[joseki]

hi, long time lurker, first time poster!

I just purchased and assembled an iRIAA and am now ready to use it to test a few home-grown phono preamps. I do have a decent scope, namely a Tek 2215 but no signal generator. I first tried to use RMAA (http://audio.rightmark.org/), but frankly even with a decent firewire sound card (M-Audio Firewire Audiophile), the square wave generation was pretty bad. I'd like to be able to generate solid 1kHz and 10 kHz square waves (preferably the latter), and I can use the sound card with RMAA for the sine curves. I have looked into a number of projects and have ordered a fair number of parts, but I haven't really gotten started yet. I'm looking for a gut feeling on which of the following approaches is most likely to work and give me the best performance. I'll save the shoot-out I was hoping to conduct for a rainy day!

sidenote: most of my research was via the links on this page:

http://www.epanorama.net/links/oscillator.html

CMOS 4047 IC-based -- Logic - Monostable Multivibrators CMOS LP Mono/Astable
 -> notes: simply add a resistor and capacitor!

CD4007UB IC-based -- Logic - Buffers, Drivers, and Transceivers Dual Comp Pair Plus
 -> notes: a few more resistors, but then I will just need to vary one resisitor and one capacitor

LMC7215 -- IC COMP MICRPWR R-R CMOS 8-SOIC
 -> notes: low power, and another design to mess around with... just add a few resistors, and vary a resistor and capacitor

CMOS 4011 NAND-gate
 -> notes: simply two NAND gates, two diodes, two resisors and a capacitor. Simple circuit!

TS555IN -- i.e. CMOS '555 timer circuit
 -> notes: Straight from the datasheet...

Any other ideas?!

Also, one last thing. Just curious if anyone else has some simple circuits used to measure, create, and diagnose problems in audio circuits or software or other things that you feel are essential to a proper audiophile's bench, I'd love to know about them, even if they are too expensive... I wonder what hagtech has on his bench 

[hagtech]

I think there is a 1kHz square wave probe calibration signal output on your scope.  Use that to feed the iRIAA.  You'd be suprised how well it can work.  Sound cards will suk in comparison.  Don't worry about the 10kHz for iRIAA.  It is only useful for amplifier bandwidth measurements.  But then again, even the 1kHz can do that.



jh

[joseki]

wow! my 18 mo. son would love all those buttons, as would I!

Ok, so I do in fact have a 500mV p-p, 1kHz calibration on my scope (although I'm not sure how to really hook anything up to it, as it seems to be made to press the end of the probe into... maybe a resistor lead will fit?) and I'm going to try and hook this up to the 50 ohm input on the iRIAA and then through the -60dB outputs to my phono stage. Seems this will net around .5mV for input into my MC stage. Seems good enough. Assuming I got this right, my question is, shouldn't 1kHz be unaffected by the RIAA curve? Seems that the curve you provide and the corresponding table lists 1000 Hz @ 0 dB. Will this measurement be all that useful?

[hagtech]

The iRIAA will pre-equalize any signal so that when played back on a phonostage you get the original back.  That is, the output of your phono should be a square wave.  If the top of the waveform isn't flat, then the phono EQ is not correct.  That's the easy way.

The other approach is to use a sound card to spit out a sinewave sweep from 20Ha to 20kHz.

jh

[dnewman]

Ok, so I do in fact have a 500mV p-p, 1kHz calibration on my scope (although I'm not sure how to really hook anything up to it, as it seems to be made to press the end of the probe into...

On scopes they are typically designed such that you pull the
business end off of your probe -- yes it should pull off -- and then
put what remains into the socket.  So, how do you make use of that?
Well, on the BNC end of the probe, you clip onto the center pin to
get the 1KHz sq. wave signal.  Use another clip to the outer casing
of the BNC to get the ground.

Cheers,
Dan

P.S. If there's a little screw head near the business end don't
immediately jump to the conclusion that you need to loosen
it to remove the end: that's likely the adjustment for
calibrating your probe....

[joseki]

so, I finally got around to hooking my iRIAA up to my two phono preamps. The first is a Whest PhonoStage.20 where the MC gain is around 65dB and my DIY project where the MC gain is around 76dB out of a balanced output. I used the outer set of jacks on both sides of the iRIAA and plugged it inline after my DAC. The iRIAA is just hanging on the wires, no case.

I am getting a buzz with both phono preamps... like a hum, but more obnoxious, so I'll call it a buzz With the Whest, I have to turn it up a bit more to hear the buzz and with my DIY the hum is noticeable quicker on the volume dial. I haven't had a chance to hook it up to anything and I double checked all the values on the iRIAA with my DMM last night, but that's about it as far as troubleshooting. I haven't tried the MM settings yet, perhaps I'll do that later tonite if there isn't something else obvious that you guys see.

dnewman: it seems that my probes aren't geniune Tek probes, perhaps that why they don't fit?

Any ideas?

[joseki]

well, same problem with the MM inputs. Also for reference, the output of my CD player is 3.0 V RMS (max). I tried to verify the values on the iRIAA with my DMM, but it gets kinda tricky as several values are in parallel and I couldn't directly verify all the values... not sure what those color bands are for anyways just kidding, but kinda hard for me to tell the colors apart...

[joseki]

any ideas?  Could it be that I simply have too much gain? I am putting the iRIAA before the phono preamp, between the CD player and the MC inputs.

[hagtech]

The iRIAA might be picking up hum from a nearby power transformer.  Or maybe it is getting into the cabling between iRIAA and phono.  Are you using coax or twisted pair?  Does moving item/cables around make any difference in the level of hum?  What if you run just one channel?

How loud is the hum compared to music level?

jh

[joseki]

The iRIAA might be picking up hum from a nearby power transformer.

very possible, so here's what I did to test. I put the iRIAA at the end of a dual 20' (foot) run of Belden 89259, which I use to run between my DAC and my preamp. This run is not really near any transformers, and is decently well shielded. I used a 0.5m pair of some Monster cable (i believe this is coax) to go from the other side of the iRIAA to my DAC. With this configuration, it did the same thing as before on both of my phono preamps using the MC inputs.

Does moving item/cables around make any difference in the level of hum?  What if you run just one channel?

no difference, and one channel does the same thing

How loud is the hum compared to music level?

The hum is much much louder than the music. The music actually sounds more like RF interference it's so faint One question, should I ground the iRIAA to the turntable earth location on the preamp, or just leave this unpopulated?

I also tried to hook my phono preamp into a headphone amplifier and it did the exact same thing. I also pulled out my receipt from Digikey and my DMM and verified all parts on the PCB as well as checked to make sure all the traces worked properly. I also did a good soaking of the PCB in anhydrous alcohol (again). I am positive that the PCB is populated properly. I could not measure the small capacitors with my DMM, but I did read the labels and they match up. I guess one of these could have gone bad, or got heated up to much, but this has never happened to me in the past, and I have a bit of experience building audio equipment.

Thanks!

[hagtech]

The problem wouldn't be any of the components on the iRIAA board.  It's just a matter of signal grounding.  Normally the ground is supplied by interconnects.  Sounds like maybe one of these is not connected.  What if you try non-DIY cables?  Just use cheap coax RCA cables, like those that come with a $29 CD player.  Then what happens?

jh

[joseki]

jim:

well, I couldn't find two pairs of cheap twisted pair cables around the house I did try another coax cable that has a metal casing that is tied to ground... I seem to have stumbled upon a way to get rid of the hum, but it's not so practical. I was accidentally holding the iRIAA in my hand and holding the casing of the cable going to the phono preamp and if I touched the input pin in the input side, things worked. I figured maybe it is my source, so I tried a DVD player and it didn't work out of the box and didn't work with the above hack. Also, I touched the turntable ground to the mounting holes which serve as a ground point on the iRIAA and the hum stopped, but unfortunately I got no signal, just a black background. I guess I need to dig up some cheap twisted pair cables to try then?

[hagtech]

Something is not grounded.  Maybe the source.  Maybe all you need to do is get a ground wire to the chassis?

jh

[joseki]

yeah! I fixed it! Burn baby, burn ...

well, the positive and negative on the RCA inputs were switched on both phono preamps. The DIY one I can understand, but the commercial one? heh...

anyways, one last question. Would it be possible for me to modify the iRIAA to have the exact same gain as my phonostage? I figured I would try using a CD source and then A/B-ing between the straight CD and the phono preamp inline ...

Thanks again!

[hagtech]

Would it be possible for me to modify the iRIAA to have the exact same gain as my phonostage?

Sure.  All you have to do is change the resistor divider between MM and MC outputs.  But that's not really necessary.  There is absolutely no need for exact gain.  So what if it is a little high or a little low?  Phonostages can handle it.

jh

[joseki]

well, I was going to hook my CD player through my preamp and then take the tape outs through the iRIAA then into my phonostage and then hook that output back into another selector on the preamp. Then if I could match the gain, I could A/B between CD and CD->iRIAA->phono stage to "listen" to the differences in capacitors, etc. when messing with my phono preamp project... maybe this wouldn't be a good comparison technique, but I was going to give it a shot...

A bit off topic, but has anyone used a FryKleaner for burning in inductors, capacitors, etc. for use in prototyping speaker crossovers? I've noticed that it's a pain to have to second guess if a capacitor is going to change or not, and was thinking about a FryKleaner for this very purpose. I've noticed that it can take capacitors 80+ hours to really settle down in a speaker crossover, especially the giant 100-200 uF polypros...

Thanks again!