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Little Dot D300 preamplifier measurements

dougi

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Little Dot D300 preamplifier measurements

I purchased the Little Dot D300 pre through an Australian reseller. It is a hybrid tube/opamp preamp. My use case was to add as a switch/level control for my analog sources (turntable and tuner) that go into the ADC of my RME ADI-2 pro. If it added tube "sound" then that might be interesting too. Specs and details are in that link and on face value, look respectable. It is intended to partner their D300 mono power amps. Note that my preamp manual says the power amp is a T class.

The preamp is apparently made to order and took a couple of months to arrive. I received it with no safety compliance labeling so I imagine the local labeling and testing requirements are being breached here. Consequently I opened it up to ensure it appeared at least to be electrically safe. Below is an overall photo of the layout. There are various corners cut but OK for the price. Note that access is from the bottom so everything is hanging on upside down from the top of the case for dear life! I wont make any further comments on parts quality or originality.

little dot layout 2.jpg


Online it says it uses 6J1 tubes as drivers. However when received it uses NOS Western Electric 408A tubes. It has a jumper on board to switch between the two types. I have no idea which the specs were written for or whether there is likely to be much difference between them. However, let's see what the performance is.

I measured it's performance using my RME ADI-2 pro. I chose a typical operating point and generally used +3dBu out to the preamp (1.1V RMS). Some tests were done with the volume "flat out" and others with 1.1V in/2V out of the preamp. Note input impedance of the RME is 9k ohm. The preamp seems specced into 600 ohms, although designed to partner the matching amps which have a stated input impedance of 10k ohm. Go figure! Presentation of the results limited by whatever REW can do. (eg I never get correct THD+N results for the sweeps so I turn noise off).

The first thing to note was that gain was observed flat out to be x4.5/13.1dB, not exactly the x2.2 of the spec, with 1.1V in going to 5V out! This should be able to drive almost any amp then though.

Total harmonic distortion vs output voltage level at 1 kHz
1 left thd.jpg

1 right thd.jpg


THD is very high but with a desirably(?) tubey domination by the second harmonic. A far cry from the spec of 0.01% (80dBr) at 2V rms at 1kHz. I could have driven it higher but kept the input at 1.1V RMS maximum (1dBFS below the +4dBu RME output range setting to avoid any possible 0 dBFS DAC strangeness).

Spectrum of noise and distortion for 1kHz at 1.1V in, 2V RMS out
little dot 1khz thdn left 2V.jpg

little dot 1khz thdn right 2V.jpg

The spectrum plots show a spray of mains harmonics, the supposedly desirable even decay of 1kHz harmonics, and an unweighted SNR of about 84dB unweighted across the audio band. It also indicates a channel imbalance of about 1dB with the left being stronger. Specced SNR was 99dB with no other info. We don't get there but it beats my phono stage and tuner by a 10dB or so margin so OK for the intended application.

Frequency response (using 96kHz sampling)
1 FR.jpg

Frequency response is OK with rolloff of 0.6dB at 20kHz and a tiny bit at 20 Hz. It again shows the channel imbalance. At least I can fix this a bit with the RME input trims or output balance controls.

THD versus frequency at 2V out
1 THD vs fr.jpg

THD vs frequency generally shows excellent consistency but with a hump at the mains frequency for higher harmonics

Seperation/Crosstalk
1 xtalk.jpg

Crosstalk is pretty average, about 60-70dB at 1 kHz, degrading to, at worst, 53dB at 20 Hz and 40dB at 20 kHz. Again likely OK for the vinyl and FM mediums of interest. (tuner is specced at 50dB at 1kHz).

CCIF IMD
1 ccif.jpg

The 1kHz difference tone is at ~-41dBr. Approximately the same as the 1kHz THD measurements. At least it is not significantly worse.

Multitone IMD
1 mt.jpg

Multitone IMD is around -60dBr (10 bit digital equivalent). I assume this might limit the amount of detail retrieval in some music, but perhaps still exceeds the practical dynamic range in vinyl and FM mediums.

Conclusions
Strangely, the results are a fair bit different from the specifications. Feel free to weigh in with what this might be due to. Valve types, change in design, to high an input level? (the inputs seem to go straight to the volume pot so the latter should not be the case in the THD level sweeps)
For my application of matching analog sources to the ADC of the RME it is fine. If I want to I can use different ranges on the RME input (+4, +13, +19dBu) then drive the preamp to get varying levels of "tube" distortion. From a casual listen I can detect no obvious coloration, but then again i can never hear most of the distortion in the various posts people on the forum hear have done! A cheap entry into the world of tube preamps. Happy to take any comments. I have more photos of the internals if there is interest.
 

Vini darko

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Be nice to have a fuse on that hot line and a safety cap across the power switch.
Measurments look about right for tube gear with little or no feedback. It's definitely going to have a sound.
 

restorer-john

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The multitone test appears broken. All those spectral lines stopping dead on -60dB, right up the spectrum?
 
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dougi

dougi

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Be nice to have a fuse on that hot line and a safety cap across the power switch.
Measurments look about right for tube gear with little or no feedback. It's definitely going to have a sound.
There's a fuse in the IEC connector. Will investigate a cap for the power switch!
 
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dougi

dougi

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The multitone test appears broken. All those spectral lines stopping dead on -60dB, right up the spectrum?
I was puzzled too. There are some that poke above though. Will try and remeasure at a higher sampling rate and fft resolution.
 

mononoaware

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It is intended to partner their D300 mono power amps. Note that my preamp manual says the power amp is a T class.

I am currently using just the D300 Monoblock Power Amplifiers to drive low-power speakers.

Funny thing is you have just the Pre-amplifier, and I have just the Monoblocks.
The supplier's are probably very confused that they are selling individually instead of both as intended.

Note that access is from the bottom so everything is hanging on upside down from the top of the case for dear life!

Yes I would share a photograph myself, but there are so many screws (just counted 10 top screws per monoblock) on the top that I worry about unscrewing the wrong ones and causing the internal components to "drop" onto the bottom inside if they are all fixed upside down to the top plate as you say.
Maybe this was intentional to prevent people from opening them up and messing around inside. . .
 
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dougi

dougi

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I am currently using just the D300 Monoblock Power Amplifiers to drive low-power speakers.

Funny thing is you have just the Pre-amplifier, and I have just the Monoblocks.
The supplier's are probably very confused that they are selling individually instead of both as intended.



Yes I would share a photograph myself, but there are so many screws (just counted 10 top screws per monoblock) on the top that I worry about unscrewing the wrong ones and causing the internal components to "drop" onto the bottom inside if they are all fixed upside down to the top plate as you say.
Maybe this was intentional to prevent people from opening them up and messing around inside. . .
Who knows! It would be interesting as, as far as I know, T class was that Tripath trademark that was only relatively low-powered and went under. So I assume it is actually a more general class D, but it would be nice to confirm. Your're right though, my supplier was confused as to who ordered what..
 

mononoaware

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Who knows! It would be interesting as, as far as I know, T class was that Tripath trademark that was only relatively low-powered and went under. So I assume it is actually a more general class D, but it would be nice to confirm. Your're right though, my supplier was confused as to who ordered what..

Regarding T class, I also read up a little about it and there were many short dismissive comments basically saying "Its Class-D end of story".
But as I understand it from longer more knowledgeable comments it is technically Class-D circuitry, but also adds further improvements/components to the design which reduce the amount of high-frequency switching noise (one of the compromise of Class-D performance).
So it is using the efficiency benefits of Class-D design, but also making an improvement and taking away/reducing one of the disadvantages of Class-D by reducing high frequency noise.

As I understand it Class-D design requires a high frequency noise filter to filter out all this "excess" noise and this very same filter sacrifices the high frequency performance/the filter simply cannot get rid of the noise in high frequency without affecting it's sound quality.
Maybe Class-T doesn't need a filter, or it uses one but it is much further up in frequency (beyond audible frequency band).

I am still a newbie here at ASR so I do not understand it all fully, but how I took it was Class-T is getting Class-D efficiency with closer to Class-AB performance.
 
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