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Old 17th July 2013, 11:13 PM   #1
emosms is offline emosms  Bulgaria
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Default do I need a cathode follower?

Hi, I built this schematic for guitar preamp
EF86 for any amplifier
I use it as a preamp to high Z input of an audio card.
The point is that it sounds pretty dull. No chime at all.. Muddy, bassy sound, no sparkle.
Tried with few ef86, old ones from different manufacturers. Mostly grey plated, one of them with silver grid around but no big difference.
I process the sound in amp simulator afterwards and I need to add a trebble booster in the chain to get smth at least close to the sound I like.

Would I benefit to add a cathode follower at the output of the circuit (mosfet)?
Now the sound goes through 680k res after C1 and then 20k log pot to ground, thats it.

Here it is the pehthode chanel bright tone with ef86:
Vox AC15 Heritage Handwired - YouTube
Some ppl didnt even believe it is true when I played them the video, they suspect there is a hidden trebble booster somewhere inside

Last edited by emosms; 17th July 2013 at 11:16 PM.
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Old 17th July 2013, 11:34 PM   #2
SY is offline SY  United States
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Yes, a cathode follower will be a good idea there. Most sound cards have a 10k (or so) input impedance and that's an extremely heavy load for that circuit.
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Old 17th July 2013, 11:35 PM   #3
kevinkr is offline kevinkr  United States
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The output impedance will be pretty high, and sound card input impedances will generally be around 10K or so. A cathode follower using something like 6DJ8/ECC88 would work, the other option would be a fet op-amp configured as a unity gain buffer with something like a 470K input impedance.
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Old 17th July 2013, 11:36 PM   #4
kevinkr is offline kevinkr  United States
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This belongs in Instruments and Amps so I'm moving it to the right forum.
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Old 17th July 2013, 11:52 PM   #5
emosms is offline emosms  Bulgaria
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Thx for moving
The sound card is for guitar, not ordinary line in or dynamic microphone.
Wonder if to start messing with the capacitors as well, the sound is really Nothing like what I hear in the demo.
If clean - it is dull. If start some distortion - it is really bassy and muddy, no clarity at all

Last edited by emosms; 17th July 2013 at 11:55 PM.
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Old 18th July 2013, 12:01 AM   #6
kevinkr is offline kevinkr  United States
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I suspect there are a number of things going on here, the best place to start might be by letting us know the sound card input impedance (resistance and capacitance) as well as that of the cable.

Did you use the recommended or similar caps, it is important in the MI world for tone..

It does sound rather like it is struggling to drive a fairly capacitive input so a CF may well help.
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Old 18th July 2013, 05:37 AM   #7
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Why does it go through a 680k resistor at all? Or do you mean that's 680k to ground?
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Old 18th July 2013, 07:02 AM   #8
emosms is offline emosms  Bulgaria
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Thx. Sounds not that extraordinary
I will try to find details for the soundcard. Now just experimenting with this: USB Guitar Link Cable Turns PC Into Amp | GeekAlerts
Didnt think it might be that bad, to cut off all the high frequencies .
---
http://www.dougcircuits.com/images/genericEF86.png
680k and 20k log pot in series, log pot to ground, wiper -> out. Primitive
Cable is a common microphone cable, nothig special or highend.
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Old 18th July 2013, 07:37 AM   #9
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Maybe the 680k needs to be bypassed with a small cap - 330pF ?
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Old 18th July 2013, 07:47 AM   #10
emosms is offline emosms  Bulgaria
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Thx, just was reading about it here:
Modifying Guitar Amp Preamp Circuitry
First thing to check!
---
Still I dont quite understand how C3 behaves in the original schematic:
http://www.dougcircuits.com/images/genericEF86.png
Better don't touch?
To my common sense and lack deep knowledge, seems that all highs go to ground through a 100/220n cap (screen cap).
But this value seems to be into the original vox schematics.
I read that in some preamps it is omitted, connecting screen grid to cathode as the suppressor grid, but haven't seen any such schematic for guitar.
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