NEW Acuhorn amplifiers 6C33C tube output transformerless

If this is really a single-ended OTL of conventional design, the 'pencil tube' is the voltage amplifier/driver tube, as it is a dual triode tube capable of driving the input capacitance of the 6C33 grids without loss of bandwidth.

The power supplies appear to be Switch Mode Power supplies, hence the wide range of AC line input frequency and voltage.

The real issue is how the tubes are coupled to the loudspeaker. The problem is that if a parafeed method is used, significant output power will be lost (I can see from internal photos that there are electrolytic capacitors connected to the speaker terminals, so this could well be the coupling means). If the speaker is placed in series with the power tubes, significant DC current will be seen through the load, causing a woofer offset (which will increase distortion in the woofer and limit excursion). A third (brilliant) method of course was developed by David Berning, although not a true OTL in the traditional sense, does allow one to built a compact amplifier with a small (air-core) "output" transformer. There were similar RF switching circuits that go all the way back to WW2; it may be that this company has found a way around the Berning patent by mining the prior art. Its all conjecture of course- the company website is severely lacking in technical information as are any of its promotional materials.

Given that the amp is operating in single-ended mode, it is also a given that it is operating class A. There is significant heat associated with this move, and the sockets appear to be ceramic, so they will have a limited service life due to the heating issues. The English text I have seen does not make sense and seems in need of better translation- it suggests that the amp makes considerably more power than it can possibly do, while in fact the dual-tube version seems to only make about 10 watts, which makes more sense.

Nice exercise in layout and the use of SMPSs to reduce weight and size. The pencil tubes might get difficult to find down the road, as they have not been made in decades. I suspect though that a 12AU7 could be made to work without much difficulty.

There are millions 6N16 russian pencil tubes out there. Nice little piece with gold grids.

Reinhard

Think the converters must be AC to DC,

Reinhard

Vicor is a USA company that makes Switch Mode Power Supplies. So yes, they are doing AC to DC conversion for filaments and B+.

At least one of the graphs above appears erroneous. This would be the frequency response curves, which shows bandwidth based on 2, 4, 8 and 16 ohm loads. Now the output configuration is as I had surmised earlier, a capacitor coupled cathode follower output. The capacitor will interact with the low frequency response more as the load impedance is reduced. So we should see that it rolls off sooner into lower impedance loads but the chart doesn't reflect that- it shows that the low frequency bandwidth is rolling off sooner into higher impedances. This leads me to suspect that the charts are not entirely accurate.

If it's a big cap the LF impedance may be pretty small w.r.t. the load, especially since the follower is probably not very low impedance itself. Still, more likely the follower interacting with the load (including the series cap) -- it would take a pretty big cap to be a low impedance at 10 Hz (1000 uF @ 10 Hz is about 16 ohms) and the graphs seem to reflect an output impedance less than half that...

Vicor also makes DC-DC converters but hard to say what exactly their power supply looks like (Vicor from line to B+, or an input transformer and bridge or line-based AC-DC stage followed by a Vicor DC-DC converter).

What about Power Output? One Watt in 8 Ohms?

Reinhard

A little hard to say from the data provided. It shows about 3 V just before the knee in the output sweep, so if that is RMS it would be just a little over 1 W into 8 ohms. And a little over 10% THD...

DonH50 said:

A little hard to say from the data provided. It shows about 3 V just before the knee in the output sweep, so if that is RMS it would be just a little over 1 W into 8 ohms. And a little over 10% THD...

Click to expand...

All that power and commensurate THD for about $5.000...

To put all this in context you have to go back and read the opening post:

Matej Isak said:

This seems like much worthy of sharing. This seems like something completely different and new. Acuhorn OTL amplifiers with an intriguing approach...

A new solution for tube amplifiers output transformerless. It is a very interesting idea to apply 6C33C tubes operating at the utmost level of Amperes in the world. The high level setup class A per tube is up 8 times greater. For example, in the S1 amplifier one tube per chanel works like traditional 8 tubes. Monoblock M4 is the equivalent of incredible 32 tubes. Acuhorn in its new amplifier series fused the newest technology with vintage audiophile tubes' qualities............

Click to expand...

I had been anticipating some new circuit topology, although hard to believe we've yet to see anything that hasn't been done with tubes such as the 6C33C. So far the graphs don't support the claims.

Comments, Matej?

Tom

1 watt at 8 ohm with 10 THD is not to me an interesting proposition. They will sell a few and fall by the wayside all that IMHO, YMMV. I sincerely was expecting a breakthrough .. Let's be honest and frank and admit this is not the case.

I will be happy to have someone convince me of the contrary.