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Tuesday, June 19, 2012

How Does it Sound ? (Part 2)


In last years How Does it Sound? Article I already wrote about what I look for in terms of sound reproduction and about the loop holes in listening tests. There had been some recent discussions on internet forums about how certain things sound, in which I participated. I'm also very often asked about my opinion about the sound of certain parts or design aspects. So I thought I summarize my thoughts about this in another article.

Most often such questions are related to a specific tube. People would like to know how a certain tube sounds, probably compared to certain other types. Very understandable because the output tube is the most obvious part in a tube amp and most often the single aspect based on which an amp or a DIY project is chosen.

Unfortunately there is no generally valid answer to this. Sure there are many opinions floating around how, say a 2A3, sounds. If you ask about the sound of a certain tube on a forum, you will get many answers. Probably some of them contradicting.

It is very common among tube enthusiasts to arrange shootouts to find the answer to such a question. We want to know which tube sounds best? Let's set up a system and hook up 211, 300B, EL34, 2A3 and a 45 amp and shoot it out. Only, it is not that easy. If you really want to nail it down how a certain tube sounds compared to another one, this is going to be very difficult. It is always the entire amp which you are listening to. And not even that alone, it is also the way this amp interacts with the speaker used and even the preamplifier. What works well in one system,.might not be so good in another.

Now let's assume you have two amps which share the same parts and circuit topology, only use different output tubes. I actually did that several times. For example a triode strapped 6CB5A and a 45. Both in a transformer coupled amp, using the same driver tube (6N7) and the same interstage and output transformers. Also the same capacitors and similar power supplies. Tube rectified in both cases with full wave TV damper bridge feeding a choke input filter. Even the same power transformer, only using diffierent taps since the 45 needs a different B+ voltage.

Now this is really a valid comparison and all we are comparing are the two output tubes, right? To a large degree yes, but there are still other factors playing a role. Both tubes have different plate resistances so they require different loading. The output transformer used (Tango XE20S) provides tapping for different impedances, so it can be adapted. Still both amps will not have the same output impedance (damping factor). Some speakers react sensitively to that, others less. So this is one aspect which has an impact on the sound. It is not a qualitative aspect as such. With one speaker a higher output impedance can be beneficial with another one it will cause a slightly worse sound.

Next is the interaction with the driver. The tubes have different grid drive requirements, different input capacitances, different tendency for grid current and need different voltage swings for the same output power. So the combination with the specific driver used is also at play. The driver might be adequate for one of the tubes, but at the verge of being stressed with the other.

Then both amps have different max. output power (by a factor of 4). The more powerful amp will be usable with a wider range of speakers. But let's assume we use a sensitive speaker and listen only at power levels below the maximum of the weaker amp. Both amps will require a different input voltage for the same output power, since the tubes have different amplification factors and the output transformers are wired for different step down ratios. So one will need more voltage swing from the preamp than the other. Of course we adjust the volume control on the preamp for the same output level between both amps. But that means we play each amp at a different setting of the volume control. Many preamps sound different at different volume settings. often it is the volume control itself. Especially resistive potentiometers tend to sound different at various settings. Also in one of the settings the distortion spectrum of the preamp might be different, this can be quite dependant on the output voltage. So we are not comparing just the 2 tubes, but we also listen to the differences of the preamp at different volume settings.

All this might sound extreme and render such listening comparisons worthless. No, that's not what I want to say. I just want you to be aware what it is you are listening too. Don't jump to conclusions too quickly. Such tests will still tell you something and the more you know the preamp, speaker and amplifier circuit and their impact on the overall sound, the more you can judge the result.

Ok, here is the answer you are waiting for: I prefer the 45, but take this with all the grains of salt mentioned above.

Just be cautious when you hear about listening test results from others, or read about them on the internet. Always take it with a grain of salt. Try to understand the set up, what was used, how the comparison was done. Otherwise findings like '300B has a beatuiful midrange but somewhat loose bass, while EL34 sounds more sterile but has a tight bass' are useless.

Recently there was a question asked on a forum about power supplies: What is better a common stereo PSU, or separate PSUs for each channel? Or rather separate supplies for each stage? At first sight this seems a very valid question. But there are many possibilities how power supplies can be implemented. A common supply can have decoupling between channels and stages or not. And most importantly: what kind of circuit is meant to be powered? Some of these aspects will have a far greater impact on the overall result than the mere decision to split the supply or not. Such questions are like asking which are the best tires for a car with 4 doors. Well, it depends.

Often people get a bit frustrated when they ask such questions to me and the answer is 'it depends' or I demand more details to be able to give a good answer. Often it just is not that simple. There is no good answer to such a general question. Because it is a poor question. I don't like to give bad answers so I either try to get the question asked in a more consice way or rather don't give an answer. Unfortunately this sometimes can be interpreted as arrogance or unwillingness to help. Be assured it is not.

I hope this post helps to understand why I'm sometimes reluctant with sound descriptions. It is because they might not be valid for other environments and most importantly they would be based on my personal subjective preferences. People are seeking simple and general answers to the question how to get good sound. If it would only be that simple. The only way to get the answer for yourself is listening and trying many things. This will enable you to build up experience and understanding in how things interact and work together. It will help also to get some intuition how to find the path in the confusing jungle of many different tube types, transformers, capacitors, circuit topologies and power supplies.

Best regards


1 comment:

  1. hi thomas i am a big fan of type 45 and 2a3 and 6a3 -i find if cable loom is neutral and pre amp also -i find quad 57 speaker will sound kind and good whereas klipschhorn will show up colouration so it is a good reviewing tool for reviewing well engineered power amps. i have heard well respected 300b. and similar to 211 -the 811 and sv572 i have only heard a push pull 211and a not respected 211 uk amp so far. i would be interested in research into the px4 .
    i will have to get you some valves for your collection -i have lots of globe 45s