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Author Topic: Help with making of a preamp  (Read 9782 times)

zyggurat

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Help with making of a preamp
« on: February 26, 2007, 04:35:23 PM »
Hello. I ´m going to make my own preamp that ´ll use TONEMENDER (tone control) http://www.runoffgroove.com/tonemender.png and FX loop called "splitter blend" http://www.runoffgroove.com/splitter-blend2.png from runoffgroove.com.
The preamp will look like this:

Input  -> Tonemender -> FX loop -> Output (to poweramp)

1.In schematic of tonemender, why is the first opamp wired as a voltage follower? Isn´t  there any gain needed for input of my preamp? Can (should) I wire this as a normally non-inverting opamp to get bit of gain?

2.The "splitter-blend" is dual fx loop , but I´m going to use only the RED part of that loop, so I´ll get single loop. Is that right? Will it work?
Another question about this fx loop is same as about the tonemender, in this RED part of fx loop there are two opamps, both wired as voltage followers.

3.If I rewire this two circuits to feed they from dual power supply, can I feed poweramp from the single PS? If yes, how should I connect their grounds together? 0V (gnd) of dual PS with - (0V) of single PS ?

4.I ´ve read somewhere of adding a cap to paralel with volume pot, to avoid highs or bass loss at low volumes, is one cap enough for that ? How can I calculate it´s value?

Thanks for all replies. ;)

teemuk

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Re: Help with making of a preamp
« Reply #1 on: February 26, 2007, 05:37:38 PM »
1. Yes, you should: The following tonestack is passive and will introduce insertion losses. Insertion losses on low amplitude signal mean added noise.
2. Yes it will. Naturally you remove the "blend" potentiometer as well. Using buffers (voltage follower) here is ok, even recommendable - assuming the signal levels are suitable of course.
3. I don't quite follow you. If you have two separate supplies (dual and single) you connect their commons together. If you have one single supply you need a virtual ground circuit.
4. Works for high frequencies, not for lows. This is known as "treble bleeder" circuit. The cap connects from potentiometer's "input" node to wiper. Calculation is tricky: You have to perform vector math to calculate impedance of parallel connection of capacitive reactance 1/(2pi*f*C) and resistance between "input" node and wiper.
http://www.allaboutcircuits.com/vol_2/chpt_4/4.html
http://www.concentric.net/~pvb/GEOM/pimpedance.html
...personally, I'd just experiment to find a suitable value. This is one of the places where SPICE simulations become very handy.
« Last Edit: February 26, 2007, 05:52:12 PM by teemuk »

zyggurat

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Re: Help with making of a preamp
« Reply #2 on: February 27, 2007, 10:34:03 AM »
Thanks a lot for your help.

zyggurat

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Re: Help with making of a preamp
« Reply #3 on: February 27, 2007, 02:48:30 PM »
I ´ve got one question else, if a tonemender will be at input of this preamp, how I should design input section (I mean first opamp in tonemender wired as volt. follower), should I take a look of some ss preamp schematics and use it´s input buffer? How the design of the input buffer affect sound of preamp? And at last, how high should be input impedance of my preamp? I´ve read that manufacturers use something about 200 - 500 kOhms , but will it be bad if I use higher value , for example 1MOhm? Thanks for replies. ;)

teemuk

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Re: Help with making of a preamp
« Reply #4 on: February 27, 2007, 03:08:40 PM »
I don't know why you would need a buffer at the input: They have their purpose inboard the guitar's from where the signal has to travel through a long wire to the next circuit. The other use of a buffer is to decrease output impedance and with an opamp that is already significantly small.

IMO, the best bet is just to turn opamp U1a into a non-inverting amplifier that has a suitable gain. There is already an input resistance defined by the parallel connection of 10M and 1M resistors (about 900k-ohms). This is quite ok for magnetic pickups. Higher values would be better but unfortunately they tend to increase noise.

zyggurat

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Re: Help with making of a preamp
« Reply #5 on: March 04, 2007, 07:33:45 AM »
Is it better to use active or passive low pass filter? I´d like to place it after tonemender, because freq. response of my speaker ends at 20kHz and I want to remove those horrible highs from 5kHz and higher. I know how to design it but don ´t know which type is better to use.

Northwave

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Re: Help with making of a preamp
« Reply #6 on: April 27, 2007, 06:15:26 AM »
Hi! First post. :D

It really depends on the following circuit. A passive lowpass will create a fairly low impedance output, which may or may not be suitable for your use (depending on the following stages input impedance). An active lowpass also gives you the opertunity for a steeper freq-response.

zyggurat

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Re: Help with making of a preamp
« Reply #7 on: May 25, 2007, 03:20:51 PM »
Here´s a schematic of simple low pass filter, that I´m looking forward to use: 



Two resistors "R" and capacitors "C" set the cutoff frequency, but what I´ve to change in schematic if I want to adjust frequency by a pot? Is there need to substitute that two resistors by a double pot ? I think this is not very good solution ... Anyone got an idea?

blindsjc

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Re: Help with making of a preamp
« Reply #8 on: September 11, 2007, 10:56:53 PM »
Hi zyggurat,
I use a tonemender as a preamp in my handmade amp.
Like this: Vulvan Distortion -> Tonemender -> TDA2050

Did you had some testing with the first opamp as a non inverting
amp? I'm really curious about how much this can make my amp
sound better. Somebody haves any info about this?

Thanks a lot people


zyggurat

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Re: Help with making of a preamp
« Reply #9 on: September 24, 2007, 01:52:55 PM »
My preamp is not complete yet, tonemender section works well, but I ´ve got some problems with effect loop section - I decided to use marshall valvestate effect loop:


I can´t find out what´s the problem , my pcb is ok and schematic looks ok too...

TO blindsjc:

If you connect first opamp as non inverting amplifier you´ll get more gain - gain of voltage follower is 1, but I think it doesn´t affect the tone too much.


teemuk

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Re: Help with making of a preamp
« Reply #10 on: September 24, 2007, 03:40:59 PM »
In my opinion, a parallel loop (which the "mixing" circuit mostly is) is not that great as one would initially think: It may turn into an oscillator if the effect in the loop causes enough phase shift, it is also less effective with equalizing effects.

Besides that issue, the circuit looks OK to me (assuming the circuit feeding it has proper output signal levels and buffering properties). Can you provide a brief description of what seems to be the problem?

zyggurat

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Re: Help with making of a preamp
« Reply #11 on: September 26, 2007, 01:33:04 PM »
In my opinion, a parallel loop (which the "mixing" circuit mostly is) is not that great as one would initially think: It may turn into an oscillator if the effect in the loop causes enough phase shift, it is also less effective with equalizing effects.

Besides that issue, the circuit looks OK to me (assuming the circuit feeding it has proper output signal levels and buffering properties). Can you provide a brief description of what seems to be the problem?

I decided to rebuild fx loop to a simple fx loop consisting of two opamp buffers that you posted in this topic http://www.ssguitar.com/index.php?topic=479.0

but now I ´ve got a problem: preamp oscillates at frequency approx. 1khz - I hear it from the speaker.
After fx loop there´s low pass filter with gain=20 and another non-inverting opamp -I ´ve tried to change it´s gain but it doesn´t affect the oscillations. I haven´t got an idea where´s the problem. :(

 

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