[Saltuk]

I found a 220v-220v 8VA isolation transformer locally(actually I am infront of the shop now) I hope, that wıll be ok to use a 8VA instead of 4VA.

If I am not mistaken the 100k resistor in the circuit is a bleeder one, so it will limit over current and let only the necessary amount of current to the earspeaker, is that right?

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif I found a 220v-220v 8VA isolation transformer locally(actually I am infront of the shop now) I hope, that wıll be ok to use a 8VA instead of 4VA. If I am not mistaken the 100k resistor in the circuit is a bleeder one, so it will limit over current and let only the necessary amount of current to the earspeaker, is that right?

8VA is fine. The 4VA model is just easy to find in the US so I guess that's why Dr. Gilmore chose it.

The 5M resistor is the bleeder resistor which controls the charge on the diaphragm. The 100k works with the 1M varistor so the bias can be adjusted.

 

[Saltuk]

Spritzer thanks for the lightning fast reply I am going in!

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif Spritzer thanks for the lightning fast reply I am going in!

No problem.

 

[spritzer]

I just wanted to post an update. I built the supply in question today with a 12VA transformer and some old parts I had and I'm only getting a max voltage of +508VDC which is not enough. The SRD-7Mk2 supply would be a better choice then as it gives both a 580v and 230v bias.

 

[Saltuk]

That was what my electrician predicted , he said that wont put out 580 only 500. He also suggested to have a custom wound transformer 220V primary 580V secondary and use some diodes on the way to convert AC to DC then limit the voltage with a zener string.

Spritzer did you use a 5 Mega ohm resistor in the end or a 4.7 one , all I can find is 4.7 mega ohm here locally. Does that 5 M resistor have any special function or any 5 or 4.7 M resistor will do the job?

The SRM-1/MK2 is quite satisfiying with 230V bias(but a little loose/not precise and tight enough) I believe even with 500 or so Volts it will be much better than my 252A, what do you think?

Which amplifier did you try the bias supply and did you try any lambda earspeakers with it? Was there an apparent improvement from the 230V ?

 

[spritzer]

That's one way of going it or just wire the isolation transformer with a 115v primary and a 230v secondary and have double the output voltage. The current draw is so low that it shouldn't matter.

The 4.7M is enough and what Stax generally uses. You could add another 300k in series if it makes you feel better but it's not necessary.

The SRM-1 is a much better amp then the 252 and the correct bias will showcase that.

I didn't try the bias supply with any headphones. I just hooked it up (with ground from the SRM-1 Mk2 PP that is in my computer rig) and measured the output. I've tried to run Pro bias headphones from normal outputs often and the difference is huge though it varies from model to model.

 

[Saltuk]

I will get the new bias supply this week, will post how it sounds compared to 230V.

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif I will get the new bias supply this week, will post how it sounds compared to 230V.

Good to hear. I've been working on a SRM-007t for the last few days and Stax uses a 4M7 5% bleed resistor in the Pro supply.

 

[Saltuk]

Yup! (4.7M right?)

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif Yup! (4.7M right?)

Yeah, 4M7 is the standard way of writing it in short hand.

 

[Saltuk]

Yesterday I prepared a cable with a single pin with the shape of stax connector and inserted it in the 252A's pro bias socket than attached the cables other end to the bias cable in the srm1/mk2 (which I cut its connection from the amplifier)

When there is no interconnect in the 252A the bias voltage drops in a few seconds and there is almost no sound, then I connected a 3.5mm to 2rca to the 252A and inserted the mini end to the computers on board sound card (inactive) after that I got a perfect, stable and full 580V bias to the srm1/mk2 and from it to my 202.

That is the biggest improvement I have ever witnessed on the Stax system. The srm1 is much more stronger, fuller and real sounding than the 252A. Especially the bass improved alot I was not expecting that good bass from the 202s.

Will get a 500V-510V bias supply soon and give impressions and compare it to the 230v and the 580V .

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif Yesterday I prepared a cable with a single pin with the shape of stax connector and inserted it in the 252A's pro bias socket than attached the cables other end to the bias cable in the srm1/mk2 (which I cut its connection from the amplifier) When there is no interconnect in the 252A the bias voltage drops in a few seconds and there is almost no sound, then I connected a 3.5mm to 2rca to the 252A and inserted the mini end to the computers on board sound card (inactive) after that I got a perfect, stable and full 580V bias to the srm1/mk2 and from it to my 202. That is the biggest improvement I have ever witnessed on the Stax system. The srm1 is much more stronger, fuller and real sounding than the 252A. Especially the bass improved alot I was not expecting that good bass from the 202s. Will get a 500V-510V bias supply soon and give impressions and compare it to the 230v and the 580V .

I think the smaller Stax amps have the same feature as the Koss E.90 where the bias supply is disengaged with no signal input. It's smart because that means less dust being attracted to the drivers.

The SRM-1 is good but it's a long way from the top...

 

[Saltuk]

I noticed a problem with the srm1/mk2 , when there is no music playing I can hear a very very low noise hum on the 202s.
It is impossible to notice it without trying to hear it when there is no music and impossible to notice when the music plays. The hum is a constant 40-50hz sound. I tried every possible combination of changes(cables, placements etc) but it only stops when I touch the chasis.
Google tells me it is ground loop. Can the normal bias cable that is cut away from the system cause that problem(like an uncompleted circuit)?

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif I noticed a problem with the srm1/mk2 , when there is no music playing I can hear a very very low noise hum on the 202s. It is impossible to notice it without trying to hear it when there is no music and impossible to notice when the music plays. The hum is a constant 40-50hz sound. I tried every possible combination of changes(cables, placements etc) but it only stops when I touch the chasis. Google tells me it is ground loop. Can the normal bias cable that is cut away from the system cause that problem(like an uncompleted circuit)?

That shouldn't be a it as the bias is always an open circuit, connected to nothing. I would try a different powercord with a 2 prong plug and also on the source. You can also a simple circuit inside the SRM-1 Mk2 to lift the ground and isolate it.