[Furlan]

Hello! 
 
Some time ago I bought an SRM-727A amplifier and headphone SR-007 MKII. 
 
I already bought the resistors to perform Spritzer Mod and i want to make the service soon. 
 
So, I have a doubt: 
 
How do I adjust the balance DC? 
Where do the adjustments? In which components? 
How do the measurement? 
Could someone explain me step by pass, please? 
 
thank you

 

[jgazal]

  Hello! 
 
Some time ago I bought an SRM-727A amplifier and headphone SR-007 MKII. 
 
I already bought the resistors to perform Spritzer Mod and i want to make the service soon. 
 
So, I have a doubt: 
 
How do I adjust the balance DC? 
Where do the adjustments? In which components? 
How do the measurement? 
Could someone explain me step by pass, please? 
 
thank you

 
http://www.head-fi.org/a/adjusting-bias-on-stax-tube-amplifiers
http://gilmore.chem.northwestern.edu/dsc_1583mod.jpg
http://img829.imageshack.us/img829/2255/p1000579o.jpg

 

[Furlan]

Dear jgazal, 
 
It was just what I needed. 
 
Thank you very much for your help.
 
Best regards

 

[duncan1]

Just a word to say I have added a stabilized  power supply using a mos-fet power transistor It is not an original design by me as there is a limit in the number of variations in solid-state design unless you make some specialized overload cut out. I used a constant current design which was pretty simple. Mos-fets are much more reliable in solid-state power supplies when used as pass transistors many normal transistors(BJT) need a lot of protection to stop them failing mos-fets dont and are used in millions of switch-mode power supplies World-wide. The result is a much reduced noise floor and power supply injected distortion products (cleaner -clearer sound ) as opposed to  simple power supply provided in a 717/727. I see many people upgrading the power supply capacitors while this is a positive thing it entails a good bit of time and work. While not a first glance looking any easier actually fitting a stabilized power supply is a good bit quicker and if you can read a circuit diagram no harder. It is easy enough to cut the pcb copper that runs to the two output fuses and solder flying leads to the power supply . It can be even simpler just fitting a pass mosfet and little more. I cut a section of the casing away next to the fuses  that allowed me to fit a small heatsink and mount the components directly onto the two mosfets. This kept the distances between components very short improving stability with no oscillation but you could make pcb to suite or even a strip board although that could introduce small amounts of capacitance. Active components must be at least 500 V working. The perceived fidelity when compared with just changing the caps is large . This is a mod you will hear right away making a large difference and will reward those with Omega 2 earspeakers with a large increase in detail due to decreased noise/distortion on the supply lines. Its no use having an amp with 0.001 thd when your power supply is outing large amounts of noise unless your amp has power supply rejection of the same amount.   

 

[Interben]

Hi guys,
I am entering the world of Electrostatics and am looking at the 717 for my solution.....
I have always loved playing with electronics and am confident to solder, replace resistors, caps etc but I really have no idea regarding how to test and measure circuits.
My question is can the 717 be easily converted from 110 to 240V?
Would it require a new transformer?
I note the caps are 400V so does that mean the circuitry would be fine with the change?
I would also like to thank you all for this thread...
As I said before I love to tinker and could see myself trying to perform the mods as described in this thread in the future .......
Thanks

 

[jgazal]

  My question is can the 717 be easily converted from 110 to 240V?
 
Easily is a relative term. You can try the following posts: 
http://www.head-fi.org/t/223263/the-stax-thread-new/23625#post_9627206
http://www.head-fi.org/t/223263/the-stax-thread-new/21285#post_9223131
http://www.head-fi.org/t/679447/stax-727-voltage-conversion-100-to-117-help#post_9758538
 
Would it require a new transformer?
I note the caps are 400V so does that mean the circuitry would be fine with the change?
 
If your model allows, you can change how primaries are wired, so secundaries will hold the same voltage. You also need to change fuses (different ratings). Be careful. Even disconnected from the mains, capacitors may be charged. The voltages inside the amp are lethal.

 

[Interben]

Awesome... That was exactly the info I need....  Also thanks for the Heads up with the capacitors .  When I was young I had a friends dad who would charge them up and throw them at us to catch... We obviously weren't that smart because we kept catching them
Thanks again jgazal....

 

[duncan1]

I had forgotten to add that the feedback resistors total 300K OHMS in the unmodded version . If you check the circuit diagram you will see that it is a     version of a widely modded standard circuit. All the other versions show the series resistance as 200 K OHMS from the output to the input I had already reduced the series resistance to 200K OHMS using a parallel resistor across one of the 150KOHMS feedback resistors in the original circuit that means that 2 resistors are paralleled in each board. With the extra feedback it does make a positive difference I still stick by the fact that if you want a major positive difference to the 727 /11  then make the power supply stabilized or at least use a pass transistor or mosfet.Believe it or not its not hard to do a even a simple design  makes a world of a difference blocking a lot of  mains harmonics and noise and distortion. Many easy to follow designs are on the web.     

 

[duncan1]

I noticed some people have trouble adjusting the balance and near zero voltages  I do not think that the ones fitted are up to the job considering the price of the total unit. To make life easier and up grade them I fitted Multi-turn presets .These allow a much finer adjustment of the voltages . They are not the large volume control types as found in high end test equipment but much smaller and narrower 1 inch and a quarter long - 3 eighths height-3 sixteenth broad made out of high impact plastic by =Paintonbourns England they are called Trimpot.I havent checked to see if the firm is still in business but I am sure there must be many US manufacturers who can supply them or equivalent ones .These are industrial types -do NOT buy ones made for the general public. There is a small adjusting screw at the end of each.

 

[duncan1]

There are many who like extreme detail both with the amp and the headphones but that does not apply to some who like a smooth musical presentation. It might not appear to some that the 717/727 boards are not really much different ,the main thing being the twin output devices on the newer board as compared to the original but the circuit of the newer board still has its original circuits still in place (except for the output circuit) . The openness and highly detailed output from the mods above might not suite some especially if they are using the latest top end Stax(too much detail) .A way round this is to re install the feedback to the input circuit please note this is not the mods as above  but also re-instating the small value capacitors which existed in the 717 which were fed back from the board output devices to the circuit feeding the input .These were series-ed using two 250 Volt caps (total 500 V working of 50 PF value) If you want the best reproduction then they must be replaced by 500 Volt Polystyrene caps with a value of 25 PF of VERY high quality , no they arent cheap but if you want the best you pay for it . If you cannot get them then look for polypropylene caps of the same high quality ,if you cannot get them the Silver Mica ,again of very high quality would do to give you an idea of the new reproduction. The result is a slight lowering of detail at the high end and a slightly more powerful bass and remember feedback tends to even out the frequency response                 some call it nested feedback. This gives a smoother quality of reproduction which can be  reproduced at a very high level without any perceived distortion in other words a very Musical reproduction  that will be welcome by some who arent convinced extreme detail =musicality. There is no problem with your foot tapping up and down with the music. I added this to the other mods  but it could be tried as stand alone ,its only 4 caps to be soldered in so it is not brain surgery but again I emphasis they MUST be 500 Volts working and of very high quality as they are stopping 375 or so volts from being fed back. Dont try and test this unless you have a digital/analogue test meter of of 100 MEG OHMS INPUT IMPEDANCE or more your 10 meg digital meter will trip the relay.

 

[fuzzzy]

Inappropriate comment removed.

 

[duncan1]

Hello fuzzzy if you have had many years of electronic service then you will know about primary feedback and secondary feedback-aka nested feedback . Everything I have stated I carried out myself on the 717 and 727. Electronics isnt some black magic anybody can do it if they read up on it and that is my whole point the inside of a power amp/pre-amp shouldnt be beyond the capabilities of the ordinary person .If they read books go to college/night school etc they soon learn that it is really easy to do.I dont patronise people I credit them with intelligence .It takes a lot of courage for somebody who paid $1000s for their  equipment to mod it themselves but if they do and dont electrocute themselves they can raise the fidelity of that product much cheaper than paying for it . As the opening warning on Head-Fi says this site isnt for semi or commercial profit but for earphone users to experience better Hi-Fi/Musicality by doing it themselves not paying another to do it.I would never patronise somebody I judge them equally as that is how life has taught me.

 

[fuzzzy]

You are right as all you say is good. No intend to patronize you at all, just a bad day at work.
Apology for my comment on your thread, I shall remove it.
 
(deleted a bunch of technical crap, never mind).

Just one more general comment to this post:
 
 
Not far above you conclude:
 
"The openness and highly detailed output from the mods above might not suite some especially if they are using the latest top end Stax(too much detail) ."
 
Really?  There is such a thing as too much detail?
But else where you discuss these mods are to improve detail and clarity. This is contradictory.
 
 
"A way round this is to re install the feedback to the input circuit please note this is not the mods as above  but also re-instating the small value capacitors which existed in the 717 ..."
 
Now continuing you suggest that for good sound to partly restore the amp back to its original configuration add backward mods towards the 717 and for "best reproduction" mod the amp again and buy expensive high quality parts, etc.

This does not make sense.

 

[duncan1]

I realise most people like the latest Stax but not everybody ,it was aimed at them. But ,yes you can have too much detail as it gets in the way of the music ,as you are listening to the detail not the music .I speak from a UK perspective as most of the "golden ears " over here come down pretty strong on it and its probable that US users like a different balance accenting the higher frequency range. Each part of a high -fi system adds or removes highs or lows to a small degree depending on the philosophy of the designer  and his choice of components. I myself have a very open system so my Stax MK1 -007,s are the correct choice to make sure the frequency response appears flat to my ears. I work on the teachings of  John Lindsay Hood who was an open system  advocator with his insistence of the different sound qualities of capacitors long before  it was applied to hi-fi.So anything I build is DC connected with top quality components and all feedback using polystyrene caps with as little value is practically possible without causing insipid oscillation. Without the latest feedback as above the 727 is very open especially if you use a stablised power supply using a  pass mosfet. which lowers power supply noise/ripple/distortion which I fitted in mine ,and its a bigger improvement than changing the caps  and for that matter easier to do . What does happen is that it is so open minute aberrations not heard because of PS noise can then be heard and you have to go to a deeper level of investigation. Adding those caps flattens out even more the frequency response as well as sounding more integrated and musical . But I know in life what one persons idea of music isnt necessarily another's that why its an option.     

 

[duncan1]

Its possible that I have not put a concise rely to Fuzzzy . When I say the latest feed-back I am referring to my own modifications to as detailed in previous posts above .The Stax corps had a "brainwave " to remove most if not all feedback from its 727 to make it more open in practice this wasnt the total success that it hoped . It was instigated when the UK hi-industry went through a -"no feed-back is good" policy  I have re-instated with mods the original 727 layout .This applies to the 2 channel cards that have DOUBLE -ie- 4 output BJT,s on heatsinks not the 717 with the single pair with no heat sinks (I of coarse added heatsinks to the original 717 as they produced some heat  in bjt,s (bipolar junction transistors ) any bjt when heated produces noise so adding heat sinks lowered the noise floor allowing more detail to be heard . So although I was re-instating the caps as per the original feedback in the 717 it was applied to my mods in the 727 so I was not going back to the original sound only modifying the boards I had already modified. This was a try and see if you like it mod which could easily be removed by unsoldering the four caps without disturbing the rest of the board . I would also add for those still confused that the original 717 had 2 feed-backs 1 overall and 2 a secondary feed-back . The difference in the 717 board and the 727 board is that the 727 has TWIN DOUBLE output bjt,s unlike the 717 with only one pair otherwise the PCB copper tracking is the same. If you read my previous posts you will find I changed the caps added a stablised mosfet power supply to isolate power supply noise as well as the feedback mods. If you can do nothing else to your 717/727 just do the stablised power supply ,it really does make a big difference and its easier than pulling apart the chassis to replace the smoothing caps just fit it BEFORE the fuses not after .