[neobop]

Imagine a 20" arm with the cart mounded in the middle...has that been tried before?   

Matter of fact it was, sort of.  Back in 1972 I believe it was, a radio station DJ in Arizona had a 28" arm custom made, only it had 2 carts one at 9" and one on the end.  The cart at 9" played a record in the conventional manner.  The one on the end played a record on an entirely different platter.  The idea was to sync a Beatles record and have one table running backwards.  That way he could switch back and forth or have them going at the same time and saying all kinds of funny and strange things.

It took the DJ a surprisingly long time to realize he could do the same thing with 2 regular tables and it would work better, but since he had a custom plinth made for the arm and two SP15 tables, he kept it that way, that is until he accidently blew up the station.  The DJ was W. Coyote.  The station was KBAM.  The arm was made by Acme Audio.
neo



 

[jschwenker]

So while I wait impatiently for my tonearm hot-rodding part from the 3D printers, here's a few other blowhard thoughts just to hold onto y'all's interest:

Tonearm bearing mass limit update:  Turns out SME has a "heavy" counterweight available for the IV arms.  While I was not able to find out what the weight weighs, or what cart weight range it covers, I was able to make some assumptions.    The standard SME IV accommodates carts from 5-16g.  I assume that the "heavy" weight may cover greater mass carts, starting with some overlap, maybe say 13-XXg.  I then measured the extent of the distance from the pivots to the counterweight center at each end of the range of adjustment.  I won't lay out all the math gobble-de-gook here but at that point one has sufficient info to calculate out several unknowns, foremost the mass of the counterweights.  The "heavy" counterweight as it turns out would be nearly 100g more than the standard.  This makes me feel somewhat better about my plan to add approx 200g to my arm's bearing load, and maybe not end up too far outside the margin for error envelopes.

Axis nomenclature:  One thing one inevitably encounters in reading about these sorts of things is some variation in terms used to describe the two axes of rotation.  SME for instance, seems to use the convention where vertical arm motion is accomplished with the "horizontal" bearings.  (meaning the bearings whose axis of rotation is horizontal)  Either way of doing things is fine as long as the reader is given some chance of understanding the chosen convention in each publication.  I'll try to be more specific as well, maybe referring to "the bearings for vertical motion", etc.

Run-out groove lateral direction inputs:  Another horizontal direction "artifact" that must be accommodated by the arm/cart is the transition on to and off of the spiral down to the run-out groove.  In the worst case the transition is made basically immediately, from a low horiz velocity to a higher one.  This means in the cart's world of horiz velocity being signal amplitude - it is effectively a square wave signal.  (though one of quite low relative intensity)  Just with the well damped SME, I currently do note some sub-woofer excursions there now.  (the sub-sonic filter I'm running is a pretty generous one at 12dB/oct & 10Hz corner frequency)

In the absolute worst case a very high lateral eff mass setup could allow the stylus to jump the groove at that point and stand the chance of encountering the pressing number engravings before one gets to ramping down the volume.  If this was really much of a problem seems like folks with straight line tracking arms, some with truly high lateral eff mass, would encounter it with some regularity.

As a related point for extreme arcane knowledge credit, does anyone know if there's a spec for that change at those transitions?  In other words, how fast or slow does the mastering engineer cause the lead velocity to go from about 5 thou/rev to 200 thou/rev?  Anyone seen this?  (or is this just inviting another groaner of a tale from Neo...   )

Cheers,  John

[neobop]

While I was not able to find out what the weight weighs, or what cart weight range it covers, I was able to make some assumptions.    The standard SME IV accommodates carts from 5-16g.  I assume that the "heavy" weight may cover greater mass carts, starting with some overlap, maybe say 13-XXg.  I then measured the extent of the distance from the pivots to the counterweight center at each end of the range of adjustment.  I won't lay out all the math gobble-de-gook here but at that point one has sufficient info to calculate out several unknowns, foremost the mass of the counterweights.  The "heavy" counterweight as it turns out would be nearly 100g more than the standard.  This makes me feel somewhat better about my plan to add approx 200g to my arm's bearing load, and maybe not end up too far outside the margin for error envelopes.

100g additional seems like a lot of weight to accommodate a cart weighing 4 extra grams?

Will the bearings for horizontal motion carry the same load as those for vertical motion?  I guess you'll find out.

As a related point for extreme arcane knowledge credit, does anyone know if there's a spec for that change at those transitions?  In other words, how fast or slow does the mastering engineer cause the lead velocity to go from about 5 thou/rev to 200 thou/rev?  Anyone seen this?  (or is this just inviting another groaner of a tale from Neo...   )
Cheers,  John

There are RIAA specs for the physical dimensions of a record including the lead-in and lead-out grooves.  Sometimes those specs were ignored.  I don't think there are specs for groove velocity at the end of the lead-in.  However, by 1984 Wylie Coyote.....
neo

[jschwenker]

100g additional seems like a lot of weight to accommodate a cart weighing 4 extra grams?

Will the bearings for horizontal motion carry the same load as those for vertical motion?  I guess you'll find out.

Hi Neo,

The SME designs tend to tuck the counterweight in extra close to the pivot axis making the CW need to be extra massive for a given cart but keeping the eff mass as low as possible.  The light end of each cart range was where I forced the transition to take place.  5g and then 13g would each need to be balanced out respectively by the CW at its most inboard position.  That 8g delta at the cart worked out to require an 85g delta at the CW.  Adding in some for the actual delta cart weight around the center of the resulting ranges, one gets in round numbers to the 100g.

The bearings for the horizontal motion also carry the bearings and yoke for vertical motion - common to many designs.  So really both sets of bearings are carrying what's going on with the cart and CW masses.  Only small advantage for the weight I'll be adding is it will be riding only on the bearings for the horizontal motion direction.  Outside of short term gross failure, not sure exactly how I'll find out.  My best guess is any change in bearing play is quite likely to be too small to measure, even with the most sensitive dial indicator or maybe to hear the effects of.  As to longer term bearing failure - well at this point maybe I've loved my LPs so much over the years that I've already gotten much more than the requisite "100,000 miles" of use out of the thing.  (nyuk, nyuk)

Yers,  John

[neobop]

Which leads to your concern about groove velocity when transitioning from lead-in to music.  Why is it that air bearing arms with high lateral mass don't have this concern?   I think it's because they don't pivot in this plane, but I don't understand all the implications of pivoting in 2 plains, tracking 3 dimensions.

Is there something unique about the Moerch arm?  I read somewhere that the DP6 has non-coincidental pivots.  Could this help when increasing lateral mass?  I'm trying to correlate pivot design/type with possible candidates for this mod.  I would guess that with the SME IV bearing strength wouldn't be too much of a concern, but maybe design is? 

We read about the arm (Rega?) in Agon thread had a new skip/mistrack with only 12g added to each side of the bearing housing.  On face value adding lateral mass seems uncomfortably similar to increased friction or overdamping.
neo 

 

[BobM]

Here's a diagram of the Moerch DP6 bearing, from their site. Not sure if this helps answer your question or not, but the bearing is described as

"One internally damped, precision ball-bearing assembly for the horizontal plane of motion. Two pivoted sapphire bearings for the vertical mode of motion which can be damped also, if desired, and then adjusted to different levels of damping. "

[jschwenker]

Hi Folks,

I think the greatest change in lateral velocity occurs going from last music grooves to the lead-out groove.  I suspect that the transition is in fact gentle enough that most all arms don't really have a problem - 'cause one doesn't seem to see it thus described.  If it were a problem it might show up first in those linear trackers with high lateral mass.  Even though they do not pivot, the whole arm mass must move to the side, being resisted by its linear inertia - its effective mass in that direction.  So I was really just speculating about a problem that may not exist in anyone's world...  If enhanced lateral mass is not a problem to them, it is pretty unlikely that any of us pivoted arm types would be able to apply enough effective mass to make it a problem for us.

One of the things I see as a little unique about the Moerch arms is:  Whether uni or dual pivot, they incorporate a housing external to the bearings.  This would make it necessary to make sure any masses added to the outside for lateral eff mass are well aligned with the vertical motion direction bearing axis, such that they have little to no effect in that direction.  And indeed per their pictures that appears to be what they do.  I also long ago had an Audioquest arm that had that same "can around the outside" approach.  So that would be a small challenge with arms of that type.

Maybe best for this application are arms where the gimbal frame that moves in the lateral direction only is available for (very carefully) attaching additional things to.  The SME has this, as well as fairly rigid mechanical design and robust bearings design.  (I am hesitating to show pictures of the SME until I can show a more complete progression from raw grain to tasty beer)  What I've seen of many Rega arms suggests that they have a very similar arrangement but it looks to me to be at least as challenging there to attach weights, with enough care to not impact other necessary attributes such as bearing preloads. 

Stolen from the Agon original post here's the sentence that causes me some wooziness, with key words given capitalization by me:  "I drilled into the <weight> bases about 1/4" and PRESS-FITTED them onto the nuts that hold the arm into the bearing yoke, so they stuck out straight sideways, like sideways spikes."  If I hit my guess, those are the same nuts that fix the preload on the vertical motion direction bearings.  It would be somewhat easy in such an operation to alter the bearing preloads by accidentally changing the nut torque very slightly or worse, temporarily flexing the assembly enough to damage the bearings.  Once they are not working quite so ideally a variety of deleterious other effects could ensue, which could easily enough include an apparent change in anti-skate required or enhanced skipping.

Together we're getting the hang of this - I think.

Cheers,  John

[Grbluen]

You're a genius John! Thanks to you, I don't believe I have to go thru any of this. I believe I can simply adjust the viscosity of the fluid at the end of the tonearm and observe the reaction at the end of the lp side!

Don

Edit: It is possible to mix different viscosities of silicone, is it not?
Edit: Silicones of different viscosities can be blended to create a third viscosity.

[jschwenker]

Hi Don,

Maybe you've got a Townshend? with the fluid trough out at the cart end of the arm?  That looks like one of the coolest setups ever. 
Easy on the g-word there... I'm not quite sure what I'd be looking for if I tried to judge arm damping by looking at that exit groove reaction.  You of course want to see no jumping out of the groove and you want to see good stylus cantilever wiggle up in the very LF audio frequency band - but those two conditions could be very widely separated in terms of damping resistance applied.  (plus my eyeballs don't work all that well at audio frequencies, nor are they calibrated there)

Good luck!  John

[Grbluen]

John,
I first brought this up on another site. The members thought I might be setting myself up to fail because of problems with off center pressings. I would think that the lead out groove would be at least as severe as a slightly off-center pressing. I would think that a problem in this area would be at least as visible as an anti-skating misadjustment.

[jschwenker]

Aha!  Now that (I think) I'm understanding a little better, yes if you are looking to test out maximal lateral shocks to the arm/cart, I definitely am of the opinion that the start of that lead out groove is a greater jolt than any off center center hole will provide in the music grooves.

[roscoeiii]

Hi Don,

Maybe you've got a Townshend? with the fluid trough out at the cart end of the arm?  That looks like one of the coolest setups ever. 
Easy on the g-word there... I'm not quite sure what I'd be looking for if I tried to judge arm damping by looking at that exit groove reaction.  You of course want to see no jumping out of the groove and you want to see good stylus cantilever wiggle up in the very LF audio frequency band - but those two conditions could be very widely separated in terms of damping resistance applied.  (plus my eyeballs don't work all that well at audio frequencies, nor are they calibrated there)

Good luck!  John

Don may or may not have a Townshend, but I do. It is a great table. I went from silicon at the tonearm pivot with a Well Tempered Amadeus, to silicon at the cart end with a Rock 7. Great stuff. You can read my impressions of the two here, so that we don't get OT:

http://www.audiocircle.com/index.php?topic=105495.0

But Townshend with the DP8 is fabulous.

[Grbluen]

Roscoe, I have just one word...

...WOW!

[roscoeiii]

Roscoe, I have just one word...

...WOW!

Thanks. I got very lucky with some steals on my vinyl set-up.

[jschwenker]

Hi Folks, 
Here's the lateral effective mass enhancement mods done to my SME IV.Vi arm.  May take a little scrolling up and down but my hope is that these pics and captions manage to communicate.  Enjoy!



Fig 1.  SME IV-Vi  outboard side view of pivot zone.



Fig 2.  Inboard side view of pivot zone.



Fig 3.  Lateral Effective Mass bracket bottom view.  (Photo is flipped upside down, as if one was holding the bracket up in the air to look under it)  (Black objects are hard enough to photograph.  With 3D printing you get the added confusion of all the shiny "filamentations".)



Fig 4.  Lateral Effective Mass bracket top view.  Bracket weighs about 25g.  40 total 1/4 inch cubes of Tungsten provide approx 195g of mass at about 2.6 inches off the rotation axis.  The bracket material integrated moment, added, probably brings that up to about 205g effective mass at the 2.6" radius.  Top of the bracket was designed to stay just below the top of the yoke - which comes fairly close to the turntable dust cover.
 (Tungsten cubes are among all kinds of handy weight distribution parts made available for the Pinewood Derby car customizing market.  Yay competitive Dads!!)



Fig 5.  Bracket mounting area, prior to installation.  Double stick tape sometimes makes an excellent, stable way to mate up parts w/o much stress transmission - and if you don't use too much area, it keeps things decently reversible!  The shoulders on the underside of the bracket will contact those tape pads.  Weight forces will press down through the relatively strong yoke assembly side frames.  Once positioned, weight, balance, cushioning and stickiness will keep the bracket assembly quietly in place.  The tie down heavy duty threads will be mostly just as safety - to keep me from accidentally knocking the assembly off.



Fig 6.  Mid-installation with heavy duty threads double strung along intended paths.  I recently cruised through a very entertaining book on knots.  A great knot for this application is called a double surgeon's knot - basically a square knot with double overhand moves. 



Fig 7.  Installation complete - arm at LP lead in groove location - outboard view.  The assembly was designed to be positioned high, primarily to free up space for the cueing lever to still be accessible.  Also for that same reason AND to keep the far side away from areas where I'll be lifting the outer LP ring clamp on and off, both arms of the bracket were clocked around clockwise, parallel to the armtube.  Nominally balanced design is maintained.  Riding on the horizontal motion direction bearings ONLY, all that counts is mass amount and horizontal distance from the axis for arm lateral eff mass enhancement.  (only downside is that the bracket covers up the cool SME model number graphics on the arm tube - Oh NOOOO!!)



Fig 8.  Installation complete - arm at LP outer diameter location - inboard view. 

Played several LP sides at the end of the evening.  Amazingly enough, everything functions perfectly well...

Finally, like to give a shout out to mgalusha.  This is a bit of payback to the circles.  I benefitted from his and others posts on Hypex ncore amp building.  I do dearly love mine! 
And then, as he would say:  "Now go listen to it..."  Next week - SME lateral effective mass enhancement - audio impressions.
Cheers,  John

[BobM]

Curious about the forward/backward placement of the weights on the right/left side of the arm. Would have thought it might be better to keep the weights aligned in the same plane. Can you elaborate a little on this decision?

Thanks

[jschwenker]

Curious about the forward/backward placement of the weights on the right/left side of the arm. Would have thought it might be better to keep the weights aligned in the same plane. Can you elaborate a little on this decision?

Thanks

Hi Bob,

The weights are in the same horizontal plane and also in a single vertical plane that passes through the rotation axis.  Being carried only on the horizontal motion bearings, it does not matter if they are either at the arm height or not, or if they are aligned perpendicular or not to the arm tube or anything.  Let's imagine you are standing on a large ball bearing table with a cinderblock held out at arm's length in each hand.  (I'm not that buff but we can dream)  Now you have a friend get down on hands and knees and try to spin the table.  He would find it about the same amount of work to get the table started no matter whether you are holding one cinder block over his head or both at 90 degrees to that.  Also, if you were to knee squat way down and still hold out the blocks - start or stop work at the bearing platform would still be the same. 

In the cases of arm designs where there is no direct access to the structure that moves only in the horizontal direction, additional constraints on the mass locations would come into play as I've described a bit in previous posts.

Yers,  John

[jschwenker]

Oops - BobM:  Also as talked of in earlier posts, I was recommending that any added mass be balanced about the bearing axis - AND balance helps ensure stability of the assembly on top of its contact surfaces.

[neobop]

It's been a week and I suspect John is working out the bugs.  This post is from the bottom of page 1 of this thread - Lateral Mass 102:

Math for masses and locations versus effective mass: 

"Effective mass" of a tonearm/cartridge can be boiled down to an equivalency to a simple mass at the cart location on an otherwise massless arm.  If an imaginary tonearm/cart had 10g of mass all concentrated only at the cart, then it would have 10g effective mass.  If it ALSO had 10g of mass concentrated at a point half way along the armtube then that mass would move half as far for a given arm rotation making it half as effective, therefore it would contribute only 5g of effective mass at the cart.  The total effective mass at the cart is therefore a sum of the effective mass at the cart location of ALL of the bits of the full tonearm/cart assembly.  (the parts that move about the pivot axes anyway)  Most arm manufacturers have calculated or measured the effective mass of their products, so all one needs to do is add the cart in directly to get the assembly effective mass number.

Boiled down from other references, the arm natural frequency per mass-spring math is given by:
fn=1000/[2*pi*square root(EM*C)]
Where Frequency fn is in Hz;  eff mass, EM is in grams;  Cart compliance C is in micro-cm/dyne or microns/milli-Newton
Starting with some realistic like numbers:  EM=12(tonearm)+8(cart)=20g;  C=15(x10^-6) cm/dyne
Therefore fn= 1000/[2*pi*sqrt(20*15)]=9.2Hz  (in the range of 9-11Hz that most folks consider ideal)

Next consider how any added masses might alter EM:
dEM=M2*L2/L1
Where dEM is the change in EM in grams;  M2 is the added mass in grams;  L2 is the distance of M2 from the pivot axis;  L1 is the distance from the pivot to the cart.  And of course the EM of the new assembly would be the original arm EM+dEM.

Let's now work out an extreme case for added mass with those realistic numbers:  As done above say for a particular arm/cart I've already calculated a natural frequency of 9.2Hz - in about the preferred range.  By the argument in my first post in this thread I might want to, at the extreme, make the lateral natural frequency as low as 0.92Hz.  Per the mass square root effect, I would need to make the lateral eff mass=10/1 squared or 100x as much EM or maybe approx 2000g of dEM.  If we had an arm that was 9 inches long and were able to place masses about 2.5 inches either side of the pivot axis, the total of the two masses would need to be 2000*9/2.5=7200g or 16 lb.  Not sure the bearings in any tonearm made would come at all close to supporting that kind of a load.  The message from this example is that it might be challenging to get too much mass in the LATERAL direction.

I next looked at the Morch website pictures for the DP-8 arm.  Making very rough estimates, it looks like the side weights added up (2 each side) are maybe 1 inch dia by 1 inch long and may be about 2.5 inches off the axis.  Assuming they may be brass (at about 0.3 lb/cu in) this leads to a total mass of about 215 grams.  From the formula above, we would have a dEM of about 60 grams.  When added to the original EM of 20, the new EM is 80g.  Using the formula for fn, we can get fn=4.6Hz, or about half of the vertical plane fn.  This could be quite an improvement in LF signal tracking - as listening test reports might suggest.

In my next post I'll share some thoughts about what might be reasonable loads to subject a given tonearm to - and what I'm threatening to do to my formerly nice arm. 

Cheers,  John

I intuitively think that an extremely low resonant frequency horizontally, is a mistake, but I don't know what is optimal for a given cart.  BTW, some carts are more compliant horizontally than vertically so added mass in that plane will have an even greater affect on those.  The target freq is 4.6Hz, which might be reasonable, but the argument for .92Hz is absurd IMO.

Increasing inertia (eff mass) makes the arm less responsive or slower to respond to the tug of the cart.  This might be a good thing horizontally, to a limited extent, giving more authority and probably detail, depending.  Too much and transient response is affected, similar to overdamping with a damping trough.  The million dollar question is what is the optimal horiz resonant freq and what is the relationship to vert?  Most cart specs don't include horiz cu.  Are there test records for horiz cu?  Maybe Ortofon uses one.

Bass tends to be less demanding of transient response and benefit from stability,  but there are limits.  It will be interesting to see how John's experiment works out.
neo

[jschwenker]

Hi Folks,

Sorry for being pokey about reporting in.  And sorry it does not seem like I'll have any heroic tales of bugs to work out.    Just been enjoying the music, and, doing a few things other than eat, sleep and audio.  Tee, hee.

The exec summary is that it sounds fabulous and works great.  The SMErch arm shows no detectable downside effects and provides a measure of solidness not before reached.

In order to provide myself with a verifiable and permanent record of the transition I turned to my trusty CD recorder unit.  (ouch!  What are those ugly two letters doing in this circle? - I was interested primarily in the lower bass.  Surely, CD recording can be pretty faithful down there.)  So anyway, I recorded, headphone monitoring only - to eliminate any possible room interplay, the Jim Keltner side of the Sheffield Drum Record.  Four tracks:  Before the mass mods, without and with the horizontal damper engaged*, and after the mods, same two damper settings.  (* horiz damper screw up out of the fluid and screw down as far as can be and then back a quarter turn to ensure clearance to the bottom of fluid trough)

Listening back with headphones (the only thing I could do late at night when I finished the transition) I was pretty thoroughly unable to hear any difference.  Only the next day, with the subwoofers doing their thing was I able to start picking up a few differences.  Areas of interest were relatively isolated kick drum hits, at approx 1:10 and 4:40 and an area of intense repeated LF drumming around 5:50.  The new setup seems to provide some true solidity to the lowest of the low frequencies and in the rapid fire sequence it seems to provide more sustained definition and control.  I was able to hear these effects even more clearly in repeated listenings with the power to my main amps turned off - just to allow me to concentrate w/o distraction on those lows. 

Folks, some things to be said here: 
A) We're really talking here about enhancing reproduction, and reproduction waveform accuracy, of the REALLY low lows.  A good example, whether you call it exactly music or not, is Eric Clapton's foot stomping the stage in "Unplugged".  The tune "Walkin' Blues" - yes that sounds gorgeously palpable now... 
B) We're also dealing with a pretty good arm to begin with here - good low/no play solid bearings, low mass, dead & stiff armtube.  It could be asking for a lot to hear day/night slam-dunk determinations of success, and especially to say how well or not it might work for others.  (that said, now that I've heard it this way, not too sure I'm in a hurry to go back...  You just may have to pry my added mass assembly off my cold dead turntable!)

My bottom line recommendations for the best one can do in this area are as follows:
NUMBER  1)  PLEASE make sure your arm/cart setup has a mass-spring natural frequency that is in the appropriate range.  Get the effective mass number for your arm and mass of your cart and the compliance of your cart.  Do the math, or easier yet go to one of the web sites with decent graphs.  Some folks succeed in solidifying the sound with adding little mass items to the headshell because they're running an arm that is light** and a cartridge that is of low compliance.  Others might experience too many skipping problems if they've got the opposite - high mass and high compliance.   (** being easier to add mass, many arm makers try for low mass designs to make themselves compatible with more carts)  Myself, with center and ring clamps helping to minimize warps, I've aimed for the absolute low end of the recommendations - sitting at about 7.5Hz, in the vert direction.
Graph sites:
http://www.resfreq.com/resonancecalculator.html
http://www.ortofon.com/support/cartridgetonearm-resonance-frequency
2)  DAMP if possible.  If your arm maker offers a horizontal damper add on, or your arm allows adding fluid - do it.  (I've got some ideas on how to go about measuring the damping forces on my arm - and will report on their appropriateness, "over or underdamped", in a future post, IF you are patient enough...)
3)  ADD LATERAL effective mass, only if you have an arm that is fairly clearly up to the task.  (and only if you are sufficiently afflicted by the DIYer's disease!) 

I do agree with neo that adding enough mass to head for 1 Hz is absurd.  Part of that math exercise was to explore the limits and see how possible it would be to add too much mass.  Looks like there's lots of room.  The 2000g (4.4 lbs) effective mass we came up with there is way more than the beefiest linear tracker pictures I've seen - where all the moving mass counts directly as lateral eff mass.  Some of those folks may encounter too much lateral friction but as we've discussed, that is a different part of the equation.

In my case by the numbers I've added about 58 g lateral eff mass, making the total in that direction about 76 g with arm/cart numbers.  With my cart at about 25 cu, fn = about 3.7 Hz or about half of the vertical direction.


Key associated equipment:
Cartridge:  Shure V15vxMr  (I managed to lay in a small stable of spare styli, when I heard they were going out of production - and am looking forward to experimenting with Peter Ledermann's best replacement/upgrade here sometime)
Other end transducers:  Thiel 3.7 mains with Thiel SSB subwoofers.   (Subs sit behind mains and are integrated with a Behringer DCX2496 crossover controller.  Subs are time aligned for filter slope delay and the distance behind speakers and fine tuned in from there with metering.)
Turntable:  Old VPI HW-19 fully TNTized.  Mods to accept ring clamp.  All sits on top of a 1Hz isolation stage.  (Aw, nuts!!  There's at least a couple of other posts I need to do...)

CD (sigh...) recorder:  Marantz CDR-510

Additional albums:  I have been tremendously impressed with the following group of albums, for their excellent production principles in this area.  In all, the drum miking and dynamics are simply "to die for".
LA4,  "Just Friends",  Concord Jazz - Direct to Disk
L.A. Four,  "Going Home",  Ai Music - Direct to Disk
Laurindo Almeida,  "New Directions",  Crystal Clear Records - Direct to Disk
"The LA4", Concord Jazz Classic

Cheers,  John