Originally Posted by mkmelt
Often there is very good synergy between the higher impedance headphones and a vintage amplifier/receiver, however, some prefer to listen to low impedance phones connected to older audio gear. So it is a good idea to audition your chosen headphones with the specific vintage amplifier you are considering purchasing and decide if you like the way this pairing sounds.
The available power from the headphone jack of most vintage units is many times what is required for almost any dynamic headphone (save for the now discontinued AKG K1000). As you stated, in these units it is often the case that the left and right channels of the main power amplifier are connected to the headphone jack using a voltage dividing network of resistors. This allows the listener to use a wider range of the volume control when setting the listening volume and also prevents an extreme mismatch in listening volume when switching from speakers to headphones.
A 30 watt per channel solid state receiver can be expected to deliver rated power when connected to 4 or 8 ohm speakers, but as the impedance of the speaker or headphone rises, the power output will become more limited such that with a 300 ohm headphone, and the additional impedance of the voltage divider network, the available power becomes limited to just 1 or 2 watts. In this case the available power is limited by the amplifier's ability to deliver the needed voltage, not the amount of current. Even so, this seemingly small amount of power is many times what is required by the headphones to achieve maximum output levels. This is also greater than the available power that can be expected from the headphone jack of more modern audio gear where an integrated circuit op-amp is capable of providing perhaps 50~100 milliwatts of power at 32 ohms.
When matching an amplifier to a pair of loudspeakers, the rated damping factor of the amplifier is an often overlooked specification. The damping factor is a specification derived from the source impedance of the amplifier when compared to a standard speaker load such as 8 ohms. When the damping factor is too low, there are speaker amplifier interactions that can effect frequency response throughout the audio band and also the woofer movement becomes uncontrolled resulting in mushy ill-defined bass fundamentals. Damping factor increases or decreases in direct relation to the impedance of the speakers. I.e., a tube amplifier might have a damping factor of 16 @ 8 ohms, but this is reduced to only 8 when driving 4 ohm speakers. This is probably too low to be a good match for most 4 ohm speakers. The damping factor would be 32 when driving 16 ohms speakers, a good match for the amplifier. In fact, many vintage speakers were rated at 15 or 16 ohms. If this same principle is applied to headphone listening, then a high impedance headphone should be a good match for use with a tube amplifier (these rarely provide a damping factor greater than 20 @ 8 ohms) or any amplifier with a somewhat low damping factor. Likewise, a low impedance headphone may not be an optimal match for an amplifier with a low damping factor. I have heard some very good high impedance headphones sound awful, with woolly, mushy, bass, just by connecting them to a particular model vintage receiver. Although the driver mass of the headphone driver is many times less than the typical loudspeaker woofer driver, I believe that the audible effect of having an under controlled headphone driver and correspondingly poor bass performance from having too low a damping factor can sometimes be heard when listening to headphones.
Originally Posted by RobxMcCarthy
However aren't most amps like those only good for 300+ ohm headphones. I was under the impression that they generally have rather high output impedance and that it would generally end up distorting at high gain. Mainly due to the fact that it's not getting high gain across the load because of the voltage division. Therefore it ends up clipping earlier.
Also, many lack the proper current supply for low ohmage headphones. V=IR I=V/R where R is lower and the voltage remains the same I increases.
Who knows though, I guess not all amps were made that way back then.
I know this is an old thread but I just picked up a recapped vintage concept 16.5 receiver. Back in the seventies the damping factor on a lot of these old receivers was for the most part less than 100. The concept 16.5 seemed to have a exceptionally high damping factor for a receiver at the time with a damping factor of over 450. I imagine that is why it sounds very impressive in the bass with my lcd2 v2 phones. The 16.5 gives me grip in the bass that I do not normally hear with the lcd2 on most dedicated headphone amps or vintage receivers of that time.
Relating to other comments in this thread you would probably have to agree that power supplies on the higher end receivers of the day would be very expensive if they were produced today and would make monster receivers of those days prohibitively expensive to produce today. The power supply in my concept 16.5 is a sight to behold more like high end esoteric amps made today.
This beast weighs in at 67 pounds and is really a struggle to wrest onto a shelf or anywhere you might want to put it.. Most of the concepts weight is in the back portion where of course the power supply is located which makes it more difficult to balance yourself when lifting this monster. I wish I knew what the spec for headphone power is on these old vintage monster receivers. The headphone amp is not a little op amp used for headphone drive on most any amp these days. Back in the day they took the signal off the main amplifier and loaded it down with a resistor for the headphone jack. The power spec for my receiver was 165 watts rms for an 8 ohm load. and 250 watts rms for a 4 ohm load. It would be nice to know the specs for a 50 ohm load on this beast but for now it sure sounds nice with my lcd2 v2s.
In the future I may investigate having at least some of the caps upgraded with higher quality caps and possibly some film caps wherever they might make the most difference.