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The SRM727 thread

kevin gilmore

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Birgir finally has his hands on the SRM727, and evidently does not like the thing at all.

Extended comments hopefully coming from the big guy soon.

Comments on the 300 should also come soon once he can get his fingers inside the

chassis to rewire the transformer.

I've come to the conclusion that the srm323 is stax's best sounding amp that is

still in production.

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Thanks for the offer Doug but I have the KGSS here and that will suffice for comparison purposes. On the whole this design is an improvement over the 717 in the same way that the KGSSHV is an improvement over the older design. It is fully CCS loaded (including the third stage), the build quality is a step up from the 717 and it looks much better to boot. That tiny amber power on led is a very neat touch. Now the bad news, Stax got the bright idea to cut the feedback loop short so it stops at the third stage (i.e. on the plugin cards) and the output stage is outside of it. I for one can't see any reason to do this unless you want an nonlinear design so you end up with an rather odd sounding amp where everything is bloated and larger than life. It even makes the SR-007Mk1 appear bright on some recordings so good sounding it is not. Another nasty side effect is the lower gain (needed for the amp to remain stable), by far the lowest of any Stax amp I've ever tried so you are pushing the volume level quite a bit even with a normal 2V source.

Now here is what you have to do to fix the amp. Stax were nice enough to leave all the feedback traces on the boards but it will be a tight fit get wires onto those gold tabs. Next up is the SRM-300 for some surgery but I might get to the 727 later today.

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Now I have the SRM-300 up and running. While it uses a Kitamura Kiden R-core transformer (not the standard Bando) Stax have maintained the same wiring setup so aside from the 117V wires cut way too short, rewiring is dead easy. Now just to be clear, the SRM-310 is the exact same amp with a different name for the export markets.

Now this amp is fairly unique in terms of what Stax normally does. It doesn't use the tried and tested SRM-1 Mk2 design (which the SRM-252 uses) but rather it is a SRM-001 on steroids, fully AC coupled and with a regulated PSU. As for how it sounds, it's not bad at all. No real bass output with the SR-007 but that could be due to the tiny output caps and the very limited power output (it consumes just 10W) but it is just fine with the Lambdas. I really like the form factor though and I hope Stax will produce the SRM-353S which also functions as a stand for the headphones.

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Now for some progress and me officially admitting that I'm a bit impatient. :palm: First off, here is how I fixed the cut windings on the transformer:


I just used some cut off resistor leads for this. Made a loop around the cut wire and clamped it in place with pliers then I flooded it with solder.

As for the purpose of this thread, how we can convert a SRM-727 into something more then it is today, I hit a bit of a snag. The resistors on these boards at least are glued in place so there is virtually no way of removing them in one piece. The plan is to remove one 150K resistor from the 300K feedback line and relocate it. Since these resistors are a pin to work with on a good day then I just did this instead:


I had to run out so I can't test it until later today.

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Ok, now I'm back so I'll continue this monologue but I can report that the mod is a complete success. No problems fitting the boards back in place and the sound is as it should be now. The amp is still on the warm side of things (pretty much the same output stage as the 717 so this is to be expected) but the bass has far more control now and the top end isn't misbehaving any more. I still have to do some more surgery to raise the gain at least 6dB to match the other Stax amps but as it stands now the 727 has mafia approval. :)

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So here is what i wanted spritzer to do. He came close.

Get some of the ultra thin copper tape with stickem on the one side, cut with a sissors to

be exactly the width of the card edge land, attach over the card edge land, and make it long

enough to go past the length of the socket. Then do the resistors as spritzer has done.

That way no pressure on the socket.

This just has to be the way to get these things to sound like what people seem to want

them to sound like.

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There is in fact no pressure on the socket if you use very little solder and solder to the very top of the gold pin.

sweet news! let the flood of spritzer modded 727s commence.

in fact you should call this the Level 1 mod and sell them for $1900. :)

If the Yen wasn't so fucked up then I'd buy a few, mod them and sell to good people. :)

Very cool, Birgir. How difficult is the mod overall?

Not terribly difficult really. These PCB's will not handle a lot of abuse so removing the old resistors is the biggest challenge, soldering in the new ones is easy. Just bend the leads to the correct form (I bent them so that they sit above the PCB but it isn't really needed) and solder them in.

Also, as Marc said, you know people and the amp is dead easy to open up (the allen key is even included) so you could just send the small plugin cards to have the mod done.

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Since Kevin asked me to check out some parts inside the SRM-300 I thought it was only fair to do some modding. :) The stock output caps are 103/630V units so I just put in some 105 (1uf) caps instead. Before:




I also wanted to show how to set the voltage of this amp. Stax placed 6 holes at the bottom of the PCB but didn't use them on this unit except for one connection. They clearly intended for the secondary wires to be soldered here and then jumpered on the other side of the board. Since there are 99% odds of me selling this amp one day it made sense to do it like this.



It's been a productive day too since I've also managed to bring a SR-Omega back from the dead. :)

Spritzer, is the 727 converted to 240v operation as well?

Yup. You can see the PCB used for voltage selection mounted to the side of the transformer. The setup is the same as above, you use jumpers to configure the secondaries and you have to move the wire from the power switch as well.

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Ok, some more progress. I took some time and fixed the gain issue of the 727, just swap out the 560ohm R5 and R6 resistors out for some 200ohm units. Again I was using RN60 pulls so they are way too big for the job...


Now the amp has the same gain as the other Stax amps, sitting at 5 on the dial with my computer source. It does sound rather good with the SR-Omega even if it does bring out their bright nature a little bit once you push the volume. There is no excessive bloom here and it has a lot of control for a commercial amp. Why Stax couldn't just have made it like this in the first place is beyond me...

As for the SRM-300, the LF353 had to go so I installed sockets and OPA2604AP's


Edit: Damnit!!! I forgot to sand the chips... :palm:

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  • 5 years later...

Sorry for the revival from the dead but I am finally contemplating doing the feedback mod to my 727 amp while the bhse is dead (bad luck, second time some shit happens with mine, one of the iec block fuses blows up right after the delayed relay switch since yesterday...).

So about the 150k resistors, 1/4W suffices?



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2 hours ago, spritzer said:

I'd use 1/2W units but quality 1/4W units should work.  That is non Chinese crap... 

I just did the mod recently and it was fun to have a proper SMD heat gun for it.  Removing the stock resistors was easy as pie. 

Thanks Birgir, I know this is almost shameful to ask but, I was looking into the zillions of resistor type on this site ( https://www.chip1stop.com ) and I am just lost about what would be fine... Actually, I could probably order some sand it someone needs at the same time...



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