All Acoustat panels can give

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thanks for the info.....I well be putting a pr 1+1 togather with there own interfaces...
I have also moved away from the C-mod crossover.....back to the first crossover less setup with the 220 mf cap........more output..an better topend......
As with a lot of things I just bout into that the c-mod had to be better....with the none c-mod..setup even the Medallion sound better......
but I am looking for input on the 121 2-B....is this a real Acoustat setup.?..was it ever used?......or is it a fake??
pic one is the 121 2-A....2 pic....is so called 2-B....I never seen this used...
 

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The Medallion used transformers with a different winding ratio for the "HF" transformer.

The 200ufd moves the HF "output" from the interface LOWER in frequency - more midbass.
No effect on the highs.

In all cases these capacitors are *best* done with polypropylene caps. So the 50ufd value, or thereabouts is easier (and less expensive) to achieve. The apparent "depth" in the sound field vs. frequency of the sound is brought back together in a single place with the polypropylene caps vs. the electrolytic + mylar bypass caps that are stock.

As far as the 121-2B? Dunno how many were made this way - but these do not show the variable HF control. Many have removed that and replaced with a fixed value of their choosing, but I am skeptical of the benefit, but it may be there.

There are other threads here which cover the various wiring schemes and mods - a search would likely turn them up. You can always comment on an old thread and it will "pop back up"?

_-_-
 
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''The Medallion used transformers with a different winding ratio for the "HF" transformer.''
Yes Bear......I saw were you are the only one ....thanks for that......I have found that took a look tested to see what was the diff in the 2 types of tran.....I agree with you... the HF tran is-dose sound diff than the frist none Medallion .....bass tran not so much..
this is not to say the C-mod dose not sound good.....it dose...but I have found as I would think anyone that gave it a try.....the none c-mod with better parts can give better sound...I see the c-mod as a eqing nom series type crossover.....I see the parallel 6ohm res an 220 cap a none crossover...that give more output... better sound.... some may say....it sounds raw....in the way a passive preamp well sound ...no eqing....were a tube are SS preamp will add eqing..... .looking at the 121 B mod.....looks like the best of all........but putting in series.....19 ohms.........I just don't know if Acoustat did this crossover...................all just one mans take on this
 
Yes those are original drawings.

If you look at the series resistor, going to the HF transformer, and make believe there is no cap across it, you can see it acts to drop the level of the HF section to some value below that of a direct connection. It also has an effect on the HF, since the load looks like a capacitor... but the effect of the caps is to "roll in" HF energy. If you took a fixed resistor and bypassed it with the caps, the effect would be to permit some LF energy, but raise the HF energy to a the equivalent of no resistor, but with the roll off down to a shelf (the resistor) at 6dB/oct below some frequency (based on the value of the cap).

But with a mid value of HF level control set, the cap, plus the rheostat (variable resistor) have the effect of a boost to HF bypassing around the resistance portion that the cap is connected across (the wiper and one side), but limited by a flat level reduction - that being set by the value of the remaining series resistance (no cap around it).

All the way up, there is little difference in level between just a cap and no resistor at all. Similarly, there is not much difference (except some LF gets through) in level with a cap fully bypassing any value resistor, since the cap "looks" like a straight piece of wire at the frequencies of interest.

The resistor may serve other functions, like making the load to the amp look more reasonable, or reducing the Q of the series filter that is formed by a cap and inductor in series...

The Medallion and the use of the smaller caps i believe was to extend the HF response above 10kHz in a better fashion than the original, and to get some midbass out of the system.

My best results with various Acoustat incarnations has been with ~175watt triode amps - by far the most musical I have heard, even better than with my own Symphony No.1 amplifier, which is quite excellent. I think if you buy or build one of this power class (811a tubes or similar are a good way to reach this power) you will be extremely happy with the speakers and not need to mess much with the interface boxes! :D
 
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What got me going on this was I got these fresh 2+2s all stock.....with the121 2a 6ohm rheostat (variable resistor) an the 220 mf-10mf poly.1mf poly....sat it aside...
I then ran just one of the stock HF tran secondary only..... to all 4 panels...with only a 1ohm 20watt res on the pos primary input....this worket great played as louds wishet a...sound vary good...no real lows but......
then I put in same place.... the Medallion HF tran......better mid.....better -more topend....to me....then I added in the C-mod crossover. to them...lost some topend..
mid sounded fat like I had added a EQ.......sounded like it was not needed for the best sound.......
Are you saying with... maybe 1ohm res on the primary side to the HF tran in 121 interface.... an the right amp would give better sound than any of the c-mod or other crossover types.....
Also I must say these old 2+2s I pic up ...have the best PVC wire I have ever had with any Acoustats panels..............I can pull the AC plug on the bias ......an these panels well play at a good output for 45 min..........I have never had this befor.....an these thing well ROCK.............just blow the doors off the soundlab M1 with there new interfaces....an these 121 interfaces are all stock the parts in the bias were replacet.....only mod ... I cut the neg input end to the 1ohm bass res lose..... so only one end was held. at the input end of the brown wire input....an ran the neg input to the freehanging end......much more better deeper bass.......thanks for your input
 
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Here is a pic....this is great mod....ezey an if you feel it not better put it right back........this 1 ohm 20watt res on the neg input.....sees all the bass mid an high end....Current passing on to the tran coming in from the amp....This res needs to Exapand & Contracts & Vibrate from sound,...when passing through it.... the stock monting bracket...stops... most of this....an it give fake smeared, bloted...one note sounding bass....also get more air around the res......this is best bass I have ever got out of any ESL ......
Have fun get better sound.......
 

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This pic come from one of Roy E moded boxes.......I just don't like to lose any output due to Absorption with silicon .....I would think it (Could) make it sound darker sounding than it needs to be.....but better than stock for sure.....
 

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I do not think the first circuit you posted is/was a factory setup from Acoustat.
The circuit was drawn and posted by Williams Audio to show what he found when opening up some Acoustats he had purchased that had obviously been worked on.
http://www.diyaudio.com/forums/planars-exotics/192497-acoustat-3-mk121c-2.html#post2644953

I can confirm that the first schematic is NOT an official Acoustat schematic.

The second schematic is an official Acoustat schematic (albeit a partial one, as it lacks the bias supply). I recognize my own hand printing, from before the days of CAD.
 
Thanks bolserst........great info

heres the rub......
this is from bolsertst posting...........
"The purpose of placing the 15ohm resistor across the primary of the high frequency transformer is to provide a constant load for the crossover capacitors so they can better filter out low frequencies from reaching the transformer."

"A second function of this resistor is to keep the high voltage output from the low frequency transformer from feeding back into the secondary winding of the high frequency transformer thru the 0.01uF capacitors and saturating the core. With this resistor in place across the primary of the HF transformer you can drive the Acoustats much harder without midrange harshness setting in."


with the c-mod cap crossover added an the 16ohm res shnt......to my ears theres top end loss.....it may vary well be....the low frequency transformer feeding back into the secondary winding of the high frequency transformer thru the 0.01uF capacitors .......because the low frc tran is feed full rang......this feed back is not all harshness or saturation....it could-must add high frc extensonion .....an with out the shnt res must
add to a more open sound.......this is not to say that the EQing done by the c-mod dose not sound good.....if you get the right cap....one you like...an res...it can give a what I well call....a ear candy type sound...
but with the 121 2a type setup.....I hear more topend ...flater sounding frc response.......Medallion high frc tran are other.....
As for the ear candy sound most like ....me too.....I have my tube-krell-preamps an amps......to add any an all EQing......I could ever need...
 

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I don't see anything that can "feedback".

What does happen with the C mod is that as you vary that level control the impedance at HF seen by an amplfier will vary. This may or may not effect the HF output, in addition to the change in level sent to the HF xfmr.

The load seen by the xover caps now has a resistive component, and also this serves to damp the primary of the transformer to some degree. Once could argue one way or the other about the best approach here, and it's not entirely out of the question (wonder if anyone has tried it - I haven't) to wire the level control the other way, with the wiper going to the xfmr and the caps feeding a fixed resistance. But of course that resistance still varies as you sweep the control since the xfmr primary is shunting. But for most practical positions of the control that would look like an L pad reversed in direction from the stock circuit: ie, caps ---> resistor in series - shunt resistor to ground ---> xfmr.

For an actual practical setting of the control the results might be identical for all purposes...

...you can get HF boost with the C mod, by shunting the wiper to the top of the control with a cap - if you select that cap carefully it will provide HF boost above some freq all the time (as if the control was all the way up but only above this caps F3) - a series resistance could shelve that, if needed. But the inflection points (F3) will vary with impedance, and the resistance selected - and so the setting of the control. But it might solve some issues with HF rolloff - IF you think you have any.
 
Here a pic of the 121 2a stock 2+2 interface..... setup with inputs for the high an low tran......for bi-wireing-bi-amping.....done by the owner....
As with all the Acoustat interfaces theres only one fuse....so he ran the bass tran from the fuse......this left the high frc tran.....with out a fuse...
This bi-amp type setup relly dose not work....... the way most think...you don't get any real gains .....that using 2 diff amps should offer..
Because... when two tran are used with one bias setup for ESL speakers...the two tran center tap must be tied togather..... so that the panels bias feed well get a full charge......any trans ran like this are not realy separated.....there still seen as one tran....
An if one the two input are reversed.....it can kill one of the two trans.....
So I set out to set this back to the stock.....one input for both tran...
But when I did this I did not put the high frc tran pos input back on the fuse....like All Acoustat 121 fuses are setups
so that left only the low frc tran on the fuse......
AS days past an I begain to feel like I was getting the best topend I have ever got out of any of these panels....I went back in to the interfaces....an found that I had forgot to put the high frc tran back on the fuse.......So I took it off the pos post......move it over to the fuse.......what the hell is going on here??
overall sound changed......if I had not new better I would not think these were the same speakers befor the change......so I put the high frc tran back ...with out the fuse......Wow...
I have posted some work I did.....running 3 panels off of just one tran... no mixer....on the secdary side.....no crossover on the primary side only a one ohm 20 watt res on the pos input winding......the tran were the first brown paper tran used in the 121 2a...then the medallion black paper high frc tran...an some ML tran used in there SL3s- An Ascent panels trans...because thay have primary cen taps....to try mirror drive.....just see how it sounded....
what I found was not much low frc.....nothing below 1-200hz......but all these tran could drive these 3 Acoustat panels are loud as I could ever need...an the 1 ohm res never got hot...fearing I could kill the trans..i put in a 3 amp fuse.....no matter how hard I drove these tran...never blue a fuse.....
I well say ...just one of any of the tran...could drive 4-6 panels vary ezely....an sound great...I was shocket.....like 8-90% of all the sound your getting from the 121 interfaces setup.......just saying...I know this dose not sound right......just try it.....youll like it...hehe
So running the high frc tran in the 121 interfaces with out the fuse is one way to get some of the best topend ever...an over all sound I have got to date......from Acoustat.......
Now I have seen were others have pulled the fuse out of the interfaces all togather....
this is NOT what I am saying.....as I feel the sound must-would be better.....I well never run my Acoustat 121 type interfaces with out a fuse on the bass tran.................
all just one mans finding
 

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Tyu, it would be best if you broke your thoughts up into paragraphs.

The way you have written, it is difficult to follow. Makes it hard to read. ok?

You can take TWO interfaces and drive 4 or 6 or 8 panels by splitting the panels between the interfaces. That's how it was done for model 6 and model 8 speakers.

Glad you are enjoying your experiments. :D

_-_-
 
I don't see anything that can "feedback".
To experience what I am talking about, take a MK-121 and a MK-121C interface and disconnect the drive signal to the HF transformers. Then, drive the LF transformer with a 1Vrms signal. The LF transformer secondary will have 200 - 250 Vrms feeding into the mixer network. A portion of this voltage will drive the secondary of the HF transformer producing alternating flux density in the core just like if it was being driven from the primary side. You can verify this by measuring the voltage induced on the primary by transformer action. Looking at a frequency toward the low end of the HF transformer's working range...say 200Hz, you will find the MK-121C has about 0.5 Vrms on the primary. The MK-121 however will have > 2Vrms, the exact value dependent on the effective load resistance of the core losses. Basically, you could drive the core of the HF transformer on the MK-121 interface into saturation without even having the primary hooked up.

The 16 ohm resistance across the primary on the MK-121C interface reduces the portion of the LF transformer output entering the HF transformer secondary by loading down the AC voltage divider formed by the capacitors and resistors in the mixer circuit. The lower the resistor value the less effect the LF transformer output will have on the HF transformer response. Sound Lab tends to use much lower value shunt resistance across their HF transformer primaries. The downside to this, is a lower impedance load on the amplifier.
 

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...so I put the high frc tran back ...with out the fuse......Wow
If you have a multimeter, you might measure the resistance from terminal to terminal on the fuse holders. I have found a few that had a bit over 1 ohm resistance which certainly will roll off the top end. Rather than just bypassing it, I cleaned the internal contacts that the ends of the fuse are clamped in and was able to get resistance back < 0.2 ohm.
 
Ok, so to simplify, the 121 interface's primary looks like an open circuit, whereas the 121C looks like it has a shunt via the 16 ohms across the primary.

It hardly matters since there is no current available, since it is being driven via 50kohm resistors, and that won't saturate the core.

There's actually a greater chance of saturating the transformer with a shunting resistance across the primary, when driven via the secondary, because that is the only way the significant current may be drawn - if there were any available. Sure there is voltage swing, but no current.

That's how I see the thing...
 
Sure there is voltage swing, but no current. There's actually a greater chance of saturating the transformer with a shunting resistance across the primary, when driven via the secondary, because that is the only way the significant current may be drawn.

Flux density in the core is dependent on voltage fed into a winding, any winding. It does not matter if there are any other windings or if there is a load connected to them or not. You are correct that the 50K source resistance will limit how far into saturation the core can be driven. For reference, the 50K source resistance on the secondary side would be equivalent to 13 ohm if driven on the primary side.

As you stated, adding a shunt resistor increases current draw from the LF transformer. But, this current draw causes increased voltage drop across the 50K resistors resulting in lower voltage applied to the secondary and thus lower flux density in the core not higher. This is what I meant by the statement "loading down the voltage divider"
 
… Run with only the "bass" xfmr, then run only with the "HF" xfmr. You'll see where the overlap is. Btw, they overlap a few octaves, it's not a standard "crossover" as many would expect.
Agreed.
Your comment reminded me that several years ago I had made some measurements as you describe. The plan was to use them to help correlate a SPICE model that could be used to experiment with component value adjustments. I never completed tweaking in the parasitic and core loss parameters, but the model already matched measurement trends pretty well. Here are a few comparison plots I had made that might be of interest.

Attachment #1: Comparison of SPICE simulation for MK-121 and MK-121C mixer output, with the HF and LF transformers driven independently.
- Notice the much broader overlap with the earlier MK-121 circuit.
- Also, notice the MK-121 HF transformer is driven all the way down to 20Hz whereas the MK-121C is rolled off below 200Hz.

Attachment #2: Comparison of SPICE vs Measured for the MK-121 mixer output, with the HF and LF transformers driven independently.
- Notice that below 200 Hz, the measured HF transformer response starts to droop as the core is approaching saturation…something the SPICE model does not consider.

Attachment #3: Comparison of SPICE vs Measured for the MK-121C mixer output with the HF and LF transformers driven independently.

Attachment #4: Comparison of SPICE vs Measured for the MK-121 and MK-121C mixer output, with both HF and LF transformers driven together.
- As Tyu mentioned, the earlier MK-121 circuit puts out between 0.5dB and 1.5dB more output in the midrange. Whether or not this is desirable from a frequency balance perspective is open to opinion.

The output from the mixer is basically a pushing match between the HF only and LF only outputs shown in Attachments #1 - #3. The resulting output at a given frequency in the overlap region is dictated by the source impedance for each of the sources. Changing the capacitor or resistor values on the primary side of the HF transformer affects not only the voltage reaching the primary, but also the source impedance (magnitude and phase) driving the mixer. As you had mentioned in a few posts, there is a lot if interaction between different circuit components…resulting in many subtle changes that are not necessarily intuitive at first.

Attachment #5: Comparison of SPICE vs Measured for the MK-121 and MK-121C HF transformer primary applied voltage, with both HF and LF transformers driven together.
- Notice that the MK-121 HF transformer is driven hard in the bass, only -3dB at 20Hz! The Cmod on the other hand nicely removes the bass handling requirements from the HF transformer and along with it the potential core saturation related distortion products.
 

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Received a PM that the legends on Attachments #4 & #5 had MK-121 & 121C swapped...apologies for the confusion.
A reminder to proofread my old plots before posting :eek:

Here are the corrected versions.
 

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bear....bolserst....thanks for your time an input......I am vary luck to have all the panels an interfaces...an other ESL setup trans......to play with an find what sound I like to day....
I find the fuse info vary good for all to know.....as I have read in some of bear post...he pull the fuses out of his interfaces.....an it is ezey to hear a diff....no dout about that.

From all the work bolserst has done for us all...to give better insite on the working of the ESL interfaces trans setup on the Acoustat 121 2a.....121c.....it come down to what sound you like.... .....an that's ezey for me to say......most others well live with what they have in there 121 interfaces........to say all the Acoustat 121 sound great for sure....121 2a-b 121c are vary cool setups....
Soundlab full rang ESL... is the only man standing...an thay have not changed interfaces much in years.......just there price....it just keep going up.....an so do the Amps it takes to drive them.......but nothing new about that..every this up....but gas.....for a min...right
 
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