input transformer for tube compressor with split secondary and magentic shield?

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MeToo2

Well-known member
Joined
Dec 30, 2009
Messages
339
Location
The Netherlands
As the title says. I'm looking for a recommendation for a (line) input transformer with split center tapped secondary for use on the input of a variable mu tube compressor (approx -60V dc on the centre tap, approx +4dBm input level)

Impedance and other specs would be something like the Edcor XSM600/10K, but with a magnetic shield.

I'm currently using this Edcor transformer and the frequency response is great down to low f and high input levels, but I have traced back some undesirable low level hum and coupling to the input trannie. Currently measuring about -60dBm at the output but I'd like -70dBm or better ;)

Yes I'll try to reduce the hum at source (suspect very strongly it's due to DC heater current rectification of a 15V AC supply for 12.6V DC heaters) so relocating mains transformer and redressing leads might be an option too that I'll look at in the meantime. Also adding some resistance to the bridge rectifier might also reduce the source.

Another alternative is to shield the existing transformer, although that could work out expensive too, without any guarantee of success.

I tried an OEP A262A3e with mu-metal shield series connected (600:25K) as a temporary test to prove where the noise coupling was (after searching all over in the HT grounding etc.). Worked absolutely fine and removed the hum entirely, but suspect it will saturate too easily at higher input levels? [spec is 1mW at 30 Hz for 1% THD]

The Jensen JT-115K-E (60) looked like the sort of thing I'd like, but it has no centre tap on the secondary.

Any recommendations of an off-the-shelf reasonably priced (±70USD) transformer for input on a vari mu?

Sowter 8540? 9062? Don't see a whole lot of love for Sowter on the DIY sites .
Lundhal LL1922 perhaps? although that's a bit more expensive.
 
hobiesound said:
no love for sowter?  :eek:  i can only say good things about sowter transformers..........

greetings,

Thomas
OK. Thanks. Just I haven't seen a whole lot of DIY designs based on them, so wondered why this was. Maybe just the geographical bias of the users on the boards I read. I dunno.
 
Don't see a whole lot of love for Sowter on the DIY sites .


???  . . . They make no less than 5 project specific "vari mu" input transformers, all with mu metal can.  I've never heard anyone post any sort of relevant reservations about their quality or sonics . . .

And there is whole page of classic vintage DIY projects using Sowter iron.

http://www.sowter.co.uk/vintage-audio-transformers.php

I believe Cinemag may have at least one that fits the bill as well.




 
lassoharp said:
http://www.sowter.co.uk/vintage-audio-transformers.php
Yep saw that page many times already, and have scanned the products on more than one occasion. I just haven't come across many posts raving about them and recommending them. Unlike UTC, Jensen, Cinemag, Lundahl, Edcor ..... Ah well. Maybe it's just me being word blind. OK OK I'll take it all back about Sowter not being loved. I promise.

Anyone got an improvement on the 8540 (600:10K CT +17dBu input max) for around the same price or slightly cheaper (100USD each inc tax & shipping)?

 
I just haven't come across many posts raving about them and recommending them. Unlike UTC, Jensen, Cinemag, Lundahl, Edcor

Ok, I'll officially rave that my Sowter Pultec OT and inductor are super fine!!  ;D

I did a 2nd Pultec with the cinemag inductor and I can't hear a difference.  I didn't seem to need the mu can so any future decisions on that product would be purely cost based.

I'm sure you know how it can be with the "raving" though.  I usually take those cartwheels with a grain of salt.  I've gotten the impression that Lundahl aims or seems to occupy an 'elitist' perception.  But that's purely a psychological reaction by me of marketing and commentary.  Could be exchange rate and shipping for US customers for Sowters.  I looked at buying their BA6A and Collins 26 interstage but at $200.00 USD I could buy an original.




 
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