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Possible to reduce phono input capacitance on C-80?

Discussion in 'Yamaha' started by ba4x, Oct 22, 2018.

  1. ba4x

    ba4x New Member

    Messages:
    4
    Hi folks. Long time lurker / google searcher of the AudioKarma forums, this is my first post. Upon searching it seems there are some threads about this topic but none containing the specific guidance I am seeking.

    I'm very happy to have put together a Yamaha stack for listening to records: P-751 turntable, C-80 preamp, and M-60 power amp. The speakers are B&W DM3000. This was all acquired over a few years through a close friend of mine who loves old Yamaha stuff, he has re-capped and cleaned up the components. I will say it sounds really great, if not a little bright at times. The phono cartridge is an Audio Technica VM540ML, which is an MM cart. I use the 47k 220pF setting on the preamp. I believe this cart + the C80 preamp pairing is the main source of the brightness, as excess capacitance can result in a spike in the HF range. The spike gets really annoying at times, and the EQ is only so good at mitigating it. I've done all I can to drive down the total capacitance (ie very short cables), and it now sits at ~290pF (220pF from the C-80, and about 60-70pF from the cables and turntable). This isn't terrible but I suspect the cart will be happier in the 100pF range. To achieve this, I'd need to get a new preamp with lower input capacitance. But here's an idea: Can I modify the input capacitance of the C-80? I realize this may be stupid or blasphemous in one way or another, and that's why I'm asking your advice.

    From looking at the service manual for the C-80, I can identify a single 220pF cap which shunts from the input signal to ground. I'm hoping to replace this capacitor with a smaller one (maybe 100pF or 50pF).

    The C-80 supports a 47k 330pF option, and I see a 100pF cap waiting to be run in parallel with the 220pF if the selector switch is set that way. So if I reduce the capacitance by any amount with my tinkering, I can always bump it up by 100pF by using this selector switch. This is my hypothesis.

    Do I have my ideas right? Will I ruin or upset some system by replacing that 220pF capacitor with a smaller one?

    I apologize if I'm muddying any concepts here. Perhaps this post should go in the DIY forum, but I thought to try here first. Thanks for your time.
     
    Last edited: Oct 22, 2018

     

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  2. ba4x

    ba4x New Member

    Messages:
    4
    Last edited: Oct 22, 2018
  3. mbz

    mbz Super Member

    Messages:
    2,174
    Location:
    Melbourne, Victoria
    Yeah, I think your right, worth a try, maybe learn something.
    Change out C149(left) and C150(right).

    Resort to the carts datasheet to identify the ideal capacitance and then work back from that, summing/estimating
    all contributions to capacitance (approx) then calculate the desired C149&150. Use a polyproplyene film, not
    boutique. Continue to use the 220pf setting, as you say keep the extra 100pf on the switch.
     
    Last edited: Oct 23, 2018
    ba4x likes this.
  4. Dr. Ear

    Dr. Ear Well-Known Member

    Messages:
    727
    Location:
    Austria
    You can safely exchange the 220 pF cap with a 100 pf film cap. I did it because my AT150MLX is also very bright. The modification did not help much. The AT540ML is also known to be a very bright sounding cartridge.
     
    ba4x likes this.
  5. 808_state

    808_state Well-Known Member

    Messages:
    702
    Have you tried adjusting the VTA on the P-751? I have a P-751 and I'm pretty sure it can be adjusted. I've heard complaints about brightness with these types of AT carts and the only time I've experienced it was when it was on the wrong arm/table. When I first got my AT150mlx I tried it on a YP-D4 and it sound terribly bright and definitely not a fit for that table/arm. Just for kicks, I moved it to my PF-800, tipped the VTA so that cart was slightly tail down and all was good with the world. Brightness transformed to detail. That style of AT cart is pretty much made for the PF-800's arm and vice versa. I've used them with both a Yamaha C-4 and C-2x preamp (very adjustable) and playing with the capacitance settings did not result in dramatic changes in sound. I know on some threads folks have tried loading the AT150mlx at 33,000 instead of 47,000, claiming that it flattens out the top end a little.
     
    ba4x likes this.
  6. ba4x

    ba4x New Member

    Messages:
    4
    Thanks very much for the tips, folks. I am eager to try the cap swap if it's clear nothing else in the preamp will be adversely affected.

    Oddly enough, the brightness has steadily become less of a problem over the weeks and months, due to changes I've made or perhaps just getting used to it. At one point (before starting this thread) I removed about 100-150pF by switching out some long cables between the TT and preamp, and this seemed to help. The cart now has 50 or 70 hours on it, so I'm deeper into the break-in period, which may be a factor. Additionally, I swapped out the M-60 amp for a (similar) MX600U, which seems to be less punishing about the brightness spike. Recently I've put more effort into damping first reflections in the room, also going for a bit less toe-in, which seems to have helped, too.

    Regarding the the turntable:
    I tried playing with the VTA a few weeks ago, and it seemed that lowering the backside of the cart resulted in a more "muddy" tone, with annoying highs still present. Certainly found a difference, but not what I needed. Maybe I can try it again. I'm back close to parallel with the tonearm. Maybe the cart breaking in has had an effect on VTA (if the suspension relaxes). I heard the AT carts are a good match for the P-751, due to a light tone arm and high compliance in the cart. Admittedly, I have not studied this very closely.

    In general I'm becoming happy with the sound. I'll try to lower the capacitance further by modifying the C80. Maybe the Y-cable trick to change the resistive loading, at some point over the winter. One day I may go for a Denon cart to try something totally different.
     
    Last edited: Nov 7, 2018

     

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