Topic: Greetings... my new mic preamp.

[digifish_music]

The RF noise is not there if I use a DC wall wart adapter or plain 9V battery. It's definitely coming from the Tekkeon battery pack.

Just a note OZPeter saw the same thing in R44 testing, when he laid a video-battery-pack next to his mic.

http://taperssection.com/index.php/topic,103270.msg1406905.html#msg1406905

BTW Off Topic: While all the EE savy people are in this thread, I found something odd with the R09HR mic in, if anyone has any ideas I'd love to hear them (not in this thread obviously ).

http://taperssection.com/index.php/topic,105893.msg1418503.html#msg1418503

digifish.

[fivefishdiy]

Can you describe the noise?  A ticking or something with a regular period of repetition?

Neither. It's more like a very high pitched whistling sound... like a faint whistling of a water kettle.

It's definitely coming from the Tekkeon. I unplug the Tekkeon, without changing the gain setting, wiring, or anything... plug in a wallwart (which is also a switching PSU design) and the wall wart didn't exhibit this said noise.

... now just to give you guys an idea, I'm splitting hair here when I said I heard the noise. At 72dB gain setting, we're talking 4,000x amplification of the signal.  If you connect a microphone at the end of that cable and the mic is picking up sound around, this noise will get buried way way under by the audio signal.

[rowjimmy]

Inaudible when there's signal present or not, it's still something to be avoided. You say it happened when the battery was atop the pre? did you pick it up when they were separated?

* rowjimmy thinks about lead-lined padding in his bag.

[fivefishdiy]

You say it happened when the battery was atop the pre?

No, when the battery was atop the input mic cables. 

The preamp's case (aluminum) is completely grounded and doesn't make the noise louder or softer. The aluminum case is shielding the pre from external RF noise.

But if the noise is injected into the mic cable coming in, then it becomes part of the signal. Amplify that little noise 4000x, and you'll hear the faint sound.  There is some CMRR going on, so the noise is reduced by the balanced input lines but my guess is the battery is not completely shielded. I'm going to try a different mic cable too.

I'd probably also try other battery brand/models (i.e. a simpler battery without push button voltage selector) to see how those behave.

[digifish_music]

You say it happened when the battery was atop the pre?

No, when the battery was atop the input mic cables. 

...

I'd probably also try other battery brand/models (i.e. a simpler battery without push button voltage selector) to see how those behave.

Perhaps it's time for the fivefishdiy high-fidelity 'battery back' to go with the preamp

Something that takes 8 AA's should provide enough juice 

digifish

[fivefishdiy]

Don't know if you saw my earlier posts about battery testing.

2 Channels, with +48V phantom power, loaded with mics, plus VU Meters = 170mA - 190mA

... using a 9V toy battery, rated 580mAh, we'll get 3.0 hrs!

Okay, parallel (2) 9-Volt Duracell batteries for a 6 hr usage life. 

[Chilly Brioschi]

I think that 4-pin XLR jack will be overkill. Since we only need 12V and a ground.  The locking Switchcraft DC jack does the job.

Now, if you're building a remote PSU that needs to supply +18, -18, +48 and GND... that 4-pin XLR jack is perfect. 

I can't imagine where I'd find that.


But back to the Pre-amp.
I'd prefer to have over-kill in durability for a measly extra $4 in parts

It wouldn't be the first Pre to run 12vdc through an 4-pin XLR if you did it.
It also allows for a combo 12/6vdc supply for, say, an external ADC or recorder.
I always upsize my power source because I get more useful life as the rechargeable cells will drop to 80% of capacity in a relatively short amount of time.

From the Archive...

4 Pin XLR Pin assignments:

6V: 
3 is hot (+ 6v)
Pin 2 is ground (-)
12V:
Pin 4 is hot (+12v)
Pin 1 is ground (-)

Designing for disposable batteries isn't the most environmentally friendly option.
If you used rechargeable 9volt cells instead, do you think it will go a full six-hours?

[fivefishdiy]

The nice thing about my design is the flexibility in customization.

It can be a custom mod offered as an option. Where the locking Switchcraft DC jack shown here, can be replaced with the 4-pin XLR.



But just to be clear, it's not going to be just a $4 addition in the price. The math doesn't work that way. Now there's (3) new holes to be drilled, and manual labor (and time) for soldering wires from the PCB to the XLR pins... which is very slow (vs. soldering the PCB mounted DC jack), screwing it on, and keeping also a new part in stock on the shelves.

4 Pin XLR Pin assignments:

6V:
3 is hot (+ 6v)
Pin 2 is ground (-)
12V:
Pin 4 is hot (+12v)
Pin 1 is ground (-)

Whose standard is this??? ^^^  Just curious.

I don't have a need for a 6V supply, so pin 2 and 3 will probably be unused.

UPDATE: Is the Panel mount XLR jack a MALE? or a FEMALE? 

Looking at the jack (rear view), which pin are you calling Pin #1?  (leftmost, or rightmost)?  and about pins 2,3,4... Clockwise? counterclockwise?

[Chilly Brioschi]

The nice thing about my design is the flexibility in customization.

Very true, and being a kit, it really lends itself to "tweaking" to one's liking

4 Pin XLR Pin assignments:

6V:
3 is hot (+ 6v)
Pin 2 is ground (-)
12V:
Pin 4 is hot (+12v)
Pin 1 is ground (-)

Whose standard is this??? ^^^  Just curious.

I don't have a need for a 6V supply, so pin 2 and 3 will probably be unused.

Yes, but it can be used for a 6vdc device, like an MTII or some other recorder.
One PS, feeding multiple components

As to who made that a standard, I am unsure.
It was posted by one of the Moderators on the board, so I'm pretty sure that it is valid.
The caveat is that the Eco-Charge four pin is wired differently, uses a jumper between two pins.
Perhaps a "de facto" standard, by a bunch of tapers who needed 6/12vdc?

Maybe someone can jump in here? Any reason the 4 Pin XLR was listed as a 12V standard?

[Gutbucket]

Actually right angle Switchcraft locking type connectors are available from Remote audio...

Really?
Cool, that's news to me. 

Couldn't find them on their site. Suppose I could drop them a line and ask though.

[fivefishdiy]

BATTERY TESTING PART 2:

I hooked up the preamp to the Tekkeon battery starting about 2pm this afternoon and left it ON all day, playing music.

It's now 10:30pm and the Battery is still going strong, and the pre still working.

(There are 7 LED indicator lights on the battery, hooked it up at 2pm with only the 6th indicator lit (i.e. 70-85% charged). Now, at 10:30pm, the 6th LED indicator is off, and the 5th indicator is lit (indicating 60-70% remaining charge). 

So far, that's more than 8 hours of operation time... And I think there's a good several more hours of battery life left here.

I'm satisfied. 

[fivefishdiy]

UPDATE: 

The Fedex guy visited me this morning with some goodies. It's the VU-Meter PCBs.  Size is about 1" x 1.5". Could be made smaller but I need 1/8" holes for mouting. 

VU has on-board voltage regulator, wide supply voltage range (up to 6 to 24VDC) and on-board trimmer for 0dB adjustment.

Built one prototype quickly to see if my design works, and yes! Success!



And a photo with the VU meter in action.  (Sorry for the lousy photo... hard to photograph it clearly and still have the LED lights visible.)

[eric.B]

I'm maybe misinterpreting the range on the second "live" VU photo, but should I be interpreting the +17 db value as = 0db/clipping?

I look forward to these updates every day!   

if it were a vu meter giving you the level of the input to a digital recorder.. then yeppers! 

[JasonSobel]

I'm maybe misinterpreting the range on the second "live" VU photo, but should I be interpreting the +17 db value as = 0db/clipping?

I look forward to these updates every day!   

if it were a vu meter giving you the level of the input to a digital recorder.. then yeppers! 

it's really going to depend on the sensitivity on your recorder or A/D converter.  For example, when the 722 is set to line-in and 0dB, it hits 0dBfs (digital clipping) at +20 dBu.  So if you hit +17 dBu on the pre-amp, you know that you'd never hit digital clipping.  (in that situation, you'd want to use some gain on the 722, so that you hit digital clipping before you overload the pre-amp).  Of course, other recorders vary on their input sensitivity, and in fact, most would probably digitally clpi well before +17dBu, in which case you know to just watch the meters on the deck or A/D converter, and as long as you weren't digitally clipping, you would know that you are not overloading the pre-amp, and you could basically ignore the LED's on the pre-amp.

[fivefishdiy]

I'm maybe misinterpreting the range on the second "live" VU photo, but should I be interpreting the +17 db value as = 0db/clipping?

No... +17dB is NOT clipping on the preamp. (Just because it's a RED led doesn't mean it's clipping.)

The max headroom of my balanced line output driver chip is +27dBu. But since I'm running at a reduced voltage supply (instead of +/- 20VDC), I would estimate my max output headroom to be +22dBu (approx. 27.6 Volts peak to peak).

So that +17dB on the VU meter is still a good 5dB away from clipping.

EDIT: So I guess using a RED LED for the 5th LED in the VU meter might be confusing... maybe I should just change the 5th LED to Yellow instead of RED?