OTC tube amp bass distortion with low impedance planars

I’ve had this Cayin HA-1A MK2 for some time now and I’ve noticed that the low bass distorts on certain songs with both my Hifiman HE6se V2 and Audeze LCD-5. It sounds flatulent or overly wet. The volume level doesn’t seem to affect the distortion and I don’t ever need to go past 10 o’clock anyway. The bass frequency doesn’t matter so much as whether the song has bass slam. Also I upgraded the two 12AU7 input tubes because multiple reviews indicated that they are the weakest link.

Does anyone know if further tube rolling would resolve this issue or if it is just a limitation of the HA-1A MK2?

It’s not an uncommon problem with transformer coupled tube amps, it’s usually a function of the transformer, cheaper transformers (and I use the term cheap loosely) often don’t have great extension, or significant distortion at either end of the frequency range. It’s often worse with larger voltage swings (read inefficient headphones).
There are other parts of the circuit that can also effect it.

The other issue which shouldn’t be at play here is headphones with significant changes impedance at low or high frequencies. It can work out for you favor with headphones like the HD600/800.
But that shouldn’t matter for planars which are largely resistive loads.

Short version it’s probably a function of the amps design and component selection, not so much something you can fix with a tube roll.


In my experience, bass bloat, rumble, and thickness is common with lower to middle priced tube amps. And with some retro-style high end amps too. This is indeed an old school tube amp stereotype. If you don’t like it, maybe try an LTA amp.


I’m with @Polygonhell and @generic – the culprit is likely the output transformers rather than the tubes. :frowning:

A couple quotes from PTA’s review of the Cayin:

“Bottom line, this thing can drive most any headphone (short of the stubborn HiFiMAN HE-6 or classic AKG K1000) …” [emphasis added]

“Cayin’s compromise is to source its own EI core output transformers…” EI-core transformers manage about 60% to 75% of the magnetic efficiency of C-core transformers, but they’re cheaper. Also, the Cayin’s minimum output impedance tap/setting is 32 ohms, which suggests they haven’t gone to extremes with the winding ratios, even though they employ a “proprietary layered winding configuration.”

With the exception of the Berning/LTA approach, tube amp circuits have been around awhile and are generally simple. Quality comes down to parts selection and execution. So, the DNA Stellaris uses output transformers from AudioNote. Decware are ultra-picky about the steel and lamination process for their UFO transformers. All ampsandsound transformers are in-house designs and over-spec’d by a factor of 2. VAC, Cary, BAT, VTL use amorphous-core transformers from Lundahl. All of these approaches are expensive, of course.

The Cayin probably drives 99% of the headphones out there very well, but you’ve got a couple of the 1-percenters in the LCD-5 and HE6se.


And I can easily hear it. Deeper, tighter, stronger bass, and smooth purity from high to low.


Thanks to all of you, especially @Lou_Ford for going the extra mile. I’d actually read that PTA review, but somehow forgot about the HE6 caveat. Oops!

This all makes perfect sense. And now I know a lot more about OTC tube amps. Including the biggest reason why the Ampsandsound Mogwai I demoed at CanJam with my LCD-5 was so great, so smooth, and so expensive.

It is also informing my next purchase. If the amp design isn’t as important, maybe I’ll get a DIY Aegis kit. Those LTA amps are certainly nicer looking though. I’m going to try as many as I can find at the next CanJam SoCal.


It’s important, the designs aren’t often complex, but a tube Amp Designer has a lot of levers to pull that affect the final sound.

The designer picks a load line, and that determines the linearity of the amps response.

The pre section, can be resistor/CCS/Gyrator or choke anode loaded, the cathode can be offset with additional power supply taps, resistors, diodes, leds, if it’s a resistor it can be completely or partially bypassed, which removes a source of hidden feedback.

Stages can be transformer, capacitively, or directly coupled.

For larger voltage output tubes you might have 3 or 4 stages.

The output tubes can be fixed voltage or resistor cathode biased.

And that’s before you get to the power supply, and in tube amps because the common designs have no way to reject power supply noise, you move all the complexity out of the audio circuit into the power supply.

The way tube heaters are powered also matters, more so for DHT tubes, and it’s more important for headphone amps because of the inherent sensitivity.

Having said that a lot of basic designs are just straight off the tube datasheet.

And component selection impacts it just as much.

But you really need to listen, because for example 300B amps can be anything from warm and ooey gooey to near SS tonality based on the decisions the designer makes.
Luckily most designers tend to have a house sound.


Unfortunately this is still true, and still my plan. The ZMF/Cayin version of the Aegis is going to be at CanJam SoCal, so that should give me an idea of what the DIY version sounds like. I’m sure Ampsandsound will be there again. I know there will be some Woo amps there. Beyond that I’ll have to see. But demo all that I can is definitely the plan. :+1:

I’ve also got a local meet coming up. The only issue there is that all of the tube amps I’ve seen there before are of the more “some dude I’ve never heard of working out his garage” type and I’m looking for something more mainstream (or make it myself).


Sure, it’s a proven, low-risk approach. Then sometimes a designer goes the extra mile to really try to optimize for a particular tube and mode of operation. So, for instance, Matt at Cascade Tubes really wanted to optimize the 6V6 in ultralinear mode for a single-ended application.

I tend to think of it as component selection and execution determine how much of the design potential can be realized. But, yeah, good parts can’t rescue a poor design. Throwing money at a problem rarely solves it.

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It’s not rocket science, you have to be aware that you’re working with high voltages, and understand the risks there.
If you’re copying a design, just be aware you need more than just a circuit diagram, grounding design is a pretty critical part of not ending up with something that hums.

My first tube amp build (outside a BHC) was a 2A3 design based on the Loftin White design (I was enamored with the direct coupling). It was ultimately successful, but I spent a lot more money than just buying something, and spent many weeks tracking down hums, working and reworking PSU a designs for heaters. It was a lot of fun, I learned a great deal about what works and what doesn’t. But I wouldn’t/didn’t do it to save money.

I assume the Aegis design includes physical layout as well as just a circuit diagram, in which case just go for it if your comfortable working with the voltages.


Probably, but I would still try to ask somebody, LordGwyn (Keenan McKnight) himself is possible, about significant differences between the DIY and commercial versions. Or maybe that’s covered in the DIYAudio thread??

Hope you find something great for those 1% cans! :grin:

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Yes absolutely, though for tube amps driving easier to drive headphones, the voltage swings are often so moderate that you can pick almost any load line you want, and it will sound OK.

I’m currently working on a hybrid amp, C3M input tubes driving a Mosfet output stage, and while I was quite picky about the load line and the pre is using a CCS to make it even more linear, the fact of the matter is in this application it never swings more than 36V Pk2Pk and you’d get pretty great linearity for almost any load line that’s in spec and not drawn at the very bottom of the curves.

I was stunned with the 2A3 amp, how little impact load had on tonality despite massive shifts in the loadline.

But yes the design has to be good before you start throwing expensive parts at it.

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Keep in mind that the commercial vendors have been at this a very long time, and did a bunch of testing before they released products. And they’ve stayed in business! So, you are seeing survivor’s bias and a lot of heavily bred and evolved designs.

I built a Bottlehead Crack and spent more on it than similar and competing OTL products (@polygonhell). It doesn’t begin to sound as good as my Decware…which cost 5x or 6x more.


Physical layout mostly on PCBs, CAD files for the chassis. And I think the most detailed build guide (100+ pages) I’ve seen.

It’s enough to make me think about it even though I really should just get myself on Decware’s waitlist. Helpful reminder from @generic. Thanks.


Yeah I wouldn’t do it to save money either. It’s more about “I made this and it sounds pretty damn good!”

It’s covered on one of the two Aegis threads on Head-fi. There are some differences. For instance the Cayin built version uses Cayin transformers. However reports are that they sound very similar, if not the same.