Turtlerock Forum

Hi all, just wondering if anyone here has built the Gyraf G9 tube mic preamp?

Heres the link:

http://www.gyraf.dk/gy_pd/g9/g9pd.htm

I am building it into an old defunct reel to reel chassis, with VU meters (got the buffers from jlm audio) and using mostly point to point wiring (tagboard) instead of PCB.

I am using the OEP input transformers as per their suggestion, but some (very much) larger (physically) output transformers I had lying around.

Any thoughts on the Gyraf G9? Quality of sound? etc?

I like the thought of tube audio stuff as I have made guitar and hifi amps before, and thought I would make this one.

Oh another question: at what voltage should I set my VU meter at 0dB, when running into the motu 828?

My next thoughts are to build the Altec 1566, see link:

http://www.tangible-technology.com/tubes/1566/1566.html

Any thoughts on that one?

Gian wrote:Oh another question: at what voltage should I set my VU meter at 0dB, when running into the motu 828?

Its a personal choice, but a while back I posted some VU test audio clips set for
0dbVU = -14dbFS. What that translates to in terms of actual voltage depends on the converter's own headroom, but in the end it doesn't matter, 'cause its all relative anyway - you just want a level that gives you the comfort of "enough" headroom when recording.

I originally set that up using an 828 as well! I now use an Apogee that has soft limiting for an extra 4db of headroom, but I still use the same -14dbFS reference.

chris p wrote:

Gian wrote:Oh another question: at what voltage should I set my VU meter at 0dB, when running into the motu 828?

Its a personal choice, but a while back I posted some VU test audio clips set for
0dbVU = -14dbFS. What that translates to in terms of actual voltage depends on the converter's own headroom, but in the end it doesn't matter, 'cause its all relative anyway - you just want a level that gives you the comfort of "enough" headroom when recording.

I originally set that up using an 828 as well! I now use an Apogee that has soft limiting for an extra 4db of headroom, but I still use the same -14dbFS reference.

Hi Chris, please excuse my lack of knowledge, but how about this for a strategy. If I try recording at different levels into my DAW, then check the waveforms to see when they distort and then set 0dB a bit before that? What do you think? Or is there an easier way?

a better way to do is to set you vu meters at the correct level which is -14db lower then where the digital clips .
there is a long essay that could be typed here
but there was an even better one in Audio technology last year by Rob Squire
anybody know which issue ..?

I believe it was Audio Technology no.51

rick wrote:a better way to do is to set you vu meters at the correct level which is -14db lower then where the digital clips .
there is a long essay that could be typed here
but there was an even better one in Audio technology last year by Rob Squire
anybody know which issue ..?

Hi Rick, I have already built the VU buffers and tested them on the VU meters I have installed on the chassis. They are very cute little boards I got from JLM Audo.

OK, tell me if I am understanding correctly. I find the maximum input voltage my converter accepts (with say a 1k sine wave and scope or multimeter). I now have a voltage reading, how do I then convert that voltage to dB?

I thought dB was a ratio of input to output using a logarithmic scale to squeeze a large range of numbers into a small range.

OK - I've looked it up for you

http://www.turtlerockmastering.com/forums/viewtopic.php?t=939&highlight=14db

You play these through your DAW and 828, and adjust your VU to read 0db.

chris p wrote:OK - I've looked it up for you

http://www.turtlerockmastering.com/forums/viewtopic.php?t=939&highlight=14db

You play these through your DAW and 828, and adjust your VU to read 0db.

Great, thanks heaps Chris. I will use these for sure.

But I would still like to understand how it works. Is this forum the appropriate place to ask? I am feeling a bit dumb.

Gian

If you contact Rob on this forum I'm sure he'll be able to provide you a copy of his article.

The short version is this:

In digital, 0db FS (full scale) is the loudest possible signal. All useful digital recordings are thus done at negative db values. Anything louder than 0db FS creates digital distortion, and that sounds like a cat scratching its claws down a blackboard - it is to be avoided at all costs.

But no signal is so predictable that you can hit 0dbFS on the nose as the maximum peak when recording. You always need to allow some headroom, so you what you do is nominate a particular lower volume as your target. This is your very own personal 0db reference. It can be anything you choose depending on your gear, but bear in mind that it must meet two competing, and in fact contradictory goals: it must be sufficiently low that the largest transient peak still falls below 0db FS, but at the same time it must be loud enough to get a decent signal to noise level into your DAW. So you want it as loud as possible but NEVER too loud. With me so far?

Well, like all such things there is a rule of thumb, and for audio recording the rule of thumb is to set your "target" recording level to -14db FS. For television its -20db FS. It can be anything, but -14db FS is a pretty good place to start. Plenty of signal to noise ratio (in fact over 100db for 24 bit converters) but still lots of headroom for that massive kick drum peak. So you play those audio files above through your DAW and calibrate your VU meters to read 0db: this sets your 0db VU = -14db FS. That way, your meters are telling you when you've got your signal hot enough (even though your DAW meters may be barely registering).

Now here's the thing: the voltage represented by -14db FS depends on your converter. While all are nominally set to work from a 0db = +4 dbu "professional" standard, each has its own maximum voltage that its converters can handle, and the designers have (or at least should have) set their 0db FS at that level. In practice its usually something between 16-24 dbu, but you don't have to worry about that: -14db is, as you state, simply a ratio, and it is this ratio that gives you the headroom you need. Whatever actual voltage is set by the converter manufacturer as 0db FS, your target is 14db below that.

Rob's article is better, but this should give you the general gist.

nice bit of info there chris!

There's a link to Rob article on JLM's webpage for the VU meters:

http://www.jlmaudio.com/VU%20Buffer.htm