Pultec dip from 20 Hz and below
- Thread starter atticmike
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ruffrecords
Well-known member
This probably depends on the source impedance of your test generator.
Cheers
Ian
Cheers
Ian
ruffrecords said:
Well, for all my other gear it's performed quite well. Just the pultec's jumping outside the picture. I've gone straight out of the lynx aurora into the EQ. Purusha's filter board had unbalanced and balanced in. I surely chose balanced since i had both the out and input populated with the lundahl. I assume the lundahl's performing that dip?
pucho812
Well-known member
well how does it perform in bypass? More over either I am reading your picture wrong or it looks like your getting a high pass filter vs a dip at 20hz
ruffrecords
Well-known member
atticmike said:
Depends. Remember the Pultec is a 600 ohm input not a 10K bridging. Most gear these days expects to drive into a 10K bridging load - who knows what the aurora will do into 600 ohms? Try it back to back with a 600 ohm resistive load. For example, if they use 22uF output caps (in each arm of the balanced out), the -3dB point with a 600 ohm load is 26Hz. With a 10K load it is 1.6Hz.
Cheers
Ian
ruffrecords said:
maybe I'm supposed to go unbalanced at the input instead of balanced? For those who're interested, despite the fact that I got that 20hz rolloff, I mixed the drums of a track by every single source (kick, snare, oh, room etc except the toms) with that one EQ and added a little bricasti m7 mojo: http://dl.dropbox.com/u/70609905/drums_pultec.mp3.
I had said it is the transformer since the original pultec also has a similar rolloff but your theory makes actually a lot of sense though... meh... I knew that thing's gonna go passive on me ;D
Mike
ruffrecords
Well-known member
MatthisD said:
If the primary really is 2H and the generator source impedance is 100 ohms then the primary impedance at 20Hz is 2*pi*f*L = 251 ohms. The loss at 20Hz is:
251/(251+100) = 0.715 = -2.9dB
which explains what you are seeing. What I don't understand is why Lundahl would make a 600:600 transformer with such a small primary inductance. Most other 600:600 transformers I have come across have a primary inductance of 10 to 20H.
Cheers
Ian
ruffrecords
Well-known member
atticmike said:
Which is another strange thing - in tube circuits at least - because it I quite clear from the Lundahl data sheet that it is intended to be used in semiconductor circuits with the transformer inductance in the feedback loop. The very low output impedance of the op amp driver shown their app note plus the feedback explains why such a low inductance is acceptable. This would not be my first choice as an input transformer.
Cheers
Ian
Kingston
Well-known member
atticmike said:
It wouldn't be the first time a very well known clone with "totally awesome sound, dude!"-reputation is found lacking in some significant area. It's rare a designer would provide any expected specifications, and even more rare that some builder would fashion a reliable test suite after the fact.
MatthisD said:
hm, just weird. Can't get my head around it at all. I could compensate for the drop with a capacitor, yet one could just use the boost to compensate for it.
About the Q, It's been mostly at 12 o'clock. Though bear in mind that I used a 5k pot for the Q range.
Kingston said:
That's what I've noticed but yet, I'm completely blown away by the EQ performing on drums.
ruffrecords said:
Well, what would've been your choice for the input trannie?
Mike
Are we doing seismic measurements, whale and elephant recordings, or studying the bottom end of the pipe organ? If not, I don't see the problem. I do think the transformer choice was made out of the choices available at the time, to get a response reminiscent of the circuit it emulates. The silly part might be that people still follow the original plan, that there's no update or real study of all the ways one might change the sound with nothing but a transformer change, for better or worse.
ruffrecords
Well-known member
atticmike said:
How about a Sowter 3603?
Cheers
Ian
ruffrecords said:
Alright, that one looks quite nice. Will it solve that input dip problem? I also found out that the output is really low, prolly has to do with the input transformer. In order to get the EQ to a proper level, I have to set my lynx inputs to -10dbv.
You probably won't want a step-up input trafo in the Pultec, as input impedance is VERY low already, approximating 100 Ohms at full-boost-high and high Q.
A 1:2 input transformer would lower this to 25 Ohms, which most outputs would have a hard time driving.
Jakob E.
A 1:2 input transformer would lower this to 25 Ohms, which most outputs would have a hard time driving.
Jakob E.
keefaz
Well-known member
And maybe the loading ability of the source equipment, no ? For example I experienced more bass from GPultec with a G1176 or API mic pre connected at its input and less bass with other gear that have electronically balanced outputs with opampsruffrecords said:
Swapped the in and out to filter-amp cabless as recommended by Purusha but then it really got bad, like up to 120hz low-cut. Meaning I had it wired up correctly before that swap.
I checked all the caps, they are all fine and at the right spot.
Maybe this build is not supposed to be run with an output Lundahl? Maybe I have to connect the xlr input to the unbalanced in since the load is 100 ohm from the lynx and not 600 and the unbalanced in has a 10k resistor which would be suitable for the lynx?
Mike
I checked all the caps, they are all fine and at the right spot.
Maybe this build is not supposed to be run with an output Lundahl? Maybe I have to connect the xlr input to the unbalanced in since the load is 100 ohm from the lynx and not 600 and the unbalanced in has a 10k resistor which would be suitable for the lynx?
Mike
alright guys, I took out the input trannie (lundahl), connected the input to the unbalanced in but nothing has changed. I've checked all my stuff and nothing occurs to be wrong. Is it just that build that is that way or have I done something wrong? I mean, this is such a simplistic build and I just can't get it working...
U got any idea where that drop might come from? :/
Also, what I noticed is that the out connection, from the filter board to the power board has load between out and ground, is that normal?
Please, help a fellow DIYer out, any input's most appreciated!
PS: Could it be caused by a bad tube? I just don't know where to go from here on ...
U got any idea where that drop might come from? :/
Also, what I noticed is that the out connection, from the filter board to the power board has load between out and ground, is that normal?
Please, help a fellow DIYer out, any input's most appreciated!
PS: Could it be caused by a bad tube? I just don't know where to go from here on ...
Are we still talking about the "problem" of the GPultec rolling off below 20Hz?
I'm not sure I understand why this gets so important?
Jakob E.
I'm not sure I understand why this gets so important?
Jakob E.
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