The Stax thread - Page 170  

You'll only need a simple voltmeter capable of measuring 600v. You adjust the pot while measuring before the 5meg resistor to get 580v or slightly less. The 5meg resistor is there only to protect you if something in the headphones breaks and they short circuit so it can be of a 1/4w rating. This is an open circuit so very little current should pass through is under normal circumstances. The transformer isn't a necessity but it is very small and cheap and it adds a level of protection. Stax didn't use transformers in the adapters and they do not bust into flame.

So....How do Stax cans work at low volumes? I have sensitive ears and need to listen at low volumes but really hate the hit sound and imaging takes when you crank the knob towards zero. Does the "faster" quality translate to better low volume quality?

There is nothing better for listening at low levels then electrostats. They have great detail and presence even at whisper levels.

THAT is the answer I needed to hear!
Thanks!

A voltmeter may not do it. You'd need a VTVM or the like (something with huge internal resistance). Otherwise, the voltmeter will ground the open bias line, and produce an artificially low reading. Use a regular voltmeter to set the bias voltage, and you could have a whole lot of bias voltage that you weren't counting on...

Thanks again ! I thoght I need a voltmeter with big input impedance to measure the bias voltage but as you say I can measure before the 5meg. What do you think about the unsteadiness of the mains supply I would think 10 percents lower bias voltage is not a serious problem (in sound quality?) but what about 10 percents higher ?

If you measure before the safety resistor a normal meter will do. You'll need a VTVM if you are going to measure at the socket.

10% doesn't matter all that much but you can adjust the multiplier a bit lower to take care of this. Exceeding the recommended maximum bias voltage is not good but I've plugged a normal bias phone into a pro socket and they are just fine. I wouldn't do it intentionally though...

Mains Bias vs Self Bias

I've just managed to get my excited little hands on a set of SR-X Mk IIIs with an SRD-7 mains-biased transformer. I wanted this set-up,even though I'd only fairly recently got some SR-X Mk IIIs with a SRD-6SB transformer, because I wanted to replicate what feckin eejet had done with his SRD-7 (see p. 121, if my memory serves me well, involving cutting out the SRD's speaker switch and PCB circuitry).

Now I haven't yet made this modification, wanting to get used to the sound of the un-modified SRD-7 first, and have spent some time today comparing the SRD-7 with the SRD-6.

I fully expected the SRD-7 to be a major improvement over the SRD-6: it is a later model, has bigger transformers and a thicker power-amp connecting cable. I have to say, though, there is not a major difference, and if anything the SRD-6 is possibly slightly more tuneful and dynamic (both SRDs being driven by a Naim 32.5/140).

Needless to say, I am a little surprised by these initial findings and am wondering if any of you regular head-fiers have carried out a similar comparison. (I should add that I'm not particularly bothered about the SRD-7 not being any better than the SRD-6: I am still looking forward to modifying it relatively easily, and getting much better sound as a result.)

There are some problems with that, so I had Sennheiser mod my set to take the higher bias, but I think that is limited to the HE90 design. I've played around with the bias a bit on some Stax phones and it doesn't make the mylar stick to the stators like the He90 does.

The only thing I can think is that it's something to do with the fact that the SRD-6 has no (dirty!) mains coming into it: the bias voltage being generated from the power amp output (?) means it's maybe cleaner?

I do have the SRD-6 (Really an SB even though it doesn't say so) and the SRD-7 mk2 if you want to give it a go in the weekend. I've not directly compared them myself but the SRD-6 has a lower noise floor since it's passive. The SRD-6 just won't power the O2s which accounts 99% of my home listening so to me it's a moot point but my SR-X Mk3 and SR-003s can be driven from either. As I said, I've not directly compared but the SR-003 sounded less appealing with the SRD-7 MK2 than with the SRM-252A. With the O2 it's the other way around but that's mainly due to the lack of drive in the SRM-252A I feel.

A simple test would be to borrow the bias supply from some other device and leave the mains unplugged. I have an SRD-6SB in the customs but I wont be able to get it till next week so no comparisons till then.