[ursdiego]

Hello!
 
The short version:
My preamp has a line-out that is:
- 75 ohms
- + - 19V
- Volume regulated
- can drive high impedance headphones (according to producer Denis Morecroft)
 
What has to be done, to "convert" it to low impedance, to drive e. g. a Grados RS-1 (without connecting an entire headphone-amp)?
 
It is a DNM 3B Primus. Its not on the internet anymore, but the specs are similar to the ones listed here:
http://www.dnm.co.uk/techspecs.html
 
I feel like adding a headphone amp of an affordable price cathegory would be a loss of sound quality, so I want to connect my headphone as directly as possible, with as few electronic steps in between, as possible.
 
The long version:
I own a very nice DNM 3B Primus Preamp, that features a volume-regulated second output (like two preamp-outputs). I would like to connect a pair of headphones to it. I feel like adding an entire headphone amp would degrade the quality. The preamp was somewhere in the 5 or 10 thousand $ category, when new (I got it second hand...), and for sure, any headphone amp in a price category affordable to me would not match the quality of this preamp.
 
However, the preamp output is rather low impedance and high power. Here are the specs I could get:
 
75 ohms
+ - 19V
More specs of similar amps here:
http://www.dnm.co.uk/techspecs.html
 
I asked the producer and he would like to exchange it to the new version, that sounds even a better of course, and that features a nice headphone output, but he said as well, that the preamp may drive a high impedance headphone directly from the (volume regulated) line out - e. g. the high impedance senns.
 
Now the thing is, I will get a Grados RS-1 soon - a typical example of low impedance can.
 
I figure that there must be a "minimalistic" way to "convert" that output to lower impedance, able to drive the RS-1, without getting it through an entire headphone amp???
 
Thanks in advance for any useful advise!

 

[ursdiego]

Hey guys, this can not be that difficult for you to answer... right ???

 

[xaval]

I'm sure someone knowledgeable will chime in. Just give them some time to do it
 
I'm currently using a ART Headtap connected to the speaker outputs of my DA60 - it powers beautifully any of my cans.

 

[wuwhere]

Instead of bringing down the output impedance of your preamp, you can add a resistor in series with the RS-1 to make it a high impedance load to your preamp. This is probably the easiest way to do it.

 

[wakibaki]

Yeah, you can simply wire in a suitable opamp configured as a unity-gain buffer.
 
I would recommend using a NJM4556 opamp, or preferably 2 in parallel, as this will provide more current drive and a lower output impedance, making it more versatile in terms of the 'phones it will drive (you never know what you may buy in future).
 
You have +/- 19V, this is slightly problematic, but only slightly. The NJM4556 is rated +/-18V but you can lose ~1.2V from the supply rails by wiring 2 rectifier diodes in series with each side, say 1N4001's. A little extra capacitance on the board will not hurt.
 
You can build the circuit on vero (perf) board, or dead-bug style, the chips are available in DIL08 packages.
 
Diagram follows...

 

[wakibaki]

 
...there you go. Break the existing ground connection to the output jack and connect it to the ground at the junction of the 47k resistors. Spider the grounds on the caps together at a single point and return that connection to the existing ground at the PSU.
 

 
w

 

[wakibaki]

Oh, don't let the wires to the +/- rails wander around untidily. Twist them together (3~4 tpi) and keep them as short as is consistent with a neat layout, you might want to dress and cable-tie them to the chassis. You may be able to solder the circuit to the output jack, otherwise secure it somewhere, maybe with doublesided sticky pads. Keep it away from any mains wiring.

 

[nikongod]

I'd use a step-down transformer off of the outputs.
 
Keep the resistors. They are likely there for stability. You or the next owner of the pre could be in for a very nasty surprise if highly capacitive cables are used which may send the pre into hard oscillation. 
 
You dont need as much voltage swing as the preamp has for anything but a buffer feeding speakers. Step it down a bit. 
 
Quote:

Instead of bringing down the output impedance of your preamp, you can add a resistor in series with the RS-1 to make it a high impedance load to your preamp. This is probably the easiest way to do it.

This makes the source impedance of the amplifier look even higher. That being said I'm not sure its a bad thing, as prefer the sound of many headphones driven from a high impedance source, but if you follow the rules regarding "proper" (says who?) damping factor it is.
 
I suppose that now is not the best time to bring up the rule dictating that headphones be driven by dedicated headphone amps?

 

[ursdiego]

Dear all
 
Thank you so much for all that!
 
In particular to Wakibaki, this looks like a complete solution! The "problem" about that is, that I will have to get into electronics... I am a DIY newbie in this area. I am however not clumsy I guess and able to understand the scheme with a bit of reading before that. But I don't know if I find an occasion to do so in a reasonable time.
 
However, did I understand you well, that this would be a modification of the preamp output? I'd prefer not to modify the preamp, but rather to build it into an external little box. I need to build a "converter" anyway, as the output comes with DIN plugs and the headphone has a standard headphone chinch.
 
So I think I need to go into reading the theoretical bases of understanding electronic schemes and then I will see if I find the nice solution to build it together.
 
BTW: Nice pictures on your Photoblog, Wakibaki!
 
Cheers! Urs

 

[ursdiego]

Hm, Wakibaki, I was now trying a bit to understand the diagram. I see that it needs a power source. Do I understand well, that your suggestion is like a simple headphone amp, like e. g. the CMoy or Grado R1 amp, just without the potentiometer?
 
I might be wrong, but this might raise again the entire question of which opamp will have what sound signature etc. My wish would have been more to have a solution that does not amplify at all, but just "converts" the output to lower ohms.
 
In my of course all too simple way of understanding electronics, I thought that maybe a resistor between the the line and the ground might at the end, lower the ohms of the output. So, somehow the opposite of what wuwhere is suggesting. However, this resistor would as well be in parallel to the headphone itself, and seen from the other side, lower the ohms of the headphone... so, not good.
 
However, I do have a headphone amplifier, and I suppose, it is not a too bad one (audio-gd fun, that is a dac and preamp and headphone amp all in one). But my question was precisely, how to avoid the headamp-step as far as possible, because I believe, that the more electronics there are in the signal path, the less accurate the sound gets to the ear.
 
Sorry for my ignorance, I am still a complete newbie to electronics

 

[wakibaki]

 
Quote:

However, did I understand you well, that this would be a modification of the preamp output? I'd prefer not to modify the preamp, but rather to build it into an external little box. I need to build a "converter" anyway, as the output comes with DIN plugs and the headphone has a standard headphone chinch.
 

 
Yes, it's a modification of the preamp. The advantage is that no additional power supply would be required. You can run the circuit from the preamp output, but you will need a power supply. PSU or batteries.
 
Quote:

Hm, Wakibaki, I was now trying a bit to understand the diagram. I see that it needs a power source. Do I understand well, that your suggestion is like a simple headphone amp, like e. g. the CMoy or Grado R1 amp, just without the potentiometer?
 

 
Yes, the main difference, apart from the pot, is that the circuit has no voltage gain, it just lowers the output impedance and makes more current available. The circuit is very close to the Grado RA1 amp, except it uses 2 opamps in parallel for more output and it has no gain. I said use 2 because they are cheap.
 
Quote:

I might be wrong, but this might raise again the entire question of which opamp will have what sound signature etc. My wish would have been more to have a solution that does not amplify at all, but just "converts" the output to lower ohms.
 

 
This opamp has a very neutral characteristic, the only effect should be that the variation in frequency response which all headphones show when driven from a high output impedance should disappear, there will be plenty of volume available and the response should be flat across the band with all but a very few types of phones. It just converts the output to lower ohms or 'amplifies' the current available. When we say that a circuit 'amplifies', this usually means that it multiplies the voltage. You have plenty of voltage, just not enough current available.
 
Quote:

I thought that maybe a resistor between the the line and the ground might at the end, lower the ohms of the output. So, somehow the opposite of what wuwhere is suggesting. However, this resistor would as well be in parallel to the headphone itself, and seen from the other side, lower the ohms of the headphone... so, not good.
 

 
You can't get the result you want with resistors alone. With a transformer perhaps, but it will not be as versatile and anyway, which transformer? I doubt you can buy a suitable one off-the-shelf, and transformers have problems (distortions and limited frequency response) of their own.
 
Quote:

However, I do have a headphone amplifier, and I suppose, it is not a too bad one (audio-gd fun, that is a dac and preamp and headphone amp all in one)..
 

 
You can pass the output from the preamp through a headphone amplifier, you will get the same result as the circuit above if the headphone amplifier has a low output impedance. You may have to turn the volume down, this may result in you being able to hear some noise (hiss).

 
Quote:

But my question was precisely, how to avoid the headamp-step as far as possible, because I believe, that the more electronics there are in the signal path, the less accurate the sound gets to the ear.
 

 
You can't make an omelette without breaking eggs.
 
I'm glad you liked the photos. Thanks.
 
w

 

[ursdiego]

Hello Wakibaki
 
Thanks a lot! You really helped me with all your explanations. I definitively want to try your circuit, and compare it to the headamp. I guess your circuit is probably the most "minimalistic" approach and that is a good thing. What I still now know, is where to get the components, and to make sure, they are of a really high quality ("audiophile" - like). So if ever you know where the good bits can be bought, let me know, thanks!
 
However, I will opt for a "out of the preamp" - solution. Theses DNM amps are a bit particular and very delicate, I don't dare getting inside with a soldering iron. This said, I need an external power source. Please let me know, are two 9V - ni-cd - blocks sufficient and last a while? Or 15 ni-mh 1.2V batteries? Or a regulated linear power supply? the latter would probably lead me to the next diy project...
 
About the opamp: Does it make a difference choosing a "high quality" opamp? As I see, they seem to have an impact, as a monster thread is dedicated only to opamps on this forum here: http://www.head-fi.org/t/432749/the-opamp-thread
 
And yes: Really nice pics! Photography is an area, where I think I have a little bit more knowledge than about electronics, even technically. I do digital as well as silver based photography. Here is an example made with a 6x7cm Plaubel Makina http://www.uth.ch/plaubeltest/Film02_18_bw_highcontrastblue.jpg and here another one with a classic 6x6cm rolleiflex: http://www.uth.ch/budapest08/Budapest_08_UTh_06.html (full size with a link below the picture).
 
Cheers! Urs

 

[wakibaki]

It's nice to get a compliment from another photographer, I see you know what you are doing. You are lucky to have the Plaubel. I used to have a Rollei and I did film photography but when I got married I lost my darkroom. I don't miss it much, I like digital. I got some other things instead.

 
Yes, you can use 2 x 9V NiCds (transistor radio PP3 types), they will last ~8 hours with the circuit. A regulated linear PSU would be good, you can buy a kit on ebay that will give you +/- 18V, this is better if you have difficult headphones but +/-9V is still fine for most. 
 
You can buy the opamp from RS (rswww.com) (you must buy 5, but the price is very low) or from ebay (Hong Kong) (not so good price + shipping). I don't know your location (Hungary?) so it's hard to make a recommendation where to buy. Look on google, you should find genuine New Japan Radio manufactured parts without trouble, they are too cheap to be worth counterfeiting.
 
Yes, the choice of opamp is very important. There are very many good opamps but few with good output current, especially DIL08 types. You will see I used 16 x NE5532s in this design:- http://www.head-fi.org/t/568055/5532-parallel-headphone-amp to get enough current and low output impedance.
 
w

 

[ursdiego]

Well, I think you certainly have done the right choice, when changing marriage against darkroom ! And darkrooms are something that can be rebuilt... mine is waiting in the basement, disassembled... but I have not lost hope to get it operational again. I hope you did not sell your Rollei however, most of them are just wonderful. I have a couple of them, going from several Rollei 35 over my grandfather's 2-eyed-Rolleiflex with 3.5 planar lens to two one eyed 6x6 rolleiflexes, one with autofocus, and a couple of nice Zeiss and Schneider lenses. So the equipment is there, now what is always difficult to find, is time, especially if free time is better spent with the family than in the lonely darkroom!
 
Back to electronics: you suggested as an opamp the NJM4556. In another thread, a fellow here wrote in a quite convincing manner, that he uses the " LM49720HA TO-99 (metal can version) because they are simply the best opamps I have heard to date ". However, what I look for is just the one that does the job at the most next-to-source way possible... So, I guess, I should stick to your suggestion of the NJM4556?
 
Cheers!

 

[Uncle Erik]

Do you realize that it might be easier to build a dedicated headphone amp?

Glass Jar Audio has good kits. You'd get a PCB and not have to source parts.

Don't get too hung up on the "purity" angle. I run headphone amps off a preamp. Can't tell much, if any, difference from running a source straight into a headphone amp.