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Showing posts with label Technical guidelines. Show all posts
Showing posts with label Technical guidelines. Show all posts

Friday, November 2, 2012

Trouble Shooting Hum


One of the most common problems an amplifier builder has to deal with is hum. There are different possible sources of hum. Trouble shooting a humming bird can be quite frustrating. So I thought I write some guidelines how to approach this. These are meant mainly for power amps. Some of the guidelines can be applied to preamps as well, but generally preamps are more difficult since they need to handle much smaller signals.

If you have sume hum, don't just blindly try any suggestions. Rather use a systematic approach to find the source of the hum and remove it. When someone asks for help on internet forums to fix some hum often a plethora of different suggestions follows without attempting to understand the cause of the hum.

In 90% of the cases hum is caused by a ground loop or improper grounding of the chassis. So check this first. Disconnect the amp from any source and short the inputs. If the hum goes away it is a ground loop in the system. If it still hums check if safety earth and chassis are properly connected. Safety earth should always have a tight connection to the chassis which in turn should have a connection to signal ground. Either directly or through some low Ohm resistor or antiparallel diodes. I prefer the straight connection.

System ground loops are caused by multiple ground connections. For example if preamp and power amp have their signal ground connected to safety earth, the interconnect between them creates a loop, which picks up hum. Have the signal ground connected to safety earth only at one place in the system, preferably preamp.

If the above did not fix the problem, there can be different other causes. A systematic approach should be applied to first identify the area where the hum is picked up. I usually start at the input side of the amp and work my way downstream, following the signal path. Connect the input tube grid to ground. If the hum goes away, the problem lies between input jack and grid (shielding, volume pot, etc). If the hum persists, move up to the next stage until you reach the output tube. This way you locate the area where the hum is picked up. Then check the stage which is affected further.

Common causes: too much ripple on B+, heater without reference to ground. Inadequate filtering of B+ can be easily verified by temporarily adding more filter capacitance. Doubling the last cap at the stage concerned should cut the hum in half. If that is the case, apply more filtering to that stage. In power amps DC heaters are normally not needed for indirectly heated tubes, yet this is the most common suggestion when someone reports a hum problem. More often it is forgotten to reference the heatesr to ground. In power amps this can be done by putting two resistors across the heater line and connect the mid point between them to ground. Even better to slightly bias the heaters positive. This can be done by connecting the midpoint of those resistors to a postive voltage of 20-30V. The volltage can be obtained by a resistor divider from B+ to ground, which at the same time can act as a bleeder. Important to bypass the resistor at the ground side with a capacitor, so that the heaters have a AC referecne to ground. The divider resistor should not be of too high resistance. The maximum allowed resistance between heater and cathode is 20k Ohm for most tubes. Elevating the heaters above ground biases the parasitic diode between heater wire and cathode positively so it won't conduct. This reduces heater-cathode leakage. I apply this technique in preamps. In power amps the direct reference to ground is usually sufficient.

With the above techniques, most of the hum issues can be solved. Of course there can be many more reasons for hum. Poor ground wire layout, coupling from power transformers, etc. Also in these cases the above approach can help to identify the area of the circuit in which the hum is coupled into the signal and narrow down the search. Also be aware that there can be several hum sources, fixing one can reveal another problem which was hidden by the sronger hum source.

Be aware that in some amps just grounding a grid might not be possible. For example in DC coupled stages or if fixed bias is used. In those cases some other means need to be found to remove the signal from the grid. Only try the approach described here if you fully understand how to do it.

This is by no means a comprehensive guidline for hum issues. A hum free amp requires careful planning and layout to start with. Some hum issues can be very tricky to hunt down. But 90% of the hum can be quickly fixed by following the hints mentioned above.

Best regards


Sunday, March 11, 2012

Filament Bias, Part 1: Concept


It seems that filament bias became very popular recently. There is a long thread on the DIYaudio discussion forum about preamps with the 26 triode and filament bias seems to be used a lot there. So I thought it would be a good idea to write a few articles about this biasing method. This one starts with the basic introduction of the concept.

I came up with the idea more than 12 years ago when I spent a lot of effort to minimize the capacitors in the signal path. I do not claim that this is an original idea. I have seen similar schemes in old textbooks especially with battery DHTs.

The most conventional and in my opinion also one of the best biasing methods is good old cathode bias. In this scheme the plate current is returned to ground through a dropping resistor which is placed between cathode and ground. In case of directly heated triodes, cathode and filament are the same electrode. A point needs to be chosen as the cathode connection. In case of AC heated filaments this can be the center tap of the filament transformer secondary or the wiper of a hum pot between the filament terminals. Filament bias is only relevant for DC heated triodes. In this case many people still put a hum pot across the filament and use the wiper as connection point. I'm of the opinion that in case you heat with DC, it should be as clean as possible so that no hum bucking is necessary and no ugly pot distrubs the signal path. I commonly choose the negative filament as the cathode point. From there you connect your cathode resistor to ground.

If the full amplification shall be utilized or if the triode is transformer coupled, it is advisable to bypass that cathode resistor with a capacitor so that the cathode has a low AC impedance to ground. Otherwise the internal resistance of the tube would be increased by roughly the cathode resistor value multiplied by the amplification factor of the tube. This would move the internal resistance into a region which is not usable with transformer coupling in most cases. In case of directly heated filaments the bypass cap will also shunt any noise voltage which might be present between the B+ and filament supplies. Such noise voltage can build up if transformers without electrostatic shields are used. Obviously this cap is in the signal path and has a great influence on the sound of a gain stage. So how can we eliminate it?

In the case of cathode bias, the plate current through the cathode resistor generates a voltage drop which elevates the cathode to a positive potential, which in turn translates to the grid being negative with respect to the cathode. The beauty of this scheme is that it lets the tube find it's own operating point. As the tube ages and emission drops, the bias voltage decreases which counteracts the aging effect somewhat. Also in case of some fault the cathode resistor acts as a safety mechanism. In order to eliminate the cthode bypass cap we also have to eliminate the cathode resistor or reduce it's resistance to a very low value compared to the plate resistance of the tube. One way to do that is fixed bias which feeds the grid with a bias voltage from a separate supply. Obviously that additional supply is in the signal path, but we wanted to minimze that. Filament bias utilizes the filament current in addition to the plate current to generate the bias voltage through a cathode resistor which now can be much smaller.

The picture above shows conventional bias vs. cathode bias. The difference is minor, the negative end of the filament supply moves to ground rather than the negative filament terminal of the tube. Now the filament current flows through the cathode resistor. With filament currents typically in the range of one to several amperes, this means we can develop 10s of volts for the bias with resistances in the 10s of Ohms. This is about a factor of 100 below the typical plate resistances and won't hurt much if left unbypassed.

Now the filament and B+ supply are referenced to the same ground, no danger that any electrostatic noise voltage builds up between the two. Of course the filament supply needs to be very clean and well filtered since everything present on top of the filament voltage will be amplified. In the schematics above the filament voltages are supplied through chokes to the triodes. This is to isolate the filaments from the supply and capacitance therein. Instead of the choke the filament bias voltage can also be supplied through a constant current source which seems quite popular lately. I still favour the passive way with good iron though.

This scheme works nicely and is proven in various configurations. It simplifies the signal path and ties filament and b+ supply nicely together. Of course the self biasing and safety aspect of cathode bias are lost. This basically acts like a fixed bias stage now. There is another disadvantage: The cathode resistor needs to dissipate a lot of power. This calculates as filament current times bias voltage. So in case of a 26 DHT we will dissipate about 10-15W in each filament bias resistor, depending on the operating point which is chosen. Worse in a 801A or 10Y: Here we get 20-50W depending on the operating point. This is some hefty power dissipation which requires serious resistors with proper heatsinking. I still use filament bias, especially with triodes like the 26 or the UX201A which only needs 0.25A filament current. With these the heat dissipation is managable. With other triodes I mostly use a scheme called ultrapath which is another way to get the cathode bypass out of the signal path. In some cases I even combine them both.

I will go a bit more into deatils in future articles and also show some actual implementation in a new preamp which I plan to build. There will also be an article about a further development beyond filament bias which I named DirectPath. This eliminates the last remaining cap in such a stage, the last B+ filter cap. Stay tuned!

Best regards


Friday, August 5, 2011

Speaker impedance and amplifier output taps


Many tube amplifers have several output taps for various speaker impedances. Typically 4 and 8 Ohms sometimes also 16 Ohms. There is a lot of confusion about which is the right tap to use. With this article I try to shed a bit of light onto this subject.

There is no technical standard which defines the parameters that would qualify an output as 4 or 8 Ohm. The difference between the two is the output impedance and matching to the output stage. Different amplifier topologies and philosophies will yield very different output impedances though. On the 8 Ohm output for example the output impedance can be anywhere between 1 and 4 Ohm for sensible designs. Even outside these limits. For the 4 Ohm tap it will be between 0.5 and 2 Ohms. As you can see there is some overlap in these ranges. So what one amplifier designer would label as a 4 Ohm output, another one would find suitable for 8 Ohm speakers only. So this can be understood more as a recommendation, not a hard rule which terminal is to be used with which speaker.

Then look at the impedance of a typical loudspeaker. The nominal speaker impedance is only a rough approximation. The impedance typically varies widely over the frequency band. A 8 Ohm speaker for example might have an impedance close to 8 Ohm somewhere in the midband. In the bass region you will typically see resonance peaks at which the impedance can rise to 20-30 Ohms or even more. In the midband and treble there can be dips significantly below 8 Ohms. Dips to 5-6 Ohms are not uncommon. Only few speaker designers linearize the impedance. Again there are no standards which define how the nominal speaker impedance is derived from the impedance curve. So again this number is widely subject to interpretation and different speaker designers will declare different nominal impedances.

Due to these two uncertainties in the impedance numbers, there is no need to slavishly connect 8 Ohm speakers to the 8 Ohm tap only. Experimenting makes sense. Connect your speakers to the tap which sounds best to you. Most often hooking a 8 Ohm speaker to the 4 Ohm tap can yield some improvement. Due to the lower output impedance of this tap, the impedance variation of the speaker will have less impact on the frequency response as with the higher output impedance, the latter can actually cause some coloration. The lower output impedance will also mean more 'control' of the amp over the speaker (better damping factor). However some speakers actually sound better with a smaller damping factor. The disadvantage of such mismatching will be that the maximum possible power output of the amp gets a bit reduced. But this is negligible in most cases.

If you have a speaker with a bi-wiring terminal, or better yet, if you build your own speakers and have full control over the crossover, there are more ways to experiment with output taps. For transformers, the relationship between winding and impedance ratio follows a square law. Besides impedances the transformer also transforms voltage and current. The ratios of voltage and current correspond linearily to the winding ratio. What does this mean? At a tapped secondary, the voltage at the midpoint (center tap) will be half the voltage which is seen across the entire secondary. The reflected impedance however will be only one fourth.

This means that in case you have 4, 8 and 16 Ohm output taps, the 4 Ohm tap is actually the center tap of the secondary winding of the output transformer. There is the same output impedance, output voltage and current across 0-4 and 4-16.

Now if you have a speaker with separate terminals for low frequency and high frequency, you can utilize the full winding, even if the speaker is not rated 16 Ohms. For this to work properly, low and high frequency sections need to be completely isolated. This can be checked with a Ohm meter. There should be an open between both ground terminals. Now you can connect the low frequency section between 0 and 4 Ohm taps and the high frequency section between 4 and 16 Ohm. Polarity needs to be observed. See the illustration below, how this looks like.

No more unused parts of the output winding, dangling in the air! You can even use the taps to attenuate the high frequency part if necessary. The output voltage at the 8 Ohm tap is about 3dB above the output voltage on the 4 Ohm tap. This is neglecting load and output impedance, depending on the actual output impedance and speaker impedance the difference will be more like 2dB or less. Calculation or measurement is necessary to get the exact difference. In most speakers the woofer has a lower efficiency compared to the rest. So tweeter and/or midrange need to be attenuated which is usually done by a resistor divider network. With a multi tapped output transformer this adaption of the levels can probably be done without resistors, if in a given system a certain combination gives the right attenuation.

Below some possibilities how LF and HF part can be hooked up to achieve different levels of attenuation:

The two alternatives in the first row provide 6dB and 3dB raw voltage difference (HF section attenuated). With real life impedances probably more like 4 and 2 dB. In the two examples in the bottom part, HF is attenuated by about 4.5 and 7,5dB (unloaded). So there are quite a few possibilities to adjust the level of the HF part through this technique.

This leaves quite a lot of room for experimentation and exploitation of all the transformer taps which might otherwise be unused. The sonic result of this technique heavily depends on the amplifier, transformer and speaker used. Keep in mind that this is playing with mismatching. In some configurations it might not work well. Also the full output power of the amplifier will not be achieved with this. If the system has enough headroom it is worth playing with this though.

In addition to purposely mismatching parts of the speaker to the amp to achieve the required attenuation, this technique can also be used to achieve correct matching, when woofer and tweeter have different impedances. For example if a 8 Ohm woofer and 4 Ohm tweeter are combined in a speaker, both can be hooked up to  appropriate parts of the secondary winding, through their respective crossovers. If 4 Ohm woofers are used with 8 Ohm tweeters, the tweeter impedance can be adapted to the 4 Ohm with a parallel resistor. Since tweeters are typically more efficient, this does not hurt. The other way around it is more difficult, you would not want to bring a 8 Ohm woofer impedance down with parallel networks to match a 4 ohm tweeter. Creative usage of output taps could make this more easy.

Best regards


Friday, January 7, 2011

The search for that 'magical' operating point


One of the most often asked questions about tube amps I get is: 'Which operating point do you recommend for tube xyz?' There is also a lot of discussions on forums about the 'optimal' operating point for certain tubes. I think it is time to present my view on this.

I'm puzzled when I see such a question posted somewhere, and straight answers are given down to the decimal point of a milliampere, without asking how the tube is loaded. I've also seen reports from people claiming that they tweaked their amp to the best possible operating point and that miniscule changes in that cause the sound to change significantly. Quite often people even think that their amp is of exceptional quality because of that. They think it must be a really good amp because it makes small changes in the operating point audible. Well, I think your design is marginal, if it is sensitive to small changes in operating conditions.

First let me point out, that the choice of a good operating point is largly dependent on how the tube is operated. Does it use a resistive load, inductive or a constant current source? What is the function of the tube under question, what constraints are there, what is the B+ voltage and in which range can it be adapted if necessary. And of course it is important to also consider the capability of the components around the tube. For example if it is transformer coupled, the current rating of the transformer is important.

If all that information is known, then the ideal operating point is chosen such that the tube's grid can be driven with about equal amplitude in both positive and negative direction before significant distortion kicks in. This point can be easily found on the loadline. But if the tube stage under question is designed with a lot of headroom as I described in an earlier post, then it will not be critical to hit the exact spot in the middle of that load line. Because of the headroom the operating point can be changed by some margin without significant changes in sound character. And this is important if you want a stable design. Tubes and also passive components age and shift their parameters with use. This results in shifts of operating points. I want my amps to have the same sound over the widest possible range of the tubes and components life time. Also in amps with passively filtered power supplies (which I prefer) the B+ voltage will change with mains voltage fluctuations. These should not impact the sound of an amp or preamp.

Let's clafrify this a bit further using a 801A/10Y linestage as an example. I chose this because in the recent days I got many questions about the ideal operating point of the 801A in a transformercoupled line stage. Let's assume the design is using a line output transformer which can be run with 20mA DC on the primary. This means it is capable to be run with up to 40mA before significant core saturation kicks in, so we have a range around the 20mA point which is about symmetrical from 0mA to 40mA. Before the transformer winders among you get excited: This is just for simplicity for the purpose here, actually things are a bit more complicated. We want to be able to use both 801A and 10Y, so we stay within the voltage limits of the 10.
We will pick 400V B+ which will result in -30V on the grid for 20mA current. In a cathode biased design this will require a 1.5kOhm cathode resistor, which will yield 30V drop with 20mA.

The graph on the left shows the plate curves of the 10. The loadlines for a resistive load (black) and transformer load (red) are drawn in. The transformer is assumed to be unloaded so the loadline is horizontal. The transformer load line is idealized. In reality it will look a bit different, but for the purpose of this topic this is fine. As the transformer secondary would be loaded with a resistance, the loadline would tilt side ways. For the resistive load a B+ of 600V would be required, while it is 400V for the transformer (neglecting copper losses, which don't play a big role in this case).
As you can see the plate curves are nice and evenly spaced between the point where they cross the red load line. We have a range of 60V peak to peak through which the grid can traverse. This means that the overload point for such a line stage would be more than 20V RMS (30V amplitude divided by square root of 2). That's what I call head room in a line stage where you typically have 2V RMS max on the input, may be 3 or 4V if you have unconventional sources with higher than normal output. The plate can swing between 150 and almost 650V without major distortion. That's 500V peak to peak or 180V RMS. Assuming a 4.5:1 lineoutput transformer (for example Lundahl LL1660) this will give up to 33V RMS output at an output impedance of about 250 Ohms. Plenty of headroom to drive even insensitive power amps and nice low Zout, so you can use long cable runs without any problem.

Now getting back to the topic of the operating point. If you shift the B+ voltage let's say down to 300V, things don't really change a lot, except the maximum possible output swing is reduced, but we have plenty of headroom to move in anyways. The same if you change the current. The operating point can be easily varied by plus or minus 25% between 15 and 25ma without significant changes. That's what I call a stable and wide headroom design. The tube can age (which typically results in less emission and less current) and the line stage will not change it's sound character much. Also no need to be picky about cathode resistors with 1% tolerance 5% is more than enough.

If you look at the resistive load line, things are a bit different. The spacing of the points where the plate curves cross the load line gets narrower as you move to the right. Here I would choose a different operating point to start with. Probably more like 300V/20mA this would move it to the more linear region. This also shows why I prefer inductive loading (CCS will be similar) or transformer loading with lightly loaded secondary.

So, if you experience significant changes in sound as you experiment with operating points, look at your design and fix the cause, rather than endlessly searching for the ideal operating point. You might be in the non linear region of the tube or some components in the design are marginal and already at the point where the current causes some stress. Move away from there, find a tube type which is more suitable for the case or find the components which are at their limits and change them.

Even if you think you have found a point which you like, think about this: As the tube is driven with signal, it moves through a whole range of points on the load line. It will behave differently at all those points. Imagine a piece of music which has some large amplitude bass notes, and some low level back ground vocals or piano is played along those notes. The low level signal portions will be amplified very differnt at the upper and lower peaks of the bass amplitudes. This is often very audible in marginal designs. When no bass note is present, vocals and piano lines might be reproduced nicely with beautiful ambience, but as the bass note kicks in the voices become less transparent and the ambience dissapears. In a good, wide head room design everything will remain stable.

I hope you enjoyed this little excursion into operating points and you will rethink and look at the bigger picture next time when you are asking yourself which operating point you should choose. When you finished a design, a good test for stability is to change the operating point and listen for changes in sound. If you hear changes with small shifts in the conditions, find and fix the cause.

Best regards


Tuesday, January 4, 2011

Gain, Headroom and Power


Thanks to all who participated in the poll so far! There seems to be a clear trend for a SE amplifier concept on a budget, followed by LCR phonos. Next is cost no object SE amps. I'll let the poll continue for a while. Let's see how it develops. If the trend stays like this. I will show the SE amp platform concept which can be scaled from very simple and low budget to all fancy, all DHT, cost no object.

In the meantime, I will write some random ramblings about some topics. These will outline some of my basic design philosophies. I don't claim these are absolute truths. There are several ways to reach good sound. What I will show is my way to good sound. At least what means good sound for me. People listen very differently and focus on different aspects of what is the 'sound' of a system. But that might be covered in another post. So let's get to the topic of this post. Gain, headroom and power. I feel there exisits some confusion about these terms, they are often interpreted wrongly or even mixed up. So here are my thoughts:


The gain of a system or component describes the amount by which it amplifies the signal from it's input to the output. So it is the ratio output voltage divided by input voltage. This is usually expressed in dB but can also be expressed as a factor. Ok, so far, so simple, what is there to misunderstand?  I have often seen that in power amps gain is sometimes confused with max. output power. People compare two amps in a system at the same setting of the volume control. Almost each amp design has a specific gain which is different from others. So in this case, one will result in a louder volume than the other. But this has nothing to do with the actual power an amp can deliver. Some amps need more input signal (are less sensitive) then others to reach full power. It is important to match the gain of a power amp to the system. That means it's sensitivity should be such that the preamp can be operated in a sensible and useful range of volume control settings. And of course the preamp needs to be able to deliver the required voltage level for full power, with some headroom (more about that later). I've seen many systems which can only be listend at volume control settings of 9 o'clock max. These systems have too much gain.

This brings me to my design philosophy about gain:

The gain of a system should be just as high as necessary, not more

What does this mean? Ideally you should be able to listen to records which have a lower than average recording level, at the loudest volumes you intend to use. To listen to these at the loudest levels you intend, the volume control should be fully or almost fully turned up. In this ideal situation, there is not a tad of gain wasted in the whole signal chain. More gain typically requires more stages, more components, let's use as few as necessary and these of the highest quality we can afford.

Why worry so much about this you might ask. Let's just stick a resistive divider somewhere if the gain is too much, or a range up to 12 o'clock on my volume control is still enough for me. Well the resistive divider will do the trick of course and might be the ticket if you have too much gain and need to reduce it somewhere. But it's just patch work and it's additional components in the signal path. An argument might also be that you just use higher mu tubes. This way you get a lot of gain without increasing the number of amplification stages. True, but higher mu tubes typically have high plate resistances which means they cannot drive a lot. No long cables for example. If you have a gain stage somewhere which really doesn't need to drive a difficult load, you can use the highest possible gain tube there. Then use that additional gain to reduce the gain requirements somewhere else, maybe by using a lower gain line out tube which will yield a lower output impedance. Or a lower gain, lower rp driver tube in the power amp, which will provide more headroom (more about that below).

I have heard claims from people saying that they generally prefer higher gain power amps in their system even if that means they cannot use the full rotation on the volume control and don't really need all that gain. In such a situation I would have a close look at the preamplifier. Resistive volume controls tend to sound quite different at certain settings. So it could well be the case that the higher gain amp just allows the preamp to be operated in the best sounding settings. It could also mean that the preamp does not have enough headroom, or has rising distortion artifacts at the levels which the amp with less gain needs to be driven to a certain power level. So that doesn't mean that the higher gain power amp is better than the others. Maybe this indicates the preamp should be changed and not the power amp.

Basic message: The overall gain of a system should be well matched to the listening habits. Each component should be chosen such that their gain characteristics match to the overall system.


First let's clarify what I mean by headroom. The headroom of a component or gain stage is the difference between the maximum output signal it can deliver to the maximum signal which is required to drive the following signal chain to full power. Here my philosophy is quite different as for gain:

The headroom of each stage in the signal chain should be as high as possible

Why this? Why being stingy with gain and generous with headroom? Quite simple: Each tube no matter how linear it is will have steeply rising distortion as you get close to the max. possible output level. This can be easily seen on the plate curves. While they might be evenly spaced around the operating point, the distances get closer and closer as you reach cut off and typically wider as you reach saturation. So let's stay far away from those corners under all conditions. Another reason is grid current. As the grid voltage get's closer to 0V grid current will set in, causing the grid to become a non linear impedance. Grid current starts not suddenly as the grid reaches 0V, but long before. Small signal tubes typically start to show grid current at voltages between -1 and -0,5V. So again let's stay away from that region, possibly by a good margin.

So how much headroom shall we choose? I consider 6dB as a good headroom. In the context of a driver stage in a power amp this means: If the output tube needs 100V peak-peak for full power out, the driver stage should be designed such that it can deliver 200V peak-peak or more. In preamps I even shoot for 12dB and more. In low budget designs, especially power amps, this might be difficult to reach. So I would be ok to compromise this aspect if on a budget.

For the complete signal chain this means that when driven to full power, the output tube will start to clip long before any other gain stages upstream are near their limit. So the distortion spectrum is largly determined by the output stage characteristics.


For power I have no straight philosophy to determine how much you should aim for. This is too dependent on the speakers you use and your listeng habits. Still this point needs some discussion. I've already written above how power sometimes gets mixed up with gain. Often people ask me for advice which of my power amp designs they should use. Of course my first question is about the power they think they need. I often get answers like 20, 30W minimum, sometimes even higher. Beeing into single ended, things get difficult with increasing power. While 20-30W are still feasable, it already gets costly in this range if you want to have a certain quality level. While more than 30W are still possible in SE, the cost rises exponentially. So my next question if I get requirements of 20W or more is about the sensitivity of the speakers used. Surprisingly often I get replies of 96, 98 or even 100dB, yet people think they need 20 or 30W of power per channel. Even if they are not half deaf hard rock head bangers or have 100 square meter lofts to fill with large orchestral music.

So where does this come from? While most often it is simple misinformation where people just don't know how much power they really need, I did experience other cases. People who have highly sensitive speakers (or at least speakers which claim to be highly sensitive) and who have tried several amps and claim that they get better sound from the higher power ones. Quite often, what they really need is a better damping factor, not higher power. Higher power amps typically (not necessarily always) have higher damping factors (lower output impedance) than the average flea power SE amp. I have actually seen SE amps which have output impedances of 5 Ohms and higher on their 8 Ohm outputs. While I don't think that huge damping factors are really necessary, a factor of under 2 ist quite low. Such amps will sound quite different on different speakers. If the speaker has variations in it's impedance curve (and most have), this will result in colorations.

While it is difficult to build a SE amp without negative feedback which yields very high damping factors, they can be done such to have at least a reasonable damping factor of say, 3 to 5. In my experience such levels are sufficient to be compatible with a wider range of even 'conventional' speakers. To reach these, higher impedance output transformers are needed. Shoot for primary impedance of 4-5 times the rp of the tube or even higher. Of course there are many other factors which determine the output impedance, like copper resistances of the windings. But the ratio between the internal resistance of the tube and the primary impedance of the output transformer is the main parameter which determines the damping factor of the amp.

A very easy test to check if your speaker needs lower output impedance can be done if the amp has different output taps. Quite often it is beneficial to connect a 8 ohm speaker to the 4 Ohm tap. More often than not, the lower output impedance of the 4 Ohm tap outweighs the disadvantage of the lower output power which results from the mismatch.

So, basic message here: Clarify how much power you really need and if your speaker needs some 'control' verify if that really means power or just low output impedance. I experienced that people often are very reluctant to consider a change in speakers. But lets face it, there are many speakers out there which are just not compatible with SE amps. And to my taste these are not the best sounding speakers anyways. So if you want to dive into the world of SE tube amplification, make sure you have the right speaker with sufficient efficiency and linear impedance curve.

This was my first post about some of my basic philosophies more will follow. If you think they make sense or if you have similar experiences, you might also like some of my circuits and concepts.

Best regards