Tame my pre amp

Discussion in 'DIY Discussion' started by T-bone Sanchez, Jun 23, 2006.

  1. T-bone Sanchez

    T-bone Sanchez

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    Please.....

    Okay got a Meixing MC-7R which came via a member of here (you know who you are :p ) I like it, I really do but the output is just too much.

    Ive tried attenuating the signal by experimenting with resistors but all I end up with is killing the dynamics at best. Its been sulking in a corner for while (my heavily modded X-Pre is back on duty without any problems what so ever).

    Manufacturers figures claim a gain of x10 and a voltage upto 40v. Im convinced that halving the gain and quatering the voltage output would do wonders but thats were my bright idea ends.

    If I dont attenuate I get a very loud buzz and early clipping if I attenuate I get what I mentioned earlier. Ive played with the pre, the power Ive changed components, Ive prayed, Ive sworn, Ive even tried voodoo (didnt help the pre but I now have hairy palms).

    Ive attached a schematic of low quality (it did travel from China) so if anyone has any ideas (Thorsten??) please help a poor amp whose life is in the balance.
     

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    T-bone Sanchez, Jun 23, 2006
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  2. T-bone Sanchez

    murray johnson

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    It's a problem. Almost any valve linestage will have too much gain and the only way round it is to use feedback or an attenuator as you are trying to do which slugs the sound. Cathode follower circuits don't sound good enough in my experience.

    Alternatives (although inappropriate here) might include using a lower gain small power triode or even an output transformer.

    The circuit looks way too complicated for my liking.
    If it were mine I'd change it for something like this.

    [​IMG]

    NB the pinouts for the 5687 are different to ECC types.

    You'll still have quite alot of gain but I'd expect it to sound much better.
     
    murray johnson, Jun 23, 2006
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  3. T-bone Sanchez

    zanash

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    What resistors did you use value and make ?
     
    zanash, Jun 23, 2006
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  4. T-bone Sanchez

    3DSonics away working hard on "it"

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    Hi,

    IF YOU LIKE THE WAY IT SOUNDS (I definitly would not, but if you do) simply attenuate the output with a voltage divider of suitably high impedance (too low and you do kill the dynamics), try 91K/10k at the input of the followingn amplifier.

    Quite frankly, I would put that preamp out of it's misery. I need to save this schematic and keep it as an example of how not to do things.

    Murray's suggested change is a good choice, though I'd probably use the Kondo Version of this circuit.

    [​IMG]

    If you do not skimp on component quality and use a switched shunt attenuator for volume this circuit could be rather good.

    Ciao T
     
    3DSonics, Jun 23, 2006
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  5. T-bone Sanchez

    murray johnson

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    Thorsten,

    How much effect on the stage gain or output impedance would you expect bypassing the cathode resistor to have?

    rgs,
    Murray
     
    murray johnson, Jun 23, 2006
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  6. T-bone Sanchez

    3DSonics away working hard on "it"

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    Hi,

    Marginal in the circuit you show.

    Kondo operates the 5687 at very low current, it is part of what makes for his house sound and I too rather like it like that.

    In Kondo's circuit with I think around 3mA the output impedance would be sky-high without cathode bypass (around Rk * (Mu+1) + Ra in parallel with Ra).

    Even in the circuit you are showing you get around 5 - 7K output impedance BTW.

    Ciao T

    PS, I could of course also throw my "el-cheapo" linestage in the ring:

    [​IMG]

    This shows a 6SN7 but a 5687 or a few other similar valves will work fine. Of course, this line stage is EVIL, because it uses negative feedback, that said, it's only around one stage and it makes the output impedance drop through the floor....

    As shown gain is around 6db with the volume fully up and 50K or higher input impedance (depends on volume setting).
     
    3DSonics, Jun 23, 2006
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  7. T-bone Sanchez

    murray johnson

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    I realised the o/p imp was quite high. I've made a couple of these for friends (with ss rectifiers-RC powersupplies etc) with shortish interconnects & the right power amp they can sound very nice though.

    I'll have to try the Kondo low current approach with the 5687 and see what the difference is.
    I serviced an M7 line not long back and it did have the 'house' character but was not stunning in other ways. I'm sure his use of Tants & the silver caps/cable contribute much as well. He also did have little regard for the lifespan of his rectifiers!

    You should try & hear that M77 though in 'known' circumstances. That is something else!

    Your circuit, with a bit of applied feedback, reminds me a little of the old AI L2.

    rgs,

    Murray
     
    murray johnson, Jun 23, 2006
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  8. T-bone Sanchez

    de'Kev

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    I have the "Valve El Cheapo" phono stage driving S&B TVC's, then into an Aleph 30 clone in parallel with a Behringer DCX2496.
    I suspect that at high volumes the phono stage is running out of oomph to drive the low impedance with the paralleled next stage.
    Would adding the "El Cheapo" line stage before the TVC be a good idea? Is there a better way of driving the DCX and Aleph? There is no way I'm going to use the DCX to drive the Aleph, I find it too veiled.

    Kev
     
    de'Kev, Jun 23, 2006
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  9. T-bone Sanchez

    de'Kev

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    I have a MingDa phono stage that I bought to replace my old JLH phono. It's not very good, so I posted on another forum and had several responses which I implemented as best I could but I was never impressed. In the end I was tempted by KYW ( Thorstens') Valve El Cheapo which absolutely murdered the MingDa and IMO is better than an EAR that I tried.
    Given my experience I would dump the line stage you have and build something along the lines suggested by Murray Johnson or Thorsten.
    If you're contemplating modifying what you have then it isn't much more of a jump to build from scratch. There is plenty of support on the forums and loads of info on the 'net.

    Go for it, I doubt you'll be disappointed.

    Kev
     
    de'Kev, Jun 23, 2006
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  10. T-bone Sanchez

    murray johnson

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    Is the audio circuit hardwired or on a pcb in the Minger?
     
    murray johnson, Jun 23, 2006
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  11. T-bone Sanchez

    de'Kev

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    pretty sure it's P2P hard wired if it's important I can take it out of it's box and have a look. Memory is failing these days, senile dementia onset can't be far off :mad: :Quad:

    Kev
     
    de'Kev, Jun 23, 2006
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  12. T-bone Sanchez

    3DSonics away working hard on "it"

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    Hi,

    Honestly, no idea. Just try it. It might be great or it might suck, I got no clue....

    Ciao T
     
    3DSonics, Jun 24, 2006
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  13. T-bone Sanchez

    murray johnson

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    apologies Kev, I meant the Meixing not the Ming Da. I was just wondering how easy it was going to be to change the linestage to 5687's.

    Senile Dementia! gets to us all. I need to take up smoking again and kill myself that way before it gets too bad.
     
    murray johnson, Jun 24, 2006
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  14. T-bone Sanchez

    T-bone Sanchez

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    Okay, so the pre's not worth bothering with, whats this 'El Cheapo'?
     
    T-bone Sanchez, Jun 26, 2006
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  15. T-bone Sanchez

    T-bone Sanchez

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    The 'minger' (quite funny) is hardwired and could be easily played with as I have been doing.
     
    T-bone Sanchez, Jun 26, 2006
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  16. T-bone Sanchez

    3DSonics away working hard on "it"

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    Hi,

    Not saying that, but the circuit is certainly rather odd... :D

    I'm not surprised it is noisy and all that.

    This one from this post:

    http://www.zerogain.com/forum/showpost.php?p=175727&postcount=6

    ?

    It is just a little project that sounds pretty nice and all that jazz.... It's low output impedance, low gain and can use actually quite a few different double triodes. Best add gridstoppers.

    Excellent choice for the Circuit is the russian 6N30 BTW.

    Ciao T
     
    3DSonics, Jun 26, 2006
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  17. T-bone Sanchez

    T-bone Sanchez

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    Well the Minger is no more, last night I set too with a soldering iron and pliers and removed all but the basic power supply. Im going to fo for one of the 5687 designs, probably the Kondo version (in memory of the man himself). I'll just gather some parts and crack on.
     
    T-bone Sanchez, Jun 27, 2006
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  18. T-bone Sanchez

    Tenson Moderator

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    You mutilated it :eek:
     
    Tenson, Jun 27, 2006
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  19. T-bone Sanchez

    3DSonics away working hard on "it"

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    Hi,

    May it's entrails rest in pieces....

    Brave, but probably well worth it.

    BTW, I would recommend 0.47uF Tinfoil Capacitors for the output coupling, to my ears at least the tinfoils have a sound not a continent of Kondo's handrolled silver jobbies and the original shown 0.1uF is a wee bit on the small value side, unless the input impedance of your amplifier is much above 100K. Kondo uses BG's on the Cathode in this design, I would recommend to you to use as well...

    Ciao T
     
    3DSonics, Jun 27, 2006
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  20. T-bone Sanchez

    raindance

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    Sorry to butt in here, what about trying different valve types with lower gain? Looking at the circuit you could decrease gain by replacing the 12AX7 (mu=100) with a 12AT7 (mu=70) or even 12AY7 (mu=44) OR another 12AU7 (mu=17 or so). Guitar amp guys do this all the time to change gain.

    Also, removing cathode bypass caps will decrease gain, but increase local negative feedback.

    Oops, I see the Minger is no more, I realise this is too late, but often the obvious gets overlooked in this fun(ny) hobby of ours!

    What the heck, while I'm here another solution is to pad down the INPUT of the power amplifier...
     
    raindance, Jun 27, 2006
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