The Stax Thread III

[Chefguru]

Apologies up-front but this is gonna be a very long post but i think its quite informative. I posed this questions asked here about different tube amp designs to ChatGPT and it spat out something pretty decent:

...

Electrostatic tube amplifiers like the KGST (Kevin Gilmore Stax Tube) and High-Amp Antares use hybrid designs that leverage the strengths of both tubes and solid-state components. This design approach creates a unique balance between tube warmth and solid-state precision – but the role of the tubes differs from what you might expect in traditional tube amplifiers. Let me break down why the “tube color” is less pronounced in these hybrid amps.

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1. How Tubes Are Used in Hybrid Electrostatic Amps:​

In amps like the KGST or Antares, the tubes are typically used in the gain stage (voltage amplification stage), while the output stage – the part that drives the electrostatic headphone directly – is handled by solid-state transistors or MOSFETs.

• Voltage Amplification (Tubes):
  The tubes amplify the audio signal to the required voltage swing necessary for electrostatic headphones (often several hundred volts). This stage sets the gain and influences the overall character of the amp.
• Current Buffering (Solid-State):
  Electrostatic drivers require very little current, but they demand high voltage swings (sometimes over 1000V peak-to-peak). The solid-state section acts as a current buffer, ensuring the high-voltage signal from the tubes can drive the headphones without sagging or distorting.

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2. Why Tubes Don’t Add as Much “Color”:​

In traditional tube amps (like for dynamic headphones or speakers), tubes handle both the voltage and current amplification. This often results in:

• Harmonic distortion (especially 2nd-order harmonics), which adds a “warm, lush, and thick” sound.
• Softer transients and rolled-off highs compared to solid-state amps.
• Natural compression and “euphonic” coloration that many audiophiles love.

In hybrid electrostatic amps, the solid-state output stage is far more linear and neutral than an all-tube output stage. This design:

• Preserves detail and transient accuracy.
• Reduces distortion associated with tube power stages.
• Keeps the sound more transparent, with less tube coloration.

Essentially, the tubes in the gain stage provide subtle harmonic richness without overwhelming the clarity, speed, and neutrality electrostatics are known for.

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3. Why People Choose Hybrid Designs:​

• Electrostatics favor neutrality and precision.
  Electrostatic headphones like Stax are prized for their speed, resolution, and transparency. Too much tube coloration could muddy the detail and reduce micro-dynamics.
• Better control over the diaphragm.
  Solid-state buffers ensure the electrostatic diaphragm moves with greater precision and less distortion than a fully tube-based design might allow.

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4. Comparing to Fully Tube Electrostatic Amps:​

• Fully tube-based electrostatic amps (like the original Stax T2) do impart more tube coloration because tubes drive the output directly. However, they often lack the same level of tightness and speed in the bass and extreme treble.
• Hybrid designs strike a balance: you get some tube warmth but retain the accuracy and punch that solid-state components offer.

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5. Why Some Say There’s No “Tube Magic”:​

• Minimal Harmonic Distortion: Since the tubes operate primarily in the voltage domain and not in the current domain (where most tube distortion occurs), their influence is subtler.
• High Feedback Designs: Some hybrid amps (like the KGST) use negative feedback to minimize distortion, further reducing the classic tube “bloom.”
• Transparency Focus: Electrostatic headphone designs aim for neutrality and realism. Excess tube coloration could actually detract from the listening experience rather than enhance it.

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6. But Do Tubes Still Matter?​

Yes – but it’s more about texture and finesse than drastic tonal shifts. Tubes in hybrid electrostatic amps can:

• Add a slight sweetness to the treble.
• Introduce subtle harmonic richness, making vocals more lifelike.
• Slightly soften the upper mids, reducing harshness.
• Improve spatial depth and enhance layering.

However, the changes are far more subtle and refined than in an all-tube system.

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7. Amp-Specific Notes:​

• KGST (Kevin Gilmore Stax Tube): Known for being very neutral and transparent. Tubes add a hint of smoothness, but the amp leans heavily on solid-state linearity.
• High-Amp Antares: Slightly warmer than the KGST but still designed to maintain clarity and precision. Tubes introduce minimal coloration, and the amp focuses on speed and accuracy.

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8. Tube Rolling – Worth It?​

• Yes, but expect subtle changes.
• Swapping tubes can adjust the flavor of the treble, midrange liquidity, or soundstage width, but don’t expect drastic shifts.
• Stick to low-noise, high-quality tubes (like NOS Telefunken, Mullard, or Amperex) to maximize detail retrieval without introducing unwanted noise.

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Final Thoughts:​

• Hybrid electrostatic amps use tubes differently from traditional tube amps – they influence the sound gently, not overtly.
• You’ll still get some of the “tube magic,” but it will feel like a sprinkle of seasoning rather than the main dish.
• This design philosophy complements electrostatics’ inherent transparency and speed, preserving their most valued traits.

If you’re seeking tube lushness and heavy coloration, a fully tube-based electrostatic amp might be more your style. But for those who value neutrality with a touch of tube finesse, your KGST and Antares are among the best-balanced designs out there.

The T2 also is a hybrid and uses mosfet

Carbon is SiC fet

Megatron is full tube
Novem is full tube
Hev90 is full tube

 

[SolarCetacean]

Apologies up-front but this is gonna be a very long post but i think its quite informative. I posed this questions asked here about different tube amp designs to ChatGPT and it spat out something pretty decent:

1. How Tubes Are Used in Hybrid Electrostatic Amps:​

In amps like the KGST or Antares, the tubes are typically used in the gain stage (voltage amplification stage), while the output stage – the part that drives the electrostatic headphone directly – is handled by solid-state transistors or MOSFETs.

• Voltage Amplification (Tubes):
  The tubes amplify the audio signal to the required voltage swing necessary for electrostatic headphones (often several hundred volts). This stage sets the gain and influences the overall character of the amp.
• Current Buffering (Solid-State):
  Electrostatic drivers require very little current, but they demand high voltage swings (sometimes over 1000V peak-to-peak). The solid-state section acts as a current buffer, ensuring the high-voltage signal from the tubes can drive the headphones without sagging or distorting.

No, that is simply incorrect on the KGST! The KGST uses transistors for voltage gain and the tubes for the output stage.

Secondly, I'm not sure why anyone would trust ChatGPT on the workings of obscure electrostatic headphone amps. The GPT output is valid for the vast majority of hybrid tube amps for regular headphones (tube voltage gain + transistor current buffer), but it is not correct for the KGST, or BHSE, or most of the Stax tube amps. I seriously doubt that the designs of the KGST or any technical discussion on the design was ever part of the training data, so I would not expect GPT to provide anything other than authoritative-sounding BS on this subject.

 

[jamiekp]

Im not sure that its true that chatgpt doesnt know anything about these amps in its training data - I mentioned the initials KGST and it knew what that stood for, who designed it etc. That said its a dangerous weapon and clearly someone who knows little about amp design shouldn't trust this blindly like i did (my apologies!):

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Apologies for the earlier confusion. After reviewing the design specifics of the KGST (Kevin Gilmore Single Triode) amplifier, it's clear that the tubes are indeed utilized in the output stage, while transistors handle the voltage gain.

Clarifying the KGST Design​

• Voltage Gain Stage: The KGST employs transistors to amplify the input signal's voltage. This approach leverages the precision and stability of solid-state components for initial signal amplification.
• Output Stage: The amplifier uses 6S4A tubes in the output stage to drive electrostatic headphones directly. These tubes operate with a constant current source, providing the necessary high-voltage swings required by electrostatic drivers.

This configuration allows the KGST to combine the linearity and low distortion of transistors in the voltage gain stage with the sonic characteristics of tubes in the output stage, resulting in a sound signature that many describe as warm and engaging.

Supporting Information​

According to Mjölnir Audio, a reputable builder of electrostatic amplifiers:

"The KGST uses the tube in a more traditional nature but with the same constant current source as the BHSE. The tubes used are the 6S4A and they handle the output directly with no extra stages needed."
Mjolnir Audio

This aligns with the corrected understanding that the tubes in the KGST are responsible for the output stage, while transistors manage the voltage amplification.

Conclusion​

Your contact from the Head-Fi forums is correct regarding the KGST's design architecture. The amplifier utilizes transistors for voltage gain and employs tubes in the output stage to drive electrostatic headphones. This hybrid design aims to blend the advantages of both technologies, offering a distinctive auditory experience.

If you have further questions or need additional clarification on amplifier designs or related topics, feel free to ask!

 

[number1sixerfan]

I would have to agree with most, on the BHSE and T2, tube changes are a bit more subtle. Separate from that, the T2 and Aeras are a bit warmer than the BHSE (Aeras a bit warmer than both), with a bit thicker timbre, so they may come off as slightly more "tubey". I have a NOS metal base set of Jan Philips EL34s in my T2 that sound pretty awesome in both it and the BHSE, but compared to modern stock mullards the difference is not that drastic honestly. It's welcome and enjoyable, but not drastic. I got them for a good price and wouldn't pay insane prices for tubes in either amp.

 

[mbrennwa]

...I have a NOS metal base set of Jan Philips EL34s in my T2 that sound pretty awesome in both it and the BHSE, but compared to modern stock mullards the difference is not that drastic...

That's what I would expect. The operation of a properly designed circuit should not be sensitive to typical variation of the devices used in the circuit. In other words, the circuit should work the same if you exchange tubes (assuming you're using healthy tubes of the same type, and bias/operating points are adjusted as intended for the circuit).

 

[kevin gilmore]

gotta go get units of health, but lests start with #1

kgst, output stage is tubes with solid state current source. its also a voltage gain stage, as in most of the voltage gain.

t8000 has tube input stage and the rest of the amp is solid state

bhse is tube output stage with solid state current source. its also a voltage gain stage as in most of the voltage gain.

megatron is all tubes. output stage also a voltage gain stage.

t2 has a cascode tube input stage. lots of solid state in the middle and a tube output stage with a solid state current source.
output stage also has voltage gain.

 

[mbrennwa]

bhse is tube output stage with solid state current source. its also a voltage gain stage as in most of the voltage gain.

Uhm, I believe that's a mixup somehow. The total voltage gain of the BHSE is 500x (with global negative feedback applied). The triode-connected EL34 tubes have a voltage gain μ = 8. This means the input stage privides more than 500/8 = 62x gain (it's actually a lot more once global feedback is factored in). The solid-state input stage thus provides most of the gain by far.

 

[Svperstar]

bhse is tube output stage with solid state current source. its also a voltage gain stage as in most of the voltage gain.

What about BH BJT?

 

[kevin gilmore]

no difference. 2sk216 and 2sj79 replaced with bipolar.

also mu about doubles when driven with decent current source.

so in megatron the open loop gain is 7 (first stage) x 15 (second stage) x 15 (output stage) == 1575
add about 6db of feedback and the closed loop gain is 500

 

[Schelp san]

I would have to agree with most, on the BHSE and T2, tube changes are a bit more subtle. Separate from that, the T2 and Aeras are a bit warmer than the BHSE (Aeras a bit warmer than both), with a bit thicker timbre, so they may come off as slightly more "tubey". I have a NOS metal base set of Jan Philips EL34s in my T2 that sound pretty awesome in both it and the BHSE, but compared to modern stock mullards the difference is not that drastic honestly. It's welcome and enjoyable, but not drastic. I got them for a good price and wouldn't pay insane prices for tubes in either amp.

As mentioned in a different thread, my experience with T2 differs a bit. I agree that rolling the EL34 on T2 does not influence the sound too much. I also found the current production Mullards being fine. However, sonically I would not like to go back to these from my Tesla double O brown base ones. But definetely there is no night/day difference.

Where I really found a playground is in the input tubes (especially the ones used in the cascoded part). Honestly, finding the "right" ones made a bigger difference in sound for me, not only talking about different flavours, but also about the sheer classic audio qualities. When I got T2 I read that it would not react a lot on rolling, so I got relatively inexpensive NOS JAN Philips 6922. With the 3-4 favourite input tube quads I use now, for me, T2 is simply on a higher level. Within these 3-4 quads, the flavour can now be changed, as they excel in different aspects.

On Megatron, I did find rolling to have a similar effect on the first two stages while it seems in comparison almost imun against changing the EL34 (3rd stage as well as the ones for the active loads). Current Mullards are perfectly fine here.

But yes, tube rolling is a rabbit hole, it can be rewarding or frustrating. That said, I would go that way any day again. I can also say that I unfortunately did not yet find a hidden gem. Most of my favorite tubes are well known but kind of rare and in case of T2 with 2 matched pairs needed not easy to find if you do not want to pay too much. Patience needed. Further it is highly recommended, especially valid for T2, that somebody reliable measured the tubes accordingly. So trusting any ebay seller with few given measurement numbers is not always a good idea.

 

[number1sixerfan]

As mentioned in a different thread, my experience with T2 differs a bit. I agree that rolling the EL34 on T2 does not influence the sound too much. I also found the current production Mullards being fine. However, sonically I would not like to go back to these from my Tesla double O brown base ones. But definitely there is no night/day difference.

Yea we definitely are on the same page here. I also would not want to downgrade from my metal bases, even though not a night and day difference.

Where I really found a playground is in the input tubes (especially the ones used in the cascoded part). Honestly, finding the "right" ones made a bigger difference in sound for me, not only talking about different flavours, but also about the sheer classic audio qualities. When I got T2 I read that it would not react a lot on rolling, so I got relatively inexpensive NOS JAN Philips 6922. With the 3-4 favourite input tube quads I use now, for me, T2 is simply on a higher level. Within these 3-4 quads, the flavour can now be changed, as they excel in different aspects.

On Megatron, I did find rolling to have a similar effect on the first two stages while it seems in comparison almost imun against changing the EL34 (3rd stage as well as the ones for the active loads). Current Mullards are perfectly fine here.

But yes, tube rolling is a rabbit hole, it can be rewarding or frustrating. That said, I would go that way any day again. I can also say that I unfortunately did not yet find a hidden gem. Most of my favorite tubes are well known but kind of rare and in case of T2 with 2 matched pairs needed not easy to find if you do not want to pay too much. Patience needed. Further it is highly recommended, especially valid for T2, that somebody reliable measured the tubes accordingly. So trusting any ebay seller with few given measurement numbers is not always a good idea.

Can certainly be the case that they can make a larger difference. I have only used 3 different input tubes, and two of them directly from Kerry's supply. I've read of input tubes causing a few issues in T2's for various users and so I've been really reluctant to go down that rabbit hole. I understand that a good part of that is really doing your due diligence and buying from very reliable sources, it's just that dealing with amp issues are such a pain (also happy to be corrected here by users that experienced this if it were more of a really specific issue not likely to happen ). I may still do so in the future.

 

[mbrennwa]

also mu about doubles when driven with decent current source.

Again, I don't quite agree. mu is a fixed parameter of a given tube type. It cannot change by the external circuit around the tube.

All one can do ist to max out the voltage gain of a circuit to the value of the mu parameter of the tube by loading the plate with a CCS. You canot get more gain from an anode follower (as in the amps discussed here).

300B (Megatron): mu = 4 typical
20B: mu = 21 typical
EL34 triode connected (BHSE, T2): mu = 8 typical

 

[icnaiwhimor1975]

the more i read about Megatron the more i want to try it..

 

[billqs]

Just got the 007's again. This one is an MK2.9 with port mod and it sounds absolutely fantastic! I'm using my Mjolnir Audio KGSSHV mini to play it.

 

[TheAbyss2022]

Does the 007 change much with DAC upgrade? Currently using the Iggy A1.