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stax srm717/srm727-11

post #1 of 64
Thread Starter 

Hello! I am relatively new to this forum .Although I have loved stax earspeakers since buying a stax lamba in 1982.and have had several the latest being the 007mk1= my old equipment was a srd7-sb.I spent some months modding the srm 717 which might interest some and I also have a new srm727-11. This I have started modding not quite the same as the "Spritzer mod" and I intend to carry out other mods like the ones on the 717.I am retired from many years working on anologue electrionics including valve and solid-state although I am "solid" behind solid-state.If any body is interested  I will continue.

post #2 of 64
Originally Posted by duncan1 View Post

Hello! I am relatively new to this forum .Although I have loved stax earspeakers since buying a stax lamba in 1982.and have had several the latest being the 007mk1= my old equipment was a srd7-sb.I spent some months modding the srm 717 which might interest some and I also have a new srm727-11. This I have started modding not quite the same as the "Spritzer mod" and I intend to carry out other mods like the ones on the 717.I am retired from many years working on anologue electrionics including valve and solid-state although I am "solid" behind solid-state.If any body is interested  I will continue.

Please do continue.  :)

post #3 of 64

Yes we are are looking forward to your findings :)

I think the stock 717 is rather excellent and better than the 727 but the latter might be slightly better still with some proper mods.

post #4 of 64
Thread Starter 

Thanks for the encouragement I dont like "reading on other peoples toes.When I bought the 727-11 in the UK the dealer was surprised I was buying it"we dont get asked for many of those"=In the UK if its not valve[tube] is not of any good.But I knew I could modify it as I had done to the 717.Yes the 717 "sounds better" than the 727-`11 as bought in the shops.But on opening it up I wasnt pleased by the active components=big publicity is given to the "dual fet" input in laymans terms okay but not in technical terms. two fets pressed together by a plasic "shrinkdown is no my idea of a dual fet. Which is two fets on the one slice of silicon. The same applies to the "dual bjt[bipolar junction transistor] same cap .The 2sa1156 were of the cheapest quality[improved in the 727] . The low-level[+/- 15volts] regulators were only one pair. Not good for power supply p overhearing >Power supply runs to the daughter boards werent decoupled at the connectors[if it is over a few w inches run distortion can be picked up by induction/transmission. The power supply ripple could be lowered by increased capacitance or a solid state regulated high voltage power supply if there was room.The high current output devices on each board dont have heatsinks although they get hot[any active device when heated gives off additional noise] The input fuse  on the 717 tends to blow this can be uprated to around 1 amp without any worries [I shorted each leg in turn of the full wave rectifiers and the fuse blew every time so its okay no other damage was done.There are other minor things but going to the 727. I tried Spritzers mod  and yes it works but if you look at the 717 negative feed back it is taken from the two 100000K series resistors[voltage limits] The 150000K resistor in Spritzers is attached to the same two resistors but at the wrong end [feedback from the output-unless it is nested feedback ] is taken to the input active device to acheive better overall fidelity .so I bought 8 0.6 watt miniture metal resistors with minute noise levels soldered two in series and soldered one end to where Spritzer has shown and the other end to the TOP of the two original resistors ie-going directly to the input fets >This reduces the level of silibance down to a very low level.I do need to point out that as Proven technically your solid-state equipment must be on for 3days to get the benefit of higher fidelity ALL solid state must be on 24/7 or you are wasting your time trying to get the highest fidelity. I intend to carry on with the mods in the 727 later.But this is enough for themoment till i spend more listening time on it.Everybody has heard of the person weith the $1000000 system .Well I want to make those that cant affoard it have the same high quality at a cheaper price-excuse the mistakes I am a qiuck clicker of keys=Duncan

post #5 of 64

Comparing the 727 and 717 is a bit fruitless since the circuits are very different and designed by different people.  The 717 was never a Stax design and built under a license so when that license ran out they released the 727 which is a souped up SRM-Xh/SRM-313. 


Placing two J74 transistors next to one another is the only thing possible today.  There are no monolithic P-channels available in the world as the LSJ109 has been delayed for years now.  It is also possible to get far better matching by hand than Toshiba ever did with the K389/J109. 


Also, please use the return key a few times... redface.gif

post #6 of 64
Thread Starter 

The two circuits different?-Only at the out-put remove the comp.capacitors and they are relatively the same. I have both boards in my hands and see they were wise enough to add heatsinks to the 727 which were left out of the 717.Unable to obtain Dualmatched monolithic j-fets+ bjts-wrong I can still obtain them  p  -Read -Horowitz and Hill: The Art of Electronics [Paul Horowitz Harvard University] After detailed technical analysis he even lists the ""best" j-fets/bjt in the US for use in audio equipment -Yes they are Very expensive but if you want the best you pay for it. And I intend to help those that dont have deep pockets.Its too bad you dont like my writing layout but you cant please every body in life and I dont intend to change .So if you cant live with it???

post #7 of 64

So what's the difference between a (modded) 727, Cavalli LL and High-amp.de Transistoramp (V4.0)?


They are supposedly based on the same basic design yet quite different.

post #8 of 64
Thread Starter 

Sorry I havent tried or seen the other two.Most of the equipment I worked on in the UK was Stax or related also Mullard design tube amps and various solid -state. and other anologue test equipment-scopes-tube+ solid state etc.If as you say the circuits are similar then the four PNP bjts are an improvement  r on the NPN bjts which are the same as the ones on the main large heat-sink[717] the bandwidth is up from 5.5mhz to 6.omhz there is negative feedback from there as the output was disconnected but now with the overall negative feedback reinstated I should probably  g diconnect that but at the moment it isnt causing a problem I aim to acheive a level of fidelity equal to equipment costing many times more. This isnt magic because those solid gold unit only sound better because of very expensive parts and better laid out PCB .Any person with some technical knowledge can do this and the cost will be low in comparison to the $$$$$ of extreme equipment.This was even accepted in the UKs Electronic World/Wireless World. I am a disciple of the late great JLH- John Lindslay Hood a UK Audio amp and others he designed an audio design engineer.He helped introduce star earthing in the UK and the acceptance of the different qualities of capacitors .At the time he was ridiculked for that view by fellow design engineers but was proved right in the end when an engineer proved by science he was right.I do not use an earth as he was right that the normal mains earth inputs loads of noise and distortion. .If needed run a wire to the garden [if you have one and hammer a spike of copper alloy three feet into the ground it will be better than any mains earth. Be aware that with the high voltages in energizers you do so entirely at your own risk.I will post a list of the best dual j-fets and bjt when I look through the book mentioned in the last post by me.-You will need a good soldering iron /as said in other posts a digital multi meter isnt ideal for testing all parts of an energizer because of the very high impedance required although it will be okay if you stick to the daughter boards [testing elsewhere operates the cutt-out.excuse the mistakes in spelling-duncan

post #9 of 64
Thread Starter 

First chance to get back.I have fitted 470UF capacitors plus polyprop bypass to the underside of the sockets of the channel boards for the +/_ 15 volts contacts.I also have fitted 100UF capacitors in the place of the 10UF ones on each board. I will be fitting 2 additional voltage regulators to the two already there to split the supply from providing both boards to each pair providing one board-this cuts down over-hearing between the channels due to the common power supply.This takes time due to waiting 3 days for the energiser to reach thermal equilibrium before judging the audio improvement As it stands just now the reproduction has changed from detailed but with a harsh sibilance to a lot more smoother and purely expressed vocals .and clearer low level detail.As the signal amplifiers are so simple they require a very high quality power supply -to stop power supply noise injection.and the ones installed arent that.This comes down to cost which is why there can be various modding "upgrades to this 717.I will get back later. 

post #10 of 64
Thread Starter 

Further to the modification of the 727.I have now added 2-additional 15V+- regulaters fitted these are TSitems no additional components are needed but I still added them .Both channels now have a pair of their own so no PS o/hearing from that source.May I add at this time that due to the 717/727 being DC connected any feedback applied by modification will change the DC voltage output at the ear-speakers.So I hope anybody who reconnects the output will -1=have to balance the two voltages-2=the working voltage will need to be reajusted so  you will need to use your digital MM at the red/white-green/yellow terminations at the main board.The transformation of the quality of reproduction is now changed completely from prominate sibilance and a slight harshness to a smooth valve like reproduction but with even deeper detection of very low level content.due to noise from the power supply having been reduced.Vocals are now "pure" and are a joy to listen to.I am not finished yet as I will be replacing the so called "dual" fets/bjt for real ones with a much highewrv -higher spec as detailed in every serious semi-conductor data books in the world.But as it sounds so good just now I will give it more listening time.My whole idea here is to give those without the money  to raise the level of the 727 up to a standard costing $10000 or more reproduction. It isnt magic its just spending a small amount of money. manufactureres build to a cost. Any amp can be uprated by those capable of working on audio electronic circuits.If I was younger I would do an upgrade service.

post #11 of 64
Thread Starter 

After a while listening I have now changed all thr  the electrolytic capacitors in the power supply .The large ones to double their value and the small ones triple their value.The feedback resiastors resistors that I added to the TOP of the two in series I have increasd the value to 750000Kohms as the 200000K ohms resisters were giving out too much deatr   detail.I have not yet changed the semiconductors .Remember to adjust the balance voltage every time you move or change the value of them.Do not connect them to the bootom or middle of the two series feedback resistors as that will make it sound as if it is smooth but on prolonged listening is actually dull.Connect ONLY to the top of the two feedback resistors.I will get back.

post #12 of 64
Thread Starter 

I have now spent some time in listening tests and the reproduced sound is now at a very high level=sibilance very low=purity of vocals excellent=plenty of detail and above all musical. I have not changed the active devices as when tested some do not conform to their identity as printed on them=eg=2sc1815 BL=standard audio bjt but not the ones fitted which are current mirrors one has beta[gain] of 20 the other the gain went off the scale of my digital tester =more than 1500. these are not standard[to the book]values and is not a darlington but are specialised .But what I have now is of a much higher standard than the original 727. I also bypassed all the electrolytics with 0.1 uf polyprop=400V working.So thats about it . I hope its of some use to those of a practical nature.

post #13 of 64
Edited by Number9 - 2/15/13 at 5:52pm
post #14 of 64

Duncan1, you seem to have said a lot here, but its nearly impossible to follow. With no spaces, line or paragraph breaks... It makes for very difficult reading!


Can you please summarize the changes you've made, and maybe your insight in modding the 727 will be better understood by everyone. If you don't want to change your writing habits to make yourself be better understood, I guess so be it. Your choice. But it does turn off people because it is considered poor form.


Commenting as encouragement rather than criticism. I'm very interested in what you have done here with this mod.



Edited by Number9 - 2/15/13 at 5:50pm
post #15 of 64
Thread Starter 

Thank you for your email NUMBER9 I dint put English phrasing high up on my list of priorities. I notice the mis-speller doesn't work right here[misses some] but that is technology for you.What I do care about is more political and not right for here. As a follower  of JLH-UK audio-design engineer[sadly no longer with us] I use his 100% tried and tested  methods in  addition to some of my own[based on his]. He was a Minimalist and DC connected where possible/Proved "capacitor sound way before it was recognised and was criticised for it[proved right in the end] .Star-Earthing etc.Every thing I posted was to "open up"the circuit while some find it a lot easier to "dumb down" a circuit introducing filters etc-wrong way to go! . It is harder opening up as it shows up even smaller amounts of noise/distortion.But what you get is a deeper revealer of detail not found in many "dumbed down" to make it smooth circuits. But I like a challenge.ALL electronic circuites from the Government down are built to a price.But in Audio why should you accept the status quo when you can spend as much time as you want AND outlay a very small amount of money to achieve a level of Fidelity that would cost  10s of 1000s of $$$$$. in the shops . The only people unhappy about that would be manufacturers and those with shares in the Audio industry.And dont think I am a lone voice.I have the copies of the magazine -Electronics World/ Wireless World  where some design engineers agree! I must be able to put my point first as then people will understand where i  "come from"-I like helping people but some see this as interference but you cant win everybody! I wanted you to read this first-Duncan1

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