Home Forums GuitarPCB Build Support Dr. Phybes output buffer

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  • #13087
    Dan
    Participant

    Hi, i have put together a Dr. Phybes phaser kit from pedalpartsaustralia.com.

    The issue: signal loss/tone suck with pedals placed after the Dr. Phybes. Big reduction in signal when the Phybes is engaged, compared with bypassed.

    It works fine when it’s the last pedal before the amp, but i’m having some issues loss of signal level, with a couple of my true bypass pedals that i want after the Dr. Phybes and before the amp. My guitar has vintage humbuckers, so i need to use a buffer after the guitar, or i get horrible tone suck from my true bypass pedals and cables. My Mooer Micro Looper must have low input impedance as is the worst offender. I’ve checked all my cables in different combinations and it’s always the same. My signal chain is:

    guitar -> 10ft of cable ->  Zenith Mini buffered bypass-> Dr. Phybes TB -> Wampler overdrive TB -> Mooer Micro Looper TB -> 20 ft of cable -> amp.

    If i remove the Wampler and Mooer pedals, the Dr Phybes works perfectly into the amp with no signal loss. However, with any other TB pedals between the Dr Phybes and the amp, the signal drops significantly (I’d estimate 10+dB) when the Phybes is engaged. Switching in the Wampler set to low gain fixes the signal loss (due to its buffered input no doubt). The signal strength is roughly equal with the Phybes engaged or bypassed. I’ve checked all the component values and everything seems fine.

    Question for you: is there a way of easily increasing the output signal from the Dr Phybes output buffer to avoid this signal loss? I don’t want extra voltage gain, rather a better low-impedance output to drive bypassed pedals and cable before the amp.

    thanks in advance

    #13093
    wilkie1
    Moderator

    The output of the Dr. Phybes is a low impedance signal provided by the Q5 PNP transistor.  From your very thorough description, it is clear that the Dr. Phybes works correctly when sending the output directly into the amp. You say that it also works fine when the Wampler is set to low gain.  If you bypass the Wampler and send the Dr. Phybes signal directly into the Moor, do you have a signal loss?  If so, that would make me believe that the input impedance of the Moor is too low and is causing the signal to be loaded down.

    I suggest you try adding a boost after the Dr.Phybes that has a variable level and a low impedance buffer.  If that cures the issue, you may like to try the GPCB STAGE 3 which is a low impedance variable level booster.  It is quite transparent and will not colour your tone.

     

    #13094
    Cybercow
    Moderator

    Wilkie1 – good advice. Thank you for that clarification. It describes succinctly what I have struggled with for a while.

    #13426
    Dan
    Participant

    It seems that the MXR phase 90 has a similar problem, people report needing a buffered pedal after the phase 90 in their signal chain. I have a couple more pedals now (just built the Blues Breaker with an Afterblaster – awesome 🙂 and a mix of factory buffered and TB pedals, and it seems the Dr Phybes is very sensitive to placement in the signal chain. I think it would benefit from a better output buffer so that its output level is not affected by the pedals after it.

    I’d like to improve the performance of the output buffer Q5 stage in the Dr Phybes, without resorting to a transistor/opamp output buffer stage as others have done to their Phase 90s.

    Looking at the schematic, it seems that the mixer/buffer transistor stage output impedance could be reduced by changing R20 from 56k to a smaller value – perhaps 5-10k? Perhaps another issue limiting the output drive current from the buffer stage could be because Q5 emitter is fed from Vb , which is limited by R29 10k resistor?

    I’m not totally familar with the common collector PNP buffer arrangement, so I’m struggling to think of suitable mods. Can anyone here advise how the Q5 stage output impedance can be lowered to improve the output drive without using a external booster/buffer stage?

    #13427
    Barry
    Keymaster

    Glad you are liking some of the other builds.

    The Phase has a higher output impedance than others. This is typically not an issue. I assume you tried a pedal with a buffer directly after it and that it improved the situation?

    Wilkie1 also previously asked “If you bypass the Wampler and send the Dr. Phybes signal directly into the Moor, do you have a signal loss?”

    Knowing some of this added info might help.

    Anyway my preference with this circuit is always pre-distortion. I really do not like it post distortion but that is perhaps because I like Pink Floyd, Robin Trower and that type of Phase sound. That said if you determine it is killing your volume and that by placing a buffered pedal after it solves these issues then I can understand what you are going for.

    Thanks

    #13428
    Dan
    Participant

    Thanks Barry, i agree that the phasor sounds better pre-distortion, however i was hoping to have some flexibility to run it “clean” into my overdriven amp.

     I assume you tried a pedal with a buffer directly after it and that it improved the situation?

    Yes, when i run the Dr Phybes straight into the buffered bypass Zenith, it works great with no signal loss at all.

    If you bypass the Wampler and send the Dr. Phybes signal directly into the Moor, do you have a signal loss?”

    Also yes.

    I just found that the gain of the output transistor stage can be adjusted by changing the 150k feedback resistor, but that seems like a band-aid solution to me.

    I wonder if there’s something up with my setup, I have a Mooer yellow compressor that works fine, until I place it before a couple of TB pedals. My FET boost totally runs out of gain when i place it at the front of my meagre 6 pedal chain, compared with a big boost if it’s run straight into the amp. Is it possible that the cheap $4 3PDT switches have some unusually high capacitance or something? It seems that any combination of my guitarpcb or commercial pedals has issues unless every other pedal is a buffered bypass.

     

    #13429
    wilkie1
    Moderator

    Any boost pedal effect would be more noticeable at the end of the chain. Switch capacitance should not be noticeable in your pedal board.  If you want to use a pedal in different positions on your pedal board, I suggest you use an Order Switching Board.  You can install one in a separate enclosure and change an effects position on the fly if desired.

    #13442
    Dan
    Participant

    Thanks for your help guys, I think this one can be closed out now.

    I’ve realised that there’s nothing wrong with my Dr Phybes output. The volume drop i was experiencing was the pretty much only with clean sounds, or into a light OD pedal. An output buffer helps a bit, but only so much. Then I tried a much bigger (0.68uF) output cap in place of the 47n, which made a huge difference. It seems the 47n high pass filter is limiting the low end output, which shows up as a volume drop when strumming a chord. Unfortunately the low end from a big output cap also renders the phase effect almost unoticeable – obviously the circuit designers knew what they were doing 🙂 As a tradeoff I changed out the output cap from 47n to 100n, and now i’m very happy with how it sounds both clean and into OD pedals. I’ve also re-ordered the pedals on my board to minimise the signals loss issues, very happy now. Thanks again

    #13482
    Dan
    Participant

    Found the problem with my setup – nothing wrong with any of my pedals, it was my patch cables. I forgot that i bought quality Mogami cable which has a semi-conductive layer over the core insulation. I didn’t remove it when making up my cables, so all but one measured 3-4k ohms core to sheath 🙂

    Fixed all the cables and now everything is working great, no more volume issues, heaps of headroom and boost level. I reverted the Dr Phybes output cap back to the stock value and it sounds excellent. Thanks again for the assistance

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