Pultec dip from 20 Hz and below

[atticmike]

Hey,

Just finished my g-pultec and love how it sounds (2 x lundhal trannies and don-audio inductor).

What I'm curious about though is the dip at 20 hz and below?

 

[ruffrecords]

This probably depends on the source impedance of your test generator.

Cheers

Ian

 

[atticmike]

This probably depends on the source impedance of your test generator.

Cheers

Ian

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 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

 

[emrr]

Looks like a typical transform rolloff to me. 

 

[ruffrecords]

This probably depends on the source impedance of your test generator.

Cheers

Ian

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?

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

 

[atticmike]

This probably depends on the source impedance of your test generator.

Cheers

Ian

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?

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

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]

Aurora output impedance is 100ohms according to Lynx. I have one myself and didn't measure a rolloff into a Pultec clone(600 ohm input). If you're using LL5402, it was measured by Jackies as having "pri 56, 8H, sec 14, 2.3H". The G-pultec schematic shows it with primaries in parallel which would be 2H or close to it. You could try bypassing the input transformer and send the signal in unbalanced and check your response for improvement.

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

 

[atticmike]

Aurora output impedance is 100ohms according to Lynx. I have one myself and didn't measure a rolloff into a Pultec clone(600 ohm input). If you're using LL5402, it was measured by Jackies as having "pri 56, 8H, sec 14, 2.3H". The G-pultec schematic shows it with primaries in parallel which would be 2H or close to it. You could try bypassing the input transformer and send the signal in unbalanced and check your response for improvement.

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

But everybody's using those lundahlls, wouldn't they experience the same culprit?

Mike

 

[ruffrecords]

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

But everybody's using those lundahlls, wouldn't they experience the same culprit?

Mike

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]

But everybody's using those lundahlls, wouldn't they experience the same culprit?

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.

 

[atticmike]

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

5402 is an output transformer 2+2:1+1, if series connected it could be called 600:150 rather than 2400:600, theres a thread discussing LL5402 in the G-Pultec (searching 'll5402 low end' should find it).

Grounding one of side of the G-Pultec input to unbalance the signal before the transformer should change the Aurora output impedance to 50 ohms so you could try that.

There could be improvement by using it backwards so that the EQ ground is connected to the ends of the paralleled primaries and the 2.3H winding as the balanced input. Where you position the ground could reduce the capacitance between the windings as mentioned in the datasheet.

Mike, regarding the sample do you have the boost set to sharp on any of your tracks?

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. 

But everybody's using those lundahlls, wouldn't they experience the same culprit?

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.

That's what I've noticed but yet, I'm completely blown away by the EQ performing on drums.

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

But everybody's using those lundahlls, wouldn't they experience the same culprit?

Mike

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

Well, what would've been your choice for the input trannie?

Mike

 

[emrr]

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]

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

Well, what would've been your choice for the input trannie?

Mike

How about a Sowter 3603?

Cheers

Ian

 

[atticmike]

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

Well, what would've been your choice for the input trannie?

Mike

How about a Sowter 3603?

Cheers

Ian

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.

 

[gyraf]

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.

 

[keefaz]

This probably depends on the source impedance of your test generator.

Cheers

Ian

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 opamps

 

[atticmike]

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

 

[atticmike]

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 ...

 

[gyraf]

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.