Are you planning to put shields around the valves?
Valves have many great attributes for audio, with their attractive distortions we like to hear, but in many high end circuits that drive sensitivity they can be Uber sensitive to RFI.
I don’t use shields on my valve headphone amp, but when I setup for high impedance headphones I can sometimes notice RF break through from mobiles and DECT phones. Even if you can’t directly hear it, it can subtly cloud the audio.
As you have probably discovered, with valve circuitry, shielding really is crucial for performance.
Enjoying my Little Dot mkiii as I type on my Senn HD650 as I type this… and I love the sound
No, no shields on the tubes. The lad who designed this Cathode Follower amp has done an amazing job. In terms of noise floor it’s completely black, more like a very good SS amp. Volume on full and you can’t tell it’s turned on.
I can have my phone next to the amp using mobile network and no noise or interference is heard on the amp. It was so good, ZMF have got together with Cayin to manufacture a commercial version!
Curious how he has managed that without sacrificing performance and cross talk issues (that is different no noise floor) with no shielding for higher impedance headphones… unless using metalicized valves ie they have the shielding built into the case of the valve… its kind of how valves work, or perhaps using a transformer to transform impedance, ie a non OTL architecture.
Most commercial valve devices not destined to look pretty for niche consumer markets used RF shielding cans, or metallicized valves because of this… certainly where higher frequencies are involved… but even some consumer products that expose the valves there are some “pretty’ shields now.
Is the design an OTL or not? It looks perhaps there are three transformers? In which case it suggests perhaps non OTL and as a consequence more optimised for lower impedance headphones? What ‘phones are you planning to use with it?
You could ask him, he’s very happy to explain the design and how it works etc. He’s on Head Fi and goes by the name L0rdGwyn. If you google Aegis Amp DIY you’ll see links to Head Fi and Audio DIY.
The amp is designed as a TOTL amp and the DIY one uses Lundahl transformers and chokes throughout (6 in total) with Jupiter caps.
The amp is a transformer coupled design. Input stage is choke loaded. Output stage is unique amongst headphone amplifiers as it is a cathode follower power stage, he says that this is, as far as he’s aware, the only headphone amp doing this at the moment. This achieves lower distortion and a low output impedance, good for driving low impedance and planars. He explains that the topology also removes a bypass capacitor from the output stage, which is a large contributor to the clarity and soundstage the amp achieves.
There is a good DIY community on the Head Fi thread and he literally provides full instructions on how this is built. A manual basically with diagrams and photos etc. He also provides a BOM listing everything needed.
It’s in the DIY section… quite a busy one for that amp… about 160 pages or more now. The commercial version has a separate thread and a lot of talk there is about the performance of the amp and tube rolling etc.
Oh yes ‘tube’ rolling (valve rolling doesn’t work does it!) can be quite fun, but also frustrating. I did finally settle on my valves on my little dot iii that matched the headphones I preferred to use with it. The little dot iii is an OTL design so sensitive to tube rolling,
It can be… thankfully, there are a lot of people providing good combinations.
So far, for Meze Elites my preference for electronic is Elrog 274b, Mullard EL38 & RCA Red Base 5691s. For acoustic, international, jazz, and pop I swap the 5691s for Mullard ECC35. Classical, swap the EL38 for EL39’s
For my ZMF Atrium Closed, I prefer the 274b + GEC KT88 with the RCA Red Base 5691’s.
That said, this amp takes most pentode power tubes due to the way it biases them… there are a few exceptions but the list of what you can use as power tubes is a very long one. You have to use 6SL7 or direct variants for input and the for the DIY version, the rectifier has to be 5v & 2a. The commercial version allows both 2a & 3a 5v tubes.
Yes in OTL designs, it’s the driver rather than the power valves that seem to most affect the balance and feel when tube rolling.
My driver pentodes I seem to remember were ex Soviet ones designed primarily for missile guidance systems… for what ever reason they sound spot on for me give me the right balance of air, insight and timing.
Yes, there’s no right or wrong way with tubes. Just different ways to find the right synergy between source, amp and headphones.
The power tubes are the main sound changers with the Aegis, but I was quite surprised by how much the rectifier and input tubes make. The 5691s and Mullard ECC35 are very different.
Edit I select a power tube then use the input and the rectifier tubes to fine tune the tonal balance and staging.
I looked at his initial circuit diagram… the output stage didn’t look like a cathode follower to me, as the transformer primary was connected between the anode and HT Powerline… was there a later design or perhaps I am wrong,
Cathode followers have a fixed anode, and the cathode follows the load line… these are typically used as buffer stages as they typically fall just under unity gain but can drive challenging loads … but protection circuitry is sometimes required to mitigate arcing risk between heater and cathode when powering up, as far as I remember.
Very good… best I’ve heard to date. You should drop him a message I’m sure he’d be happy to explain why it’s a cathode follower design and what’s doing. I’m sure he has a few times on the forums.
