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 mod_evil 6th May 2006 06:47 PM

Power Transformers in Output!

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.

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?

Felipe Navarro.

 Merlinb 6th May 2006 07:17 PM

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.

 Brian Donaldson 6th May 2006 07:28 PM

Your cans have a much higher imp than 8 ohm. I would suggest a OTL cathode follower with a LTP input to preserve absolute phase and give you a few db gain.

 Merlinb 6th May 2006 07:37 PM

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.

 Brian Donaldson 6th May 2006 07:49 PM

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?

 mod_evil 6th May 2006 08:29 PM

Hey all,

My Sennheisers has 300ohm impedance.

Merlinb,
If I use a 300ohm impedance, can I use the 12v segundaries???

What transformer i need?

Thanks,
Felipe Navarro.

 ilimzn 6th May 2006 08:54 PM

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.

 Merlinb 6th May 2006 10:08 PM

Quote:
 Originally posted by ilimzn [B]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.
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.

 Brian Donaldson 6th May 2006 10:34 PM

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.

 pedroskova 6th May 2006 10:53 PM

Use a parafeed topology with power toroids as OPT's , i.e., a choke-loaded tube that is cap coupled to your transformer(no dc to deal with). The smaller power toroids have bandwidths out to 100k.

Search the web for "parafreed" . Plenty of info out there.

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