[darren adcock]

Hi title doesn't really cover my question. In a modular synth I would like a bunch of pre amps/boosts or amps to control the outputs from various audio signal sources. I have built a couple of small single op amp mixers but I still don't get amplification that can help me match signal LVLS (for example however much I love the sines that come from resonant parts of filters I can never get a signal to match their volume, getting a sine wave to compete with a square wave is also a problem). Is my solution to build seperate amps for all of these or I am overcomplicating things?

Thanks in advance

Darren

 

[BobK]

Active filters can include gain, even adjustable gain.

Bob

 

[darren adcock]

I will look into that thank you.

 

[darren adcock]

Just to report back. I tried to understand the filter I am using to see where the gain can be adjusted but alas I couldn't work it out. Ended up realising I could just increase the gain on the signal in opamp which solved the problem of mixing both the sine resonance and signal in. Thanks for advice, I have contacted soundtronics to see if they can point me in the right direction based on their circuit. If they do I'll post the circuit up here.

Best

Darren

 

[Audioguru]

You did not post the schematic of your filter so we are guessing that it is a bandpass resonant filter of some kind.
Most active filters oscillate if their gain is a little too high.

I made a bass boost circuit for my audio system. It is a Sallen-Key active highpass filter with a gain adjustment. When the gain of its opamp is 1 then the frequency response is flat. When the gain is close to 2 times then frequencies near the cutoff frequency are boosted. When the gain is 3 times or higher then it oscillates.

 

[darren adcock]

Thanks Audio guru. I will post the schematic asap.

 

[darren adcock]

Here is the schematic of the VCF I have built. It is two files and I will also include the MFOS article pg link. The article does mention gain for the resonance but I got confused as it goes into the LM13700N and I couldn't follow the termilogy used when describing the op amp and LM13700N's relationship. A side note is that the LM13700N has been quite a revelation for me, look forward to working with them more.

http://musicfromouterspace.com/inde...G=VCFJAN2006/VCFJAN2006.html&VPW=2038&VPH=784

 

[darren adcock]

You did not post the schematic of your filter so we are guessing that it is a bandpass resonant filter of some kind.
Most active filters oscillate if their gain is a little too high.

I made a bass boost circuit for my audio system. It is a Sallen-Key active highpass filter with a gain adjustment. When the gain of its opamp is 1 then the frequency response is flat. When the gain is close to 2 times then frequencies near the cutoff frequency are boosted. When the gain is 3 times or higher then it oscillates.

I'll put your circuit together later on today so I understand it better. Thanks for including the graph also that helped a lot.

 

[Audioguru]

I have seen many circuits that use the LM13700 IC and I have its datasheet but I never used one.
Did you know that the TL08x is a "general purpose" opamp and can be fairly noisy but a TL07x is a TL08x selected for low noise? I used many TL07x opamps in my audio career.

I just looked at Digikey. The TL072 low noise opamp costs less than the TL082 noisy opamp.

 

[darren adcock]

That's good to know thank you. As it happens I have been buying the tl07 series instead as it's cheaper.... I read in various places the LM324 was preverable for audio purposes. Do you have a preference between this and the TL074? When i'm looking at datasheets which bit tells me how noisey they are? What is this noise?

 

[darren adcock]

What is it you build? specialise in?

 

[Audioguru]

I worked with Pro sound systems, very high quality huge intercom systems and deluxe tele-conferencing systems.

The lousy old LM324 quad and its sister the LM358 dual are noisy, have crossover distortion and have trouble with frequencies higher than only 2kHz because they were one of the first opamps to use low power (low current).

ST Micro lists the LM358 Equivalent Input Noise voltage at 55nV/root Hz when its input is nearly shorted with a 100 ohm resistor to ground, they do not say how much noise current into a high resistance because it will be very high and they cheat on distortion and wrongly say 0.02%. The TL07x lists Equivalent Input Noise voltage at 18nV/root Hz when its input has a low resistance and its its noise current at almost zero into a high resistance. The distortion of a TL07x is listed at 0.003%. The TL07x works well up to 100kHz.

There are many audio opamps that are better than the TL07x. The OPA134, OPA2134 and OPA4134 list noise voltage at 8nV/root Hz, noise current at almost zero, distortion at 0.00008% and work well up to 250kHz.

 

[darren adcock]

Thanks.

That's hugely helpful.

Hadn't come accross the OPA series.