RAAL-requisite: The Official VM-1a Thread

Our all new standalone ribbon headphone amp the VM-1a features a dedicated variable mode tube circuit design exclusively for driving ribbon headphones. Capable of operating in Ultralinear, Triode or Pentode mode, this top of the line amp delivers uncompromising performance RAAL’s SR-1a headphones.

This custom built amp features variable mode select for: Ultra-Linear, Triode, and Pentode Operation modes. This direct drive vacuum tube amplifier is compatible with all of RAAL-requisite’s ribbon headphones. It has selectable SR-1a open-baffle compensation and CA-1a non-compensation for both outputs. It also has a 24-step precision attenuator which is ideal for precise channel matching. On top of that, it has transformer balanced inputs (left/right) and outputs for simultaneous dual headphone operation. Toroidal output transformers and the toroidal power transformer are all magnetically shielded and epoxy cased. Lastly, the amp allows you to select between 115V and 230V voltages, which is great for those who want to use the amp in different regions of the world.


  • Ultra-Linear

    • Power Output: 15W

    • Bandwidth Frequency Response: 20Hz - 70kHz, -3dB

  • Triode

    • Power Output: 10W

    • Bandwidth Frequency Response: 10Hz - 70kHz, -3dB

  • Pentode

    • Power Output: 20W

    • Bandwidth Frequency Response: 30Hz - 50kHz, -3dB


  • Input Tube: 1x Tung-sol GTB 6SN7 Dual-Triode

  • Splitter/Driver Tubes: 2x Apex Matched Mullard EL34

  • Output Power Tubes: 4x Apex Matched Mullard EL34 Quartet

  • Input Impedance: 50K ohms

  • Input Sensitivity: 2V

  • Distortion: < 1%

  • S/N Ratio: 72dB

  • Weight: ~29lb


First, welcome to the forum!

And then, regarding the VM1a itself …

Since I’m not really in a position to “demo” this anymore … at least not without just buying it, and relying on the return policy, I have some questions prior to going down that path.

I assume the signal path is, essentially:

Balanced input into a transformer. Single ended output from the transformer into the 6SN7. The 6SN7’s output goes into the first two EL34s to split phase and return to balanced/differential signals. And then output from those first two EL34s into a balanced/differential power stage of the second, quad, set of EL34s?

If so …

  1. That should result in reduced even-order harmonics at the output (i.e. lower distortion); is that the actual end result?

  2. Is there a reason (beyond the high-end market wanting balanced inputs) to take a balanced input and convert it to single-ended, rather than just taking a native single-ended input?

And then …

How does this behave if you connect a conventional headphone to the ribbon output (via an adapter, such as that supplied with the HSA-1b)?

Is that a supported scenario?

I’d assume higher noise (and power) levels if so, since my guess would be that this unit does not have the attenuation circuit (due to presumed lack of internal space) that the HSA-1b includes to make it more suitable for conventional, relatively sensitive, headphones?


Thanks, Torq!

Glad to be here!

You are correct about the signal path.

  1. Yes. Normal push-pull behavior.

  2. Yes. The reason for using input transformers is that we wanted galvanic insulation of audio ground. This is a full blown speaker-kind of amp that is speacialized for our purpose, so it’s not a champ in low-noise department. We need all the help we can get to keep the noise floor low, so we’ve had to make our audio ground completely insensitive to user’s system ground, with possible ground loops, etc.

And then…

Using conventional headphones via an adapter is definitely possible, but we can’t guarantee the perception of the noise floor and what SPL you will get. It depends on headphones, but this is what we have:

We have 20W of output on VM-1a at our load of 0.4 Ohms. Way more power than needed, so it’s incredibly powerful with our headphones, plenty of reserve for the most demanding peaks.

However, that is 2.8Vrms. Is that enough or not to whom, I don’t know.

It does not have the attenuation circuit.

In any case, nothing will happen to the amp itself, as it has a fixed ballast load inside that always loads the output transformer with a safe load that doesn’t make it “open secondary”.

So, trials are definitely possible, but this type of use was not on our radar. The intended purpose of this is to offer a system, in combination with SR1 headphones, that exceeds the performance of the most famous amp-headphones specialized combos in existence, except we’re not selling it exclusively as a combo.

I hope this answers your questions. If you have more, please ask!


Alex from RAAL-requisite


Hello @Aleksandar_R welcome to the community here. Thank you for the detailed answers to @Torq questions which are of interest to all of us.

Having enjoyed the SR1a through different amplification up to now I can’t wait to hear it through your flagship amplifier.

Thank you for doing what you do!


Thanks, @Aleksandar_R , much appreciated!

So … if 2.8 Vrms is the output limit, the following table shows the maximum achievable dB/SPL level for some current flagship and/or harder-to-drive headphones:


  • The maximum SPL shown will be 120 dB/SPL, even if the amplifier could push the headphones harder without hitting its voltage limit. In these cases, the actual required voltage should be less than what the amplifier can deliver and the actual voltage required is shown.

  • Calculations are done to the highest whole dB/SPL level that can be achieved without exceeding the amplifier’s voltage limit. For example, the LCD-4 requires 2.52Vrms to hit 112 dB/SPL - but to hit 113 it would require 2.83Vrms, which exceeds 2.8Vrms, so 112 dB/SPL is treated as the maximum (and the actual voltage required is indicated).

  • This is an academic exercise and is NOT intended to indicate suitability of the VM1a for any given conventional headphone, only the maximum SPL you could expect to achieve. Noise levels and other factors have not been considered; thus you may wind up with suitably loud replay but with audible noise with some headphones.


Wow @Torq!

Your table is great!

Well, the noise floor will be the limiting factor here. as you pointed out, so I wouldn’t keep high hopes that VM-1a is usable with any of those headphones.



Hello @MokhaMark,

thank you for enjoying the SR1a and your warm welcome!




Yes, welcome.

Question; can the VM-1a run on KT88/6550’s or just KT77/EL-34’s?


For those wondering, got the answer from Danny. No!

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At one time, we were considering possible compatibility with KT88/6550. Eventually, it was decided to optimize the circuit for EL34 only. I am waiting for more details to add to my reply and will follow up.
Best, Danny

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One thing I am familiar with from my ownership of a power amp that can utilize either EL-34 or 6550’s is the biasing requirement is much higher with the later type.

Strongly considering this amp, I’d love to hear how a set of Gold Aero KT77’s or SED Winged “C” EL34’s would sound in it.