Comparing battery-powered phono pre-amps, from RS ($$) to ASR ($$$$) ... and beyond!

[D-500 Flogger]

Finally, finally I've almost completely wrapped up all things having to do with fully sussing the power subsystem for the Rat Shack (Rat's Hack? :0) phono preamp.

But having slept on all this & re-reading last night's postings, I was struck by the fact that I was describing the power delivery problem as if I was going to have to supply the same amount of *current* demand at 20 Khz as I was going to need to deliver for all the percussion below 120 Hz.

Before I go any further, I do believe & adhere to the concept that the circuit must provide as close to equal voltage gain across the 30Hz-20KHz frequency range. (And supposedly this circuit does this within a +/- 1dB tolerance.)

But in terms of what is going to be actually demanded from the power supply? How big & how often? Well, if I know *this*, then assuming I have to decide where my finite power supply upgrade budget goes I can now make a more informed decision about where to properly allocate the funds: 33% batteries/66% capacitors, 50/50 split, or 66% batteries/33% caps?

Given the above, upon reflection I decided that last night I solved the power supply upgrade problem *before* understanding what demand(s) were going to be presented to it. Good news, I went on a read-about & came up with a detailed AES study on this very issue:


(Taken from this AES article -- a deep read!)
There is a *lot* of info in this one graph. This is good info for us, amp builders, people choosing speaker components, etc.


(See link above)

Personally, from a lifetime of listening to music, I know that while I enjoy a good 100-watt bass thump in the chest, (over & over)
10 watts of voice / lead guitar (in a small room) is right around the point where I start fishing for ear plugs, and 5 watts of power at 8 KHz & above will drive me out of the room immediately. For a bunch of reasons best left for another post elsewhere, with conventional loudspeakers it takes a lot of electrical watts to deliver any amount of bass acoustic watts. (Owners of properly set up K-horns excluded.) Even with (12) 12" servo-controlled woofers, Arnie Nudell had something like 2Kw of power on hand in order to deliver his goal of the sonic power of a full 96-member orchestra with his IRS-V. (!)

Meanwhile, due to the size of the transducers vs. the wavelengths being generated, midrange & tweeters are much more efficient at turning electric watts into acoustic watts....

...but I digress. My current understanding is that the battery/battery pack is going to provide the juice for the relatively long/loud bass & percussion, whereas C11 will support the immediate needs of all the low wattage/delicate high frequency details. (Thinking of all the magical air in the 'tizz' of a brushed cymbal or drumhead that is always among the first thing to get mangled by a brick-wall filter or lossy .mp3 file...or substandard power supply. :0(

****

I know it sounds strange, but just because we need to uniformly amplify (in the voltage domain) all the signals across the audio frequency spectrum, this doesn't mean that the actual watts of power demanded by typical 'popular music' are spread uniformly across the audio spectrum.
Quite the contrary.

Food for thought...and I'm done thinking about the power subsystem. On to more interesting stuff, promise! :0)

Cheers --

 

[laatsch55]

LOL!! Good reading D3..... now get to measurin!!

 

[D-500 Flogger]

Since we've already looked at optimizing the power subsystem, let's take a look at how to best lower the noise floor.


To implement arrow #3, we need to replace the resistors in the pre-amp. (At the very least, the 2 resistors in the signal paths, R3 & R4.)

Big picture, ignoring the max effort/exotic stuff, today's resistors fall into 3 basic camps:

* BROWN body: The old carbon 'composition' resistors. When you read about "Excess noise" in a technical white paper, these are the resistors that, due to the way that they are made, are guilty of generating so much of the hiss that we hear in vintage equipment.

* TAN body: These are the next generation carbon film resistors, and more importantly, were used in our phono preamp. Better than the old stuff, but not as good as the metal films.

* LIGHT BLUE body: These are the new metal film resistors. Thermally stable, *much* quieter than the carbon-based stuff. Originally there was a significant price premium, but this has all but disappeared over the years. Depending upon where you buy them, these now can cost anywhere from 15 cents apiece & up. NOTE: Some guitar amp guys fret over the fact that these might make their amps lose their signature 'tone' by being too clean/too sterile, but since we are trying to let the recording provide whatever tone it has, we should just upgrade the resistors from carbon film to metal film & enjoy the lower noise floor.

FWIW, here's a good depiction of where the 'excess noise' comes from in an old carbon composition resistor:

Because of the 'discontinuous' nature of the individual carbon particles in these composition resistors, the more the current flow, the more the excess noise developed -- essentially we are hearing the tiny arcing between adjacent carbon granules. (NOTE: Not to be confused with thermal noise, which is a constant based upon the actual electrical resistance value of the resistor. ALL resistors (including metal film) suffer from thermal noise -- and the lower the resistance, the lower the thermal noise.) But the real difference between carbon comp & metal film is that the metal film doesn't suffer from 'excess noise'. Keep this picture in mind when you are wading through the Analog Devices article in the capacitor post...

****

For what it's worth, here's a typical discussion about what resistor to use in an audio circuit: https://forum.audiogon.com/discussions/top-resistors

****

And the 4th arrow on our big-picture graph involved lowering the noise floor the last possible amount by replacing the (4) original 2SC1740S transistors with the 2SC1740LN (Low Noise) version. (Of course, since we're doing this 20-30+ years after the fact, this low noise part number is obsolete, so now what we have to do is find a modern-day equivalent to this version of the transistor & acquire those instead.)

****

And assuming all the parts we purchase meet specification, and we are careful with replacing the old parts with the newer, quieter parts, we will have taken a fairly quiet phono preamp and made it even less noticeable to the listener.

Shhhhhh...

 

[D-500 Flogger]

Now that we've stretched the signal/noise ratio of this phono preamp with an improved power supply to handle the music transients, and then lowered the noise floor with metal film resistors & low-noise transistors, now it's time to change/improve the remaining 'sound' of our phono preamp.

The sound I am referring to are the capacitors in the signal path. Referring to the schematics we've been using, in *each* channel there are 2:

L channel - C1 on the input, and C10 on the output

R channel - C2 on the input, and C13 on the output

Good news! Instead of us playing the telephone game by my paraphrasing a subject matter expert, please stop here & first read in the attached file what John Curl had to say about the issues with different types of capacitors. (Note: The capacitor section starts on p. 12)

****

OK. For those who want to better understand what John Curl (and Gepetto) are trying to share with us about the choices they have made in their designs, here's a quick listing of articles that will help us appreciate what they have to work with:

Capacitor articles from the Objective / Analytical Perspective:

1) Analog Devices - Application Note AN-348 -- "Avoiding Passive Component Pitfalls"
(Discussion/measurement of several errors + Dielectric Absorption or 'Soakage'. Varies from "0.02% for Teflon...to up to 10% for electrolytics." (!)

2) Another article on Dielectric Absorption (newer, easier to read graphs)

3) For those who are just getting into the 'vintage audio' hobby, this entire video from "Mr. Carlson's lab" will show you what we are up against when it comes to aging capacitors and their affect on both sonics & reliability. (And if you don't have time to watch the whole thing, his o-scope demo on capacitor microphonics starting at the 20:22 mark makes a convincing case.)

4) A gentleman by the name of Cyril Bateman did an exhaustive series of articles back in '02-'03 for Electronics World about "Capacitor Sound".

Here's just one screen capture where he measures the distortion of a 1v signal by an electrolytic capacitor vs. a film cap.

Here is where to go in order to check these out: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles

And for those who find a more Subjective approach more intuitive to choose a capacitor to run your music through, here's a couple more articles:

5) Reasoned rationale for choosing the right passive components: https://bext.com/replacing-passive-components-to-improve-sound-quality/

6) "Miniature electrolytic capacitors shootout" - Article actually written for headphone amps, but a good read if we stay with electrolytic coupling caps in the phono preamp? NOTE1: Although he shares at least 9 different "listening impressions", it doesn't look like he got a chance to try a good 'bipolar electrolytic' -- elsewhere people have written that IF you are using electrolytics in your signal path, then the bipolar setup is the preferred way to implement. NOTE2: As with most subjective articles, this eventually devolves from shedding light to heat on the subject -- too bad, a wasted opportunity. Still the first part is worth a view.

https://www.diyaudio.com/community/...otout-best-for-coupling-to-headphones.319979/

****

From the Rat's Hack phono preamp perspective, we have 2 caps in the signal path of *each* channel, so given everything I've read this is where we have the opportunity to make the biggest improvement in the overall clarity/musicality of the device.

Hope this helps...

3D

 

[J!m]

Metal film resistors are a given. As you point out, in a new build, the price difference to stuff an average board is less than a cup of coffee.

Metallized film capacitors- if you want exotic, I think the Mundiorf in the red wrapper are the nicest. That says, I rarely spend that extra money because the difference is so small.

But, another advantage is how tightly tolerances these things get. A 1%tolerance film cap is not that exotic. Channel balance gets easier. Behavior much more predictable.

When I couldn’t get specific parts for my phono preamp, I went tighter on tolerance and/or up in voltage or amperage. Price difference from a 2% metal film resistor to a 1% is less than a dime. And I have a few that are 0.1% tolerance…

 

[D-500 Flogger]

Re: Renewing/upgrading capacitors --

In order to actually replace the old caps, you have to source/purchase new ones. Problem is, it seems that anything that earns a good reputation & there's a demand for, the counterfeiter b@st@rds are quick to respond with their bad chemistry cr@p made to look like the originals at 1st glance. (!)

To try to combat the problem, the legit manufacturers try to defend their reputation by marking the top & bottom covers with unique stampings. (Supposedly the counterfeit operations either don't bother or can't duplicate these markings accurately.)

For what it's worth this chart is supposed to be something you can you use to help prove/disprove that the capacitors in question are legit or not:



****

The bad chemistry thing. A few years ago while working on other people's PCs I've seen electrolytic capacitor failures. It seems that the root cause was a case of commercial espionage gone wrong. (In the form that the formula for the high-performance low-ESR electrolytic paste was mis-copied.)
A few years ago this problem was widespread enough that it was referred to as the Capacitor Plague.

Supposedly this is all better...but nevertheless it looks like you are taking a real chance with off-brand electrolytic caps, much less counterfeits.

Buyer beware --

 

[D-500 Flogger]

Now that we've reasoned our way through this phono preamp in the old school method, there is still one more thing that this simple, finite device can provide us -- that is, a really simple circuit that can make a big difference in the sound of our main system...so simple that we can actually use it to help us learn about SPICE?

I really think so. For the self-starters out there here are the first 2 links to get you on your way. And the price of admission? Free! The learning curve?
Steep. But worth it.

1) Where to get it? From Analog Devices, no less: https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

2) In addition to everything you can download, watch, and play with on link #1, here's an 8-minute video; the simplest-possible intro to firing this up:

(Think baby steps.)

It's a brave new world, but there are a effton of videos that pick up from here and take you a long ways into circuit simulation. I, for one, won't be using SPICE in anger anytime soon...but down the road I absolutely will...for it will be a lot cheaper to try the top 3 parts that gave me what I wanted in my simulation...instead of buying a dozen different parts & soldering them in 1 at a time and figuring out the circuit empirically in the physical realm. (!)

EDIT: Not just the money savings, but the time savings in playing the "what-if" game?

Remember my penchant for playing the min-max game in order to better understand/troubleshoot something? With 'SPICE' (LTSPICE) I could sub in different quality caps in different places in a circuit and see where the circuit 'doesn't care' vs. where the circuit is extremely susceptible to capacitor quality. To go from a basic conceptual familiarity to a deep comprehension via simulation would be as fun for me as what others derive from the immersive PC gaming experience. (Which I am a complete novice...never had any extra time to dabble in the genre...)

3D

 

[D-500 Flogger]

LOL!! Good reading D3..... now get to measurin!!

...And that's all I've got for this battery phono preamp project -- for awhile anyway.

The good news is that a ton of gear has arrived for the lab, so I am going to refocus on physically assembling / repairing / proving the repeatability of this lab for the next while.


Then the 3 amps, the preamp, the speakers...and then once *all* that is back online, then I'll get to come back & turn all this phono preamp theory into reality for myself -- for listening to Aja on vinyl will literally be the cherry on top of this huge project. (At least the TT is ready & waiting. :0)


Meanwhile, if Derek92994 & others want to take this project & run with it in the interim, I will be all eyes & ears.

Best of luck, mate! :0)

Cheers --

 

[NeverSatisfied]

...And that's all I've got for this battery phono preamp project -- for awhile anyway.

The good news is that a ton of gear has arrived for the lab, so I am going to refocus on physically assembling / repairing / proving the repeatability of this lab for the next while.
View attachment 60598

Then the 3 amps, the preamp, the speakers...and then once *all* that is back online, then I'll get to come back & turn all this phono preamp theory into reality for myself -- for listening to Aja on vinyl will literally be the cherry on top of this huge project. (At least the TT is ready & waiting. :0)


Meanwhile, if Derek92994 & others want to take this project & run with it in the interim, I will be all eyes & ears.

Best of luck, mate! :0)

Cheers --

You know if learning had been this much fun when I was in school, I might have stuck with it and got a degree.
I did get my RNE (red neck engineering) degree but that came from the school of hard knocks.
3D your effort and enthusiasm is appreciated, I understand a little more now than I did before, thanks.

 

[derek92994]

3D, that is excellent, my cigarette went out multiple times during reading . Your contribution is much appreciated. I am working 12 hour shifts, so not much time to do anything. On a positive note the other 3 phono pres arrived. The best I can do for now with the 30 minutes I have before bed, is hook two of them up and dedicate each to one channel. I have also purchased some fresh duracell ultra 9 volt batteries which are supposed to be a step up from the standard copper top.
More when I get some time to play with these. Cheers from Australia Mate!

 

[derek92994]

Hooked up the two pres, both loaded with a duracell ultra battery. Two audio grade 32k resistor plugs on unused inputs (its all I had handy). Earth wire running from Turntable then another wire to share the earth to the 2nd pre. Put some Dire Straits on, Japanese pressing, not bad. Its going to get better too with upgrades!

 

[derek92994]

Meanwhile the Project Tubebox DS phono pre sits there snickering

 

[D-500 Flogger]

Meanwhile the Project Tubebox DS phono pre sits there snickering

derek92994, is this your phono preamp that the RS preamps are being compared to? And are you still running it off of the lab DC power supply?

If so, then this is *excellent*, for this means that you are comparing these little boxes to a high quality preamp! For example, looking at the specs they are claiming a noise floor of -93db. (seriously good for a phono preamp!)

****

You see, these days there are a lot of households where a Rat Shack battery-powered phono preamp would be the best thing in the house...so where do you go from there? Since you've rolled tubes & are running your Tubebox off of a lab power supply, you obviously know how to get the very best out of it. So this is a golden opportunity to carefully document the sound of the 'as purchased' RS phono preamps against your gold (glowing?) standard. Difference in perceived noise floor, the sound of music, female vocal, solo piano, your favorite songs, etc..

And then, as you make changes to the RS preamp, try to characterize any improvement (or loss of ground) compared to the previous state of the preamp.

With such a good preamp to compare against, it should make hearing the differences a simple matter. It will be fascinating to see just how far you can take it!

3D

PS - really enjoyed those photos showing the Dual Mono setup. Some viewers may stumble upon this and feel a wave of nostalgia... :0)

 

[derek92994]

I went back to running the tubebox ds off the standard switch mode psu due to space constraints, it still sings nicely and sounds more full than the rat shack battery pres. That being said, considering what the battery pres are, they do very well, but of course this will improve with mods.

Currently running sovtek 12ax7 lps valves, they are close to the genalex gold lion valves, which have a certain edge to them. Cant really describe what the difference is, both sets of tubes sound good. There is also the stock electro harmonix valves which came with a kit line level preamp that I built, have not run those much as I almost immediately upgraded to the sovtek/genalex valves.

She has some dust build up from non use but who knows, there could even be upgrades to this in the future.

 

[J!m]

Not sure about today, but Sovtek and Electro-Harmonix were the exact same tube (made by Sovtek). Sovtek also marketed the re-issue EM stomp boxes because they owned the IP.

I was a dealer back when we had the guitar shop and we sold a LOT of sovetek tubes. Trey from Phish got tubes from us. He would use a set of four for two shows and then change them… we had a huge hoard of low hour matches quads that my partner got, since he actually played guitar.

 

[derek92994]

Ok, here it is, the board on the left is the one I have sourced in Australia, the board on the right is the rat shack one that 3D sourced.

 

[laatsch55]

Are C9, C12 ceramic?

 

[J!m]

They look like it to me but what do I know?

voltage is low so everything should be metallized film in my opinion.

 

[George S.]

Don't overlook using silver mica capacitors for the lower values.

 

[J!m]

Do they have compelling specifications for those locations?