1:1 output transformer question

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi!

I would like to try a SS amp with 1:1 output transformer. :bigeyes:
Yes, yes I know it is a stupid idea, but I have heard so many oppinions pro and contra about output transformers, that now I would like to test it myself.
I now the sound of the direct coupled and also the capacitor coupled amps. Now here is the time to get acquainted with the transformer coupled amps.:D
I would like to try it, for example with the good reference amp the JLH69 (if it is possible at all.... ) or with another ca. 15W capacitor coupled SS amp instead of the output caps.
Could somebody help me how can I calculate the parameters of such an 1:1 output transformer?

Greets:

Tyimo
 
Hi Sajti!

The design should be same as the tube output transformers. You know the input, and output voltages, and the impedances. The structure must be same as well

Could you be a bit more detailed?

One of my amps has:
- +/- 12Vrms output
- 2A bias
- 8R speaker

I don't know the output impedance of my amp.
I use 4.700uF output caps and I would like to try the 1:1 output transformer in stead the caps.

Tyimo
 
Tyimo said:
Hi Sajti!



Could you be a bit more detailed?

One of my amps has:
- +/- 12Vrms output
- 2A bias
- 8R speaker

I don't know the output impedance of my amp.
I use 4.700uF output caps and I would like to try the 1:1 output transformer in stead the caps.

Tyimo

If You have symmetrical PSU, You don't need output capacitor. There is no chance for the output transformer, if You have DC offset on the output of the amplifier.

So first You have to get the size of the iron, which is depended by the power. If You have it, You can count the number of the turns, which is depended by the voltage. In practice the primary, and the secondary turns can be same

sajti
 
if you have a cap coupled output, and are just wanting to try an output transformer, just put the transformer primary in place of the speaker, and the secondary to the speaker. you will need to keep the cap in circuit, since you have a DC level at the junction between the output halves. a 1:1 toroid is easy, but time consuming to make. you'll want to calculate the inductance of the primary to have an XL of at least 4 ohms at 20 or 30 hz. also you really should have "kickback" diodes across the output transistors, because if the speaker leads ever go open for any reason, your load will be almost purely inductive, and the reverse emf will show up across the output transistors. if you have a bipolar supplied amp (no dc offset on output, + and - power supplies) just hook up the primary across the speaker output. the other way to do it is to use a push-pull arrangement with split phase drive (as is done with tubes). that way, you will need a split primary winding, and all of your output devices can be one polarity (PNP or NPN) with a single power supply feeding at the center tap, and the output transistor collectors at the ends. a lot of early SS amps were designed this way, before the idea of complementary symmetry and quasi comp-symm amps came about. it was a holdover from tube amps, and it worked well.

one caveat.... if you try this with a cap output amp, you will have a problem at one frequency, where the combination of the capacitor and the transformer become series resonant, and the amp will see a short across it's output (the total load impedance will be the total of the cap's ESR and the DC resistance of the transformer primary). since different amps use different output caps, and you don't yet know the inductance of your winding, you may want to sweep the LC circuit with a function generator to see where that resonant point is (hopefully, it's somewhere outside of the audio spectrum). also with series resonance, the voltage across the cap may be much higher than the working voltage of the cap
 
So first You have to get the size of the iron, which is depended by the power. If You have it, You can count the number of the turns, which is depended by the voltage. In practice the primary, and the secondary turns can be same

Thanks Sajti!

So in my case:
-iron size for 15W/channel
-number of the turns for 12V or 24V?

Maybe last time I wrote wrong: my amp is a Mosfet follower and need +/-12V input voltage swing.
I have 2A bias and +30Vdc. So I get:
Upeak=(0,5*30)-2=13V and Urms=13*0,707=9,2Vrms and
Prms(U)=(9,2*9,2)/8=10,6Wrms.
With 8 ohm load, my amp is voltage limited to 10W.

There is no chance for the output transformer, if You have DC offset on the output of the amplifier.

Yes, but I saw the Zeus amp's SE version from Susan Parker and it is working fine. http://www.susan-parker.co.uk/zeus-se-amp.htm

Tyimo
 
Hi unclejed613!

you'll want to calculate the inductance of the primary to have an XL of at least 4 ohms at 20 or 30 hz. also you really should have "kickback" diodes across the output transistors, because if the speaker leads ever go open for any reason, your load will be almost purely inductive, and the reverse emf will show up across the output transistors.

Thanks for the help!

you will need to keep the cap in circuit.....

This what I want to avoid.....:xeye:

Tyimo
 
Tyimo said:


Thanks Sajti!

So in my case:
-iron size for 15W/channel
-number of the turns for 12V or 24V?

Maybe last time I wrote wrong: my amp is a Mosfet follower and need +/-12V input voltage swing.
I have 2A bias and +30Vdc. So I get:
Upeak=(0,5*30)-2=13V and Urms=13*0,707=9,2Vrms and
Prms(U)=(9,2*9,2)/8=10,6Wrms.
With 8 ohm load, my amp is voltage limited to 10W.



Yes, but I saw the Zeus amp's SE version from Susan Parker and it is working fine. http://www.susan-parker.co.uk/zeus-se-amp.htm

Tyimo



Hi,

please note, that Susan's amplifier has single power supply, and the transformer is designed as part of the follower. Your mosfet follower has current source (Ithink), and if You connect the output transormer, You will short the ouput of Your amplifier. To use the output transformer You have to redesign the whole follower as Susan's amplifier.

Sajti
 
Hi Sajti!

Susan's amplifier has single power supply, and the transformer is designed as part of the follower. Your mosfet follower has current source (Ithink), and if You connect the output transormer, You will short the ouput of Your amplifier.

Now it is clear and I understand. You and unclejed613 have true, I have to use bipolar (+/-) supplied amp (no dc offset on output, + and - power supplies). Maybe I could try it with the Hiraga Monster!:)

To use the output transformer You have to redesign the whole follower as Susan's amplifier.

I think I could! It would be something like a choke loaded Mosfet follower!!
I can calculate and redesign the bias etc.
It need a 120mH/0.5R/3A stable EI airgaped choke. So with transformer the primary and secondary's value would be the same and equal, it isn't it???

Thanks a lot!

Greets:

Tyimo
 

I think I could! It would be something like a choke loaded Mosfet follower!!
I can calculate and redesign the bias etc.
It need a 120mH/0.5R/3A stable EI airgaped choke. So with transformer the primary and secondary's value would be the same and equal, it isn't it???

Thanks a lot!

Greets:

Tyimo [/B]



Not necessary to use choke If You use transformer! The transformer subsitute the choke, with some more features! my experience, that vertical MOSFETs sound better is the bias is higher. So with transformer output, You can use lower rails, and higher current

Sajti
 
Not necessary to use choke If You use transformer!

Yes of course. I meant only that the transformer primary's parameters need to be like the chokes has.


So with transformer output, You can use lower rails, and higher current

I would like to ask something about choke loaded amps power calculations, maybe you can help me!
I know, if I ignore stuff like saturation voltage and losses in the choke, just divide the rail voltage by 1,414, square the result and divide by the load impedance.

(+U/1.414)2/Rload

But, what about the current??? How will I know that my amp is voltage limited or current limited into any given load?????


Greets:

Tyimo
 
The choke load, and the transformer load different. The choke do AC isolation only, but the transformer transform the load impedance to, the follower.
If You have choke, and 4ohm load, the follower will see 4ohm load, if the inductance of the choke is high enough.
If You have 2:1 transformer, and 4ohm load, the follower will see 8ohm. Of course the inductance of the transformer must be enough to handle 20Hz too.
I'm not familiar in choke load.

sajti
 
Hi,
How will I know that my amp is voltage limited or current limited into any given load
measure the voltage into various load resistances. it will allow calculation of output impedance at the higher load values, but as the load resistance falls the calculated impedance will rise, indicating that current is becoming limited.
If You have 2:1 transformer, and 4ohm load, the follower will see 8ohm.
transformers are specified in either ohms or turns ratio. If you see "2:1" it is more likely to be turns ratio. Impedance conversion is (turns ratio) squared.
The 2:1 transformer will present a 16ohm load, when the real load is 4ohm (in the bandwidth where it is effective).
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.