A Primer On OTL Circuits

Before we get into specifics on output transformer less (OTL) circuits, first let us consider what the output tube must do that is different from amplifiers with output transformers. Most tubes want to work at 400-600 volts on the plate, an idle current of 50 mA and see a transformer that is 20-30:1 in turns ratio. Whether the amplifier is single ended (SE) or push pull (PP) the same conditions apply. All SE amplifiers are Class A. PP amplifiers can be A, AB1 or AB2 which is just a matter of how much idle current they run (NOTE: I did not say bias. Bias is the negative voltage on the grid).

A typical 25:1 output transformer steps down the voltage by 25 and steps up the current by 25. People tend to think mostly about the voltage, but the current is just as if not more important. People talk a lot about matching the impedance of the speaker to the impedance of the tubes. This is foolishness. The tubes do not have a desired impedance, that is determined by the voltage and currents in the circuit. The typical load for an EL-84 is 8,000 ohms, however the transformer in the Music Reference RM-10 is 13,000 ohms for the same tubes, but the voltages and currents are quite different. The difference allows me to get 35 watts out of a pair safely.

On the matter of impedance, the 26DQ5 output tube has an impedance of thousands of ohms, but it is more capable of driving an 8-ohm load than the 6AS7 which is just a few hundred ohms. So, I hope you can see that impedance matching does not matter. If you start reading a paper about matching impedance note that it only affects damping and is usually not low enough to provide adequate damping without feedback. Feedback is necessary in OTL amplifiers for them to have decent performance. Feedback is not evil if it is properly done. The advantage of OTL amplifiers is that there is no transformer in the feedback loop and, if the loop is short, feedback is easily applied and not problematic.

To make a good OTL you need a tube that can conduct appreciable current close to one amp and not be damaged in the process. The 6AS7 will pass one amp but little sparks come off the cathode like a 4th of July sparkler. Those cathode flakes rapidly reduce the emission and thus the current. Horizontal output tubes (mistakenly called "video tubes") such as the 26DQ5 were designed to conduct high peak currents and do not shed cathode material even at 1.5 amps.

Now one must arrange the tubes in some kind of circuit that connects them directly to the load. Since transformers are out, standard Push Pull is also out as it requires a center tapped transformer. What is left is to either stack the tubes or put them in a bridge. While one can make a single ended OTL, that is foolish as the idle current must equal the output current. The dissipation in the tube would be too large.

The single ended push-pull (SEPP) is another obvious choice, but the self-bias resistor is a problem. The Music Reference ESL direct drive amplifier is SEPP, but it runs at 2500-5000 volts. SEPP amplifiers work great up there but are worthless at voltages needed for 8-ohm speakers. A SEPP is a stacked tube output circuit where the top tube is self-driven so that one only needs to drive the bottom tube. No phase inverter is needed. It is a great circuit when the parameters are right, but they are not for 8-ohm speakers.

Now we have it down to two circuits. The totem pole (Futterman) has a top tube that pulls the speaker up positive (P) and a bottom tube that pulls it negative (N). This is exactly what happens in a standard transistor amplifier. However, most solid-state amps have P and N complementary transistors, we have only N type tubes. Therefore, we must figure a way to drive the two tubes equally even though the top and bottom tubes have very different drive and DC requirements.

The main accusation leveled at totem pole outputs is that the top tube never gets the proper drive which is equal to the bottom drive plus the output voltage which is large and varies with the load. If the Futterman circuit is looked at casually that appears to be the case, and many assume that the Futterman uses large amounts of feedback to correct this situation. However, a properly constructed Futterman circuit has equal drive with no feedback at all. What I notice is other writers must first damn the Futterman to present their "correct drive" which sometimes is not correct at all. I see these mistakes all over the Internet by somewhat respected sources (ex. Bruce Rozenblit’s analysis of the Futterman: http://www.stereophile.com/content/transcendent-t8-otl-monoblock-power-amplifier-futterman-redux).

The other configuration of the OTL is taken from the Electro-Voice Wiggins Circlotron circuit (http://en.wikipedia.org/wiki/Circlotron) and was made popular by Atma-Sphere. This brief article is worth reading and has a schematic and some interesting numbers. I believe the original Electro-Voice driver had some bootstrapping, but in any event, you can see it takes 2 or 3 dual triodes to drive the output tubes.

As a designer my circuit choice, like Futterman’s, is the totem pole configuration. However, the Futterman circuit had internal capacitor coupling and a big one on the output which limited the low end. The last thing we want is a big electrolytic between our amplifier and speaker. My circuit results in a low powered, ultra-short path amplifier with minimal parts, no output capacitor, DC coupled internally from input to output, self-balancing via a servo, single bias adjustment, and a single output pair using 26DQ5 tubes. Via its direct output it can source 1 amp of current. It can also supply 100 volts into a high impedance load. If one slows the servo, the amplifier is capable of DC response

Circuit Description

The driver consists of a single 6GH8 tube for each channel in a very short path, DC coupled internally. The pentode section of this tube provides the entire voltage gain of the amplifier which is about 500x or 56 dB. The triode section is connected as a bootstrapped split load phase inverter which, contrary to some misinformed individuals, provides equal drive to the triode connected output tubes. These tubes provide the current gain and can source one ampere from the positive or negative supply.

Feedback is a controversial subject among audio enthusiasts and is often looked upon as being bad rather than good. Many amplifier designers tout the fact that their circuits have no feedback. In recent issues of Stereophile Magazine there have been several reviews of amplifiers which have feedback switches so that the listener can determine for themself the effects of feedback. In two examples I have read, 2 dB of feedback is used, which is virtually nothing. In each case the reviewer reported that this small about of feedback did not please them and they preferred the sound without feedback. I cannot imagine properly applied feedback of 2 dB would worsen the sound of the amplifier as reported unless it was very poorly applied. It would take careful listening to hear it at all being vanishingly small. In addition, why even bother with 2 dB when it takes 6 dB to cut distortion in half and double damping, 2 dB does nothing.

The application of feedback is a tricky thing. Amplifier circuits do not have a place marked “put the feedback here.” One must create a place to inject the feedback. I imagine the amplifiers previously mentioned might have created this injection point with the switch thus modifying the circuit entirely when the feedback is switched in. The designer of the $75,000 amplifier in one of the reviews I read admits putting the feedback switch on the amplifier to show people how bad feedback is. Well, one can certainly install a switch to make an amplifier sound bad.

In my circuit the input tube literally rides on the output terminal. So as the input rises the output follows it perfectly in phase. Rather than being an injection point for feedback this point exists in the fundamental circuit. This allows the amplifier to react immediately. In conventional amplifiers the feedback comes through the output transformer with considerable phase shift that is load dependent. The feedback then needs an injection point which is often the cathode of the input tube or grid of a differential amplifier. Internal delays in the loop (phase shifts at low or high frequencies) can cause the negative feedback to become positive and make the amplifier oscillate at low or high frequencies or both. Many amps are unstable without a load, and many have low frequency instability that causes the woofer cone to wander about in its rest position. My OTL amplifier will not oscillate into any load.

To obtain low distortion, low noise, and good damping the circuit was developed with the shortest and fastest path possible. It is desirable in an amplifier to have DC coupling to produce the best bass and wide open-loop bandwidth so that feedback can be applied and stability into a variety of difficult loads is guaranteed. My amplifier has a bandwidth from below one Hz to above half a megahertz. The result is that distortion is lower than most conventional amplifiers at 0.5% or less, the damping is higher at 20, and the frequency and power response are far wider as previously stated. The tubes deliver their power directly to the speaker with no intervening coupling capacitor or relay.

The power supply consists of a 300 VA toroid, bridge rectified, and connected to two 2700 uF filter capacitors for exceptional energy storage. There are separate supplies for the driver, bias, and servo. The servo is a simple integrator that always keeps the DC at the output below 20 mV. It does this very slowly and much slower than the lowest bass note from any instrument. Bias is set on the negative going tube and, due to DC coupling, the positive tube adjusts itself to equal idle current.

A high inductance, low loss tapped choke provides protection from DC should a tube fail. Fuses prevent damage to other components if that should occur. With only a pair of output tubes per channel, tube failures are reduced and locating a bad tube is simplified. The choke is in parallel with the output of the amplifier to ground. It is unique in that it has three taps which act as an impedance converter allowing for more power and lower distortion than would otherwise be available without it. As such the amplifier can output 10 watts into a variety of speaker loads from 1 ohm to 32 ohms. The impedance converter can be bypassed, but the result is the amplifier power is reduced to 3.5 watts for 8 ohm loads and doubles as the impedance doubles up to about 32 ohms. The impedance converter does not harm the sound of the amplifier in any way and simply conducting a test comparing the tapped outputs versus the direct output will confirm that claim.

[Source: circa 2014]