300B OPT 8 ohms for headphone amp?

[merlin el mago]

Could the 8 ohms drive 300 ohms headphones?

 

[JonSnell Electronic]

The basic rule of thumb with impedances is if the source is equal to or less than the receiver then that is an ideal situation and no losses will be incurred.
If you are driving your headphones and no loudspeaker as a load, as it is low volume, I would use a 15 Ohm dummy load of about 2Watts in size, just to keep the valve amplifier happy and your headphones will work well.

 

[audiowize]

How much power do the 300 ohm headphones need? Use that to estimate the voltage required. A 300B amp will make about 8V at the output.

 

[merlin el mago]

The basic rule of thumb with impedances is if the source is equal to or less than the receiver then that is an ideal situation and no losses will be incurred.
If you are driving your headphones and no loudspeaker as a load, as it is low volume, I would use a 15 Ohm dummy load of about 2Watts in size, just to keep the valve amplifier happy and your headphones will work well.

Thank you. What's the purpose of 15 ohm 2W resistor in parallel with the OPT 8 ohms source?

 

[Tom Bavis]

A minimum load provides some worst-case protection - if a tube amplifier is driven into clipping with no load, extremely high voltages can occur which can damage the output transformer. Unlikely but possible.

A further issue is that headphones are MUCH more sensitive than speakers, and hum and noise may be objectionable. An attenuator will solve this.

 

[PCL200]

Thank you. What's the purpose of 15 ohm 2W resistor in parallel with the OPT 8 ohms source?

Suppose the impedance ratio of the OPT is 5K:8

What impedance do you think the 300B will see at the primary if there is 300 Ohm instead of 8 Ohm connected to the secondary?

 

[daanve]

You're asking Felipe a lot...
(Psst Felipe...the answer is 187500 ohm....don't tell any further...).

 

[Pano]

The 300B seems like overkill. No way you need that much power for headphones.

 

[audiowize]

A minimum load provides some worst-case protection - if a tube amplifier is driven into clipping with no load, extremely high voltages can occur which can damage the output transformer. Unlikely but possible.

A 300B isn't going to do that.

 

[drjlo]

The 300B seems like overkill. No way you need that much power for headphones.

I guess that depends on which headphones. Abyss off the speaker posts.

 

[merlin el mago]

The 300B seems like overkill. No way you need that much power for headphones.

45?

 

[wattsman]

How about 71a

 

[gmant123]

71a or 31 are good options...

 

[merlin el mago]

Both needs opt right. What value? Any proved schematic? Are easy to source & cheap.

 

[merlin el mago]

No schematics?

 

[PCL200]

Yes schematic!

It is from here (use Google Translate to translate from Dutch to Spanish and mail Rob for questions):

DHTRob - Een Single Ended hoofdtelefoonversterker met de '71A en de '27 (1)

 

[merlin el mago]

Thank you.

 

[randytsuch]

Couple old threads on the subject
thread on output impedance for cans

What output Z for headphone amp?'

For my headphone amp, I had electraprint wind iron for something like 50 ohms on the secondaries. I use a 45 in a tubelab se amp.

 

[stevenrotterdam]

If you buy a nice transformer for a 300B, just use it with a 6EM7 and save yourself a lot of money.

 

[nicoch58]

6EM7 is a good tube , the pw side is like an 2A3 but isn't a DHT
merlin I suggest a nice 45 power amp with a opt with a custom headphone tap ,or a voltage divider as randy .....
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