Transistor equivalent

Nikolai Petrenko · Nov 30, 2015

I have heared about zobel circuit : normally the value of R is half value of speaker's impedance, wrong?
I will use 1mF (for 8ohm spkr) or 2mF (for 4ohm ones) output capacitors on satellite amp because the cut-off frequencies is below 20Hz, subwoofer too. But I think bridge circuit without output caps (like BTL or OCL) will be better for very low frequencies.

Nikolai Petrenko · Nov 30, 2015

audioguru said:
Here in the West, electronic parts are very inexpensive because money here grows on trees (it is easy to make a lot of money working here). I am usually shocked to hear that many people on the other side of the world that I have never seen get paid in a week about what I got paid in a couple of hours. Now I am retired and my government is paying me a comfortable pension, I can buy almost anything I want (but not a Lamborghini car yet). We pay a lot for government taxes but there is plenty of money to pay for it.

We had a special shopping day recently called Black Friday. Stores were selling large 4K Ultra High Definition flat screen TVs for the price paid by working for a few hours. Amplifiers were selling with so many channels (7.2) that there are speakers all over the place.

I want to create a channel in Youtube about DIY anythings I have known, but my family condictions not allow it. I think I will make a little money by make circuits or fix failed devices as my add "income" to develope my projects or plans.

MrAl · Nov 30, 2015

spec said:
Hmm! heady stuff- sounds like you have studied this quite a lot.

In a word, State Variable Feedback. Oh wait that's three words
One day i decided to try this for a simple buck switching regulator. I was after super compliant response. What i found was that although the controllability gramian indicated that this should be possible, after looking at all the circuit nodes carefully it was found that method assumed that we had signal levels everywhere in the system that were actually needed. For example, one node might reach as high as 25 volts peak when the gramian was satisfied, but in the actual system we might only have a 10 volts supply! That was the practical end, but we could get that super great response if we actually did allow that 25v peak by increasing the power supply voltage. That's only one example with made up numbers, but in reality the voltage could have needed to go to 100v or even 1000v. The only way to tell is to look at every node. But the main point was, given that required 25v was available we actually obtain the great response.

I see your approach about having a speaker with characteristics tailored for control. These days you dont have to worry too much about efficency and transient power demands- that can be relatively easy to sort. I'm not sure about this, but I think the speakers in the aticles in the mag I mentiond had an auxilary coil for monitoring what the speaker was doing.

Good point about not mentioning mic- I was speculating. Controlling the cone is a feasible task as far as I can tell. Also, as you imply, one of the reasons why this servo thing hasn't taken off is the cost as normal open-loop speakers can be pretty good and not fantastically expensive like they used to be- relative to salaries.

I have rewound a few speakers in the past- you have got me thinking.

Slight change of subject but still speakers- are you ready for this. A speaker chassis mounted in an airtight box and all the air pumped out. Forgeting the practical aspects, for the moment, it would have a lot of advantages; no reflections from the rear for example, so you would loose the boxy sound. And I don't think the resonant frequency of the cone would be upped like is a nomal box because a vacuum has 100% compliance.

Perhaps we could design a new speaker sysytem- vacuum and servo. A a sub-sonic servo could place the cone in the centre of its travel under quesent conditions- one of the problems of the approach.

The extra coil sounds interesting, although it would only allow monitoring of the AC response. Any DC information would be lost. A physical sensor would overcome this.

A speaker box with all the air pumped out? I dont think so
Air is the medium in which sound waves travel. Without air, no sound

audioguru · Nov 30, 2015

Hi Spec,
You probably remember when the first solid state audio amplifiers used a quasi-complementary output similar to the one in the LM3886 IC amplifier because in those days a good PNP power transistor was not available or was very expensive.
I guess the same occurs in ICs. It is also called a "compound pair" or a "Sziklai pair":

spec · Nov 30, 2015

MrAl said:
In a word, State Variable Feedback. Oh wait that's three words
One day i decided to try this for a simple buck switching regulator. I was after super compliant response. What i found was that although the controllability gramian indicated that this should be possible, after looking at all the circuit nodes carefully it was found that method assumed that we had signal levels everywhere in the system that were actually needed. For example, one node might reach as high as 25 volts peak when the gramian was satisfied, but in the actual system we might only have a 10 volts supply! That was the practical end, but we could get that super great response if we actually did allow that 25v peak by increasing the power supply voltage. That's only one example with made up numbers, but in reality the voltage could have needed to go to 100v or even 1000v. The only way to tell is to look at every node. But the main point was, given that required 25v was available we actually obtain the great response.

Very interesting. I wish I had paid more attention to control theory- may be I will gen up on it one day.

MrAl said:
The extra coil sounds interesting, although it would only allow monitoring of the AC response. Any DC information would be lost. A physical sensor would overcome this.

Yes, that's what I though. An auzilary voice coil is only a half-way house!

MrAl said:
A speaker box with all the air pumped out? I dont think so
Air is the medium in which sound waves travel. Without air, no sound

That is the very objective. No rear sound, but front sound normal, which is what you listen to (forget ported cabs for the time being) No rear sound elliminates the three disadvantages of cabinets:

(1) No rear sound, thus no reflections in cabinet, therefore no boxy sound.

(2) No air borne sound forces on the cabinet, one again because no air. The result is no panel flexing which colours the sound. The mechanical coupling cause by the speaker being mounted still exists but this can be greatly reduced by gel gaskets and the like which are well documented.

(3) As the compliance of a vacuume is 100% the resonant frequency of the speaker chassis will not be lowered like it is in a closed cabinet. This gives more bass extension.

No, the main problem with the vacuume cabinet, neglecting the practical aspects, is that a loudspeaker chassis mounted in a vacuated cabinet will, if left to its own devices, semi half wave rectify the sound. The cone would have to overcome air pressure (14.7 lb per sq inch) on its forward travel, but on its rearward travel the air pressure would greatly assist. I haven't got to the bottom of this aspect yet, but it did occiur to me that your control stuff could sort this out.

audioguru · Nov 30, 2015

Hi Nikolai,
When I was 14 I picked all the money off nearby trees until there was no more. I mowed grass and cleaned windows to make some money, then I got the idea of collecting money for people to park their cars in the nearby parking lot between the hockey arena and my home. I pretended that I worked at the office where there was the parking lot. I collected a lot of money in maybe 20 minutes before each game.

audioguru · Nov 30, 2015

The resonant frequency of a speaker is increased when it is mounted in a sealed enclosure. The enclosure is usually filled with sound absorbent material to eliminate the boxy sound and it lowers the resonant frequency a little because the sound waves in the absorbent material are converted into heat waves that travel slower.

spec · Nov 30, 2015

audioguru said:
Hi Spec,
You probably remember when the first solid state audio amplifiers used a quasi-complementary output similar to the one in the LM3886 IC amplifier because in those days a good PNP power transistor was not available or was very expensive.
I guess the same occurs in ICs. It is also called a "compound pair" or a "Sziklai pair":

Yes, remember it well AG- those were the days. No PNP transistors- what a pain!. In the UK at that sort of time there was great discussions about adding a power diode in the driver emitter of the Sziklai pair (no idea how you pronounce that name). The optimum seemed to be a 1N4001 type with a 2K7 resistor in parallel.

Many discussions and distortion plots in the mags. Being a purist, I used the e/b junction of a power transistor- never heard any difference though. Mind you, the transducers were nowhere near as revealing as now, apart from electrostatics that is. I couldn't even afford the glossy sales brochure for one of those at the time

I had an RCA dual concentric 8" speaker. Boy did I love that. Class A, 5W single-ended 6V6GT valve amp too- none of this fancy stereo stuff either.

You guessed it, speaker went up in smoke one day- I was upset for a month. But then my Dad took pity on me and gave me an 8" Wharfedale dual-concentric, complete with cambric negative roll surround- nothing better, until one day ...

spec · Nov 30, 2015

Nikolai Petrenko said:
May "ground loop"? I have test on small TDA2822M amp, if I wire the ground like a loop, the amount noise will increase.
You can explain all reasons, I feel interested in learning about electric, my favor led me to good at physic in the class. (I am 14 years old, and like a young bull )

14 years old , you amaze me. I was thinking more like 20.

You must be very bright. When I was 14, I had trouble figuring Ohms law and would never have dreamt of going into designs like you are- just lights, electric motors, and buzzers then. Keep up the good work- you will become an electronics genius and make much NVD: then maybe buy Linh kiện điện tử TuHu (https://mualinhkien.vn/)

You read my posts? Not bored- they are very long

MrAl · Nov 30, 2015

spec said:
Very interesting. I wish I had paid more attention to control theory- may be I will gen up on it one day.

Yes, that's what I though. An auzilary voice coil is only a half-way house!

That is the very objective. No rear sound, but front sound normal, which is what you listen to (forget ported cabs for the time being) No rear sound elliminates the three disadvantages of cabinets:

(1) No rear sound, thus no reflections in cabinet, therefore no boxy sound.

(2) No air borne sound forces on the cabinet, one again because no air. The result is no panel flexing which colours the sound. The mechanical coupling cause by the speaker being mounted still exists but this can be greatly reduced by gel gaskets and the like which are well documented.

(3) As the compliance of a vacuume is 100% the resonant frequency of the speaker chassis will not be lowered like it is in a closed cabinet. This gives more bass extension.

No, the main problem with the vacuume cabinet, neglecting the practical aspects, is that a loudspeaker chassis mounted in a vacuated cabinet will, if left to its own devices, semi half wave rectify the sound. The cone would have to overcome air pressure (14.7 lb per sq inch) on its forward travel, but on its rearward travel the air pressure would greatly assist. I haven't got to the bottom of this aspect yet, but it did occiur to me that your control stuff could sort this out.

Hi,

I guess i did not understand the intent of your physical construction idea. You said pump the air out, but now i am not sure where you are pumping the air out of. Are you saying just the back of the speaker then? If so, wouldnt that pull the cone in toward the back?
Maybe you are suggesting to use a vacuum as the insulation, similar to a thermos bottle. But then the sound would still reflect off the inside panels. But feel free to elaborate a little more.

spec · Nov 30, 2015

MrAl said:
Hi,

Hi MrAl

I guess i did not understand the intent of your physical construction idea. You said pump the air out, but now i am not sure where you are pumping the air out of. Are you saying just the back of the speaker then? If so, wouldnt that pull the cone in toward the back?
Maybe you are suggesting to use a vacuum as the insulation, similar to a thermos bottle. But then the sound would still reflect off the inside panels. But feel free to elaborate a little more.

I see, your point of view now. My fault! May be I didn't explain fully. What I am proposing is an airtignt enclosure (cabinet) just big enough to house the speaker chassis. Also the speaker cone is airtight. The speaker chassis is mounted to the cabinet with a gasket so that the enclosure is now airtight. You then evacuate the air from the cabinet. Of course the speaker cone will be pushed back untill it hits the stops.

Thats where the servo comes in. Under quescent conditions the servo puts enough current through the speaker coil to centralise the cone ( the standing bias in fact). If you then imposed a normal audio signal on top of the bias current I am not sure what the speaker would do and it is the bit I will have to think about.

Assuming the cone moves differently forwards than it does backward, I was thinking that a control loop could linerise the cone movement.

Different subject, but you have got me thinking about a cabinet like a Thermos flask with evacuated walls... Hmmm

(PS: this vacuume cabinet is a crazy idea and just for discussion. There are many practical problems like the static force on the cone (area of cone in inches x 14.7 pounds: thats some load!)

MrAl · Nov 30, 2015

spec said:
I see, your point of view now. My fault! May be I didn't explain fully. What I am proposing is an airtignt enclosure (cabinet) just big enough to house the speaker chassis. Also the speaker cone is airtight. The speaker chassis is mounted to the cabinet with a gasket so that the enclosure is now airtight. You then evacuate the air from the cabinet. Of course the speaker cone will be pushed back untill it hits the stops.

Thats where the servo comes in. Under quescent conditions the servo puts enough current through the speaker coil to centralise the cone ( the standing bias in fact). If you then imposed a normal audio signal on top of the bias current I am not sure what the speaker would do and it is the bit I will have to think about.

Assuming the cone moves differently forwards than it does backward, I was thinking that a control loop could linerise the cone movement.

Different subject, but you have got me thinking about a cabinet like a Thermos flask with evacuated walls... Hmmm

(PS: this vacuume cabinet is a crazy idea and just for discussion. There are many practical problems like the static force on the cone (area of cone in inches x 14.7 pounds: thats some load!)

Hi,

Yeah that's what i was thinking. If you pull enough air out you've got one heck of a force pushing it in, which has to be overcome again and again just like air pressure. Imagine having it filled with sponge attached to the cone that shrinks, pulling the cone back. The coil current has to counter that completely. Depending on the surface area, that could be a lot of current required. Could it be regained? Maybe, but it sounds pretty high to begin with so im not sure how this would work.

spec · Nov 30, 2015

I have plenty more hair-brained ideas

Been thinking- yes a mechanical restoring force at the back of the speaker- good idea

spec · Nov 30, 2015

Assume 5" speaker:

Cone area = 19.63 sq" * 14.7 lb sq "= 289 lbs....hell

Nikolai Petrenko · Nov 30, 2015

audioguru said:
Hi Nikolai,
When I was 14 I picked all the money off nearby trees until there was no more. I mowed grass and cleaned windows to make some money, then I got the idea of collecting money for people to park their cars in the nearby parking lot between the hockey arena and my home. I pretended that I worked at the office where there was the parking lot. I collected a lot of money in maybe 20 minutes before each game.

LOL , I have done this work only for 1 time at the circus show.

Nikolai Petrenko · Nov 30, 2015

audioguru said:
The resonant frequency of a speaker is increased when it is mounted in a sealed enclosure. The enclosure is usually filled with sound absorbent material to eliminate the boxy sound and it lowers the resonant frequency a little because the sound waves in the absorbent material are converted into heat waves that travel slower.

I will MDF to make speaker boxes and fill half box with natural cotton to make the sound waves change into thermal waves

Nikolai Petrenko · Nov 30, 2015

spec said:
14 years old , you amaze me. I was thinking more like 20.

You must be very bright. When I was 14, I had trouble figuring Ohms law and would never have dreamt of going into designs like you are- just lights, electric motors, and buzzers then. Keep up the good work- you will become an electronics genius and and make much NVD: then may be buy Linh kiện điện tử TuHu (https://mualinhkien.vn/)

You read my posts? Not bored- they are very long

Thanks to my father teach me. Before June, I mostly play with junks

audioguru · Nov 30, 2015

Nikolai Petrenko said:
I will MDF to make speaker boxes and fill half box with natural cotton to make the sound waves change into thermal waves

If you make sealed speaker boxes then you should use speakers designed for a sealed box (very low resonance frequency). Other speakers work well in a ported speaker box if the port and box size are what are recommended by the speaker manufacturer.

spec · Nov 30, 2015

Nikolai Petrenko said:
I mostly play with junks

UPDATE
Fully woken up now- got it! You didn't do electronics, you just had fun.

Nikolai Petrenko · Nov 30, 2015

spec said:
UPDATE
Fully woken up now- got it! You didn't do electronics, you just had fun.

junks here are LED, small motor, batteries, and sometime blow up capacitors

Transistor equivalent

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