Questions about treble boost mod

Discussion in 'Fisher' started by thornev, Apr 26, 2018.

  1. thornev

    thornev Well-Known Member

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    I've attached the images about the subject mod which I found elsewhere in this forum. Questions...

    1. Is this one board enough for this mod or does the mod require left and right channel boards built the same way? The schematic refers to left and right dual op-amp IC's.

    2. I count 4 input and output connections in the schematic: front level control, to other channel, to original bias lead, and new preamp output jack. Can someone be more specific about where in a 500-C these connections need to be made? I assume the "bias lead" connection can be anywhere that bias power can be tapped.

    3. There are multiple TL082 op-amps. I bought TL082BIYDT and they look way too small and don't fit on my breadboard where I'm previewing the setup. I then purchased LF353N op-amps and am waiting for delivery. Can someone be specific about which TL082xxx is correct? I use mouser.com.

    4. I'm using the attached completed board as guidance for how the mod should look. Does the build on the board accurately reflect the schematic? I would think so but want a sanity check.

    The original post is here:
    http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/page-3, post 41

    Thanks for helping. Thorne
     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    It's not a Treble boost. It's a unity gain buffer between the preamp and the final amp. As I understand it, you have to re-wire the space expander jacks to become "PRE-OUT" and "MAIN IN" jacks, and the buffer helps with the impedance mismatch between outboard amp or preamp.

    Yes you need an opamp for each channel. As the TL082 is a DUAL opamp, just one physical opamp is sufficient.

    As for the hookup, I can't say as I've not done the mod.
     
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  3. Dave451

    Dave451 AK Subscriber Subscriber

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    Thorne,
    You are a busy fellow! I can provide some feedback on a couple of your questions, based on my own experience installing Dave's mods, including this one:

    1. Yes, you only need one board for both channels. The one op amp chip has two amp circuits--you duplicate the channel inputs and outputs using the different sets of op amp connections shown on the schematic. There is only one set of power wiring, powering both sides. I mounted the 1000 uF axial off the board on a tie strip due to its size.

    Here is the link to my set of 400 mods, with post 60 showing the op amp install: http://audiokarma.org/forums/index.php?threads/newly-acquired-fisher-400.736364/page-3

    2. Here's the ope amp chip from Mouser that I used:
    512-LF353N
    LF353N
    Dual Operational Amp

    3. There are 5 connections: 1 power, 2 inputs and 2 outputs. No experience with the 500 wiring, though.

    Good luck with it--These mods really made my 400 sing!
    Dave
     
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  4. Dave451

    Dave451 AK Subscriber Subscriber

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    While not a treble boost, the unity gain amp addresses Miller capacitance on the 12AX7 audio amp and therefore prevents treble roll off, if I recall correctly.
    Dave
     
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  5. thornev

    thornev Well-Known Member

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    Hmm. I don't want PRE-OUT and MAIN-IN jacks as I'm fine with using the AUX and PHONO jacks. I am interested in anything that improves the sound and that's what attracted me to this mod. Does this mean if I want the sound improvement that comes with this mod I have to use the space expander jacks to connect any external devices such as a CD player or turntable?
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    NO. In addition to Dave451's Miller explanation, The buffer normalizes the impedances so that you can use the preamp of the 400 with any other amp, or another preamp with the 400's amp. It's a UNITY GAIN component which means 1 in - 1 out=NO GAIN. Gain in this case meaning amplification. The space expander jacks get rewired so you can use them as PREOUT (the end of the preamp) and MAIN IN (start of the amplifier section). Both are moved to after the buffer and before the amp so the jumpers now act as a strap between the preamp and the amp as in Solid state Receivers and amps that have them.
     
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  7. thornev

    thornev Well-Known Member

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    Ah, I think I get it now.. Thanks Larry and Dave. This mod CONNECTS (or rather. REWIRES) the 2 stages of the 500-C to achieve unity gain which improves the sound. There's no need to use the space expander jacks at all. I like it !
     
  8. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    So,Thorne, being a long-time musician, likely exposed to (over) amplified music for years, how's your ears' upper frequency response? It'd be a pain to do these mods & not get the full experience. I know mine ain't what it used to be. But , you being the inveterate tinkerer, you go for it!:music:
     
  9. thornev

    thornev Well-Known Member

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    Dave451 - Your post 66 in that thread (your set of 400 mods) confuses me. You say you didn't install the preamp-in/amp-out mod, but you did install the buffer. Does this mean you can install the buffer in 2 different places - connecting the preamp-in and amp-out stages, and connecting to the volume control as in the pictures you posted in that thread? Or are these 2 different mods? All the pictures I've seen when the preamp/amp mod is discussed show the board mounted next to the volume control. On my 500-C the volume control and space expander jacks are not near each other. Maybe that's why I'm confused?
     
    Last edited: Apr 26, 2018
  10. thornev

    thornev Well-Known Member

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    Cat - Yes, my high-end frequency detection via my ears has been damaged over the years and people who listen to music with me might claim that I add too much high-end to the music. But that's how I compensate - I turn up the tweeters. The way I measure how much additional high end is needed is by adding until I can hear the cymbals or other high-end instrumentation clearly. I can still hear the high-end; I just need to crank it up. And I still like listening to music loud, just not so loud that my ears start to bleed. ;)
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    You don't have to re-wire the SpaceXpander jacks to use this, but installing this part would allow you do that if you wanted to use the Fisher's preamp to drive another power amp. If you have no plans to ever do this, there isn't any particular need to change that part of the amp.
     
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  12. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Yep, I'm with ya on the treble boost, mines always maxed on whatever I'm listening to.
    Too many years riding noisy ol' shovelhead h-d without ear plugs plus playing with guns hasn't exactly helped my ears.:eek:
     
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  13. thornev

    thornev Well-Known Member

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    Cat - You shoulda been a Goldwing Man. (sorry, couldn't resist)
     
  14. thornev

    thornev Well-Known Member

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    Thanks, gadget. Being that's the case, what do I do with the part of the schematic that says "New Preamp Output Jack"? I just leave the left channel op-amp pin 1 going through the .22 cap, 100 ohm resistor and 220K resistor, and right channel op-amp pin 7 with same parts going to ground?
     
  15. Dave451

    Dave451 AK Subscriber Subscriber

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    Thorne,
    You are correct, I installed the unity gain op amp buffer (once) without installing the mod for the pre-out/amp in of the space expander jacks (although I may eventually), as per Gadget's input.
    Dave
     
  16. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Thorne, I rode a gold wing once & about fell asleep! I'm on a '05 triumph speedmaster now, my divorce present to myself. bought new, 40k+ miles & counting...:banana:
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thorne; Follow the space expander wiring on the changed schematic for that mod. IIRC it dead ends back on the Volume control input. That's where you wire in your opamp output.
     
  18. thornev

    thornev Well-Known Member

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    Location:
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    Thanks, Larry. Assuming you're talking about the schematic I posted in post# 1 above, all it displays is "front level control." I downloaded one of Dave's images for his 400 and it looks like there are 2 wires that go to the 2 center tabs on the volume control. I don't know if that applies to my 500-C too. My question #2 in post #1 requests specific instructions on where the 4 ends in the schematic are actually attached.
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    1.) The buffer goes between the Volume control and the 2 shielded cables running to the Input stages of the amp (Driver/phase inverter tube). 2.)The inputs to the buffer are connected to (1ea) the volume control Wiper (center tab/pin). 3.) Outputs on the buffer then connect to the (1Ea) shielded cables to the Driver stage. 4.) "POWER in" can take off at the IBAM board on your Franken500c. As for the "To Other channel" I have no clue.
     
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  20. thornev

    thornev Well-Known Member

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    Thanks, Larry. I assume "to other channel" means to one of the IN2 pins on the IC chip, but + or - I don't know which.

    "franken500c"? That's funny.
     

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