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Old 31st March 2012, 12:53 PM   #1821
Magz is online now Magz  United States
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Quote:

I have a question, still at a deadlock on a design decision. I will be using a stepdown permalloy output transformer (parafeed) its a custom design from ElectraPrint but very sensitive picking up hum from the mains transformers. Really all opt at preamp levels pick up EMI (60 hz hum) from the B+ transformers, so I always use a separate chassis to separate the OPT's from any main transformers (need to especially with sensitive Grado's.)

But I haven't done a DHT build like this and adding the coleman regulators really adds complexity to a 2-case build.

It looks like with the SSHV2, we can have a larger distance between the regulator board and the amp section which helps. But the coleman regs still need to be in the amp chassis.

Is it OK to have the prefiltered supply to the coleman regs in the amp chassis share the same umbilical cord as the prefiltered B+ ?

My goal is one umilical cord. Has anyone found a means to do this with sub -90dB hum ?
Can Electraprint put your custom transformers in a mu-metal can? That ought to help with noise pickup. The Lundahl LL1676s I'm using for parafeed output are in such cans being that they were originally designed as low level microphone step-up transformers, and they are dead quiet, even though they are next to a strong AC source in my preamp-on-a-board.
Just a thought...

Last edited by Magz; 31st March 2012 at 12:56 PM.
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Old 1st April 2012, 01:52 AM   #1822
regal is offline regal  United States
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Quote:
Originally Posted by Magz View Post
Can Electraprint put your custom transformers in a mu-metal can? That ought to help with noise pickup. The Lundahl LL1676s I'm using for parafeed output are in such cans being that they were originally designed as low level microphone step-up transformers, and they are dead quiet, even though they are next to a strong AC source in my preamp-on-a-board.
Just a thought...
Its a good idea but we have no luck finding a can to fit these, do you know of a vendor who sells mu-metal OPT cans ?
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Old 1st April 2012, 02:28 AM   #1823
Magz is online now Magz  United States
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Originally Posted by regal View Post
Its a good idea but we have no luck finding a can to fit these, do you know of a vendor who sells mu-metal OPT cans ?
No, but you could make your own cans or shields using TI-Shield, which doesn't lose its shielding properties when it's bent or soldered. It's permalloy clad with copper on both sides. You can get a sheet of it from Michael Percy Audio, which will be more than enough. I have used TI-Shield in various audio components and it works well on both magnetic and electrostatic fields. Be sure to ground the can/shield.

http://www.emsclad.com/examples/emi-rfi-shielding.html


.

Last edited by Magz; 1st April 2012 at 02:35 AM.
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Old 1st April 2012, 03:32 AM   #1824
regal is offline regal  United States
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Thankyou
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Old 1st April 2012, 05:48 PM   #1825
Magz is online now Magz  United States
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So back to my 26 preamp.

My plan is to use the Bent Audio Slagleformers AFTER the output transformers. These are specified to handle up to 10V signal levels. I figure this will give several advantages:

1) increased signal to noise ratio as the preamp noise will be attenuated by the same amount as the signal, which does not happen with the VC at the input

2) increase in the effective input impedance of the following component by the impedance ratio of the autoformer, especially at low volume settings

3) decrease in the output impedance of the preamp by the impedance ratio of the autoformer, especially at low volume settings

My only concern is whether the LL1676 output transformer can handle the inductive load of the autoformer - has anyone here used this sort of arrangement? Any other watchouts from experience?
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Old 1st April 2012, 07:26 PM   #1826
vgeorge is offline vgeorge  Greece
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I use the autoformers at the output.
LL1660pp to Intact audio AVCs.
Very happy with the combination, and better than a pot at the input in my opinion.
My AVCs are around 20H, if I remember correctly, but can be stacked to higher or lower inductance.
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Old 1st April 2012, 08:50 PM   #1827
Magz is online now Magz  United States
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Cool, I think the Intact are the same autoformers as the Bent.
Thanks for the feedback.
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Old 2nd April 2012, 07:29 AM   #1828
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Originally Posted by dhtrob View Post
Hi Bas,
Here: Verpas - Groothandel in doppen en kunststofproducten
Lots of different sizes etc etc..
Rob
Rob
did you buy 1000 of these for 360 - that would be a good chance to pinch some from you.
Or did you find a retailer? I found Technigro who charges 2,60 per piece!!! Absurd pric imo.

But interesting. I discarded a microphonic pre after a lot of annoyance
Albert
nl
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Old 3rd April 2012, 04:41 PM   #1829
jamesdb is offline jamesdb  United States
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Default Preferences for Suitable 26 HV Power Transformers

While looking for a suitable HV power transformer for the 26, would the Antek AS-1T175 toroid be a good choice? Antek - AS-1T175 It is 100VA with an electrostatic screen between the primary and secondary, and a magnetic shield around the outside, and has two 175V-0V windings at .28A so separate B+ supplies can be had for each channel. I have some EZ80and EZ81 rectifier tubes and was considering a hybrid rectified supply since it also has two 6.3V 3A windings. Or would a simple single LCLC tube rectified supply using the concentric wound Hammond 369EX (190VCT) Hammond Mfg. - Universal Primary - "Classic" Power Transformers be preferred over a toroid & dual supplies?
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Old 3rd April 2012, 05:20 PM   #1830
vgeorge is offline vgeorge  Greece
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I would go for EI transformer.
Depending on your target B+ and rectifier tube used, 190V maybe on the low side if you go LCLC.
Better to simulate with PSUD II.
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