17 August 2013

Project "Mistral MM-1 Hybrid Tube Amplifier Modification"





I bought this hybrid tube amplifier made by Mistral Audio (http://www.mistralaudio.com/Stereo.html) from the shop Precision Audio in Adelphi (affiliated to Ban Leong Brothers Singapore) slightly over a year ago. It sounded really good when I first got it and only cost $169SGD (!!!!) which is the cheapest power amplifier I have seen in the entire building (The Adelphi has many...many high end audio shops with 4-5 digit product price tags).

It uses a pair of 6N1 triodes as well as a pair of 6P15 pentodes for the preamplifier section. The power output utilizes a pair of LM1875 solid state amplifiers capable of 20 Watts each.

After using it for a year I noticed the sound quality started to deteriorate really fast. It started buzzing, humming and distorting intermittently and really got on my nerves and my ears. I did some research and I found out quite a lot of disappointing things from many different forums, mainly:

-It isn't really 'made' by Mistral Audio. This amplifier is a generic Chinese designed and manufactured amp apparently in the same factory as where they build the other Mistral Audio amplifiers that cost more than a thousand bucks. This amplifier that costs $169SGD is made in that same factory (i think?) but wasn't designed by Mistral Audio; it was designed by a nameless company and they just slapped their brand on it...together with a few other brands such as Jaycar (model AA-0474) in Australia which they are discussing on this forum: 
and it is also branded as Marriola on Amazon.com:

-All the tube heaters are actually over voltage and are running much hotter than they should be. I found this out from the audiokarma forum link above as well as from this experienced person's (cool386) website: 

This means that the tubes will fail and burn out much sooner than their normal lifespan, bringing the sound quality down the longer they are run. They are not supposed to be as bright as they look in this picture:


-Also, only half of each triode is actually hooked up to the preamplifier circuit, meaning that while the tubes definitely do contribute to the sound, they are not actually fully utilized.


---------------
The bottom line is that the amplifier doesn't sound bad...it doesn't sound bad at all! But in order to get it working consistently at its full potential, a few modifications had to be made.

The first modifications I did was to reduce the heater voltages. I assumed the tubes were all burnt out after spending a year running hot and ordered a completely new pair of (Bulgarian/Ukrainian NOS tubes) 6N1s and 6P15s from eBay. The forum members on the audiokarma forum suggested putting voltage drop resistors which would work but after tracing the preamplifier PCB I could not understand the logic behind their suggested resistor placement and came up with a new one.

The amplifier can be dismantled easily; just remove all the screws on the edges of the bottom and the entire bottom plate comes off. Use a marker to mark all the connectors you see so you will be able to identify which connector goes back to which later on during reassembly.



The preamplifier PCB can be accessed by removing all the tubes from their sockets and unscrewing the 12 nuts holding the PCB in place.


Be sure to click on the image above to get a closer view. The socket pins bounded in the red squares are the tube heater pins. From cool386's post on this amplifier he stated that the power from the heaters come straight from the 16VAC transformer winding. This 16VAC is fed through a diode to produce about 8.5V RMS which is above the required 6.3V heater voltage of the tubes.

I got 4 x 3.3ohm ceramic resistors rated for 4 Watts each and connected each tube heater in series with the 8.5V RMS source. This would drop about 2V for each tube, bringing it down to a more tolerable 6.5~V whilst each resistor would burn about 1.5~W each.



I apologize for my poor MS paint skills but I think this is enough to illustrate the modification. Cut the traces with a penknife at the red X's and wire up the resistors accordingly. All the resistors are of the same value and the blue colored one is only for the sake of clarity. I scraped away the track coating in the right yellow circle and soldered the resistors to the copper track directly as there were no pads available there. I placed heat shrinks on the legs of the resistors to prevent any shorts.The completed modification is as shown:




The next thing I looked at was the power amplifier stage which holds the solid state amplifiers as well as the power supply:


Since the warranty for this amplifier was over, I decided to overhaul all the cheap electrolytic capacitors on this board to audio grade ones. There were also more electrolytic (decoupling?) capacitors on the top of the preamplifier board to be replaced as well:


The replacement capacitors of my choice were the Nichicon "Muse" Electrolytic capacitors which are supposedly of 'audio grade'. I felt more comfortable having these high quality components rather than the chinese "Robicon" (ripped off from Rubycon?) and ChengX capacitors.





so it was out with the old and in with the new:



Some of the new capacitors were bigger than the old ones thus some slight bending and twisting was required to make them fit back into the chassis comfortably.




The tubes (6N1p pictured) also arrived and I replaced the stock Chinese ones with these NOS Russian ones which are supposedly military grade and more hardy:




After going through all that trouble, I switched on the amp...and...the buzzing and distortion was still there!!! How frustrating!!! You can hear the buzzing and distortion I am referring to in the before and after video at the end of this post.

I realized the distortion only comes during quiet passages of certain songs that were played. The noise got louder every time I increased the treble potentiometer so I traced the circuit from the pot and found that it led to a pair of JRC4558D op amps (the red squares in the picture below) on the PCB, one connected to the treble pot and the other to the bass pot.


I did some research and my description of the distortion matches what is known as "crossover distortion" (http://en.wikipedia.org/wiki/Crossover_distortion) where the distortion mainly occurs during low signal (soft passages in a song) voltages in a "push pull" amplifier but disappears in normal to loud voltages. I won't go into much detail about this but I spotted two transistors down the signal path that might suggest this is a push pull amplifier. Before attempting to try to change the transistors I decided to change the 4558 op amps first just to see if that fixes anything.

I removed the op amps and soldered on 8 pin sockets in their places. After trawling through many forums and webpages I found a suitable upgrade in the form of the LM833N op amp and plugged them in.




I switched on the amp and played the same song that was giving me so much distortion before and...it was perfect! Changing the op amps solved the 'crossover distortion' completely!



I did not connect the LED lighting for the VU meter because I felt it was far too glaring and spoiled the look of this otherwise good looking amp. The tubes also lost their lightbulb glow and is now glowing at what seems to be the correct brightness. The sound of the amp has also improved by a great amount and I don't mean it in the pretentious "the high end is clearer and the soundstage is wider etc etc" kind of way...I mean it REALLY improved and the video below proves it. Be sure to select and watch it in the highest definition to get the best audio quality.

I'm not going to state that the 4558D op amps are better than the 833N because of the sheer amount of arguments I have read on forums all around. I will only state that the issues I had with the 4558 might be because the pair of op amps I replaced were perhaps from a defective batch because they did not sound this way when I first bought the amp. But from the datasheet the 833N claims to be low noise and that justified a try and I'm happy I did.



Take note that the video was recorded using an external mic set on the speakers and that makes it sound worse than it actually does. But you should be able to hear the buzzy distorted vocals that are no longer buzzy after the modifications are made.

*EDIT*: unfortunately I realize that youtube isn't exactly the best place to compare audio quality. the buzzy distorted vocals before the modification sound way worse in the .wav file i saved but I think youtube downsizes the audio quality to 128kbps or so, thus smoothing over the buzzy vocals and making the distortion not that obvious..but take my word for it that switching the op amps in this situation really helped rectify the problem

50 comments:

  1. Another great post,informative and cheerfully presented. I have often wondered about those Chinese amps while browsing/shopping the internet.
    I'm glad you were able to make the best of yours.
    Thanks for posting.

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    Replies
    1. thank you for the comment! I must admit that this was really an impulse buy because I have never seen any amplifier with tubes (hybrid or not) selling for such a price..heck I can barely find any portable headphone amplifiers for the same price..

      there are many other brand-less tube amplifiers made by generic Chinese companies...some are high performers with prices to match, while others have really shoddy quality and this amplifier is somewhat in between..but I have no complaints after the modifications done in this post.

      Delete
  2. Hi P, great post which proves you really can make a silk purse from a sow's ear ! I have the very same amp but it is badged as a Marriola av 101 and I plan on doing these mods as soon as I figure out why it does not work. It lights up but no sound ! Could it be one of the four OP amps (4 x TDA 2030 secured on large heat sink)

    thanks in advance, Baz.

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    Replies
    1. Hi Baz, thank you for the compliment! I can't remember the details that clearly because I worked on this some time ago, but referring to the pictures I have posted above I remember that only 2 x LM7812 op amps are secured to the large heat sink as seen in this picture here:

      http://3.bp.blogspot.com/-2-LeCV3buW4/Ug4029LfUEI/AAAAAAAAAbo/REl6Bpn77Qo/s1600/DSC03446.JPG

      I didn't touch those op-amps in my amplifier because I had no reason to suspect they were faulty. Did you verify that all your connections like the line-in and output connections are all hooked up properly? Did you select the correct output channel as well (Aux/CD/DVD/etc)? I usually always change the electrolytic capacitors first in all my restorations/repairs because they're usually the most likely to fail (especially the dodgy Chinese made ones), perhaps you should do that as well just like I did?

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  3. Actually now that I turn the volume up real loud and listen to the video at 1080p I can actually hear the differences I am trying to let people hear.. I couldn't hear it when I played it back from Youtube previously a few months ago, but it is really there! Buzzy without modifications and clear with modifications.. whew..

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  4. Hi could you post the name of speakers that you've used with this amp? I plan to buy speakers for this amount, but I am little confused about proper impendance...

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    Replies
    1. Hello, I use a pair of Monitor Audio BR1s, I believe they have an impedance of 6ohms, but the amp should be able to drive speakers of 4 - 16ohms as stated in the manual if I am not mistaken..

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    2. Yes it stated 4-16 ohms, but I wasn't for sure if the values are ok... I have another question about capacitors that you've changed - I wanted to buy them but I wanna know te proper capacity and voltage for them. Could you specify parameters for them and voltage?

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    3. There were a lot of capacitors with different values and tolerances, maybe numbering about 30-40 (can't remember exactly) or so, and I don't have the list with me anymore as this was some time ago so you will need to open up the amplifier and see for yourself which, from your comment I take is what you're intending to do already anyway. Are you intending to modify your Mistral MM-1? Or one of the variants that the MM-1 also belongs to?

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    4. Yes main goal is improve sound, also to reduce heater voltage. I also have this buzzing sound, but only on left channel - here probably it is because I am using thin cable connected to left speaker. I am gonna to update it. Also I have problem on choice what kind of speakers to get... IDK if better is get some vintage speakers @96dB/m (new one is expensive as hell) - it will improve sth or not... I don't know.
      Another question about tubes that you've use 6n1-WI - so you have on stock chinese 6n1? - how about of heating (temerature) the bulbs - is it noticable? I have 6n2-t - specs stated that 6n2 should heat up to 100-110 celsius but 6n1 up to 180 celsius.
      Thanks :)

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    5. Hmmmm.. I'm pretty sure the cable wouldn't be a factor causing the buzzing whether it is thin or not, cables don't really contribute much over such a short distance anyway so you should rule that out. It would most probably be a faulty capacitor or amplifier chip like in my case.

      The tubes do get really bright and hot without any modifications to the amplifier, although that is what tubes normally do, this was visually beyond the norm. I had one 6n1 tube fail after a few months of use, there was an intermittent popping sound from one channel and I traced the problem to one of the 6n1 tubes.

      I don't understand your question "so you have on stock chinese 6n1"? I replaced both sets of tubes (6n1 and 6p15) with more hardy Russian military grade ones which I bought from a seller on eBay after making the modifications so they should (hopefully) last me a long time.

      Delete
    6. Meaning about "stock" 6n1 i ment question if you have "chinese" 6n1 i found that MM-1 and other brands of this amp have different version of tubes... I have 6n2-t you 6n1. Refering to specs http://radiolamp.ru/rl-sprav/index.php?to=triod&lamp=6n2p 6n2p is better than 6n1p - better for example time of life, also is noticable cooler than 6n1p, also have better gain (i am not sure what it is for). 6n2p are little more expensive than 6n1p - I am gona get it and test it out.

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  5. Little update I've get tubes - 2x 6p15p-ew, 2x 6n2p-ew and as reference I've also get 6n1p-ew. All tubes are paired and from same date. Also i must admit that I've also get vingtage speakers that are not very efficient ~91 dB - 3 way with 25 cm woffer. Standard tubes have problem with proper "drive" of the bass. 6n2p-ew refered to standard tubes is more detailed, amp also get more dynamics, scene also gets more width. So no way to back to standard ones. 6n1p-ew refered to 6n2p-ew gets more warm as i mentioned before - also 6n2p-ew is little bit more dynamic than 6n1p-ew. So my recomendation is 6n2p-ew. Now I am gonna to change the capacitors and op amps - since left channel is little buzzing - but lower than on standard tubes.

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  6. Just wondering how much it cost you to change the component? and is it worth the money? I am actually looking for an amp . . . looking at Marantz PM6005 (ard S$ 460+) . . Went to Ban Leong, they show me this amp and another, model DT 307B / 309B (ard S$ 400), sound is quite good. . . got recommendation?

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    Replies
    1. hey there, if you're getting it brand new then i would not recommend it at all. if i knew i had to make such modifications to this amplifier when i bought it brand new then i would never have gotten it.. just doesn't make sense to buy a product with a design flaw. i'll recommend the Marantz solely based on the reliability of solid state over vacuum tubes but if you love to tinker with things and you have a tight budget then perhaps you can find a second hand Mistral MM1. I don't know anything about the DT307B, but I did see it in Ban Leong and I believe the power output isn't that great (can't really remember).

      The modifications I made to my amplifier cost me less than $30, but it took quite a bit of man-hours just to research and perform the modifications.

      Delete
  7. Hey dude, got myself a mistral recently, and I must say, it sounds a lot better than some of the other amps I've had, and whats better is that the lil bugger drives my floorstanders, and don't need to put it more than 30% volume to listen comfortably. I was wondering, since you modified yours to 'make him better', would you mind doing it for me as well? I am in Malaysia, and will pay for it.

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    Replies
    1. sorry I only post these things up as a guide and nothing more :) just follow my guide if you're competent with some soldering and wiring and you should be fine :)

      Delete
  8. Hi, I have buy the amplifier "Marriola AV-101", I have discovered which is the same at "Jaycar AA-0474" and "Mistral MM-1". In my opinion it sounds good! I wanted to ask, if i change only the JRC4558D with the LM833N, i can feel the difference? A flaw that I found is the hand of the "vu meter" moves very little, it is normal?
    Sorry for my bad english. Ernesto

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  9. Sorry, I have another question; the shop near my house has only resistence 5w 3,3ohm and not 4w. They are good all the same? Thank you

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    Replies
    1. hello! 3.3ohm and rated for 5W is perfectly fine.. 5W just means it can tolerate burning 5 watts of heat so the more the merrier..

      I can't say that changing the op-amps to LM833N made it sound 'better' because it sounded quite bad with the original JRC4558D amps on mine were probably defective.. what I can say is that I can't hear anything wrong with the LM833N's and I think I know what sounds good and bad.. I suggest if you don't have any issues with the JRC4558D then you don't need to change. The data sheet of the LM833N mentions that they are 'low noise' though.. seems pretty silent to me when there's no signal.

      If your VU-meter moves very little it could be your input signal is really faint.. if you have to move the volume knob more than past 12 o'clock if you're playing audio then your source is probably really soft..

      Delete
  10. Hello,
    Do you have a diagram for this amplifier?

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    Replies
    1. hmmm no i do not, it is really obscure but i haven't worked on it for a long time so there might be others online now that have worked on something similar and have produced some circuit diagram of some sort since. what are you intended to do with it?

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  11. for the tube replacement
    6N1 and 6P1 what part model name to replace them, and where to buy?

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  12. my VU-meter is always stayed high, even play or not play music. Please advisde

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    Replies
    1. ands also, amp really heat up, I place on the carpet, after 1 hr play, my carpet heat up like heater :
      any modified for it? like big heatsink?

      Delete
  13. This comment has been removed by the author.

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  14. Hi!
    How anode voltage?
    Something seems to me that these lamps are purely decorative.

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  15. Wow, can see a huge difference in the brightness of the tubes as well!!

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  16. What is the name of the song you are playing? I would like to purchase it. Thanks!

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  17. This comment has been removed by the author.

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  18. I can't read the rating on that cap. Is it 220 nf, same as .022 uF? Makes me think crazy thoughts, like attempting paper in oil tone capacitors.

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  19. Wow! Impressive mods! I was thinking of purchasing one of these but I do not have the time (nor arguably the skills---my last foray into building a Radio Shack transistor radio from a kit ended badly 30+ years ago!) to undertake this kind of a project now. Thanks for posting!

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  20. NE 5532s are far better opamps then the 833s you used -and cheap too!

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  21. Anyone know if the Monoprice version suffers from the same over-voltage issue?
    (www.monoprice.com/product?c_id=109&cp_id=10919&cs_id=1091903&p_id=13194)

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    Replies
    1. The Monoprice version of this amp has the same voltage issue. Monoprice also slightly changed the circuit board, so some of the circuit traces are different. You can't go exactly by the instructions above of where to cut and where to solder, but it wasn't too hard to make it work. The pins for the tube heaters are still the same.

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  22. Hi - I just picked up the monoprice version, loved the look of this amp and checked it would accept up to 220v on the power input. Well, I got the whole thing back to the UK now, but it doesn't - the power transformer is only able to accept 0 - 115v! I've called monoprice and they are willing to give me a full refund, but that isn't the point... Does anyone have experience or can help me source a new transformer that would accept up to 220v? I've searched a number of reputable suppliers here in the UK (RS for example) and found similar transformers, but none that match this one with the 2 x 16vac output.
    Any help would be greatly appreciated!!

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  23. I performed the same mods to the monoprice version of this tube amp and I couldn't believe the difference. Changing out the cheap caps with Nichicon audio grade caps made all the difference. This cost me about $15 to do, worth it in my opinion. Here's my cap list with part numbers from Digikey.
    4 493-10364-1-ND CAP ALUM 4.7UF 20% 25V RADIAL
    6 493-10930-1-ND CAP ALUM 100UF 20% 25V RADIAL
    1 493-10887-1-ND CAP ALUM 1000UF 20% 25V RADIAL
    9 493-10359-1-ND CAP ALUM 10UF 20% 25V RADIAL
    2 493-10944-1-ND CAP ALUM 1UF 20% 50V RADIAL
    2 493-10936-1-ND CAP ALUM 47UF 20% 25V RADIAL
    2 493-10999-ND CAP ALUM 4700UF 20% 25V RADIAL

    ReplyDelete
    Replies
    1. This comment has been removed by the author.

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    2. Don't go by this list I took mine apart monoprice version and on the boards print it states 24Volts but the 47uf caps are 35v and the 4.7uf are 50V not 25v like stated in this list also the big caps are 4700uf 35V NOT 25volt.

      don't know if the voltage rating matters though maybe somebody will reply with a answer for this.

      above list of caps

      I performed the same mods to the monoprice version of this tube amp and I couldn't believe the difference. Changing out the cheap caps with Nichicon audio grade caps made all the difference. This cost me about $15 to do, worth it in my opinion. Here's my cap list with part numbers from Digikey.
      4 493-10364-1-ND CAP ALUM 4.7UF 20% 25V RADIAL
      6 493-10930-1-ND CAP ALUM 100UF 20% 25V RADIAL
      1 493-10887-1-ND CAP ALUM 1000UF 20% 25V RADIAL
      9 493-10359-1-ND CAP ALUM 10UF 20% 25V RADIAL
      2 493-10944-1-ND CAP ALUM 1UF 20% 50V RADIAL
      2 493-10936-1-ND CAP ALUM 47UF 20% 25V RADIAL
      2 493-10999-ND CAP ALUM 4700UF 20% 25V RADIAL



      see this list below is not correct.
      AnonymousJanuary 24, 2016 10:04 AM

      I performed the same mods to the monoprice version of this tube amp and I couldn't believe the difference. Changing out the cheap caps with Nichicon audio grade caps made all the difference. This cost me about $15 to do, worth it in my opinion. Here's my cap list with part numbers from Digikey.
      4 493-10364-1-ND CAP ALUM 4.7UF 20% 25V RADIAL
      6 493-10930-1-ND CAP ALUM 100UF 20% 25V RADIAL
      1 493-10887-1-ND CAP ALUM 1000UF 20% 25V RADIAL
      9 493-10359-1-ND CAP ALUM 10UF 20% 25V RADIAL
      2 493-10944-1-ND CAP ALUM 1UF 20% 50V RADIAL
      2 493-10936-1-ND CAP ALUM 47UF 20% 25V RADIAL
      2 493-10999-ND CAP ALUM 4700UF 20% 25V RADIAL

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    3. as mentioned in the comments above->
      "The Monoprice version of this amp has the same voltage issue. Monoprice also slightly changed the circuit board, so some of the circuit traces are different. You can't go exactly by the instructions above of where to cut and where to solder, but it wasn't too hard to make it work. The pins for the tube heaters are still the same."

      so if you are performing this modification to the Monoprice version of this amp then it will be your responsibility to check the values of the components before carrying it out.

      that being said, I have ran this amplifier for nearly 3 years since I performed this modification and the tubes are running fine so far unlike how it was unmodded where they would already have burnt out after a few months. Capacitors in the power and signal lines are also performing fine with no visible issues.

      the voltage stated on capacitors are just the rating, thus it is generally okay to exceed the value with no issues, but never okay to replace it with a lower rating than the original.

      Delete
    4. Hi today I replaced the FG muse caps that are on the tubes board with 10uf 35V elna silmic II caps and it sounds much better than the FG gold muse caps did..... I am wondering if I should swap out the 4 FG caps by the op amps 2 1uf and 2 10uf and put the silmic II in that section also ...wondering what it would do to the sound.

      Delete
  24. Hi do you know what wires are the right and left channels that come out of the preamp section?
    I really want to use rca connectors from the tube preamp section and connect it to my mosfet adcom amplifier if possible.
    THANKS

    ReplyDelete
  25. Hello! I have a question (I state that I'm a beginner in audio...and sorry for my english): I see in 7th picture that there is a LM7812 (+12V) and a LM7912 (-12V) voltage regulators...what I want to know if those regulators feeding the two LM1875 amplifiers for a total power voltage of 24V, because I know that the total output power of the amplifier depends on it...so, with "only" 24V how much undistorted power can give the amplifier, with a load of 4Ohm? Thanks!

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  26. To me the sound was the same before and after the modifications.
    I have one and I am quite happy with it for its purchase price.

    People are never satisfied If you want pure audiophile sound prepare
    to pay 5 figure $$$$$.

    ReplyDelete
  27. Hi I beg to differ I did not use the caps from the guy's list above because his list consists of the gold series caps with the lower grade audio series caps...I used all the gold series and then I replaced the caps on the tube board with elna silmic II's and the sound stage is much more open and it sounds much better.... I know I ran it on very high end speakers.

    ReplyDelete
  28. Very nice mods. I was thinking of purchasing one of those amps on eBay.

    ReplyDelete
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  30. Dude, you have absolutely no idea of what you are actually doing.

    ReplyDelete
  31. Dude, you have absolutely no idea of what you are actually doing.

    ReplyDelete
  32. Does anyone have a picture of how the monoprice board should be wired up to drop the heater voltage? I can't quite figure it out with the PCB change.

    ReplyDelete