Superlux HD-681 modifications

[superluxman]

Hi Guys!
I am from Ukraine, so don't pay attention to my clumsy English.
I have bought Superlux HD668B. They are looks very brightly for me. Here is their frequency response:

They also has a high frequency peak.
I use the software filter to correct them. It looks like this:

But it is not comfortably. Because I use monitors simultaneously.
I want to use a hardware filter like HD681 filter. But HD668B has different impedance - 56 Ohms, and different inductivity (I don't know the value, but I can measure it if it needs). I do not know whether inductance of headphones influences on a filter or not. But I built a circuit in Multisim, and judging on that a result appeared not such as in the example HD6812.pdf. (I am not a professional electronics, and have install Multisim yesterday just to try
Circuit:

Diagram:

The first question is what I did wrong?
The second question is what I have to change in the circuit in order to get a result similar to the software filter for my HD668B?

[Deleted]

The schematics you drew are completely different from how they should be.
The series filter (L1,R1, C1) needs to be connected across the sinewave source but in your schematic it is in parallel with the parallel filter.

The HD668B I had did not have the peak at 7 kHz in fact it sounds just like the SR850 and IMO doesn't need a notch filter but the top end needs to be lifted.
The filter needs to recalculated for 64 Ohms (56 ?) in both the HD681 type filter and SR850 filter.

Does it sound exactly the way you like with the software filter ?

This is a concept for a HD681 type notch filter for HD668B acc. to your findings and altered to the advise I would give.

[superluxman]

Thank you for your prompt reply!
HD668B are indeed less cut the ears with such software filter. You also can see their frequency response on superlux web site. It differs slightly from that given me. But the point is made. HD668B have 56 Ohm impedance. And if I correctly understood the principle, I will could calculate the appropriate filter itself.
Here is a half of your scheme:

Here is my circuit:

It seems to me they looks the same. Is not it? And the result:

I would be grateful for an explanation of my error.

[superluxman]

Oh, sorry. I understand when I am wrong. I replaced the source and a headphone...

[superluxman]

Now I can calculate my own filter.
Thank you for your help!

[Deleted]

Indeed you swapped the phone and source side.

The filter as I have drawn is the correct one for the HD668B.
The series filter L1,R1,C1 only have a function with sources (headphone amps with output resistances between 10 and 120 Ohm) and is optimized.

The filter is calculated for around) 60 Ohms HD668B
The only things that can/could be changed is the 68 Ohm in the parallel filter.. bigger resistance = more filtering, smaller value = less filtering.

[superluxman]

I found a best solution for my ears. It is looks like this.
Software filter:

A circuit:

Hardware filter:

And how it looks in the frequency responce (approximately):

I tried to use a filter (different types of software VST filters) with different target frequencies. On my opinion this is a best target frequency value.

My source is Infrasonic Quartet.I do not know its output resistance. Do you think it satisfies the condition of 10-120 Ohms?
And one more question. Coils of the headphones must have some inductance too. Do you measured their inductance? If so, what is the value? Does it influence to the filter?

[Deleted]

R1 C1 L1 filter won't work properly.
As you might have noticed if you leave these parts out in the simulation the filter does not change.
In order to configure this part of the filter (which I already did for you in the posted schematic) you need to insert a resistor of 120 Ohm between the + of V1 and the filter and do simulations with this value and smaller values to see the curve change.
Run it again with the values you use and my proposed values and see how the curve remains similar shaped with my design and alters considerably with your values.
Getting this part of the filter right is essential as you do not know the value of the output resistance (which is easy to determine) and this can be anything between 0 Ohms and 120 Ohms.
The filter needs to work correctly on all output resistances.

the 70 Ohms can be 68 Ohms resistor as this difference is unnoticeable.

Shifting the filter towards 8kHz maybe even 9 kHz will work better IRL as the dip at 4.5kHz will be better compensated by the added 7 kHz.

The inductance of the driver does not influence the filter.
The inductance cannot be measured with an L meter by simply attaching it as the meter will give improper readings due to the 56 Ohm resistance.

You can only determine it with a tone generator and scope or AC voltmeter that must be lin to 100 kHz.

[superluxman]

Yes, you are right. R1 C1 L1 filter quite doesn't work. I haven't noticed this before. There are no difference I am insert it in the circuit or not. I followed your advice and insert a resistor of 120 Ohm between the + of V1 and the filter. But no change:

Am I wrong again? How to make (on this scheme) for work properly filter R1C1L1?
You have made a concept filter for HD668B. How I can see you chose the target frequency is 8.8 kHz. But have you seen a measured frequency response of HD668B? There are in the superlux site ( www.superlux.com.tw/index.do ) a frequency response most of headphones. Judging by the frequency response of the HD668B (from the superlux site) target filter frequency should be 8 kHz:

But to my ears sounds better with the filter is 7.4 kHz:

Frequency response, which I quoted earlier (yesterday) was measured in Russia. In Russia Superlux headphones are sold under the brand called Axelvox.

[Deleted]

the input of the XBP1 must be connected to the other side of the 120 Ohm.
If you want to lower the freq. of the filter simply increase the value of the capacitor from 680 nF to 820 nF or if you want it at 7 kHz 1uF

[superluxman]

Thanks for helping me. I configured my filter and I think it is correct:

I am going to use a potentiometer (R9 on the scheme) to adjust the headphone volume. How do you think 10 kOhm will be enough?

[Deleted]

I certainly won't do that as you not only change the volume but a lot of other parameters too.
especially the bass will become very different.
You wont see that in the simulation as the hump around 100Hz in the impedance graph is not simulated.
If you like to adjust the volume passively behind an amp I would search for an L-Pad or 100 Ohm (max) potmeter and connect it like a volume control.

Prefered method is adjusting the volume of the headphone amplifier itself.

[superluxman]

OK, I don't want to use a potentiometer any more
I looked at the available components. And I modified the circuit slightly:

But there is one problem - the resistance of 3.3 mH inductor is much more than 2,5 Ohm. Minimal resistance of such inductor, what I found - 6.56 ohm (axial inductor). How will this affect the frequency response? How I should install this resistance in the circuit to see its effect?

[Deleted]

L3 = 3.3mh+6 Ohm + 100 Ohm so in practice 3.3mH + 106 Ohm so you don't have to worry about the coil resistance in this (L3 C3 and R5) series circuit.
The 470uH is more critical for proper operation of the filter as L3 C3 and R5 only do their job as compensation for the increased resistance of the parallel filter (L4,C4,R6) + driver resistance and has some influence on the curve..

So a resistance of L3 upto 10 Ohms is NO problem at all.

[superluxman]

Another question. There are resistors with different power ranging from fractions of watts to tens of watts. What is the minimum resistors power necessary to choose for the filter?

[Deleted]

Since only a small part of the frequency band passes the resistors (around 7kHz) and the maximum power the headphones can handle is around 0.5W to 1W resistors between 0.25W and 0.4W are already heavily overdimensioned.
The energetic levels around the 7kHz band will be way less then 0.1W in any case when driving these cans to ear shattering levels.
So you can use standard resistors (0.3W to 0.6W)

[superluxman]

The last question I think
I found the inductor 3.3 mH (6.56 Ohm), the maximum current is 0,2 A and the inductor 470 uH (1,15 Ohm), the maximum current is 0,47 A. Can I use the inductors with such maximum current strength? What is the minimum current strength of inductors can be used?

[Deleted]

When pumping a staggering 1W of energy around 7kHz in the headphone (this would probably be around 10 to 100W musical signals) the 3.3mH must be able to withstand 56 mA.

The 470uH when punmping 1 Watt continuous in it on every frequency but the centre frequency the minimal current must be 177mA.

Mind you the voicecoils will be glowing with this power already.

So 3.3 mH minimal 0.1A and 470uH minimal 0.25A is already more then adequate.

[superluxman]

In the forum I found a photo of the filter with capacitors, which are not multi-layered ceramic:

What kind of capacitors on the photo? Will it be better to use them in the filter?

[Deleted]

They could be used for reasons of having those caps lying around...

Some say polyprop sounds better (in this picture).
Some will say mica or silver-mica is better, yet not available in these values.
multitlayer ceramics (X7R is preferred over Z5U) are small and can be used inside the headphone without taking up too much space.
There are also smaller sized polypropyline avaiable.
The voltage (which determines the size) is not of importance.

For external filters the sky is the limit (money wise) and can be constructed to your audio religion.

[yellowjeep]

For the orginal HD681 filter from the very first page, say I want to attenuate the ~7.25 kHz frequency 3db rather than 6db? I replace the resistor in parallel part of the filter with one of lower value correct? What value would I use?

I tried to search the thread but with no luck...

[Deleted]

Parallel filter (the one in series with the driver) between 27 and 33 Ohm
series filter (on the input of the filter) R = 68 Ohm

[yellowjeep]

Cool thanks Frans.

[blade]

wow..this is an extensive thread about Superlux HD681. Reading so much positive comments, I have ordered myself a pair of HD681 and also a pair of HD668 too, they are really that cheap for me to just grab 2 one shot. now anxiously waiting for the delivery =)

[Deleted]

I have moved some posts about measuring output resistance of amps to the DIY section in a new thread:
rockgrotto.proboards.com/index.cgi?board=m&action=display&thread=6226

Solderdude