 48V PSU.

I'll build a second prototype of the switchboard and test my theory. This may delay launch of the pre. How long? I don't know. We'll see how the new testing goes.

Sorry for the delay....

Use these machined sockets for your chips you will not have any issues.
http://www.king-cart.com/phoenixent/product=SOCKETS+DIP+IC+MACHINED/exact_match=exact

[fivefishdiy]

I'm already using those kind of sockets.  Thanks. 

Still looking at the weird problem. I'm beginning to think it's not caused by the PSU. I think it's related to the phantom power wiring fix I did.

[Church-Audio]

I'm already using those kind of sockets.  Thanks. 

Still looking at the weird problem. I'm beginning to think it's not caused by the PSU. I think it's related to the phantom power wiring fix I did.

Yes it might be somehow your resistors are becoming microphonic... Try a bit of hot glue on them to dampen them to your circuit board.

Chris

[SparkE!]

I'm already using those kind of sockets.  Thanks. 

Still looking at the weird problem. I'm beginning to think it's not caused by the PSU. I think it's related to the phantom power wiring fix I did.

Yes it might be somehow your resistors are becoming microphonic... Try a bit of hot glue on them to dampen them to your circuit board.

Chris

Capacitors are more likely to be microphonic than resistors, but you're right.  Parts can be microphonic and the usual way of dealing with it is to glue them to the rest of the circuit so that the whole thing moves together.

[fivefishdiy]

There's nothing microphonic about this preamp. This is all solid state, not toobs.

And no need to hot glue anything. I don't do that.

Now, certain ceramic capacitors in SMD format can have piezoelectric effect that can be heard as popping noise when dropped but I'm not using SMD parts but all PTH parts. PTH parts don't have this capacitor piezo problem (the legs absorb the vibration... but not the same case with SMD parts).

So I repeat... it's not MICROPHONIC noise or something.

I think I know what the problem is (and it's only happening on one channel and not the other)... but haven't got some free time to test my solution yet.... I'm swamped packing, filling up orders this past week and finishing a media website for the Olympics coverage.  (I have a real job, this electronics business is just a sideline.)

We'll get to the bottom of it.

[Church-Audio]

There's nothing microphonic about this preamp. This is all solid state, not toobs.

And no need to hot glue anything. I don't do that.

Now, certain ceramic capacitors in SMD format can have piezoelectric effect that can be heard as popping noise when dropped but I'm not using SMD parts but all PTH parts. PTH parts don't have this capacitor piezo problem (the legs absorb the vibration... but not the same case with SMD parts).

So I repeat... it's not MICROPHONIC noise or something.

I think I know what the problem is (and it's only happening on one channel and not the other)... but haven't got some free time to test my solution yet.... I'm swamped packing, filling up orders this past week and finishing a media website for the Olympics coverage.  (I have a real job, this electronics business is just a sideline.)

We'll get to the bottom of it.

Electronics do not have to have tubes to have a problem with microphonics. I am sure you will get to the bottom of it. 



"Microphonics describes the phenomenon where certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term is derived by analogy to older microphones where that behavior is inherent in the design, while with modern electronics it is sometimes an intentionally added effect.

When electronic equipment was built using vacuum tubes, microphonics used to be a very serious design problem. The charged elements in the vacuum tubes would vibrate and the motion would change the distance between the elements, producing charge flows in and out of the tube in a manner identical to a capacitor microphone. A system sufficiently susceptible to microphonics could experience feedback.

With the advent of solid state electronics (transistors), this major source of microphonics was eliminated but smaller sources still remain. The ceramic dielectrics used in high-K capacitors ("Z5U" and "X7R") are piezoelectric and will directly transform mechanical vibration into a voltage in exactly the same fashion as a ceramic microphone. Wiring and cables can also exhibit microphonics as charged conductors move around and various materials can develop triboelectric ("static") charges that couple to the electronic circuits.

The sound of guitar amplifiers that incorporate the electronic chassis into the same cabinet as the speaker are susceptible to microphonics. Though a guitar amplifier's microphonics distortion is sometimes appreciated as part of the "special sound" of a guitar amplifier, a faulty vacuum tube or other component can cause out of control feedback. Unwanted microphonics-related audible distortions can often be alleviated by using commercially available vacuum tube dampers"

[fivefishdiy]

I'm sure it's not microphonics, since it only happens when Phantom Power is ON and only on one of the channel. With PP off, it's normal...

The other channel is behaving normally with either PP on or off.

if it's microphonics related, it will be happening on both channels, with either PP on or off.

[fivefishdiy]

Okay, I had a couple of hours free time this afternoon and worked on the prototype.

Mystery solved.

I found out it was two separate issues... and both issues were on the same channel! 

One channel was kinda like "superchannel" and was immune from all these stupid problems. The other channel was like a problem child on crack. <shrug>

1. Because of my original error in drawing the schematic (and thus the error on the PCB design), and trying to jury-rig a solution, etc... etc... well... that kludge solution worked for the superchannel but it didn't work on the crack channel. Weird. Don't ask me... I don't know why. The 2nd problem channel doesn't have a reliable startup of it's phantom power. Sometimes there is PP, sometimes not.

So this time, I really fixed the wiring by doing it the way it's supposed to be in the schematics. YES! It now works reliably, and there is 48V phantom power reliably on both channels, everytime. The DC-DC converter sounds happy too (which was a clue for me), and the DC waveform on the oscilloscope looks way way much better.... the way I expected it.

2.  POPPING SOUND. Another mystery that only happens on the crack channel. The DC-DC converter was just fine. As I said, it's not microphonics. And no need to hot-glue anything. Don't want that ugly stuff on my board.

The problem was the BurrBrown chip which I'm using as a DC servo. (yeah, my pre has no capacitor in the signal path between the mic preamp and the output driver... I'm using a DC servo). I guess this particular chip is marginal because as soon as I replaced it, everything was quiet and back to normal again. Tapping the board, and dropping it from 1 to 1.5" on the table was just fine. No loud popping sound, followed by some crackling noise. Sounds great!

So now, the prodigal son, errr... channel has come back to the fold and everything is working fine again.

I'll continue monitoring in the next few days to see if any new problem is discovered.

BTW.... running the pre on 5.0VDC input using the Tekkeon battery and with phantom power ON...and  IT WORKS, my power-hungry FET condenser is happy and working. I'm very pleased with that.

[sanaka]

Just reread this whole thread again, and am more jazzed than ever about this project.

Just a couple quick Q's for the 5Fish man  :

- Will you fix the phantom resistor location problem on a new PCB revision? I assume so because you've emphasized that you usually go through several board revisions before releasing a project officially, but I didn't see where you mentioned it specifically regarding this problem. I feel a little bad asking, because I know it means junking a bunch of otherwise good boards...

- Have you settled on a definite set of uses for the 6 switches/positions? Sorry I missed it if you have. Do you have a photo of the face with the toggles instead of the pushbuttons? Like a couple others did, I'm doing my own (possibly annoying to you, sorry!) Photoshop mockup of the TS-2 front faceplate.

Peace,
Sanaka

[fivefishdiy]

I assume so because you've emphasized that you usually go through several board revisions before releasing a project officially, but I didn't see where you mentioned it specifically regarding this problem.

Yes, there'll be new PCB revisions. I already have some of the changes done in CAD for the 2nd PCB revision... about 12 or so items before I discovered the PP resistors error. The rest of the PCB design is good.  I think these boards (rev 1) can still be used, just need a small perf PCB for the input XLR section.

Most of the other changes on the new PCB revision 2 are minor... i.e. shifting parts a few millimeters here and there, adding extra pads for test points, or "future use", moving the mounting hole backwards, adding new mounting holes for additional support in the back, shifting locations for 1 molex connector, etc.  minor stuff.

Have you settled on a definite set of uses for the 6 switches/positions?

VU Meter on/off
CH1 phantom power
CH1 polarity reverse
CH2 polarity reverse
CH2 phantom power
Main power switch

[sanaka]

VU Meter on/off
CH1 phantom power
CH1 polarity reverse
CH2 polarity reverse
CH2 phantom power
Main power switch

Cool. The polarity switch certainly won't hurt anything, but probably won't be used much in the field (which probably has been mentioned). Personally, I'd probably rather have the polarity jumpered/fixed inside the case. This would eliminate two toggle switches and two holes on the front plate, and not require reworking the PCB (of course it's another option you could offer, but I'd just ditch it for the TS-2). Or some other feature could replace the polarity control's front panel real estate with few complaints I bet 

SO, here goes Sanaka's cartoonish Photoshopped mockup of the TS-2 front panel, which may or may not be helpful in any way :



Also, there was some discussion of the power connector and how it would be nice if it were right angle and locking. Again, don't know if this is any help, but these connectors at McMaster Carr look pretty solid, though pricey. Particularly, part# 6897K83 looks like a great panel connector, if one wanted to interface the matching RA plug to their power source (images show 4 pole and 3 pole versions, whereas 2 pole is good for just 12V DC to the preamp).



G'Night!

Peace,
Sanaka

[jerryfreak]

ha! i order from mcmaster all teh time, and have never once though of using them as an electronics supplier!

[NOLAfishwater]

I second Sanaka's feelings on the polarity switch. Never used one, would be better if it were a -20 pad for line in from a hot SBD patch.