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Adding a meter to a preamp

@Boswell

or any of our other resident electronics wizards..

I have this ADK AP1 mic preamp that I love, but it lacks metering. There is nothing there to indicate what my input/output levels are.

I'm wondering how easy it would be to attach a meter, even a simple VU, that could help me determine what the input/output signal level is.

Is there a way I could attach a basic meter to this unit via alligator clips, or something that I could easily remove? I don't want to have to get into soldering anything, or connecting this permanently in any way.

Or would this just turn out to be a PITA?

Thoughts?

Comments

Boswell Sat, 06/27/2015 - 08:50

Basic meters such as home multimeters are not a lot of use for audio, as they lack any sensible dynamics. There are plenty of mediocre DIY designs around for crude level indication, but not many decent commercial units that are external products calibrated specifically for VU metering, and those that I know of are not particularly low cost.

The [="http://www.markertek.com/product/ru-sm16a/Attachments/Specifications/RDL/RU-SM16A-Specifications.pdf"]RDL RU-SM-16A[/]="http://www.markerte…"]RDL RU-SM-16A[/] at about $230 (needs wall-wart PSU) from [[url=http://="http://www.markerte…"]Markertek [/]="http://www.markerte…"]Markertek [/]might do what you want. It's dual-channel (intended for stereo), but you can feed a mono source into both channels and have one channel set to VU and the other set to peak so you get more indication of signal dynamics. It has a 4dBu switch setting, but can be calibrated for other nominal levels. Stupidly, it has only terminal-block inputs, so you would need to do something like modify an XLRM - XLRF cable to have 3-conductor T-off going to the meter.

BTW, metering should only be attached to the output of a pre-amp.

DonnyThompson Sat, 06/27/2015 - 23:49

As usual, You always clear things up, Bos. I was hoping I could take a spare VU meter (working) for my Revox B77 ( LOL, also working) that I have lying around, and simply attach it to points inside the pre, but if that's not feasible, I guess the option you mentioned is a little more than I want to get involved with... or spend, either.

But I sure do appreciate the info. :)

Boswell Mon, 06/29/2015 - 09:02

Wow, great photo - but without any idea of scale, it looks a bit like something you just stripped out of your bathroom!

No, that's not going to work as a VU meter without some electronics to drive it, as the ballistics of VU metering is all-important. If you are interested in persuing the project, I could do some searching around to see if there are DIY designs that might work with this meter, but otherwise, I'm sorry to say that I don't think it's a runner.

pcrecord Mon, 06/29/2015 - 12:52

If you don't find an affordable meter, you could write down some guide to use.
Exemple, you put the output to minimum or very low and analyse the signal while turning up the input until it breaks into distortion and note in your DAW what level it gives you.
My idea is if everytime you setup your pre, you start at the same output level and turn up the input you will be able to chose the level in relation to the breaking point. (before, on or past that point)
Then once the input is set up well, you just turn up your output level to the desired recording level...
Am I lost ?? Do that make sens ?

DonnyThompson Tue, 06/30/2015 - 01:03

Thanks for weighing in guys...

Bos... I appreciate your offer to research this for me... but don't worry about it. It's not that critical. But I sure do appreciate your willingness to help, my friend. ;)

Marco: I'm using a Line Level on the output of the pre to my Presonus i/o. The variable with this is that it's the ADK, with the swappable trannies, so the level changes depending on what combination of input XFO and OpAmp's I am using.
But, your advice is still solid, in that I could set up a DAW project template with multiple different tracks, and by using the line level input on my i/o, use the pre with different trannie combinations, and adjusting the gain of the pre, I could find out "which does what", and notate the details about the gain in relation to the transformers for each scenario.

Not as easy as a meter - LOL - and more time consuming, but... it would still work.

Thanks guys... :)

d.

DonnyThompson Tue, 07/28/2015 - 13:45

Jensenmann, post: 431122, member: 18198 wrote: In theory you could wire a real VU meter parallel to your output. In practice the back-EMF of the metermovement will induce distortion into your signal. We don´t want that hence the need for a buffer between output and meter. The best DIY solution I found for that is by JLM Audio: http://www.jlmaudio.com/shop/vu-meter-kits.html

Thanks for the link!

I see they have several models to choose from; is there a particular meter/model I should be looking for - as you said, that has a buffer - which would alleviate the distortion caused by the meter movement?

or would I be better off using a two piece method, so would something like this: http://www.jlmaudio.com/shop/vu-buffer-kit.html

work well with something like this: http://www.jlmaudio.com/shop/jlm-large-vu-meter.html

Is this a combination that would work for what I want to do?

Or, am I better off to look for a one-component solution, using a meter with a distortion buffer that is built-in?

Thanks again, Jensenmann!

Jensenmann Wed, 07/29/2015 - 00:00

Any real VU meter (or fake, if you prefer their ballistics) can be chosen if you use it together with the vu-buffer-kit.
I´m currently not aware of one-component solutions (LED meters excluded). They´d be seriously expensive, anyway.

I´ve been using the large VU and the vu buffer kit in various instances and they work great.

eric labrie Sun, 03/13/2016 - 06:00

If you want to track the output with a vu, you have 2 option. One is to use a true vu meter, rated about 20micro amp IIRC, rated for 600ohm output with an external 3k6 serie resistor. You have tp use an rectified vu meter. Note this will add a little distortion due to the diodes. The other option is to use a driver board to implement the vu meyer. Thevsignal stay clean, you can somewhat adjst your 0 db, but you need something like 9 to 15v to drive it, which can be problematic sometimes when there is no rails in the psu for such voltage.

DonnyThompson Wed, 03/30/2016 - 02:39

eric labrie, post: 437094, member: 49774 wrote: If you want to track the output with a vu, you have 2 option. One is to use a true vu meter, rated about 20micro amp IIRC, rated for 600ohm output with an external 3k6 serie resistor. You have tp use an rectified vu meter. Note this will add a little distortion due to the diodes. The other option is to use a driver board to implement the vu meyer. Thevsignal stay clean, you can somewhat adjst your 0 db, but you need something like 9 to 15v to drive it, which can be problematic sometimes when there is no rails in the psu for such voltage.

Thanks Eric.

But I've kinda given up on the idea, and instead have taken note of the varying differences of XFO's and OpAmps, and that method seems to be working okay.

I understand why the AP1 doesn't have metering; when Jon Erikson designed it - with the help of Justin Morse; and Larry Villella from ADK talked about the AP mic pres and priorities ( in his own words, "We spent so much money on the engine there was none left to put into a shiny paint-job and chrome bumpers...") they ended up spending a lot of money to hand-fabricate it, using top of the line standards, so costs had to be cut on "extras" - like a VU/LED, as well as on the cosmetics, which is understandable, as these things don't contribute to the quality of the sound.

It's a very "sparse" looking preamp, but they put the money into where it truly counts - under the hood. Top quality components and wiring were used, and in the end, the quality was to the level that it couldn't compete with other cheaper mass produced preamps - it seems as though everyone wants "Made in The U.S.A." products, but they want them at Chinese prices. Personally speaking, if I found a pre that sounded great, I wouldn't care if it were mounted in a pine box with hand written magic marker lettering.

;)

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