Page 2 of 2 - Transformers Audio Book Thread - posted in Transformers Discussion: My youtube posts got deleted. Im trying to recreate them. Here is the new link for The Battle For Cybertron. Megatrons Fight For Power Part 1Megatrons Fight For Power Part 2I lost my original copy of Decepticons Underground, and youtube deleted it. Anyone have a copy made? Audio Frequency (AF) Transformers work at frequencies between about 20Hz to 20kHz and are used in audio amplifier circuits, they were essential in valve (tube) designs for matching the high impedance outputs of theses amplifiers to low impedance loudspeakers, but transistor amplifiers have much less need for output transformers. Free stories for kids of all ages. Audible Stories is a free website where kids of all ages can listen to hundreds of Audible audio titles across six different languages—English, Spanish, French, German, Italian and Japanese—for free, so they can keep learning, dreaming and just being kids. Transformers Lightning Strike Audio Cassette by Ladybird booksChildhood throwback, The Transformers children's books by ladybird came with an audio cassette. Thank you for downloading Audiobook Transformer for Mac from our software portal. The package you are about to download is authentic and was not repacked or modified in any way by us. The download version of Audiobook Transformer for Mac is 1.4. The software is periodically scanned.
Reply | You aren't logged in. (Guest)1Hello fellow radiophiles:
Exact replacements for audio output transformers in old tube radios are often hard to find. Some of the problem is that there is some competition on the market from the audiophile world.
But there is a general purpose multi-Impedance transformer type that has not been 'discovered', and is easy to find at modest prices and in new production.
I am talking about the so-called '70V Line Matching Transformer'. These transformers are marketed for public address use, as you might find in large stores and other public spaces.
The engineering concept behind these transformers is that the power amplifier puts out several watts of power at high impedance, and each of the remote speakers, which could be located hundreds of feet away, has it's own step-down impedance matching transformer.
The reason for this arrangement is low loss transmission of power over long distances with wires that have significant resistance. This is the same reason why Nikkola Tesla transmitted power from Niagara Falls hydro-electric power station over long distances with high voltage transmission lines over a century ago. This concept became universally used.
For example, consider an 8_Ohm speaker driven by 100feet(30m) of #26 (0.4mm diameter) wire. The resistance in 200feet of this wire is 8_Ohms. This means that half the power would be lost in the wire resistance. The problem gets worse with large installations drivine several speakers, where the net parallel speaker impdedance could drop to less than 1_Ohm.
If the same 100 foot speaker wire is used to drive a 1kOhm load, the 8_Ohms of wire resistance are negligible.
Application to tube radios
These transformers come with several primary and secondary taps, so that the output power delivered from the public address amplifier to the speaker can be adjusted without the loss of a rheostat . The power levels that are marked in the primary winding assume an 8 Ohm or 4 Ohm load to the appropriate secondary tap.
I added voltage, or turns, ratio markings to the impedance markings on the box schematic diagram:
The primary impedance is usually not stated in the service data, but tube data sheets often contain this information for individual tubes:
These two examples for a 6L6 and a 1C5, require primary impedances of 2.5K and 8K, respectively. The 2k and 8k taps of the example transformer would work for these two cases. If the requirement is for a 3k tap, you can experiment with the 2k tap and 4k tap for the sound you like better. Perhaps you will not notice a significant difference between the two taps. Be sure to insulate any unused tap wires, which must remain unconnected.
Low frequency response
The measured secondary inductance at the 8 Ohm tap is 4.5mH. The inductance at the various taps is directly proportional to the tap impedance. This means that the primary inductance at the 8kOhm tap is 4.5H.
The inductance establishes the low frequency cutoff point from F=R/(L*2*pi) as 8_Ohm/(2*pi*4.5mH)=282Hz, if the primary transformer is driven by the high output impedance of a pentode or a bipolar transistor. By high output plate impedance, it is meant that this impedance is greater than the impedance seen at the primary, which can be taken to the the stated impedance on the box, if under ideal 8 Ohm or 4 Ohm load conditions.
A lower output plate impedance driving the transformer, such as that at the plate of a power triode will extend the low frequency cutoff. For example, if a triode with an 8kOhm plate impedance drives the 8K tap, the low frequency cutoff is extended to 141Hz. This happens because the 8k of the triode combines in parallel with the 8k load impedance for an effective resistance term R=4k in the general formula F=R/(L*2*pi) for the low frequency cutoff point.
Negative feedback from the speaker terminal to the the input of the amplifier also extends the low frequency cutoff.
In general, good results can be obtained for transformers of the same or smaller physical size, as long as the correct primary and secondary taps are used.
The Physical size establishes the maximum amount of magnetic flux that the transformer can store. The highest flux requirements are at low frequencies, so the size of transformer affects low frequency power handling ability and distortion.
Push-Pull substitutions
The primary taps are arranged in 1.4X step turns ratio or voltage increments. This means that every two steps the turns ratio is 2X. This is ideal for a Push-Pull substitution. A few examples of Push-Pull combinations for various output tube impedances.
| Transformer tap in Ohms | Voltage or turns ratio to output | 2k in p-p | 1k in p-p | 500 in p-p |
|---|---|---|---|---|
| 8000 | 32x | Plate A | ||
| 4000 | 22x | Plate A | ||
| 2000 | 16x | Center Tap | Plate A | |
| 1000 | 11x | Center Tap | ||
| 500 | 8x | Center Tap | ||
| common | 0 | Plate B | Plate B | Plate B |
This table shows three possible impedance configurations for push-pull. The push-pull impedance in the table headers is the plate load impedance that each plate will see, assuming that the other plate has the high impedance of a pentode.
The 2kp-p example would be good for a 6L6 push-pull configuration. The last example column is more likely to be useful in a p-p transistor amplifier substitution.
A note about output impedance requirements for audio amplifiers
The first consideration for the load impedance of output amplifier is the impedance that can deliver the highest level of undistorted power.
This is a very different requirement than matching the high output impedance of a pentode or transistor to an equally high load impedance.
The maximum power that the amplififer can deliver can be first determined from the the curve family of the tube or transistor, such that the load impedance can be represented by a load line that connects the point of lowest voltage and highest current with the point of highest voltage and lowest current.
The following curve family has a 4kOhm load line that crosses pentode curves of a substantially higher impedance. The more horizontal the curve, the higher the impedance.
The two blue dots mark load line limits over which the curve spacing is even enough for low distortion. The red dot marks the plate supply voltage of 105V and 43mA of quiescent current.
Regards,
-Joe Boron mass number.
This article was edited 23.Nov.09 09:31 by Joe Sousa .
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