Hi all,
after 2 days rush into basic stuff (stuff I don't know at all or have forgotten), this is what I put up...
An fx loop for bass amp.
Some concerns..
- is it ok to split the signal after the input buffer? Ok to use decoupling caps at the next modules inputs?
- I am going to split the signal again, after the mixer output
(xlr bal_out and crossover)
- do I need R Load at the send output, or anywhere else?
- anything else wrong you may point out..
Happy Holidays
after 2 days rush into basic stuff (stuff I don't know at all or have forgotten), this is what I put up...
An fx loop for bass amp.
Some concerns..
- is it ok to split the signal after the input buffer? Ok to use decoupling caps at the next modules inputs?
- I am going to split the signal again, after the mixer output
(xlr bal_out and crossover)
- do I need R Load at the send output, or anywhere else?
- anything else wrong you may point out..
Happy Holidays
Remove S.R5, B.R2 and RL.
After M.C1, M.C2, S.C3 there should be a resistor connected to mass. ~ 47kOhm.
And better replace the TL072 with an audio OP.
After M.C1, M.C2, S.C3 there should be a resistor connected to mass. ~ 47kOhm.
And better replace the TL072 with an audio OP.
Thx 🙂.
When we split the signal to two or more following modules, do we have resulting input impedance of
Zin1 || Zin2 ... || Zinx || RL? (Simply as resistors in parralel).
And what could be the purpose of RL (a resistor at opamp's output to gnd)?
When we split the signal to two or more following modules, do we have resulting input impedance of
Zin1 || Zin2 ... || Zinx || RL? (Simply as resistors in parralel).
And what could be the purpose of RL (a resistor at opamp's output to gnd)?
If we have a decoupling capacitor - it forms a highpass CR chain with the following load impedance.
Provided we don't know the following Zin or it is variable:
- isn't it a good idea to secure the roll-off with our own set RLoad ???
(just thinking of possible use).
About the decoupling caps - could I have all internal connections DC coupled?
I mean, to leave out P.C1, M.C1, M.C2.
The input buffer also could go without it, cause the preamp certainly is going to have cap at output.
The crossfader output... I don't know..
If I am going to decouple it, better to put cap here, instead of putting 2, 3 caps after the signal is split.
Not a great idea. Keep those resistors. A small build-out resistor in series with the opamp output isolates it from subsequent capacitive loading (for example, guitar cables plugged into the FX loop), and helps to ensure the opamp remains stable.
A TL072 is an audio op am. For guitar circuits, it's an excellent choice. Don't replace it, it's a complete waste of money.
-Gnobuddy
add some resistors >=1MOhm to the +Inputs and ground of SU1B and RU1B, this will reduce potentiometer "crackle"
Coupling caps, I mean DC decoupling.
In general, is it a good Idea to strive for DC coupling as a first option and think about AC coupling only when necessary??
Is it good to keep all opamps in the circuit DC coupled?
I will probably put caps on Send and Return. It is sane and conservative when you connect 'some' external device.
The whole circuit would be for sure AC coupled to the previous module.
And I might use AC coupling at the circuit output or somewhere at the next stages, to prevent DC offset build up.
(anyway, all modules are about unity gain..)
p.s. anyway2.. the power amp circuits have an input cap... anyway... 😀 😀
Last edited:
For guitar preamps? No, not at all!
There is a type of amp where DC coupling is a good idea. All modern solid-state power amplifiers for Hi-Fi use large amounts of global negative feedback. In this case, it is necessary to minimise phase shifts within the amplifier, otherwise you will get an oscillator rather than an amplifier. DC coupling helps to reduce unwanted phase shifts near the low-frequency end of the audio range.
Short answer, no, it's not. 🙂
Longer answer, keep things DC coupled if it simplifies the circuit and does not cause any unwanted side effects, otherwise AC couple.
Electronics is too complicated to reduce everything to a handful of simple rules.
-Gnobuddy
Well, this is a starting point and a proper mindset.
Keep it simple and add stuff only if necessary 🙂.
About M.C1 and M.C2 - if using AC coupling to minimize pot travel noise...
The pot is still DC coupled to the opamp input.
So, AC coupling to minimize pot noise is not an argument for AC coupling there?
About the phase switch... If I put apropriate value electrolytic cap in series with a large resitor, and the switch across the resistor:
- would that minimize switch popping?
- I hope that shunting the opamp input via elec cap would work app. the same.
Keep it simple and add stuff only if necessary 🙂.
About M.C1 and M.C2 - if using AC coupling to minimize pot travel noise...
The pot is still DC coupled to the opamp input.
So, AC coupling to minimize pot noise is not an argument for AC coupling there?
About the phase switch... If I put apropriate value electrolytic cap in series with a large resitor, and the switch across the resistor:
- would that minimize switch popping?
- I hope that shunting the opamp input via elec cap would work app. the same.
As long as there is no DC voltage across the two ends of the pot, you should be okay.
If the opamps have BJT input stages, there will be a tiny DC (bias) current flowing in or out of the opamp input pins. In practice, I think this current is usually so small that it doesn't cause audible scratching sounds.
Your schematic shows a TL072, which has JFET inputs, so there is virtually no bias current (thousands of times less than for a BJT input opamp), and no worries about scratching sounds.
In your circuit, P.C1 is already grounded through P.R3, so closing P.SW1 shouldn't cause any abrupt DC voltage change across P.C1.
However, the opamp will have its own internal DC offset voltage between the inverting and non-inverting pins, and it is possible that this might cause a pop when P.SW1 is opened or closed.
Why not put a capacitor in series with P.SW1? Something with a reactance much less than P.R1 at the lowest frequency of interest.
-Gnobuddy
Cap in series with the switch yields bad results on simulation.
I will leave it as is.
Finally, I could omit P.C1 (phase switcher input ac coupling), M.C1, M.C2 (crossfader inputs), right?
Or it is good ti leave pads on the pcb... The whole stuff is going to be modullar. About 9-10 pcbs to design (incl. psu and preamp) .
With options for ac caps, but as little AC caps as possible to be soldered ��
I will leave it as is.
Finally, I could omit P.C1 (phase switcher input ac coupling), M.C1, M.C2 (crossfader inputs), right?
Or it is good ti leave pads on the pcb... The whole stuff is going to be modullar. About 9-10 pcbs to design (incl. psu and preamp) .
With options for ac caps, but as little AC caps as possible to be soldered ��
Why would you want an FX loop on a bass amp? FX loops are really only needed for guitar, so that FX pedals can be used to shape a distorted signal from the preamp. Not a common scenario for bass - and I'm speaking as a bass player, as well as guitarist.
Or put a high value (1 meg, say) resistor across the switch, as well as the cap in series.
Without P.C1, any DC offset from the output of opamp S.U1.B will contribute to any pop/thump/click issues when P.SW1 is opened or closed.
Personally, I would leave those caps in circuit - they make sure there is no DC on M.RV1 (less chance of scratchy operation).
No prototype? No breadboard testing? Ten PCBs straight away, for an untested circuit? Bit of a gamble, that.
-Gnobuddy
There are some genres of music that use heavily distorted bass guitar. Heck, heavily distorted everything.
Not my cup of tea, but there it is.
Links:
1) Unlocking The Secrets Of Bass Distortion | Seymour Duncan
2) Blend Clean and Distorted Bass for Better Tone - inSync
-Gnobuddy
B.C3, B.C4 and R.C3, R.C4 connections are wrong!
pin8 should be connected directly to +16 and C3 should connect to Pin8 and ground. Same as Pin4 should be directly connected to -16 and C4 should connect to pin4 and ground
pin8 should be connected directly to +16 and C3 should connect to Pin8 and ground. Same as Pin4 should be directly connected to -16 and C4 should connect to pin4 and ground
Good catch! I didn't even notice that drawing error.
I suspect it is only an error in where the "+18" and "-18" legends were placed on the drawing. They should have been near pins 8 and 4 rather than the ground ends of those decoupling capacitors.
-Gnobuddy
- Status
- Not open for further replies.