Need help/recommendations on rebuilding phono stage

  1. ashpool
    I've made it a project to rebuild a Grado PH-1 and (hopefully) improve its sound. I've ordered the parts to build a +/- 12V Dynahi PSU and now I'm looking into replacing the components inside. I know the PSU is a little overkill for this application, but I've never built a power supply with opamps in it before and I thought I'd give it a try...[​IMG]

    I've taken the PH-1 apart and have attempted to draw a circuit diagram of one of the channels so that I can plan a parts order:

    [​IMG]

    I have quite a few questions that, hopefully, somebody can help me with:

    1. Does this even look right? I've checked the diagram against the pcb, but I could have made a mistake. Once upon a time, I understood the basics of RC networks. These days, however, the RIAA equalization portion of this circuit just makes my head hurt.

    2. How do I calculate gain of this circuit? How much will I have to pay attention to the ESR of C6 and C4 when I choose replacement parts? The current gain on the "high" setting is too low for low output moving coil cartridges and I'd like to make sure the gain will be high enough before I order parts.

    3. I assume R14 is there to bring down the input resistance for proper cartridge loading. 47k seems like a pretty standard input impedance for moving magnet cartridges, but totally wrong for just about any moving coil cartridge. I think the input impedance for my current phono stage is around 500 Ohms on the moving coil setting. I have to admit that I don't really understand the principles behind what makes for a "good" cartridge loading. Can anybody explain the black art of cartridge loading to me?

    4. What are some good brands/types of resistors and capacitors for this application? This is the first project I've had that involves anything that looks like a crossover or filter.

    5. Are the 100nF power rail capacitors necessary with a high-quality power supply and is there any benefit to removing them? I think the ones I pulled out are ceramics. Do they have to be ceramics?

    Any help I could get would be great. I only know enough about electronics to get myself in trouble. This project is an attempt on my part to remedy that...[​IMG]
     
  2. Voodoochile
    Just a precursory warning to any respondants in this thread: there is to be NO DISCUSSION of any PCB manufacture for the PH1, nor any group buy for such a PCB. This thread is about modding potential and circuit operation, not about cloning or fabbing any PCB for this product.

    ashpool- I realize you are neither asking about nor promoting any PCB, but history shows that it will happen soon enough.
     
  3. 00940
    I think I had found this schematic on diyaudio some times ago, if that helps :

    [​IMG]

    error on the schematic though : 2700nf should be pf
     
  4. Garbz
    IIRC the Grado PH-1 Doesn't even follow the RIAA curve very well. Unfortunately I need to rush off but I'll run the parts through a simulation and get back to you this evening.
     
  5. ashpool
    Quote:

    Originally Posted by Garbz
    IIRC the Grado PH-1 Doesn't even follow the RIAA curve very well. Unfortunately I need to rush off but I'll run the parts through a simulation and get back to you this evening.



    Thanks! What sort of software do you use to do run this sort of simulation?

    I found this page that explains a little more about active RIAA equalization with opamps.

    It appears that all the equalization is being done via feedback and C2 (in my hand-drawn schematic) acts as a coupling capacitor. I'm a little confused why this capacitor is needed if the PH-1 uses a +/- power supply. Won't the opamp swing around ground and thus have no DC offset?
     
  6. Garbz
    Anyway it seems that my elec1000 lecture is about exactly this topic, hence i'm here in the library looking for something to do instead.

    Firstly it seems I was wrong the PH1 does follow the curve pretty well. However I will still recomend looking at a different project. I built a variant of the El-Cheapo http://www6.head-fi.org/forums/showt...ight=el-cheapo which is very similar in design to the Grado PH1 but follows the RIAA curve exactly (0.1db as apposed to about 0.3db). It has a few features such as better loading of the MM cartridges, and a class-A bias do dramatically improve the sound. The first link on that thread is the original El-Cheapo article. The original also has a MC input to properly load MC cartridges and apply the appropriate input to bring both to the same level.

    I would personally take a mix the original El-Cheapo input (for MC support), with the first variant (class-A bias, and better output coupling cap), and on the end of that bolt a rumble filter. The schematics for my design doesn't have that in there, but I did add it afterwards since I noticed that when the cat jumped on the table the speaker drivers went nuts. That can't be healthy. Anyway it's just a simple RC highpass network tuned to 20hz.

    To your questions:

    Quote:

    Originally Posted by ashpool
    1. Does this even look right? I've checked the diagram against the pcb, but I could have made a mistake. Once upon a time, I understood the basics of RC networks. These days, however, the RIAA equalization portion of this circuit just makes my head hurt.



    Yes this looks right for an RIAA curve, and it does match the normal simplicity of the Grado Products.

    Quote:

    Originally Posted by ashpool
    2. How do I calculate gain of this circuit? How much will I have to pay attention to the ESR of C6 and C4 when I choose replacement parts?



    The gain of this circuit is a complex combination of all the parts (resistors and capacitors in the feedback loop). I'd personally just simulate it and hope for the best. The gain should be quoted at 1khz per standard.

    Quote:

    Originally Posted by ashpool
    The current gain on the "high" setting is too low for low output moving coil cartridges and I'd like to make sure the gain will be high enough before I order parts.



    Moving coil cartridges should not be loaded the same way as Moving Magnet cartridges. I'd sugest looking at another equally simple project like the El-Cheapo and taking that MC circuit infront of this project.

    Quote:

    Originally Posted by ashpool
    3. I assume R14 is there to bring down the input resistance for proper cartridge loading. 47k seems like a pretty standard input impedance for moving magnet cartridges, but totally wrong for just about any moving coil cartridge. I think the input impedance for my current phono stage is around 500 Ohms on the moving coil setting. I have to admit that I don't really understand the principles behind what makes for a "good" cartridge loading. Can anybody explain the black art of cartridge loading to me?



    A cartidge is esentially a magnet and an inductor. As the magnet moves through the inductor it converts a magnetic field into an electric current and volah, output is low and needs to be amplified.

    Unfortunatly there is no such thing as a perfect inductor. The cartridge can be electrically modelled as a resistor perfect resistor, capacitors and inductor in parallel, and a resistor in series. So now when you attach it to the following input stage of an amplifier the frequency response is greatly effected as the model interacts with the load. 47k is a very standard value to make this interaction lower, but some different values exist and many phono stages have switches to compensate. Often a capacitor is also included in parallel to the resistor to further improve the loading. IMHO the omission of this part shows just how much thought Grado put into their products, but that's just me [​IMG]

    Quote:

    Originally Posted by ashpool
    4. What are some good brands/types of resistors and capacitors for this application? This is the first project I've had that involves anything that looks like a crossover or filter.



    Film caps should be used throughout, and where the value for thoes are too small silva mica caps. Actual brand isn't important. My phono stage sounds like gold and I'm using all generic components in the signal path. (except for a wima cap I had on hand) The most important thing is because of the VERY high gains we are using here the parts must be carefully matched. I wasn't to worried about frequency response being perfect between speakers so I only matched the resistors as close as can be. But if you want to skip this Holco produces some 0.1 and 0.01% resistors if the budget allows.

    Quote:

    Originally Posted by ashpool
    5. Are the 100nF power rail capacitors necessary with a high-quality power supply and is there any benefit to removing them? I think the ones I pulled out are ceramics. Do they have to be ceramics?



    No they don't have to be ceramics. Infact they can be anything but their value must be small. Is the powersupply going to be right next to the opamps? Like withing a cm? Wires on circuit boards are slightly inductive. Sudden transients at higher frequencies attempting to pull current across them can cause sagging in voltages. These capacitors are there and must be placed as close as possible to opamps supply pins to prevent this and interference to other parts of the circuit.

    Quote:

    Originally Posted by ashpool
    Thanks! What sort of software do you use to do run this sort of simulation?



    Any spice simulator. Texas Instruments makes a free one with sufficient fucntionality for this. Search for TINA on the texas instruments site.

    Quote:

    Originally Posted by ashpool
    It appears that all the equalization is being done via feedback and C2 (in my hand-drawn schematic) acts as a coupling capacitor. I'm a little confused why this capacitor is needed if the PH-1 uses a +/- power supply. Won't the opamp swing around ground and thus have no DC offset?



    C2 does act as a coupling capacitor. The problem here is that the gains used in phono stages are really high. And the bass frequencies extending to DC are about 20dB higher then that again. All opamps have an input offset between a few micro Volt to milli Volt. This is amplified by the high gains and causes the opamp put DC on the output (in my case using OPA637 I have about 600mV in each channel). These should be blocked before leaving the amp.
     
    Desi likes this.
  7. ashpool
    First of all, I'd like to say thanks for answering all of my questions so thoroughly.

    Quote:

    Originally Posted by Garbz
    Firstly it seems I was wrong the PH1 does follow the curve pretty well. However I will still recomend looking at a different project. I built a variant of the El-Cheapo http://www6.head-fi.org/forums/showt...ight=el-cheapo which is very similar in design to the Grado PH1 but follows the RIAA curve exactly (0.1db as apposed to about 0.3db).



    That's a handsome looking piece of equipment[​IMG]. My intentions with the PH-1 are to use it as a learning exercise; my goal is to learn more about DIY and audio tweakery than anything else. Fortunately(?), the PH-1 is a simple device. I really want to understand what's going on and the reasons for any audible improvements I may make. Perhaps once I'm done with the PH-1, I'll look into making something more complicated that I will actually intend to use after I'm done. My current phono stage is actually a very nice EAR 834P that has been modified by Scott Frankland from Wavestream Kinetics. At my current level of understanding, I'm not sure I could DIY myself a phono stage that approaches this one.

    Eventually, I want to be comfortable enough with theory that I won't feel like I'm going to kill myself with the high voltages involved in DIY tube projects.

    Quote:

    Originally Posted by Garbz
    Moving coil cartridges should not be loaded the same way as Moving Magnet cartridges. I'd sugest looking at another equally simple project like the El-Cheapo and taking that MC circuit infront of this project.



    Interesting. Do you know where I could get more information on the principles involved? I may just leave the PH-1 loaded for MM, but I'd be interested in finding out more about this aspect of phono stage design.

    Quote:

    Originally Posted by Garbz
    IMHO the omission of this part shows just how much thought Grado put into their products, but that's just me [​IMG]



    The PH-1 is the only Grado product I've owned, and I'm not particularly impressed. It was bought when I first got a turntable and before I started down the dark path of DIY audio... I'm a little bit embarrassed that I paid as much as I did for it. However, I'm not at all nervous to be ripping it apart and mucking around inside, now that I've had a look at it[​IMG]

    Quote:

    Originally Posted by Garbz
    No they don't have to be ceramics. Infact they can be anything but their value must be small. Is the powersupply going to be right next to the opamps? Like withing a cm? Wires on circuit boards are slightly inductive. Sudden transients at higher frequencies attempting to pull current across them can cause sagging in voltages. These capacitors are there and must be placed as close as possible to opamps supply pins to prevent this and interference to other parts of the circuit.



    Good to know. I've always wondered what the little caps were for.

    Quote:

    Any spice simulator. Texas Instruments makes a free one with sufficient fucntionality for this. Search for TINA on the texas instruments site.



    Unfortunately, I can't seem to find any good SPICE software for OS X. I'll have to find a friend with a PC. How would I go about comparing a simulated circuit against the RIAA curve? I would imagine it's a little more complicated than hitting the "how good is my phono stage?" button.

    Quote:

    C2 does act as a coupling capacitor. The problem here is that the gains used in phono stages are really high. And the bass frequencies extending to DC are about 20dB higher then that again. All opamps have an input offset between a few micro Volt to milli Volt. This is amplified by the high gains and causes the opamp put DC on the output (in my case using OPA637 I have about 600mV in each channel). These should be blocked before leaving the amp.



    If C2 is used for coupling, how important is the value? Would I be able to use a larger value, or would that be detrimental to the sound?
     
  8. ashpool
    Quote:

    Originally Posted by Garbz
    Any spice simulator. Texas Instruments makes a free one with sufficient fucntionality for this. Search for TINA on the texas instruments site.



    *sigh*. I just spent some time in TINA, modelling the PH-1 circuit. I tried, but I couldn't manage to get anything to show up on the virtual scope, even when I hooked the function generator straight into the scope. I'm sure I'm missing something obvious...

    I looked at the spacing required by the case and tried to find the closest values in the Percy catalog to the original parts that would still fit in the case. For some reason, every value I found is slightly off from the originals:

    C4 - 13nF
    C6 - .0024uF

    R8 - 332K
    R9 - 27.4k

    Would these new values throw off the RIAA equalization much? I didn't realize that fitting new parts into the case would be so tough.

    Also, would changing the value of C2, the coupling capacitor, to 6.8uF (from 5.14uF), effect the sound much?
     
  9. Garbz
    Quote:

    Originally Posted by ashpool
    That's a handsome looking piece of equipment[​IMG]. My intentions with the PH-1 are to use it as a learning exercise; my goal is to learn more about DIY and audio tweakery than anything else. Fortunately(?), the PH-1 is a simple device. I really want to understand what's going on and the reasons for any audible improvements I may make. Perhaps once I'm done with the PH-1, I'll look into making something more complicated that I will actually intend to use after I'm done. My current phono stage is actually a very nice EAR 834P that has been modified by Scott Frankland from Wavestream Kinetics. At my current level of understanding, I'm not sure I could DIY myself a phono stage that approaches this one.



    This is the reason i sugested the El-Cheapo variants. In it's most basic form, i.e. the original one without the MC input stage, and without the uselessly complicated output coupling it's exactly like the Grado PH-1. Infact the only difference is there's no gain switch (which is useless anyway since as you realised the PH1 doesn't load MC cartridges properly), one extra capacitor in parallel allowing a fine value to be obtained which doesn't exist in the normal selection, and an extra resistor at the end causing a high frequency roll-off of the eq curve following the RIAA-Ext curves instead of the original curves.

    I suggest whatever you build build it on a perfboard. When it's working research into class-A bias like in the PPA and pimeta boards, and then put a constant current accross the output for a fantastic sound improvment. Then investigate a rumble filter research how they work and slap that on the output, and finally put a MC stage on the input. You will in the end have turned your PH-1 into a Very fine unit almost identical to the El-Cheapo [​IMG]


    Quote:

    Originally Posted by ashpool
    Interesting. Do you know where I could get more information on the principles involved? I may just leave the PH-1 loaded for MM, but I'd be interested in finding out more about this aspect of phono stage design.



    No I don't know the details of where to find info. All I know is that MC cartridges have a much lower output and a different loading requirement specified on the cart. I also know that my cart while not being MC MM or VR, is closest to MM in required loading specs so I left the MC stage off my Phono Preamp.

    Quote:

    Originally Posted by ashpool
    Unfortunately, I can't seem to find any good SPICE software for OS X. I'll have to find a friend with a PC. How would I go about comparing a simulated circuit against the RIAA curve? I would imagine it's a little more complicated than hitting the "how good is my phono stage?" button.



    Actually you already know where to find out how to do this. http://www.euronet.nl/~mgw/backgroun...kground_3.html you linked to contains the formula for a 3 pole active EQ like the one in the PH-1 and El-Cheapo. Page 1 on that site contains the forumla for the ideal RIAA eq curve and ideal RIAA-enhanced EQ curve. With excel you can build a table of values, subtract them from each other take the log of them and graph the deviation in decibels, or you could just download a pre-designed one like I did : http://www.garbz.com/amp/phono/phono...compliance.xls, where I slotted in the original and my El-cheapo values. I may think i'm good at excel but I draw the line at making something that already exists [​IMG]

    Quote:

    Originally Posted by ashpool
    If C2 is used for coupling, how important is the value? Would I be able to use a larger value, or would that be detrimental to the sound?



    C2 does not effect the output of the opamp. It only really comes into play with the following stage's input impedance. If you've built a CMoy you'd remember that C1 interacts with R2 to form a high-pass filter, and if the values were wrong you'll loose bass. Well you can use that fact to your advantage and turn that coupling cap into a rumble filter with a 6db per octave drop by tuning it to 20hz. The formula: C = 1/(2*pi*20*(ROut||Zamp) ) where ROut is a resistor to ground after C2, something like 20k or so, ans Zamp is the input impedance of the amplifier. (btw incase you don't know, a||b = (a*b)/(a+b).

    Ideally though you want to keep this a low value below 5uf allowing you to use film capacitors as electrolytics caps are more detrimental to the sound.
     
  10. ashpool
    Quote:

    Originally Posted by Garbz
    Actually you already know where to find out how to do this. http://www.euronet.nl/~mgw/backgroun...kground_3.html you linked to contains the formula for a 3 pole active EQ like the one in the PH-1 and El-Cheapo. Page 1 on that site contains the forumla for the ideal RIAA eq curve and ideal RIAA-enhanced EQ curve. With excel you can build a table of values, subtract them from each other take the log of them and graph the deviation in decibels, or you could just download a pre-designed one like I did : http://www.garbz.com/amp/phono/phono...compliance.xls, where I slotted in the original and my El-cheapo values. I may think i'm good at excel but I draw the line at making something that already exists [​IMG]



    Along with having a Mac, I don't have Excel[​IMG]. I managed to get the spreadsheet working under OpenOffice. If I used the spreadsheet correctly (using the non-inverted sheet), the original values that I measured from the PH-1 look like this:

    [​IMG]

    And a new set of proposed values (not the ones I proposed previously, as they kind of suck, according to the spreadsheet) look like this:

    [​IMG]

    The new values stay closer to the 0dB line, but don't have the same nice flat section that the original values have. Is it more important to have a flat section, or to be closer overall, without the flat section?

    Quote:

    Originally Posted by Garbz
    C2 does not effect the output of the opamp. It only really comes into play with the following stage's input impedance. If you've built a CMoy you'd remember that C1 interacts with R2 to form a high-pass filter, and if the values were wrong you'll loose bass. Well you can use that fact to your advantage and turn that coupling cap into a rumble filter with a 6db per octave drop by tuning it to 20hz. The formula: C = 1/(2*pi*20*(ROut||Zamp) ) where ROut is a resistor to ground after C2, something like 20k or so, ans Zamp is the input impedance of the amplifier. (btw incase you don't know, a||b = (a*b)/(a+b).

    Ideally though you want to keep this a low value below 5uf allowing you to use film capacitors as electrolytics caps are more detrimental to the sound.




    I used a DMM to measure the input resistance of my line stage to be about 100k with the line stage turned on and selected to the measured input. I know resistance isn't quite the same thing as impedance, but that's the closest thing my equipment will measure.

    Please tell me if this math looks right:

    Since there is no resistor to ground after C2 on the PH-1, the calculation for C = 1/(2*pi*20*1000). That means I need a ~8uF cap to tune the output of the PH-1 to 20Hz, right?

    With the original 5.1uF coupling capacitor, that means the frequency cutoff of the PH-1 in my system was 1/(2*pi*5.1x10^-6*1000), or roughly 31Hz. I think the biggest capacitor I can fit into the PH-1 housing is a 6.8uF Axon metallized polypropylene. That would place the cutoff frequency at 1/(2*pi*6.8x10^-6*1000), or ~23Hz. Close enough, yes?

    Would it be better to use a lower value film & foil capacitor, or use the higher-value metallized polypropylene? I don't have enough experience to know how the different capacitor types affect sound.
     
  11. Garbz
    Well it depends on your equipment. For instance the El-Cheapo values were almost perfect to 0.05% for the RIAAE curve. However my cartridge is very mellow, so I actually changed the RIAA curve to give it a bit of snap going nearly 1db up at 2.5k. This is just like using an EQ to tune your speakers as normal except that since this curve is part of the record system it doesn't come with the usual drawbacks. If you have a perfectly flat cart, and perfectly flat speakers then try and get it as close as possible. Even if you do you may still wish to tune to personal tastes. In reality though you won't be able to hear a 0.2db deviation. Differences in people's ears, more then account for this already, and if you use speakers chances are you can't get much more acurate then 2-3db in a room anyway.

    Which line stage do you use? Input impedance should be specified in the manual or online. All equipment I've seen has.

    The math is wrong or the input impedance you speced is wrong. 100k is 100000 so using an 8uf cap would give: 0.2Hz = 1/(2pi*100000*8E-6). Also it's worth mentioning that this gives absolutely no compliance. If you move the phono stage to a different amp (future upgrade or whatnot) then the filter changes. That is why it's a good idea to put a resistor from the output to ground. 10-20k or so. Amplifiers vary in input impednace from 10k to 500k. Putting a 15k resistor on the output means this variance changes to: 6k-14k a much narrower range to effect your filter.

    Also film caps are all very similar in sound quality. Some argue polypropelyne caps sound the best, but so far I'm hard pressed to find any difference between all the film caps, polystyrine, polypropelyne, polyester, etc. Just keep values in the film cap range so you don't need to use electrolytics.
     
  12. ashpool
    Quote:

    Originally Posted by Garbz
    If you have a perfectly flat cart, and perfectly flat speakers then try and get it as close as possible. Even if you do you may still wish to tune to personal tastes. In reality though you won't be able to hear a 0.2db deviation. Differences in people's ears, more then account for this already, and if you use speakers chances are you can't get much more acurate then 2-3db in a room anyway.



    Hehe. I won't claim that my system is perfect in any way[​IMG]. If that's the case, I'll just see what the values I speced out sound like and then tweak from there.

    Quote:

    Originally Posted by Garbz
    Which line stage do you use? Input impedance should be specified in the manual or online. All equipment I've seen has.



    I have an Air Tight ATC-3. I'm in love with the line stage, but the manual is a terse deal written in japanese and half-heartedly translated into english. The english specs don't mention input impedance, but I just noticed that the japanese specs listed above them have an extra bullet point that, for some reason, hasn't been translated into english: "<characters I don't understand>: 100k". 100k it is.

    Quote:

    Originally Posted by Garbz
    The math is wrong or the input impedance you speced is wrong. 100k is 100000 so using an 8uf cap would give: 0.2Hz = 1/(2pi*100000*8E-6). Also it's worth mentioning that this gives absolutely no compliance. If you move the phono stage to a different amp (future upgrade or whatnot) then the filter changes. That is why it's a good idea to put a resistor from the output to ground. 10-20k or so. Amplifiers vary in input impednace from 10k to 500k.



    You're absolutely right. I have no idea why I was plugging 1k in where I should have been plugging in 100k[​IMG]. This time, with the right math, tuning C2 to my system would yield: ~79nF = 1/(2*pi*20*100000). That means that on my system, the original PH-1 was tuned to: ~0.3Hz = 1/(2*pi*100000*5.1x10^-6). Does that seem more correct?

    I had forgotten that the PH-1 might not be used on this exact line stage until the end of time[​IMG]. How important is it that the resistor to ground is in the 10k-20k range? I crunched some numbers and found that if I want to use a 1uF capacitor, the resistor to ground needs to be ~39k (when I approached the equation from the other end, I was getting very uncommon capacitance values). Using a 39.4K resistor, I get these cutoff values for the two extremes of the range you describe and my own system:

    10k Ohm system: ~20Hz = 1/(2*pi*1x10^-6*(10000||39400))
    500k Ohm system: ~4.4Hz = 1/(2*pi*1x10^-6*(500000||39400))
    My 100k Ohm system: ~5.6Hz = 1/(2*pi*1x10^-6*(100000||39400))

    Are these more reasonable numbers? Is the nearly 2 octave interval between 20Hz and the cutoff frequency on my system too big?

    Wow. I seem to be an endless source of questions[​IMG]. Thanks so much for hand-holding me through this process, Garbz.
     
  13. Garbz
    Yeah this compliance thing for the filter is a pain in the arse. I'll show you some examples. A 10k, 50k, 100k & 500k stage, firstly with a 10k to ground, then 50k to ground:

    10k to ground:

    10k : 40hz = 1/(2*pi*0.8uF*(10k||10k))
    50k : 24hz = 1/(2*pi*0.8uF*(10k||50k))
    100k : 21hz = 1/(2*pi*0.8uF*(10k||100k))
    500k : 20.3hz = 1/(2*pi*0.8uF*(10k||500k))

    50k to ground:

    10k : 109hz = 1/(2*pi*175nF*(50k||10k))
    50k : 36.5hz = 1/(2*pi*175nF*(50k||50k))
    100k : 27.3hz = 1/(2*pi*175nF*(50k||100k))
    500k : 20hz = 1/(2*pi*175nF*(50k||500k))

    Since the filter looses it's effictiveness if it starts cutting the bass, we can assume that it becomes useless above 25hz. Here you can see that the 10k resistor with an appropriate cap allows you to use 50-100k amps. Whereas the 50k resistor will get dominated by smaller amps and becomes useless even for 100k. Welcome to the world of compromises [​IMG], which audio is all about.

    I'd look at the highest value film cap you can get / use, and work backwards from there. I can get 4.7u caps in my collection so i'll work with that:

    1700 = 1/(2pi*4.7uf*20hz), now use this as a base ideal value for your worst case senario 500k impedance: so roughtly 500k||1.7k = 1.8k. So now with 1.8k as your resistor to ground:

    amp : result:
    10k : 22.2hz = 1/(2*pi*4.7uF*(10k||1.8k))
    100k : 19.2hz = 1/(2*pi*4.7uF*(100k||1.8k))
    500k : 18.9hz = 1/(2*pi*4.7uF*(500k||1.8k))

    Rumble filters need to be as tightly setup as possible. This will in the end be a very primitive filter since it drops off very slowly relative to frequency, but the idea is that vinyl imperfections or vibrations in the stand don't ruin your speakers. Start with a capacitor and work out a resistance. From there work out what resistor + amp combination you need to get the compliance right. But in general larger capacitors improve compliance.
     
  14. ashpool
    Here's just a little status update on this project.

    I rebuilt the PH-1 from parts ordered from Percy Audio, with a few alterations to the design:

    1. I changed the cartridge loading resistor to 511 Ohms, out of curiousity. Garbz said earlier that MC cart loading wasn't quite the same as MM cart loading, but I thought I'd see what would happen anyways. I may switch out the resistor for a 47K later and see what the difference is.

    2. I used a 1uF coupling capacitor instead of a 5.1uF coupling cap. To compensate for the lower capacitance, I tied the output of the cap to ground with a 39.2K resistor. Choosing a 1uF cap gave me a much bigger range of capacitors to choose from, though it didn't really provide me with a rumble filter. Previously, the only ~5uF caps that would fit in the PH-1 housing were Axon/Solens caps. I ended up using 1uF Auricaps.

    3. Replaced the dual-opamp with a pair of OPA627s.

    I can't really say what #1 did to affect the sound of the PH-1, because it was part of the big re-build. It doesn't seem to have done anything *bad*, however.

    As for #2, I suspect I have discovered the reason why Grado didn't add a resistor to ground after the coupling capacitor; once I added the resistor to ground, the PH-1 seemed to get a bit noisier. The noise now has a noticable component that sounds like 60Hz hum. I assume that the resistor is doing something to cause the ripple from the wall-wart to become more noticeable. Hopefully once I finish the Dynahi PSU (still waiting on heatsink and toroid), the hum and noise will go away. I may also try to build a battery-pack power supply for the PH-1.

    #3 was actually something I did before I first posted here about rebuilding the PH-1. It only seemed to do good things for the overall sound.

    I just did this work last night, so the newly rebuilt unit only has a few hours of play on it. The sound is still changing. I'll post about how it sounds once it starts to settle down.

    Thanks to Garbz for helping me so much with this project. I'm pretty sure I would have done something really bad without his help...[​IMG]
     
  15. Garbz
    You're welcome. Though the thought that the wallwart is putting a hum on the ground plane is worrying. You sure you didn't connect it to a supply rail [​IMG], sorry had to be said, stupider things have happened.


    When it's all up and running apply a Class-A bias to the chips. The circuit can be copied directly from the PPA or MINT designs. All you need is 2 FETs. Personal opinion is it made it well worth it.
     

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