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OTL headphone amp - lower impedance?

Some years ago, I built this headphone amp/line stage. I built it as drawn, plugged in a pair of 250ohm headphones and was happy. But it was designed with 600ohm headphones in mind, and over the years I've got my hands on headphones with lower (32ohm Grado/55ohm AKG) impedance. So I think some soldering is the way forward. I'm not a designer, only a builder, so I'd really appreciate a little help here :)

In the article it says to bypass R6 with a 1000uF/25V cap to lower output impedance, and I guess the output cap should be a fair bit bigger as well. But how big? Also, I'd like to be able to still use the 250ohm phones...
 

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U230421

Disabled Account
2017-02-07 1:54 pm
Output cap is depending on load (resistance).
With 600Ω load and 10µF (C2), the roll off frequency is (1 / (2 * π * 600 * 10E-5) ) 27Hz.
With 250Ω load fo = (600/250 * 27) 65Hz.
Ditto 55Ω -> 300Hz, 32Ω -> 500Hz.


If you want 20Hz with 32Ω, C2 must be (1 / ( 2 * π * 32 * 20) ) 250µF (220µF).
Check the DC voltage at the left side, some 150V to expect.
So C2 would be 220µF/250V at least. Considering the supply voltage, I'd recommend 220µF/350V.


edit: pi show up as 'π'
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
The only DC path through the tube-stack goes through a 10k resistor. Even if the tubes were low-low impedance you could only flow 30mA. The tubes will take-up a lot of volts, and 6SN7 is more a 10mA tube. I doubt there is 20mA peak current possible.

"BIG" sound at 32 ohms can mean 150mA peak. Most users will be well served by 40mA peak. <20mA peak is not cheesy but not ample. Medium-loud in 32r may be fine, Loud may clip.

Bypassing R6 may give a hair more gain but will not increase Power. You probably don't want gain on low-Z phones. However it is a trivial experiment to try.

The cap computation should include the non-zero output impedance of the amplifier. However 220uFd is ample and not a big cost.
 
At the risk of excommunication (cough, ahem…), why not deploy a pair of small toroidal power-transformers-as-impedance-matchers? Cheap, requires no circuit changes, accomplishes the OP's end run.

Triad VPT48–520 comes to mind. 50 VA, so can handle a few watts all the way down to 20 Hz. 120 V primary, 24 volt secondary (in one configuration).

Using the usual “input impedance multiplier = VPRIM² / VSEC²” relationship, then (120 ÷ 24)² = 5² = 25.

If the design point was 600 Ω say … 600 ÷ 25 = 24 Ω. Well, that's within shooting distance of 32 Ω for the Grados!

Moreover, the DC isolation capacitor C₆ remains its as-shown value, because the reflected load is still 32 × 5² = 800 Ω.

At $17.50/ea, qty 1 to 4, … it seems like such an obvious idea! At least worth the very modest investment, and simplicity-of-trying. Heck… could just put the pair of them in a separate “impedance matching” box. Wouldn't even have to fiddle with the existing OTL.

OK, I've got my shields up. Expecting incoming tomatoes any time now.
(Goat ducks!)

Just Saying,
GoatGuy ✓
 
At the risk of excommunication (cough, ahem…), why not deploy a pair of small toroidal power-transformers-as-impedance-matchers? Cheap, requires no circuit changes, accomplishes the OP's end run.

Triad VPT48–520 comes to mind. 50 VA, so can handle a few watts all the way down to 20 Hz. 120 V primary, 24 volt secondary (in one configuration).

Using the usual “input impedance multiplier = VPRIM² / VSEC²” relationship, then (120 ÷ 24)² = 5² = 25.

If the design point was 600 Ω say … 600 ÷ 25 = 24 Ω. Well, that's within shooting distance of 32 Ω for the Grados!

Moreover, the DC isolation capacitor C₆ remains its as-shown value, because the reflected load is still 32 × 5² = 800 Ω.

At $17.50/ea, qty 1 to 4, … it seems like such an obvious idea! At least worth the very modest investment, and simplicity-of-trying. Heck… could just put the pair of them in a separate “impedance matching” box. Wouldn't even have to fiddle with the existing OTL.

OK, I've got my shields up. Expecting incoming tomatoes any time now.
(Goat ducks!)

Just Saying,
GoatGuy ✓

I've thought about transformers too, so no tomatoes from me :) I made an Excel sheet a long time ago to calculate primary/secondary impedances from toriodal power transformers, I have to see if I can find it.

I've also found some 2W line transformers that look interesting: CUI30A02E | Line Input Transformer 1 + 1:2.7 + 2.7 for 70 Ω/600 Ω | RS Components

It can be connected as 70Ω:600Ω, so it should do the trick? Not a perfect 32Ω match, but a heck of a lot closer. The frequency response looks very good too, flat from 20Hz-20kHz when connected like this.