Single Ended Amplifier Concept, Part 1

Hi!

In the poll most visitors voted for single ended amplifiers. Most of them for a low budget amp, but many also for a cost no object SE amplifier. So let's see how we can cover both with a single concept which can be adapted to various budgets and will deliver exceptional sound quality within a given cost range. 'Low budget' has quite a different meaning for different people. So let's discuss this first. In a DIY project there is a wide range of possibilites to influence the budget. The more time you are willing to put in to search for cheap parts, the lower you can go. Since this concept needs to be reproducable we need to resort to readily available parts. The foundation of a good amplifier is the iron: output transformers, interstage transformer (if any), power supply transformer and power supply chokes.

As I mentioned already in the post about the 6CB5A, the idea for this concept was born on the german tube forum Röhren und Hören. There was a thread about a DIY amplifier concept which should be fairly easy to build, affordable and provide excellent and hum free sound. The desired cost range was determined through a poll. The result was a budget of 1000 Euros for all electronic parts, including tubes but without chassis material. This should be for a stereo amplifier. Of course there was a wide spread in the votes for the budget, so some flexibility was desirable to be able to scale the cost down to 500 Euros by selection of cheaper parts or a simplified concept and also to scale it up by using better parts, building mono blocks, external power supply, etc. But the requirement was that the base concept for the cost of 1000 Euros should deliver exceptional sound quality and come with very good parts.

There was a clear preference for a single ended amplifier concept. But not too low in power output, something which gets closer to 10W than to 1W.

Many people would have liked the 300B tube but that would have taken out a big portion of the available budget. Especially if there is the desire for some spare tubes. And no way to even think about original manufacture Western Electrics with the given budget. And in my opinion: If you want 300B sound go for original WE 300Bs (not the reissues of the 90ies) but that will be covered in a later post. Most people are too focussed on just the output tube anyways. It is the whole concept which determines the sound quality. Driver stage at least as much as the output stage. And of course the power supply. Most designs have 4-5 parts in the signal path per stage. Well optimized designs get it down to three. All of them have an equal influence on the sound quality. This is a very simplified view, but if you look at it this way, the output tube maybe contributes a third to the overall quality of the output stage. Power amps have 2-3 stages, so best case, the output tube makes up one sixth of the overall sound. And this is not even counting the power supply! Therefor equal effort was spent in this concept to get all parts on a comparable level. I'd rather listen to a well designed amplifier with a lesser tube than the 300B but with solid iron and capacitors, than a 300B amp with cheap output transformers, electrolytic caps and a marginal design.

There was another reason not to go for a directly heated triode like the 300B. The amplifier should be as hum free as possible even on sensitive speakers. A directly heated triode would have required DC filament supplies, except maybe the 45 or 2A3 which run on 2,5V filaments. But these were ruled out due to their low output power. A DC filament supply would have added cost and complexity. An indirectly heated triode would be as simple to use as it can get with regard to heating.

How that lead to the choice of the 6CB5A was described in the tube of the month post from last week. Besides exceptionally low cost, the 6CB5A has another advantage. It's operating points and requirements to the output transformer are very close to that of the 300B. So the same concept could be very easily changed to the 300B output triode and a comparison between the too would be very easy, even allowing the comparison in the same amp, with the same parts, except for an additional filament supply for the 300Bs.

With the tube cost beeing so low, that left almost the entire budget to spend on high quality parts, especially the iron. But before we come to the choice of parts, the basic architecture needs to be defined. The number of stages in an amplifier has a major impact to the complexity (and also cost). This is also dependent on the gain requirements. If you read my post about gain, headroom and power, you'll remember that my philosophy is to use only as much gain as is necessary, with as much headroom as possible. This lead to the choice of a two stage concept (driver and output stage). To keep complexity low, the driver should be supplied from the output stage B+ via a separate decoupling circuit (RC or LC). Interstage transformer coupling would yield good headroom from a given B+, better than RC coupling. Also transformer coupling was not very widely used ta that time in Germany, so such a solution would bring some new concepts into the scene. Of course also because I always got the best sonic results from transformer coupling.

The requirement for the concept to be fairly easy to build naturally leads to cathode bias as the method to maintain the operating points. This avoids additional supplies for bias, and any complexity to ensure the right sequencing  order of the supplies during turn on. Just a single B+ supply for both channels. Tube rectification will take care of delayed and slowly rising high voltage. In order to use something better than average, a nice, classic choke input filter approach was selected for the HV supply. Again to keep it simple also for beginners to build, no regulation in the pwer supply just good solid passive filtering.

So the basic architecture was defined. A two stage transformer coupled concept, using indirectly heated tubes, cathode biased with a single tube rectified  and choke input filtered B+ supply and AC heating throughout. No silicon at all in the entire amplifier.

Where does this leave us with the budget? Here is a raw calculation: A good choice for excellent sounding transformers at moderate cost is Lundahl . They have a wide range of suitable tarnsformers. For the primary impedance requirement (3-5k) of the output transformer the LL1663 or LL1664 would be suitable. That is about 250 Euros the pair. The LL1660 interstage tarnsformer is about 180 the pair. A heavy duty power transformer for the PSU would be around 100-120. Chokes come at 50-75 each, depening on supplier. At least 3 chokes would be required. One for each channel for decoupling. One in the common PSU, better two since additional smoothing might be required due to choke input. That is 200-300 Euros for the chokes. This sums up to 750 Euros max. For the iron which leaves 250 Euros for the rest. Since tube cost is low, this allows even for some nice oil caps.

Here is a sketch of the schematic of the concept so far:

Straight forward circuit, a separate choke in each channel which allows the use of a common supply with minimal interaction between the channels. The driver stage B+ is derived from the same supply via it's own RC filter segment. In a more elaborate implementation this could be upgraded to LC, but since we are on a moderate budget, let's stick with RC here. But wait, there is one unusual aspect which is not commonly seen: The capacitors from B+ to cathode in both output and driver stage. This is the so called 'ultrapath' concept. The origins of this approach go back to the engineers from Western Electric. Lynn Olson covered this on his website in an article Western Electric - Rosetta Stone for Triodes. As far as I'm aware the first person to mentioned this approach again in 'modern' times and who re-introduced it to vacuum tube audio is Jack Elliano of Electra Print. He is also the one who named it 'ultrapath' in an article in the magazine Vacuum Tube Valley.

What ultrapath basically does is to provide a 'shortcut' for the signal path. Normally the signal would traverse from the tubes plate through the primary winding of the coupling transformer to B+. From there through the power supply (usually the last cap in the filter chain) to ground. From ground through the cathode resistor and/or the cathode bypass cap (which usually is an electrolytic) to the cathode of the tube. The ultrapath cap is usually fairly low in value and a high quality cap can be chosen. It bypasses the cathode circuit with it's electrolytic alltogether. Depending on the circuit, tube and output transformer, often the cathode bypass cap can be left out with the ultrapath connection. For clarity it is left in the above scheme.

There is quite a lot of misunderstanding out there about the purpose of the ultrapath cap and it also got some bad press recently. In my opinion it is a very effective and cheap way to boost the performance of any transformer coupling stage by reducing the components in the signal path. It can also be used to reduce powers supply rejection, since it couples residual ripple from B+ to the cathode. The ratio of ultrapath and cathode cap can be chosen such that ripple is cancelled out. But this is not the purpose of this approach here. We only use it to control the signal path. Especially if the cathode cap is omitted, ultrapath will require a very well filtered and hum free B+ supply, since ripple is coupled to the cathode. Hence the provision for the second choke in the PSU, which will be 3 LC stages if the separate decoupling chokes per channel are counted.

If the available budget is much lower, the circuit can be significantly reduced, by abandoning the ultrapath concept and changing the interstage coupling to RC. For further cost reduction, the individual decoupling filters can be replaced by a single electrolytic with high capacitance. Of course this will have an impact on the resulting sound quality. The change to RC coupling will also reduce the ehadroom in the driver since the driver tube will operate on about half the voltage. The other half will be consumed by the plate load resistor.

Here is the conceptual schematic of the low cost version:

Such a concept only needs 4 pieces of iron: 2 output transformer, power transformer and one choke. The choke could even be left out, but let's not make it to primitive it should still sound good. If a less oversized power transformer is used, the iron set would be around 250 for the output transformer (let's still use something very good like the Lundahls), 50 for the choke and 100 for the power transformer. That's 400 Euros for the iron set. Another 100 - 150 should be enough for the tubes, sockets, resistors and capacitors.

And the beaty of this: You could start with the simple RC concept and later upgrade to transformer coupling.

In the next parts of this series we will fill the concept with a bit more flesh. I will write about the driver tube selection and sizing of the resistor and capacitor values. After that I will present some power supply concepts for this amp and will also show how the design can be easily converted to use directly heated triodes like the 300B, 45, 2A3 or 801A in the output stage. But even there the journey will not end, the concept can be enhanced to an all DHT amplifier with directly heated triodes in the driver stage as well. Stay tuned!

Best regards

Thomas