HD83, how to use low and high impedance cans?

[A Stuart]

You would lose the best part of 20 decibels of signal level with this resistor. Also the headhones would see a very high impedance source so volume might vary strongly according to the headphone impedance at different frequencies.

I was just trying to work out if you could use a switch. I can't see that you could arrange it without the signal passing through about 6 switch contacts per channel.

A compromise could be to hardwire the windings in series (ie leave them unchanged ) but either via a switch or a second socket, feed the low impedance phones from a single set of the windings.
Admittedly the headphone would see 4x the resistance in the windings but I think, correct me if I am wrong, it would not upset the impedance seen by the amplifier. I dont know about inter-winding capacitances etc etc, but on a basic level, magnetically, the primary does not know if the secondary is drawing eg 1mA x 100 turns in one wire or 4 x (0.25mA x 100 turns) in parallel. It would be just like a thicker-wired version of the same 100 turns.
I don't believe you should lose signal level other than that attributable to the resistance of the non-paralled winding.

Again correct me if I'm wrong but I can't imagine headhone currents overheating a transformer winding.

Alastair.

PS if you used a switch, take care to arrange it so as not to short out the other three windings!

[John T]

Originally Posted by Claes

Those pics are a little hard to read, you would not happen to have higher resolution images that are easier to read?

The bandwidth of the board is a limiting factor. I'll email you the pdf file of the complete instructions.

John.

Ace John, I really appreciate your help here, it means that I can start thinking about this two weeks earlier than I thought, since I am going to Ireland in two weeks, and then the instructions will come with me home.

Cheers.

I appreciate the previous idea about how to wind things, but one q.
If I find a really good resistor that is frequency "independant" so that the
resistance does not vary with the frequency, then it should not add any
problems, other than the volume control being turned up a bit more. But, that
should not be a problem with a sensitivity of 104 dB/mW and the amp can give 1000 mW. Hmm, maybe that is a solution that will work, or is my reasoning wrong? Keep'em coming, the advice that is.

[A Stuart]

Originally Posted by Claes

If I find a really good resistor that is frequency "independant" so that the resistance does not vary with the frequency

I did not mean the resistor would change. It would stay near enough a constant pure resistance at all frequencies under consideration.

However, assuming the impedance graph of headphones is vaguely similar to the graphs you see of loudspeakers, you might for example at one frequency have a 64 ohm headphone in series with the 220 ohm resistor, and at another frequency a 16 ohm headphone in series with the same 220 ohm resistor. Thus the attenuation provided by a constant resistance could be very uneven.

Alastair.

Ah, that is how you mean it.
Well, would not a transformer winding see the same issue then?

[A Stuart]

To some extent yes, but surely nowhere near the extent of 220 ohms.
I can't say just how audible it would be for any given value.
Even with feedback, output impedances are never zero ohms.
You could measure the DC resistance of a single secondary winding. As stated originally I suspect the ac behaviour would be near unchanged.

If you already have two output sockets (can't remember the design offhand), it would be easy to switch the hot side of your low impedance headphone socket between the single winding terminal and a 220 ohm resistor to the top end of the secondaries , and hear any sound difference.

Does anyone else have a knowledgeable input on the wisdom of taking the output off a solitary secondary winding, leaving the other three secondaries redundant?
Alastair.

[sharkey22]

Hi, I am currently facing the same issue as the o/p, wanting a simple method of switching impedance for different headphones with my HD83.

It seems to me that the ideal solution would be to add DIP switches (perhaps on an additional board) to the HD83 so that it functions the same as the later HD3S.

My amp has the HD83 board in the HD3S chassis so I presume my kit was supplied around the time of design change.

Would it be possible to purchase the HD3S PCB only? Who would I contact to ask? Is the HD3S circuit identical to the HD83?

Could some kind member provide the manuals for both versions so I can work out my next move.

Thank you.

[John Caswell]

Hi Claes/Sharkey, et al,
The main problem is that HD83 is hard wired for whatever impedance phones you have or decide upon. Either parallel for low Z headphones, series/parallel for medium Z headphones, or series for high Z headphones.
The simplest way would be to wire the tx for 32R and wire that to one headphone socket then add a series resistor of 220R to both channels and wire that to another socket, ie effectively in parallel but "stood off" by the 220R. All that you should notice is that the 250R 'phones would be lower level.
It is not a "good" solution by any means but try it and see, after all we are not talking lots of power so you may be able to get away with no resistor whatsoever and drive both your headphones from a low Z source. You may be surprised at the results! On the other hand you may not

John

[Pingushome]

Hi,
May be a bit late but if you still need the different output impedances this may help.

I read on here recently, but cannot find the thread now, that an alps switch from HiFi collective could be used if you use the new switchable board
If you can find the thread it has a link to the switches I mentioned

It may be that you can convert the pcb you already have to use a switchable impedance.

I’m sure someone will chip in with more info

Martin

[Pingushome]

Ha ha ha found the above ref

It was in my thread about HA and sent by Greg
So here it is

By the way, you could make a reasonable improvement to sound by simply replacing the DIP switches with a quality selector switch. An Elma would do a nice job. Have a look here.
https://www.hificollective.co.uk/swi...-switches.html

Hope that helps if you still need it

Marin