Radio engineering speakers 35ac 1 modernization scheme. Speakers S90: technical specifications, diagram. DIY speakers. Correct AC filters

Radiotehnika 35AS-012, Radiotehnika S-90, Radiotehnika 35AS-012, Radiotehnika S-90

Currently I am the proud owner of Radiotehnika S-90 speakers.

Consideration of acoustics in normal condition

First, you should specify the full name of the acoustics - 35AC-012. From their number it immediately becomes clear that we are dealing with top-class acoustics, by Soviet standards, that is, with acoustics that have very high characteristics. It should be noted right away that by Soviet standards this was not the best acoustics, but an ordinary ordinary workhorse. There were acoustics that had a more balanced sound, for example the same Cleaver/Corvette 35AC-008.

But as they say, we have what we have. Let's go back a little to the moment of their purchase. I bought them from a friend of mine for $50, when I came to him, when I saw the decorative grilles protecting the sound-emitting heads, I wanted to cry, they were dented and very cruelly (mainly the grilles of the high-frequency and mid-frequency speakers were damaged). But this didn’t scare me, since what was sold on our market cost at least $100, and the quality of the speakers deserved no more than 3, and in these speakers the speakers looked like 5. In general, I took these speakers to my home. When connecting them to an amplifier the sound was quite decent. But nevertheless, two drawbacks should be noted, 1 of which is inherent in all 35AC-012, and as it turned out, in all its 35AC clones, to one degree or another.

The first of the shortcomings that simply killed me on the spot was an incomprehensible overtone when the woofer was working, very similar to the fact that something was stuck to the speaker from behind, and now it was vibrating, as it turned out later it was a drop of solder that stuck to diffuser from the reverse side. The second drawback was precisely in the mid-frequency dynamics 15GD-11A - according to the old standard and 20GDS-1-8 according to the new one (these speakers came in a large number of modifications for this reason, which ones are extremely difficult to track). And again, a small digression in which I will say that the differences in the standards lie in the designation of power, that is, according to the old standard, the rated power of the speaker was indicated, and according to the new standard, the rated power is indicated (from the audio engineering course:

The rated power of the speaker is the power of the speaker, when supplied, it operates with harmonic distortion levels not exceeding the permissible
Nameplate power (often also called noise): this is the level of power supplied to the speaker at which the level of harmonic distortion is equal to ten times the level at rated power).

An additional division into the frequency ranges of the heads was also introduced, which was now indicated in the name of the speaker, in particular this is the third letter.

So the disadvantage of this speaker is that it often begins to resonate at high volumes and thereby spoils the sound picture, and as you know, the mid-frequency speaker is crucial in the formation of the sound picture.

Let us now consider in order all the speakers that we have in stock:

1)Low frequency - 30GD2, also known as 75GDN-1-4(8):

Purpose - use in closed and phase-inverted remote acoustic systems of household radio equipment of the highest complexity group as a low-frequency link when working indoors. The loudspeaker head is of electrodynamic type, low-frequency, round, with an unshielded magnetic circuit. The diffuser holder is made by injection molding from aluminum alloy. The cone diffuser is made of impregnated paper pulp. The suspension is toroidal-shaped and made of rubber. The centering washer is made of impregnated fabric.

I would also like to add that the speakers have a relatively heavy dome and a rubber surround, which spoils the quality of the bass; it becomes less continuous and booming than speakers with a lighter moving part and a foam surround. But it should be taken into account that the bass is influenced not only by the design, but also by the acoustic design itself, for this reason these problems can be eliminated a little and the speaker will play decently. On the other hand, due to the rubber suspension, the speaker turned out to be very reliable and practically indestructible, while the foam suspension soon crumbles due to the presence of sulfur in the air and the speaker requires repair.

Purpose - use in closed and phase-inverted remote acoustic systems of household radio equipment of the 1st and 2nd complexity groups as a mid-frequency link when working indoors. The loudspeaker head is of electrodynamic type, mid-frequency, round, with an unshielded magnetic circuit. The diffuser holder is made by injection molding from aluminum alloy. The conical diffuser and spherical cap are made of impregnated paper pulp. The toroidal-shaped suspension is made of polyurethane foam. The centering washer is made of impregnated fabric.

Here is actually a photo of this miracle of technology:

It is worth saying that at a good volume it distorts the sound considerably, but as practice has shown, this problem is very easily solved and, at that, quite simply.

Purpose - use in closed acoustic systems of household radio equipment of the highest complexity group as a high-frequency link when working indoors. The loudspeaker head is of electrodynamic type, high-frequency, round, with an unshielded magnetic circuit. The mounting flange and acoustic lens are made of plastic. The dome-shaped diaphragm with suspension is made on the basis of polyethylene terephthalate.

In general, they sound good, but the filters are tuned close to the resonant frequencies.

Upon closer examination of the acoustics (especially from the inside), you begin to be horrified by the build quality, for this reason we will begin to refine it. We will refine it according to the simplest possible scheme, without interfering with the filters, since without specialized equipment there is nothing to do there. For anyone interested, here is the acoustics diagram:

Revision 35AS-012

I will describe in order all the stages of improvement that my speakers went through:
1. Disassembly:

First of all, we take them to a secluded place (meaning a room) in which our experimental subjects will not be accessible to children (if any) and other family members. We lay the speaker system on its back and begin to disassemble it.
Now remove the decorative covers from all the speakers and put them aside.

Here they are:

Then we take out the speakers. ATTENTION: when unscrewing the bass speaker (the treble and midrange speakers are secured with the same screws as the decorative cover, and the woofer is mounted separately from the cover), be extremely careful, because if the screwdriver comes off, you will disfigure it. Then we use a soldering iron to unsolder the wires connecting the filter and the speakers, and safely hide the speakers in a secluded place.

We remove the bass reflex cover and take out the bass reflex itself, and this must be done as carefully as possible, since we are working with plastic, and it can easily break. Then we hide these details in a secluded place.
Now let's take on the regulator/controllers of the HF/MF sections. To dismantle them, you need to remove the decorative plug in the center of the regulator, then unscrew the exposed screw and remove the regulator handle. After this, carefully pry up the remaining plastic lining using two chisels and carefully remove it, then unscrew the 4 screws securing the attenuator itself and now you can push it inside the case. We push it out and unsolder it from the filter. We put it aside, in the future you will need to work a little magic on it. By the way, the joint between the attenuator body and the speaker body is generously covered with a sealing viscous substance; I personally reused it when putting it back in place, but you can use sealant or plasticine.
We take out the cotton wool bags that are supposed to be in your speaker system and set them aside.
We dismantle the panel with filters; it is screwed to the body with screws, having previously unsoldered the wires from the output at the back of the speaker system. We put it aside, since we will spend a lot of time working with them.
Finally, remove the terminal panel from the back cover of the speaker and set it aside.

It seems like a lot of work has been done, but in reality it is just a drop in the ocean. More interesting and time-consuming work lies ahead.

2. Restoration of appearance:

For this purpose, we take the grilles and covers we removed from the speakers earlier, level them, carefully sand them, degrease them and paint them several times with car paint (which is in cans) several times and leave them to dry. I’ll make a reservation right away that I restored the grilles only for the reason that I have a small child who can damage the speakers, otherwise the simplest solution would be to abandon the grilles as such, since they only introduce disadvantages into the sound, think for yourself.

3. Refinement of the speaker system housing:

Here everything is actually very simple, and is carried out in several stages:

If desired, the body can be strengthened. What will this give us? Clearer and smoother bass, since the cabinet panels will vibrate less and, accordingly, will introduce less overtones into the bass component of the sound. How to do this? This is purely a matter for everyone, since as many people there are as many decisions. In general, it all consists of installing spacers, installing additional corners at the joints of the walls of the speaker system, and installing stiffeners on the walls of the speaker. Personally, I limited myself to gluing in additional corners at the joints. You can also tightly glue all joints. Unfortunately, I can’t show you a photo since the entire speaker system is already damped with foam rubber.
Sealing all joints and seams. It is done very simply using various materials. For example, I used plumbing sealant. The procedure is simple: cover the joints with sealant and carefully spread it with your finger, thereby tightly sealing any cracks.
At a hardware store we buy foam rubber 10mm thick (personally, I chose this thickness, don’t take too much as it will suffocate the body) and glue it to all walls except the front. In this way we dampen the body, thereby increasing its virtual volume.

To do this, we buy terminal blocks with gold-plated universal-type connectors in the store. Since the S-90 terminal block itself is large, and the new ones are small, we remove the connectors from the terminal blocks and install them on the body of the S-90 terminal block. Then we lubricate the installation area with sealant (don’t be sorry, wipe off the excess later) and put it all in place, tighten the screws. Here's a photo of what you should get:

5. Let’s move on to reworking and replacing the filter:

First of all, carefully examine the filter, pay attention to the fastening of the parts, since often the inductors were fastened with metal screws, which immediately disrupts the filter settings.
If there were problems with the fastener, complete it by eliminating metal parts from the fastener. There are also cases of assembling the filter on a metal plate, then transfer the filter to a plywood panel.
We pick up a piece of paper, a pen, and carefully redraw all the elements of the circuit, restoring, so to speak, the filter circuit itself, because the parameters of the speakers were wrong and for this reason the filter circuit could have been changed at the factory. By the way, we exclude the attenuator from the circuit, it simply spoils the sound.
Now we take a soldering iron (preferably 100 watts) and disassemble the filter, or rather, simply remove all the jumpers that were installed from the factory.
Now we assemble the filter, instead of jumpers we will now use a cable made of oxygen-free copper with a cross-section of 4 mm 2, the cable can be bought at any car audio store. It should also be noted that you shouldn’t buy a very expensive cable, as the changes in sound quality will be insignificant, but the costs are simply colossal.
After assembling the filter, we solder the wires that will go to the speakers based on: for the low-frequency link 4 mm 2, for the mid-frequency link 2.5 mm 2, for the high-frequency link 2 mm 2.
We put the filter in place, and then solder the terminal blocks to it (observe the polarity, otherwise you will lose the sound picture).
The very last step is to run the wires to the speakers, secure them and cover the filter with foam.

You'll end up with something similar to these photos:

6. Attenuator installation:

We remove all resistance from it.
Let's put it in its place.
We carefully seal it.
Additionally, we cover it with foam rubber (I only covered it on the front wall)
We install all the decorative panels completely.

7. Installation of the bass reflex:

Everything is simple here, we put it back on the sealant, carefully make sure that it is not pinched anywhere by the foam rubber, as this will throw off its setting.

8. Reinstall the bass reflex cover:

We install it the same way as we removed it, just install it with sealant and new screws, since the panel itself often rattles in the bass. Seal the joint between the panel and the bass reflex well.

9. We proceed to installing the dynamic heads in place:

A) Install the HF head:

1) We remove the travesty of the seal that is on it (some kind of rubber or cardboard at the back).
2) Cut out a new seal; a mouse pad, in particular a black porous base, is perfect.
3) Solder the wires to the speaker and install it in place.
4) Put the decorative trim (grid if desired) in place and tighten it tightly with the screws.

B) Install the midrange head:

1) We make a cylinder from foam rubber, such a size that our box will fit into it. We place this cylinder inside the speaker and pass a cable through it, which we take out.

2) Pass the wire through the box (most likely you will have to widen the hole), then put the box in place, adjust the length of the wire and seal the hole into which the wire is passed.
3) Solder the wires to the speaker.
4) Now the crucial step is damping the mid-frequency head. To do this, we sew a cylinder of foam rubber, such a size that it fits tightly onto the speaker frame and covers all the windows.

5) Fill the box with cotton wool, having previously fluffed it.
6) Put the dynamic head, grille (optional) and frame in place and tighten it.

1) First, put back the bags of cotton wool that were removed during disassembly of the speakers. Solder the wires to the head. I tied the wires that are soldered to the head to the frame so that they do not hit the diffuser, because there is a possibility that when you put the speaker in place, the wires will bend and fall into the window of the diffuser holder.

2) We make a gasket from a porous material, for example
Apply window seal and carefully place the speaker in place.
3) Tighten the mounting screws. Do not apply much force, then the speaker will be spring-loaded by the gasket and this will reduce the energy of vibrations transmitted to the body.
4) Put the grille (optional) and decorative trim in place. If you do install a grille, then I advise you to cut out small triangles from foam rubber and place them on the speaker at the place where it is attached; this will eliminate the vibrations of the grille, and therefore eliminate overtones at high volumes.

I came up with this solution a long time ago, look at the photos for more details:

Conclusion:

After the modification, all the listeners (there were not many of them, about five, but I asked for the most honest information from them) noted more delicate and soft bass, a much cleaner middle, the highs remained practically unchanged (it seemed to me that they became a little cleaner). Also, the acoustics began to calmly reach higher volumes.

In conclusion, I would like to say that the proposed method is the cheapest, simplest and most accessible. All components, of course, can be modified or changed several times. For example, instead of foam rubber, you can use felt (natural), this, in theory, will give better results than foam rubber; it is also a good idea to use vibrating mastics. Many people advise replacing the 15GD-11A with a 5GDSH broadband receiver, in my opinion this is a bad idea, but it’s everyone’s business. 10GD-35 - it is recommended to treat it with a notch filter, 15GD-11A should be modified based on halves of a tennis ball (by the way, the idea is quite interesting, I haven’t done it myself since I don’t have such speakers in stock).

At the moment, there are two 6GD-13 speakers available (considered the best in Soviet times), so perhaps I’ll post what happened sooner rather than later.

And perhaps finally I’ll say that if you are a fan of MP3 music and you have inexpensive source equipment, then maybe you shouldn’t bother so much, although I heard differences when extracting sound from a Creative Sound blaster 24 bit sound card. It should also be noted that the room in which you listen to music has a huge impact on the sound.

Listening to music on my 35AC-1, every day I became more and more tired of their sound. I have long heard about the modification of the speaker system, but for some reason I didn’t pay much attention to it before. But then I decided to try it anyway. After surfing the Internet, I found a lot of information about modifications to the 35AC-212 (S-90) and 35AC-1, and I got down to business.

1. Body.
The first thing I did was disassemble the speaker system. I unscrewed the front panel, the dynamic heads, took out the cotton wool, removed the filter and the bass reflex.

Then I strengthened the back wall of the box, installing two slats across the entire body with a thickness and width of 20 mm and a spacer (see photo), closed the holes on the front panel where there was a glass from the mid-frequency head, a high-frequency head and switches. I filled 10mm thick plywood with PVA glue. In place of the glass, I cut out new holes for the midrange and tweeter. (see photo). I placed a plastic cup (similar to the S-90) on the midrange driver, but it is usually better to use a wooden one. Inside, I covered all the walls with 18mm foam rubber (I couldn’t find any felt), but before that I needed to lay the HF and MF head wires under it (I didn’t do this, I was in a hurry). The bass reflex was shortened by 10cm by tuning it to a different frequency (mumbling less) and also covered with foam rubber on top. The box was covered with black covering fabric.

2. Filter.
I started by throwing out the tone switches and unnecessary resistors; for this you may need a circuit (it’s not hard to find on the Internet). After that, I moved all the filter parts onto a wooden board made of 8mm plywood. It is not recommended to weld inductors; they must be glued. I soldered all the connecting wires of the filter to an audio cable (at least the cheapest one). I soldered the wires that lead to the heads directly to the filter and not through each other. For the low-frequency driver, a thick wire is no less than 2 mm?, for the mid-frequency driver, no thinner than 1.5 mm 2, and 1 mm 2 for the high-frequency head (in the photo, the wires for the midrange and high-frequency heads are thin, I wanted to try it and installed what was there, and later replaced it).

The filter was screwed onto the bottom wall of the speaker.

3. Vata.
From one sausage that I took out, I sewed a square-shaped pillow and threw it on top of the column. On the other I made a small pad (about 0.5 sausages) and placed it on the bottom on top of the filter.

4. Midrange.
I didn’t have a midrange driver at all. I installed the 10GD-34 only after finishing it (by gluing a cap made of a tennis ball, I also found an article about this modification on the net), and also closed the windows in the diffuser holder of the head with foam rubber. I covered the inside of the glass with felt and filled it with cotton wool.

5 . The final part.
I soldered the dynamic heads while maintaining the switching polarity. The dynamic heads were screwed through rubber gaskets. I turned on the music and began to enjoy a completely different sound. I myself didn’t believe that my 35AC-1 could play like that.
P. S. I advise you to redo one speaker first so that you can compare the sound of one and another speaker system.

I have long wanted to remake the 3AC-305 acoustic speakers from the old VEGA 323 stereo. So that they work as shelves for a home theater. And finally it’s ripe so that it doesn’t take up space in the pantry. I decided to take the AC type - a closed box.

During the modernization process (it turned out to be global), all that remained of these speakers were lacquered panels and decorative plastic frames, the rest went to the trash heap!

To begin with, everything had to be completely disassembled, glued, assembled and strengthened, since the cases had cracked at the seams over time and creaked. The front panel is made of 18 mm plywood, the back panel is made of 10 mm plywood. Inside the case, I generously poured anti-gravel in 3 layers (BODY950) and covered it with foamed polyethylene (the material, so to speak, is not important - it doesn’t stick well, but what was at hand), you can use any convenient material, the main thing is to extinguish the reflection from the walls of the case. It would also be advisable to install spacers inside the case - one between the front and rear panels, the second between the side ones, but I did not do this.

Now we have a fairly strong and solid speaker body. After final assembly and installation of the heads, we had to slightly adjust the volume of the housing to the working volume of the woofer using synthetic padding. I just added and subtracted for the optimal bass sound.

I used speakers from ASALAB as emitters. LF - MB1345.4, HF - T252.1.4. I made the crossover filters myself, the crossover frequency was 3 kHz. The internal volume was approximately 9 liters, taking into account padding polyester and speakers.

As a result, I got very decent-sounding acoustics, I didn’t even expect it!

The filter was assembled according to the following circuit diagram:

And initially there was no filter at all - there was only one speaker. Perhaps the appearance will not seem very good to some, but there was no such task - to lick it to a shine. Yes, there are nuances that you begin to hear after listening for some time. The best thing to do is to add notch chains to the filter for each speaker. On the HF, loosen the tweeter a little, although this can be done

11-01-2009

Modification of acoustic systems Radiotehnika 35AC-012 (S-90)

Radiotehnika 35AS-012, Radiotehnika S-90, Radiotehnika 35AS-012, Radiotehnika S-90

Currently I am the proud owner of Radiotehnika S-90 speakers.

Consideration of acoustics in normal condition

The first of the shortcomings that simply killed me on the spot was an incomprehensible overtone when the woofer was working, very similar to the fact that something was stuck to the speaker from behind, and now it was vibrating, as it turned out later it was a drop of solder that stuck to diffuser from the reverse side. The second drawback was precisely in the mid-frequency dynamics 15GD-11A - according to the old standard and 20GDS-1-8 according to the new one (these speakers came in a large number of modifications, for this reason it is extremely difficult to track which ones you have). And again, a small digression in which I will say that the differences in the standards lie in the designation of power, that is, according to the old standard, the rated power of the speaker was indicated, and according to the new standard, the rated power is indicated (from the audio engineering course:

The rated power of the speaker is the power of the speaker, when supplied, it operates with harmonic distortion levels not exceeding the permissible
Nameplate power (often also called noise): this is the level of power supplied to the speaker at which the level of harmonic distortion is equal to ten times the level at rated power).

An additional division into the frequency ranges of the heads was also introduced, which was now indicated in the name of the speaker, in particular this is the third letter.

Let us now consider in order all the speakers that we have in stock:

1)Low frequency - 30GD2, also known as 75GDN-1-4(8):

Here is actually a photo of this miracle of technology:

It is worth saying that at a good volume it distorts the sound considerably, but as practice has shown, this problem is very easily solved and, at that, quite simply.

In general, they sound good, but the filters are tuned close to the resonant frequencies.

Revision 35AS-012

I will describe in order all the stages of improvement that my speakers went through:
1. Disassembly:

First of all, we take them to a secluded place (meaning a room) in which our experimental subjects will not be accessible to children (if any) and other family members. We lay the speaker system on its back and begin to disassemble it.
Now remove the decorative covers from all the speakers and put them aside.

Here they are:

We remove the bass reflex cover and take out the bass reflex itself, and this must be done as carefully as possible, since we are working with plastic, and it can easily break. Then we hide these details in a secluded place.
Now let's take on the regulator/controllers of the HF/MF sections. To dismantle them, you need to remove the decorative plug in the center of the regulator, then unscrew the exposed screw and remove the regulator handle. After this, carefully pry up the remaining plastic lining using two chisels and carefully remove it, then unscrew the 4 screws securing the attenuator itself and now you can push it inside the case. We push it out and unsolder it from the filter. We put it aside, in the future you will need to work a little magic on it. By the way, the joint between the attenuator body and the speaker body is generously covered with a sealing viscous substance; I personally reused it when putting it back in place, but you can use sealant or plasticine.
We take out the cotton wool bags that are supposed to be in your speaker system and set them aside.
We dismantle the panel with filters; it is screwed to the body with screws, having previously unsoldered the wires from the output at the back of the speaker system. We put it aside, since we will spend a lot of time working with them.
Finally, remove the terminal panel from the back cover of the speaker and set it aside.

It seems like a lot of work has been done, but in reality it is just a drop in the ocean. More interesting and time-consuming work lies ahead.

2. Restoration of appearance:

3. Refinement of the speaker system housing:

Here everything is actually very simple, and is carried out in several stages:

If desired, the body can be strengthened. What will this give us? Clearer and smoother bass, since the cabinet panels will vibrate less and, accordingly, will introduce less overtones into the bass component of the sound. How to do this? This is purely a matter for everyone, since as many people there are as many decisions. In general, it all consists of installing spacers, installing additional corners at the joints of the walls of the speaker system, and installing stiffeners on the walls of the speaker. Personally, I limited myself to gluing in additional corners at the joints. You can also tightly glue all joints. Unfortunately, I can’t show you a photo since the entire speaker system is already damped with foam rubber.
Sealing all joints and seams. It is done very simply using various materials. For example, I used plumbing sealant. The procedure is simple: cover the joints with sealant and carefully spread it with your finger, thereby tightly sealing any cracks.
At a hardware store we buy foam rubber 10mm thick (personally, I chose this thickness, don’t take too much as it will suffocate the body) and glue it to all walls except the front. In this way we dampen the body, thereby increasing its virtual volume.

5. Let’s move on to reworking and replacing the filter:

First of all, carefully examine the filter, pay attention to the fastening of the parts, since often the inductors were fastened with metal screws, which immediately disrupts the filter settings.
If there were problems with the fastener, complete it by eliminating metal parts from the fastener. There are also cases of assembling the filter on a metal plate, then transfer the filter to a plywood panel.
We pick up a piece of paper, a pen, and carefully redraw all the elements of the circuit, restoring, so to speak, the filter circuit itself, because the parameters of the speakers were wrong and for this reason the filter circuit could have been changed at the factory. By the way, we exclude the attenuator from the circuit, it simply spoils the sound.
Now we take a soldering iron (preferably 100 watts) and disassemble the filter, or rather, simply remove all the jumpers that were installed from the factory.
Now we assemble the filter, instead of jumpers we will now use a cable made of oxygen-free copper with a cross-section of 4 mm 2, the cable can be bought at any car audio store. It should also be noted that you shouldn’t buy a very expensive cable, as the changes in sound quality will be insignificant, but the costs are simply colossal.
After assembling the filter, we solder the wires that will go to the speakers based on: for the low-frequency link 4 mm 2, for the mid-frequency link 2.5 mm 2, for the high-frequency link 2 mm 2.
We put the filter in place, and then solder the terminal blocks to it (observe the polarity, otherwise you will lose the sound picture).
The very last step is to run the wires to the speakers, secure them and cover the filter with foam.

You'll end up with something similar to these photos:

6. Attenuator installation:

We remove all resistance from it.
Let's put it in its place.
We carefully seal it.
Additionally, we cover it with foam rubber (I only covered it on the front wall)
We install all the decorative panels completely.

7. Installation of the bass reflex:

Everything is simple here, we put it back on the sealant, carefully make sure that it is not pinched anywhere by the foam rubber, as this will throw off its setting.

8. Reinstall the bass reflex cover:

We install it the same way as we removed it, just install it with sealant and new screws, since the panel itself often rattles in the bass. Seal the joint between the panel and the bass reflex well.

9. We proceed to installing the dynamic heads in place:

A) Install the HF head:

B) Install the midrange head:

1) We make a cylinder from foam rubber, such a size that our box will fit into it. We place this cylinder inside the speaker and pass a cable through it, which we take out.

2) Pass the wire through the box (most likely you will have to widen the hole), then put the box in place, adjust the length of the wire and seal the hole into which the wire is passed.
3) Solder the wires to the speaker.
4) Now the crucial stage is damping the mid-frequency head. To do this, we sew a cylinder of foam rubber, such a size that it fits tightly onto the speaker frame and covers all the windows.

5) Fill the box with cotton wool, having previously fluffed it.
6) Put the dynamic head, grille (optional) and frame in place and tighten it.

I came up with this solution a long time ago, look at the photos for more details:

Conclusion: