LL1515 on a micpre, anyone?

[theaterguy]

Hello everyone, an old newbie here.

I'd like to build a simple micpre, like this:

https://docs.google.com/viewer?url=...mers.com/wp-content/uploads/2014/08/as064.pdf

I have couple Lundahl LL1515's but there is no datasheet available. It is a microphone input trafo, 1+1:7 (200 ohm : 10k). Primary series/parallel? Some 10-60k resistor between secondary? I have a schematics where LL1515 is feeding a differential 5534 pair but I don't want to go that route.



The rest is easy, I have PSU's with phantom and so on already.

 

[jabe]

Seems like a solid plan. Assuming you're using the SSM2017 or similar (INA217?) per that whitepaper or something similar I'd personally wire the primary parallel, unless you specifically plan on using it with low output mics.
In addition to the load resistor it might be worth experimenting with a zobel. Obviously the datasheet is not available but Lundahl states "for replacements, please study LL1530 or LL1576" and they recommend a zobel of 10k/220pf for the former in 1:3.5 and (seemingly) 30k/220pf for the latter.

 

[theaterguy]

Seems like a solid plan. Assuming you're using the SSM2017 or similar (INA217?) per that whitepaper or something similar I'd personally wire the primary parallel, unless you specifically plan on using it with low output mics.
In addition to the load resistor it might be worth experimenting with a zobel. Obviously the datasheet is not available but Lundahl states "for replacements, please study LL1530 or LL1576" and they recommend a zobel of 10k/220pf for the former in 1:3.5 and (seemingly) 30k/220pf for the latter.

ok. Or should I just double those cap and resistor values shown in the differential schema?

 

[Newmarket]

I suggest that the scheme shown posits a performance increase to an existing input stage using SSM2017 or similar.
But starting from scratch I'm not seeing the advantage over a simple unbalanced gain stage post transformer.

 

[jabe]

Or should I just double those cap and resistor values shown in the differential schema?

Those seem pertinent to that circuit, so it's hard to say without seeing the rest of it. But I'd wager no.
The absence of a zobel in that snippet suggests maybe it doesn't need one, but I freely admit I'm stabbing in the dark without a datasheet for the Lundahl. Ideally you'd want to test for ringing with an oscilloscope but you might not be set up for that.
And Newmarket is right. Unlike an unbalanced gain stage the SSM2017 doesn't need an input transformer at all. Far be it from me to dispute Jensen, but I suspect you won't gain a lot from it. Is there a reason for going this route other than that you have the Lundahl and want to keep it simple?

 

[theaterguy]

Good points. Yes I'd like to use those Lundalhs and there is some advantages to use a trafo. Now it escapes my knowledge why Jensen grounds other leg and does not do it balanced.

Here is what comes after a Lundhal. After IC1&2 there is another 5534. It is from a desk built to Finnish Broadcasting company. I have still few fully functional channel strips but it's pointless to repair those anymore.
Another idea is use those caps and resistor values and go balanced to SSM.

 

[Matt Syson]

The slightly unique winding regime (dual bobbin) of Lundahl MAY be an influence on the Finnish design which is also good due to other aspects as overall CMMR may be better than other winding techniques.

 

[jabe]

I believe those resistors are for input bias current and their optimal values would depend on the circuit. See for example the SSM2017 datasheet, which recommends values "up to 10 kΩ, but they should be kept as small as possible to limit common-mode pickup".
The caps are, I'm guessing, to safeguard against RF noise. They may not be necessary but can't hurt.