input transformer - mumetal casing etc.

[Alexontherocks]

Dear All, investingating the possibility of building a pmillet universal PP amplifier

Push-pull KT88 class A amp with "universal" driver PCB

I researched one of the most expensive components (on the PCB that is)...the input transformer.

either a sowter or lundhal will do according to pete millet. Here is the link to the sowter product

3575

To reduce cost and shipping problems I would have liked to have all the transformer parts made and shipped through edcor electronics. I have contacted them in relation to the sowter part (to have it "cloned") and the reply is the following:

1. They don't use mumetal cans or electrostatic shielding
2. "What we can do is the 1:1 ratio PC and PCW series transformers (on our website) and could put a external copper shield on the transformer for an additional cost"

I am no engineer and would like to ask some input to some of the guys on the forum. Can such a solution work? Waht would be a good choice among the EDCOR products to emulate the charaacteristics of the sowter part?

Thanks
Alex

 

[Alexontherocks]

one of the solutions might be this:

EDCOR Electronics Corporation. PCW10K/10K

or

EDCOR Electronics Corporation. WSM10K/10K

comments?

Thanks again
Alex

 

[male42]

If I understand you correctly, then the input transformer should have a screen. preferably of magnetic material. such as permalloy, nickel-containing alloys. sometimes requires a dual screen. copper in this case is not suitable.

 

[Alexontherocks]

The sowter seems to have one, I guess the edcor should have one as well...

what would be the effect from using copper instead of a megnetic material?

 

[pieter t]

No screening from low frequency stray fields.

 

[male42]

you need to protect the transformer coil from external parasitic interference with magnetic origin. such as power transformer. Copper does not have this property

 

[Alexontherocks]

Is there anyway to obtain sheets of magnetic material and have them formed to create a screen?

In conclusion you all foresee problems with using the EDCOR alternative, correct?

I have plenty of tools and machines (a friend has a CNC/metalwork shop) so forming, bending and such would not be a problem...

 

[SY]

Go Cinemag. Prices are very reasonable and the quality is as good/better than Sowter. The input transformer I've been using seems identical in performance to the Jensen JT11P1 at half the price.

 

[Alexontherocks]

cinemag's site seems to be down...

 

[SY]

Audio Transformers By Cinemag Inc.

Came right up for me.

 

[Alexontherocks]

now it works....must have been my wireless connection.

is there any model which could be considered a direct equivalent of the sowter part?

 

[male42]

I have made myself such transformers. and recommend a dual screen .. need to check in practice

 

[Alexontherocks]

Cinemag replide and stated that the CMLI-15/15B would be a good choice to replace the sowter part.

http://www.cinemag.biz/line_input/CMLI-15-15B.pdf

is the dual shielding necessary from a practical point of view?

 

[SY]

It can't hurt! I've gotten such excellent noise rejection performance from the Cinemags (and Jensens) that I just go with it and try not to question their wisdom.

 

[kevinkr]

Dual shielding is quite important if the transformer is a conventional EI core type which has poor rejection of external magnetic fields. Toroids and C-cores tend to need less shielding as they seem to contain their own magnetic fields more effectively and in so doing are also generally less susceptible to external fields.

I use transformer coupling extensively in my system, and use shielded transformers everywhere except in the driver stage and OPT. The IT is an unshielded Lundahl C-core which given the high signal levels is not a problem. I do go to some trouble to keep power transformers far away from these transformers if possible.

 

[trobbins]

This paper from 1938 includes a good level of practical measurements to guide those wanting to diy. I may be wrong, but I think the casing metal sheet used for some old floppy disk drives was made from a better magnetic shielding type of steel - it was certainly quite maleable for bending in to shapes.

http://dalmura.com.au/projects/Magnetic shielding of transformers at audio frequencies July 1938.pdf

 

[Wavebourn]

I once bought on eBay SMD - mount transformers made for DSL modems. They are tiny, but have 12 Henry windings, both input and output. Stacking such transformers I am getting different ratios. Then found good cases for them: bought a lot of motors made for cassette recorders.

 

[ruffrecords]

one of the solutions might be this:

EDCOR Electronics Corporation. PCW10K/10K

or

EDCOR Electronics Corporation. WSM10K/10K

comments?

Thanks again
Alex

The trouble with Edcor is they are relatively low inductance transformers - that's why they are cheap. The two mentioned have primary inductances of 8H and 11H respectively. At 20Hz these look like about 1K ohms rather than 10K so unless you can be sure your driving source impedance is a lot less than 1K at 20Hz you will certainly have poor low frequency amplitude and phase response as well as distortion.

Cheers

Ian

 

[DF96]

I have plenty of tools and machines (a friend has a CNC/metalwork shop) so forming, bending and such would not be a problem..

I understand that mu-metal loses much of its permeability when machined or bent, so it needs to be processed again after forming.

 

[Gluca]

I have been running 15/15B since ages ... I own 2 pairs. Good stuff.