How class D amplifier may react to headphone

[A Surfer]

Hopefully this post is helpful/interesting to others and not just myself. I have recently purchased an SMSL A300. It suits my needs quite well, but I am hoping to extend it's functionality. Years ago I had a speaker tap cable made so that I could drive headphones directly from the speaker terminals of my NAD M3. Brilliant, worked like a charm for many different headphones. I have taken my main rig down so while I still have the M3, I am not currently using it.

I have the A300 in use as my amplifier that I use when playing my drums along to music with, and the a300 is absolutely adequate if perhaps even good. I have the original speaker tap cable still and I have been wondering about using it with the A300 just for fun. I have access to several headphones, although right now I only have a Sony Z7 MKII and a first generation Verum 1 on hand, so those would be the headphones I would like to drive directly from the A300 speaker terminals.

My question is, given the performance parameters of the A300 and Class D amplifiers in general, if one did use the A300 as described for headphones, in particular the two that I described, how would you expect the A300 could perform? Would the frequency dependance get worse, better or stay the same when the load was a headphone? Thanks for any input, much appreciated.

 

[MaxwellsEq]

How does the tap work? If it's a resistor-divider it's likely to perform worse than a dedicated low output impedance headphone amplifier regardless of what amplifier is driving it.

 

[raindance]

The load would be a speaker tap cable plus headphone. So you'd need to know the impedance of each and then contact the amp manufacturer or run tests to see what the behavior looks like into that load.

Basically you'd be throwing away power to heat up some resistors and you'd likely increase the frequency response / impedance dependency.

 

[A Surfer]

How does the tap work? If it's a resistor-divider it's likely to perform worse than a dedicated low output impedance headphone amplifier regardless of what amplifier is driving it.

The tap is simply cables that plug directly into the speaker terminals, no resistors or anything electrical in-between the amp and the headphone. I did this for years with many headphones, probably 10 different models with my NAD M3 (180watts into 8ohms) and never had a poor outcome in terms of sonic performance or damage as I was always careful with turning the volume control down before hitting play.

 

[DVDdoug]

The amplifier will probably be fine. Amplifiers are usually OK with higher impedance loads, or no load, no matter the "class". (You can't use it in bridged mode since regular headphones share a common ground for left & right.)

And I wouldn't worry about paralleling the headphones & speakers (again because of the higher impedance, probably 32-Ohms or more).

If there is no resistor network to reduce the voltage, the things I'd be concerned about are excessive noise (since headphones are more sensitive than speakers), and accidently frying the headphones (or your ears!) with excess voltage. Headphones don't get as much wattage as speakers, again because of the higher impedance but it could still be too much.

 

[A Surfer]

The load would be a speaker tap cable plus headphone. So you'd need to know the impedance of each and then contact the amp manufacturer or run tests to see what the behavior looks like into that load.

Basically you'd be throwing away power to heat up some resistors and you'd likely increase the frequency response / impedance dependency.

Thanks for taking the time to respond, but as above, no resistor box used, just speaker cable. I had Norne Audio build me the speaker tap cable. Well made cable.

 

[raindance]

If there are no resistors in the cable then you'll likely blow up your headphones and hear a lot of noise.

 

[raindance]

But if you're careful, try it out and see.

 

[A Surfer]

The amplifier will probably be fine. Amplifiers are usually OK with higher impedance loads, or no load, no matter the "class". (You can't use it in bridged mode since regular headphones share a common ground for left & right.)

And I wouldn't worry about paralleling the headphones & speakers (again because of the higher impedance, probably 32-Ohms or more).

If there is no resistor network to reduce the voltage, the things I'd be concerned about are excessive noise (since headphones are more sensitive than speakers), and accidently frying the headphones (or your ears!) with excess voltage. Headphones don't get as much wattage as speakers, again because of the higher impedance but it could still be too much.

Thank you,
Yes I was more concerned about the likelihood of excessive noise, probably more than the fear that the load would be subject to problematic changes in frequency response due to the load reactivity of the A300 (although it still might be a factor). One follow-up question. when you wrote this :And I wouldn't worry about paralleling the headphones & speakers (again because of the higher impedance, probably 32-Ohms or more)." did you mean that I wouldn't need to disconnect the speaker wires from the terminals and could actually have the speaker tap cable connected at the same time (it uses banana plugs).

Thanks very much. And thanks to the other members who have also replied. I appreciate all of the input.

 

[A Surfer]

If there are no resistors in the cable then you'll likely blow up your headphones and hear a lot of noise.

The noise is more of the concern. As mentioned, I have used this direct speaker terminal connection with many headphones for several years without any issues or even close calls (I'm very careful). The NAD M3 is considerably more powerful than the A300. Thank you for the concern and warning though.

 

[Zapper]

The amplifier will probably be fine. Amplifiers are usually OK with higher impedance loads, or no load, no matter the "class". (You can't use it in bridged mode since regular headphones share a common ground for left & right.)

And I wouldn't worry about paralleling the headphones & speakers (again because of the higher impedance, probably 32-Ohms or more).

If there is no resistor network to reduce the voltage, the things I'd be concerned about are excessive noise (since headphones are more sensitive than speakers), and accidently frying the headphones (or your ears!) with excess voltage. Headphones don't get as much wattage as speakers, again because of the higher impedance but it could still be too much.

The specs in this post say the A300 is a BTL (bridge tied load) design. The OP shouldn't use his speaker tap cable because it will short two of the amp outputs together, which will at best sound bad and at worst blow the amp.

Correction: According to the manufacturer website, BTL is an optional mode and not the normal stereo mode. So the above concern would not apply.

I would check the resistance between the two (-) speaker terminals with an ohmmeter (with the amp on). If the measurement is not ~0 ohms then I wouldn't use the cable.

 

[raindance]

Check that the negative terminals of your tap cable do not tie left and right ground together before you try it!

 

[Zapper]

Check that the negative terminals of your tap cable do not tie left and right ground together before you try it!

They would have to because headphones generally use a 3-terminal connector with a common ground. The question is whether the amp outputs have a common ground, i.e. shorted together. If they do then the cable is OK to use.

 

[A Surfer]

Damn, I'll have to go back and do the research again. I remember years ago when the cable was made, Trevor at Norne Audio asked if it was common ground or not before making it. I had NAD electronics tell me the answer, but it was years ago now so I don't remember. I am pretty sure that the M3 was common ground though, not being a truly balanced design internally, just using two mono amplifiers.

 

[MaxwellsEq]

The tap is simply cables that plug directly into the speaker terminals, no resistors or anything electrical in-between the amp and the headphone. I did this for years with many headphones, probably 10 different models with my NAD M3 (180watts into 8ohms) and never had a poor outcome in terms of sonic performance or damage as I was always careful with turning the volume control down before hitting play.

Ah! So not a tap at all, just a wire. A tap is something different.

Imagine a 12" water pipe runs through your land and you want to take a small percentage of the water flowing in it, that's the same as an electrical "tap" where you extra a fraction of the whole. Your cable can't do that, it's just a cable. I hope you didn't pay more than a couple dollars for it.

 

[A Surfer]

Ah! So not a tap at all, just a wire. A tap is something different.

Imagine a 12" water pipe runs through your land and you want to take a small percentage of the water flowing in it, that's the same as an electrical "tap" where you extra a fraction of the whole. Your cable can't do that, it's just a cable. I hope you didn't pay more than a couple dollars for it.

Gotcha. I'll have to take pictures. It is actually a well constructed cable. 12 feet divided into two sections and it seems that all the elements were quality. I'm sure there was plenty of profit made, but given that it enabled me to use my M3 for headphones and never need to buy a headphone amp, and that it was so well made and will last for years to come, I still feel it was money well spent.

But no, it was certainly not a few dollars, nor should it be. The workmanship is excellent and certainly wasn't slapped together in an hour.

 

[somebodyelse]

Also consider the effect of the unusually high load on the output filter. The A300 has a peaking frequency response with the 8R load. 32R will make this somewhat worse.

 

[A Surfer]

Also consider the effect of the unusually high load on the output filter. The A300 has a peaking frequency response with the 8R load. 32R will make this somewhat worse.

This is the absolutely the type of interaction effect that I am wondering about. I'll know how it all shakes out on the weekend when I should have a chance to try this out.

 

[Randolf]

A simple resistor voltage devider is a reasonable way to connect headphones to power amplifiers. Directly connecting the headphone is dangerous for your ears, heaphone or amplifier (shared ground of 3 pin headphones). I would recommend to check out https://diyaudioheaven.wordpress.com/headphones/power-amp-adapter/ . I just build a voltage devider to drive my Grado GS1000i from my Yamaha R-N803D (Class A/B, unbalanced, shared ground): https://homeaudio.jimdofree.com/headphones/ Building the devider incorrectly, using the wrong type for your amplifier type or connecting in incorrectly may damage your amplifier!

 

[MaxwellsEq]

A simple resistor voltage devider is a reasonable way to connect headphones to power amplifiers. Directly connecting the headphone is dangerous for your ears, heaphone or amplifier (shared ground of 3 pin headphones). I would recommend to check out https://diyaudioheaven.wordpress.com/headphones/power-amp-adapter/ . I just build a voltage devider to drive my Grado GS1000i from my Yamaha R-N803D (Class A/B, unbalanced, shared ground): https://homeaudio.jimdofree.com/headphones/ Building the devider incorrectly, using the wrong type for your amplifier type or connecting in incorrectly may damage your amplifier!

Because the OP mentioned a "tap" in the title, what you describe above is what I mistakenly thought he already had, rather than just a wire!