Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Tube amp output transformer impedance matching
New Posts  All Forums:Forum Nav:

Tube amp output transformer impedance matching

post #1 of 2
Thread Starter 

I'm trying to make some sense out of OPT impedance matching to headphones and the more I read up on it, the more befuddled I get.


About the project:  I'm building my first tube amp for headphones, to make things easy I purchased PCB's for a well documented first build, the tube lab SSE speaker amplifier, info about it here:




It's geared towards speaker amplifiers and starts out as a basic module ( SE output transformer based) design to which optional 'modules' such ar a choke, rectifier (ss or tube), volume control...can be added.  Construction is very well documented on the site.


Bear in mind, I know nothing of this stuff...but I'm learning.  Design options that I've taken so far:


- Power supply is going to be the Hammond 374BX transformer (I'm in a 230 V country), 200 VA, 375-0-375 V @ 175 mA, 6.3V/6A, 5V/3A.

- The project uses the 12AT7 driver tube, I'm going to stick with that for the sake of simplicity.

- As power tubes I've -provisionally, see below- settled on the KT-88,  if I run these at a B+ of 450V with a 560R cathode resistor the power output should be 5.24W if Z(load) is 5000R


Where things start getting hard is in the selection of the output transformers, the options on the site basically settle on a couple of transformers that present a 5000 Ohm primary load, assuming an 8 Ohm secondary speaker load.


The thing is, the secondary load is not going to be an 8 Ohm speaker but my 50 Ohm HE-6 planar.  The good thing is that I only want to use it with the HE-6 and as it's a planar, the load is purely resistive and there are no changes over the frequency.


The way I understand things is that the transformer doesn't have an own impedance, rather it reflects the load impedance based on the square of its turns ratio and 'presents' that to the tube. This means that, with the transformer in question having a 25 turns ratio, the load presented to the tube would be (25^2 x 50)= 31.25 kOhm.  The anode resistance of the KT-88 when wired as a pentode, is about 12 kOhm if I read the datasheet correctly (obviously this will depend on the operating point to some extent).


Is this going to be a problem or will it just have an influence on the power available to the phones (which should be ample I imagine).  If it's a problem, should I go for a custom transformer with a lower winding ratio to get a better match?


I've read about a million pages with all kinds of theories and opinions ranging from 'just hook it up, you'll have less power available but it are headphones so it should be fine' over 'for best power, primary impedance should be 2xRa, for lowest distortion 4xRa' to 'you'll lose bass'. As for the last argument, I suppose this can be covered to some extent by picking sufficiently powerful OPT's (heavy iron, in other words).


Any tips/insights?  I don't intend to build 'the ultimate' headphone amp, this is just a first -learning- project but obviously I'd like to be able to drive my HE-6 well.  Money's not really a constraint here but as it's a learning project and -unless thing go disastrously wrong- will be followed by others I'd rather not go to custom would silver wire Lundahl transfos.

post #2 of 2
Yes, the 50 Ohm load will be reflected as 31K (assuming you did the math right.) However, OPTs have an inductance which limits the load that can be reflected. If an OPT is rated at 5K, that would generally mean that it has an inductance such that it appears as 5K at some frequency. The equation for this is Z = 2piFL, so if the OPTs are 5K at 20Hz, then they have an inductance of ~40H. If trying to reflect 50R to appear as 31K, that means your bass will start rolling off around 120Hz with this load. If you want to use this design with OPTs made for speakers, your best bet is probably to put a large wattage resistor in parallel with the phones to make the load as expected.

As for the proper load, the 2x or 4x rp stuff is for triodes. In triode mode, 5K is plenty for these tubes. Required load vis-à-vis rp is different when pentodes are used (and 5K is likely OK there, too). This amp can switch between modes, so you should try both ways.
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Tube amp output transformer impedance matching