Introduction

Did you ever dream about having a nice valve headphone amp? I did. Several times in fact. I’m not keen on the OTL designs and yet, owning a pair of Grado’s SR80i 32Ω headphones, I’ve been looking at different topologies and designs for headphone stages. 

Building a good headphone amp is a real challenge. The low impedance of most of the headphones out there (with exception of 600Ω ones), makes the task a very hard one for any valve. A simple stage with a transformer is very attractive. I found this transformer from Sowter specially designed for headphones which attracted me at fist sight. Having a mu-metal core provides a great frequency response. It can easily achieve 30kHz bandwidth. Having a mu-metal core provides a low distortion which is very attractive. The transformer isn’t cheap but you can’t cut costs on this if you’re looking to implement a high-quality amp for your headphones. Even 600Ω are not easy to drive by valve circuits such as White Cathode Followers and SRPPs. There are plenty of circuits out there (some not good at all) but I decided this time to explore and experiment a bit (as usual).

Pushing the gyrator

I wanted to keep the design very simple. I could have gone down the route of a simple transformer loaded, but transformers which can have similar wide bandwidth, gapped for DC and have amorphous core, are more expensive. A classical approach is a choke loaded parafeed design. Again, high quality chokes are expensive as well. You could go with an CCS load, but you want to take advantage of a mu-follower setup so you can provide low output impedance and minimise impact of a lowish (e.g. 4KΩ) load to the driving valve. 

A gyrator load has proven to be outstanding. Both in terms of performance as well as sound. Rather than a CCS, I opted for my beloved gyrator.

So, how can we build a single stage (a.k.a “spud”) which can drive these headphones in parafeed mode?. there are several high-mu triodes (or triode-strapped pentodes) which can do the job here. 

Parafeed cap

The resistance of the windings reflected to the primary is about 576Ω. If the load is 32Ω, the transformer is configured in 12:1, so the total load presented to the stage is about 5,184Ω. The output impedance of the gyrator is 1/gm. Despite expected gm is about 40mS for VDS=8V as per the BF862 data sheet, I’d expect it to be not even half of that at low VDS. So If we consider a conservative 20mS, the output impedance of the gyrator stage is around 50Ω. So for a pole at about 10Hz, we need the capacitor to be about 3uF. A 2uF should work well and provide the -3dB point about 12Hz. On my tests I used a 1uF Oil cap I had at hand. I would invest in a good quality film or oil cap. A +300Vdc part is needed and the higher the better. 

If you are using high-efficiency headphones (e.g. SPL>90dB) you don’t need much gain on this stage so the spud design is easily achievable with a wider range of triodes (or triode-strapped pentodes). I found that my headphones are really loud with 70-80 mV. If you consider the peaks and headroom needed, you need to design for 150 to 200mV output at low distortion. A gain of about x30 will give full tilt with an input of 500mV. Plenty enough. You can easily go with a low-mu DHT (e.g. 4P1L) and have enough to drive this with a 2V source. 

I tried several valves here, and here is my top 3:

1. 6Z49P-DR: amongst my favourite Russian pentode. Triode strapped can do x45-x50 gain. it’s extremely linear and sounds amazing in my experience. 
2. 12HL7: It has a reputation and recently highlighted in the DIY Audio community. With lower gain, it is very linear. Needs some more current than the 6Z49P though
3. 6e5P/6e6P: my favourite ones. Both are great valves and have mid-mu (30-ish) so will work fine here.
4. 6J52P: I love it but the parameters are all over the place. You won’t need this much gain.

After several tests on the bench I settled for the 6Z49P-DR and the 12HL7. I prefer the 6Z49-DR though. The cathode bias is with SiC diodes. You can do what you like here. I even try a degen resistor of about 82Ω. Didn’t see much improvement in distortion and you trade off some gain. 

On the workbench I use the fix bias supply of the curve tracer to find the sweet spot and then I change it to the cathode bias arrangement I want. That’s the way generally I try different things. 

How does it measure?

Here is a sample of the great performance. The 12HL7 is a tad more linear, but not much. See how low the distortion is at ridiculous output levels. 2V represents 125mW which will leave you deaf:

At peak (e.g. input 1V and output of 4V (or 500mW) which is the maximum the transformer can handle, the distortion is just 0.32%

How does it sound?

I like the sound of my Grado’s. I played some of Trane’s “Blue Train”, and the powerful bass notes of the intro are very detailed. I generally test with horn playing as take me back to my musician days when I played the sax. I can recognise subtle differences in the reed’s timbre and I have to say the sound of this headphone amp is delightful. Of course, it will rely heavily on the headphone’s quality.

Next steps

Build it on a nice case, but I haven’t got the time. Hopefully soon…