Preamps: Hot Take for Headphone Enthusiasts

Sovrce Gear
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Disclaimer: I’m only talking about headphone chains. Speaker people read if you want to, but it’s a totally different game.

That being said…

I have found that a Preamp is CRITICAL, and deserves as much attention as anything else in your chain.

Why? I’ll present 3 common situations.

  1. The signal coming out of your source is too hot. A 2V source going into an AMPLIFIER is just not optimal for the specific requirements of headphones. Your prized headphone amp at 9 on the pot is not ideal, to put in kindly.

If you have high quality volume control on your source, or your amp properly accepts 2V+, such that your amp can operate in its sweet spot, this doesn’t apply to you.

  1. Your destination to source impedance ratio is likely too low. 10:1 is OK. 100:1 or more is better. Below 10:1 is unacceptable.

  2. You’re going unbalanced into your amp. Balanced signals are a better design. If your source does not output a balanced signal (many don’t) and your amp does take a balanced signal, you need to make that happen.

Corollary 1: Active Preamps are inferior to the best Passives (read below) because they introduce another active stage. You risk losing as much as you gain.

Corollary 2: Passive Transformer preamps (aka TVCs) are superior to all others because a proper design will get you 1, 2 and 3, passively, as a matter of course. They are the right tool.

Looking at the HiFi preamp market, I cannot believe how many offerings cannot fix 1,2,3… and want to charge you 5 grand to not do so… AND they introduce an active stage :expressionless:

Don’t get me wrong, designers are clever. They can and will minimize the drawbacks of an active Pre. My point is that it is not optimal and often way more costly than it needs to be.

I’m not here to recommend specific devices either; I’m not a salesman, and part of the satisfaction is finding your own components. You want high quality transformers that will allow you to adjust gain DOWN. You do not want just a box with a resistor/potentiometer.

Adding the right Pre to my system has added girth and impact to bass, tonal focus and density across the frequency spectrum, and the ability to listen at low volumes with all elements from softest to loudest perfectly clear and separate. Tutti elements with all instruments at once fill the soundstage with force.

One more thing. If you’re happy with what you got, and it doesn’t follow the above, then great. Don’t let me rain on your parade. This is just food for thought. Do you.

In closing, the right Pre will allow you to grab system synergy out of thin air if the specs on your gear mismatch as outlined above. IMO,YMMV and all that BS.

What has been your experience with TVCs? What is your source? I’m exclusively using CD or SACD, mostly the latter on my Denon player.

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I have no experience with pre-amplifiers, but now that I have a DAC with a hot output, I’m beginning to agree with your view. Though I’m unsure yet what the difference in sound is with my headphone amp at 9 o’clock instead of say 12-13 o’clock.
Using the single ended out of the dac does give me more playroom in my volume. So trying this instead of using the balanced out, which is more hot.
Further thoughts I have is to use one of the open baffle headphones out there (RAAL/mysphere) who would give me theoretically even more play on the volume dial to get sound at good level.
The guy from SW1X has a video on what makes a good pre-amplifier on his site (scroll down on page link).
SW1X PRE III Line Pre-Amplifier - SW1X Audio Design
Using the SW1X VDT with downloaded music on internal HD myself, which ive found to be more to my liking.

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It’s not so different for speaker people.

I’ve no experience with TVC type pres but given your 3 common issue situations, if you’re not in a situation where those issues exist in the chain that you’re trying to put together, there may be more benefit with active solutions that with passives. Just a situation of chaining the right gear together to maximize synergy and if you’ve done that, then you’re in a better situation.

I will add that a preamp, is something that you may want to dabble into in stages and in a 2-CH environment where amps are more common they’re needed so it would be unwise to jump intot he high end right at the beginning if you’re still trying to put your chain together.

That said though, I didn’t reach my system’s full potential, or at least where I’ve been happiest with it until I put preamp in line and every pre I’ve purchased that has been a dollar value step up has brought me a noticeable step up in performance as well. Everybody’s situation is different and what I’m trying to say in not as many words is that a preamp has the potential for great rewards but it will require the most amount of matching of synergy to extract the most out of it.

So do your research when you’re at that point.

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10:1 is plenty for most amplifiers.
Input impedance is “important” because you get an implicit divider between your output impedance and input impedance. Low input impedances mean your source has to provide more current, and it overlays the shape of the output impedance curve onto the FR of the amplifier.
At 10:1 you impose 9% of that, at 100:1 it’s <1%
Though generally for well behaved sources and amplifiers it’s close to flat across the audible range anyway, and 9% of ~0 is pretty close the the same as 1% of 0.

You also unfortunately can’t tell how the impedances will interact with just a single measurement. I can pretty trivially design an amplifier that has high input impedance at say 1KHz where it’s generally measured but varies pretty dramatically at frequency extremes, you’d hope most commercial amplifiers don’t do that, but they can.

IMO Some of the best sounding amps don’t have balanced circuits, balanced cabling isn’t going to help very much in those cases.

On transformer based pre’s, they can be very good, they definitely have a sound, in some cases they are more colored than active solutions.
The best preamps in speaker systems I’ve heard have all been active, I spent a lot of time dicking around with passive solutions many years ago under the impression that they should be technically “better”. I consider all of that wasted time, good active solutions have always sounded better to me.
But YMMV.

One last thing on the volume point.
Because most headphone amps have the volume Pot at the start of the circuit, And your just amplifying the signal after that Pot, how hot the input is really only dictates which bit of the Pot your going through. Analog pots do have sweet spots so it can help to attenuate before it enters the amp, but for devices with stepped attenuators, it really doesn’t matter if your at 9 O’clock or 3 O’clock on the dial.
Some amps do put the Pot between the stages, it’s arguably the best place to put it, and in that case you do get different distortion profiles from the first stage by changing the input level, but that leaves the amp open to clipping for higher level inputs and but it’s VERY unusual in headphone amps. You do see it in some active pre amps though.

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I feel as though preamps have always been sort of a focus when it comes to traditional speaker setups, but is for sure something more neglected within the headphone space. Personally I think it would really depend on your chain and your equipment. Most headphone amps are designed to take a direct signal from a dac or another source closer to line level, and have built in stages internally. The quality of those stages may drastically differ from design to design, but I wouldn’t say a preamp is needed or desirable in every headphone chain out there. Most headphone amp designers aren’t really designing for one to be placed in front of their amp and aren’t designing their input stages to fully take advantage of one either. Not to say it’s bad to put one in or that it won’t help, but I don’t think I’d say it’s inherently necessary either. If you can bypass a mediocre volume pot in a headphone amp or elsewhere in the chain it’s worthwhile to do so if you can, but you just want to make sure you’re actually getting sonic improvement in doing so because it’s not guaranteed

IMO the hard part about adding a preamp into a headphone chain is just finding the right time and place to do it, because from my experience you end up best off with a preamp on the same level/caliber as your amp and dac, otherwise it’s just inserting a bottleneck into your chain in most cases imo. There are times where even a more simplistic/basic preamp can help if you’ve got too hot of a source or a big impedance problem, but I feel as though those are mostly edge cases rather than the norm. It just ends up being harder to justify than in traditional 2channel because you may find that putting your money elsewhere in the chain would offer more sound per dollar benefit vs a speaker chain where you’re running power amps that you have to use a preamp in already and rely more on the quality of said preamp for their end performance.

At least based on my experience I would absolutely not say that passive preamps are the end all be all, far from it. I will absolutely say that I’d rather have a cheap passive preamp over a cheap active preamp, since I feel as though a good active preamp is generally more expensive than a quality passive unit (an example would be if I had under 1k to spend on a preamp, I would go with a passive design personally for a headphone amp). But once you reach into some of the nicer active preamps, I haven’t really found the desire to go back to any of the passives I’ve tried. It all really depends on the rest of the system and your preferences as well though, there’s no one size fits all preamp imo.

TVC are really cool and can be worth it for sure, although imo the biggest downside to a TVC is that the quality of it basically entirely banks on the transformer, and good transformers aren’t cheap either lol. Also, I would say they’re actually one of the most colored sounding preamps as well, you really hear that transformer. That’s not a bad thing, but a lot of the TVC preamps I’ve heard imparted a lot of their own individual character into the chain, and I think that’s important to make sure to account for when picking one.

I would also be pretty hesitant to say balanced is always better, it will really depend on the design of the equipment at hand. But I feel like that’s another discussion entirely lol. I do think for the most part that on internally balanced equipment, the balanced output is always going to be better than a single ended output on a balanced dac/amp/preamp. But a single ended design can be just as good if not better sonically than a balanced one. I would only really see the argument if you’re running very very long cable runs, or need it more as a problem solver in terms of ground loops or interference (which balanced cabling don’t always fix anyways for one reason or another). All depends though

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I like this point a lot, because it speaks to some ideas I have kicking about. I’ve been exploring amplifier circuits a lot both as stand alone things and as part of mixer/summing mixer set ups and something I think happens in hifi is we tend to conflate gain and attenuation into one thing - not least because that is how the buttons and dials on amplifiers tend to work. I hope I am not telling too many people something they already know but…

For anyone not familiar, generally in audio design, gain is defined as adding power and always requires an active amplifier circuit. Attenuation is then the reduction of volume, usually by placing one or more resistors in the path.

As @Polygonhell says, most common hifi amplifiers place attenuation before gain. You reduce the incoming signal before reamplifying it with a gain circuit.

signal in → attenuation → voltage amp → power amp → signal out

A lot of amplifiers are two stage, one for increasing the voltage and another to increase the power. There are good reasons for this to do with signal integrity. Essentially this means that all amps of this type are a passive preamp followed by a powered gain stage.

An advantage of placing the attenuation like so:

signal in → voltage amp → attenuation → power amp → signal out

is that you create an impedance buffer which means that your attenuation pot is always working with a fixed incoming impedance. As most modern amps are using an opamp for the voltage amplification stage this is super trivial to achieve - it is something opamps are very much suited for. However it does make the power amp stage more complex as now that has to be able to deal with a variable impedance on the incoming signal.

Summing or line mixers for this reason are often as follows:

2 + signals in → 2 + voltage amps → 2 + attenuations → 2 + impedance buffers → power amp → signal out

What we mean when we say a source is too hot then is that the incoming signal is too loud for the attenuation at the front end of the amplifier. The very simplest way to deal with this is literally well chosen resistors in the signal path immediately before the volume pot.

There is potentially one significant drawback to placing a passive variable resistor before an amplifier. Depending on the design, if the amplifier does not have at least some kind of buffer, you could easily have two variable resistors in series which can sometimes do funky things to your nice clean signal.

For these reasons, if I were to use a passive preamp I would always prefer to choose a ladder stepped attenuator. Fewest parts in the signal chain, fixed impedance input for the amplifier.

Final point, for anyone who wanted to know what gain tends to mean in hifi terms, it refers usually to changing the route of the signal through the voltage amplification stage in order to change how much this is amplified. This can be in fixed amounts or could also be done with a second pot.

Also like, guys, this is the only place I ever get to geek out about this stuff, you have no idea how much I appreciate that :smiley:

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This is a bit off topic but.

For most designs, by adding another divider in front of the existing pot, you will reduce the input impedance of the amp it’s in front of.
In most preamp circuits, the input impedance is largely a function of the pot, by stacking them you effectively put some portion of them in parallel with each other which results in a lower impedance as seen from the output of the source.
In a lot of cases it doesn’t matter all that much but it can.

As an example in my speaker system I used various Passive Preamps between my CD player and Power Amp for a long time, before going active, and that was a case that mattered, because the CD player had a tube output buffer, which probably put in the the 2K+ output impedance ballpark, and while the Power amp was tube based and therefore probably had input impedance in the 100’s of K/MegaOhm range, by putting the passive pre in front of it, I was dropping that to whatever the total impedance of the Pot was (probably 20-50K).

There was a dramatic improvement when I swapped to an active pre.

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yep I use preamps in all 3 of my headphone chains I also use DDCs in all 3 as well for my sources. Streamer → DDC → DAC → Pre → Tube + SS amps for all 3. It’s worked well for me thus far.

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Thanks for your perspectives, everyone.

I think on this topic they are different. I can easily imagine an additional active stage on the way up to speaker level being beneficial. Not so cut and dry with Headphones.

Unfortunately, yes. I have found lots to like and I think others should thoughtfully explore what’s possible here.

…

I want that incremental benefit. I’ve also noticed a pattern where I get better bass response and more realism at a higher ratio.

I’d bet you’re right, and I’ll edit the language in my original post… if you love an unbalanced amp, then you need to cater to what it does.

I will say, and this ties into those incremental gains above, that if balanced inputs are available then that would be preferred.

Another benefit of transformers is that they electrically isolate source from amplifier. Combine that with balanced cables and it might help explain why this is some of the quietest listening I have ever done.

Just like incorporating higher resolution music into your experience, all this “negligible benefit” stuff has to add up. For me it does.

…

Let’s take my specific example. The output of my SACD player gets dropped -30 db(!), from 2 to 0.06V, before entering my amp. That is a requirement of my system. If I have a resistor doing this job, my precious Beethoven turns into heat. By having a transformer do the job, I conserve almost all power by simply turning voltage to current. The amp will do what it will, but is it not better to give a transformed signal of equal power rather than a weakened signal?

I think there are real performance consequences here.

I think you have to decide on components you won’t change, because you like them, and then bring them to their full potential. I’m enjoying that. I’m also finding things I happen to love have high ceilings.

Endgame is a live experience of the best music I can find in physical format, through headphones. First came the amp/headphone pairing, then source, finally the preamp to marry them.

Now I can blow all my money on out of print music.

I think it’s hard to assume much at all about what’s going to be plugged into a headphone amp. Will the headphones be 20 or 200 ohms? How much current will they require at that impedance? Is the source going to be fixed 2V, 4V, or will the operator have continuous control (a pot)? I do think for all amps there is an ideal signal to feed it with a given set of headphones, and for me preamps can help bridge the gap between what your source is and what your amp wants. But like you say it depends on what you already got.

In your headphone chains?

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I would agree with that, I would consider a preamp (for a headphone chain) more of a last step to put the finishing touches on a system and extract the most potential performance, just have to make sure to get the rest of the chain up to snuff and make sure that the system is ready to take advantage of a good pre and it will actually benefit from one

I don’t personally like using the endgame word but sure lol, it’s absolutely satisfying to reach a setup that you simply just enjoy without thinking about anything else

Not really imo, most manufacturers expect a line level signal from a dac or cdp, maybe output from a phono preamp. Not as often a varied signal from a preamp.

Those are all primary factors taken into account when designing a headphone amp, designers will create an amp that either fits their use case with what they use and prefer, or design it to reach its best performance with a specific category or type of equipment, which is why we see so much variety in amplifier design, there are many many ways things can be done, and typically there’s reason behind doing so. I guess I can’t say too much though since I don’t design them myself lol. But I do know most amps, especially those in the high end, are not designed to be one size fits all, and are meant to have great synergy with some pieces but not others, because otherwise you end up with jack of some master of none which typically isn’t what most discerning listeners or enthusiasts are after. And the ways they accomplish it can sometimes be unorthodox or counterintuitive lol

There absolutely is an ideal sort of signal for each amps input, but I don’t know if I would say a preamp is mostly a solution to that either. It really all depends, I don’t like to generalize, I think it would really come down to a case by case basis

In both headphone and speaker chains. I did use passive preamps of all types (pot in a box, resistor/stepped, LDR, TVC, etc), but eventually I found that the only preamps I kept around for sonic benefit ended up being active, and I kept passives more around for hard and fast problem solving rather than inclusion for best sonic performance. But that was my chain, my preferences, and my equipment so ymmv. I do think that passives can fit into some chains very well that I might prefer over an active solution, but I didn’t find that as common for most of what I’ve listened to.

I think you’re conflating specs with sound, he’s speaking in terms of harmonic distortion rather than subjective quality, and I wouldn’t tie those two together personally. I think one of the things you may not be considering might be the introduction of transformers into your chain via your TVC which will color the sound in the way you describe imo, but that’s not a bad thing at all, assuming this is with a TVC. It’s really not easy to do apples to apples comparison since each circuit will be different, with different synergy between what you test

But everything is give and take, if we’re talking pure measurements for harmonic distortion, I would imagine the signal running through a transformer would measure less “clean” than without especially if you’re saturating it, you may be reducing THD by having more theoretically ideal impedance but introducing it back in the chain by the means of the transformers in the tvc itself lol (along with a transformer having reduced linearity and higher susceptibility to noise, and the noticable sonic differences that occur running anything through a transformer).

There’s a reason why most manufacturers with a focus on actual sound quality aren’t always making attempts to gain the best ratios or best specifications because tradeoffs have to be made elsewhere in order to achieve that end up reducing subjective performance in their own way. There is a reason why designing for ideal measured/theoretical performance doesn’t always/typically pan out to best results in practice lol, if it did, this hobby would be very very different. This goes beyond what I feel qualified to speak on though lol

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Good question! I’m learning things to answer properly which is cool.

I think it depends on what you mean by performance consequences? Anything you are going to do to attenuate the signal is going to potentially change the signal and will produce heat as a by-product. Either in the attenuator or in the amplifier immediately afterwards.

In most (non tube) amplifiers, transistors, either discrete or as part of an op-amp, are used to increase the signal. Transistors work with very low currents so in order to have control over the signal gain without distortion they need to have theoretically infinite input impedance. This is achieved in practice by using feedback loops and other techniques to use a bias current that cancels the input current out.

As you increase the input current so the bias current increases. This will lead to more heat build up in the components, particularly the transistors. If the current gets too high for the bias capabilities it will leak into the transistors and start to create distortion.

Most of the time none of this matters too much as the current on an audio signal is very low. The voltage is the signal we are concerned with preserving and the current is more the means of moving it along. Usually if you use a transformer to attenuate you would also want it to impedance match the output.

Your suggestion is to keep the same power as the input. 30dB is 1 watt of power and if you want to attenuate that without any change in overall power then in order to go from 2V to 0.06V you are going from 0.5A to 16.7A which is a significant jump and definitely into the realm of needing to know if the following amplifier is able to deal with it.

So I think it is not that a passive transformer attenuator is inherently bad, it is just more complex. It has a greater effect on a following amplifier than a resistor based attenuator and getting it right needs more work and costs significantly more.

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Just quoting this specifically because it was the last post in the thread.

But why does anyone care.
While there are tolerances here and things do change with temperature, transistor curves, resistances, it’s not like cooler is better (within reason).
As for dropping a Line level signal (lets call it 2V) into a 100K input impedance device even if you drop 100% of that signal into the resistor, you’d be looking at having to dissipate (2*2)/100000 or 0.04mW, which likely wouldn’t even raise the temperature of the resistor measurably.

The big difference with Transformers Volume controls or Autoformers for that matter, is they change the demand on the device as you change the volume level.
If the device has a class A output section your just moving the heat into the device, for Class B output sections it will reduce the devices power draw, but at the levels of power were looking at it’s irrelevant, the device should be designed to handle any current draw it’s likely to see with a LOT of headroom.

The downside to a TVC/AutoformerVC is that transformers do not have flat response with respect to frequency, it’s why tube amp output transformers get expensive, your paying for the materials to minimize the high and low frequency roll off.
For a volume control with minimal current draw, a design can probably have pretty good bandwidth without getting very exotic, but it will have some impact even if it’s outside the normal audio range.

All the TVC’s I’ve heard have some impact on the sound, as I stated in my original post they are IME often more colored than active preamps. Some of them sounded very good, certainly better than most of the resistor based passives I’ve tried in my system. But I’ve always preferred good active pre’s.

To the point I’ve considered building an amp with a Transformer based volume control, instead of a Pot. But your kind of stuck having to wind your own and I’d need to be in the right mood to do the research to do that right.

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Unless it pushes a component outside tolerances I don’t think anyone minds. Most of the time it doesn’t matter but this example of preserving all the power rather than just transforming the voltage signal made it relevant to the point :slight_smile: Putting over 16A into a device is a pretty significant amount given that a lot of amplifier components will be specced for 1/4 to 1/2 a watt maximum.

Edit to add: I’ve probably gotten too caught up in the technical bits and pieces. I should probably just leave it be as I am not sure it needs the amount of thought I’ve been giving it today lol. Learnt a lot though :smiley:

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I like that approach and appreciate the perspective. Better to let curiosity lead than to pretend you know everything.

Yeah good point.

I’m just wishing there was more TVC discussion in general, partly because of that (good) “color”.

This is one of only 2 threads where they have been discussed at all…

It’s a shame because I’ve found so much happiness by adding one… And my system didn’t sound like shit before.

They solve problems and are a super elegant thing to add to your chain, assuming you can get it to work out.

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I didn’t really pay attention to it until this thread but the Vinnie Rossi LIO, one of the preamp options for it is a TVC. I’ve been trying to sell mine for a while and reading through this thread is giving me urges. lol

Interesting concept for the LIO as it has a VR, tube and TVC options. I understand now why he offered the three distinct options for the pre stage.

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They’re still a bit niche, and of course not a lot of lower priced options.
They can be a very good solution, but like anything else a lot depends on what components it’s stuck between.

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