OP AMP Tone Control Circuit Problems

Hi guys, This is my first post to these forums but I've been a reader for a long time. I have a quick question for the troubleshooters out there. I recently created an Op Amp based tone control circuit. The first attempt I used a TL072 Dual Op Amp and I coulden't get it to work at all. So I switched to an LM358 Dual Op Amp and I got it to work... Kinda... The thing is when I turn the Treble Pot CCW to boost the Treble Frequencies it distorts the audio. The Bass control on the other hand seems to make no difference at all on the tone. I'd post the schematic I used but I'm not sure how to post them on here. I'll try and describe it though. I used the standard Baxandall model for the circuit with 10k and 100k resistors and a 50k pot for each (treble and bass) my Treble has a .001uf Cap in series with the Wiper and the Bass has a .047uf Cap in parallel to the Pot. I was kind of ghetto in my testing and I've never had to use LM358's so I used a standard 9V battery. Do 358's need Dual PS's? The Datasheet showed V+ and Gnd so I figured it was a standard supply. If I left out any info that you guys need to know just ask and I'll get back. Your help would be GREATLY appreciated because I hope to combine this circuit with another for a bigger project I had in mind. Thanks!
 
DC Coupled? I guess I really should put a cap on the input and output I diden't think about that. Plus yeah I'll hook it to a real power supply tomorrow. Just out of curiosity why would I want bypass (or filter if you wanna consider it that) caps across my battery? Batteries are pure DC as far as I know. I'll go back and look at it tomorrow and try and add the schematic to this post.
 

janusz

Member
2004-10-05 9:45 am
Perth
Hi Nick,

If your pots are 50k each then what is 100k resistors doing there? Take a look at the attached tone control schematics based on 50k pots. This circuit with NE5532 OP AMPs should work very well with PS from +/- 15V up to +/-21 V. You may reduce LF boost/cut by replacing 4.7k resistors with higher values - eg 10k. To better understand how tone controls work you may read Mark's Garvin article on this topic - just search the web for it.

You do not need to use NE5532 in this circuit. You may use either single or dual OP AMPs. The choice of alternatives to NE5532 is quite large. You may opt for the top OP AMPs such as OPA627 (this one works best when forced into class A) or much cheaper OP285 (dual), OPA2604 (dual) OPA604 (single), OPA132 (single), OPA2132 (dual) etc. TLs are not the best choice in this application because of their higher noise level but otherwise these work fine.

True, that some OP AMPs are not performing best at very low PS voltages but I do not think that +/-4.5V is the main problem here although without seeing your schematics it is a guess. Nevertheless, tone controls should use the highest possible PS voltage to prevent early clipping – do not forget that at the max boost the output voltage at high and/or low frequencies is a few times that at the centre frequency (about 1kHz) so with a high output CD player, volume control after tone controls and low PS voltage you may overdrive input and output capability of any OP AMP straight away.

Cheers,
 

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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Nick,
Your signal common is also your negative supply. Therefore your circuit wil clip off all the negative excursions. The common mode now includes the negative rail. The only solution would be to create a midpoint and use that as your signal common. This puts the input and output of the op amp at ½ the supply, requiring capacitive coupling. As I mentioned earlier, these op amps don't work at their best at +/- 4.5 VDC (with a fresh battery). But at least you would get sound.

Connecting a real bipolar supply will help. Bypassing. As a battery discharges the AC impedance goes up, a capacitor will extend the useful battery life. Local supply bypass capacitors are used often and are just good design practice. Why not solve a problem before it exists so you don't have to rediscover the wheel?

-Chris