Tuesday, November 17, 2020

John Linsley-Hood Class A Headphone Amp

Having been a happy user of a John Linsley-Hood (JLH) 1969 Class A amp cloned by Nobsound, I was intrigued by the JLH Class A headphone amp boards featuring discrete circuitry (no ICs or op-amps) advertised on eBay. In the midst of the 2020 lockdown, I ordered this $25 pre-assembled board, which arrived in a padded pouch within 10 days. I also got a surplus 24 VCT @ 2A power transformer for another $15.

!!!WARNING!!! 
The voltages in this project are potentially lethal! Proceed at your own risk!


Typical of Chinese kits, this board didn't come with instructions or any type of documentation. But the board is marked for an experienced DIYer to decipher and set up in no time. Unlike the original JLH 1969 design, which needed DC blocking capacitors at the output due to the single rail supply, this headphone amp module was updated to work with a dual rail (+/-) supply so the output capacitors can be omitted.
 

Unfortunately, I couldn't get stable zero DC offset at the output terminals. Trimming for zero volts after 30-45 minutes of warm up was not a guarantee that in another 30 minutes it'll still be zero - I've measured as much as 0.3V.😞


Since I didn't want to risk frying my headphones and classic speakers, I installed 2500uf @ 16V electrolytic blocking capacitors at each channel’s output. Then, I bypassed the electrolytic caps with hermetically sealed 4uf paper caps just like I did to the Nobsound NS02g.  

Top trace = audio generator
Bottom trace = amp output

In spite of the DC blocking caps at the output, the square waves still show excellent bandwidth, transient response with no trace of ringing. 

Before clipping, I measured 250mW/channel @ 4 ohms, 500mW/channel @ 8 ohms and 950mW/channel @ 16 ohms, 1.2W/channel @ 32 ohms,  395mW/channel @ 150 ohms, 190mW/channel @ 330 ohms and 115mW/channel @ 560ohms. I pretty much exhausted the resistor values from my parts bin to emulate the nominal impedance of typical headphones. 

The 115mW output into a 560 ohm load shouldn't worry headphone enthusiasts since this amp drove my Austrian AKG K240/600 ohm Monitor cleanly beyond SPLs my ears could tolerate. It also had no issues driving less efficient ortho-dynamic headphones like the Fostex T50RP MKII below, courtesy of my friend Nate. Thanks, buddy!


This amp definitely outclassed the Rat Shack mini amps I'm fond of. Its class A design's purity and transparency puts it on another league. Unfortunately, Class A amp designs require a more substantial power supply, which is incompatible with portability. 

The character of this amp is exactly like that of its bigger brother, a fatigue-free solid state amp that almost (but not quite) succeeds in sounding like a good tube amp!


BTW, did I mention that it can also drive my Altec 755As in Silbatone cabinets?


10 comments:

  1. Hi Joseph,
    Looks interesting and fun - thanks for being an enabler! Mine will arrive sometime by January. ;-) I love the Altecs being driven by this big power in the video! What is on the board under the turntable, next to the RS phono pre?
    Be well, and keep enabling!
    Pete

    ReplyDelete
    Replies
    1. Hopefully your JLH pouch will arrive sooner than later. Next to the RS phono pre is the holiday special. ;)

      Stay safe, Pete!

      JE

      Delete
  2. It arrived yesterday - not as long a wait as I expected! So don't be discouraged by the long expected wait time. I looked around my stuffand didn't find the 12-0-12 transformer I thought I had, so I'll have to get one. What amperage do you think is appropriate? Thanks, and be well!

    ReplyDelete
    Replies
    1. The pouches are surprisingly quick. I used a 24VCT @ 2A, which I think is overkill. 1A would've been sufficient.

      Delete
  3. with a 24V 1A should produce how much potency per channel?

    ReplyDelete
    Replies
    1. Power output into different loads are listed on the second paragraph below the square wave pictures. I just replied to your other query in you tube.

      Delete
  4. I found this blog when I was searching for info about building this amp, and it's very helpful, thank you. I have my amp at first test now, and it sounds surprisingly good for such a cheapie.

    Interestingly, I don't get the DC offset problem. It starts at around 200mV at power on, but it drops steadily and settles at around 2-3mV after about 15 minutes. I will still add blocking caps, though, as a precaution against anything going wrong.

    Thanks again for sharing.

    ReplyDelete
    Replies
    1. The DC offset variation is perhaps due to the variable quality of parts used in these cheap kits. I'm glad you found this useful.

      Thanks for stopping by and happy listening!

      JE

      Delete
  5. Hi, I builded same amp years ago and I used UPC1237 Headphone Protector to protect my headphones. Reading this blog I'm thinking about adding second output thru blocking caps. Could you explain 2500uf/16V electrolytic blocking capacitors bypassed with 4uf paper cap?
    Keep up with good work! Thx 

    ReplyDelete
    Replies
    1. To my ears the 4uf PIO bypass caps remove some of the harshness inherent in electrolytic caps. But just like anything in the audio hobby, YMMV!

      Delete