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 Post subject: RS SA-150 mods continue!
PostPosted: Wed Mar 01, 2017 10:58 pm 
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I finally got around to modding the phono section of this little beauty; I didn't go as crazy about the caps as I did for the amp section but did use the same PRP low-noise resistors. Again, the results were outstanding-maybe next will be the PS section, I'm thinking of using a 4s Lipo battery.


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PostPosted: Fri Jun 09, 2017 2:29 pm 
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This mod was as good as the amp section for improving the sound. Hooked up to a good turntable it sounds very nice but it'll need some hours on it. Next is the power supply section and much better speaker wire, I believe there's big gains there over the skinny lamp cord.


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PostPosted: Sun Jun 11, 2017 8:34 am 
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It looks like the same two transistor circuit in their 9V battery powered RIAA boxes they used to sell.

Here's one that's been pulled from the rat shack box it comes in...

probably built by the same people/company.

The zener (D103) probably is helping out as a budget grade regulation. So I hope you are being careful with the buffer capacitance levels, as over-sizing it can cause too much current draw through the zener (elevated voltage in the zener part of the circuit), and pop it.

Try lifting the zener off the circuit board a bit so it can have better air cooling, and thus be able to operate better, or more stably near it's max. Just a precaution that tends to help in these sort of mods, regarding long term reliability.


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PostPosted: Sun Jun 11, 2017 10:23 am 
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Thanks for the tips, I've used the same capacitor values in this section so as not to stray from the original design, just to improve it. That, and the fact I'm not capable of designing a phono preamp....Yeah, the Zener does look a bit small and lost in there, I'd replace it if I were to do anything. I'm glad I got into this really for several reasons; as a Radio Shack fan and as a learning experience but mostly to get the truth out of modding equipment. This corny little amp with my M-7 speakers now equals or beats a lot of my gear, up to about five watts anyway.


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PostPosted: Sun Jun 11, 2017 2:27 pm 
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When I say lift it off the board, just re solder it so it has air all around it and it is not contacting the board so much. This gives it better air flow and thermal relief. However, it looks like it may not need it, as the unit (over it's life so far) suffers no board burn around the zener. It will very likely remain fine if you did not alter the buffer capacitor values.

Over stressed, they tend to look like this (the glass zener diode with the blue on it is the source of the board burn):

Image

And now you know that the last stage of audiophile aspirations, is about walking down the road of buying modified gear and/or modifying said gear with premium parts. It can get tricky, yes, but we can learn to be wise about it, over time, in either the buying or doing the modifying. Bigger risk, bigger return.

Right now I've got a NAD 3120 that is 3/4's done just like that. Everything done with 'best quality' parts, and a modified output amplifier section. Polystyrene caps, all non-magnetic parts, everything with the dials turned to 11+. Even the fuses are custom.

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PostPosted: Sun Jun 11, 2017 9:44 pm 
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Well Ken, since you mentioned it I looked into Zeners as regulators and I see things like using multiple red or IR LEDs, an LM329, which ones would be in avalanche or zener mode at what voltage- I don't even know what voltage it is yet but the Vishay ones look good I think.

I had also thought of doing the big job on a NAD amp, a 304 using the kit from Condor Audio, would you have an opinion on that? It looks spendy. Thanks.


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PostPosted: Mon Jun 12, 2017 4:19 am 
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Question from left field: would a half inch of bare wire, soldered to each lead of the Zener diode, vertical to the board, do anything useful to dissipate heat?

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Alec


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PostPosted: Mon Jun 12, 2017 5:09 am 
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Alec_124c41 wrote:
Question from left field: would a half inch of bare wire, soldered to each lead of the Zener diode, vertical to the board, do anything useful to dissipate heat?

Cheers,
Alec


It might, but it does nothing to deal with the basic problem of too much current flowing through a slightly underspeced device and the lack of airflow underneath it. A clip-on heatsink might be a better temporary solution.

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PostPosted: Mon Jun 12, 2017 8:05 am 
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Alec_124c41 wrote:
Question from left field: would a half inch of bare wire, soldered to each lead of the Zener diode, vertical to the board, do anything useful to dissipate heat?

Cheers,
Alec


yes it would help with heat, but it is best to them go to a zener with a higher wattage rating and then solder that new one, about 3-5mm off the surface of the board. Not too high up as that is no good either - also, to not use jumpers. The whole zener cascade idea is like a controlled flowing river, with the control being in the flow and choke points pools, levels, gating, etc.

One can get a small amount of certain types of noise over that of a traditional regulator, but traditional regulators, when installed and used by the given tech book circuit, they will generally inject notably more distortion into transients--as that is where their correction takes place. That is what they do.. Which is wrong, as we'd hear the smearing as increased detail when it is just distortion. So zeners can make things more 'greyish' in sound quality but they don't screech like a textbook regulator implementation, and transient signal relationships are more linear (less disturbed and colored) with respect to one another. A more complex true regulator circuit can sound better but it takes careful design. Zener regulation is the same, in that implementation is also important. But the rules for zener regulation are fairly simple and the parts count and cost is embarrassingly low. Or one can improve the capacitor and resistor quality in the zener circuit for what might be considered a better circuit at the same price, by that point.

I'm sure someone can dispute what I've said but the simple zener circuits can sound better, to my ears.

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PostPosted: Mon Jun 12, 2017 8:29 am 
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With regards to zener noise, I recall reading that in the early days of digital a zener was used to dither the recorder bit streams....dunno if that's still being done. Zeners are supposed to be noisy, but if 96dB SNR is still possible it's a pretty small amount of noise.

Also, another fellow posted a page with o'scope jpegs showing the results of cascading a zener after a series pass regulator. He used a stereo receiver with a blocking cap to amplify the noise on the output....very impressive reduction, I must say....'ghostly quiet' was the way he described it.

I always wanted to try it but never did. Shunt regulators are supposed to be much better than series, though much less efficient.

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PostPosted: Mon Jun 12, 2017 1:24 pm 
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The PRP resistors I used are 1% non-magnetic metal film and they make a huge difference, like that between a pig squealing and a bird singing. The caps were the best I could fit, in my opinion. I would agree that just a simple, high-quality replacement for the diodes would be in order. What I didn't use were Vishay MKP1837 film caps, either for bypass or replacements (in the amp section) as they were horrible, I've read that they were the go-to cap so there you go. I guess you can't believe everything.

So I play a stock LAB-500 turntable with it now and the whole system is pretty groovy, along with an SCT-33 three-head tape deck and a 31-2020 equalizer through the tape loop only, not the turntable. I can almost stop messing with it now and just listen!


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PostPosted: Sun Jun 25, 2017 9:47 am 
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The lamp cord speaker cables had to go, so I found a package of Belden CAT5e Bonded Pair and wired it in, first to the speaker switch board which turned into a bit of a fail then just straight out. I can get rid of the switch assembly and connect directly to new switches in the future. Those are four pairs of solid wire BTW, I'll have to find some strain reliefs for them later.

As for the difference, I would say the sound was cleaner, had faster attack and at first maybe lost some bass but after listening for a while maybe it was more balanced and the highs, when I think they couldn't get better just did. Actually a bit more in the lower midrange as well. I wish I had a spectrum analyzer to back it all up really. I had just changed the drivers again putting the PRO X-7 woofers in and then the M7 "B" tweeters back in and that seems to be the magic.

After all this I'd conclude that it would be more than worth it to invest in a small pair of very good bookshelf speakers if you had any doubts, these are my project of course but for a small room little speakers will more than suffice, I wonder what something like Totem Mites would sound like! If mine have gone this far they should be truly amazing.


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PostPosted: Tue Jun 27, 2017 7:12 pm 
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If the unit is fused, try a very long time delay fuse, they tend to not 'crunch' the transients as much as a standard delay fuse.

There are some non-magnetic shurter/cooper-bussman/etc fuses at digikey, get the ones with the 1,000a-10,000a peak transient handling capacity.

Those types tend to be the base design for the audiophile fuses. Probably half of their perceived sound qualities, in the less expensive given varieties and brands.

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PostPosted: Wed Jun 28, 2017 9:36 am 
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No fuses or relays in this thing, I like it! Minimalist design for sure, in fact my best amp is like this. True, power can go through a fuse, maybe try a solid copper rod and take your chances....


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