Rouge Un - my attempt at a red Lundahl + 5534 preamp

[thomasdf]

True

 

[gyraf]

Did anyone figure out the specs on the pumpkin?

1:2+1? Am I right to suppose that it's a simple quadfilar wind?

What inductances? What DC resistances?

/Jakob E.
(who now have a machine for this sort of work)

 

[thomasdf]

AFAIK precise specs are still a mistery ? I was OK to buy one or two and have them checked by someone like you or CJ, but they are nowhere to be found... Maybe someone with an appropriate Focusrite piece of equipment / serial number can get one from Carnhill directly ?

Some info here, but not much https://groupdiy.com/threads/anyone-have-any-info-on-a-carnhill-vt30408-tranny.9703/

 

[gyraf]

So 1:2+1 confirmed
Then +20dBu handling at primary - but at what frequency? 10Hz? 10KHz?
And 600 Ohms secondary expectancy - this puts the secondary DC resistance in the 25-50 Ohms range

Could any of you open your ISA110 and confirm?

 

[KrIVIUM2323]

(who now have a machine for this sort of work)

Off topic: Jakob, I wondered what was this strange machine you used in one of your Youtube videos, now i got the answer!

I could try another Lundahl too, i've got some 1517 waiting for some use. It'll be limited to +18dbu on primary though, max 24dbu on series connected secondary (-2db difference to original spec of Isa range).

 

[gyraf]

yup, like seen in here:

I wouldn't worry at all about levels beyond +24 - nearly all analogue gear I've tested sounds best around +12 to +15 anyway, these are the sensible work-levels to aim for

/Jakob E.

 

[thomasdf]

Yes, and we are talking microphone preamp here, I'd be worried if my levels got up to +24 right at the output of the preamp haha !


Edit : not related but I just finished building a CNC and I'm experimenting with PMMA / Plexiglass cutting / engraving, enjoying it a lot .. and can't wait to get at aluminium, either with this very one or a larger future build ...
Rolling your own transformers and engraving front panels ... living the dream !

Cheers

 

[KrIVIUM2323]

Yes, and we are talking microphone preamp here, I'd be worried if my levels got up to +24 right at the output of the preamp haha !

Did i expressed i want to run this hot?
No, my concerns are relative to headroom, which in my experience is one of the main difference between semi pro and 'pro' gear( Neve designed gear-or same quality range- in particular). One of the reason my little sister could make a good sounding record/mix into a pro control room, despite it's not her field of expertise at all.

I agree 2 db is not a big deal but it's already lower than the original module, hence overall feel might be different wrt an original.

It depends the way you track and with which material ( i'm more into metal than chamber music). I must admit i don't like to treat to much at takes (compression or limiting) and because i used first generation digital (16bit) at beginning of my carreer i tend to run relatively hot into AD. No more than 6db left before clipping. With the occasional peak which can pass through it can land into the +20db range easily. It doesn't let many headroom left.
It can be argued that with 24bit i could lower level but it's my habits.

 

[krabbencutter]

Well, the pumpkin is intersting for sure, but it is hard to find ... the website on the previous page does not have them in stock.
2503 does not seem to be a direct drop in replacement ? It is missing a tap on the output ?

@krabbencutter is having trouble with his AT8 rendition so maybe it's a safer bet to stick to the OG design topology.

I guess I'll have unused LL1539 maybe I can sell them to buy something elese

I hate having different options haha

The AT8 seems to work as intended. But I'm having trouble figuring out the correct cmrr trimming. Official documentation is very sparse but I'll hopefully be able to make progress soon. You can also adapt the circuit to build a standard cross-coupled output stage (like the SSM2142/DRV134).

 

[ruffrecords]

yup, like seen in here:

I wouldn't worry at all about levels beyond +24 - nearly all analogue gear I've tested sounds best around +12 to +15 anyway, these are the sensible work-levels to aim for

/Jakob E.

I suspect health and safety would have something to say about holding a faceplate down by hand on a working CNC bed.

Cheers

ian

 

[gyraf]

Yeah, but luckily its so low powered on all axis's that its not a problem stopping it by one hand. It's an old machine, set up for engravings only

Hands on the faceplate is for adjusting the +/- 0,05mm that panel thickness varies, that would otherwise result in poor looking engravings...

 

[thomasdf]

The Pumpkins were back in stock here so I bought 3 of them. One left apparently :
https://www.audiotestserviciotecnic...arnhill-vt30408-output-transformer-focusrite/
My idea is to use a pair, and have the third one available for analysis.. DC resistance, inductance, and maybe unwind
I’ll let you know when I get them, and we’ll see how we can organize that !
Cheers

 

[gyraf]

..very interesting development indeed.

Note that if you put e.g. 10 extra turns of your own onto the toroid, on top of existing windings, you can measure what sort of inductance these 10 turns brings you

Then, using https://powermagnetics.co.uk/calculator/ you can deduct the A(l)-value of the core

Now, for every "official" winding you can just measure inductance, and above calculator will estimate the number-of-turns of said winding

If you rough-estimate the length of each turn, you can now calculate wire length on each winding.

Having the wire length, we just need to measure DC resistance of the winding to get a good estimate of wire thickness..

And voila - a fully non-invasive dissection

/Jakob E.

 

[thomasdf]

Interesting ! Thanks for sharing... I'll give it a try. But I am totally open to ship the 3rd transformer somewhere for someone to measure / dissect it
A DIY fueled toroid clone / replacement would be very nice.

 

[KrIVIUM2323]

The Pumpkins were back in stock here so I bought 3 of them. One left apparently :

Not in stock anymore some hours later... Are people watching this thread?!

 

[thomasdf]

I had suscribed to an email alert for when the item was back in stock. Honestly I never thought they would have some other units one day, but who knows. When I received the email this morning and clicked the link, they had 4 in stock, I bought 3. I am not surprised there is none left, there seems to be a demand for these.
Hopefully the analysis of the design will allow us to come up with an alternative ? I’d be happy to help with this, at least.
I will let you guys know when I get them, and will gladly work on a 500 version featuring this transformer and/or a GDIY version clone