A friend called me, who is possible use Power Transformers in Output of Tubes.
Fine.
I'm thinking to make a amplifier for mine Sennheiser HD580. With EL84 Pentode in SE.
What they think about this? I have two transformers with 127-220 primaries and 12+12 2A segundaries!
The same friend called me, who I need a LOW voltage ( 6 ~ 7v ) secundaries to can use power transformers in the output.
Can anybody help me?
Thanks in advance,
Felipe Navarro.
Fine.
I'm thinking to make a amplifier for mine Sennheiser HD580. With EL84 Pentode in SE.
What they think about this? I have two transformers with 127-220 primaries and 12+12 2A segundaries!
The same friend called me, who I need a LOW voltage ( 6 ~ 7v ) secundaries to can use power transformers in the output.
Can anybody help me?
Thanks in advance,
Felipe Navarro.
I have done this (mine was push pull though). Here's how to choose a transformer:
Decide what primary impedance you want. It might be 5200 ohms for an EL84.
Divide by speaker impedance, it might be 8 ohms.
5200/8 = 650
Find the square root- because you need to find the TURNS ratio from the IMPEDANCE ratio, and one is the square of the other.
sqaure root of 650 = 25
So turns ratio of transformer is 25:1
Divide the primary VOLTAGE of the transformer by the turns ratio. You said you have a 220V transformer, so:
220/25 = 8.8V
So you would need a 220 to 8.8V transformer.
You can work backwards of course too, so:
A 220V : 9V transformer would be close = 4700 ohms primary impedance.
A 220V : 6V transformer would be 10000 ohms primary impedance, also pretty good.
Your 220V : 12V transformer would be 2600 ohms = too low! BUT- connect to a 16 ohm speaker doubles it = 5300 ohm = ok!
However, it will sound BAD for hifi.
Its ok for LoFi, like guitar amps.
For a single ended design, use a transformer capable of twice the wattage than you need, so for an EL84 say a 12VA transformer- nice and big.
Decide what primary impedance you want. It might be 5200 ohms for an EL84.
Divide by speaker impedance, it might be 8 ohms.
5200/8 = 650
Find the square root- because you need to find the TURNS ratio from the IMPEDANCE ratio, and one is the square of the other.
sqaure root of 650 = 25
So turns ratio of transformer is 25:1
Divide the primary VOLTAGE of the transformer by the turns ratio. You said you have a 220V transformer, so:
220/25 = 8.8V
So you would need a 220 to 8.8V transformer.
You can work backwards of course too, so:
A 220V : 9V transformer would be close = 4700 ohms primary impedance.
A 220V : 6V transformer would be 10000 ohms primary impedance, also pretty good.
Your 220V : 12V transformer would be 2600 ohms = too low! BUT- connect to a 16 ohm speaker doubles it = 5300 ohm = ok!
However, it will sound BAD for hifi.
Its ok for LoFi, like guitar amps.
For a single ended design, use a transformer capable of twice the wattage than you need, so for an EL84 say a 12VA transformer- nice and big.
There is nothing obviously worng with the circuit, except the standby switch. Disconnecting valve cathodes while HT is applied will damage valves over time. A standby switch should be in the HT supply, before or after the rectifier.
Are you quite sure the oscillation is coming from the output stage? You're not simply hearing oscillation from one of the pre amp stages?
I would perhaps reduce the size of the capacitors C14 and C15 to 33nF to avoid low-frequency oscillation. Also check them for leakage.
Are you quite sure the oscillation is coming from the output stage? You're not simply hearing oscillation from one of the pre amp stages?
I would perhaps reduce the size of the capacitors C14 and C15 to 33nF to avoid low-frequency oscillation. Also check them for leakage.
Oh yea, I also forgot, SE and power trans for OPT is a bad combination. I've heard toroids can make pretty good push-pull opt , but are very intollerant of DC bias, and EI OPT's will have very poor high freq response.
If you parafeed a toriod, you can get decent results, but for HP amp, dump the transformer and use a 10W or so triode cathode follower. Maye a 6SA7?
If you parafeed a toriod, you can get decent results, but for HP amp, dump the transformer and use a 10W or so triode cathode follower. Maye a 6SA7?
First of all you cannot simply use a power transformer as an output transformer for a SE amp. A SE amp has DC bias current going through the transformer, power transformers are not designed for DC bias (no airgap), hence you can't use them here.
For a PP amp you may be able to use a power transformer as Merlinb suggested. Assuming you can balance the DC currents ideally, a toroidal transformer may provide more than decent results!
The 'use twice the VA rating if it's SE' mentioned by Merlinb is downright WRONG. In PP amps, the size depends on the maximum flux chosen and the lowest frequency you want full power at. For 20 Hz you need a transformer that would have about 6 times higher VA rating if it was used as a power transformer - In THEORY. Various other considerations may require an even bigger core. For SE things are even more complex and because an airgap is needed, the transformer is even larger. 10W SE output transformer cores could easily be used as 100, in some cases even 200VA power transformers if their airgap was removed.
Finally, if you wish to drive 300 ohm headphones with an EL84, you should not need an output transformer at all, if you use the proper topology.
For a PP amp you may be able to use a power transformer as Merlinb suggested. Assuming you can balance the DC currents ideally, a toroidal transformer may provide more than decent results!
The 'use twice the VA rating if it's SE' mentioned by Merlinb is downright WRONG. In PP amps, the size depends on the maximum flux chosen and the lowest frequency you want full power at. For 20 Hz you need a transformer that would have about 6 times higher VA rating if it was used as a power transformer - In THEORY. Various other considerations may require an even bigger core. For SE things are even more complex and because an airgap is needed, the transformer is even larger. 10W SE output transformer cores could easily be used as 100, in some cases even 200VA power transformers if their airgap was removed.
Finally, if you wish to drive 300 ohm headphones with an EL84, you should not need an output transformer at all, if you use the proper topology.
ilimzn said:
You CAN use them, it will simply be EXTREMELY inefficient. I was assuming this was for a small LoFi design or experimentation- for HiFi or prolongued use I'd agree with you obviously. At the end of the day, a transformer is nothing more than some copper around some iron. You can abuse it as much as you want and get it to 'work', but it won't work 'well' granted. It's just a matter of what you want to achieve.
When I was about 10, I wrapped about 40 turns of #14 wire around a 1/2"carrage bolt, then another 10 turns of #14 on top of that. I then plugged the first set of windings into the wall outlet. Yes, I was trying to make a transformer. I would say it didn't work, but you may disagree.
With the core saturated, the inductance is greatly deminished and you're heating a tube anode more than anything else. You will get some garbled, highly distorted noise out, but I wouldn't call it working.
With the core saturated, the inductance is greatly deminished and you're heating a tube anode more than anything else. You will get some garbled, highly distorted noise out, but I wouldn't call it working.
I typically assess my power transformers for this duty before sorting the rest for landfill 
You can put a resistor in series with the primary, equal to about your output stages desired load. Sweep a sinewave across the spectrum and find the frequencies where the resistor drops all the voltage.
Some power transformers don't have enough inductance to be useful at the bottom end, and some have too much interwinding capacitance to be useful at the top end.
Of all the power xfmrs I look at, I find more that are useful as chokes, and some that could maybe be used as interstage xfmrs
You can put a resistor in series with the primary, equal to about your output stages desired load. Sweep a sinewave across the spectrum and find the frequencies where the resistor drops all the voltage.
Some power transformers don't have enough inductance to be useful at the bottom end, and some have too much interwinding capacitance to be useful at the top end.
Of all the power xfmrs I look at, I find more that are useful as chokes, and some that could maybe be used as interstage xfmrs
For 300 ohm headphones, EL34 seems like overkill. You only need about 1/2 watt of OP power, which I think you could get from a commonly available double triode, such as a 12AU7 or 6SN7, operating in class A push-pull. That would enable you to use a toroidal power transformer as the OP transformer. I don't know how good the bandwidth would be but it would certainly be better than an EI tranny.
The toroidal transformer would need a split primary, such as you can find in a "universal" power transformer rated for 115v/230v, with a secondary voltage rating selected according to the ideal plate-to-plate loading of a 12AU7/6SN7 in class A push-pull. Perhaps someone can suggest a suitable ratio?
The toroidal transformer would need a split primary, such as you can find in a "universal" power transformer rated for 115v/230v, with a secondary voltage rating selected according to the ideal plate-to-plate loading of a 12AU7/6SN7 in class A push-pull. Perhaps someone can suggest a suitable ratio?
I'am inclined to agree with people when they say that no output transformer is the best with a headphone amp. With a 300ohm load this should be fairly easy. Look to the Headwize site for good circuits.
However mains transformers can be used for outputs with a few provisos.
EI can be used but sound crap (to much interwinding capacitance).
Toroidals can sound excellent but in SE can only cope with AC, therefore Parafeed is essential. As said before, think 4-6x the required watts in VA rating. Even then there will be acertain amount of roll off in the bass. Because parafeed is been used then the roll off can be compensated by forcing the transformer- parafeed capacitor to resonate at about 10Hz, this will pick up the bass nicely. I have done this in my main 807 amp and it sounds very good.
I have also built a simple headphone amp with ECL82's. I used 240:24V transformers at about 30VA, again there is low frequency roll off, but again this can be compensated for. I used 200V B+ with a 3K3 resistor as the anode load.
In all cases you can expect a better high frequency response than from a conventional airgapped transformer. They would make an excellent companion to horn loaded speakers where the bass is already missing.
I am currently experiementing with toroidals in a 6AS7 PP amp. The higher the ouput current, and lower the output voltage - the better the expected results are likely to be.
Shoog
However mains transformers can be used for outputs with a few provisos.
EI can be used but sound crap (to much interwinding capacitance).
Toroidals can sound excellent but in SE can only cope with AC, therefore Parafeed is essential. As said before, think 4-6x the required watts in VA rating. Even then there will be acertain amount of roll off in the bass. Because parafeed is been used then the roll off can be compensated by forcing the transformer- parafeed capacitor to resonate at about 10Hz, this will pick up the bass nicely. I have done this in my main 807 amp and it sounds very good.
I have also built a simple headphone amp with ECL82's. I used 240:24V transformers at about 30VA, again there is low frequency roll off, but again this can be compensated for. I used 200V B+ with a 3K3 resistor as the anode load.
In all cases you can expect a better high frequency response than from a conventional airgapped transformer. They would make an excellent companion to horn loaded speakers where the bass is already missing.
I am currently experiementing with toroidals in a 6AS7 PP amp. The higher the ouput current, and lower the output voltage - the better the expected results are likely to be.
Shoog
Friends,
A friend have alerted me who I'm not need a output transformer to driver my headphone.
I was win two ECC82, it isn't the best tube. I will mount a amplifier for my Sennheiser and put some photos and questions in here.
ilimzn, what topology you think is the best to I use with a ECC82 to driver my fones?
Shoog, have you a schematic of this amp with ECL82?
Thanks in advance for all replyes.
Felipe Navarro.
A friend have alerted me who I'm not need a output transformer to driver my headphone.
I was win two ECC82, it isn't the best tube. I will mount a amplifier for my Sennheiser and put some photos and questions in here.
ilimzn, what topology you think is the best to I use with a ECC82 to driver my fones?
Shoog, have you a schematic of this amp with ECL82?
Thanks in advance for all replyes.
Felipe Navarro.
This headphone amp can be used with an ECC82 without any major changes http://headwize.com/projects/showfile.php?file=cmoy5_prj.htm.
Heres a link to the original thread in which I designed it.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=63258&perpage=10&highlight=&pagenumber=6
I since modified the load on the triode to 100K and the cathode resistor to 1K, this allowed the plate to plate feedback to be increased a little without swamping the driver triode. You could probably reduce the load and cathode resistor further.
Shoog
http://www.diyaudio.com/forums/showthread.php?s=&threadid=63258&perpage=10&highlight=&pagenumber=6
I since modified the load on the triode to 100K and the cathode resistor to 1K, this allowed the plate to plate feedback to be increased a little without swamping the driver triode. You could probably reduce the load and cathode resistor further.
Shoog
If I manage to separate the E-s from the I-s in a power transformer (E+I) and put an air (paper) gap, I could use it as an OPT?
The transformer I intend to use looks to be very responsably built; each layer in the primary/secondary is isolated with paper, the output voltage is 40V (from 220), so I cand modify the ratio to 20:1 or similar.
The downside I see, is that the primary/secondary are wired in one piece and not P=4*800....
The transformer I intend to use looks to be very responsably built; each layer in the primary/secondary is isolated with paper, the output voltage is 40V (from 220), so I cand modify the ratio to 20:1 or similar.
The downside I see, is that the primary/secondary are wired in one piece and not P=4*800....
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