I am currently preparing to build a Mesa Mk4 preamp.
(basically Mk2C with switching circuit of Mk4)
I‘ll omit the stock loop and reverb,
and add an ZL fx loop after GEQ.
However, since the GEQ circuit itself has a low impedance output, It could be FX Send with just a simple volume adjustment.
Actually, the Fx Loop in the Mk5 is configured in this way, with a tube recovery stage.
Is it okay to use the recovery stage of the ZL loop in the same configuration? (and omit the send stage)
The switch that shorts the 3.3k is a part of the -10dB / +4dB switch in version 4.1.
Furthermore, is there some better choice for recovery stage circuit?
I think high impedance output of this circuit is for internal use, not that good choice for external preamp output. and I think the beauty of ZL loop circuit is in the send stage.
Maybe add a source follower..?
Fx loop configuration in Mesa Mk build
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Fx loop configuration in Mesa Mk build
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Stevem
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Re: Fx loop configuration in Mesa Mk build
Whenever someone wants to add and or even use an existing effects loop I need to ask if you intend to use anything wired into it that has any time delay to it.
Reason being that any time delay put on your signal that is again distorted by a down stream amp stage will sound pretty bad, at least by most people’s standards.
Even a chours can sound crappy even though it’s a very fast repete time .
Reason being that any time delay put on your signal that is again distorted by a down stream amp stage will sound pretty bad, at least by most people’s standards.
Even a chours can sound crappy even though it’s a very fast repete time .
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Not screaming like the passengers in his car!
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Re: Fx loop configuration in Mesa Mk build
Yes, that’s why the ‘FX Loop’ exists in the last chain of preamp..Stevem wrote: ↑Tue Jun 17, 2025 4:35 pm Whenever someone wants to add and or even use an existing effects loop I need to ask if you intend to use anything wired into it that has any time delay to it.
Reason being that any time delay put on your signal that is again distorted by a down stream amp stage will sound pretty bad, at least by most people’s standards.
Even a chours can sound crappy even though it’s a very fast repete time .
Other than that, fx after recording or load box. I already have reactive load and Ox Stomp that has built-in reverb and delay, and it’s better sounding than fx loop.
However I want to implement loop function on my build, because.. I just want to make the things look ‘handy’
Well, compared to vintage amps like Plexi, Mesa Mk series don't rely that much on power amp distortion.
Re: Fx loop configuration in Mesa Mk build
I realized that I don’t need the return stage, since this is a preamp version. It was stupid to implement whole FX loop circuit in the stand alone preamp..
Now I need a proper line out with adjustable level, and it’s a little bit problematic to add a pot after GEQ output simply. Maybe the side effect is subtle, however I think it’s not ideal.
There’re some thoughts;
It would be slightly better for avoid ground loops, to use same PSU of GEQ for output buffer.. I think.
My GEQ supply is rectified from seperated winding, not like original that is bias tap inside of HV secondary.
And the output buffer of GEQ would not need to be another hundreds voltage circuit.
So another BJT emitter follwer buffer with volume control.. since the output impedance of GEQ is low, I don’t need to use FET buffer, which is difficult to find higher voltage(40v) and P channel one.
Maybe I could convert the GEQ circuit to positive supply and copy some J201 FX send circuit.
Or just add a IRF820 source follower..
Now I need a proper line out with adjustable level, and it’s a little bit problematic to add a pot after GEQ output simply. Maybe the side effect is subtle, however I think it’s not ideal.
There’re some thoughts;
It would be slightly better for avoid ground loops, to use same PSU of GEQ for output buffer.. I think.
My GEQ supply is rectified from seperated winding, not like original that is bias tap inside of HV secondary.
And the output buffer of GEQ would not need to be another hundreds voltage circuit.
So another BJT emitter follwer buffer with volume control.. since the output impedance of GEQ is low, I don’t need to use FET buffer, which is difficult to find higher voltage(40v) and P channel one.
Maybe I could convert the GEQ circuit to positive supply and copy some J201 FX send circuit.
Or just add a IRF820 source follower..
Re: Fx loop configuration in Mesa Mk build
To me, R42 becomes a 10k pot for the send level, then stick a common collector MPSA20 stage after it as a buffer.
Few fx pedals put out sufficient signal level to drive a power amp to full output, so an additional recovery stage may be a good idea. eg a MPSA20 common emitter stage set up for a gain of 5 to 10, or whatever suits the desired loop level.
If a recovery gain control is desired, it's a matter of preference whether to insert that at the input or output of the recovery stage.
Few fx pedals put out sufficient signal level to drive a power amp to full output, so an additional recovery stage may be a good idea. eg a MPSA20 common emitter stage set up for a gain of 5 to 10, or whatever suits the desired loop level.
If a recovery gain control is desired, it's a matter of preference whether to insert that at the input or output of the recovery stage.
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Re: Fx loop configuration in Mesa Mk build
Thank you for the reply!pdf64 wrote: ↑Wed Jun 18, 2025 8:47 am To me, R42 becomes a 10k pot for the send level, then stick a common collector MPSA20 stage after it as a buffer.
Few fx pedals put out sufficient signal level to drive a power amp to full output, so an additional recovery stage may be a good idea. eg a MPSA20 common emitter stage set up for a gain of 5 to 10, or whatever suits the desired loop level.
If a recovery gain control is desired, it's a matter of preference whether to insert that at the input or output of the recovery stage.
Someone gave me some advice that simply adding a pot won't be ideal because of the total send impedance shifting through pot sweep and loading effects on it.
Maybe I could ignore those and simply put a pot, but it’s new build, so far I have a chance to improve some.
Contrary to what I mentioned before, it's not completely pointless to add recovery stage. If I have to use old tube amp that has only passive return(P.I. input), it would be better to have the recovery stage..
It won't happen to me right now though, just in case
Actually all of my preamps that I’v built have FX loops.
So...
I'm trying to make a FX loop with BJT.
Normally BJT won't be good choice for this..
The GEQ is on 'negative' 40v supply.
For larger input headroom, I'll increase the voltage even higher than stock. probably around 60v. (I have a seperated 50v winding)
Under these conditions, it was not easy to find proper FET or something. There are OPAMPs that operate at 60V, but a bit expensive, cost time to source, larger footprint(no dual one)..
And if necessary, I would convert the whole circuits to positive supply.
However I'm not dealing with high impedance signal, I think PNP BJT would be good. BC556 is cheap, it can handle upto -65v.
.. To be honest, as an incompetent amateur, it's not easy for me to create a 'new' circuit.
Well it's not 'new', just a patchwork. Even though, I'm not sure about this
R6 is output pulldown of the GEQ circuit I posted before.
it was 10K, but 100K on some other schematics.. since 100K is more handy with additional resistor divider, I chose 100K.
Although I have drawn the recovery stage, it is not necessary that much.. as I mentioned ealier.
If the circuit design is not good enough, I would just omit the recovery and implement only the line out.
I'd be grateful for some advice.
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Re: Fx loop configuration in Mesa Mk build
..The FX send is not that big concern since it’s simple buffer, but I’m not sure about the performance of this BJT return stage.. it’s rarely used and bettered by FETs and OPAMPs.
I don't know well but I guess a discrete circuit with more sophisticated design would be needed to achieve clean recovery(especially with -10dB level) with BJT..
I should look into some mic preamp circuits.
It would be harder to compensate after the build if I stay with negative supply, because the limited choice of components.
So I decided to convert whole circuit to positive supply and adopt more verified circuits. There is no MOJO in negative supply design in earlier Mesa Mk, and it changed to positive supply after Mk5..
I don't know well but I guess a discrete circuit with more sophisticated design would be needed to achieve clean recovery(especially with -10dB level) with BJT..
I should look into some mic preamp circuits.
It would be harder to compensate after the build if I stay with negative supply, because the limited choice of components.
So I decided to convert whole circuit to positive supply and adopt more verified circuits. There is no MOJO in negative supply design in earlier Mesa Mk, and it changed to positive supply after Mk5..