Damping transformer ringing (Zobel network question)

[Gold]

The AP P1 was set to 40 ohm output impedance. That is how I have it set under normal circumstances. After installing the zobel, the top end flattened out on both the AD2 and the P1. I was only looking at the square wave on the AD2 so I don’t have the same sine sweep measurement on both. I should do that. I also don’t know if the midrange dip was the same on both.

It’s a 1:1 transformer. I don’t know which is supposed to be the primary and which is supposed to be the secondary. If I have if backwards could that make a difference?

 

[abbey road d enfer]

It’s a 1:1 transformer. I don’t know which is supposed to be the primary and which is supposed to be the secondary. If I have if backwards could that make a difference?

It shouldn't. However, I have seen transformers where the nominal loss (loss that happens when the xfmr is connected to nominal source and load impedance) was compensated by giving more turns to teh secondary. In that case there would be a minuscule difference.

 

[Gold]

A resistive load was the ticket. Thanks Henry L! I ended up using the same zobel jig and used a jumper instead of a capacitor.

 

[Gold]

-3dB around 50K. +26dBu @50Hz for 1% THD.

 

[HenryL]

You're welcome Glad it worked out for you.

 

[Gold]

If I have a 1k5 resistor across the secondary does that mean the source sees an input impedance of 1k5 in parallel with the input impedance of the line input?

 

[JohnRoberts]

yes.... for 1:1 trafo

JR

 

[abbey road d enfer]

If I have a 1k5 resistor across the secondary does that mean the source sees an input impedance of 1k5 in parallel with the input impedance of the line input?

Yes. I think 1.5k for taming a line input xfmr is brute force. How much loss does result from it?

 

[Gold]

yes.... for 1:1 trafo

JR

Yes. I think 1.5k for taming a line input xfmr is brute force. How much loss does result from it?

I'm getting a 1.5dB loss. I don't think I can use this transformer in this situation. I was planning on selling it but I can imagine a user having problems with such a low line input impedance.

 

[abbey road d enfer]

I'm getting a 1.5dB loss. I don't think I can use this transformer in this situation. I was planning on selling it but I can imagine a user having problems with such a low line input impedance.

You said it came from a VMS74, correct? Do you have the schematic of the circuit?

 

[Gold]

You said it came from a VMS74, correct? Do you have the schematic of the circuit?

I’ve been looking but can’t find it. I found the old cutterhead feedback amplifier circuit. That transformer was terminated with a 2k2 R. That is internal to the rack and doesn’t take input from the outside world. It’s fed from the cutterhead feedback coil.

This transformer could be that transformer but I think it’s the line in transformer. The line in transformer was just mounted on the frame and could feed the HF limiters if they were fitted or directly into the line in. I can’t find a drawing that shows that.

 

[HenryL]

1k5 does seem a bit low even to my novice's eye, though a 1.5 dB loss in itself might be quite easily liveable with?

I assume these are for line inputs (?)

Does the equipment you'll be feeding it with have 40ohms or less Zout? If so then it may not be an issue?

 

[Gold]

I’m putting this up for sale. I wouldn’t feel comfortable selling it that way. I wanted to use what I had but it’s looking like that’s not going to happen. The Lundahl’s I have aren’t 1:1. Neither are the Tamura’s I have.

I ordered a pair of CineMag’s which is probably the right thing to do anyway.

 

[waltzingbear]

I’m putting this up for sale. I wouldn’t feel comfortable selling it that way. I wanted to use what I had but it’s looking like that’s not going to happen. The Lundahl’s I have aren’t 1:1. Neither are the Tamura’s I have.

I ordered a pair of CineMag’s which is probably the right thing to do anyway.

I could send you a pair of 111C repeat coils, would that do?
Cheers
Alan

 

[Gold]

I could send you a pair of 111C repeat coils, would that do?
Cheers
Alan

The last pair I sold had UTC LS30’s. That’s what I used for years. I’m not looking to spend 111C money. No one can complain too much about a Cinemag.

 

[abbey road d enfer]

A 111C as an GP line input xfmr is inadequate, for two reasons:

• First, it is way oversized
• Second, it is optimized for 600 ohm matching, which is a thing of the past

The original Neumann (Haufe?) xfmr is not optimized for its task. I believe Neumann designers used what they had in the factory parts bin, didn't dedicate time to devise an optimized part, since it was used parcimoniously (only two in a $100 000+ rig).

 

[Gold]

A 111C as an GP line input xfmr is inadequate, for two reasons:

• First, it is way oversized
• Second, it is optimized for 600 ohm matching, which is a thing of the past

The original Neumann (Haufe?) xfmr is not optimized for its task. I believe Neumann designers used what they had in the factory parts bin, didn't dedicate time to devise an optimized part, since it was used parcimoniously (only two in a $100 000+ rig).

The size isn’t a problem because the case would fit them. The UTC LS30’s I used as input transformers for years were line matching 600 ohm. They worked well. The Cinemag transformer closest to the original Reichenbach is a 1:1 600 ohm model.

 

[abbey road d enfer]

The size isn’t a problem because the case would fit them.

The larger core may result in increased Barkhausen effect. Indeed, the Barkhausen effect makes it seldom heard.

The UTC LS30’s I used as input transformers for years were line matching 600 ohm. They worked well. The Cinemag transformer closest to the original Reichenbach is a 1:1 600 ohm model.

Being designed for 600 ohm matching, they have a different optimization than those designed for bridging and may be difficult for some sources..
I just warn not replacing blindly a line input xfmr with one of those without proofing performance.

 

[HenryL]

I'm getting a 1.5dB loss. I don't think I can use this transformer in this situation. I was planning on selling it but I can imagine a user having problems with such a low line input impedance.

Not wanting to become a wasp at a picnic here as you seem to have the input you were seeking, or perhaps not, if you'll pardon the joke, but I'm slightly curious as to why you've abandoned the idea so quickly and would be interested if you'd clarify.

Do you mean you were putting together this whole thing in order to sell it and that in that case you didn't think the low Zin looked good enough for where you were going to pitch it? If so then I kind of get it, though obviously there is plenty of (sought after) gear with more old fashioned eg.600ohm interfaces, and as long the purchaser is aware of the need for a low-z source...

I assumed you did the frequency analysis with 40ohm source because that was in line with the kind of gear you'd connect to it. Having achieved the kind of flat freq response you wanted with that kind of source then having to make up a dB or so of gain doesn't seem that onerous ?

 

[Gold]

Yes, I’m putting it together for sale. I don’t assume a knowledgeable buyer. I’m not so keen on having the low load impedance and I’m not so keen on having a 1.5dB level drop. I wouldn’t want that if I was going to keep it.

QE solid state gear isn’t 600 ohm. I don’t want to make it so it is.