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stax srm717/srm727-11 - Page 3

post #31 of 64
Thread Starter 

Yes Dave. exactly right.also for the adjustment when hot .Solid -state audio equipment i should be left on 24/7 and give it 3 days at least to judge it . If it is kept being switched off/on then you will never hear the best fidelity.Valves[tube] isnt the same doesnt need as long to judge it .valve equipment heats up a lot quicker and the actual amplification is usually all valve so other parts dont need to heat up. I am talking a pure valve set up not a hybrid valve/so;id-state.Once f valves are "run in"for the first time of use the fidelity doesnt change much on switch-on only thing to watch is the bias adjustment/unequal gain which will increase distortion and every piece of valve amps are heavily dependent on the quality of the out-put transformer although in energisers they wouldnt normally be used.Reproduction should be a lot clearer and open [smaller detail heard] but that depends on not only the other mods but the quality of your other equipment.My own-build buffer is 2 individual amps with their own ott PS and an very low distortion audio chip in each /plugged into gold-plated contact sockets so chips can be changed to suite. JLH proved to his own satisfaction that at low signal voltages it was near impossible to tell the difference between them and a discrete circuit.Star earthing polyprop caps used as bypass for electrolytic caps-105 deg high quality double voltage regulators[2 stage drop in voltage] this gives me a neutral output although if some like it warm then just change the chips to suite.I should add if you are not changing the caps in the PS then at least change them [as I have done in the channel cards to 100uf-10uf as fitted is not good enough and change bypass caps to 0.1uf polyprop /soldered to the back to give room to fit the larger capacitors[side-ways] The other 2 small /very low value caps I changed to polystyrene.Duncan

post #32 of 64

that 500k resistor had better be rated for at least 400 volts.

otherwise you are going to be burning up front end fets.

post #33 of 64
Thread Starter 

 Kevin- I have been running the 727-11 for over a month  with no problems! The difference to the "bog standard" 727-11 is "night and day" .It is "well seeing" that Stax has cut-corners with the quality of components and the amount used.

It is "cheapskate" and not top hi-Fi just to use 1 pr of 15V regulators for BOTH channels . I am sure you know about channel overhearing via the power supply . In this case the low voltage PS to the critical input

This lowers the perceived stereo  effect and openness.And that's just 1 comment /many others are there.

I listen on Stax 007-MK1s that people say can be recessed -not on my system.I go for openness and minimalism and my own build mono-buffers are on near as straight through as you can get .

If it makes people feel better then okay buy 500V -500K resistors/ or  2-250k-250V working I dont have problem with that.After ALL the mods it produces upmarket reproduction which is a mile better than the standard item.

Even the dealer in the UK whom I bought it from couldn't figure out why I was buying it as -his words-nobody buys this amp here! They all buy tube amps. But I knew what I was getting and in the future will be making some more mods.  The ONE advantage   that the channel circuits have is that they are so simple with little active devices in the signal path that they reproduce a clearer sound without being "processed" by many stages.

post #34 of 64
Thread Starter 

After extensive listening tests  I have found that  -in my ears- That I am hearing too much detail-NOT because of uneven FR as NEG-feedback flattens the FR but because of increased CLARITY-drop in noise/distortion.

I could hear analogue drum skin bounce and every vocal articulation from the singer somewhat like the OM2-009s. on my 007s-MK1 AS  I like a fully musical presentation I therefore increased the feed-back resistors value to 1MEG-OHM this cured it.

I also realise this amount of detail is what a lot of people want so its "horses for  courses"

post #35 of 64

In effect, increasing the value to 1 Meg Ohm does what? Increase negative feedback?

post #36 of 64
Thread Starter 

NO reduces the Neg-feedback.The reproduction was too "crystal clear"-too much feed-back for me as the sibilance  was also reduced to a minute sharp point. which isnt really  correct musically but it gave highly detailed reproduction  .Even with the increased value of the resistors there still is a lot of detail but the sibilance now sounds "normal" and more natural.

The 727-11 circuit is not the same completely as the 727-1 or 717 although more than half is the same .The presentation in the standard version 727-11 is slightly more open than the others. I have made it [if you do all the mods] a lot more open but with much higher fidelity.   achievable  because of the lowering of noise/distortion. 

If you just change the resistors you would hear more clearly the noise which affects the sibilance making it rougher.Some people dont like sibilance at all but in that case what you would be listening to wasnt  Natural musical reproduction Just listen to a live performance in a hall or football stadium.and you will hear the difference.But there are people who cant live with it so its up to the individual what makes him/her happy.

the PS is very basic and I doubled the capacitance . But really a stablised PS is the ultimate answer. But changing the 2- 10 UF caps on each board to 100UF-400V helps[fitted sideways.Change the bypass low values to high quality  poly-prop and the 2 smaller value comp.  caps to polystyrene -fit both types on the rear of the PCB [to allow the 100UF caps to fit flat. 

post #37 of 64
Thread Starter 

I have now spent time constantly listening  and it is now exactly as I want it to be. Natural musical reproduction smooth but not too smooth so that all fine detail can be heard completely natural.Even 50s rock-n-roll not remastered  to digital now sounds alive and you get a fine sense of that era.

I have a wide taste in music only not keen on jazz. and classical music played has a sense that you are part of the audience.

There is no similarity between the standard 727-11 and what I am listening to now. I have an old but upmarket record -deck and it is not too dissimilar to that- considering I use a CD player that's saying something. Reproduction is on a par with a $10000 unit.

All this can be done by anybody who     knows electronic  components . All you need is a digital MM to adjust the DC V+DC balance .

People dont realise that you can with some work improve the fidelity of Audio equipment beyond the makers set level. It isnt magic just a bit of work.

post #38 of 64

Hi Duncan, this is very encouraging news. With DRPs help, I think its very easy to understand your variation to the NFB or "Spritzer mod is all about.

 

The power supply changes are much less clear to me. It is obvious that there is need for improvement in the PS (as you've stated several times). But what specifically should be done? 

 

You have said:

 

But changing the 2- 10 UF caps on each board to 100UF-400V helps [fitted sideways].

Change the bypass low values to high quality poly-prop and the 2 smaller value comp. caps to polystyrene-fit both types on the rear of the PCB [to allow the 100UF caps to fit flat. 

 

Can you please elaborate in such a way that this can be easily replicated by someone much less knowledgeable (like myself)?

 

THanks.

post #39 of 64
Thread Starter 

HelloNumber9-When a audio circuit is designed  many things need to be taken into mind including the amount of neg.feed-back.The same applies if you change or amend an existing circuit design

The parameters of the  original circuit are different from the modified circuit. The reason I do not supply photos is simple .Unusually I have never got round to buying a digital camera.

On the front of the PCB are 2- 10UF caps-unsolder them-also remove the two small by-pass caps and the 2 even smaller comp.caps.

The 100UF caps are big so must be fitted side-ways-either use stiff solid core wire to solder them in[to support them] or super glue them and use flexible wire.

On the reverse of the  PCB -solder in 2 poly-prop-400V -0.01UF bypass caps-the 2 smaller comp.caps- solder in 2- polystyrene caps.same value as the old ones.= labelled-221J=220PF

This allows the large 100UF caps to be flat on the PCB.

Get back if more info required.

post #40 of 64
Thread Starter 

Just a word to let you know Number 9 still sounding great!

      tried out over the days in all types of music very detailed with a realistic reproduction[sounds natural]

      You get "the feel" of the atmosphere of the music it isnt just a "smooth sound" which can mean you arent hearing deep detail.and therefore cant feel the emotion in the music.
 

post #41 of 64
Hi Duncan, thanks forms sharing your mod. I think I will indeed try it at some point.
post #42 of 64
Thread Starter 

Number 9- I realise my problem is no photos . And this can cause problems in people understanding what I post.

                    At the first opportunity I will buy a Digital camera.

post #43 of 64

I recently compared a modified 727 (original version of the mod) against my own 717

 

It is a bit fiddly as I constantly need to pull out and connect everything and the gain is also different so difficult to match precisely. The volume pot is disabled in both amps.

 

Even so, overall I would say they are quite close in sound.

 

The 727 is a bit more neutral and a bit more precise in the bass so maybe has a little bit better clarity.

Instrument positioning is a little different maybe the 727 has a slight edge again. Maybe.

 

But with the SR007 the modded 727 does sound a little more anemic and I somehow prefer the 717s presentation a bit more it has a slight bloom to the midrange that matches it well.

With SR Omega SR Sigma Pro and SR Lambda I prefer the 727 but I wouldn't be unhappy with the 717 either.

 

 

Considering the 717 costs less to buy and needs no fix at all to sound good (the stock 727 is quite different) it is still great value in my opinion.

 

Both are excellent solid state amps it would be interesting to see what else duncan1 did to his 727 and what those mods can lead to in terms of sq.


Edited by padam - 4/19/13 at 4:57am
post #44 of 64
Duncan, fascinating mods.

Would you expect to see similar gains in sound quality by making analogous capacitor upgrades to the SRM 007t(II)? Or is the tube circuit an entirely different beast?
post #45 of 64

Put the largest cap you like in there but it won't change how badly overdriven the tubes are. 

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