What do we think of this headphone amp with replaceable opamp

[Tomás Mulcahy]

Saw this on Amazon, while searching for a suitable amp for high impedance headphones.

What does the hive mind think? Is this a bog standard circuit? Better than a Focusrite Scarlett?

[Hugh Robjohns]

Hard to know, but it seems to have positive reviews, and you could return it if its no good.

Looking at the circuit board, I'd guess the output from each channel of the NE5532 drives a pair of small push-pull transistors (Q1/2 and Q3/4) to boost the power out to the phones, which is a pretty standard approach. They claim 1.3W into 32 ohms which sounds a little optimistic, but possible.

It looks like it has a DC-DC converter to provide the power rails with a wide input range, and while it can be powered from a 5V USB source, I expect it would perform better from a 12 or 15 V DC source.

The blurb is angled chinese-english and the specs are dubious... 70dB snr seems poor... so I'd buy, try and return if necessary.

Nob-sound is a bit off-putting!

[Eddy Deegan]

Nob-sound is a bit off-putting!

The beauty is in the ear of the behold... er, nevermind

[Sam Spoons]

Nob-sound is a bit off-putting!

But may be entirely appropriate

[Rockrooms]

Saw this on Amazon, while searching for a suitable amp for high impedance headphones.

What does the hive mind think? Is this a bog standard circuit? Better than a Focusrite Scarlett?

I've got one, bought for considerably less from Aliexpress. I'm not using it for critical listening, but it works far better than it should being powered off of a USB port (just part of a particular setup for quick audio testing) and performance improves if you give it a 12V supply.

I've not swapped out the NE5532s, I doubt I'd hear a difference beyond placebo effect. At £47 from Amazon it's not an impulse buy, but at £18 it was a no-brainer.

I did give it a quick run through on an Audio Precision and for the price, it's pretty good. I do like the stepped volume control, perhaps not to everyone's liking.
It might be bog standard, but it's a pretty shiny and clean bog

[Wonks]

It's actually cheaper in Ireland than the UK (both prices on Amazon), so they've probably taken the dollar price and used the same for € and £.

The LM4562 seems to be a favoured replacement op-amp. But if you get the headphone amp from Ali Express, you could afford to spend a few Euros on some alternative op amps to try.

[Hugh Robjohns]

I've not swapped out the NE5532s, I doubt I'd hear a difference beyond placebo effect.

I get a little grumpy at random chip swapping. Op-amps are not lego blocks, and good audio design is about carefully optimised circuitry around the devices in use, taking into account all relevant parameters.

The 5532 is a very transparent op-amp when used properly — just ask Doug Self! — but there are alternatives with even lower noise or different distortion characteristics (like the LM4562) which may be preferred in some applications... if the budget can take it.

But choosing one over another for an existing board really requires the schematic, an understanding of the circuit parameters, and the requirements of the opamp. Ideally test equipment should also be available to evaluate any performance changes qualitatively, as well as to confirm system stability.

[Martin Walker]

I get a little grumpy at random chip swapping. Op-amps are not lego blocks, and good audio design is about carefully optimised circuitry around the devices in use, taking into account all relevant parameters.

...Ideally test equipment should also be available to evaluate any performance changes qualitatively, as well as to confirm system stability.

I remember talking to the designer of DIY synth hardware a few years ago to query the value of a small feedback capacitor around an opamp, which I felt restricted its bandwidth rather too much.

He told me that he'd done this deliberately, because some builders randomly swapped his perfectly respectable chosen opamp with other more expensive models offering significantly higher slew rates, which resulted in them bursting into continuous ultrasonic oscillation.

So he compromised this capacitor value to avoid getting complaints from opamp swappers!

[Tomás Mulcahy]

I did give it a quick run through on an Audio Precision and for the price, it's pretty good.

Oh cool. Can you share any measurements?

[Tomás Mulcahy]

Nob-sound is a bit off-putting!

LOL. Thank you for the review!

[James Perrett]

Is this a bog standard circuit?

Probably capable of delivering more current than some (like the Behringer HA400 for example) but you don't actually need that if you are driving high impedance headphones so most of what it does is wasted in your application.

[ajay_m]

The original concept of an op amp (short for operational amplifier) was as a component in analogue computers, where the opamp allowed the implementation of integrators and differentiators, rather than primarily being a unit of gain, though this was of course also useful.
It was therefore designed from the start to deal with differential inputs and to operate down to DC.

Many of the specifications that make an opamp 'better' are not necessarily of prime importance in audio applications. For example, input offset, common mode ratio rejection (CMRR) and drift are not of great importance in single-ended AC-coupled circuits that operate from 20Hz-20KHz. But when connecting the opamp to, say, a silicon photomultiplier for a gamma ray spectrometer they might be critically important.

While other parameters such as noise and slew rate are of some importance, even a modest opamp such as the NE5532 has an open loop gain of around 1000 at 10KHz according to its datasheet, and it would be typical to use negative feedback to reduce that gain substantially, perhaps to 10 or so for headphone amplification or even less, since (as in this application) the opamp's role is not primarily to add voltage gain but to act as a low impedance current source potentially to an output stage external to the opamp (as here), but presenting a high impedance input.

Even modest opamps have noise figures such that in this role the noise contribution from the opamp is going to be very low.

If you did replace the opamp with a 'better' one then either you'd reduce the noise floor from negligible to more negligible, or you would push the frequency response out from 'more than adequate' to 'even more than adequate' but at the risk, as Hugh says, of hitting a pole in the combined complex impedances presented by the circuit board and opamp combined that could easily lead to high frequency oscillation. Some opamps are not, for example 'unity gain stable' and others are 'uncompensated' i.e don't have intrinsic rolloff capacitors, because they are intended for specific applications where the designer is responsible for managing circuit impedances across the passband. Just dropping one of these 'better' opamps into a circuit that was never designed for it, could easily worsen things dramatically.

EDIT: One reason to choose an opamp with really low noise figures might be in a summing amplifier such as a multi-channel mixer, where adding, say, 48 channels of audio from relatively low-noise sources might lead the designer to prefer opamps which keep the summed noise as low as possible. That said, apart from the intrinsic noise introduced by the signal source, such as a microphone, even passive components such as resistors do have non-negligible noise, so it's a complex design tradeoff that can be argued many ways.

[ef37a]

I would also question the (usually complete bllx) "Class A" claim?

I am with Hugh and others about the ad hoc swapping of op amps.
Instability is a possibility but the expected noise improvement may well not materialise? Yes, the 4562 is a few dB quieter than the already very quiet 5532/5534 but not always and not everywhere. Self has been mentioned and he shows that in the particular application of a moving magnet PU pre amp the NE5534 is the quietest of the bunch because its V&I noise is optimised for that particular (rather high) source impedance.

It is almost always the case that the designers of equipment knew what they were about MUCH better than some geek in a shed!

Dave.

[Tomás Mulcahy]

Is this a bog standard circuit?

Probably capable of delivering more current than some (like the Behringer HA400 for example) but you don't actually need that if you are driving high impedance headphones so most of what it does is wasted in your application.

I am not sure about this. Sonarworks recommend a headphone amp with Beyerdynamic DT880 pro 250 ohm. I've used them with a Focusrite Scarlett Solo Gen 2 and the volume had to be up full to get a useable level. Whereas the same device has no trouble with my Sennheiser HD25ii 75 ohm (if I put it up full with those I'd be deaf).

[Martin Walker]

James is correct - high current capability will allow higher volume with low impedance headphones, but for higher volumes with high impedance headphones you need larger voltage swings.

So, the fact that you can run this headphone amp with higher voltage supplies is more relevant in your case.

[Drew Stephenson]

I am not sure about this. Sonarworks recommend a headphone amp with Beyerdynamic DT880 pro 250 ohm. I've used them with a Focusrite Scarlett Solo Gen 2 and the volume had to be up full to get a useable level. Whereas the same device has no trouble with my Sennheiser HD25ii 75 ohm (if I put it up full with those I'd be deaf).

I suspect that's probably more down to the fact that the 2nd gen solo is bus powered from a type-B connection. My first gen 8i6 will happily deliver ear-damaging volumes into 250 Ohm Beyers.

[Hugh Robjohns]

Yes, high-impedance phones need high voltage power rail(s) to get decent volume, and that's not always practicable in devices powered from a low voltage supply.

This nob-sound thing appears to have a DC-DC converter, which suggests its working with a high-ish power rail, but I doubt it's as high as you'd typically find in a decent mains-powered headphone amp.

[James Perrett]

This nob-sound thing appears to have a DC-DC converter, which suggests its working with a high-ish power rail, but I doubt it's as high as you'd typically find in a decent mains-powered headphone amp.

Though there's no reason why they couldn't bump up the voltage to something like 30V with the DC-DC convertor (I'm assuming they are using a single supply because I only see one obvious inductor and two big output capacitors).

Confirmed at

https://www.audiosciencereview.com/foru ... ube.57389/

where there are measurements and more technical detail.

Whether this device is better than what you have already totally depends on the design of what you have already. Personally I would say that it would give little or no improvement over the average audio interface headphone output but I probably don't listen at really high volumes. The use of output capacitors means that low frequencies are going to be slightly reduced with standard 32 ohm headphones which may be important if you are creating dance music where bass level is important.

[Martin Walker]

The use of output capacitors means that low frequencies are going to be slightly reduced with standard 32 ohm headphones which may be important if you are creating dance music where bass level is important.

However, looking at the test results on your very welcome link James, with a 33ohm load the frequency response only measures -1dB down at 5Hz, so I doubt that's a worry in practice.

[Folderol]

A possible point of concern is that uptick in the frequency response just above 80kH - suggests marginal stability.
Indeed, why haven't they tamed the bandwidth well below that frequency?

[James Perrett]

However, looking at the test results on your very welcome link James, with a 33ohm load the frequency response only measures -1dB down at 5Hz, so I doubt that's a worry in practice.

The graph that I looked at shows 0.5dB down at 30Hz - and an 0.5dB difference is certainly audible with a good monitoring system. I'd expect a well engineered amplifier to be almost flat between 20Hz and 20kHz.

[Martin Walker]

My apologies - I was looking at the 300/150 ohm curves

[Hugh Robjohns]

I think Will is talking about this frequency response plot:

Which, after rolling off gently from 20kHz, has a rather alarming rise at 80kHz towards a resonant (?) peak at maybe 100kHz or higher — just above the measurement range.

That kind of thing is never good, and highlights the importance of looking beyond the wanted bandwidth for unwanted artefacts.

It's hard to tell, but there is a hint of an uptick above 80kHz in the noise-floor plots too, which suggest a possible self-oscillation problem, rather than a resonance problem.

[ef37a]

So, it is definitely NOT class A and it delivers less than half its advertised power but the video tells me is is a remarkable good device even at the very low price. I find Amazon have them at £44.00 Hmm, tempted but...MOT next month!

There is that problem at ~80kHz. Couple of 27 puffs?

Will mentioned "out of audible response trimming"? The argument has raged for ever whether we need amps to go much beyond say 30kHz and has never, to my knowledge ever been resolved? I don't think there is any other area of "control" engineering where systems are made to respond to signals way outside what they will experience? Examples are bumps in the road and waves in the sea.

Dave.

[James Perrett]

I find Amazon have them at £44.00 Hmm, tempted but...MOT next month!

I'm not sure that there's any point buying one Dave. I'll bet you've probably got something lying around that will work just as well.