ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« on: June 25, 2004, 12:17:51 AM »
What is the desired input signal level on this unit?

Here's the deal when I come out of my D/A (Digi 002) into the Pultec if the signal level is over -7 I get distortion.

As well When I come out the level is just as low....   And I have to boost it with the A/D input mic pre...

Am I hooking this up worng somehow ? Balanced out, Balanced in.

Any ideas?

-Chuck


rafafredd

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #1 on: June 25, 2004, 02:02:15 AM »
maybe the digidesign outputs can´t drive the input transformer. Are you using a 600 ohms primary input transformer?

Check on the DA manual, what´s the output impedance...

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #2 on: June 25, 2004, 02:19:15 AM »
I'll check.

I'm using the Lundahls (IN and Out) as spec'd by the Gyraf DIY projects...

Thanks I'll get back to you about the impedance.

-Chuck

Kev

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #3 on: June 25, 2004, 02:26:55 AM »
You need to understand your 002.

Don't laugh.
I've had many a flame over at the DUC over this sort of stuff.  With both 888's and 001's.  Many people just don't get it.  It is all very well to quote 0dbV or 0dbU but what does it all mean ?

It will involve using two passive VU meters with switchable 600 ohm termination.

I can explain and describe a procedure if you like.   :cool:
Kev
DIY Factory

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #4 on: June 25, 2004, 02:41:24 AM »
Yes please do !

The lame manual only goes so far to say that the XLR outputs are wired to match the impedance of a mic and not line level.

I've tried every combination of TRS and +4 to -10 etc.. But still the audio is will distort when cranked. And the output is always too low.

When I tested it on the scope with a signal generator I got VPP unity gain thru the device. It must be the interface?

Thanks

_Chuck

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #5 on: June 25, 2004, 03:21:21 AM »
I only have a multimeter here at home so I stuck that on it.

1. The input signal coming from the 002 D/A reads 1-3 VAC.
2. The output of the first Transformer, input to the passive EQ reads 0.1 to 0.3 VAC.
3. The main output reads 10 to 30 mVAC .

Attenuates by a factor of ten after each transformer. Has anyone seen something like this before?

Keep in mind I put a signal generator on it earlier and feed it 1-2 VPP and got unity gain thru.

?????

-Chuck

DrFrankencopter

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #6 on: June 25, 2004, 09:11:07 AM »
Possibly a dumb answer, but is there a chance that you've wired your transformer as a 10:1 instead of a 1:1?

I used lundahl's in and out on my pultec, and the level was almost unity direct through.

Cheers,

Kris

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #7 on: June 25, 2004, 12:41:15 PM »
No I used the PCB board provided by Gustav. There was only one way to drop in the transformers.

Besides I saw unity gain with the sifnal generator.


Any other ideas?

-Chuck

sismofyt

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #8 on: June 25, 2004, 12:48:52 PM »
Are you sure you're running balanced in/out with the 002? Maybe it has wannabe floating pseudo differential circuits :? Check the cables too..

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #9 on: June 25, 2004, 01:27:42 PM »
Not sure I understand? floating ?

I tried grounding negative side of the input and output to see if it truely wasn't balanced but no change.

The 002 says it has TRS balanced INs and outs.


Any other Ideas ?

-chuck


sismofyt

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #10 on: June 25, 2004, 01:30:21 PM »
I just meant not real balanced operation. That sorta cicuit can act funny with trannies. Hmm.. I dunno what's wrong :? Have you tried bypassing the trannies and going unbalanced?

soundguy

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #11 on: June 25, 2004, 01:49:06 PM »
chuck-
are you talking about -7 on a digital scale?  If so, distortion  might make sense, your zero should be between -18 and -12dB...  Look for 1.2volts dc and see if your input is still distorting.  OdB on a VU meter is about 1.2volts.

I hope Im remembering that correctly...

dave

chips are good with dip...

sismofyt

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #12 on: June 25, 2004, 01:54:15 PM »
You mean 1,2V _AC_, not DC, right? I don't get '-7 on a digital scale' :?

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #13 on: June 25, 2004, 02:34:59 PM »
Yeah I just meant -7 on my fader display in Protools.

I haven't had a chance to put it back on the scope.

I'm think it's gotta be an impedance issuse?
Does anyone know the impedance of the DIGI 002?

-Chuck

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #14 on: June 25, 2004, 02:43:51 PM »
From the DIGI site ...



002

Output Impedance:  50 ohms

Dynamic range:
Main Outputs: 114 dB A-weighted, 112 dB (unweighted)
Outputs 3-8: 114 dB A-weighted, 112 dB (unweighted)

Input Impedance:
Inputs 1-4 (XLR): 2 kohm
Inputs 1-4 (1/4"): 10 Mohm  


Would this cause the results I am seeing?

-Chuck

sismofyt

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #15 on: June 25, 2004, 02:46:25 PM »
Have tried using some of the other inputs? I don't think those 1/4" 10MOhm inputs are balanced?

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #16 on: June 25, 2004, 02:50:53 PM »
Yup I tried all the inputs,

But yes the are supposed to be TRS balanced inputs.

-Chuck

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #17 on: June 25, 2004, 04:13:08 PM »
I am now seeing the same error on the scope with a signal generator!!!

This means something shorted and the second transformer is pulling the signal down somehow...

I am thinking there is something up with the main board. I have to wait for it to cool down to take it apart.

-Chuck

ChuckD

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #18 on: June 26, 2004, 01:49:40 AM »
Am I retarded?

Fixed !!!!

What was wrong? The damn secondary step up to 220 transformer was disconnected when I was tidying things up!

When I analysed the passive circuit I found that it does by the nature of the "not so true" bypass attenuate by a factor of approx 10 in bypass mode. But then the Tube gain stage gives that back.  

A truely clever design but then when I realized this I figured out that the tube gain stage wasn't working. So I probed around very catiously and didn't see that 220 anywhere! Problem found and solved.


One thing to note. Since Bypass is not a true bypass and will still go through the gain stage the EQ can be used in Bypass mode to add a Little "Tube warmth" to a dry digital recording track.


Time to make another and go stereo!!

-Chuck

Kev

INPUT and OUTPUT levels low in Gyraf Pultec??
« Reply #19 on: June 26, 2004, 07:41:29 PM »
Great that things are fixed
Please do consider a couple of passive VU meters with a termination switch.
everyone should have some as they are just too useful not to.
send me an email if you are keen.  Can't be bothered getting into a Mike T.   0db  blah blah ... thing here.
He is right
 BUT we all have to deal with the equipment we have.
Learn the rules
then break them if you need to.
Kev
DIY Factory


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
1392 Views
Last post April 19, 2008, 05:46:50 AM
by ioaudio
6 Replies
2028 Views
Last post August 11, 2008, 09:21:44 AM
by martthie_08
6 Replies
2303 Views
Last post August 19, 2008, 03:15:14 PM
by crisotop
12 Replies
3743 Views
Last post August 27, 2012, 02:34:12 PM
by EmRR