Rod Elliot P06 phono preamp build

[jkhuri44]

Hi All,

I am starting a DIY phono preamp:

http://sound.whsites.net/project06.htm

...and finding it a little difficult following the instructions for it, which seem to be written in Electronic-ese

I'm posting a few pics and asking a few questions once i go through the documentation for the millionth time...

But for now...this is a pic of the PCB layouts with wiring drawn in as if staring into the box...
https://1drv.ms/b/s!AnPSi5tW1m-wgizml42c4z5zpTuo

PINK = all grounding wires
GREEN = all audio signal wires
CYAN = all neg and pos power wires

The PCBs are:
TOP LEFT = subsonic filter
TOP RIGHT = Phono preamp module
BOTTOM = Power module.

Does this wiring look correct?? (Note the designer has requested not to share the values and such so , sorry for the white boxes)

Curious if anyone has tackled this specific project before?

Cheers,

J.

[chrisp]

Hey Jamil - great to hear from you, its been a while.

(1) Twist those + and - input wires together as they travel from socket to board

(2) Likewise, don't take the J3 and J4 G wires on the output card to your grounding post, those are ground references for the output jacks - so twist the R and G wires from J3 over to the R output jack and similarly for the J4 L outputs. You then don't need to take ground leads from the output jacks to your grounding post.

(3) ... and twist the wires from the rumble filter to the RIAA card, i.e. Out2 Sig and G on the P99 go to R and G on the RIAA card - the G's do not go to the grounding post.

(for 1+2+3) Alternatively you can get better results using a shielded cable for the jack to board and board to board audio connections - the shield goes to G and the core to the signal pin.

WHY? You have AC floating around inside this box, which will generate RF interference. You want twisting (or better, shielding) to stop that RFI getting into your signal.

(4) To minimise signal wiring (which also will act as an antenna) I would rotate the RIAA board 180 degrees from that shown.

I get that you are trying to get star grounding happening, but that does not always mean running every G wire to a post - it depends what that wire is doing!

(5) Power issues: Just connect the G, + and - of the P05 output to the respective P99 and P06 power inputs - I would actually create a chain for the three wires from P05 to P99 to P06, that would look neater, especially if you nudge the P05 board across to the left hand side - that will also allow you to run the AC wires straight along the left hand side of the box.

(6) I assume you are using a 12-16V AC wall wart? The AC wires go to AC1 and GND, NOT AC1 and AC2, and the INPUT GND most certainly does not go to the post. See the 'single wiring' on the ESP page.

(7) What you should do, though, is use isolated jacks for your power, input and output jacks - these ensure that there is no electrical connection between the chassis and the jack's G lug

So putting this all together, you do not need a grounding post at all! WHY NOT? You have no actual earth connection in this diagram - what you have is a reference voltage called 'ground' (more accurately 0V compared to the +/- volts) created by the bridge rectifier (the four diodes in the P05 card) - this is called a floating ground, and it may or may not be electrically the same voltage as the earth pin in your 240V socket. It will in fact be half way between the cycles of the AC supply assuming that the capacitors and power regulators are exactly in spec - and most capacitors are +/- 20% !

You have a grounding post on your chassis when your power supply has an actual earth connection that then allows you to make sure the chassis and controls never become live - vitally important when you have a 240V AC supply. In this instance with a floating supply, you keep the chassis isolated as much as you can.

Hope this all helps. Oh, and remember not to insult the ladies on this forum - "Hi gents" will get you ignored by some people out there with awesome skills.

Regards

[jkhuri44]

hey chris,

far out thanks for taking the time to have a look! That's a hugte response, i'll have to read it carefully when i get home tonight!

Sorry i edited my greeting, i didnt mean to offend!

[chrisp]

Feel free to post any follow up questions - it's how we all learned this stuff.

I would to add that the NE5532 is a very common dual opamp and IMHO does an entirely adequate job, despite some posts you might find on the net - I saw it called the cockroach of opamps on one site because it is so common.

That said, there are more niche audio opamps out there - I tend to go with the Burr Brown OPA2134 and/or the LM4562 - both are drop in compatible with the NE5532 specified in this project.

For more discussion, see the website http://www.stereo.net.au/forums/topic/103321-if-you-have-the-best-dual-op-amps-why-not-use-them/

In short, do not solder opamps directly to your PCB - always use an IC socket so you can try out different ones to see if they suit better.

[Text_Edifice]

On opamp selection - I found this interesting

http://nwavguy.blogspot.com.au/2011/08/ ... ments.html

[jkhuri44]

Hi all,

chris thanks again for that legendary write up, ive had a splitting headache all day, but i did read through it all, you probably have answered most of my questions, tho i do have a few more to come!

I just want to read through it again with all the diagrams in front of me to process what you're saying, once again thanks so much!

oh,ALSO....i forgot to actually add on my diagram, i actually bought some thin aluminium, as I will be cutting a small internal wall to separate the power section from the rumble filter/preamp modules.

I guess i could ask a few more questions which

**why exactly there might be 2 x sig connections on the top left PCB??

**in the documentation there are 2 pcb holes which i'm asked to "install a wire link"...i assume this is just soldering a shielded cable to these 2 holes? Does normal signal cable suffice for this?

**There is reference to "shielded cable" for audio, and "hook up wire"....if i walk into jaycar and ask for either of these, am i going to be understood? (sorry this seems like a dumb question)

** I listen to a lot of electronic music with super low bass....i have a few choices for the filter cut off....what is the general conccensus of what constitutes turntable rumble?? im assuming 5-10hz....?
I'm planning to cut off at 17.8hz.. (assuming this is reasonable)

**On the power module, i'll be installing heat-sinks on some ICs...using mica washers and plastic washers between them...i have to then check with an ohmeter to check no conduction between the "tabs of the voltage regulators and heatsink" im assuming that's done by putting one pin of my ohmeter on the bare metal part of the voltage regulator and each of the 3 legs (one at a time) of the IC...?? (sorry again, seems like a newbie question).

**last question...for now...
is it easy enough to add a power on LED ...as well as a power switch on a project like this? or will there be a bunch of hidden caveats....(i dont know if me blanking out the pcb's is not helping here!)

once again thanks so much guys for chiming in...greatly appreciated!!!

[chrisp]

**why exactly there might be 2 x sig connections on the top left PCB??

No idea. Though it sounds silly, it might be nothing more than keeping the part count down and avoiding construction errors by having all the connectors 3 pin, rather than some 3 pin and others 2 pin.

**in the documentation there are 2 pcb holes which i'm asked to "install a wire link"...i assume this is just soldering a shielded cable to these 2 holes? Does normal signal cable suffice for this?

A wire link is often nothing more than the trimmed leg off a resistor used elsewhere in the circuit. It's a way of getting signals around the board without the expense of a 2-sided PCB. Just bend the ends into the two holes, and solder it in.

**There is reference to "shielded cable" for audio, and "hook up wire"....if i walk into jaycar and ask for either of these, am i going to be understood? (sorry this seems like a dumb question)

Yep, you will be understood. Hookup wire is simply a wire (usually braided fine strands) with a plastic coating for insulation. You will use this for the power connections in your project. Shielded cable has a core wire (again usually braided) with a plastic coating for the signal, and a shield cable layer that forms a cylinder around that core, again coated for insulation, for the ground or shield connection. The shield protects the core from RFI. You will use this for the audio connections in your project.

** I listen to a lot of electronic music with super low bass....i have a few choices for the filter cut off....what is the general conccensus of what constitutes turntable rumble?? im assuming 5-10hz....?
I'm planning to cut off at 17.8hz.. (assuming this is reasonable)

One man's rumble is another's bass! No, not really - the truth is that any rumble under 20Hz on a record is almost certainly unintentional, so your number sounds reasonable to me. Bear in mind that the quoted frequency is the -3db point - the filter actually cuts in a little higher than this, so if you have lots of really deep bass that is getting a bit cut you could go for a slightly higher value filter cap from the project's table. But I would start off with the value you suggest.

Another way of thinking about this is to set the value based on how low your monitors can go. The idea of a subsonic rumble filter is more about protecting the speakers than musicality.

**On the power module, i'll be installing heat-sinks on some ICs...using mica washers and plastic washers between them...i have to then check with an ohmeter to check no conduction between the "tabs of the voltage regulators and heatsink" im assuming that's done by putting one pin of my ohmeter on the bare metal part of the voltage regulator and each of the 3 legs (one at a time) of the IC...?? (sorry again, seems like a newbie question).

No, because the bare metal back of the power regulator is almost certainly the same as one of the pins - for the LM7815, for example, the regulator back metal is connected with the centre GND leg.

What you want is infinite resistance between any regulator leg and the heatsink itself, but do not overly sweat this. Mica washers with plastic screws and nuts will do the job admirably, or I use sticky mica washers that are peel and stick.

**last question...for now...
is it easy enough to add a power on LED ...as well as a power switch on a project like this? or will there be a bunch of hidden caveats....(i dont know if me blanking out the pcb's is not helping here!)

Yes, very easy.

For the Power switch - simply wire in a SPST (on-off) switch into the wire that goes from the power jack to the P05 PCB AC1 pin, as in Jack --> Switch --> AC1.

For the LED - Two extra components, the LED and a resistor. A typical red LED likes a current of about 20mA, and you have a power supply of 30V (-15V to +15V). Now Volts = Current * Resistance, so to calculate the right resistor voltage you have 30 = 0.02 * R, which means R is about 1.5 KΩ From the DC side of you power board, then, add

V+ --> (1K5 resistor) --> (A LED K) --> V- (Make sure you get the LED the right way around, the longer leg is A, the shorter leg is K).

Good luckski

[chrisp]

... and I would try it without the internal wall - lots of effort but unlikely to be much benefit in what is a low voltage application. Just keep the AC wires running along that left hand side and you should be fine.

[jkhuri44]

Hi all,

chris thanks again for your help in the posts above, i've gone through them carefully and all makes pretty good sense.
I will have to re-draw my diagram just to make sure i'm doing it all OK.

one thing i have to ask though is...will i notice a good reduction of noise if i build a wall separating the power module from the subsonic filter + amp module? (I have the tools and aluminium ready - but it will be a PITA to install and route the cables through holes and grommets)...but i do want this thing to sound as good as possible...

Another question for now....the bill of materials i have doesnt really say specifically the types of capacitors used.
in the photos below, im guessing all the boxy shaped caps are "polypropolyne caps?"




doing my parts shopping now, and will get started asap!

unfortunately will have to follow up with a few PCB testing questions later on!

[chrisp]

Go with the Bill of Materials (BoM) supplied with the kit, if there is one.

The cylindrical ones in the photos are electrolytic - these are typically polarised so it matters which way around you put them. The yellow and cream ones are MKTs (metabolised film capacitors) and these are not polarised. The squarish green ones indeed look like polyesters, but MKTs would work here as well. Those two green box caps are in the circuit just before the output, so if you want to go down the 'which audio caps are best' rabbit hole, that is where to spend your dosh. Guitarists swear by Sprague Orange Drops, but those with money to burn go for paper in oil at $20-$30 per capacitor! Either way it would be a bit of a kludge to fit the on, but certainly doable. Personally I would have been tempted to draw in an electro cap at this point in the circuit, (pos side towards the opamp output) but what do I know? Darn it, just go with a polyester like in the photo.

Two points from Rod's webpage are important -

Update - 15 Aug 2003 ... For some unknown reason, some suppliers no longer stock 82nF capacitors, and several constructors have had difficulty sourcing them. There is an answer, and it actually improves the EQ accuracy (albeit very marginally). R8 (L&R) may be changed to 750 ohms (a standard E24 value), and C4 (L&R) changed to 100nF. If 750 ohms is not available from your supplier, use 2 x 1k5 resistors in parallel.

and

The low value capacitors [ie all non-electro cars] should be 2.5% tolerance if obtainable, otherwise you may be able to measure a selection of standard tolerance caps to find those which are closest to the required value. Some deviation from the ideal RIAA equalisation curve will occur if these caps are too far from the designated values. More important is matching between channels - this should be as accurate as possible. CP emphasis added.

My view if that the sheet metalwork is not likely to significantly improve S/N in this project if shielded cable is used for the audio connections. Your AC power runs only along one side of the box and is not high voltage, nor are there any power transformers in this build, so RFI is not going to be the same sort of issue where you have 240V AC and big transformers in the same box as the audio signals.

[jkhuri44]

hey chris,

yes you did mention that it wasnt crucial to shield this project, like you said no 240V mains which is what I've seen in photos online hence why i got hung up on it. Cool!

Thanks for bringing my attention to those 2 points. i must have missed them...i dont know how many times i have to read his damn write up! haha.

Re caps...the BOM doesnt explain for ALL parts what to buy, his communication is a little confusing at times....he's included Voltaage and type of some caps, but not all. So, my best bet is just going off the pics...i can see Polys, some small orangy.brown ceramics and also electrolytics.

I've found poly caps like those pictured on RSonline which is good, however, does capacitor tolerance play a big role in selection? I'm buying all my resistors with a tolerance of 1%....is a 5 or 10% tolerance on a cap, good or bad? when it comes to quality, because I cannot find any 1% on rs...and cannot find the values I need on Element14 (what a nightmare of a site!)

[chrisp]

Hi again Jamil

Capacitor tolerance varies greatly depending on the construction - and on how much you want to pay - but generally will not be as accurate to value as a 1% resistor.

[jkhuri44]

i'm assuming since the literature says nothign about capacitor tolerance, i do not need to worry about it.
i cant even find 1% tolerance 150nF polyprop caps (blue ones) for example...i guess ill forget about it haha

[chrisp]

Yep! Don't stress about it.

Or here's a challenge - caps generally do one of four things in a circuit -

(a) they are part of a high or low pass filter network - hint: look for cap-resistor combinations to ground

(b) they block the DC voltage component of a mixed AC / DC signal and pass the AC - hint: the cap is in series with the signal path

(c) they smooth out DC voltage - hint: the cap goes from a power trace to ground

(d) they regulate timing by slowing quick things down - Hint: the cap is in parallel with opamp feedback or sits across the pos and neg of some signal processing.

What are each of the caps in your circuits doing? Bearing in mind that tolerance to stated value is really most critical for use (a), and to some extent (d), which caps should you focus on? And where a cap is there for one thing (e.g. DC blocking), so it have any other effects (e.g. acting as a filter)? Are these other effects likely to be of importance?

These are the sort of questions circuit designers face. For example, a DC blocking cap on signal output will often also act as a high pass filter in conjunction with the output impedance resistor, but choosing the cap value appropriately means that you can move the filter cutoff point well outside the audio range.

Take a look at your RIAA preamp circuit. C5 (1uf) looks like a DC blocker, doesn't it, and indeed that is its function. But then you have R9 at 100K as the output impedance resistor, at the two in combo make an HPF. Is this a problem? Well, the filter's cutoff frequency Fo is calculated as

Fo = 1 / SQRT( 2 * pi * R * C)

so plugging in the numbers R=100,000 (ohms) C = 0.000001 (farads) gives a cutoff frequency just over 1 Hz - so while the C5 / R9 combo will act as a unintended (but unavoidable) HPF filter across the PCB's signal output, it will pass all signals over 1 Hz and only filter those below - in other words, not a problem!

[jkhuri44]

far out chris, you might need to change your sig quote mate I'm learning a lot and hugely appreciate your help....u'll need to pm me your address (a case of your fav beer will be delivered to your doorstep!)

started doing my order at the moment, trying to buy decent brands at good prices is a bit of a head ache!!!
what are your experiences in shopping websites???

I found element14 an insane headache finding parts and proper values, RSonline is fantastic to use and seem to find everything i need...however (can of worms ahead)...

there seems to be 2 camps when it comes to audio quality and components....
parts that are the right ratings will do the job just as well as "x" name brand....

VS

"x"brand sounds way better....

Should i care that i'm buying resistors that "arent" - vishay/panasonic/nichicon...(insert name brand)?

What are people's genuine experience trying different brands??? (i'm in the camp of ....buy the right ratings, Wattage, tolerance, and type of semiconductor VS making sure it's a brand).

i feel like i'm overstaying my welcome in this thread!! jeez....

[chrisp]

Jamil, I get my reward every time I listen to Mandela, so don't sweat it.

Your local Jaycar will sell the 1% resistors, MKTs and power regulators (78xx and 79xx) - don't sweat brands on that stuff, its all generic. That said you can get higher tolerance (5%) MKTs from more specialist suppliers. Jaycar can also sell you some DIL8 IC sockets (go for the gold pin ones).

In terms of other components, WES, E14, Mouser, Digikey, RS, Altronics - in no particular order, I've ordered from all of 'em from time to time.

I do tend to go for name brand electrolytic caps like Panasonic or Vishay within reason. But go first and foremost with value and voltage rating, and also look at operating temp range for the caps. ICs are pretty standard from the specialist suppliers above, go with whatever is actually in stock!

[jkhuri44]

so, with capacitors.

i noticed on rs online and the other sites that you can filter results for a particulat voltage to be VDC or V
am i understanding correctly that the ones without "dc" are meant to be for "ac" applications?

That leads me to then ask....
on my board, i have 1 x power supply PCB which requires AC caps? then all power signals going FROM the power PCB would be needing DC caps?

or does this not matter at all? I've been doing my shopping at the moment with ALL AC caps...but this has m thinking i need to change some.

[jkhuri44]

never mind...i answered my own question
found another forum post ...its really annoying searching using wrong terms on google...grr!

[jkhuri44]

thankfully i've got all my parts ready to order shortly from RS online
there are a few polypropylene caps i've found hard to find ...and just found 1 seller on ebay with them in Aus at reasonable prices....does anyone know however if a project specifies 50 or 63V power ratings on a cap...is there any harm in getting one which is 400V for example?

my logic is that having voltage to spare shouldnt be bad, but something else tells me there might be a hidden caveat!

also, my turntables need to hook up to a grounding post of some sort....(there's a thumb screw type) which is on quite a few DJ mixers...in my case, what exactly is this connection supposed to connect to? seeing as i'm not creating some sort of common ground on the case (as per chris' comment above)?

[chrisp]

JK, your project is 30V so any cap with specs equal to or greater than that will be fine. Yep, even 400V.
The exception to this is electrolytic which tend to increase in size and price as the voltage spec increases, so you look for a closer match there - other caps its fine so long as you are over 30V.

The ground in this project is floating, so it is not an ideal point to connect the TT ground wire. If you can, take that straight to your preamp ground pole.

[jkhuri44]

Thanks for the response chris.

Physically how would you actually connect the TT wire, it has a U shaped terminator which works well with thumb screw screw termination blocks on turntable mixers....

would I need a way to screw that to the outside of the case? then run a wire from that point to the ground on the preamp?

[chrisp]

... or just run it straight to the preamp and not bother with any connector on this case. Will it reach? If not, then I'd do a bit of a hack - cut the TT ground wire a couple of cm short of then U connector, then and patch in an extension bit of wire (you can use some of that spare hookup wire) so that it can reach the preamp.

from TT ---------X --------- new bit of wire ----------- X -- U connector

Its mainly a hum thing and it needs a proper ground, not a floating ground. That said, people have reported no hum just leaving the ground wire disconnected, and some have reported getting hum by connecting the ground wire! It all depends on the TT and preamp wiring. You might try it leaving the wire disconnected as see how you go, but my experience is that hum and noise is better when the TT ground is properly earthed.