How to: transformer-based impedance-matching box

[Les_Garten]

Any you guys know what's in this box?

[url=http://www.head-fi.org/forums/6245035-post760.html]Head-Fi: Covering Headphones, Earphones and Portable Audio - View Single Post - REVIEW: Audiotailor

[nikongod]

Its a pair of transformers. Maybe autoformers. Maybe with loading resistors/networks maybe not.

I'l post pictures of my own attempts when I get home.

You can get some very solid ideas from the mogran jones clone DIY article/schematics at headwize.

Edited December 18, 2009 by nikongod

[n_maher]

I'm not technically proficient enough to be able to say for sure one way or the other but I can't see how parking a pair of transformers on the output of an otherwise non-transformer coupled amp can be a good thing.

[Dusty Chalk]

It brings the impedance up, I believe is the intention. (I.E. OTL amps typically need higher impedance loads, this gives it to them, but then you can plug a lower impedance headphone like a Grado or an AT into it.)

[dsavitsk]

I'm not technically proficient enough to be able to say for sure one way or the other but I can't see how parking a pair of transformers on the output of an otherwise non-transformer coupled amp can be a good thing.

Why? That's all a parafeed amp is. Or the Maple Tree stuff. Or the L'espressivo. It's just a matter of using the right transformer.

[Les_Garten]

I thought some of you guys might have done this before. I haven't been able to find any pix of the inside yet.

[dsavitsk]

The problem I'm trying to solve is the hum from my DV 337SE that occurs with Low Impedance cans.

It looks like it's already transformer coupled. The proper solution is to just fix the hum. It could be power supply noise, heater noise, grounding, or magnetic coupling.

[Les_Garten]

It looks like it's already transformer coupled. The proper solution is to just fix the hum. It could be power supply noise, heater noise, grounding, or magnetic coupling.

It seems that it is a Family characteristic of this amp in that it hums a "little" with Low impedance cans.

Here's the inside of the amp for anyone interested. The JPG is big, beware.

http://www.turbonet.biz/misc/337seReview/AAA_1430.JPG

[dsavitsk]

It looks like it's already transformer coupled.

I take that back. It looks like power transformers on both sides, and tubes in the middle? Does it really take 2 power cords? Odd.

Anyhow, it looks like a solid state bridge rectifier sunk on the side and a CRCRC filter for B+. It also looks like AC heaters. Either of those could cause hum. To lower B+ ripple, you could add a choke -- it will not only reduce ripple, but it will also take some edginess off the sound. Heaters are a little harder as the 6080's draw a lot of current which means big diodes and a lot capacitance. It is also tricky to get 6.3VDC out of a 6.3VAC winding.

Honestly, I'm not sure I'd bother.

[Les_Garten]

I take that back. It looks like power transformers on both sides, and tubes in the middle? Does it really take 2 power cords? Odd.

Anyhow, it looks like a solid state bridge rectifier sunk on the side and a CRCRC filter for B+. It also looks like AC heaters. Either of those could cause hum. To lower B+ ripple, you could add a choke -- it will not only reduce ripple, but it will also take some edginess off the sound. Heaters are a little harder as the 6080's draw a lot of current which means big diodes and a lot capacitance. It is also tricky to get 6.3VDC out of a 6.3VAC winding.

Honestly, I'm not sure I'd bother.

Hey, yeah it has 2 Power cords, each feeding a Transformer. I was looking at those heater runs as well, but they would be kinda a pain to change.

[spritzer]

Replacing those copper bus bars with twisted wire should help with the hum.

[n_maher]

Why? That's all a parafeed amp is. Or the Maple Tree stuff. Or the L'espressivo. It's just a matter of using the right transformer.

It's the one size fits all approach that has me doubting. I'm sure that it could be designed to work well, but with any amp?

[Les_Garten]

Replacing those copper bus bars with twisted wire should help with the hum.

Hey, you're referring to the heater fead? It would probably be a lot quieter with a whole rewire job I guess. But it wouldn't look as kewl!

[spritzer]

Yup the heater feed. It makes sense to twist all AC wires to minimize hum and that setup is just asking for trouble.

[Les_Garten]

Yup the heater feed. It makes sense to twist all AC wires to minimize hum and that setup is just asking for trouble.

It does look like a Freakin' Antenna Array in there doesn't it!

[dsavitsk]

It's the one size fits all approach that has me doubting. I'm sure that it could be designed to work well, but with any amp?

Yeah, well, I doubt there is anything that will work with every amp. But, you can probably come close. Pretty much any off the shelf amp can drive a 300 ohm headphone. So, with a 300:32 step down and you can probably get decent results.

[Les_Garten]

Yeah, well, I doubt there is anything that will work with every amp. But, you can probably come close. Pretty much any off the shelf amp can drive a 300 ohm headphone. So, with a 300:32 step down and you can probably get decent results.

How inexpensive could that be done?

[nikongod]

The really cheap speco T-7010 configured as autoformers (10K tap to output of amp, each other tap except 4ohm selectable by switch). One could make up their mind about what they like and do away with the switch, but I cant.

[Les_Garten]

NG,

Autoformers is a newb concept to me. I read up a little on a number of sites and graps the concept as espoused here:

Zero Autoformers, Impedance Multiplying Transformers [English]

The commercial implementations for speakers is mega expensive!

However, not sure how you have those Transfos, switch jacks etc in you box wired.

Do you have a drawing, like for Autoformer Dummies?