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The SRM727 thread


kevin gilmore
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Is there a place on the 727's board where a normal bias line could be tapped? I will be adding normal bias to my amp since I only have normal bias phones right now. I could just tap the 120v line and put in an SRD style bias supply but if there is a place on the board that could be tapped for the bias supply that would require less components that would be cool to know.

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Oh I know. Believe me, that part of the operation is already complete. I installed the six pin sockets last night. I like the daughter board they are using now. The old jumpers between the sockets were far from elegant. 

Edit: So if I'm understanding you spritzer, I'll basically be building the SRM-1 Mk2 low bias supply. B+ from the 727 is 350v correct? So a voltage divider for this would be B+ into a 180K resistor with a 345K resistor to ground then a .1uf capacitor to ground before a 4.7m resistor to the bias pin.

Voltage divider bias circuit..png

Edited by Arthrimus
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My step down transformer arrived today and the SRM-727a is singing. I'm still waiting for a couple of the components for my normal bias supply to get here so for now I'm borrowing the bias supply from my SRM-1 Mk2. I also volume matched them with a 50hz sine wave and a voltmeter and am doing some A-B comparisons. So far what I've learned is that the SRM-1 Mk2 is a really good sounding amp. I'm really not that impressed by the 727. Of course I haven't yet performed the feedback and gain mods, and I only have a pair of SR-Lambda NB to test with but still. I think in stock form the SRM-1 Mk2 probably sounds better than the 727a. This is all part of my multi year plan to acquire an Omega 2 so it was a necessary upgrade regardless. 

Off to do some mods!

727 srm1mk2 a-b.jpg

Edited by Arthrimus
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There is. The ground lugs on both amps are bonded together.

 

Edit: The mods are complete and it sounds a lot better now. This new version of the board has hardly any glue on the resistors so I was able to remove them with little damage. The feedback resistors tested good so I just reused them. The bass struck me as the most problematic thing with the amp before the mod, but now it's under control and sounds really good to these ears. The gain fix was also substantial, before the mod the 727 had to be turned past 2:00 to match the SRM-1 Mk2 at noon, now the situation is almost exactly reversed.

I still don't think it sounds substantially better than the SRM-1 Mk2 but as I said before this was a necessary upgrade for my future plans. 

727 mods.jpg

Edited by Arthrimus
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Well I don't know how many normal bias only SRM-727s exist, but I have one. I got the bias board built and installed today. I'm still waiting for one of the resistor values to arrive here, so for the moment I've ghetto rigged a couple of 100K resistors in series to get 200K on the first resistor, and I've switched the second resistor in the divider to 300K because I needed some for my SRX plus build and I figured I'd kill two birds with one stone. B+ runs a little hot on my amp at 365v so all together this setup yields a 225v Bias. I've got some 165K resistors enroute to replace the rigged 200k resistor on the input side so the final board should yield around 235V bias which I'm thinking will be just fine. 

The board is mounted to the side of the chassis on some nylon standoffs. Ground and B+ were tapped off of the test points on the board as indicated in the picture below. For ground you could actually just ground to the chassis using brass standoffs and connect ground on the bias board to one of the standoffs. I didn't have any brass standoffs so I just ran a wire for ground.

She's powered up and my Lambda NBs are singing with the amp completely untethered so I'm happy. Mod success! If anybody else wants to build a bias supply like this the above schematic and values are correct as confirmed by spritzer. 

I'd also like to amend my previous comments on the sound of this amp. It's been a slow burner for me, but I've really warmed up to the sound. It's a little warmer than the SRM-1 MK2 which I think in short term A-B testing makes the 727 sound a little veiled. But actually all of the detail is there, it's just not as in your face as with the SRM-1 Mk2.

I'm not great with all the audiophile lingo, so it's tough for me to describe exactly what this amp does, but I guess if I had to generalize I'd say it sounds more natural and effortless than the SRM-1 Mk2. It's strength is especially apparent with percussion instruments. I've listened to this amp for around 20 hours at this point, going through a significant chunk of my music collection that I'm really familiar with, and I prefer it's presentation over the SRM-1 Mk2 on everything I've heard so far. 

Well I think I'm done here, this is my new main amp, at least until I build the SRX Plus, but that will probably be a while.

727 post mod inside.jpg

727 post mod singing.jpg

Edited by Arthrimus
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  • 10 months later...
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  • 1 month later...
On 6/23/2016 at 11:25 PM, Arthrimus said:

Well I don't know how many normal bias only SRM-727s exist, but I have one. [Details redacted]

Thank you Arthrimus for posting all of this about your bias mods! I haven't learned enough to even think about trying it myself (I can follow most of it, but I wouldn't have thought about grounding the cases together, for example). It's great being able to see what's possible if I put in the work to get better. A year ago you'd have lost me at "bias voltage" :)

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  • 4 months later...

Well, I just received mine, it is the last version as well. I think with the Lambda Signature there is nothing that strikes out as particularly bad about it in stock form (it seems somewhat less synergistic with the SR-404, where it gives more of an impression of being rolled off, as Tyll wrote in his shootout). Although the more I listen to it, the more that "half blandness" starts to stand out. And it doesn't quite have the mid sweetness of the hybrid variants either, so I can see why people dislike it, it is neither this nor that.

If I use it in bypass mode with XLR input anyway, is there any benefit or adding in the extra gain as well besides the feedback mod? In theory is it better to feed it more input level or less? Or it doesn't really matter?

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3 hours ago, padam said:

[…] somewhat less synergistic with the SR-404, where it gives more of an impression of being rolled off

[…]If I use it in bypass mode with XLR input anyway, is there any benefit or adding in the extra gain as well besides the feedback mod? In theory is it better to feed it more input level or less? Or it doesn't really matter?

I'm pretty sure it's the 404 itself that rolls off the lowest of the low end. I always thought of "synergy" as a purely subjective thing, I don't think you can attribute an objective effect to it.

I assume you're using an 404 and a Lambda *PRO* Signature? If so, cool! That's my setup exactly! I think the two sets of headphones compliment each other quite well, any time I get tired of the sound of one, the other is always exactly what I want to change to! 

I was under the impression that the 727 gain mod made the amp unstable and it wasn't recommended, but don't put a ton of stock in that until someone else can confirm. If you're using an active preamp, the stock 727 can handle enormous amounts of power, so if you really need more gain, you can put it upstream easily enough. I'm using an ODAC -> passive attenuator -> 727, and I haven't once needed or wanted more gain than I have; I suspect that might become a concern when I upgrade to an SR-007 in ten years after I've saved up enough!

I do think you will like the results of the feedback mod based on your description, since your negative impressions are exactly what the mod fixes, subjectively to me at least. It's a really easy job, assuming you have soldering/de-soldering experience. I did it in a half hour, after getting over my nervousness about working on something so expensive and so critical to my day-to-day life. 

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The more I listen to it, the more it reminds me of those movie scenes where everything is there, nice and tidy and looks good etc., and yet there is a suggestion that something is not quite right. That's how I can sort of describe this "semi-bland, semi-dynamic" sound. Sometimes I think it works quite ok actually, I prefer to preserve my hearing and keeping things at lower volumes (the bloom/lack of control becomes more apparent, when pushing it more)

I already have some experience from a few years back, through a local friend, I had the pleasure of testing with a set of 009s it for a week or so, that seemed like a bit better match, but the mod still improved it. After the mod, it just gains back it solid-state-ness, more grip on the sound. It sounds very very similar to a 717 - slightly clearer, less diffused stage and a bit less bass bloom - and that was described as not far off the T8000 (by KG). Just wanted to know if there is any (theoretical) sonic benefit to raising the gain and feeding more voltage from the source side, that's all, with the feedback reinstated, the stability problems should be absent. I also have a dynamic amp (a CEC HD53N) with a gain switch and it certainly sounds a bit different when switching those modes and changing the input level, I think I like it with low gain and with 4V RMS input which is the maximum recommended one for XLR input and adjusting the rest on the amp. But with the Stax amps it always seems best when the attenuator is bypassed. Probably best to leave that gain as-is, the adjustment remains easier.

Yes, although so far in my world every Lambda lived as a shadow under the original, except the latest ones, which I haven't heard yet. That's my assessment anyway.

I prefer the bass quantity of the SR-404s but they lack the smoother, more musical voicing of the Lambda Signature (it is more like 'rough and ready'). The trouble with the latter is still the same - it just tilted a bit too much on the lightweight/bright side for me and the mids are close, but yet not quite there. So back again where I've started a few times already. That is, the NB Lambdas just seem to hit the nail on its head. And they weren't even designed to be like that, just a tamed down derivative of the NB Sigmas that accidentally managed to sound really good and less 'Lambda-like' in some aspects. Unfortunately, a recent factory visit article confirmed what was already known anyway, while they do acclaim older models, but they are not looking backwards (meanwhile, Leica makes a shitload of money re-making old lenses)

The thought of the HE60 popped into my head many times, but I think besides the construction I would have similar 'too bloody bright' complaints to the Siggy, but this one I may seek into after all.

So I think I will have to have the "full Arthrimus" mod done as well.

And of course after all this hassle comes the point, when I realize that all this is nothing else, but a gateway drug to a world of DIY designs...:frantic:

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You've got a normal bias jack on your 727? If so, that's awesome! I'd love to do that to mine so I can get in on this NB Lambda Signature action too. Where do you see a 4 V rms limit on the XLR input? The owner's manual just says 30 V and doesn't specify a particular input, which seems to imply it refers to both.

Anyway, I once had that notion that higher input might change the sound characteristics in some way, but the more I learn, the more I fail to imagine how that could be. That in itself doesn't mean much, but then with the fact that nobody with a better understanding than me has explained this phenomenon either… Until someone can explain otherwise I don't think it makes a difference. I chalk it up - like so many other ideas I'm unlearning - to some combination of tribalism and wishful thinking meeting in an echo chamber.

I call this monster Trifucking! 

;)

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8 hours ago, ComradeTexan said:

You've got a normal bias jack on your 727? If so, that's awesome! I'd love to do that to mine so I can get in on this NB Lambda Signature action too. Where do you see a 4 V rms limit on the XLR input? The owner's manual just says 30 V and doesn't specify a particular input, which seems to imply it refers to both.

Anyway, I once had that notion that higher input might change the sound characteristics in some way, but the more I learn, the more I fail to imagine how that could be. That in itself doesn't mean much, but then with the fact that nobody with a better understanding than me has explained this phenomenon either… Until someone can explain otherwise I don't think it makes a difference. I chalk it up - like so many other ideas I'm unlearning - to some combination of tribalism and wishful thinking meeting in an echo chamber.

I call this monster Trifucking! 

;)

Not yet, maybe next week. And there is not much point in running the Signature at normal bias, it won't sound very good, it is intended for the original SR-Lambda (I have three Lambdas right now)

I meant that for my other dynamic amp, not the Stax amp. I will probably leave the gain as-is (for now), so that hopefully I will be able to do a direct A-B to a stock unit at the distributor to  evaluate how much better it really is. As-is, it does a good job of taming down the etch in the Signature (not so much on the SR-404), it is just the lack of overall fullness, which is a bit of a shame, but it does its magic with symphonic music.

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  • 3 weeks later...

The amp mod is now done and normal bias added in as well (thanks everyone for the contribution). It runs pretty hot for being quite big and solid-state, maybe I'll move the whole setup away from its current location.

Well, shoot me and, but I think the Lambda Signatures actually sounded better in its stock NFB from, there was a bit of bass bloom, warmth and also sparkle to its midrange (but not peaky) that was actually quite pleasant. This is smooth but also thin and lean, not sure about the matching on this one. The SR-404s have improved, the bass is now more in control. If they were my own headphones I could potentially get used to them, but I don't think they are my cup of tea. Because...
I just prefer the SR-Lambdas (what a surprise), they are just better balanced and the slight upper bass emphasis makes them fuller, but the 'unforced', yet very clear midrange is my favourite bit. Shame I couldn't try them unmodded, it may have been interesting. I feel they sound leaner and brighter than with the SRA3S. Those original Hitachi tubes just seem to have superb synergy with them, while the phones are able to provide plenty of 'magic' on their own, there was something particularly alluring with the midrange, which is not quite there here, it's just more 'plain'.

I will say though that the SRA-3S wasn't as clear, spacious or detailed as the modded 727 (it's great to have the option of disabling the Alps pot and I also prefer balanced connection), so maybe it's a sound that will grow on me as time goes. I do remember that the old version KGSSHV sounded even brighter than this.

Maybe a CCS modified SRM-T1(S) / SRM-007t with some nice tubes is able to sound more pleasant than this with the Lambdas while costing about the same or even less, but this is all just speculation based on one opinion at the other place.

All I know is that the SR-007 Omega IIs (any variant) are definitely much better with the modded 727, overall it just a rather neat 'little' amp and at a good price point as long as one likes a more solid-state sound. Hopefully I can do a direct side-by-side with a stock demo unit to do this properly. It would be great just to really see how much difference is actually there, what differences I've ignored, and how much is just imagination.

Edited by padam
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Glad to hear your thoughts! My 727 also runs notably warmer after the feedback mod, but it's constant whether its idling or driving two pairs of Lambdas. It worried me at first, and I've thought about adding a cooling fan, but mine's run continuously for six months and the only problem is keeping my cat from discovering this new warm thing she could sleep on and get her fur shoved down into.

Agreed about the value of the pot bypass, I don't use mechanical pots at all in the signal path if I have any choice. The 727 uses a version of the Alps RK27 that was never sold to consumers AFAIK (the key feature is the dual knobs, one per channel). The RK27 has a beautifully smooth feel, but even the cheap 21-way DACT switch I made into a preamp provides superior sound quality. I'm about done building an LDR-based preamp volume attenuation device that I expect to sound even better.

@padam what does NFB mean, and what mods are done on your Lambda Signature?

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