Microphone input transformers

pinksoir

A while ago I posted about an old tube R2R my friend found in a skip that I want to mod into a pair of microphone amps. I have all the schematics, but I can't for the life of me work out/find out what sort of mic input transformers I need. The dude who made the schematics recommends cinemag CMMI 10C or 7C transformers. The only place I can find these is direct from manufacturer, which is based in the States. For his original he used input transformers from a Shure microphone amp but used cinemags for the later ones.

Can anyone give me a lowdown on input transformers? I have no idea about them at all. There's a place in the UK that manufactures and sells them called Sowter, but I don't know what specs I should be looking at.

peter05

Sowter is your best bet if you can get them. I use the bigger ones in my vari mu compressor, and sowter are used in the fairchild. You might need to bulk buy them though. I would use Sowter or lundahl mostly for trannies in an audio chain.

Is it a PCB mount you need?

Contact Sowter they are very good at getting back to you and helping you get the right trans.

pinksoir

Cheers Peter.

Canford sell them: http://www.canford.co.uk/SOWTER-AUDIO-TRANSFORMERS

I just don't know which ones to buy. I can't find any there with similar specs to the Cinemag ones recommended by the modder. But yeah, I should just email Sowter, they'll know best.

peter05

Post the schematic. You will need to match the relays. Without that info you will be shooting in the dark.

I think the lundahl is LL1544a.

pinksoir

Cheers,

The schematic is at the end of this page (it's a zip file):
http://beyondsanityproductions.com/index.php?option=com_content&task=view&id=13

peter05

pinksoir wrote: »

Cheers,

The schematic is at the end of this page (it's a zip file):
http://beyondsanityproductions.com/index.php?option=com_content&task=view&id=13

I don't see you having any luck using Sowter for it. Send them an email anyway they might recommend something compatible to the cinemag.
They don't seem to have anything matching your turns ratio 1:20/10

I'll have a better search later tonight for you it might turn up something.

pinksoir

Bummer. Yeah that's what I was worried about. I'll fire them a mail. From what I can gather, cinemag do ship out single or small orders. Jensen too. But it might not make much sense economically... I'll see what Sowter say anyways.

Thanks again.

TroutMask

Try these guys

http://www.oep.co.uk/products.php?cat=6

they are distributed through Radionics and Farnell AFAIK.

They're inexpensive, and will give you an idea whether the project is going to sound good or not. If it sounds good, you can then consider upgrading to Jensen or similar. The audio traffos come multitapped, so you could probably match your impedance by manipulating the wiring of the coils.

pinksoir

Cool. Ta very much.

TroutMask

Jensens (or any traffo worth its salt) are seriously expensive - like the guts of 100 yoyos per channel. Before the shipping and import duty. Good irons costs, which is why I recommended the lower cost OEP for a test run. OPE are still good BTW - BSS used to use them in their awesome AR116 DI boxes.

pinksoir

Yeah, i didn't really want to spend over the odds on getting input transformers really. The Sowter ones are cheap enough at ~€40 each, but unfortunately I don't think that have the right specs. The ones you linked look like they do though, which is cool. I'll have to have a closer look when I get a chance.

Tbh, the preamps I'm making aren't going to be super high quality, so 100 quid a channel seems like a bit of overkill! The dude who wrote the guide on how to do it used trannies from an old Shure mic mixer and by all accounts it worked fine. He upgraded to Cinemags down the line.

peter05

If you change your mind on the Sowter or if you can get them, I'll be ordering 4 at the end of April. Might give us a discount for a bulk buy, Save on postage also.

Let me know if you get sorted in the mean time.

@Troutmask I have been searching high and low for a Vari cap 10 - 47pf. Would you know if I could use a 9 - 50pf instead as it is far easier to find.

pinksoir

That's definitely something i'd be interested in Peter. i haven't gotten round to emailing them yet, but i will do tomorrow hopefully.

TroutMask

peter05 wrote: »

If you change your mind on the Sowter or if you can get them, I'll be ordering 4 at the end of April. Might give us a discount for a bulk buy, Save on postage also.

Let me know if you get sorted in the mean time.

@Troutmask I have been searching high and low for a Vari cap 10 - 47pf. Would you know if I could use a 9 - 50pf instead as it is far easier to find.

Hi Peter, yes that will work perfectly. Most caps are 10% tolerance (e.g., a 100 pf cap can lie in the range 90 - 110 pf). Often there is a 20% tolerance. Only 'precision' and/or 'mil spec' (military specification) components have really accurate tolerances, and these don't usually find their way into audio gear. Some manufacturers bulk buy large quantities of components and measure them to find the components that are actually the value stated. You'll need to check the voltage too, you probably know that already. If tone (mojo & all that) is an issue, then the cap type can be a factor. PM for more on this if you want.

peter05

No tone. Just calibration of headroom. Cheers Troutmask.