Glad to see you had the PCB made. Again it was good of you to put the history of the circuit on the PCB. Shows good character.
I wonder if changing R1 & R4 indicates that there is a stability/ringing problem that needs investigating in the interaction between the PSU and the IC?
It's possible that snubbering of the main bridge rectifier is not optimal - I used a 25A, 800V device instead of 4 x MUR8x0 8A fast-recovery devices. This might be ringing on turn-on/turn-off, which could probably be easily cured with the discrete fast-recovery devices.
Anyway, the carbon-films at R1, R4 have cured it almost completely for the LM318 +/- 12V rails, so it's not a concern as of now. The LM3886 Howland section appears to be immune to this ringing, in any event.
Related stuff:
1) I had the privilege of auditioning the bookshelves developed by diyAudio member "soundsgreat" today. Quite extraordinary sonics - it's a 2-way rear-ported design with a stock 5.25" mid/woofer and a dome tweeter of his own design, with a 2nd-order butterworth crossover at ~2.5 kHz. With the MyRef RevC version 1.3, it utterly destroys just about everything I've heard - my JBL LX2002 bookshelves, which are about the same size, now sound very ordinary and disconnected/incoherent in comparison.
2) Bare Version 1.3 PCBs will be made available shortly - I'll make a posting in the Group Buy section in a few days. These are tin-lead finish, not immersion gold, so it's unfortunately not RoHS compliant - EU members, please excuse me. Tentative pricing is $8 each + shipping (probably $8 to $10 for a few PCBs by Airmail to North America). All existing parts sets from earlier Group Buys will fit comfortably on this PCB, and physically larger parts will fit at many locations (e.g. C3, C8 are 35mm footprints; C1, C2 can accommodate 16mm diameter Black Gates, Cerafines and MUSEs with 7.5mm lead pitch).
Hi guys
This is my first post on forum
I would like to post some credits:
To my friend Dario(Clave Freman)for his wonderful work in the BoM,and for making me discover this excellent amplifier
To Madisonear,for his work on BoM
To Mauro Penasa,for sharing with us,his magic design
To Udaley,for the very beautiful PCB
To others Guys in this thread, for sharing their passion in this hobby
To DIYAudio, for all the valuable information it contains
Many thanks
In atthached some poor imagines of my MyRef,w/o case...
Sorry for my poor english
Marco
This is my first post on forum
I would like to post some credits:
To my friend Dario(Clave Freman)for his wonderful work in the BoM,and for making me discover this excellent amplifier
To Madisonear,for his work on BoM
To Mauro Penasa,for sharing with us,his magic design
To Udaley,for the very beautiful PCB
To others Guys in this thread, for sharing their passion in this hobby
To DIYAudio, for all the valuable information it contains
Many thanks
In atthached some poor imagines of my MyRef,w/o case...
Sorry for my poor english
Marco
Attachments
Hey Linuxguru
Anything I need to change from the kit I picked up from you late August? or did you supply me the right R1 and R4? i will be building mine later this week.
Cheers
Hi Gobble - I think you have the right R1/R4 (carbon films); your kit was filled after my posting here. Anyway, just check that the 1k, 2W resistors are smooth and cream-coloured with non-magnetic leads - those are carbon films. If they're rough and grey-coloured with magnetic leads, they're metal-oxide which probably need to be substituted with carbon films.
Howdy, boys. Haven't posted in so long because I've been busy. To see why, please read on.
I have just completed testing another batch of caps for the C13 position in these MyRef C amps. The amps I am currently using are very close in configuration to the most recent group buy. I would say the differences are insignificant for the purpose of this test, which was to select the best sounding cap for DC blocking at the input.
First of all, let me say that, based on extensive listening over a very long period and the simulations provided earlier, it is now obvious that my initial choice of 0.68uF is too small. After trying many different caps at larger values (1.5 – 2.2uF), it is readily apparent that, although the -3dB point may be acceptable, the phase distortion introduced by smaller caps extends far higher than I originally thought it would. With larger caps in place, I could immediately hear more flesh on the bones of all music as high up as the lower midrange. My first conclusion is that 1.5uF is the minimum size to consider for use in this position. The small caps did not produce the fullness, the foundation, of voices and many instruments.
With that in mind, I cannot claim that there was a valid evaluation of the bass of some of the smaller caps I tried.
I used all kinds of music for testing, from symphonies to many types of chamber music, old monaural and modern stereo jazz, and some popular music. I listened in stereo, as well as with one channel signal fed to both speakers, and to one speaker at a time. I listened at my usual position about ten feet from the speakers, and nearfield to eliminate room effects.
All caps were broken in for a minimum of ten hours, or to the point where I no longer heard them changing, which varied from cap to cap. ALL caps exhibited a great deal of initial change, but all of them eventually settled down to a constant sound.
The system is a Marantz SA-11S1 SACD player, a Sonic Euphoria passive preamp, B&W 802 Matrix speakers with outboard crossovers, and all wire, from source to speaker, is OCC solid core copper with Teflon insulation in various configurations. Most connectors are Vampire or Furutech gold-plated copper.
I maintained one amp with my reference cap, so I always had a stable reference for comparison. In some cases, this probably affected imaging and soundstage, but I worked hard to overcome this by listening to one speaker at a time for at least part of each test.
In chronological order, the caps I tested: Bennic, ClarityCap SA, Audio Cap Theta, Mundorf Silver/Oil, Fostex, Mundorf ZN, Obbligato, and Sonicap Gen I.
Bennic 2.2uF is adequate but rather rough in the highs/mids, with somewhat flabby bass. It made sound, yes, but it didn’t make much music. I’ve used these in crossovers, but I would never consider using them in this critical position. They weren't awful, but why bother?
ClarityCap SA 2.2uF was an improvement over Bennic. The highs and mids became much smoother, and the bass tightened somewhat. There is not much detail, and there is very little spatial information. Overall kind of muddy, but at least it sounded like music. Again, these might be acceptable in a crossover, but not here.
Audio Cap Theta 1.0uF was better yet, with super detail and tight bass. Mids are okay, but there is so much detail in the highs that it sounds mechanical. Clinical is the best word to describe it. If you’re a detail freak and don’t ever hear live music, these might be okay. There is a nice sense of depth to the image and space around the instruments, but the highs are really too prominent.
Mundorf S/O 0.68uF is very nice, with superb retrieval of inner detail and spatial information. These took the longest of any cap to settle down, with the sound changing almost every day for several weeks. There is a slightly etched quality to the highs, although the mids do not exhibit any grain, and the bass is lacking. Of course, this is too small a cap, but it’s the biggest I could afford. Compared to a different cap of the same value, the bass is weak, although what is there is very well controlled. I liked the detail and spatial qualities a lot, and it took careful listening to determine that it is somewhat colored and artificial simply because it tips the entire frequency spectrum upward. These would make terrific bypass caps in a tweeter crossover.
Fostex 0.68uF is my current reference. I can’t say enough good things about this unknown cap, but I’ll start with this: it is the most balanced, detailed, musical cap I have heard. There are probably better caps, but not at this price. Again, this value is too small to produce the deepest bass, but what is there is very accurate and controlled. The mids are tonally perfect, with every nuance of every instrument coming through clearly. The highs are smooth yet extremely detailed. Every cymbal has a distinct ring and shimmer, just like real ones, with none of the white noise so common in detailed high frequencies. Brass has bite, strings can be velvety or edgy, woodwinds are in the room, voices are full and smooth, and percussion, including piano, can be startling. There is no grit or grain. Large scale choral works and solo instruments are produced equally clearly. If I can find any fault with this cap, it is that it produces a somewhat shallow soundstage, with the image almost stuck to the plane of the speakers. There is some space there; it just isn’t terribly deep. However, air around individual instruments is very clear. Dynamics are excellent, with the leading edge of every note played with powerful realism. I got a chance to hear my amps with these caps in them in two other systems with very high quality electronics and speakers, and these amps easily bested an Aragon 8008BB and Herron monoblocks in the highs and mids.
Mundorf ZN 1.5uF is a very nice cap. The sound is clear and smooth. Tonally accurate midrange is slightly forward, with very strong presence, but not objectionably so. The bass is well controlled. Highs are slightly truncated, with a minor lack of transparency, yet the lower treble is somewhat exaggerated, with a hint of sibilance. I liked this cap a lot, and I would be satisfied with it if I didn’t have the Fostex for comparison. Good spatial information, with voices and guitars jumping out of the speakers into the room. There is just a little bit of “hi-fi” sound to this cap. It colors the sound ever so slightly to make it just a bit more exciting, more present.
Obbligato 2.2uF, supplied with the most recent group buy, is also a very pleasant cap. Tony Gee and others have rated this cap highly, and I initially liked it a lot. However, it definitely colors the sound by adding some form of ambience, almost a reverb effect. This initially sounds very smooth, three dimensional and realistic, with lots of air and space. After extended listening to a variety of music, though, I realized that this cap produces the same effect with every type of music: it doesn't retrieve ambience and spatial cues; it creates them. There is a strong coloration present, with everything sounding like it was recorded in an empty church. I know it’s preposterous, but it seems as if the sound is bouncing around inside that big metal case. If that’s how you want all of your music to sound, this is a good cap. Tonality is pure, and bass is pretty good, if just a bit flabby because of the reverberation. The leading edge of notes is rounded off, and the highs don’t have ultimate transparency because of the same effect. All cymbals sound the same, with no distinctive ringing or shimmer. At times I thought I detected some midrange resonance, almost an uneven “cupped hands” effect to saxophone and vocals. I hope this cap sounds better in a speaker crossover, or else previous evaluations are woefully inaccurate. Tony Gee calls it “analogue”. I call it artificial.
Sonicap Gen I 1.5uF is the biggest surprise of all. This is very nearly the equal of my reference Fostex, and it improves on the single area where the Fostex is weak. I tried this one last, and almost skipped it because I was tired of testing instead of listening, and who would expect this comparatively inexpensive, tiny cap to be the best of the group? Well, it is, especially considering the cost. Frequency response is dead flat as far as I can tell, with no exaggerations or omissions. It might lack the tiniest bit of transparency of the Fostex, but it has somewhat better spatiality, which, in my book, is more than a fair trade-off. I listened so very carefully and could detect only a slight loss of sparkle in the very highest frequencies. There is no harshness, no grain, no glare, but plenty of ring and shimmer to cymbals. The midrange is sweet and tonally accurate, portraying every instrument and voices very truthfully. Bass is controlled and full. There is plenty of space around instruments, and nice depth to the soundstage. Dynamics are realistic, with a powerful leading edge at all frequencies. This is my new reference. It costs one-fifth of the Fostex, no more than the Obbligato. I haven’t discovered a better bargain in audio in quite some time. For those of you in the latest group buy, please shell out another $15 or so to get full performance out of your MyRef; the Obbligato, although interesting in its own way, is in no way the equal of this Sonicap Gen I. Amazing to think that I almost quit before trying it…
Peace,
Tom E
I have just completed testing another batch of caps for the C13 position in these MyRef C amps. The amps I am currently using are very close in configuration to the most recent group buy. I would say the differences are insignificant for the purpose of this test, which was to select the best sounding cap for DC blocking at the input.
First of all, let me say that, based on extensive listening over a very long period and the simulations provided earlier, it is now obvious that my initial choice of 0.68uF is too small. After trying many different caps at larger values (1.5 – 2.2uF), it is readily apparent that, although the -3dB point may be acceptable, the phase distortion introduced by smaller caps extends far higher than I originally thought it would. With larger caps in place, I could immediately hear more flesh on the bones of all music as high up as the lower midrange. My first conclusion is that 1.5uF is the minimum size to consider for use in this position. The small caps did not produce the fullness, the foundation, of voices and many instruments.
With that in mind, I cannot claim that there was a valid evaluation of the bass of some of the smaller caps I tried.
I used all kinds of music for testing, from symphonies to many types of chamber music, old monaural and modern stereo jazz, and some popular music. I listened in stereo, as well as with one channel signal fed to both speakers, and to one speaker at a time. I listened at my usual position about ten feet from the speakers, and nearfield to eliminate room effects.
All caps were broken in for a minimum of ten hours, or to the point where I no longer heard them changing, which varied from cap to cap. ALL caps exhibited a great deal of initial change, but all of them eventually settled down to a constant sound.
The system is a Marantz SA-11S1 SACD player, a Sonic Euphoria passive preamp, B&W 802 Matrix speakers with outboard crossovers, and all wire, from source to speaker, is OCC solid core copper with Teflon insulation in various configurations. Most connectors are Vampire or Furutech gold-plated copper.
I maintained one amp with my reference cap, so I always had a stable reference for comparison. In some cases, this probably affected imaging and soundstage, but I worked hard to overcome this by listening to one speaker at a time for at least part of each test.
In chronological order, the caps I tested: Bennic, ClarityCap SA, Audio Cap Theta, Mundorf Silver/Oil, Fostex, Mundorf ZN, Obbligato, and Sonicap Gen I.
Bennic 2.2uF is adequate but rather rough in the highs/mids, with somewhat flabby bass. It made sound, yes, but it didn’t make much music. I’ve used these in crossovers, but I would never consider using them in this critical position. They weren't awful, but why bother?
ClarityCap SA 2.2uF was an improvement over Bennic. The highs and mids became much smoother, and the bass tightened somewhat. There is not much detail, and there is very little spatial information. Overall kind of muddy, but at least it sounded like music. Again, these might be acceptable in a crossover, but not here.
Audio Cap Theta 1.0uF was better yet, with super detail and tight bass. Mids are okay, but there is so much detail in the highs that it sounds mechanical. Clinical is the best word to describe it. If you’re a detail freak and don’t ever hear live music, these might be okay. There is a nice sense of depth to the image and space around the instruments, but the highs are really too prominent.
Mundorf S/O 0.68uF is very nice, with superb retrieval of inner detail and spatial information. These took the longest of any cap to settle down, with the sound changing almost every day for several weeks. There is a slightly etched quality to the highs, although the mids do not exhibit any grain, and the bass is lacking. Of course, this is too small a cap, but it’s the biggest I could afford. Compared to a different cap of the same value, the bass is weak, although what is there is very well controlled. I liked the detail and spatial qualities a lot, and it took careful listening to determine that it is somewhat colored and artificial simply because it tips the entire frequency spectrum upward. These would make terrific bypass caps in a tweeter crossover.
Fostex 0.68uF is my current reference. I can’t say enough good things about this unknown cap, but I’ll start with this: it is the most balanced, detailed, musical cap I have heard. There are probably better caps, but not at this price. Again, this value is too small to produce the deepest bass, but what is there is very accurate and controlled. The mids are tonally perfect, with every nuance of every instrument coming through clearly. The highs are smooth yet extremely detailed. Every cymbal has a distinct ring and shimmer, just like real ones, with none of the white noise so common in detailed high frequencies. Brass has bite, strings can be velvety or edgy, woodwinds are in the room, voices are full and smooth, and percussion, including piano, can be startling. There is no grit or grain. Large scale choral works and solo instruments are produced equally clearly. If I can find any fault with this cap, it is that it produces a somewhat shallow soundstage, with the image almost stuck to the plane of the speakers. There is some space there; it just isn’t terribly deep. However, air around individual instruments is very clear. Dynamics are excellent, with the leading edge of every note played with powerful realism. I got a chance to hear my amps with these caps in them in two other systems with very high quality electronics and speakers, and these amps easily bested an Aragon 8008BB and Herron monoblocks in the highs and mids.
Mundorf ZN 1.5uF is a very nice cap. The sound is clear and smooth. Tonally accurate midrange is slightly forward, with very strong presence, but not objectionably so. The bass is well controlled. Highs are slightly truncated, with a minor lack of transparency, yet the lower treble is somewhat exaggerated, with a hint of sibilance. I liked this cap a lot, and I would be satisfied with it if I didn’t have the Fostex for comparison. Good spatial information, with voices and guitars jumping out of the speakers into the room. There is just a little bit of “hi-fi” sound to this cap. It colors the sound ever so slightly to make it just a bit more exciting, more present.
Obbligato 2.2uF, supplied with the most recent group buy, is also a very pleasant cap. Tony Gee and others have rated this cap highly, and I initially liked it a lot. However, it definitely colors the sound by adding some form of ambience, almost a reverb effect. This initially sounds very smooth, three dimensional and realistic, with lots of air and space. After extended listening to a variety of music, though, I realized that this cap produces the same effect with every type of music: it doesn't retrieve ambience and spatial cues; it creates them. There is a strong coloration present, with everything sounding like it was recorded in an empty church. I know it’s preposterous, but it seems as if the sound is bouncing around inside that big metal case. If that’s how you want all of your music to sound, this is a good cap. Tonality is pure, and bass is pretty good, if just a bit flabby because of the reverberation. The leading edge of notes is rounded off, and the highs don’t have ultimate transparency because of the same effect. All cymbals sound the same, with no distinctive ringing or shimmer. At times I thought I detected some midrange resonance, almost an uneven “cupped hands” effect to saxophone and vocals. I hope this cap sounds better in a speaker crossover, or else previous evaluations are woefully inaccurate. Tony Gee calls it “analogue”. I call it artificial.
Sonicap Gen I 1.5uF is the biggest surprise of all. This is very nearly the equal of my reference Fostex, and it improves on the single area where the Fostex is weak. I tried this one last, and almost skipped it because I was tired of testing instead of listening, and who would expect this comparatively inexpensive, tiny cap to be the best of the group? Well, it is, especially considering the cost. Frequency response is dead flat as far as I can tell, with no exaggerations or omissions. It might lack the tiniest bit of transparency of the Fostex, but it has somewhat better spatiality, which, in my book, is more than a fair trade-off. I listened so very carefully and could detect only a slight loss of sparkle in the very highest frequencies. There is no harshness, no grain, no glare, but plenty of ring and shimmer to cymbals. The midrange is sweet and tonally accurate, portraying every instrument and voices very truthfully. Bass is controlled and full. There is plenty of space around instruments, and nice depth to the soundstage. Dynamics are realistic, with a powerful leading edge at all frequencies. This is my new reference. It costs one-fifth of the Fostex, no more than the Obbligato. I haven’t discovered a better bargain in audio in quite some time. For those of you in the latest group buy, please shell out another $15 or so to get full performance out of your MyRef; the Obbligato, although interesting in its own way, is in no way the equal of this Sonicap Gen I. Amazing to think that I almost quit before trying it…
Peace,
Tom E
Thanks for the compliments.
Dario, I believe that the Mundorf ZN is a very nice cap. The coloration it makes is subtle and might even sound really good to some ears on some systems--they're all different. The only thing it lacks that the Sonicap has is clearer highs, more transperancy, and the Sonicap midrange seems more balanced with the rest of the frequency spectrum. If that is important to you (I'm sure it is!), there is no doubt that the Sonicap is worth trying. I know it will cost more for you because of shipping, but, after all the other experiments and tweaking you've done to get the best sound from your amps, I think it's worth the few bucks (really, they're quite inexpensive for as good as they are) for you to try them. If you don't like them in your amps, I bet you can find a place for them in some other component. They are the smallest of all the caps I've tried.
C13 is the single most important part that determines how this amp sounds.
Peace,
Tom E
Dario, I believe that the Mundorf ZN is a very nice cap. The coloration it makes is subtle and might even sound really good to some ears on some systems--they're all different. The only thing it lacks that the Sonicap has is clearer highs, more transperancy, and the Sonicap midrange seems more balanced with the rest of the frequency spectrum. If that is important to you (I'm sure it is!), there is no doubt that the Sonicap is worth trying. I know it will cost more for you because of shipping, but, after all the other experiments and tweaking you've done to get the best sound from your amps, I think it's worth the few bucks (really, they're quite inexpensive for as good as they are) for you to try them. If you don't like them in your amps, I bet you can find a place for them in some other component. They are the smallest of all the caps I've tried.
C13 is the single most important part that determines how this amp sounds.
Peace,
Tom E
Ive been going through a lot of threads for this amp and still have a few question.
Will 2 Antek 200va 25v toroid be good for 6 ohm speakers?
R2 says "R2 value is better lower <12k" and i'm not sure what this means. the resistor is 33k in the pack, is this the one im support to use??
thank you
Will 2 Antek 200va 25v toroid be good for 6 ohm speakers?
R2 says "R2 value is better lower <12k" and i'm not sure what this means. the resistor is 33k in the pack, is this the one im support to use??
thank you
I consistently see Madisonears review stating that all the 2u2F input coupling caps do not perform well.
I also see that most of the smaller 0u68 to 1u5F coupling caps perform quite well to very well.
I said way back that 2u2F was too big in comparison to the negative feedback cap.
When comparing different value of input caps then the Rin value must be adjusted to bring the filter frequency back to a standard value to allow comparisons using a fixed NFB cap value.
This may not change any of the review opinion, but with different filter values, the comparison as it stands, is little more than guessing at what has affected the resulting sound.
I also see that most of the smaller 0u68 to 1u5F coupling caps perform quite well to very well.
I said way back that 2u2F was too big in comparison to the negative feedback cap.
When comparing different value of input caps then the Rin value must be adjusted to bring the filter frequency back to a standard value to allow comparisons using a fixed NFB cap value.
This may not change any of the review opinion, but with different filter values, the comparison as it stands, is little more than guessing at what has affected the resulting sound.
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Just a note about that.
In this moment I'm again using the 10nF FKP2.
With C13 in place the sound balance changes a bit, particularly in the bass area (more bass).
It seems that C13 has an influence on LM318's behaviour.
With a DC Coupled MyRef a C7 value of 33nF gave the same sound balance of no C7 at all.
With C13 in place and 33nF C7 bass is too much (and a bit muddy) and different than no C7 at all (!) and 10nF is again the right value...
If someone wants to play a bit with C7 values it would be welcome
I stated clearly that 0.68 is far too small. I don't know for sure that 2.2 is too big. I understand that Rin will affect the low freq cut-off for different value caps. You are correct that I made no attempt to compensate for smaller caps by changing Rin, so of course the filter performed differently. However, I did try to compensate for smaller caps in the way I listened to them.
That's why I confined most of my comments to mids and highs, and stated that evaluations of bass might not be valid, especially for smaller caps. Most of the larger caps were also cheaper and probably inferior, selected because I could afford to buy them. More expensive caps of higher quality were chosen in smaller values only to save money. In that respect, there was some inference about possible effects of smaller caps on bass response, based on what was audible. Regarding midrange and high freq's, there was no "guessing" involved; these were carefully observed subjective evaluations with no predisposition toward any one cap. I think the unbiased nature of the experiment is indicated by the unexpected results.
It does not necessarily follow that a cap that sounds better in the upper frequencies will also produce better bass (if of adequate size), but it certainly seems likely. The low cost caps of larger value were so bad that it wouldn't matter if the bass were the best on the planet--you still wouldn't want them in your amp.
If someone wants to try Sonicap, perhaps they could get 2.2uF and let us know how it sounds, especially the bass. After extended listening to both monoblocks now fitted with 1.5uF Sonicaps, I am more strongly convinced that these are the best choice for airy, accurate, transparent, smooth, dynamic highs and mids. However, perhaps they're still not big enough, as the bass starts to fall off around 80Hz. It doesn't completely disappear, but it's not as powerful as what I've heard with other amps in my system. Maybe it's just the amp itself.
Peace,
Tom E
That's why I confined most of my comments to mids and highs, and stated that evaluations of bass might not be valid, especially for smaller caps. Most of the larger caps were also cheaper and probably inferior, selected because I could afford to buy them. More expensive caps of higher quality were chosen in smaller values only to save money. In that respect, there was some inference about possible effects of smaller caps on bass response, based on what was audible. Regarding midrange and high freq's, there was no "guessing" involved; these were carefully observed subjective evaluations with no predisposition toward any one cap. I think the unbiased nature of the experiment is indicated by the unexpected results.
It does not necessarily follow that a cap that sounds better in the upper frequencies will also produce better bass (if of adequate size), but it certainly seems likely. The low cost caps of larger value were so bad that it wouldn't matter if the bass were the best on the planet--you still wouldn't want them in your amp.
If someone wants to try Sonicap, perhaps they could get 2.2uF and let us know how it sounds, especially the bass. After extended listening to both monoblocks now fitted with 1.5uF Sonicaps, I am more strongly convinced that these are the best choice for airy, accurate, transparent, smooth, dynamic highs and mids. However, perhaps they're still not big enough, as the bass starts to fall off around 80Hz. It doesn't completely disappear, but it's not as powerful as what I've heard with other amps in my system. Maybe it's just the amp itself.
Peace,
Tom E
we are commenting on different aspects.
I am saying as long as the comparison used different filter values then readers cannot be sure the sound differences were due solely to the cap make or type.
You are saying there were clear differences. I accept that, My question comes back to certainty over why there were differences?
I don't know for sure that 2.2 is too big.
1.5uF Sonicaps, .................. perhaps they're still not big enough,
C9 = 220uF
R10 = 390r
RC = 86ms giving F-3dB~1.9Hz
C13 = 2u2F
R13 = 100k
RC = 220ms giving F-3dB~0.7Hz
The ratio of RCs ensures that some AC voltage is generated across the electrolytic.
Even when C13 = 1u5F, the RC ratio still allows AC voltage on the electrolytic.
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From Mauro's Technical Document:
• Bandwith (-3db typical): 2Hz-70Khz
• Maximum Power (8ohm): 40Wrms
• Maximum Power (4ohm): 56Wrms
• Dumping Factor (8ohm): >200
• S/N ratio (600ohm): >96 dB non pesato
• THD typical (20Hz-20Khz, 1-40W 8ohm <0.05%
And C13 in the original MyRef is 1uF...
Tom, which resistors are you using in R1, R4? Which components differs from the GB?