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# 5K Impedance Audio output Amplifier to 8 ohms

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#### gary350

##### Well-Known Member
Online audio amp transformer calculator is giving me crazy numbers. It says 5K ohm primary and 8 ohm sec. = 25 to 1 ratio. Is that true??? What about the 360vdc 130ma tube. Tube manual says 5K plate transformer.

Information online says #26 enamel coated copper wire resistance determines 5K ohms. 5k x 24.5 ohms per ft = 122,500. ft of wire on the primary. That is 23 miles of wire that will never fit on the transformer. We are talking about Impedance not ohms?

Audio Amplifier is 70w.

What is the correct math to make a 5K audio output transformer pri. for a tube amp, 8 ohm sec. for the speaker?

Why are you trying to make an audio transformer, which is a specialized type and difficult to make?
What would you use for the magnetic core?
Much easier and better to buy one.

The impedance ratio is the square of the turns ratio.

25:1 turns = 625:1 impedance. 625 x 8 = 3125, so it's not an exact match but in the right region.

And remember impedance is inductive (or capacitive) reactance, not wire resistance!

The primary impedance will be (or close to) the load impedance x transformer impedance ratio.

The impedance ratio is the square of the turns ratio.

25:1 turns = 625:1 impedance. 625 x 8 = 3125, so it's not an exact match but in the right region.

And remember impedance is inductive (or capacitive) reactance, not wire resistance!

The primary impedance will be (or close to) the load impedance x transformer impedance ratio.

Impedance ratio is the square of the turns ratio.

25:1 turns = 625:1 impedance

625 x 8 ohms = 3125 ? 8 ohms x 625 = 5000 ohms primary.

Ok, I think I did not understand the 25:1 answer on the audio transformer calculator.

I'm still not sure I understand this?

I know the transformer needs more than 1 turn of wire on the secondary. How is the starting point determined? Can I guess and use 20 turns on the secondary then 20 x 25 = 500 turns on the primary?

Can I guess and use 20 turns on the secondary then 20 x 25 = 500 turns on the primary?
Sure you can guess, but that seldom gives correct results when doing electronic design.

You need enough primary turns so that the maximum applied AC voltage (power) at the lowest audio frequency of interest (typically 20Hz) doesn't saturate the core.
What are you using for a core?

Sure you can guess, but that seldom gives correct results when doing electronic design.

You need enough primary turns so that the maximum applied AC voltage (power) at the lowest audio frequency of interest (typically 20Hz) doesn't saturate the core.
What are you using for a core?

That is a good question. The only way to know what size EI core I need is to know total number of turns on both windings and wire size to make sure it will fit in the slots. Here is an EI core that just happens to be handy, slots measure .5625" x 1.6785". Homemade cereal box cardboard coil form has to fit too. Once I learn the total number of turns it won't be any different than making a step down voltage transformer. I have plenty of #24 enamel coated copper wire I need to be using. #24 wire is .020" diameter, 46 turns per inch. Cereal box cardboards real name is, chip board.

I use to have 100s of saved transformers in 5 gallon buckets. Garage walls had no wall covering 50 years ago so I built shelves between all the 2x4 boards in the walls. I set 200 lbs of transformers on the shelves to get all those heavy 5 gallon bucket off the floor and out of my way. Then when I finally had money to cover walls with plywood I decided it was too much work to haul all those transformers away so I covered them up with 1/2" plywood that was about 1982. My son lives in that house now walls all have work benches and shelves now. Some day someone will find all this transformers. LOL

Here are my 50 year old notes, how to wind transformers, step up or step down voltage transformers. I could never find information how to wind audio transformers, it was not in college text books, college teacher did not know either. The engineer at local Thordarson Meissner Inc was going to tell me about audio transformers when he had time but that factory closed a few months later.

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The only way to know what size EI core I need is to know total number of turns on both windings and wire size to make sure it will fit in the slots. Here is an EI core that just happens to be handy, slots measure .5625 x 1.6785.
You can't randomly choose the core size as that looks way too small for a 70W.
You might look up the size of a 70W audio transformer to get an idea of the size you need.
You need to know at what flux level the core saturates and then work back from there for the maximum power and minimum frequency for the transformer along with the inductance to keep the magnetizing current below saturation.
It's somewhat of an iterative process from there to determine the number of turns and the wire size needed to keep the IR losses at an acceptable level.
It's certainly not a trivial task.

You can't randomly choose the core size as that looks way too small for a 70W.
You might look up the size of a 70W audio transformer to get an idea of the size you need.
You need to know at what flux level the core saturates and then work back from there for the maximum power and minimum frequency for the transformer along with the inductance to keep the magnetizing current below saturation.
It's somewhat of an iterative process from there to determine the number of turns and the wire size needed to keep the IR losses at an acceptable level.
It's certainly not a trivial task.

People on DIY Audio forum said, audio output transformers are 5 times over size so peak pulses are not cut off. 70 watt amp needs a 350 watt audio output transformer. This 60w circuit drawing will be easier to build than the 70w I don't want all the extra options. The EI core in the picture is rated 120 vac 700w. I'm not sure how 120vac 700w relates to a 300w audio transformer 350vdc 130ma.

What EXACTLY are you trying to build? - assuming it's a HiFi amp (rather than a guitar amp) then just buy a transformer - it's critical part if you want to get decent quality from it, just 'chucking windings' on a core is going to produce a crappy transformer.

What EXACTLY are you trying to build? - assuming it's a HiFi amp (rather than a guitar amp) then just buy a transformer - it's critical part if you want to get decent quality from it, just 'chucking windings' on a core is going to produce a crappy transformer.
Any suggestions as to where to get such a transformer? It would be a very specialised transformer.

I repaired a clock radio with a blown transformer many years ago. It had 9 V for the radio, 5 V for the clock LEDs and 12 V for the clock electronics, which also provided the timing. I couldn't find a direct replacement could only find the usual centre-tapped transformers in the same frame size. I bought a 5 or 6V one, used a voltage doubler for the radio supply, and had a separate tiny transformer for the clock.

Any suggestions as to where to get such a transformer? It would be a very specialised transformer.

Valve output transformers are fairly easily available, and still a pretty 'standard' item. There's quite a decent market for home made valve amplifiers.

However, they aren't cheap - but valve output transformers were never cheap.

Here's one random example from google:

Even RS Components sell a small range of valve output transformers.

However, I'm fairly dubious gary350 has his specs. right, as it would be rare for him to do so

Gary, why are you making a 62 years old vacuum tube amplifier today? Where will you buy replacement tubes for it as they burn out? Why spend a lot of money on a rare old output transformer for it? Does your car use an old steam engine and burn oil or is it a modern Tesla electric?

I made a Heathkit tubes amplifier 61 years ago. I thought it sounded great then I took it to a MacIntosh amplifier clinic for it to be tested. It had fairly bad distortion, they said it was because it needed replacement tubes. I replaced the output tubes then it tested with low distortion. A few months later I had it tested again and it had distortion again because the "new tubes" were worn out again. I soon sold the amplifier to an old geezer and bought my first solid state receiver that I still have today. It is 59 years old and still works but its selector switch is worn out.

There is a "stereo store" near me that sells solid state amplifiers that have old tubes glowing on top. They sell a lot of them to people who like old retro stuff.

Gary, why are you making a 62 years old vacuum tube amplifier today? Where will you buy replacement tubes for it as they burn out? Why spend a lot of money on a rare old output transformer for it? Does your car use an old steam engine and burn oil or is it a modern Tesla electric?

I made a Heathkit tubes amplifier 61 years ago. I thought it sounded great then I took it to a MacIntosh amplifier clinic for it to be tested. It had fairly bad distortion, they said it was because it needed replacement tubes. I replaced the output tubes then it tested with low distortion. A few months later I had it tested again and it had distortion again because the "new tubes" were worn out again. I soon sold the amplifier to an old geezer and bought my first solid state receiver that I still have today. It is 59 years old and still works but its selector switch is worn out.

There is a "stereo store" near me that sells solid state amplifiers that have old tubes glowing on top. They sell a lot of them to people who like old retro stuff.

I bought this 60w stereo amplifier, connecting an input takes 10 seconds it is no fun, connecting an output takes a few seconds that is not fun either. The circuit board has built in power supply all it needs is 22 VAC connected to it. The only fun part of this project was winding a 100w transformer for 22vac. Transformer was so easy it was no challenge at all, about 1 hour working winding it on the lathe. Heat sinks was not much of a challenge either. I connected some speakers and a turn table and I can make the walls shake and the neighbors can probably hear it. LOL. This project was too easy.

I don't care how much tube amp costs, I need a fun winter project. Winter will be over soon and I won't be a prisoner of the house again until about December. I have a lathe the transformer is not really a hard job. I already have tooling for other transformers I made. Tube amp needs a 350 VDC power supply transformer too and 6v for tube filaments. Tube rectifier will be replaced with bridge rectifier.

March 1st time to start planting the back yard vegetable garden. Onions, carrots, garlic, potatoes, corn, tomatoes, peppers, beans, melons, more. May 1st time to start camping on the lake. I probably won't get to work on electronic projects again until Dec or Jan. I use to never let weather both me, I am starting to act like old people. LOL.

Your Yuanjing amplifier is solid state with no vacuum tubes. Google shows many but none look like yours.

Your Yuanjing amplifier is solid state with no vacuum tubes. Google shows many but none look like yours.

I suggest you read his post properly!.

Gary said that the 60W Chinese amplifier with four amplifier ICs was no fun. Maybe it is a car 4 channels amplifier?
Gary wants a 62 years old vacuum tube amplifier to build.

Gary said that the 60W Chinese amplifier with four amplifier ICs was no fun. Maybe it is a car 4 channels amplifier?
Gary wants a 62 years old vacuum tube amplifier to build.
Stereo 30w each channel. 4 ICs on PC board with 2 speaker connections, how does that work? 15w each IC ??? No circuit drawing. No instructions either. Not much to build, only a few wires & PS required. A lot of people probably like projects with not much work. Building a case for the amp with knobs is probably more of a challenge than a PS transformer. Instructions are on the PC board.

I found your amplifier in Amazon on the other side of the world (.in).
It is stereo (one IC for left and one IC for right) plus a mono sub-woofer two ICs in parallel to drive a mono 4 ohms woofer).

The transformer MUST have a centertap.
If the amplifier heatsinks are large enough it can drive 4 ohms stereo speakers and a 2 ohms woofer with almost double the power.

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DIY Audio forum is talking about winding your own 5k tube audio transformer. I joined the forum but I am not allowed to talk, I keep getting a message that says, you have no privileges yet.

This is copy and paste from DIY Audio.

Don't try to calculate. Inductance will depend on many parameters. Some of these parameters are easy to determine (cross-section, magnetic path length), but others not so (mu, stacking factor, effective gap). Furthermore, inductance depends much on frequency and AC voltage amplitude. So, wind a test winding and measure it under your specific conditions.

You can use whatever number of turns you find convenient for testing. I regularly use 10 turns, as it makes calculations easy and straightforward.
The level of excitation depends on your application, and your test should match it for a good accuracy: an incorrect level could cause a near 1:10 error ratio.

L =( 1.257*mu*N^2*Ae)*10^-8/Lm

where,
N^2 is the squared number of turns, mu is the effective permeability (I suggest using the small signal permeability for SE application which usually spans from 200 to 450 otherwise for PP use 1500 for small signal to 5000 for max value, with minimal gap), Ae is the effective core cross-section (measure the inner core surface and multiply by the stacking factor of 0.95) in square cm, Lm is the average magnetic path length in cm which you can compute by taking the nominal size of the lamination (it's the width of the centre leg of the E) and multiply by 6. The factor 10^-8 will give you the result in Henry.

Wound 500 turns and measured inductance. If inductance = 7H. Plugging that in gives mu = 4690 which would be close to expected results.

Audio transformers are undistinguishable from power trafos. Apply the same math using the lowest frequency you want the trafo be able to pass and the desired induction. When unknown data, I use 8000 gauss or 0.8T (the same). Use the AC RMS voltage at the primary. And in case of DC premagnetization, don't worry. You can test and adjust gap it in situ.
The issue with this bobbin with separate space for primary and secondary will be the increased leackage inductance. IMO I would wound 1/4 pri in each semi-bobbin (let me create a neologism). Thus, 1/2 secondary in each sector after isolation, and thus, end with 1/4 pri in each them.
This wise you mantain toguether low leackage inductance and low capacitance.

This trafo reused core from verical deflection and perform ultralinear using ECL85 pentode without issues.

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This reminds me when I worked at the transformer factory the engineer had 3 transformers made. Next day he tested all 3 transformers then threw all 3 in the trash. He had 2 more transformers made and tested them. Then they ordered a production run of 500 transformers.

What are these math symbols? ^ *

Arrow up = ?

* = ?

I think / is divide?

What is mu?

What is the formula for mu?

^ is often used as the symbol for "raised to the power of" so N^2 means N squared. N^3 would mean N cubed.
10^-8 means 0.00000001

* is used as the multiply symbol so 2 * 3 = 6 would be an example.

mu is the permeability. https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
I think the equations are using mu (sometimes written as μ) to mean the relative permeability, which is how much more magnetic flux would be in a material than would be in vacuum in the same conditions.

SE and PP are references to types of material that the core could be made out of.

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