Warwick Hellborg preamp transformer question

IMG_2379.jpeg
So this is the input board
XLR to transformer - mono Jack to IC201 to get to mic level …
Then the relay is to switch between them - what’s the stuff on the right hand side for? Power to the relay? The +0v and -0v is something I’ve not seen before!
 
View attachment 93580
So this is the input board
XLR to transformer - mono Jack to IC201 to get to mic level …
Then the relay is to switch between them - what’s the stuff on the right hand side for? Power to the relay? The +0v and -0v is something I’ve not seen before!
Looks to me like those are the letter O and not the number 0. So possibly an abbreviation for positive and negative Output voltage?

Reasonable to assume that's the rail voltages for the NE5532
 
makes sense - those 100R resistors are going to drop it slightly from +-15
Also have a 40Hz LPF on each rail going on. 100R shouldn't alter the V but will current limit protection to some extent. Well. After 150mA or so. That's when you'll see V drop.
 
It had been a while since I heard back from Haufe, but they emailed me today informing me that upon further investigation it turns out ST 8161 is a customer-specific item and they are not allowed to sell it to anyone else. They offered to try to find me a similar unit, but I already have a few hundred similar units on a shelf so I don't really need theirs.
 
It had been a while since I heard back from Haufe, but they emailed me today informing me that upon further investigation it turns out ST 8161 is a customer-specific item and they are not allowed to sell it to anyone else. They offered to try to find me a similar unit, but I already have a few hundred similar units on a shelf so I don't really need theirs.
Well - first of all thank you Nathan
Even if they can't sell something with you, what you managed to find out from them means we can work out what the design decisions would have been....
Its specs as you found are very very close a Neve line input transformer, wired backwards. So either that's chance or they were trying to get close to a Neve line input transformer - which sounds like something Jonas Hellborg might have asked for...

So that's enough to recreate the preamp section.
The EQ section, was good but no so great, and inductors are a) expensive and b) a wee bit beyond my capabilities/undersanding at the moment
and the DI transformer is a custom transformer, (possibly neve flavoured), but essentially any DI transformer that will work from the output of a NE5532 will be fine ...


My plan then is to make a thing.
Transformer- preamp stage - output and DI out
- omit the EQ, mute and effects loop.

I'll use it end of my pedal board chain - though I'm tempted for pedalboard use put it in a box with an Input buffer of some kind into switchable loop infront of it. Still working out if that's a good idea!
 
I’ve got the basic circuit into KiCad now to build it as a preamp…

Thanks to @vigilante398 I know that a neve style line input transformer will get me in the ballpark - so carnhill or cheaper OEP equivalent

Output Di transformer is from the output of a 5532 stage - and I should be able to work out what I need…

Inductors I know nothing about, the low mid inductor is similar to the neve eq 10H tapped inductor, but not going past the 7H tap and with the taps at different places.
The hi mid is 640mH centre tapped at 320mh…

So question for the hive mind …
Preamp pedal for bass - Should I bother the the eq?
If yes any good sources of inductors for our kinda diy needs? My search finds (I’m in the U.K.) and don audio but not much else - it feels like I’m missing some vital search term. I’ve a feeling custom inductors might end up quite expensive! Are there any obvious suppliers? Have I missed the eq inductor section of Tayda?
 
I know AML as they are fairly local to me.
And seen CAPI - the challenge will be getting them to the same/similar values as the hellborg ones


Alternative I guess would be a non-inductor based semi parametric mid control
 
You could absolutely do the standard neve RLC filtering and match/expand the hellborg frequencies. The inductor is the only pricey but, otherwise it's film caps and resistors and a rotary switch in the feedback loop of an opamp.
See the second page of schematics here
 
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