[Saltuk]

Hi,

today I recieved the SRM-1/MK2 (A series) standart bias that I won on the electronicbay.

I am planning to use it with my SR-202 pro bias earspeakers.

The SRM-1/MK2's bias is 230V so the SR-202 (580v) doesn't sound its best (worse then my pro bias SRM-252A).

As far as I know it is possible to convert a 230V bias energizer to a 580V bias one.

Here are some photos of the SRM-1/MK2 . The blue cable in the 2nd photo is marked as Bias on the PCB.

Any help will be highly appreciated.

Thanks.

 

[spritzer]

This must be one of the very early models as there is no empty space on the PCB for a pro bias supply. It also looks like the stock bias is take of the HV line you have to build something from scratch. If the HV of the transformer is 200v+ you could use a 190v zener string and a voltage tripler. Slightly below 580v but swapping a few different zeners might give you the exact voltage.

 

[Saltuk]

Thanks spritzer' I am running the amp in 220v (i can run it on 240v too) So does that mean that the HV transformer is 200V+ ?

If I understand corectly the zener string will limit the constant voltage to 190 and then I will add a tripler circuit to raise the voltage to 570 so in theory it will sound %98.3 as good as a pro bias SRM-1/mk2.

Where should I add the circuits? For example would it work if I cut the blue cable (the bias cable) and add the zener string (closer to the pcb) than add the Voltage tripler(closer to the plug)?

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif Thanks spritzer' I am running the amp in 220v (i can run it on 240v too) So does that mean that the HV transformer is 200V+ ? If I understand corectly the zener string will limit the constant voltage to 190 and then I will add a tripler circuit to raise the voltage to 570 so in theory it will sound %98.3 as good as a pro bias SRM-1/mk2. Where should I add the circuits? For example would it work if I cut the blue cable (the bias cable) and add the zener string (closer to the pcb) than add the Voltage tripler(closer to the plug)?

Since you have a 200v+ voltage coming from the wall you can build the bias supply freestanding with voltage from the wall powering it. You could also build the variable supply Dr. Gilmore posted some years ago. I should have the design somewhere around here...

You got the operation but since zeners are most likely matched to +/-5% you should be able to get 580v by trying a few different units.

The circuit is very small and can be installed on the side panels which run the length of the amp without much trouble.

 

[spritzer]

Here it is:

You could skip the transformer since you have 220v to start with.

 

[Saltuk]

Thanks again ,
as far as I understand this was designed with 110V mains in mind so the part on the left (transformer?) is not required for me(sorry for repeating what you say but I am trying to make it clear for me)

So when I get this circuit built , what I have to do is to attach it to the amplifiers transformer(the bando d-25 thing at the back ?) and I believe there are dedicated minus and plus cables coming out of the transformer through the pcb for the bias, I will disconnect those cables from the transformer(bando) and attach the circuit to the transformer instead of the cables going through the pcb for the bias. And solder the new bias cable coming out of the new circuit to the bias part of the socket?

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif Thanks again , as far as I understand this was designed with 110V mains in mind so the part on the left (transformer?) is not required for me(sorry for repeating what you say but I am trying to make it clear for me) So when I get this circuit built , what I have to do is to attach it to the amplifiers transformer(the bando d-25 thing at the back ?) and I believe there are dedicated minus and plus cables coming out of the transformer through the pcb for the bias, I will disconnect those cables from the transformer(bando) and attach the circuit to the transformer instead of the cables going through the pcb for the bias. And solder the new bias cable coming out of the new circuit to the bias part of the socket?

The transformer is a small 4VA model by Hammond which has two 110v winding on both the primary and the secondary. Here it's used to step up 110v up to 220v but it can also be used as an isolation transformer by inputting 220v and outputting the same.

You don't touch anything in the amp except the bias wire leading to the sockets. The new bias supply should draw power from the wall (it could use the same fuse though) and be totally independent from the rest of the amp.

 

[Saltuk]

So, I should disconnect the blue wire (labeled as bias) from the pcb and add the circuit to it and power the circuit from the wall that outputs 220V is it true?

But isn't the bias 580V DC? Dont I have to convert the AC power from the wall to DC?

And I belive there are 2 zeners in the circuit (the triangles with the short line on the upper corners?) are they 190V ?

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif So, I should disconnect the blue wire (labeled as bias) from the pcb and add the circuit to it and power the circuit from the wall that outputs 220V is it true? But isn't the bias 580V DC? Dont I have to convert the AC power from the wall to DC? And I belive there are 2 zeners in the circuit (the triangles with the short line on the upper corners?) are they 190V ?

You simply cut the blue wire (reminds me of some old movie...

) and the new bias supply is connected to that wire.

The bias is indeed +580VDC but the two diodes on the supply act as a half-way rectifier and thus turning the AC input into DC. There are no zeners in this supply, the triangle with a line in front of it is a diode. Just use something like 1N4007 which is good for 1000v if I remember correctly. The rest of the parts are just capacitors (again 1kv) and resistors (the 1M is a variable resistor).

 

[00940]

for safety's sake when servicing the amp, I certainly would use an isolation transformer and not run the thing straight from the wall... At 4va it won't be expensive.

 

[spritzer]

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif for safety's sake when servicing the amp, I certainly would use an isolation transformer and not run the thing straight from the wall... At 4va it won't be expensive.

The transformer is very cheap and galvanic isolation is always a good thing to have. A fuse is also mandatory.

 

[Saltuk]

I completely understand the stand alone bias supply now , thanks alot again.

Will post again when the circuit is built.

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif I completely understand the stand alone bias supply now , thanks alot again. Will post again when the circuit is built.

Cool

 

[Saltuk]

One more thing ' can you specify the the pro bias in detail (curren,voltage,polarity etc.')

I visited the electrician that repaired my 252A and he suggests a different approach .So it would help me to have the exact proporties of the pro bias . As far as I know what I need is a constant , stable +580V DC . Does the pro bias have a spesific current (is the current determined by the power of the transformer?) Can there be too much power for the stax pro bias? (like using too powerfull transformer)

And spritzer, in dr.gilmore's drawing does the transformer feed the circuit with 220Vac (110V as primary 220V as secondary?)

 

[spritzer]

Quote:

Originally Posted by Saltuk /img/forum/go_quote.gif One more thing ' can you specify the the pro bias in detail (curren,voltage,polarity etc.') I visited the electrician that repaired my 252A and he suggests a different approach .So it would help me to have the exact proporties of the pro bias . As far as I know what I need is a constant , stable +580V DC . Does the pro bias have a spesific current (is the current determined by the power of the transformer?) Can there be too much power for the stax pro bias? (like using too powerfull transformer) And spritzer, in dr.gilmore's drawing does the transformer feed the circuit with 220Vac (110V as primary 220V as secondary?)

The bias is +580VDC and the current is very low though the exact amount will depend on the driver feeding off the supply. You can of course have more current as the phones will only take what they need and the bleeder resistor controls that.

As can be seen in the picture the secondaries are serial connected so the output is 220v.