[urs]

Dear all,
I’m currently considering to up-dating my STAX-compatible HeadAmp with an extra- option of a variable Polarization Voltage .
Reportedly, the SR-007 Mk2 may benefit from such a higher voltage

Who has experience with such a feature ?

Thanks & Regards

Urs

 

[AlexmodNew]

It is possible, but in a light limits. Volume will rise with rising the bias because it is in direct relation. But higher Bias can result to reaching of the polarization limit inside the gaps between membrane and stators. Therefore spark can ocures (especially at high humidity environment), and it can possible damage expensive headphones. Also membrane can reach their moving limit (on LF) and stuck to stator. Also leads to possible damage. So you can try it but carefully. And keep in mind that a present good sound from your 007 with high BIAS can be the last one.

If you really want to try I suggest at first putting additional gaskets between membrane and stators from both sides. You can cut it from PE or even paper. It can solve at least 1 problem (stucking to stators on a very high volume at low frequencies) but it will also reduce the volume (because gaps will be higher) and you need to rise Bias to compensate that.

 

[urs]

Sorry- I should change "Polarization Voltage" to BIAS Voltage
Urs

 

[urs]

Alex,
Thanks - yes I'm aware of the risks.
I have currently just one (positive) report of this procedure and for STAX ONLY for the 007 - other brands may behave different.
Reportedly the 007 has a somewhat bigger gap between membrane and stators than the other (newer?) models

regards
Urs

 

[AudioCats]

I have a DIY box ( with Lundahl LL1620's inside), with multiple bias voltages. The voltages are taken off a zener diode string so should be relatively accurate.

The effect of different bias is easily auditable but not huge. There is not that much difference between 580 and 620v.

If you just want to try a different voltage, the easiest way is probably by using a bias-adj box, this box has a pack of battery inside to provide the differential voltage. Say, some 9v batteries in series, 3x will provide around 30v to be added/subtracted in the bias line.

The proposed bias-adj box:
--- box has a short input pig-tail which plugs into a stax-competible amp.
-- inside the box, signal lines go straight to the output socket.
-- The bias line from the input pigtail goes to the battery pack; the other end of the battery pack goes to the bias pin of the output socket. Change the # of 9v batteries to get higher/lower final bias voltage at the output socket. The batteries will last basically forever since there is no current drain.

switches can be added to pick which differential voltage (say, 2x or 4x 9v) or change the direction (add or subtract from the amp's bias voltage).

 

[urs]

AudioCats,

Thanks for your interesting feedback.
My HeadAmp needs a "Full-Service" anyway, hence I'll have this mod done by the Orig. Manufacturer, together with other up-dates.

But allow me one more question: HOW do you measure the BIAS voltage ? without phones plugged in - or with ?

Thanks & regards
Urs

 

[AudioCats]

the main advantage of a bias-adj box is it being an add-on device it will work with any existing amp.

I don't have a meter with high enough input impedance to measure the voltage that an amp's bias pin, I think something like 100M ohms is needed, the highest I got is10M.
Hence the use of a zener string in my DIY transformer box, I could just count the # of zeners and add up the voltage.

The bias-adj box will rely on the batteries inside to provide the differential voltage to be added or subtracted to whatever bias comes in from the input pig-tail.

 

[AlexmodNew]

Hello.
Effect is subttle because difference in Bias is too small. SPL directly depends from Bias voltage, it is on multiply factor. 620v\580 is only about +7% of SPL rising, almost undetectable. +3dB louder is equals to 580x2=1160volts. And +3db at SPL rise is only "very slightly louder" like +1hour on volume or even less.

Please check my older messages ( i wrote only few here). Later i described how to encrease loudness of 007 by removing 2 of 4 PFE spacers inside driver. It is eguals to approximately +3dB rise in efficiency (estimated). But it can be performed by your own risk, sure. You can destory headphones. Best way is to disasssemble the driver, exactly measure all gaps, calculate dangerous bias voltage for that gaps and use that as a high limit for encreasing (or sure that is is higher than 580v after reducing the gaps by removing the spacers if you want to try it) All formulas are known, can be found in clever books. Just look for them.

 

[urs]

Alex,

Thanks for your advice !
As I'm NOT a DIY man, I'll won't touch my 007 ...

Regards
Urs

 

[urs]

VERY preliminary first impression with a Malvalve HA Three-MK4 "special" on my 007 MK2:
An increase in BIAS Voltage does increase volume slightly.
AND: I feel a touch better definiton in bass and maybe also in treble as well.
HOW much the BIAS increase is, I can't say, as I have no measurement.

Regards
Urs