[dlaloum]

Hi Folks,

Did a brief comparison of the Revox vs the JVC yesterday.... was wondering about some anomalies and wanted to make sure my measured results were good...
(my results were good, and identical between the tables...)

In the process I took another look at the bottom end frequencies....

The same cartridge running in both arms (Shure V15HRP with N97xE-SAS mounted) - responded identically in the high frequencies to loading changes... (what I was measuring). It also responded as expected in terms of arm/compliance resonance and arm/compliance damping.

What I noted that I had not noticed before was that the boost the I get, centered on 300Hz (with an octave in either direction - so 150Hz to 600Hz) is greater on the JVC.
This makes me suspect that this boost is therefore an arm/headshell/cartridge resonance.... the Revox has it, but limited to around 1db boost.... the JVC has it with a peak of around 2db - both using the same test record.

Is this common?
Are there particular tweaks/mods to control it?

This clearly is a substantial contributor to the Revox sounding somewhat more neutral than the JVC....

It does appear to vary a little based on cartridge/headshell used on the JVC - with some combos the boost extends further... sometimes as far as 100Hz to 900hz. (I have not done much experimenting on the Revox, as changing cartridges on it is quite a painful process... which is why I purchased the JVC).

Another frequency anomaly (colouration) that contributes to the vaunted "vinyl sound" ?  - especially if this is common to many/most S-Arms...

bye for now

David

[neobop]

Hi David,
You're talking about a difference of 1dB?

Is there a correlation of headshell properties (like mass) that extend the frequency?

Are you using the damping on the JVC?

Taking a quick guess, I'd say the mass difference is bumping the upper bass and the damping is extending the frequency range rather than adding to the bump. 1dB isn't much.
neo

[dlaloum]

Hi Neo,

No that's not it...

I investigated the LF arm resonance... the Revox is a magnetically stabilised unipivot linear tracker - so it has some damping... but is not really a damped arm - Resonance is at 10.8Hz - and quite evident even with the the brush down.
On the JVC resonance was at just under 9hz, and with the combination of arm damping and the damping brush in place - was completely controlled.

When running the LF sweeps to identify the resonance, no 300Hz(ish) harmonics were to be seen... (on either arm)

I am suspecting the arm/headshell interface on the JVC as the frequency appears to coincide to what a number of places have mentioned as being the frequency where that flexing occurs/shows up.

I am guessing of course! - But the difference in frequency response between the two tables is quite clear - even if it is only 1db's worth....

1db is particularly noticeable when you are optimising the setup for a cartridge and attempting to achieve  +/-0.75db 100Hz to 15kHz (my self appointed preliminary target...)

The other difference I noticed was that there was higher level of bass between 20Hz and 50Hz on the Revox than on the JVC - this may perhaps be due to the damping on the JVC.... ie it may be an illusion of more bass - pleasant nevertheless, but it may imply reduced control. - and this does indicate an impact where the albeit mild damping of the Revox is extending the LF resonance - but that effect peters out by the time it gets to 50Hz.... and the boost on both TT's appears to start from around 150Hz. - So I doubt that that is the issue.

On the JVC bass levels between 20Hz and 70Hz are noticeably lower than on the revox - so I doubt the JVC's frequencies above 100Hz are at all influenced by the LF resonance.

At 1.3g VTF the Shure/SAS is easily tracking through HFN +16db track (3rd tracking test) - but clearly fails the +18db test - although there is no needle jumping (the Revox arm would never allow that.... Teutonic control and all that - you understand !   )

I did see a slight 300Hz tonearm resonance mentioned in relation to the Revox arm - it was one of the issues tackled by Empire in their modifications to the Revox TT - they released their version of the Revox under the Empire brandname.... the Delrin arm was replaced with solid brass (much high mass of course!) - and extensive other mods were involved.... so the 300Hz hump on the Revox does not surprise me - but the fact that it is 1db lower than on the JVC... that I find surprising.... and points to an issue of some sort.... (I think).
I cannot imagine how I would go about controlling a resonance in the Revox arm - all 2" of it...

But I figure there must be things that people have successfully tried on such a standard item as a Japanese S-arm.... it might be as simple as using the right headshell, or putting some of my favourite modelling clay in the join between arm and headshell? or....?

bye for now

David

[BaMorin]

Hi,  are you considering arm tube resonates? pivot bearing resonates? arm to plinth feedback?  Rega, on their newest tables finally decided to add a "superstructure" between the main bearing in the plinth to the arm mount on the table to get a continual energy loop, something AR did in 1961.

[dlaloum]

 
Honestly I am open to suggestions....

It is not a "noise" in that it would come up on my various tests as a seperate signal...

it is definitely a response to stylus activity - identified using pink noise tracks to measure frequency response. - Sound was being monitored through headphones, so as to avoid any potential feedback issues. So it is a resonance.

Question is where is it resonating? How to identify the source?

I have considered adding some plasticine / modelling clay to the arm mounting (between arm and plinth, and in hollows of the molding) - plasticine is a very effective damper for frequencies of around 300Hz....

But if at all possible I would like to work out where the resonance could be coming from first - then I could focus on that area!

I have used the van-alstine recommended dot of modelling clay on the arm tube to reduce resonances - did that some months back - clearly has not affected this particular resonance much if at all...

What exactly do you mean by continual energy loop... (and how would I achieve that, assuming it is possible with this plinth/arm...)
The arm is a magnetic servo damped arm.... but I know nothing about arms, never having taken one appart before.... and being wary of taking apart a complex example like this one as a first attempt!


open to suggestions

David

[BaMorin]

Hi David, if you can send me an e-mail address, I can send you a link to a site written by a ME from MSU (Michigan State University) that has a pretty easy read and follow of how energy transfers through boundarys, various vibrational models of 1st, 2nd, 3rd orders, etc. His site can't be published in an open forum due to intellectual property rights, but it's free for personal consumption.  Just don't pay any attention to how "fouriers" are formed and discuss that on the other site you visit    .  you'll be persued by the "hounds of hell" 

Marc

BaMorin@AOL.com

[BaMorin]

pursued .......with a U, not an E........too much coffee

[dlaloum]

I've come to accept typos... partly because when typing on my lounge room system, I use a wireless keyboard that sometimes drops characters.

And as I type fairly quickly (a consequence of a touch typing course at high school many years ago..... - all the girls took the typing course.... wasn't a hard choice metalwork class with the "blokes" or typing with all the "chicks"  .... but as it works out, it was one of the most usefull classes I ever took!) I sometimes overlook the typos...

[neobop]

David,
I wasn't talking about the LF resonance necessarily, although that might be a factor. Effective mass is really moment of inertia and that has an effect on the sound. I don't know specifically if it would account for a 1dB difference at that bump. Maybe it is flexing type resonance at 300Hz.

I was surprised that you measured LF resonance at 10.8Hz on the Revox. I plugged in some numbers on the VE resonance calc and came up with arm mass of -3.33g.    Whoops, when they say down under they're not kidding.
It must be that other stylus, has a cu of 15 to give you a 2.45g arm mass? Doesn't sound right either. Maybe those magnets are stronger than you think.
 

The difference in bass performance is also interesting. I wonder how much damping on the JVC is effecting that. I think the inevitable conclusion is that you need a med mass arm, as a control.
neo

 

[dlaloum]

Mmmm that had not occurred to me.....

Test was done using the Technics SH90S p-mount headshell = 8.5g, + V15HRP = 6g.... total 14.5g...

Not that far off from the OEM headshell at 11g + V15V @ 5g = 16g

In terms of mass, I should be right in the design sweet spot of that arm !?!

I had not done the sums on the Revox LF resonance I must admit - although I was surprised to find it so high up! - The SAS stylus has compliance of 22cu
Using arm mass = 4g and cartridge mass = 8g the VE resonance tool comes up with 10Hz.... seems pretty close to the mark.

Seems about right at 8to 9Hz on the JVC arm too....

[*Scotty*]

dlaloum,the broad band resonance you have measured with the JVC arm is a low Q resonance and I believe it will be very hard to eliminate or even lower much.
If the arm tube is hollow and has no internal damping material present you might consider drilling two small holes in the arm tube. One hole just ahead of the collar at head-shell end and one at the other end as close to bearings as possible.
 The holes need to be sized to accommodate the size nozzle that is on a can of polyurethane foam insulation. You may be able to neck it down a little bit by inserting a doubled ended 1/8inch hose barb used for connecting together the vinyl tubing used to supply air to aquariums. A little squirt of foam sufficient to fill the arm tube and would damp the tube itself. 
 The downside is that now the wiring in the arm tube will be encased in foam and not air, this may negatively impact the sound. Obviously this is a one way street and irreversible once done.
 I suspect that in fact the resonance you are measuring is a by-product of the damping applied to the arms bearing system, if this is the case the system is pretty well damped and will be hard to alter or improve upon. If you were to succeed in lowering the resonance by 1.25dB the new curve or bump would probably conservatively cover 75Hz to 1200 Hz. 
 This will be an interesting experiment at the very least and I can't say that injecting foam into the arm tube will alter the resonant behavior of the arm/bearing system at all.
Good luck with this most interesting challenge.
Scotty

[dlaloum]

I am hesitant to do an irreversible mod.... especially when parts for this table are likely to be "unobtainium".

I have seen arm "tape wrapping" as another damping mod...

To keep mass down, I have seen people use teflon plumbers tape and use it to "bandage" the arm...

Has anyone got experience of what frequencies these mods affect?

Also if the resonance is due to flexure in the HS/Arm joint, then an arm treatment would have no effect....

 

[*Scotty*]

This may sound crazy but you might try making a home-brew constrained layer damping tape.
 Take oven strength heavy-duty aluminum foil and heavy duty double faced tape and make sandwich out of it. You now have a damping tape that could be made 1/4in wide and wrapped around the tonearm. You could even make it four layers thick if necessary and still add very little mass to the system. Double faced foam mounting tape could also be tried as alternative to the regular double face tape. Self adhesive Copper foil used for alarm traces on windows could also be used, it would have a higher mass than the aluminum foil. A final approach would be to substitute the copper foil used in copper foil inductors, this might be the most effective combination of materials even though it will be more difficult to wrap around the arm.
Scotty

[dlaloum]

I quite like the concept...

But the question remains - what frequencies do these solutions damp?

Not much point to it in my case if it's peak damping performance is at 5kHz (for example..)

Especially as I run high compliance cartridges, and lowering mass is one of my main tweaks/improvements! - so if the F damped is outside the desired zone, I would be adding mass to no benefit.... going backwards!
Not an issue for a mid or low compliance MC.... - but I have lots of top notch high compliance MM/MI cartridges and only 2 mid compliance MC's...

[*Scotty*]

I agree if the tonearm goes clunk clunk when you tap on it instead of ting ting the tape isn't going to help much. And it may lower the tone arm low frequency resonance into an undesirable region.
Scotty

[neobop]

I recommend against irreversibly putting spray foam in your arm tube. The only benefit is structural - rigidity. As Scotty suggested, it would also act as a dielectric. I don't think that kind of resonance has much to do with it anyway. If it did, the foam might just shift the frequency of the resonance. You could try heat shrink or various wraps. I think they tend to be more effective with microphonics and would deaden the overall sound. Origin Live cuts slots in the bottom of a Rega arm tube for resonance control. That seems to me the best approach, depending on the arm tube. John TCG knows of some material for applying dots. I'm just getting around to checking it out.

I suspect the cart has at least a 1dB rise in that region and the mass + damping of the JVC brings it up to 2dB. I've noticed that in undamped arms (which I have a few), a higher cu cart in a heavy arm often sounds dull, thick and sluggish. This is irrespective of any LF resonance. There might be a slight rise in the upper bass/mid region. IMO People who (successfully?) use high cu carts in heavy arms tend to go to great lengths to compensate for inappropriate MOI (mass). Electrical damping is usually reduced with very high value loads and VTA/SRA is set way high. I don't agree with this approach.

This is something rarely discussed but MOI is what effective mass is all about. Why does effective mass decrease when weight is added to the counterweight and it is moved closer to the pivot? I would say that usually a cart with 22cu might not be the best match with a 17 or 18g arm. I'm not talking about LF resonance. (Yours seems surprisingly high for a 22cu cart) I mean FR response and transient response. Also, the weaker performance of the JVC on the deep bass could be due to the high mass restricting the greater stylus/cantilever movements required. Do you usually take measurements on the Revox or other arms?  That kind of comparison would be really interesting and would shed some light on this.

BTW, I plugged the wrong cart in the VE eff mass calc. I thought you might find it funny.
neo

[dlaloum]

I tend to agree with you with regards to MOI...

I purchased the JVC so I would have an exchangeable headshell table to use for easy measurement and experimentation with differing cartridges.

The Revox is a royal PITA to change cartridges on - definitely NOT something to be done casually or regularly.

Due to our predilection for high cu MM's I also focused on obtaining lighter headshells - the original OEM HS for the arm is 11g.... I have a number of headshells of different types in the 6g to 9g range.
With some of the lighter cartridges (OM with weight removed) mounted on my lightest HS, I also had to track down some lighter Counterweights  so the arm would balance... - Which allows me to select whichever counterweight will balance at the closest point to the pivot.

End result is I normally shave around 4g to 6g from the standard OEM effective mass of the arm.
It still is NOT a low mass arm but I can get it down to low-mid mass.

The Revox arm is definitely ultra low mass at 4g.... and even with p-mounts which are relatively porky 6g cartridges and then the p-mount adapter, the end result is still well within low mass territory.

The same cartridge in the Revox also shows a slight upper-bass/mid hump - but now the peak is only 1db rather than the 2b peak I get in the JVC.

Also my own testing with various electrical loads tends to lead towards lower resistance rather than higher in many cases... so I don't think I am underdamping.... - and so far I have stuck with "factory spec" VTA/SRA - top of cartridge/arm level when playing.

Mass vs Bass - you think? - I'm not so sure about that equation... The servo damping controls resonance motion of the arm in that LF range...
And theoretically speaking... assuming a flat record.... the arm should be immobile - allowing the needle/cantilever to move in the groove from a stable overhead support.
The only problem I see with excess mass is that the arm responds more slowly to vertical (warp) motion - requiring that motion to be absorbed by the cantilever suspension.
I may be missing something - but I am not getting the conceptual link between increased mass and decreased bass...

Most of my measurements have been done using the JVC as that is why I purchased it... but I have been planning to get back into the P-mount cartridges more heavily - and part of that will involve some back to back comparisons between the two tables...which will be interesting.

I also found the vintage reference to the Revox mods done by Empire - the flaw they were working on was a 30Hz resonance not a 300Hz resonance.... which may explain the slight increased bass I saw on the Revox - will have to measure again and take a look at that.
Among other major mods, Empire also fitted the Revox with beefier damping magnets - to cater for the Empire/Benz MC cartridges...
This seems to imply to me that the level of damping applied in the system is probably best suited to lighter cartridges (5g and down?) - and in p-mount mode, the Revox will be running effectively with 8g cartridges... There are just so many variables with vinyl...

The 30Hz resonance may be less evident or even absent with an appropriately light cartridge.... (Ultra400 might be perfect, or the V15VSAS, or the OM ) - where the damping can work more easily on a lower mass system. But the process of changing cartridges....

In any case I had been starting to get a handle on the various dips and humps in standard MM frequency response measurements - the low-mid hump was my outstanding unexplained item... and the variance between the two arms, has shown me that it is at least partly Arm/TT related, as opposed to cartridge/cantilever.

It is by no means an offensive effect - although inaccurate from an absolute perspective - the Turntable sounds very good.

I think I will try the least intrusive damping mods first.... suggestions that sound promising:
1) Teflon/Shrink Wrap the arm tube
2) Damp connection point btwn Headshell and armtube (will experiment with some plasticine to replace the normal washer.....

As I am measuring the results with a real time spectrum analyser, I will know very quickly whether it is working or not.


Another thought: The 1db boost centered on 300Hz on the Revox could potentially be related to the p-mount adapter, and its flex/vibration... This is something I can test by using an identical P-mount and 1/2" mount cartridge pair with the same stylus.... and seeing how it differs.... another exploration....


bye for now

David

[neobop]

Mass vs Bass - you think? - I'm not so sure about that equation... The servo damping controls resonance motion of the arm in that LF range...
And theoretically speaking... assuming a flat record.... the arm should be immobile - allowing the needle/cantilever to move in the groove from a stable overhead support.
The only problem I see with excess mass is that the arm responds more slowly to vertical (warp) motion - requiring that motion to be absorbed by the cantilever suspension.
I may be missing something - but I am not getting the conceptual link between increased mass and decreased bass...

David

"The servo damping controls resonance motion of the arm in that LF range..."

How does it do that?

I know it's counter-intuitive but I think it can be true. We tend to associate mass with solidity and more authoritative bass, why? I've noticed that sometimes reducing VTF increases bass. Thinking of mass or MOI in a vertical mode only, is a mistake IMO. Tracking is 3 dimensional and then some. Grooves are cut at 45 degrees so that makes a sphere of tracking - 3 dimensions. The arm and cart are constantly moving toward the spindle by being dragged by the stylus. We think of the cart/arm relationship as stationary to conceptualize, but in fact it is never stationary while in play. Records are rarely flat and that vertical motion is actually angular because the arm is constantly moving inward.

If you look at it in 2 dimensions - as a stylus tracing horizontal groove modulations, then bass requires long excursions. I think this would be optimised with lower MOI, not higher. That is, until you reach an under-damped situation with too little mass for a given cu. 
neo

[dlaloum]

"The servo damping controls resonance motion of the arm in that LF range..."
How does it do that?

Sorry - I assumed you knew about the JVC Servo arm(s)...

Here is the ad flyer diagram:


And the tonearm in the metal:



It actively senses resonance electro-magnetically and damps it the same way. (Similar to the Sony Biotracer, and the Denon Servo Damped arms...) - the Servo damping is I believe limited to frequencies below 100Hz... I vaguely recall reading somewhere that the later QL-Y77/66/55 version extended the effect to a higher frequency...

The level (aggressiveness?) of the damping is controlled by a dial labelled "Q" - the manual suggest Q=VTF for a starting point.

It has two sets of servo's - horizontal and vertical. (they are also used for queuing and moving the arm)

bye for now

David

[neobop]

Tell you the truth, I never owned an arm w/resonance control.

"It actively senses resonance electro-magnetically and damps it the same way. (Similar to the Sony Biotracer, and the Denon Servo Damped arms...) - the Servo damping is I believe limited to frequencies below 100Hz..."

If resonance is sensed, it can be from any plane or direction, not just vertical. This mechanism being active could be responsible for reduced output below 50Hz.  Actually, I think it would be a prime candidate for that. I would also guess that it can't be entirely defeated by turning down the Q control. I can't read Japanese and the translator didn't recognise it, so I don't really have a good idea how it works. Never the less, simple logic would suggest that, even without another arm with a little less mass as a control. Either the low bass on the Revox is exaggerated, or it is deficient on the JVC, or both.

The affect of the damping is below 100Hz, but damping usually limits amplitude of resonance and spreads it out over a much wider band of frequencies. It could be responsible for an extra 1dB rise at that hump at 300Hz. It's only an octave and a half above that 100Hz cut-off limit for the damping.

The servo damping is a very useful feature on a somewhat upscale deck like this. It allows the use of a wider range of carts. It will also help prevent woofer pumping in less than ideal physical set-ups. Most popular music has little content below 50Hz anyway. Bass controls were often turned up ('70s?) and for most users satisfaction levels were high.
neo