Michael Fidler Spartan 20 Phono Stage Review

[NoxPlague]

I don't think it's a good idea to include these, as they do confuse people a bit, and can also be conflated with the total system capacitance which would lead to poor results.

The small capacitor that yields most of the input capacitance of 120pF is 100pF, and has to be connected after the RF stop resistor of 100 ohms or so to prevent radio-frequency interference in the VHF band from getting onto the input where it would be envelope-detected as audible hash. If it were to be routed through a switch, the extra inductance would make it a less effective shunt.

A lot of the preamplifiers advertised as 100pF are in fact 120pF or more, as you've got to factor in the PCB track-to-ground capacitance, op-amp input, and so on...

Adding more capacitance can be done quite easily, but adds complexity to the design, and only really affects the response above 10kHz by a few dB at most. Most modern cartridges work best with about 200-300pF, so I don't really see a need for it if we have 120pF from the preamp and then another 80-150pF from the tone-arm/cabling. Adding it would just increase the size/complexity of the board, enclosure, generate long e-mail exchanges about setting it, give people the opportunity of setting too much of it and complaining etc.

If it's not necessary, then it's best avoided. Keep the phono-stage simple so it can be plugged in and work straight away with the vast majority of devices, with the loading question completely out of mind.

The same goes for MC loading, which doesn't even really affect the frequency response until it drops below the coil resistance or less. You need a low enough resistance value that lets the resistor dissipate the resonance of the cartridge inductor against the load capacitance. The load capacitor selected for its highest value such that the appropriate resistance is about ten times the coil resistance to keep insertion loss under 1dB. If you design the input amplifier correctly, then the thermal noise of coil resistance will dominate if it increases beyond 30 ohms, such that the noise penalty from insertion loss is mitigated for higher coil resistance.

If you let the user select the MC load resistance value, then they'll either increase insertion loss and harm the signal-to-noise ratio of the device, or create an underdamped RF peak at the coil-inductance/load-capacitance resonance point that will be detected on the input and lead to complaints. Changing the loading alters the level at the input slightly, but perhaps enough for people to convince themselves they have a 'preferred' amount that will probably provoke trouble... Most preamps on the market use pretty brutal (and rather noisy) resistive series RF stopper networks on the input amplifier, preventing the resonance from getting through, at a massive noise penalty which is only acceptable because the market has such low expectations with the intuitive (but incorrect) explanation that the level is lower and therefore noise is correspondingly higher.

While selectable loading is very easy to implement, it is not necessary in my opinion and actually highly undesirable from a customer-facing point of view.

I need to hide my Spartan 20 in a cabinet. I need to run a 4 foot cable from my turntable to the Spartan phono preamp.

Currently I'm using the https://snakeoilsound.com/products/pair-of-copperhead-rca-cables-4-in-15-ft Snakeoil Tiapan cables based on the Sommer cable, I believe it's this cable: https://www.sommercable.com/en-us/M...-2-x-0-50-mm2-FRNC-OE-6-50-mm-black/200-0151F which states

Capac. cond./cond. per 1 m (audio) [pF]: 80 Capacity wire/wire at 1 ft. (audio) [pF]: 24.384 Capac. cond./shield. per 1 m (audio) [pF]: 145 Capacity wire/electic screen at 1 ft. (audio) [pF]: 44.196

So my understanding is that I'm adding about 170pf of capacitance with my cable to the 120pf of the Spartan 20 = ~300 pf load on my Audio Technica VM760SLC.

Is my math and thinking correct?

How critical is lowering the capacitance? Snakeoil sells the German-made ultra-low-capacitance Sommer Carbokab 225 for about 4x the price of the Taipan (already own):

Capac. cond./cond. per 1 m (audio) [pF]:	46
Capacity wire/wire at 1 ft. (audio) [pF]:	14.0208
Capac. cond./shield. per 1 m (audio) [pF]:	90
Capacity wire/electic screen at 1 ft. (audio) [pF]:	27.432

thoughts?

 

[Michael Fidler]

I need to hide my Spartan 20 in a cabinet. I need to run a 4 foot cable from my turntable to the Spartan phono preamp.

Currently I'm using the https://snakeoilsound.com/products/pair-of-copperhead-rca-cables-4-in-15-ft Snakeoil Tiapan cables based on the Sommer cable, I believe it's this cable: https://www.sommercable.com/en-us/M...-2-x-0-50-mm2-FRNC-OE-6-50-mm-black/200-0151F which states

Capac. cond./cond. per 1 m (audio) [pF]: 80 Capacity wire/wire at 1 ft. (audio) [pF]: 24.384 Capac. cond./shield. per 1 m (audio) [pF]: 145 Capacity wire/electic screen at 1 ft. (audio) [pF]: 44.196

So my understanding is that I'm adding about 170pf of capacitance with my cable to the 120pf of the Spartan 20 = ~300 pf load on my Audio Technica VM760SLC.

Is my math and thinking correct?

How critical is lowering the capacitance? Snakeoil sells the German-made ultra-low-capacitance Sommer Carbokab 225 for about 4x the price of the Taipan (already own):

Capac. cond./cond. per 1 m (audio) [pF]:	46
Capacity wire/wire at 1 ft. (audio) [pF]:	14.0208
Capac. cond./shield. per 1 m (audio) [pF]:	90
Capacity wire/electic screen at 1 ft. (audio) [pF]:	27.432

thoughts?

You could try RG59 coax with RCA connectors soldered on the ends for 60pF/m. I use RG58C, which is a bit higher around 80pF, or URM76 at 100pF. Thick ground conductors also help a lot with ground-current conversion into common-mode interference.

I wouldn't recommend going over 250pF for the AT cartridges (mine are seeing about 220pF in use).

Having said all of that, 120cm is quite a long length for any turntable cable. I would recommend 60cm or less, as the length on the turntable side is usually far more troublesome, but this may not fit in with your situation.

 

[NoxPlague]

You could try RG59 coax with RCA connectors soldered on the ends for 60pF/m. I use RG58C, which is a bit higher around 80pF, or URM76 at 100pF. Thick ground conductors also help a lot with ground-current conversion into common-mode interference.

I wouldn't recommend going over 250pF for the AT cartridges (mine are seeing about 220pF in use).

Having said all of that, 120cm is quite a long length for any turntable cable. I would recommend 60cm or less, as the length on the turntable side is usually far more troublesome, but this may not fit in with your situation.

Is there an easy way to measure the actual pF capacitance?

For example would I be able to use a multimeter at the output on the Spartan 20 to measure the pF?

 

[Michael Fidler]

Is there an easy way to measure the actual pF capacitance?

For example would I be able to use a multimeter at the output on the Spartan 20 to measure the pF?

I wouldn't connect a capacitance meter to the S20's input, as this would probably overload it and therefore give a false reading (or potentially even cause damage to the IC).

I'd use a standard capacitance meter to measure the cable capacitance when disconnected.

 

[lini]

(...) Is my math and thinking correct? (...)

Most likely not, 'cause the cables with two inner conductors plus shield are typically cinfigured in a pseudo-balanced fashion, so that the actual capacitance will be roundabout the sum of the conductor-to-conductor and the conductor-to-shield capacitance (be the shield connected on one end only or on both ends). So if one thrives for low cabling capacitance in combination with a regular (= non-balanced) phono input, a regular, low capacitance coax cable type will pretty much always be the better choice.

And if you can solder, it's not difficult to make a good phono cable yourself. If you'd prefer a twin cable type, you could for example choose the Mogami W2947 and terminate that with the priceworthy, but decent Amphenol ACPR RCA plugs.

Greetings from Munich!

Manfred / lini

 

[MattHooper]

First congratulations on what seems to be an excellent product!

The same goes for MC loading, which doesn't even really affect the frequency response until it drops below the coil resistance or less. You need a low enough resistance value that lets the resistor dissipate the resonance of the cartridge inductor against the load capacitance. The load capacitor selected for its highest value such that the appropriate resistance is about ten times the coil resistance to keep insertion loss under 1dB. If you design the input amplifier correctly, then the thermal noise of coil resistance will dominate if it increases beyond 30 ohms, such that the noise penalty from insertion loss is mitigated for higher coil resistance.

If you let the user select the MC load resistance value, then they'll either increase insertion loss and harm the signal-to-noise ratio of the device, or create an underdamped RF peak at the coil-inductance/load-capacitance resonance point that will be detected on the input and lead to complaints. Changing the loading alters the level at the input slightly, but perhaps enough for people to convince themselves they have a 'preferred' amount that will probably provoke trouble... Most preamps on the market use pretty brutal (and rather noisy) resistive series RF stopper networks on the input amplifier, preventing the resonance from getting through, at a massive noise penalty which is only acceptable because the market has such low expectations with the intuitive (but incorrect) explanation that the level is lower and therefore noise is correspondingly higher

I have a question about this.

My turntable uses a Benz Micro Ebony L cartridge:

BENZ MICRO EBONY L – Benz Micro North America

benzmicro-northamerica.com

My pre-amps are a Conrad Johnson premier 16 LS2 tube preamp, and a Benchmark LA4 preamplifier.

I use a JE Audio HP10 phono amp:

https://www.je-audio.com/wd/ni/FILE-0000000211/20240111-204055_001.pdf

(it probably measures worse than your product )

It offers five different impedance settings from 10 ohms to 500 ohms. (MC)

When I play with the impedance settings I perceive the sound changing - the higher the impedance setting the tighter and brighter the sound, the lower the impedance setting seems to become more lush and rolled off sounding and slightly less focussed sounding, less focussed bass.

Are you saying in the above that audible differences with the different impedance settings is implausible?

Or have I misunderstood?

Thanks.

 

[Michael Fidler]

First congratulations on what seems to be an excellent product!



I have a question about this.

My turntable uses a Benz Micro Ebony L cartridge:

BENZ MICRO EBONY L – Benz Micro North America

benzmicro-northamerica.com

My pre-amps are a Conrad Johnson premier 16 LS2 tube preamp, and a Benchmark LA4 preamplifier.

I use a JE Audio HP10 phono amp:

https://www.je-audio.com/wd/ni/FILE-0000000211/20240111-204055_001.pdf

(it probably measures worse than your product )

It offers five different impedance settings from 10 ohms to 500 ohms. (MC)

When I play with the impedance settings I perceive the sound changing - the higher the impedance setting the tighter and brighter the sound, the lower the impedance setting seems to become more lush and rolled off sounding and slightly less focussed sounding, less focussed bass.

Are you saying in the above that audible differences with the different impedance settings is implausible?

Or have I misunderstood?

Thanks.

I've addressed this here...

From the customer's perspective: we could say that it's entirely a placebo that lets them adjust in what they believe to be right and thus confirm their own presuppositions about it. Or they're making small changes to the level at the input (but not frequency response or distortion), with moderate adjustments to the load resistance and perceiving those level changes as sounding 'better', 'more direct', 'harder', 'softer', etc.

Changing the loading on a 12 ohm cartridge coil from 200 ohms to 60 ohms would increase insertion loss by 1dB, enough to cause a perceived change in sound quality that may not be interpreted as the level change that it is. In extreme cases, where the preamp has very poor overload margin (if passive RIAA equalisation is used, for instance), then increasing the loading to bring the nominal level at the input down could also mitigate preamp overload, leading to improvements in sound quality.

Like you say, 100-150 ohms works perfectly for almost all LOMC cartridges (a few very-low-output outliers are best dealt with using step-up transformers into MM inputs IMO), and I would agree with you 100%. 100 ohms is enough resistance to keep insertion loss below 1dB with a typical 12 ohm coil (Audio Technica), low enough to absorb RF peaks to prevent resonant detection, and allows a sufficiently large load capacitor to shunt away HF/VHF/UHF radio-frequency interference.

 

[MattHooper]

I've addressed this here...

Thanks, but I’d already read that. The reason I was asking for clarity is because… you’re an engineer and I’m not so everything you’re saying of course makes perfect sense to you, but it’s full of so much engineering speak I’m having trouble interpreting it.

So are the type of impressions I described implausible?

Thanks

 

[Michael Fidler]

Thanks, but I’d already read that. The reason I was asking for clarity is because… you’re an engineer and I’m not so everything you’re saying of course makes perfect sense to you, but it’s full of so much engineering speak I’m having trouble interpreting it.

So are the type of impressions I described implausible?

Thanks

The sound is changing because you are making slight level changes as a result of the insertion loss varying, with over 3dB for a 5 ohm cartridge brought down from 100 to 10 ohms. This has much the same effect as making minor adjustments to the volume control on your amplifier, with the major disadvantage of the level being reduced at a highly noise-sensitive point in the signal chain where the voltage is miniscule.

 

[MattHooper]

The sound is changing because you are making slight level changes as a result of the insertion loss varying, with over 3dB for a 5 ohm cartridge brought down from 100 to 10 ohms. This has much the same effect as making minor adjustments to the volume control on your amplifier, with the major disadvantage of the level being reduced at a highly noise-sensitive point in the signal chain where the voltage is miniscule.

OK, thanks.

So as I understand it, you’re saying the only changes that would be possible amount to volume changes.

Interesting because the differences I perceive do not mirror those of volume changes - brighter, tighter images, tighter bass - they are not the same changes as when I change the volume with my remote. And the changes seem distinct irrespective of the volume I’m playing back.

I feel fairly confident in what I’m hearing, but of course it could be a bias effect. It would be interesting to do a blind test. But if I understand, right switching the impedance could cause volume changes that might be interpreted as changes in the sound rather than just volume changes. Therefore, any blind test should be able to change impedance without changing the volume.

I’m not able to do a blind test on this at this time.

But if somebody we’re going to do that, would the changes in volume/output when changing impedance settings show up with a voltmeter measuring at the speaker terminals?

If so, then it would be a question of whether the person doing the switching could compensate with the preamplifier control.

(just thinking off the top of my head)

 

[Michael Fidler]

OK, thanks.

So as I understand it, you’re saying the only changes that would be possible amount to volume changes.

Interesting because the differences I perceive do not mirror those of volume changes - brighter, tighter images, tighter bass - they are not the same changes as when I change the volume with my remote. And the changes seem distinct irrespective of the volume I’m playing back.

I feel fairly confident in what I’m hearing, but of course it could be a bias effect. It would be interesting to do a blind test. But if I understand, right switching the impedance could cause volume changes that might be interpreted as changes in the sound rather than just volume changes. Therefore, any blind test should be able to change impedance without changing the volume.

I’m not able to do a blind test on this at this time.

But if somebody we’re going to do that, would the changes in volume/output when changing impedance settings show up with a voltmeter measuring at the speaker terminals?

If so, then it would be a question of whether the person doing the switching could compensate with the preamplifier control.

(just thinking off the top of my head)

Yes, at the input only the signal level would be changing, with frequency response and distortion being much the same.

However, depending on the performance of the signal chain, reducing the operating level may also alter sound quality. The preamp you reference uses a valve section and specs distortion rather vaguely at 0.05%. I referenced a case where the level change could result in a change in overload behaviour due to the relative distortion thresholds moving further down the signal chain and in the preamplifier section. This would not be the case for a low-distortion signal path, but all bets are off if there is significant distortion generated at normal listening levels.

You could set the reference level using a test disc with a constant 1kHz tone + measuring the voltage at the speaker terminals.

 

[Soniclife]

Interesting because the differences I perceive do not mirror those of volume changes - brighter, tighter images, tighter bass - they are not the same changes as when I change the volume with my remote. And the changes seem distinct irrespective of the volume I’m playing back.

The differences you report are exactly the type reported with imperfect level matching, that then goes away when the levels are correctly matched.

If you want to see if something else is at play record the output at different loadings, and compare with deltawave. It will match the levels for you.

 

[MaxwellsEq]

Interesting because the differences I perceive do not mirror those of volume changes - brighter, tighter images, tighter bass - they are not the same changes as when I change the volume with my remote. And the changes seem distinct irrespective of the volume I’m playing back.

This is probably THE most important point about perception. Tiny changes in level, which we are totally unable to perceive as volume changes can have a significant impact on how we hear things.

This has been common knowledge since at least the 70s. The first time I think this was captured in an article involving Bob Carver doing amplifier comparison tests. The article is now behind a login, but this a quote from it:

"When we were finally able to get the output levels of the two power amplifiers exactly matched, there was absolutely no audible difference when switching between them while listening to either white noise or music. During the adjustments of the amplifiers, it was demonstrated dramatically that minute differences in volume level (sound quantity) that are too subtle to be heard as such are interpreted by the ear as "obvious" differences in sound quality. Everyone was startled by the effect — everyone, that is, except Larry Klein, who had touched upon the phenomenon some time ago in his Audio Questions and Answers column."

 

[MattHooper]

The differences you report are exactly the type reported with imperfect level matching, that then goes away when the levels are correctly matched.

Yes I’m aware that in direct comparisons even slight differences in level matching can lead to a variety of different impressions.
And that can include even differences and sound staging and all that.

So this is why I’m saying “ it seems” in terms of referencing my perception.

But when I say I find my perception of the different characteristics to be “ interesting” with regard to the explanation of “ slight changes in volume” is that… it does not seem to be based on slight changes of volume.

By that I mean:

If I was arriving at my impressions strictly through what I perceive when keeping the same volume control on my pre-amplifier and only changing the impedance settings, then attributing the changes I perceived to tiny volume changes could make sense.

So let’s take that as a given.

However, I’m not drawing that perception simply on keeping the volume constant on the preamplifier. By that I mean the Sonic changes I’m talking about SEEM to be there whatever the volume I’m using on the pre-amplifier. Turning the volume up or down does not make one impedance setting sound like the other. The characteristics on one impedance seem to be maintained as I turn the volume up and down.

That’s why I say it’s “ interesting” to me.

If there is a bias effect involved - and of course that could be the case - it doesn’t seem to be fully explained by teeny changes in volume due to impedance settings.

I think they would have to be something else involved if it’s a bias effect - something like an expectation effect or whatever.

If you want to see if something else is at play record the output at different loadings, and compare with deltawave. It will match the levels for you.

Thanks. Normally, this is something that I would probably like to investigate on my system, but I’m unable to at this point.

 

[sq225917]

Mc loading with measurements from jim hagerman.

Hagerman Technology LLC: Cartridge Loading