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Pete B

Dynaco A-25 Restoration - Measurements - Comments

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Thanks for the new Dynaco section Mark, are you going to move the A-25 posts here or should I copy them?

Pete B.

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Hi Vern,

Yes I also like Dynaco equipment and agree that we should have a dedicated section here.

A few nice links:

http://home.indy.net/~gregdunn/dynaco/components/speakers/

Another, Note that the A-50 XO schematic seems to be incorrect:

http://www.t-linespeakers.org/classics/dynaco.html

I'm reverse engineering a pair of A-25s just because I always thought they had interesting qualities. I wrote this about them some time ago:

Compared one A-25 with the 25F-EW (claimed to be the modern replacement woofer) to another with the original 25TV-EW and they are significantly different. The 25F-EW is more efficient making the speaker sound shouty in the 1 kHz region, and less well integrated with the tweeter. The stronger magnet provides more damping and there is less deep bass output with the 25F-EW. Checked the efficiency difference with warble tones and the difference is right on 3dB at 250, 314, 400 Hz, then it varies between 1 to 3 dB for the rest of the bands up to 2 kHz.

I believe this makes the 25F-EW a replacement for the A25XL which was 3 dB more efficient and used a ceramic magnet woofer. See the A25XL reference here.

http://www.t-linespeakers.org/classics/dynaco.html

Pete B.

>Hi there;

>

>I know that there is at least one other member who is very

>interested in classic Dynaco.

>

>If there is enough interest from the members, perhaps, Mark,

>may create a Dynaco discusion group.

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I wrote this to another group:

I've been curious about compliance shift over time in rubber

surrounds in the hope of finding trends in materials,

manufacturers, or the environmental exposure.

Some surprising information. I have a pair of Dynaco A-25s

that I bought recently purely for the purposes of reverse

engineering. The woofer used in the early production model

was a SEAS 25 TV-EW with an ALNICO magnet. SEAS has a

suggested ceramic magnet replacement 25 F-EW which is what

was used in one of the systems. It is interesting that both

are well within specification going by the free air resonance

one from 1971 and the other from 1990.

SEAS:

25 TV-EW date code 1971 measured Fs = 23.6 Spec = 20-25 Hz

25 F-EW date code 1990 measured Fs = 27.4 Spec = 26 Hz

I noticed in listening to these systems that the one with the

ceramic magnet was significantly more efficient and measured

it midband as 3dB more. The data below shows only 2dB but it

is a theoretical midband value. I believe that the cone

behavior is slightly different providing a bit more output

midband. This is probably the driver used in the A-25XL model

from Dynaco which is claimed to be 3dB more efficient than the

original.

Measured T&S parameters:

SEAS 25 TV-EW UNIT SAMPLE: PLB#1 1/12/06

UNIT DATE: 25 week of 1971

Rubber surround part number: SR 231/1

Effective cone diameter = 21 cm measured

Effective cone area = 285 cm^2 old SEAS spec, 350 cm^2 new

(25F-EW)

seems the old spec of 285 cm^2 was an error and might explain

the difference in moving mass below:

Measured SEAS

Delta M 15.75 Spec

Fshift -16%

Fs 23.6 20-25

Vas 205

Re 5.7

Qe .48

Qm 4.2

Mms 37.5 30

no .36

SPLref 87.6 88

Bl 6.6 .8

Qts .43

Cms .

====================================================================

SEAS 25F-EW(H250) UNIT SAMPLE: PLB#2 1/13/06

UNIT DATE: 48 week of 1990

Rubber surround part number: SR 231/1

Effective cone diameter = 21 cm measured

SEAS Spec effective cone area = 350 cm^2

Measured SEAS

Delta M 15.75 Spec

Fshift -15%

Fs 27.4 26

Vas 144 175

Re 5.5 5.8

Qe .49 .39

Qm 3.9 3.8

Mms 39.6 33

no .58

SPLref 89.6 89

Bl 8.8

Qts .44 .35

Cms .85

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Thanks Vern, nice to hear from you.

I'm just copying these old posts over:

The original woofer sounds much better, more low end extension (not that it has much to start with) and probably less top end on the woofer so that it transitions better to the tweeter. The ceramic woofer seems to be correct for the A-25XL which had a different tweeter, and perhaps crossover so it's just not right here. I might try a compensation network on the woofer to get a better match, but I'll probably look for the correct woofer on e-bay.

SEAS suggests the 27TFFC as a replacement tweeter acording to Madisound and I have a feeling it should work well given what I've seen from measurements.

Pete B.

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I posted this on another forum and it applies to the A-25 because it uses such a large vent:

I've been meaning to write more about aperiodic systems for some time now. Dick Pierce does a fine job here:

...a variovent can be accurately modeled using a standard vented model (4th order) with the following adjustments:

1. Since the vent diameter is relatively large and the vent length essentially the thickness of the panel it's mounted in, the actual vent mass is quite low, that results in a box resonance which is much higher than would be considered reasonable for a vented system, which means that a system aligned with such a port would indeed have a very strange response, if it were not for the fact that:

2. With the damping in the vent, the port losses are extremely high, the resulting Qp is VERY low (around 1-2 max), and thus the action of the port at box resonance (at the high frequency it has) is significantly attenuated to the point where the contribution to the system's total volume velocity is essentially attenuated to insignificance.

The result is a vented system with a high Fb that has a very low Qb. Such systems, while still technically 4th order, approach 2nd order behaviour at and below resonance for a significant range.

What advantage does this have? Well, with normal woofers, it's not dear. It has no efficiency advantage over properly designed closed boxes, it does not have the bandwidth or efficiency advantages of lower-loss vented systems. It might have an advantage when you are forced to use a high-resonance, high Qts woofer (just like some of the woofers Dynaudio makes, for example).

But, magic it is not.

Dick Pierce

Loudspeaker and Software Consulting

17 Sartelle Street Pepperell, MA 01463

-----------------------------

I believe that Dick Pierce was referring to the A-25 which does have high port area, but there were others with much less area that might work better. The Audio Concepts sub based on the DV-12 woofer from many years ago is an example. I plan to measure the A-25 some day, but that will likely be far in the future.

Pete B.

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I did more work on these some time back and I've been meaning to post about it here.

I did a quick simulation with approximate models of the drivers in order to determine the attenuation level of the 5 switch settings.

I also determined the values for a voltage divider to match the newer, more efficient woofer to the old design.

I just want to get these into good working order and plan to get back to them someday and really dig in with more acoustical measurements.

I implemented the resistor divider for the new woofer and compared the two systems. The system with the newer woofer sounded as if the tweeter was significantly more efficient. This system on normal sounded like the original system full up. I thought it was a tweeter problem, then decided to measure the woofers above 2 kHz. I know these drivers fairly well and expected them to have similar cone behavior, which turned out to be wrong. The newer woofer is much more efficient above 2 kHz, between 3 and 5 dB as I recall, probably due to a stiffer cone. I had planned to buy a replacement woofer on e-bay, but wanted to determine what was needed to properly fit the newer woofer to the old design. I've decided that it's not worth the effort at this point since a more complicated crossover network will be required for the woofer.

I finally purchased an original type woofer on e-bay, and will test it soon.

Pete B.

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I was recently asking about people's thoughts concerning tilt back for the Dyna A-25's. There was a reason.

The Spica TC-50 claims to be, and is to some extent, a linear phase design. It uses a crossover that is a non-standard design and requires time delay on the tweeter, or advance on the woofer depending on how you look at it.

I found, in simulation, that I could also obtain a fairly linear phase response with the A-25's by advancing the woofer in time as would be seen by tilting back the enclosure, or listening in a position that places the woofer closer to the listener.

The Spica uses a first order network on the tweeter as does the A-25. It also uses an approximate 2nd order network on the woofer, however it too is nearly first order electrical as a result of the values chosen. Spica claims a Bessel response on the woofer, but that is not what I saw in simulation.

I have noticed for years that the A-25's sound best tilted back and was not aware of this nearly linear phase characteristic.

Pete B.

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Hi Pete;

Nice write-up, as usual.

I wonder what tilting the enclosure does with the tweeter now being in more direct eyeshot as well.

The tweeters dispersion was satisfactory but never outstanding.

With the time tilt and beaming maybe you have just discovered a new speaker POOGE at no cost.

Have you ever tried Double Dyna's?

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Hi Vern,

I tilted them years ago just to find the best listening angle. Yes, I do think being more on the tweeter axis helps with the directional tweeter. Forgot to mention that I'm not convinced it's the near linear phase property that provides the good sound since the frequency response also varies strongly with vertical angle.

I never have heard doubles, I take it you have? How are they?

Pete B.

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Hi Pete;

Always nice to read your write-ups, Pete.

I personally have not heard double Dynaco A-10, A-25, A-35 or A-50's, yet.

I have enough A-25's, both versions, to try something in the near future.

I plan on seeing if I can hear a difference between, the Seas and Scan, versions.

It is particularly noteworthy, that, since the Double Advent's were written up in the Absolute Sound, many decades ago, that no other reviewer or manufacturer ever did a double's trial of any model, that I am aware of.

Since I started comming here, a few members have tried and commented briefly on several smaller, the AR-LST's are the biggest, speakers.

There was something, "magic", with the Larger Advents, it seems.

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Hi Vern,

I'd like to hear your impressions comparing the two and for any stacked combinations you try, should be an interesting read.

I think many people did the double Advent because it was suggested by Kloss.

There is almost "magic" when speakers are paired up, their SPLs add in the same way as in phase voltage sources, and therefore the acoustical power goes up by a factor of 4, or 6 dB, while pulling, if the amplifier can handle it, only twice the electrical power due to the load impedance being one half. The efficiency nearly doubles.

Also, the maximum SPL goes up by a factor of 4 rather than just 2 because of this effect. The volume displacement is doubled, which raises Max SPL by 6 dB, not 3 dB as might be expected on an intuitive basis.

This is highly unintuitive and part of the detailed explanation is that direct radiator drivers are so inefficent to start with.

Many incorrectly associate this with mutual coupling which is not the reason. There are AES papers covering this in case anyone is thinking of challenging the claim.

Pete B.

This subject is often misunderstood and I often find people on the internet, sometimes professionals, who try to contest and debate the matter. I just came across this writeup about Professor W. B. Wadsworth who I was lucky enough to study Audio and Acoustics with:

http://www.auditory.org/asamtgs/asa94mit/3pNS/3pNS3.html

I was surprised to hear some years ago that often students at the best universities have little access to some of the world renouned professors who are shielded by graduate students. I did project work with Professor Wadsworth, many years after his work in the 1950s, and if he heard me mention something like mutual coupling, or back EMF showing that I didn't fully understand the material, he'd sit me down in his office and explain it until I did understand it, sometimes referring me to the literature. He thought it important to know the history and give credit to original ideas, he told me of the very early work by Rice and Kellogg for example:

http://history.acusd.edu/gen/recording/loudspeaker.html

I was lucky because he was a briliant and patient gentleman, even other professors would smile and say, he's on another level, his understanding was so deep.

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Hi Vern,

Just waanted to mention that if I sound cranky above it is not directed at you in anyway, just based on past exchanges with a few others. Always nice to hear from you.

And all the best to everyone for the 4th!

Pete B.

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My reply to a few questions in another thread:

I mentioned about the sound differences in the other thread, about 3 dB more output 200 to 500 Hz, and even more above, about 5 dB more above 2 kHz. Magnet is stronger so the Q is lower resulting in less low bass.

Any film cap will do just fine, I didn't have a 1 uF so I used a pair of .47 uF Panasonic Polys in parallel.

I used Axons to replace the 5 uF, and would have used them for the 1 uF if I had them. Anything over a 100V rating is fine.

The cap just adds some "air" to the sound, not like turning up the treble or the switch on back, because it's only lifting the very high end. More like moving forward in your seating location. It is subtle but important for realism.

Pete B.

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Post by Vern copied here, and my answer follows:

>Hi Vern,

>

>Thanks again very much, nice of you to comment.

>I mentioned about the sound differences in the other thread,

>about 3 dB more output 200 to 500 Hz, and even more above,

>about 5 dB more above 2 kHz.

Hi Pete;

Will this cause a boomier bass or more mid range presence?

Original Dynaco A-25's were noted as having a extremely smooth response in this area.

Is 3db double the output in this regard?

In my reading of old Dynaco literature yesterday, I came across a report of a 9khz and a 10khz resonance, one each from two different sources.

One source still tested and claimed outstanding transient response at the 10Khz, at the same time as it also showed a 10khz resonance photo.

Can we hear this resonance as a screech, buzz or rattle?

As always, Pete, a very informative write-up from you, thank you.

Magnet is stronger so the Q is

>lower resulting in less low bass.

>Any film cap will do just fine, I didn't have a 1 uF so I used

>a pair of .47 uF Panasonic Polys in parallel.

>

>I used Axons to replace the 5 uF, and would have used them for

>the 1 uF if I had them. Anything over a 100V rating is fine.

>

>The cap just adds some "air" to the sound, not like

>turning up the treble or the switch on back, because it's only

>lifting the very high end. More like moving forward in your

>seating location. It is subtle but important for realism.

>

>Pete B.

>

Have a good one.

Vern

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Hi Vern,

The new driver is about 3dB more efficient in the pass band piston range. By this we mean above the bass region where the box and fundamental resonance come into play and below frequencies where the cone does not approximate a point source very well, and/or has breakup modes. This is very roughly 150 to 400 Hz, and if this was all that was different, we could turn up the level on the tweeter to match it and we'd have a system that was 3 dB more efficient across the audio band. However, it measures even more efficient around 2kHz and up by about 5 to 6 dB so yes it will be much more forward in the presence region. The stronger magnet and lower Q reduces the low bass output, relative to the pass band, which I've not measured, but is probably around a few dB.

+3dB in level for equal voltage input is twice the acoustic power. It's like driving the original woofer with an amplifier with twice the power. It would be nice to find a way to make this higher efficiency driver work and take advantage of the higher efficiency.

If you were to compare a system with the replacement driver by matching levels in the piston range where it is 3 dB more efficient, as I did, you'd hear a few dB less bass around 60 Hz, and 2 to 3 dB more output around 2 to 4 kHz, going from memory here. It is more forward sounding and not in a good way at all. Turning the tweeter level all the way up on the original system made them sound much closer.

I don't know about any 9-10 kHz resonance, it's probably inaudible if it's just a few cycles of ringing. Was this tone burst testing, as was common in the old days? I don't think I've seen any A-25 reviews in a very long time, except for those in Stereo Review for the A-25 and A-50 which are not very detailed and mostly praise them. I did look on the web recently. Who were the authors of these reviews?

Pete B.

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Hi Pete;

>I don't know about any 9-10 kHz resonance,

My comment was from an Audio magazine un-dated review about a 9khz small peak.

They found a 10khz small peak in the June/69 issue of Stereo Review and more interesting, they also did a tone-burst-response-photo at that same 10khz peak.

Obviously it had no negative effect, as it was published in Dynaco's 22 page brochure D669.

it's probably

>inaudible if it's just a few cycles of ringing.

You appear to be absolutely correct in your comment, Pete.

Was this tone

>burst testing, as was common in the old days? I don't think

>I've seen any A-25 reviews in a very long time, except for

>those in Stereo Review for the A-25 and A-50 which are not

>very detailed and mostly praise them. I did look on the web

>recently. Who were the authors of these reviews?

>

>Pete B.

Also High Fidelity magazines review July/69, there was no mention of any peak.

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Hi Vern,

Thanks for the dates of those reviews. I might look them up one day.

I do remember the old Dynaco literature that mentioned tuning each A-25 with a 5 Hz square wave. I think this was marketing hype, every A-25 I've seen has been stuffed the same way. I do also think they showed the tone burst tests. I do think I have that around here somewhere but I've not had my finger on it in some time.

Best,

Pete B.

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>Hi Vern,

>

>Thanks for the dates of those reviews. I might look them up

>one day.

>I do remember the old Dynaco literature that mentioned tuning

>each A-25 with a 5 Hz square wave. I think this was marketing

>hype, every A-25 I've seen has been stuffed the same way. I

>do also think they showed the tone burst tests. I do think I

>have that around here somewhere but I've not had my finger on

>it in some time.

>

>Best,

>Pete B.

Hi Pete;

You are very welcome.

I feel that they did perhaps have to do a bit of fiddling around for the prototypes.

Afterwards they quite likely ordered either rolls of a certain thickness and cut them into pieces or ordered the pieces in bulk.

I am also certain that they would have done spot checks, to be certain of their suppliers quality control.

It would be interesting for an employee of that company to reply.

Also a write-up regarding the relationship of Scan, Seas and Dynaco, would be of great interest, as well.

As a side note, the A-35 and the A-50 had an internal divider/partition with a vent opening, no external vent openings.

I believe yhat they used a felt type material, as I remember, to cover one side of the internal opening, not fibreglass.

Those memories are from the early '70's, I am certain that there were no details that I forgot about these models.

The divider/partition in the A-50 was done in two internally different locations, as well.

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Interesting Vern,

Yes I'd agree about prototypes and spot checks, makes sense.

I'd read about the partioned cabinets for the A-35 and 50 but have never had a look inside one.

Thank you,

Pete B.

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>Interesting Vern,

>

>Yes I'd agree about prototypes and spot checks, makes sense.

>I'd read about the partioned cabinets for the A-35 and 50 but

>have never had a look inside one.

>

>Thank you,

>Pete B.

Hi Pete;

AR used to show in a photo, that of a tester, using a stethoscope to test an AR speaker for cabinet resonances.

I felt, that, they were saying, that, they tested each and every one.

Maybe, I wasn't the only one thinking that, at the time.

Buried somewhere, is all of the measurements from an A-25, A-35 and A-50 cabinet from when I serviced them.

I also have Dynaco literature which shows drawings of different A-50 partition locations, as well as different versions of the basic Dynaco 400 amplifier.

I think sometimes the engineering, manufacturing and sales lose touch with each other, the sales have brochures drawn up pre-matureley.

The material that covered the internal vent opening was, typical felt material, it covered the internal hole.

I never tried to remove the felt to see if it was on the other surface of the partition.

When I dig up those notes, I'll add to this topic in the future.

I have A-25's, A-35's and A-50's, I tend to just use them and not take them apart.

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A few comments about voice coils:

Years ago in my early days experimenting with speakers I noticed that even some inexpensive but decent drivers such as the $2.49 CTS 4.5" driver used in the *ose 901 employed what appeared to be an aluminized paper voice coil former. Whereas many of the European drivers used a paper former.

I was given a pair of A-25s where one of the woofer (25TV-EW) voice coils was burnt out. I know that it had a paper former because I removed it and it was burnt with the windings coming loose. I measured the gauge of the wire, counted the turns, and wound a new coil on aluminized paper and repaired the driver.

I also used the Phillips AD8060 8" woofers in pairs and we would often smell something burning when we turned up the system. These failed more slowly, over many years, with the paper former just looking browned and cooked like toast, and the enamel on the wire completely black with the windings mostly shorted.

A good heat conductor is needed to sink the heat out of the voice coil. This is well known today and even most tweeters employ solid aluminum voice coil formers or at least aluminized paper.

Note also that the 1.5" tweeter in the A-25 had a paper only former, and the larger than usualy size helped to give it some thermal capacity.

I believe that the 25F-EW woofer has a better former, either aluminum or aluminized paper as it has a higher power rating. The A-25XL system also has a higher power rating.

Pete B.

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