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...
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...
Attachments
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'
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'
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.
"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 ✓
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.